Hubbry Logo
Nicotinamide adenine dinucleotideNicotinamide adenine dinucleotideMain
Open search
Nicotinamide adenine dinucleotide
Community hub
Nicotinamide adenine dinucleotide
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Nicotinamide adenine dinucleotide
Nicotinamide adenine dinucleotide
Nicotinamide adenine dinucleotide
View on Wikipedia
from Wikipedia

Nicotinamide adenine dinucleotide
Skeletal formula of the oxidized form
Skeletal formula of the oxidized form
Ball-and-stick model of the oxidized form
Ball-and-stick model of the oxidized form
Names
Other names
Diphosphopyridine nucleotide (DPN+), Coenzyme I
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.000.169 Edit this at Wikidata
KEGG
RTECS number
  • UU3450000
UNII
  • InChI=1S/C21H27N7O14P2/c22-17-12-19(25-7-24-17)28(8-26-12)21-16(32)14(30)11(41-21)6-39-44(36,37)42-43(34,35)38-5-10-13(29)15(31)20(40-10)27-3-1-2-9(4-27)18(23)33/h1-4,7-8,10-11,13-16,20-21,29-32H,5-6H2,(H5-,22,23,24,25,33,34,35,36,37)/t10-,11-,13-,14-,15-,16-,20-,21-/m1/s1 checkY
    Key: BAWFJGJZGIEFAR-NNYOXOHSSA-N checkY
  • InChI=1/C21H27N7O14P2/c22-17-12-19(25-7-24-17)28(8-26-12)21-16(32)14(30)11(41-21)6-39-44(36,37)42-43(34,35)38-5-10-13(29)15(31)20(40-10)27-3-1-2-9(4-27)18(23)33/h1-4,7-8,10-11,13-16,20-21,29-32H,5-6H2,(H5-,22,23,24,25,33,34,35,36,37)/t10-,11-,13-,14-,15-,16-,20-,21-/m1/s1
    Key: BAWFJGJZGIEFAR-NNYOXOHSBR
  • NAD+: O=C(N)c1ccc[n+](c1)[C@@H]2O[C@@H]([C@@H](O)[C@H]2O)COP([O-])(=O)OP(=O)([O-])OC[C@H]5O[C@@H](n4cnc3c(ncnc34)N)[C@H](O)[C@@H]5O
  • NADH: O=C(N)C1CC=C[N](C=1)[C@@H]2O[C@@H]([C@@H](O)[C@H]2O)COP([O-])(=O)OP(=O)([O-])OC[C@H]5O[C@@H](n4cnc3c(ncnc34)N)[C@H](O)[C@@H]5O
Properties
C21H28N7O14P2+ (oxidized)
C21H29N7O14P2 (reduced)
Molar mass 664.4 g/mol (oxidized)
665.4 g/mol (reduced)
Appearance White powder
Melting point 160 °C (320 °F; 433 K)
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 Not hazardous
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
1
0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism.[1] Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an adenine nucleobase and the other, nicotinamide. NAD exists in two forms: an oxidized and reduced form, abbreviated as NAD+ and NADH (H for hydrogen), respectively.

In cellular metabolism, NAD is involved in redox reactions, carrying electrons from one reaction to another, so it is found in two forms: NAD+ is an oxidizing agent, accepting electrons from other molecules and becoming reduced; with H+, this reaction forms NADH, which can be used as a reducing agent to donate electrons. These electron transfer reactions are the main function of NAD. It is also used in other cellular processes, most notably as a substrate of enzymes in adding or removing chemical groups to or from proteins, in posttranslational modifications. Because of the importance of these functions, the enzymes involved in NAD metabolism are targets for drug discovery.

In organisms, NAD can be synthesized from simple building-blocks (de novo) from either tryptophan or aspartic acid, each a case of an amino acid. Alternatively, more complex components of the coenzymes are taken up from nutritive compounds such as nicotinic acid; similar compounds are produced by reactions that break down the structure of NAD, providing a salvage pathway that recycles them back into their respective active form.

In the name NAD+, the superscripted plus sign indicates the positive formal charge on one of its nitrogen atoms. A biological coenzyme that acts as an electron carrier in enzymatic reactions.

Some NAD is converted into the coenzyme nicotinamide adenine dinucleotide phosphate (NADP), whose chemistry largely parallels that of NAD, though its predominant role is as a coenzyme in anabolic metabolism. NADP is a reducing agent in anabolic reactions like the Calvin cycle and lipid and nucleic acid syntheses. NADP exists in two forms: NADP+, the oxidized form, and NADPH, the reduced form. NADP is similar to nicotinamide adenine dinucleotide (NAD), but NADP has a phosphate group at the C-2′ position of the adenosyl.

Physical and chemical properties

[edit]
Further information: Redox

Nicotinamide adenine dinucleotide consists of two nucleosides joined by pyrophosphate. The nucleosides each contain a ribose ring, one with adenine attached to the first carbon atom (the 1' position) (adenosine diphosphate ribose) and the other with nicotinamide at this position.[2][3]

The redox reactions of nicotinamide adenine dinucleotide

The compound accepts or donates the equivalent of H−.[4] Such reactions (summarized in formula below) involve the removal of two hydrogen atoms from a reactant (R), in the form of a hydride ion (H−), and a proton (H+). The proton is released into solution, while the reductant RH2 is oxidized and NAD+ reduced to NADH by transfer of the hydride to the nicotinamide ring.

RH2 + NAD+ → NADH + H+ + R;

From the electron pair of the hydride ion, one electron is attracted to the slightly more electronegative atom of the nicotinamide ring of NAD+, becoming part of the nicotinamide moiety. The remaining hydrogen atom is transferred to the carbon atom opposite the N atom. The midpoint potential of the NAD+/NADH redox pair is −0.32 volts, which makes NADH a moderately strong reducing agent.[5] The reaction is easily reversible, when NADH reduces another molecule and is re-oxidized to NAD+. This means the coenzyme can continuously cycle between the NAD+ and NADH forms without being consumed.[3]

In appearance, all forms of this coenzyme are white amorphous powders that are hygroscopic and highly water-soluble.[6] The solids are stable if stored dry and in the dark. Solutions of NAD+ are colorless and stable for about a week at 4 °C and neutral pH, but decompose rapidly in acidic or alkaline solutions. Upon decomposition, they form products that are enzyme inhibitors.[7]

UV absorption spectra of NAD+ and NADH[image reference needed]

Both NAD+ and NADH strongly absorb ultraviolet light because of the adenine. For example, peak absorption of NAD+ is at a wavelength of 259 nanometers (nm), with an extinction coefficient of 16,900 M−1cm−1. NADH also absorbs at higher wavelengths, with a second peak in UV absorption at 339 nm with an extinction coefficient of 6,220 M−1cm−1.[8] This difference in the ultraviolet absorption spectra between the oxidized and reduced forms of the coenzymes at higher wavelengths makes it simple to measure the conversion of one to another in enzyme assays – by measuring the amount of UV absorption at 340 nm using a spectrophotometer.[8]

NAD+ and NADH also differ in their fluorescence. Freely diffusing NADH in aqueous solution, when excited at the nicotinamide absorbance of ~335 nm (near-UV), fluoresces at 445–460 nm (violet to blue) with a fluorescence lifetime of 0.4 nanoseconds, while NAD+ does not fluoresce.[9][10] The properties of the fluorescence signal changes when NADH binds to proteins, so these changes can be used to measure dissociation constants, which are useful in the study of enzyme kinetics.[10][11] These changes in fluorescence are also used to measure changes in the redox state of living cells, through fluorescence microscopy.[12]

NADH can be converted to NAD+ in a reaction catalysed by copper, which requires hydrogen peroxide. Thus, the supply of NAD+ in cells requires mitochondrial copper(II).[13][14]

Concentration and state in cells

[edit]

In rat liver, the total amount of NAD+ and NADH is approximately 1 μmole per gram of wet weight, about 10 times the concentration of NADP+ and NADPH in the same cells.[15] The actual concentration of NAD+ in cell cytosol is harder to measure, with recent estimates in animal cells ranging around 0.3 mM,[16][17] and approximately 1.0 to 2.0 mM in yeast.[18] However, more than 80% of NADH fluorescence in mitochondria is from bound form, so the concentration in solution is much lower.[19]

NAD+ concentrations are highest in the mitochondria, constituting 40% to 70% of the total cellular NAD+.[20] NAD+ in the cytosol is carried into the mitochondrion by a specific membrane transport protein, since the coenzyme cannot diffuse across membranes.[21] The intracellular half-life of NAD+ was claimed to be between 1–2 hours by one review,[22] whereas another review gave varying estimates based on compartment: intracellular 1–4 hours, cytoplasmic 2 hours, and mitochondrial 4–6 hours.[23]

The balance between the oxidized and reduced forms of nicotinamide adenine dinucleotide is called the NAD+/NADH ratio. This ratio is an important component of what is called the redox state of a cell, a measurement that reflects both the metabolic activities and the health of cells.[24] The effects of the NAD+/NADH ratio are complex, controlling the activity of several key enzymes, including glyceraldehyde 3-phosphate dehydrogenase and pyruvate dehydrogenase. In healthy mammalian tissues, estimates of the ratio of free NAD+ to NADH in the cytoplasm typically lie around 700:1; the ratio is thus favorable for oxidative reactions.[25][26] The ratio of total NAD+/NADH is much lower, with estimates ranging from 3–10 in mammals.[27] In contrast, the NADP+/NADPH ratio is normally about 0.005, so NADPH is the dominant form of this coenzyme.[28] These different ratios are key to the different metabolic roles of NADH and NADPH.

Biosynthesis

[edit]

NAD+ is synthesized through two metabolic pathways. It is produced either in a de novo pathway from amino acids or in salvage pathways by recycling preformed components such as nicotinamide back to NAD+. Although most tissues synthesize NAD+ by the salvage pathway in mammals, much more de novo synthesis occurs in the liver from tryptophan, and in the kidney and macrophages from nicotinic acid.[29]

De novo production

[edit]
Some metabolic pathways that synthesize and consume NAD+ in vertebrates.[image reference needed] The abbreviations are defined in the text.

