Cytochrome P450 3A4 (abbreviated CYP3A4) (EC 1.14.13.97) is an important enzyme in the body, mainly found in the liver and in the intestine, which in humans is encoded by CYP3A4 gene. It oxidizes small foreign organic molecules (xenobiotics), such as toxins or drugs, so that they can be removed from the body. It is highly homologous to CYP3A5, another important CYP3A enzyme.

While many drugs are deactivated by CYP3A4, there are also some drugs that are activated by the enzyme. Some substances, such as some drugs and furanocoumarins present in grapefruit juice, interfere with the action of CYP3A4. These substances will, therefore, either amplify or weaken the action of those drugs that are modified by CYP3A4.

CYP3A4 is a member of the cytochrome P450 family of oxidizing enzymes. Several other members of this family are also involved in drug metabolism, but CYP3A4 is the most common and the most versatile one. Like all members of this family, it is a hemoprotein, i.e. a protein containing a heme group with an iron atom. In humans, the CYP3A4 protein is encoded by the CYP3A4 gene. This gene is part of a cluster of cytochrome P450 genes on chromosome 7q22.1. Previously another CYP3A gene, CYP3A3, was thought to exist; however, it is now thought that this sequence represents a transcript variant of CYP3A4. Alternatively-spliced transcript variants encoding different isoforms have been identified.

CYP3A4 is a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases that catalyze many reactions involved in drug metabolism and synthesis of steroids (including cholesterol), and other lipids.

The CYP3A4 protein localizes to the endoplasmic reticulum, and its expression is induced by glucocorticoids and some pharmacological agents. Cytochrome P450 enzymes metabolize approximately 60% of prescribed drugs, with CYP3A4 responsible for about half of this metabolism; substrates include acetaminophen (paracetamol), codeine, ciclosporin (cyclosporin), diazepam, erythromycin, and chloroquine. The enzyme also metabolizes some steroids and carcinogens. Most drugs undergo deactivation by CYP3A4, either directly or by facilitated excretion from the body. Also, many substances are bioactivated by CYP3A4 to form their active compounds, and many protoxins are toxicated into their toxic forms (see table below for examples).

CYP3A4 also possesses epoxygenase activity in that it metabolizes arachidonic acid to epoxyeicosatrienoic acids (EETs), i.e. (±)-8,9-, (±)-11,12-, and (±)-14,15-epoxyeicosatrienoic acids. EETs have a wide range of activities including the promotion of certain types of cancers (see epoxyeicosatetraenoic acid). CYP3A4 promotes the growth of various types of human cancer cell lines in culture by producing (±)-14,15-epoxyeicosatrienoic acids, which stimulate these cells to grow. The CYP3A4 enzyme is also reported to have fatty acid monooxgenase activity for metabolizing arachidonic acid to 20-Hydroxyeicosatetraenoic acid (20-HETE). 20-HETE has a wide range of activities that include growth stimulation in breast and other types of cancers (see 12-hydroxyeicosatetraenoic acid).

The CYP3A4 gene exhibits a much more complicated upstream regulatory region in comparison with its paralogs. This increased complexity renders the CYP3A4 gene more sensitive to endogenous and exogenous pregnane X receptor (PXR) and constitutive androstane receptor (CAR) ligands, instead of relying on gene variants for wider specificity. Chimpanzee and human CYP3A4 are highly conserved in metabolism of many ligands, although four amino acids positively selected in humans led to a 5-fold benzylation of 7-BFC in the presence of the hepatotoxic secondary bile acid lithocholic acid. This change in consequence contributes to an increased human defense against cholestasis.

Fetuses do not express CYP3A4 in their liver tissue, but rather CYP3A7 (EC 1.14.14.1), which acts on a similar range of substrates. CYP3A4 increases to approximately 40% of adult levels in the fourth month of life and 72% at 12 months.