Most organisms synthesize NAD+ from simple components.[4] The specific set of reactions differs among organisms, but a common feature is the generation of quinolinic acid (QA) from an amino acid – either tryptophan (Trp) in animals and some bacteria, or aspartic acid (Asp) in some bacteria and plants.[30][31] The quinolinic acid is converted to nicotinic acid mononucleotide (NaMN) by transfer of a phosphoribose moiety. An adenylate moiety is then transferred to form nicotinic acid adenine dinucleotide (NaAD). Finally, the nicotinic acid moiety in NaAD is amidated to a nicotinamide (Nam) moiety, forming nicotinamide adenine dinucleotide.[4]

In a further step, some NAD+ is converted into NADP+ by NAD+ kinase, which phosphorylates NAD+.[32] In most organisms, this enzyme uses adenosine triphosphate (ATP) as the source of the phosphate group, although several bacteria such as Mycobacterium tuberculosis and a hyperthermophilic archaeon Pyrococcus horikoshii, use inorganic polyphosphate as an alternative phosphoryl donor.[33][34]

Salvage pathways use three precursors for NAD+.

Salvage pathways

[edit]

Despite the presence of the de novo pathway, the salvage reactions are essential in humans; a lack of vitamin B3 in the diet causes the vitamin deficiency disease pellagra.[35] This high requirement for NAD+ results from the constant consumption of the coenzyme in reactions such as posttranslational modifications, since the cycling of NAD+ between oxidized and reduced forms in redox reactions does not change the overall levels of the coenzyme.[4] The major source of NAD+ in mammals is the salvage pathway which recycles the nicotinamide produced by enzymes utilizing NAD+.[36] The first step, and the rate-limiting enzyme in the salvage pathway is nicotinamide phosphoribosyltransferase (NAMPT), which produces nicotinamide mononucleotide (NMN).[36] NMN is the immediate precursor to NAD+ in the salvage pathway.[37]

Besides assembling NAD+ de novo from simple amino acid precursors, cells also salvage preformed compounds containing a pyridine base. The three vitamin precursors used in these salvage metabolic pathways are nicotinic acid (NA), nicotinamide (Nam) and nicotinamide riboside (NR).[4] These compounds can be taken up from the diet and are termed vitamin B3 or niacin. However, these compounds are also produced within cells and by digestion of cellular NAD+. Some of the enzymes involved in these salvage pathways appear to be concentrated in the cell nucleus, which may compensate for the high level of reactions that consume NAD+ in this organelle.[38] There are some reports that mammalian cells can take up extracellular NAD+ from their surroundings,[39] and both nicotinamide and nicotinamide riboside can be absorbed from the gut.[40]

The salvage pathways used in microorganisms differ from those of mammals.[41] Some pathogens, such as the yeast Candida glabrata and the bacterium Haemophilus influenzae are NAD+ auxotrophs – they cannot synthesize NAD+ – but possess salvage pathways and thus are dependent on external sources of NAD+ or its precursors.[42][43] Even more surprising is the intracellular pathogen Chlamydia trachomatis, which lacks recognizable candidates for any genes involved in the biosynthesis or salvage of both NAD+ and NADP+, and must acquire these coenzymes from its host.[44]

Functions

[edit]
Rossmann fold in part of the lactate dehydrogenase of Cryptosporidium parvum, showing NAD+ in red, beta sheets in yellow, and alpha helices in purple[45]

Nicotinamide adenine dinucleotide has several essential roles in metabolism. It acts as a coenzyme in redox reactions, as a donor of ADP-ribose moieties in ADP-ribosylation reactions, as a precursor of the second messenger molecule cyclic ADP-ribose, as well as acting as a substrate for bacterial DNA ligases and a group of enzymes called sirtuins that use NAD+ to remove acetyl groups from proteins. In addition to these metabolic functions, NAD+ emerges as an adenine nucleotide that can be released from cells spontaneously and by regulated mechanisms,[46][47] and can therefore have important extracellular roles.[47]

Oxidoreductase binding of NAD

[edit]
Further information: Protein structure and Oxidoreductase

The main role of NAD+ in metabolism is the transfer of electrons from one molecule to another. Reactions of this type are catalyzed by a large group of enzymes called oxidoreductases. The correct names for these enzymes contain the names of both their substrates: for example NADH-ubiquinone oxidoreductase catalyzes the oxidation of NADH by coenzyme Q.[48] However, these enzymes are also referred to as dehydrogenases or reductases, with NADH-ubiquinone oxidoreductase commonly being called NADH dehydrogenase or sometimes coenzyme Q reductase.[49]

There are many different superfamilies of enzymes that bind NAD+ / NADH. One of the most common superfamilies includes a structural motif known as the Rossmann fold.[50][51] The motif is named after Michael Rossmann, who was the first scientist to notice how common this structure is within nucleotide-binding proteins.[52]

An example of a NAD-binding bacterial enzyme involved in amino acid metabolism that does not have the Rossmann fold is found in Pseudomonas syringae pv. tomato (PDB: 2CWH​; InterPro: IPR003767).[53]

In this diagram, the hydride acceptor C4 carbon is shown at the top. When the nicotinamide ring lies in the plane of the page with the carboxy-amide to the right, as shown, the hydride donor lies either "above" or "below" the plane of the page. If "above" hydride transfer is class A, if "below" hydride transfer is class B.[54]

When bound in the active site of an oxidoreductase, the nicotinamide ring of the coenzyme is positioned so that it can accept a hydride from the other substrate. Depending on the enzyme, the hydride donor is positioned either "above" or "below" the plane of the planar C4 carbon, as defined in the figure. Class A oxidoreductases transfer the atom from above; class B enzymes transfer it from below. Since the C4 carbon that accepts the hydrogen is prochiral, this can be exploited in enzyme kinetics to give information about the enzyme's mechanism. This is done by mixing an enzyme with a substrate that has deuterium atoms substituted for the hydrogens, so the enzyme will reduce NAD+ by transferring deuterium rather than hydrogen. In this case, an enzyme can produce one of two stereoisomers of NADH.[54]

Despite the similarity in how proteins bind the two coenzymes, enzymes almost always show a high level of specificity for either NAD+ or NADP+.[55] This specificity reflects the distinct metabolic roles of the respective coenzymes, and is the result of distinct sets of amino acid residues in the two types of coenzyme-binding pocket. For instance, in the active site of NADP-dependent enzymes, an ionic bond is formed between a basic amino acid side-chain and the acidic phosphate group of NADP+. On the converse, in NAD-dependent enzymes the charge in this pocket is reversed, preventing NADP+ from binding. However, there are a few exceptions to this general rule, and enzymes such as aldose reductase, glucose-6-phosphate dehydrogenase, and methylenetetrahydrofolate reductase can use both coenzymes in some species.[56]

Role in redox metabolism

[edit]
A simplified outline of redox metabolism, showing how NAD+ and NADH link the citric acid cycle and oxidative phosphorylation[image reference needed]
Further information: Cellular respiration and Oxidative phosphorylation

The redox reactions catalyzed by oxidoreductases are vital in all parts of metabolism, but one particularly important area where these reactions occur is in the release of energy from nutrients. Here, reduced compounds such as glucose and fatty acids are oxidized, thereby releasing energy. This energy is transferred to NAD+ by reduction to NADH, as part of beta oxidation, glycolysis, and the citric acid cycle. In eukaryotes the electrons carried by the NADH that is produced in the cytoplasm are transferred into the mitochondrion (to reduce mitochondrial NAD+) by mitochondrial shuttles, such as the malate-aspartate shuttle.[57] The mitochondrial NADH is then oxidized in turn by the electron transport chain, which pumps protons across a membrane and generates ATP through oxidative phosphorylation.[58] These shuttle systems also have the same transport function in chloroplasts.[59]

Since both the oxidized and reduced forms of nicotinamide adenine dinucleotide are used in these linked sets of reactions, the cell maintains significant concentrations of both NAD+ and NADH, with the high NAD+/NADH ratio allowing this coenzyme to act as both an oxidizing and a reducing agent.[60] In contrast, the main function of NADPH is as a reducing agent in anabolism, with this coenzyme being involved in pathways such as fatty acid synthesis and photosynthesis. Since NADPH is needed to drive redox reactions as a strong reducing agent, the NADP+/NADPH ratio is kept very low.[60]

Although it is important in catabolism, NADH is also used in anabolic reactions, such as gluconeogenesis.[61] This need for NADH in anabolism poses a problem for prokaryotes growing on nutrients that release only a small amount of energy. For example, nitrifying bacteria such as Nitrobacter oxidize nitrite to nitrate, which releases sufficient energy to pump protons and generate ATP, but not enough to produce NADH directly.[62] As NADH is still needed for anabolic reactions, these bacteria use a nitrite oxidoreductase to produce enough proton-motive force to run part of the electron transport chain in reverse, generating NADH.[63]

Non-redox roles

[edit]

The coenzyme NAD+ is also consumed in ADP-ribose transfer reactions. For example, enzymes called ADP-ribosyltransferases add the ADP-ribose moiety of this molecule to proteins, in a posttranslational modification called ADP-ribosylation.[64] ADP-ribosylation involves either the addition of a single ADP-ribose moiety, in mono-ADP-ribosylation, or the transferral of ADP-ribose to proteins in long branched chains, which is called poly(ADP-ribosyl)ation.[65] Mono-ADP-ribosylation was first identified as the mechanism of a group of bacterial toxins, notably cholera toxin, but it is also involved in normal cell signaling.[66][67] Poly(ADP-ribosyl)ation is carried out by the poly(ADP-ribose) polymerases.[65][68] The poly(ADP-ribose) structure is involved in the regulation of several cellular events and is most important in the cell nucleus, in processes such as DNA repair and telomere maintenance.[68] In addition to these functions within the cell, a group of extracellular ADP-ribosyltransferases has recently been discovered, but their functions remain obscure.[69] NAD+ may also be added onto cellular RNA as a 5'-terminal modification.[70]

The structure of cyclic ADP-ribose

Another function of this coenzyme in cell signaling is as a precursor of cyclic ADP-ribose, which is produced from NAD+ by ADP-ribosyl cyclases, as part of a second messenger system.[71] This molecule acts in calcium signaling by releasing calcium from intracellular stores.[72] It does this by binding to and opening a class of calcium channels called ryanodine receptors, which are located in the membranes of organelles, such as the endoplasmic reticulum, and inducing the activation of the transcription factor NAFC3[73]

NAD+ is also consumed by different NAD+-consuming enzymes, such as CD38, CD157, PARPs and the NAD-dependent deacetylases (sirtuins, such as Sir2.[74]).[75] These enzymes act by transferring an acetyl group from their substrate protein to the ADP-ribose moiety of NAD+; this cleaves the coenzyme and releases nicotinamide and O-acetyl-ADP-ribose. The sirtuins mainly seem to be involved in regulating transcription through deacetylating histones and altering nucleosome structure.[76] However, non-histone proteins can be deacetylated by sirtuins as well. These activities of sirtuins are particularly interesting because of their importance in the regulation of aging.[77][78]

Other NAD-dependent enzymes include bacterial DNA ligases, which join two DNA ends by using NAD+ as a substrate to donate an adenosine monophosphate (AMP) moiety to the 5' phosphate of one DNA end. This intermediate is then attacked by the 3' hydroxyl group of the other DNA end, forming a new phosphodiester bond.[79] This contrasts with eukaryotic DNA ligases, which use ATP to form the DNA-AMP intermediate.[80]

Li et al. have found that NAD+ directly regulates protein-protein interactions.[81] They also show that one of the causes of age-related decline in DNA repair may be increased binding of the protein DBC1 (Deleted in Breast Cancer 1) to PARP1 (poly[ADP–ribose] polymerase 1) as NAD+ levels decline during aging.[81] The decline in cellular concentrations of NAD+ during aging likely contributes to the aging process and to the pathogenesis of the chronic diseases of aging.[82] Thus, the modulation of NAD+ may protect against cancer, radiation, and aging.[81]

Extracellular actions of NAD+

[edit]

In recent years, NAD+ has also been recognized as an extracellular signaling molecule involved in cell-to-cell communication.[47][83][84] NAD+ is released from neurons in blood vessels,[46] urinary bladder,[46][85] large intestine,[86][87] from neurosecretory cells,[88] and from brain synaptosomes,[89] and is proposed to be a novel neurotransmitter that transmits information from nerves to effector cells in smooth muscle organs.[86][87] In plants, the extracellular nicotinamide adenine dinucleotide induces resistance to pathogen infection and the first extracellular NAD receptor has been identified.[90] Further studies are needed to determine the underlying mechanisms of its extracellular actions and their importance for human health and life processes in other organisms.

Clinical significance

[edit]

The enzymes that make and use NAD+ and NADH are important in both pharmacology and the research into future treatments for disease.[91] Drug design and drug development exploits NAD+ in three ways: as a direct target of drugs, by designing enzyme inhibitors or activators based on its structure that change the activity of NAD-dependent enzymes, and by trying to inhibit NAD+ biosynthesis.[92]

Because cancer cells utilize increased glycolysis, and because NAD enhances glycolysis, nicotinamide phosphoribosyltransferase (NAD salvage pathway) is often amplified in cancer cells.[93][94]

It has been studied for its potential use in the therapy of neurodegenerative diseases such as Alzheimer's and Parkinson's disease as well as multiple sclerosis.[4][78][95][75] A placebo-controlled clinical trial of NADH (which excluded NADH precursors) in people with Parkinson's failed to show any effect.[96]

NAD+ is also a direct target of the drug isoniazid, which is used in the treatment of tuberculosis, an infection caused by Mycobacterium tuberculosis. Isoniazid is a prodrug and once it has entered the bacteria, it is activated by a peroxidase enzyme, which oxidizes the compound into a free radical form.[97] This radical then reacts with NADH, to produce adducts that are very potent inhibitors of the enzymes enoyl-acyl carrier protein reductase,[98] and dihydrofolate reductase.[99]

Since many oxidoreductases use NAD+ and NADH as substrates, and bind them using a highly conserved structural motif, the idea that inhibitors based on NAD+ could be specific to one enzyme is surprising.[100] However, this can be possible: for example, inhibitors based on the compounds mycophenolic acid and tiazofurin inhibit IMP dehydrogenase at the NAD+ binding site. Because of the importance of this enzyme in purine metabolism, these compounds may be useful as anti-cancer, anti-viral, or immunosuppressive drugs.[100][101] Other drugs are not enzyme inhibitors, but instead activate enzymes involved in NAD+ metabolism. Sirtuins are a particularly interesting target for such drugs, since activation of these NAD-dependent deacetylases extends lifespan in some animal models.[102] Compounds such as resveratrol increase the activity of these enzymes, which may be important in their ability to delay aging in both vertebrate,[103] and invertebrate model organisms.[104][105] In one experiment, mice given NAD for one week had improved nuclear-mitochrondrial communication.[106]

Because of the differences in the metabolic pathways of NAD+ biosynthesis between organisms, such as between bacteria and humans, this area of metabolism is a promising area for the development of new antibiotics.[107][108] For example, the enzyme nicotinamidase, which converts nicotinamide to nicotinic acid, is a target for drug design because this enzyme is absent in humans but present in yeast and bacteria.[41]

In bacteriology, NAD, sometimes referred to factor V, is used as a supplement to culture media for some fastidious bacteria.[109]

High-cost unlicensed infusions of NAD+ have been claimed in the UK to be "clinically proven" and "effective" treatment for alcoholism and drug abuse. NAD+ is not approved or licensed for medical use in the UK; there are likely breaches of advertising and medicines rules, and no proof that treatments work. Medical experts say "It's complete nonsense" ... "It's untested and unproven. We don't know anything about its efficacy or long-term safety". A November 2024 study, cited 700 times, claiming that NAD+ levels in lab rats decreased with age was withdrawn after images were found to have been manipulated, and underlying data was not provided to the publishers on request.[110]

History

[edit]
Arthur Harden, co-discoverer of NAD
Further information: History of biochemistry

The coenzyme NAD+ was first discovered by the British biochemists Arthur Harden and William John Young in 1906.[111] They noticed that adding boiled and filtered yeast extract greatly accelerated alcoholic fermentation in unboiled yeast extracts. They called the unidentified factor responsible for this effect a coferment. Through a long and difficult purification from yeast extracts, this heat-stable factor was identified as a nucleotide sugar phosphate by Hans von Euler-Chelpin.[112] In 1936, the German scientist Otto Heinrich Warburg showed the function of the nucleotide coenzyme in hydride transfer and identified the nicotinamide portion as the site of redox reactions.[113]

Vitamin precursors of NAD+ were first identified in 1938, when Conrad Elvehjem showed that liver has an "anti-black tongue" activity in the form of nicotinamide.[114] Then, in 1939, he provided the first strong evidence that nicotinic acid is used to synthesize NAD+.[115] In the early 1940s, Arthur Kornberg was the first to detect an enzyme in the biosynthetic pathway.[116] In 1949, the American biochemists Morris Friedkin and Albert L. Lehninger proved that NADH linked metabolic pathways such as the citric acid cycle with the synthesis of ATP in oxidative phosphorylation.[117] In 1958, Jack Preiss and Philip Handler discovered the intermediates and enzymes involved in the biosynthesis of NAD+;[118][119] salvage synthesis from nicotinic acid is termed the Preiss-Handler pathway. In 2004, Charles Brenner and co-workers uncovered the nicotinamide riboside kinase pathway to NAD+.[120]

The non-redox roles of NAD(P) were discovered later.[3] The first to be identified was the use of NAD+ as the ADP-ribose donor in ADP-ribosylation reactions, observed in the early 1960s.[121] Studies in the 1980s and 1990s revealed the activities of NAD+ and NADP+ metabolites in cell signaling – such as the action of cyclic ADP-ribose, which was discovered in 1987.[122]

The metabolism of NAD+ remained an area of intense research into the 21st century, with interest heightened after the discovery of the NAD+-dependent protein deacetylases called sirtuins in 2000, by Shin-ichiro Imai and coworkers in the laboratory of Leonard P. Guarente.[123] In 2009 Imai proposed the "NAD World" hypothesis that key regulators of aging and longevity in mammals are sirtuin 1 and the primary NAD+ synthesizing enzyme nicotinamide phosphoribosyltransferase (NAMPT).[124] In 2016 Imai expanded his hypothesis to "NAD World 2.0", which postulates that extracellular NAMPT from adipose tissue maintains NAD+ in the hypothalamus (the control center) in conjunction with myokines from skeletal muscle cells.[125] In 2018, Napa Therapeutics was formed to develop drugs against a novel aging-related target based on the research in NAD metabolism conducted in the lab of Eric Verdin.[126]

See also

[edit]

References

[edit]

Further reading

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Nicotinamide adenine dinucleotide

View on Grokipedia
from Grokipedia
Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to , present in all living cells and consisting of two —one containing an base and the other a base—linked by their groups. It exists primarily in oxidized (NAD⁺) and reduced (NADH) forms, where NAD⁺ acts as an by accepting electrons during reactions, while NADH serves as a by donating them. Discovered in the early and fully characterized in , NAD plays an essential role in energy production pathways such as , the , and . Beyond redox functions, NAD⁺ serves as a critical substrate for non-redox enzymes, including sirtuins, poly(ADP-ribose) polymerases (PARPs), and , which regulate processes like , , , and . NAD⁺ levels are maintained through from , the Preiss-Handler pathway from nicotinic acid, and the salvage pathway recycling , with the latter being predominant in mammals. These pathways are regulated by enzymes such as (NAMPT) and nicotinamide mononucleotide adenylyltransferases (NMNATs). Substantial evidence indicates that NAD⁺ levels decline with age, with declines more pronounced in sedentary or impaired individuals, while older adults who exercise maintain levels similar to younger people; typically by ~50–65% from young adulthood to older age in human tissues such as skin, liver, muscle, brain, and blood plasma, though this is not universal across all tissues or studies and human data remain limited compared to rodent models. This decline, attributed to reduced biosynthesis (e.g., lower NAMPT activity), increased consumption by enzymes like CD38 and PARPs, and inflammation, contributes to metabolic dysfunction, genomic instability, and age-related diseases including neurodegeneration, cardiovascular disorders, and cancer. This age-associated depletion impairs and PARP activities, leading to reduced mitochondrial function and increased . In the context of dementia, particularly Alzheimer's disease, NAD⁺ and its precursors such as NMN and NR are essential for cellular energy production, DNA repair, and sirtuin activation; preclinical models demonstrate that boosting NAD⁺ reduces neuroinflammation, mitochondrial dysfunction, and DNA damage. Some preliminary human trials in aging and mild cognitive impairment have shown safe increases in NAD⁺ levels but no clear evidence of cognitive reversal, with therapeutic evidence remaining primarily preclinical. Therapeutic strategies focus on supplementation with NAD⁺ precursors like (NMN) or (NR), as direct oral supplementation with NAD⁺ is ineffective due to poor bioavailability and degradation in the digestive system. NAD⁺ is a dinucleotide coenzyme and not a peptide. Oral precursor supplementation is generally well-tolerated in human trials, with mild side effects such as nausea, fatigue, headache, bloating, flushing, and stomach upset; higher doses may increase the severity of these effects, and long-term safety is not fully established. While these supplements safely raise NAD⁺ levels and show preliminary benefits including improved insulin sensitivity and mitochondrial function in some human studies, evidence for broad anti-aging or disease-reversing claims remains preliminary and inconclusive, with most robust evidence coming from preclinical models. Direct intravenous NAD⁺ injections are also used in some wellness and clinical contexts as an alternative administration method, but high-quality evidence remains limited as of 2026, with small studies and anecdotal reports suggesting potential short-term benefits while large-scale randomized controlled trials are lacking; risks include acute side effects during administration such as nausea, vomiting, flushing, chest tightness, abdominal pain, headache, dizziness, and fatigue, with long-term effects unknown and the approach not FDA-approved for anti-aging or most claimed uses.

Structure and Properties

Chemical Composition

Nicotinamide adenine dinucleotide (NAD) is a dinucleotide coenzyme composed of two units: (NMN) and adenosine monophosphate (AMP), connected via a pyrophosphate linkage between their 5'-phosphate groups. The NMN portion consists of a base attached to a sugar and a group, while the AMP portion features an base linked to a sugar and a group. This structure positions the nicotinamide ring at one terminus and the adenine ring at the other, forming the molecular formula C21H27N7O14P2 for the oxidized form, NAD+. The key functional groups in NAD contribute to its biological reactivity. The pyridine ring of the moiety serves as the site for (H-) acceptance during reduction reactions, enabling NAD+ to act as an in metabolic processes. In contrast, the ring facilitates specific binding to enzymes through hydrogen bonding interactions, ensuring precise recognition and orientation within active sites. NAD exists in two interconvertible forms: the oxidized NAD+, where the nicotinamide ring carries a positive charge as a ion, and the reduced NADH, in which the ring accepts a to form a neutral structure. This transformation alters the molecule's electronic properties without disrupting the overall dinucleotide backbone.

Physical Characteristics

Nicotinamide adenine dinucleotide (NAD) exists in oxidized (NAD⁺) and reduced (NADH) forms, each with distinct molecular weights of 663.43 g/mol and 665.44 g/mol, respectively. NAD⁺ and NADH exhibit high in , exceeding 50 mg/mL at 20°C for NAD⁺, while NADH is similarly soluble but often prepared in mildly alkaline conditions to enhance stability. In contrast, both forms show limited in organic solvents such as and DMSO, remaining largely insoluble. The spectroscopic properties of NAD facilitate its detection in biological assays. NAD⁺ displays a UV absorption maximum at 259 nm with a molar extinction coefficient (ε) of 16,900 M⁻¹ cm⁻¹, whereas NADH absorbs maximally at 339 nm (ε = 6,220 M⁻¹ cm⁻¹) and exhibits with excitation at 340 nm and emission at 460 nm. These characteristics, arising from the and moieties, enable sensitive UV-Vis and fluorescence-based quantification without interference from other cellular components. NAD⁺ remains stable in aqueous solutions at neutral and for up to one week, but it degrades through of the linkage under acidic or highly alkaline conditions. NADH is more labile, prone to oxidation by atmospheric oxygen, particularly at low or elevated temperatures, leading to conversion back to NAD⁺.

Redox Forms and Reactions

Nicotinamide adenine dinucleotide (NAD) functions primarily as a coenzyme, existing in oxidized (NAD⁺) and reduced (NADH) forms that interconvert during cellular . The core reaction involves the transfer of a hydride ion (H⁻) from a substrate, equivalent to the addition of two electrons and a proton, represented as: NAD++2e−+H+⇌NADH\text{NAD}^{+} + 2\text{e}^{-} + \text{H}^{+} \rightleftharpoons \text{NADH} This half-reaction has a standard reduction potential E∘′=−0.320E^{\circ\prime} = -0.320 V at pH 7 and 25°C, indicating that NADH is a strong reducing agent capable of donating electrons to a variety of acceptors in biological systems. The mechanism of this interconversion proceeds via direct hydride transfer from a substrate to the C4 position of the nicotinamide ring in NAD⁺, forming NADH. This transfer is stereospecific, with enzymes exhibiting either A-side (pro-R) or B-side (pro-S) specificity at the C4 position, ensuring precise control over the redox process. A representative example is the reaction catalyzed by lactate dehydrogenase, where pyruvate is reduced to lactate: Pyruvate+NADH+H+⇌Lactate+NAD+\text{Pyruvate} + \text{NADH} + \text{H}^{+} \rightleftharpoons \text{Lactate} + \text{NAD}^{+} This reversible reaction illustrates the role of NAD⁺/NADH in substrate oxidation-reduction, with the enzyme utilizing B-side specificity for hydride transfer. The energetics of NADH oxidation are highly favorable, particularly when coupled to the electron transport chain. The complete oxidation of NADH by oxygen (NADH + H⁺ + ½ O₂ → NAD⁺ + H₂O) yields a standard free energy change ΔG°′ = -52.6 kcal/mol at pH 7, providing sufficient energy to drive the synthesis of approximately 2.5 molecules of ATP per NADH through oxidative phosphorylation.

Cellular Distribution

Intracellular Concentrations

Intracellular concentrations of nicotinamide adenine dinucleotide (NAD) vary significantly across cell types, compartments, and organisms, reflecting its central role in cellular . In mammalian cells, the total NAD pool typically ranges from 0.2 to 0.5 mM, with free NAD+ concentrations measured at approximately 100–120 μM in the nucleus and 50–100 μM in the . Mitochondrial NAD+ levels are generally higher, often exceeding 250 μM, while nuclear pools are around 70–120 μM; these compartmental differences can vary by tissue, such as being twofold higher in mitochondria of mouse skeletal muscle compared to other compartments. In yeast, such as , intracellular NAD+ levels are notably higher, exceeding 500 μM and contributing to a total pool of about 1–2 mM under standard growth conditions. The NAD+/NADH ratio is a key indicator of cellular state, with cytosolic ratios in healthy mammalian tissues estimated at 500–700:1, favoring the oxidized form to support oxidative reactions. In mitochondria, this ratio is lower, typically around 7–10:1, due to higher NADH utilization in the . These ratios shift dynamically under stress conditions; for instance, hypoxia increases the NADH/NAD+ ratio by slowing the tricarboxylic acid cycle and elevating NADH production, thereby reducing the overall NAD+/NADH balance. Precise quantification of NAD levels relies on established analytical techniques. (HPLC), enzymatic cycling assays, and liquid chromatography-mass spectrometry (LC-MS) are commonly used for their sensitivity and ability to distinguish NAD+ from NADH and other metabolites. These methods allow for accurate measurement of total and free NAD pools, often requiring cell extraction or noninvasive approaches like 31P magnetic resonance spectroscopy for assessment. Several factors influence intracellular NAD concentrations. Aging is associated with a progressive decline in NAD+ levels across various human tissues, including skin (at least 50% decrease over adult aging), liver (approximately 30%), skeletal muscle, brain (10–25%), and blood plasma, with overall reductions ranging from 10% to 80% from young adulthood to older age. This decline correlates with hallmarks of aging, such as mitochondrial dysfunction, reduced DNA repair, chronic inflammation, and metabolic dysregulation. Causes include reduced biosynthesis due to lower nicotinamide phosphoribosyltransferase (NAMPT) enzyme activity, increased consumption by enzymes like CD38 and poly(ADP-ribose) polymerases (PARPs), and inflammation. However, variability exists across tissues and studies, with clearer declines in certain tissues like muscle and brain but inconsistent results in others, and human data remain more limited compared to rodent models. Dietary niacin intake modulates these pools, as supplementation can elevate NAD+ by 1.3- to 2.3-fold in muscle tissue, counteracting deficiencies from low-niacin diets.

Compartmentalization and Dynamics

Nicotinamide adenine dinucleotide (NAD) is predominantly localized in the and mitochondria of eukaryotic cells, where it supports essential reactions and energy metabolism. In the nucleus, NAD pools are maintained to facilitate sirtuin-mediated deacetylation and other non-redox functions, with concentrations estimated around 100 μM in cell lines. Mitochondrial NAD levels are similarly compartmentalized, often comparable to or slightly higher than cytosolic pools, enabling efficient activity; recent studies highlight mitochondrial NAD+ import via transporters like SLC25A51, complementing local synthesis. In contrast, NAD presence in the (ER) and Golgi apparatus is minimal, as these organelles rely more on localized precursors rather than substantial NAD reservoirs. NAD itself lacks dedicated transporters and cannot readily cross cellular membranes due to its size and charge, necessitating compartment-specific synthesis or precursor shuttling. Intracellular NAD is primarily produced locally through nicotinamide mononucleotide adenylyltransferase (NMNAT) isoforms: NMNAT1 in the nucleus, NMNAT2 in the , and NMNAT3 in mitochondria. Precursors such as (NMN) and (NR) can be transported via specific transporters (e.g., Slc12a8 for NMN in some tissues) or diffuse, allowing conversion to NAD within target compartments. This indirect ensures spatial regulation of NAD availability without direct flux of the dinucleotide. NAD dynamics are characterized by rapid turnover, with half-lives ranging from approximately 1 hour in mammalian cells to 15 minutes in high-flux tissues like the liver, reflecting constant consumption and resynthesis. In metabolically active organs such as the liver, NAD pools turn over quickly to meet demands from reactions in and the tricarboxylic acid cycle, as well as non- processes like poly(ADP-ribosyl)ation. This balance is achieved through salvage pathways that recycle , preventing depletion despite high usage rates. Turnover varies across tissues, with slower rates (up to 15 hours) in less demanding environments, highlighting adaptive compartmental kinetics. Regulation of NAD pools involves ectoenzymes like , a major NADase that hydrolyzes extracellular NAD into products such as cyclic ADP-ribose, thereby limiting precursor availability and contributing to intracellular NAD decline, particularly during aging. CD38 expression increases with age, exacerbating NAD depletion in tissues by up to 50%, and its inhibition restores levels, underscoring its role in pool . This age-related increase in CD38 activity, along with elevated PARP activation due to DNA damage and reduced efficiency of salvage pathways from lower NAMPT activity, contributes to overall NAD+ decline, which is linked to aging hallmarks including mitochondrial dysfunction, impaired DNA repair, chronic inflammation, and metabolic dysregulation. Caveats include tissue-specific variability and the relative scarcity of comprehensive human data compared to animal models. This extracellular degradation indirectly modulates intracellular dynamics, as reduced salvage substrates diminish resynthesis, while CD38 knockout models show 10- to 30-fold elevations in tissue NAD. Such regulation ensures controlled NAD signaling without direct intracellular breakdown.

Biosynthesis

De Novo Pathway

The de novo biosynthesis of nicotinamide adenine dinucleotide (NAD⁺) in mammals occurs primarily through the , initiating from the L- and culminating in NAD⁺ production. This pathway begins with the oxidative cleavage of to N-formylkynurenine, followed by conversion to , 3-hydroxykynurenine, and 3-hydroxyanthranilic acid. Subsequent steps involve the formation of 2-amino-3-carboxymuconate semialdehyde, which can spontaneously cyclize to or be diverted to other metabolites; is then phosphoribosylates to nicotinic acid mononucleotide (NaMN). NaMN is adenylated to nicotinic acid adenine dinucleotide (NaAD) and finally amidated to NAD⁺. An alternative route to NaMN involves dietary nicotinic acid via the Preiss-Handler pathway, though this is distinct from the primary de novo route originating from . Key enzymes in the mammalian de novo pathway include tryptophan 2,3-dioxygenase (TDO2), predominantly expressed in the liver, and (IDO1/2), which is more ubiquitous and inducible. Downstream enzymes encompass 3-monooxygenase (KMO), kynureninase (KYNU), 3-hydroxyanthranilate 3,4-dioxygenase (HAAO), and quinolinate phosphoribosyltransferase (QPRT), the rate-limiting step converting to NaMN. The final steps utilize adenylyltransferases (NMNAT1-3) to form NaAD and NAD⁺ synthetase (NADSYN1), a glutamine-dependent amidotransferase that incorporates an group using as the nitrogen donor. These enzymes are compartmentalized, with early steps in the and mitochondria, and later ones shifting to the nucleus or mitochondria depending on isoform. In humans, the de novo pathway plays a minor role in overall , contributing less than 15% to the total NAD⁺ pool, though its contribution is higher in the liver where it serves as the primary source. The liver and, to a lesser extent, the are the main sites of this synthesis, with hepatic production supporting systemic NAD⁺ levels via release of precursors like . Regulation is influenced by availability, as dietary intake limits flux through the pathway, and by inflammatory signals that upregulate expression via cytokines such as interferon-γ, diverting toward production during immune responses. This induction can enhance de novo to meet demands in but may deplete for protein synthesis. While minor compared to salvage pathways that recycle NAD⁺ breakdown products, the de novo route provides essential backup during precursor shortages.

Salvage Pathways

In mammals, the salvage pathways represent the primary route for nicotinamide adenine dinucleotide (NAD+) , degradation products and precursors to maintain cellular NAD+ pools, accounting for over 85% of total NAD+ production in most tissues. These pathways predominate over from , especially under normal nutritional conditions, by reutilizing (NAM), (NR), and nicotinic acid (NA) derived from NAD+ consumption or dietary sources. The (NR) pathway begins with the uptake of NR via equilibrative nucleoside transporters (ENTs), followed by phosphorylation to (NMN) catalyzed by kinases ( and NRK2). This step allows NR to bypass the rate-limiting enzyme nicotinamide phosphoribosyltransferase (NAMPT), which is often impaired in age-related decline, enabling NR → NMN → NAD⁺ synthesis even under conditions of NAMPT dysfunction. NMN is then adenylated to NAD+ by adenylyltransferases (NMNAT1, NMNAT2, or NMNAT3), which localize to different cellular compartments. is ubiquitously expressed, while NRK2 predominates in tissues like heart, , and , enabling tissue-specific NAD+ replenishment. In the nicotinamide (NAM) pathway, NAM—generated from NAD+-consuming reactions like sirtuin deacetylation—is converted to NMN by (NAMPT), the rate-limiting enzyme. NMN proceeds to NAD+ via NMNAT enzymes, forming a recycling loop that sustains NAD+ levels during high metabolic demand. NAMPT is highly expressed in liver and but lower in and , influencing tissue vulnerability to NAD+ depletion. Inhibition of NAMPT, as explored with compounds like FK866, depletes NAD+ in cancer cells reliant on salvage synthesis, linking pathway dysregulation to tumorigenesis and offering therapeutic . The nicotinic acid (NA) pathway, also known as the Preiss-Handler route, initiates with NA phosphoribosyltransferase (NAPRT) converting NA to nicotinic acid mononucleotide (NaMN). NaMN is then transformed to nicotinic acid adenine dinucleotide (NaAD) by NMNAT, and finally to NAD+ by NAD+ synthetase (NADSYN1). NAPRT expression varies by tissue, with high levels in liver and , making this pathway prominent in those organs for handling dietary NA. NR and NMN serve as effective oral supplements to boost NAD+ levels, with NR demonstrating bioavailability and conversion efficiency in clinical studies, often used to counteract age-related NAD+ decline. NMN can be directly converted to NAD⁺ via NMNAT, bypassing NAMPT and providing therapeutic utility in conditions of impaired NAMPT activity, such as aging. These salvage pathways are upregulated during nutrient stress, such as glucose deprivation, through increased NAMPT expression to enhance NAD+ recycling and support metabolic adaptation.

Metabolic Roles

Redox Metabolism

Nicotinamide adenine dinucleotide (NAD) plays a pivotal role in catabolic pathways by facilitating the transfer of reducing equivalents as NADH, which is generated during key oxidative steps. In glycolysis, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) oxidizes glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, reducing NAD⁺ to NADH. Similarly, in the tricarboxylic acid (TCA) cycle, isocitrate dehydrogenase converts isocitrate to α-ketoglutarate, producing NADH; α-ketoglutarate dehydrogenase oxidizes α-ketoglutarate to succinyl-CoA, yielding another NADH; and malate dehydrogenase oxidizes malate to oxaloacetate, generating NADH. During β-oxidation of fatty acids in mitochondria, the acyl-CoA dehydrogenase step produces FADH₂, but subsequent dehydrogenase reactions, including 3-hydroxyacyl-CoA dehydrogenase, reduce NAD⁺ to NADH. The accumulated NADH donates electrons to the electron transport chain via complex I, driving proton pumping and ATP synthesis through oxidative phosphorylation. In anabolic processes, the NAD system supports biosynthetic reactions, often in reverse of catabolic steps. utilizes the reverse GAPDH reaction, where 1,3-bisphosphoglycerate is reduced to glyceraldehyde-3-phosphate by NADH, consuming reducing power to build glucose from non-carbohydrate precursors. This step highlights NADH's role in maintaining balance during energy-demanding synthesis. For lipid biosynthesis, while direct NAD⁺ involvement is limited, the system indirectly supports by linking catabolic NADH production to NADPH generation via transhydrogenase activity or shuttles. A phosphorylated variant, NADP⁺, operates a parallel system tailored for , distinguished by a group on the . In the , and 6-phosphogluconate dehydrogenase reduce NADP⁺ to NADPH, providing reducing equivalents for synthesis and defense. NADPH also powers reductive steps in and synthesis, ensuring ample reducing power for anabolic flux without competing with catabolic NADH demands. The NAD⁺/NADH ratio integrates these pathways by regulating metabolic flux. A high NADH level, indicative of reductive stress, inhibits TCA cycle enzymes like isocitrate dehydrogenase, slowing oxidative to prevent over-reduction. Conversely, an elevated NAD⁺/NADH ratio promotes catabolic efficiency and ATP production, fine-tuning across cellular compartments. This dynamic balance ensures coordinated and in response to nutritional and energetic cues.

Oxidoreductase Binding

Nicotinamide adenine dinucleotide (NAD⁺) serves as a critical coenzyme in enzymes, particularly dehydrogenases, where it participates in hydride transfer reactions. The primary structural motif for NAD⁺ binding is the Rossmann fold, a β-α-β secondary structure element consisting of two parallel β-strands connected by an α-helix, repeated to form a dinucleotide-binding domain. This fold positions the moiety and of NAD⁺ through bonds and hydrophobic interactions, while the linkage is accommodated in a positively charged groove formed by glycine-rich loops, such as the GXGXXG motif. The Rossmann fold was first identified in the crystal structure of , where it enables specific recognition of the coenzyme's ADP-ribose portion, facilitating without covalent attachment to the enzyme. Enzyme specificity for NAD⁺ is governed by the orientation of the nicotinamide ring within the , determining the of transfer from NADH. Dehydrogenases are classified as pro-R or pro-S specific, referring to the facial selectivity at the C4 position of the nicotinamide ring: pro-R enzymes transfer the on the Re face, while pro-S enzymes transfer the Si face . This specificity arises from the dinucleotide-binding domain's architecture, which orients the nicotinamide syn or anti to the , ensuring stereospecific and preventing non-productive binding. For instance, the bridge interacts with conserved or residues, stabilizing the extended conformation of NAD⁺ and enhancing transfer efficiency.67225-4/pdf) Representative examples illustrate the diversity of NAD⁺-oxidoreductase interactions. In horse liver , a zinc-dependent , NAD⁺ binding to the Rossmann induces a conformational shift that positions the catalytic Zn²⁺ for substrate coordination, with the metal ion polarizing the alcohol substrate for hydride abstraction to NAD⁺. Similarly, in , substrate binding triggers an induced-fit mechanism, closing the cleft around NAD⁺ and the substrate, which enhances specificity and by excluding solvent and aligning the hydride donor-acceptor . These mechanisms highlight how binding site dynamics optimize NAD⁺ utilization in redox reactions. Unlike prosthetic groups such as flavins, which are tightly bound or covalently linked to enzymes like flavoproteins for repeated intra-enzyme cycling, NAD⁺ functions as a loosely bound coenzyme that dissociates after each , allowing its recycling across multiple enzymes in metabolic pathways. This diffusible nature enables NAD⁺ to shuttle electrons between distant oxidoreductases, contrasting with the fixed role of prosthetic groups that remain enzyme-associated.

Non-Metabolic Functions

ADP-Ribosylation and Signaling

ADP-ribosylation represents a fundamental non-metabolic function of NAD⁺, serving as a post-translational modification that transfers ADP-ribose units from NAD⁺ to acceptor proteins, thereby releasing nicotinamide (NAM) as a byproduct. This process is catalyzed by a family of enzymes known as ADP-ribosyltransferases (ARTs), which in mammals include the poly(ADP-ribose) polymerase (PARP) family, also referred to as ARTDs. The modification exists in two primary forms: mono-ADP-ribosylation (MAR), involving the addition of a single ADP-ribose unit, and poly-ADP-ribosylation (PAR), characterized by the formation of linear or branched chains of ADP-ribose. Mono-ADP-ribosylation (MAR) is mediated by several ARTD enzymes, such as PARP10 (ARTD10) and PARP14 (ARTD1), which utilize NAD⁺ to attach a single ADP-ribose moiety to specific residues on target proteins, including arginines, glutamates, and aspartates. For instance, PARP10 performs MAR on substrates involved in immune , thereby modulating protein function through altered interactions or localization. This modification plays a role in fine-tuning cellular responses, distinct from the more extensive chain-building activity of PAR. In contrast, poly-ADP-ribosylation (PAR) involves the iterative addition of ADP-ribose units to form polymers, primarily catalyzed by and PARP2, which account for the majority of cellular PAR synthesis. , the most abundant and well-studied member, is activated upon binding to DNA strand breaks, leading to its automodification and the PARylation of other proteins to facilitate repair complex assembly. During severe genotoxic stress, hyperactivation of can consume up to 90% of intracellular NAD⁺, resulting in rapid depletion that compromises cellular and contributes to pathways such as parthanatos. Beyond modification, products participate in intracellular signaling. Free ADP-ribose, generated as a degradation product of PAR chains or directly from MAR, functions as a second messenger by activating transient melastatin 2 (TRPM2) channels, thereby mobilizing Ca²⁺ from intracellular stores and amplifying stress signals. Additionally, the NAM byproduct from both MAR and PAR inhibits deacetylases, creating a regulatory feedback loop that links to broader metabolic signaling networks. Physiologically, ADP-ribosylation is integral to the DNA damage response, where PARylation by recruits repair factors like XRCC1 and DNA ligase III to single-strand breaks, ensuring genomic integrity. In inflammation, and other ARTDs, such as PARP14, modulate pathways like activation; for example, MAR by PARP14 enhances type I responses during viral infection. Excessive during inflammatory or exacerbates tissue damage through NAD⁺ depletion, highlighting its dual role in adaptive signaling and pathology.

Sirtuin Activation and Aging

Sirtuins, a family of seven mammalian proteins (SIRT1–7), function as NAD⁺-dependent deacetylases that catalyze the removal of from residues on proteins, thereby regulating diverse cellular processes. The core reaction involves the of NAD⁺, where the transfers the from an acetylated substrate to the ADP-ribose moiety of NAD⁺, producing deacetylated substrate, (NAM), and O-acetyl-ADP-ribose (OAADPr). This process can be represented as: NAD++acetyl-lysine→deacetylated lysine+OAADPr+NAM\text{NAD}^{+} + \text{acetyl-lysine} \rightarrow \text{deacetylated lysine} + \text{OAADPr} + \text{NAM} NAM acts as a competitive inhibitor of sirtuins by binding to the enzyme's active site and preventing further catalysis, thus providing a feedback mechanism to modulate sirtuin activity based on NAD⁺ availability. Among the sirtuins, SIRT1 primarily resides in the nucleus and cytoplasm, where it deacetylates histones to influence gene expression, particularly genes involved in stress resistance and metabolism. For instance, SIRT1-mediated deacetylation of histone H3 and H4 promotes chromatin condensation and transcriptional repression of genes that drive cellular senescence. Mitochondrial sirtuins, such as SIRT3, SIRT4, and SIRT5, localize to mitochondria and regulate metabolic enzymes; SIRT3 deacetylates and activates proteins like acetyl-CoA synthetase and superoxide dismutase 2, enhancing fatty acid oxidation and antioxidant defenses. These roles position sirtuins as key integrators of nutrient sensing and energy homeostasis. Aging is closely linked to declining NAD⁺ levels, which progressively reduce activity and contribute to age-related pathologies. Substantial evidence indicates that NAD⁺ levels decrease during aging, though not universally across all tissues or studies. In humans, levels drop substantially from young adulthood to older age in tissues like skin (at least 50%), liver (approximately 30%), muscle, brain (10–25%, with whole-brain NAD⁺ declining progressively), and blood plasma (sharp reductions). Studies in and humans show that NAD⁺ concentrations can drop by up to 50% between young adulthood and old age, leading to diminished deacetylation capacity and impaired mitochondrial function. Causes include reduced biosynthesis (e.g., lower NAMPT enzyme activity), increased consumption (by CD38 and PARPs), and inflammation. Caveats include tissue-specific variability—decline is clearer in certain tissues (e.g., muscle, skin) but inconsistent in others—and more limited human data compared to rodents. Moreover, regular physical activity mitigates this decline, with exercise-trained older adults exhibiting NAD⁺ levels in skeletal muscle comparable to those of younger individuals, whereas sedentary or impaired individuals show more pronounced reductions. This NAD⁺ depletion correlates with increased , genomic instability, reduced DNA repair, inflammation, and metabolic dysfunction, that sirtuins normally mitigate. Caloric restriction (CR), a dietary intervention known to extend lifespan in multiple species, elevates NAD⁺ levels through upregulation of (NAMPT), the rate-limiting enzyme in the NAD⁺ salvage pathway. Enhanced NAMPT expression under CR conditions increases NAD⁺ biosynthesis, thereby boosting activity and promoting longevity pathways, such as improved insulin sensitivity and reduced . This mechanism underscores sirtuins as mediators of CR's anti-aging effects. Emerging research on NAD⁺ precursors like (NR) and (NMN) demonstrates their potential to restore function and extend lifespan in animal models. In mice, long-term NMN supplementation mitigates age-associated weight gain, enhances energy metabolism, and increases median lifespan by approximately 8–29% in various strains, effects attributed to elevated NAD⁺ and reactivated sirtuins. Human clinical trials post-2020, including randomized controlled studies, indicate that NR and NMN supplementation safely raises blood NAD⁺ levels by 130–150%, yielding metabolic benefits such as improved insulin sensitivity and reduced in middle-aged and older adults, though direct lifespan data remain unavailable. These findings highlight NAD⁺ boosting as a promising strategy for countering age-related sirtuin decline.

Extracellular and Clinical Aspects

Extracellular Actions

Nicotinamide adenine dinucleotide (NAD⁺) is released into the from neurons and immune cells through regulated mechanisms, including efflux via Connexin-43 hemichannels and passive leakage during cell lysis under stress or inflammatory conditions. In neurons, NAD⁺ is co-released with neurotransmitters from synaptic vesicles, contributing to local signaling in the . Immune cells, such as monocytes and T cells, actively secrete NAD⁺ via or diffusion through pannexin and connexin channels, particularly during activation or injury. Extracellular NAD⁺ exerts its effects by engaging purinergic receptors, including P2Y (e.g., P2Y1, P2Y11) and P2X (e.g., P2X4, P2X7) subtypes, often through its hydrolysis product ADP-ribose, which triggers calcium influx and pro-inflammatory signaling in target cells. Additionally, ectoenzymes such as and CD157 on cell surfaces hydrolyze NAD⁺ to produce cyclic ADP-ribose (cADPR) and nicotinic acid dinucleotide (NAADP), which act as potent second messengers for intracellular calcium release. These enzymes are highly expressed on immune and neural cells, enabling precise modulation of extracellular NAD⁺ levels. Key functions of extracellular NAD⁺ include calcium mobilization in , where CD38-mediated generation of cADPR and NAADP amplifies intracellular , influencing gliotransmission and neuroinflammatory responses. In the , NAD⁺ modulates T-cell activation by activating P2X7 receptors on regulatory T cells, leading to their depletion and enhanced antitumor immunity, while hydrolysis products promote T-cell proliferation and metabolic reprogramming. These actions position extracellular NAD⁺ as a (DAMP) that fine-tunes immune responses. Degradation of extracellular NAD⁺ occurs rapidly via ecto-NADases, including , CD157, and ecto-ADP-ribosyltransferases (ARTCs), which prevent its accumulation and mitigate excessive signaling. This breakdown is tightly linked to , as elevated NAD⁺ levels during tissue damage trigger ectoenzyme activity, producing metabolites that either amplify or resolve inflammatory cascades, such as through generation via CD73. In pathological states like or cancer, dysregulated degradation contributes to immune dysregulation.

Therapeutic Implications

Nicotinamide adenine dinucleotide (NAD+) modulation has emerged as a promising therapeutic for various diseases, primarily through precursors that boost NAD+ levels or inhibitors that target NAD+-dependent pathways. NAD+ boosters, such as (NR) and (NMN), have been investigated for their ability to elevate cellular NAD+ concentrations, with NR holding self-affirmed (GRAS) status by the FDA for use in foods and supplements; as of September 2025, the FDA has also declared NMN lawful for use in dietary supplements. These precursors bypass impaired nicotinamide phosphoribosyltransferase (NAMPT) activity, a common issue in aging and disease: NR is converted to NMN via nicotinamide riboside kinase (NRK) enzymes and then to NAD+ via nicotinamide mononucleotide adenylyltransferase (NMNAT), while NMN is directly converted to NAD+ by NMNAT. In animal models, NR and NMN supplementation restores NAD+ levels, activates sirtuins, improves mitochondrial function, reduces inflammation, and extends healthspan. Clinical trials have demonstrated that NMN supplementation enhances muscle insulin sensitivity and remodels muscle structure in individuals with metabolic disorders, suggesting benefits for conditions like . For instance, a 2021 study in postmenopausal women showed NMN improved insulin sensitivity, while a 2023 trial indicated reductions in body weight and improved profiles in adults. Clinical trials have confirmed that both NMN and NR reliably increase NAD+ levels in humans, primarily measured in blood and peripheral blood mononuclear cells, with associated metabolic benefits such as improved insulin signaling, better muscle NAD+ metabolome, and reduced inflammation markers. Both are precursors that the body converts to NAD+, increasing circulating NAD+ by 100–150% or more at typical doses and leading to comparable benefits in energy and cellular repair. Preclinical studies suggest that NMN may provide faster and more sustained NAD+ boosts in certain tissues like the brain, muscle, and heart, while NR performs well in others such as the liver, though human data on tissue-specific effects remains limited and further research is needed. Supplements intended to increase NAD+ levels typically contain precursors such as NR or NMN rather than NAD+ itself, as direct oral administration of NAD+ is poorly absorbed due to instability in the gastrointestinal tract and limited cellular uptake. Oral supplementation with NR and NMN is generally well-tolerated in human clinical trials, with no serious adverse effects commonly reported at typical doses. Mild side effects have been noted in some cases, including nausea, bloating, itching, sweating, headaches, fatigue, flushing, and stomach upset; higher doses may increase the incidence or severity of such effects. While considered possibly safe for short- to medium-term use based on available trials, long-term effects are not fully established and require further investigation. Beyond pharmacological interventions, lifestyle factors can naturally boost NAD+ levels by activating synthesis pathways sustainably without side effects. Combining aerobic and strength exercises upregulates intracellular NAD+ through enhanced biosynthesis, as evidenced in human studies showing increased nicotinamide phosphoribosyltransferase expression. Intermittent fasting and caloric restriction elevate NAD+ levels, promoting sirtuin activation and metabolic reprogramming in preclinical and clinical models. Adequate sleep supports circadian regulation of NAD+ metabolism, maintaining optimal levels via sirtuin-mediated pathways. These interventions coordinate anti-aging effects and improve overall healthspan. Direct administration of NAD+ via intravenous (IV) injections has been explored as an alternative to precursors, potentially offering rapid increases in NAD+ levels. This approach is marketed for benefits including boosted energy, enhanced cognitive function, anti-aging effects, improved metabolism, and support for addiction recovery. However, high-quality evidence remains limited as of late 2024/early 2025, with no major new clinical trials, systematic reviews, or regulatory changes identified in 2025 or 2026. Small human studies and anecdotal reports suggest potential short-term improvements in fatigue, mood, or withdrawal symptoms, but large-scale randomized controlled trials are lacking, and benefits are not well-established. Animal studies show promise for cellular repair and longevity pathways, but translation to humans is uncertain. Preclinical and limited human studies suggest potential benefits including increased energy and stamina through enhanced ATP production and mitochondrial function, improved cognitive function via enhanced mental clarity, focus, and brain repair mechanisms, and anti-aging effects supporting DNA repair, reducing inflammation, and promoting cellular regeneration. For instance, IV NAD+ has been shown to improve mitochondrial efficiency and reduce oxidative stress in cellular and animal models, with small human cohorts reporting minor cognitive improvements. Acute side effects during administration can include nausea, vomiting, flushing, chest tightness, abdominal pain, headache, dizziness, and fatigue. Long-term risks are unknown, with concerns about potential oxidative stress, liver strain, or other effects from supraphysiological doses. NAD+ injections are not FDA-approved for anti-aging or most claimed uses and are often provided off-label in wellness clinics. Researchers and experts call for more rigorous studies to establish efficacy and safety. In contrast, NAD+ pathway inhibitors have shown efficacy in oncology. NAMPT inhibitors, such as FK866, deplete NAD+ in cancer cells by blocking the enzyme nicotinamide phosphoribosyltransferase, leading to antitumor effects in preclinical models of chronic lymphocytic leukemia (CLL), gastric cancer, and neuroendocrine tumors. FK866 has demonstrated selective cytotoxicity in NAPRT-deficient gastric cancer cells and enhanced efficacy when combined with other therapies in lung and prostate carcinomas. Similarly, poly(ADP-ribose) polymerase (PARP) inhibitors like olaparib exploit NAD+ consumption in DNA repair, proving effective in BRCA1/2-mutated tumors by inducing synthetic lethality; olaparib significantly improves progression-free survival in germline BRCA-mutated metastatic breast, ovarian, and pancreatic cancers. Therapeutic applications extend to neurodegeneration and cardiovascular diseases, where age-related NAD+ decline contributes to pathology. In , NAD+ levels decrease in brain tissue, exacerbating mitochondrial dysfunction, , and ; supplementation with NAD+ precursors has normalized these features, reduced neuroinflammation, mitochondrial dysfunction, and DNA damage in preclinical models, and improved mitochondrial stress responses. Human trials, such as a 2025 randomized trial of NR in older adults with mild cognitive impairment, have shown safe increases in NAD+ levels with evidence of biomarker stabilization (e.g., a 7% reduction in pTau 217 vs. an 18% increase with placebo, p=0.02) but no significant cognitive improvements (e.g., no between-group differences in RBANS scores, p=0.55); evidence for cognitive reversal remains lacking, and therapeutic efficacy is still largely preclinical, with larger trials needed. For cardiovascular conditions like , NAD+ deficiency impairs and , but boosting NAD+ via NR has shown safety and potential to preserve in clinical trials, with preclinical data indicating reduced and enhanced mitochondrial function. Intravenous (IV) NAD+ infusions remain controversial, particularly for treatment, with claims of reducing cravings and withdrawal symptoms supported by preclinical studies and small-scale human pilots, such as a 2022 pilot study of 50 patients showing significant craving reduction (p=1.063E-9) and no relapse, but lacking robust large-scale evidence as of 2026 and raising ethical concerns due to unproven and potential risks such as nausea, vomiting, flushing, chest tightness, abdominal pain, headache, dizziness, fatigue, and high costs. Typical protocols for IV NAD+ therapy in addiction treatment involve doses ranging from 500–1,500 mg per session, infused over 2–8 hours daily for 3–10 days initially, followed by maintenance boosters every 1–2 months as needed. This is an emerging therapy with limited clinical evidence, primarily from case series and clinic protocols, and it is not approved by regulatory bodies such as the FDA for this use. Historical case series from 1961 reported complete removal of cravings and withdrawal in over 100 cases with minimal side effects, but modern randomized controlled trials are needed. As of 2026, no large-scale trials support IV NAD+ for , and regulatory bodies have not approved it for this use. Challenges in NAD+ therapeutics include poor of direct NAD+ supplementation, as the molecule is unstable and poorly absorbed orally, necessitating like NR or NMN, which achieve modest increases (e.g., 22-142% in blood NAD+ after weeks of NR dosing). Age-related NAD+ decline, observed across tissues due to increased consumption by enzymes like PARPs and sirtuins, presents a key therapeutic target; companies like Napa Therapeutics, founded in 2018, are developing small-molecule drugs to modulate NAD+ metabolism and mitigate this decline in age-related diseases.

Historical Development

Early Discovery

The discovery of nicotinamide adenine dinucleotide (NAD) began with investigations into the mechanisms of in the early . In 1906, British biochemists Arthur Harden and William John Young demonstrated that extracts required a heat-stable, dialyzable factor to support efficient of glucose to alcohol and . They termed this factor "cozymase" or "coferment," distinguishing it from the heat-labile enzyme, and showed that its addition restored fermentative activity to boiled juice. This finding marked the first recognition of a coenzyme essential for enzymatic , laying the groundwork for understanding NAD's role in metabolic processes. During the 1930s, advances in isolation techniques revealed more about cozymase's properties and distribution. Otto and his colleague Walter Christian successfully isolated cozymase from horse red blood cells and yeast, linking it to the "yellow enzyme" they had identified earlier in studies of and dehydrogenation reactions. 's work demonstrated that cozymase acted as a hydrogen carrier in oxidation-reduction processes, facilitating the transfer of electrons in respiratory s, which earned him the 1931 in or for elucidating the nature and of the respiratory . In 1937, Conrad Elvehjem identified nicotinic acid as the pellagra-preventive factor (niacin), paving the way for linking cozymase to derivatives. These studies established cozymase—later known as coenzyme I or diphosphopyridine nucleotide—as a ubiquitous component in animal and microbial cells, critical for and respiration. In 1936, the structure of NAD was determined independently by and Hans von Euler-Chelpin as a dinucleotide composed of and moieties linked by bonds. In 1938, researchers Francis M. Strong and Esmond E. Snell contributed to characterizing cozymase by identifying it as a containing nicotinic acid, using microbiological assays to link the coenzyme's activity to derivatives. These empirical findings were recognized through Nobel awards, including the 1929 Chemistry Prize shared by Harden and von Euler-Chelpin for their foundational work on and coenzymes. By the early , the compound was formally named nicotinamide adenine dinucleotide (NAD) to reflect its precise chemical identity, following recommendations by the International Union of Biochemistry.

Key Biochemical Advances

The elucidation of NAD⁺'s chemical structure and its central role in reactions marked pivotal advances in the mid-20th century. In 1936, NAD⁺ was identified as a dinucleotide by Hans von Euler-Chelpin and independently by Otto Warburg, building on its initial recognition as a coferment. Warburg's work in demonstrated that NAD⁺ functions as a coenzyme in dehydrogenation reactions, facilitating in metabolic pathways such as and the , which earned him the in or in 1931. This revelation transformed understanding of , showing NAD⁺/NADH as essential carriers in over 500 enzymatic reactions. Advancements in NAD⁺ biosynthesis pathways further illuminated its metabolic integration. In 1958, Jack Preiss and Philip Handler delineated the three-step Preiss-Handler pathway, converting nicotinic acid to NAD⁺ via nicotinic acid mononucleotide and nicotinic acid adenine dinucleotide, primarily in prokaryotes and as a salvage route in eukaryotes. Concurrently, the from was outlined in 1965 by Hayaishi and colleagues, involving eight enzymatic steps to produce , then nicotinic acid mononucleotide. The salvage pathway gained clarity with the 2004 discovery by Bieganowski and Brenner that (NR) is directly phosphoribosylated by NR kinases (NRKs) to form (NMN), bypassing traditional routes and highlighting dietary NR as a potent precursor. These pathways underscored NAD⁺'s dynamic turnover, with cellular pools maintained through to prevent depletion. The expansion to non-redox functions represented a paradigm shift in the late 20th century. In 1963, Chambon et al. identified poly(ADP-ribose) polymerase (PARP) activity, where NAD⁺ donates ADP-ribose units for DNA repair and chromatin modification, revealing its role beyond electron transfer. The 2000 identification of sirtuins as NAD⁺-dependent deacetylases by Imai, Armstrong, Kaeberlein, and Guarente linked NAD⁺ to gene regulation and aging, with SIRT1 deacetylating histones and transcription factors using NAD⁺ to produce nicotinamide and O-acetyl-ADP-ribose. Subsequent work in 1989 by Lee et al. uncovered cyclic ADP-ribose (cADPR) as an NAD⁺-derived second messenger regulating calcium signaling. These discoveries elevated NAD⁺ from a metabolic cofactor to a signaling hub, influencing longevity, stress responses, and disease.

References

  1. https://www.acs.org/molecule-of-the-week/archive/n/nicotinamide-adenine-dinucleotide.html
  2. https://www.nature.com/articles/s41598-019-49547-6
  3. https://pmc.ncbi.nlm.nih.gov/articles/PMC7963035/
  4. https://pubmed.ncbi.nlm.nih.gov/31207144/
  5. https://www.aging-us.com/article/103218/text
  6. https://pmc.ncbi.nlm.nih.gov/articles/PMC7442590/
  7. https://pmc.ncbi.nlm.nih.gov/articles/PMC8747183/
  8. https://pmc.ncbi.nlm.nih.gov/articles/PMC4911708/
  9. https://www.nature.com/articles/s41392-020-00311-7
  10. https://www.science.org/doi/10.1126/sciadv.ady9811
  11. https://alz-journals.onlinelibrary.wiley.com/doi/10.1002/trc2.70023
  12. https://www.sciencedirect.com/science/article/pii/S0006295222000405
  13. https://pmc.ncbi.nlm.nih.gov/articles/PMC9495723/
  14. https://pmc.ncbi.nlm.nih.gov/articles/PMC9036060/
  15. https://pmc.ncbi.nlm.nih.gov/articles/PMC10240123/
  16. https://pmc.ncbi.nlm.nih.gov/articles/PMC9454445/
  17. https://pubmed.ncbi.nlm.nih.gov/8749365/
  18. https://content.byui.edu/file/a236934c-3c60-4fe9-90aa-d343b3e3a640/1/module8/readings/atp_nad_fad.html
  19. https://pubchem.ncbi.nlm.nih.gov/compound/5893
  20. https://pubchem.ncbi.nlm.nih.gov/compound/439153
  21. https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/product/documents/668/557/n8285pis.pdf
  22. https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/product/documents/592/242/n9535pis.pdf
  23. https://www.selleckchem.com/datasheet/nad-S251805-DataSheet.html
  24. https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/product/documents/138/043/n4505pis.pdf
  25. https://www.researchgate.net/publication/342039911_THE_APPROACH_FOR_EXPRESS_SPECTROMETRIC_DETERMINATION_OF_THE_REDUCED_FORM_OF_NICOTINAMIDE_ADENINE_DINUCLEOTIDE_NADH_CONTENT
  26. https://www.sigmaaldrich.com/US/en/product/mm/481911
  27. https://pmc.ncbi.nlm.nih.gov/articles/PMC11597533/
  28. https://pmc.ncbi.nlm.nih.gov/articles/PMC3680915/
  29. https://www.ncbi.nlm.nih.gov/books/NBK557536/
  30. https://www.ncbi.nlm.nih.gov/books/NBK9903/
  31. https://clintransmed.springeropen.com/articles/10.1186/s40169-016-0104-7
  32. https://pmc.ncbi.nlm.nih.gov/articles/PMC7502477/
  33. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0106496
  34. https://journals.physiology.org/doi/full/10.1152/ajpendo.00054.2012
  35. https://pmc.ncbi.nlm.nih.gov/articles/PMC3683958/
  36. https://www.nature.com/articles/oncsis201550
  37. https://www.pnas.org/doi/10.1073/pnas.1417921112
  38. https://www.mdpi.com/1422-0067/25/4/2325
  39. https://pmc.ncbi.nlm.nih.gov/articles/PMC9512238/
  40. https://pmc.ncbi.nlm.nih.gov/articles/PMC10692436/
  41. https://www.sciencedirect.com/science/article/pii/S155041312030190X
  42. https://www.cell.com/cell-metabolism/fulltext/S1550-4131(15)00266-1
  43. https://www.cell.com/trends/biochemical-sciences/fulltext/S0968-0004(20)30145-6
  44. https://www.nature.com/articles/s44324-025-00067-0
  45. https://pmc.ncbi.nlm.nih.gov/articles/PMC6530925/
  46. https://www.nature.com/articles/s41392-020-00354-w
  47. https://pmc.ncbi.nlm.nih.gov/articles/PMC7053056/
  48. https://www.tandfonline.com/doi/full/10.3109/10409238.2015.1028612
  49. https://www.sciencedirect.com/science/article/pii/S1550413122004958
  50. https://pmc.ncbi.nlm.nih.gov/articles/PMC2852209/
  51. https://pmc.ncbi.nlm.nih.gov/articles/PMC5842119/
  52. https://pmc.ncbi.nlm.nih.gov/articles/PMC9316858/
  53. https://www.nature.com/articles/s41467-023-37744-x
  54. https://www.nature.com/articles/s42255-025-01373-z
  55. https://www.nature.com/articles/s41467-019-13668-3
  56. https://www.cell.com/cell-metabolism/fulltext/S1550-4131%2824%2900013-5
  57. https://pubmed.ncbi.nlm.nih.gov/5451100/
  58. https://pubs.acs.org/doi/10.1021/ja00007a002
  59. https://pubs.acs.org/doi/10.1021/acs.biochem.7b00446
  60. https://pmc.ncbi.nlm.nih.gov/articles/PMC5817062/
  61. https://onlinelibrary.wiley.com/doi/abs/10.1002/9780470015902.a0000637.pub2
  62. https://pmc.ncbi.nlm.nih.gov/articles/PMC4477203/
  63. https://pmc.ncbi.nlm.nih.gov/articles/PMC8003356/
  64. https://www.nature.com/articles/s41467-019-08530-5
  65. https://pubmed.ncbi.nlm.nih.gov/25534250/
  66. https://pmc.ncbi.nlm.nih.gov/articles/PMC11107671/
  67. https://pubmed.ncbi.nlm.nih.gov/23102634/
  68. https://www.cell.com/fulltext/S1097-2765%2804%2900082-6
  69. https://genesdev.cshlp.org/content/20/21/2913.full.pdf
  70. https://www.ijbs.com/v07p0575.htm
  71. https://pmc.ncbi.nlm.nih.gov/articles/PMC3943707/
  72. https://pmc.ncbi.nlm.nih.gov/articles/PMC6577427/
  73. https://pmc.ncbi.nlm.nih.gov/articles/PMC4112140/
  74. https://www.ahajournals.org/doi/10.1161/CIRCRESAHA.118.312498
  75. https://pmc.ncbi.nlm.nih.gov/articles/PMC5419884/
  76. https://www.sciencedirect.com/science/article/pii/S1550413116302443
  77. https://pubmed.ncbi.nlm.nih.gov/35010977/
  78. https://www.liebertpub.com/doi/10.1089/rej.2017.1975
  79. https://pmc.ncbi.nlm.nih.gov/articles/PMC5668137/
  80. https://www.sciencedirect.com/science/article/pii/S2161831323013595
  81. https://www.tandfonline.com/doi/full/10.1080/10408398.2024.2387324
  82. https://pmc.ncbi.nlm.nih.gov/articles/PMC8038981/
  83. https://pmc.ncbi.nlm.nih.gov/articles/PMC8371318/
  84. https://portlandpress.com/biochemj/article/393/3/697/79190/Extracellular-NAD-regulates-intracellular-calcium
  85. https://www.fda.gov/food/gras-notice-inventory/recently-published-gras-notices-and-fda-letters
  86. https://www.nutraingredients.com/Article/2025/09/30/fda-declares-nmn-lawful-in-dietary-supplements/
  87. https://pmc.ncbi.nlm.nih.gov/articles/PMC4077918/
  88. https://pmc.ncbi.nlm.nih.gov/articles/PMC5854409/
  89. https://www.science.org/doi/10.1126/science.abe9985
  90. https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2800829
  91. https://www.nature.com/articles/s41514-022-00084-z
  92. https://www.nature.com/articles/s41467-018-03421-7
  93. https://pmc.ncbi.nlm.nih.gov/articles/PMC9158788/
  94. https://pmc.ncbi.nlm.nih.gov/articles/PMC10684646/
  95. https://pmc.ncbi.nlm.nih.gov/articles/PMC11205942/
  96. https://pmc.ncbi.nlm.nih.gov/articles/PMC8444956/
  97. https://www.cell.com/cell-metabolism/fulltext/S1550-4131(17)30670-8
  98. https://pmc.ncbi.nlm.nih.gov/articles/PMC10361580/
  99. https://www.nature.com/articles/s41598-022-18272-y
  100. https://pmc.ncbi.nlm.nih.gov/articles/PMC11365583/
  101. https://pmc.ncbi.nlm.nih.gov/articles/PMC10639164/
  102. https://pmc.ncbi.nlm.nih.gov/articles/PMC8842632/
  103. https://pmc.ncbi.nlm.nih.gov/articles/PMC7752291/
  104. https://pmc.ncbi.nlm.nih.gov/articles/PMC10273285/
  105. https://pmc.ncbi.nlm.nih.gov/articles/PMC12177089/
  106. https://aacrjournals.org/clincancerres/article/20/18/4861/13389/On-Target-Effect-of-FK866-a-Nicotinamide
  107. https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2022.970553/full
  108. https://www.gastrojournal.org/article/S0016-5085%2818%2934544-X/fulltext
  109. https://www.nejm.org/doi/full/10.1056/NEJMoa2105215
  110. https://www.nature.com/articles/s41388-024-03227-6
  111. https://pmc.ncbi.nlm.nih.gov/articles/PMC8369418/
  112. https://www.nature.com/articles/s41419-024-07062-1/
  113. https://www.science.org/doi/10.1126/sciadv.ady9811/
  114. https://alz-journals.onlinelibrary.wiley.com/doi/10.1002/trc2.70023/
  115. https://clinicaltrials.gov/study/NCT05040321/
  116. https://www.pnas.org/doi/10.1073/pnas.1718819115
  117. https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.117.032626
  118. https://www.jacc.org/doi/10.1016/j.jacbts.2022.06.012
  119. https://link.springer.com/article/10.1007/s40256-025-00764-7
  120. https://www.addictioncenter.com/treatment/nad-therapy/
  121. https://www.cns-center.com/blog/nad-iv-therapy-addiction-recovery-chicago
  122. https://www.jinfiniti.com/nad-for-addiction/
  123. https://www.fagronacademy.us/blog/nicotinamide-adenine-dinucleotide-nad-a-review-on-dosing
  124. https://sa1s3.patientpop.com/assets/docs/136179.pdf
  125. https://pmc.ncbi.nlm.nih.gov/articles/PMC7278809/
  126. https://pmc.ncbi.nlm.nih.gov/articles/PMC9474872/
  127. https://www.theguardian.com/society/2025/feb/23/its-not-ethical-and-its-not-medical-how-uk-rehab-clinics-are-cashing-in-on-nad
  128. https://www.npr.org/sections/health-shots/2019/08/22/741115178/addiction-clinics-market-unproven-infusion-treatments-to-desperate-patients
  129. https://kffhealthnews.org/news/addiction-clinics-expensive-unproven-infusion-treatments-desperate-patients/
  130. https://www.nature.com/articles/s41514-023-00134-0
  131. https://www.aboutnad.com/blogs/blog/why-you-can-t-take-nad-directly-and-the-best-precursor-to-boost-nad-effectively
  132. https://www.jinfiniti.com/nad-injections-vs-oral-nad/
  133. https://www.fightaging.org/archives/2018/08/napa-therapeutics-formed-to-develop-drugs-to-influence-nad-metabolism/
  134. https://pmc.ncbi.nlm.nih.gov/articles/PMC5521000/
  135. https://pubmed.ncbi.nlm.nih.gov/20718025/
  136. https://www.sciencedirect.com/topics/neuroscience/nicotinamide-adenine-dinucleotide
  137. https://www.nobelprize.org/prizes/medicine/1931/summary/
  138. https://www.nobelprize.org/prizes/chemistry/1929/ceremony-speech/
  139. https://pmc.ncbi.nlm.nih.gov/articles/PMC5599801/
  140. https://doi.org/10.1016/0006-291X(63)90024-X
  141. https://doi.org/10.1038/35016506
Add your contribution
Related Hubs
User Avatar
No comments yet.