Modeccin is a toxic lectin, a group of glycoproteins capable of binding specifically to sugar moieties. Different toxic lectins are present in seeds of different origin. Modeccin is found in the roots of the African plant Adenia digitata. These roots are often mistaken for edible roots, which has led to some cases of intoxication. Sometimes the fruit is eaten, or a root extract is drunk as a manner of suicide.

Modeccin consists of two subunits that are bound by a disulfide linkage, the intact protein has a molecular weight of approximately 57-63 kDa. When treated with mercaptoethanol the chains can be dissociated into two subunits, subunit A with a mass of 25-28 kDa and subunit B with a mass of 31-35 kDa.

The A-chain is called the effectomer and possesses ribosomal-inactivating properties. The B-chain contains the carbohydrate binding site and it is termed the haptomer. While the intact toxin molecules have potent cytotoxic effects on cells, they exhibit no ribosomal inactivating activity on ribosomes in a cell-free system. By contrast, reduction of the toxin with a disulfide reducing agent creates the opposite effects. Reduced, dissociated toxin subunits inhibit ribosomal activity in cell-free systems, but they have no effect on intact cells.

The reason for these properties is due to the toxin's mode of action. Toxin molecules bind through saccharide recognition sites on the B-chain to particular β-galactosyl-containing glycoprotein or glycolipid components on the surface of cell membranes. In animals that are sensitive to these toxins these polysaccharides are present in virtually all cell types. The toxin binds to cell-surface polysaccharide receptors with a high affinity (Ka in the range of 107–108/M). When the toxin binds to the cell, the A-chain enters through either active transport or endocytosis. Once inside the cell the A-chain enters the cytoplasmic space, binds to the 60S ribosomal subunit and enzymatically inactivates it. The mechanism is catalytic because of this one toxin molecule is enough to disrupt protein synthesis and kill the target cell.

Cytotoxic lectins including modeccin act in a similar manner as ricin, a well understood toxic lectin, though each one has a different saccharide binding specificity.

Cytotoxic lectins include ricin, abrin, modeccin, volkensin (least toxic, 10 and 40 times less cytotoxic than ricin and modeccin respectively.) and viscumin (10 times less cytotoxic than ricin).

Comparison of the parenteral lethality of ricin and related toxins in laboratory mice

Synthetically produced toxins and genetically engineered toxin chimeras are areas of emerging interest because of their possible application as new medical modalities (e.g., IgTs) and powerful research tools, as well as their potential misuse as toxin weapons to confuse traditional medical countermeasures (Olsnes and Pihl, 1986; Millard, 2005). Hybrid molecules were prepared from the A- and B-chains of the toxic lectins ricin and modeccin by dialyzing mixtures of isolated chains to allow a disulfide bridge to be formed between them. Whereas the hybrid consisting of ricin A-chain and modeccin B-chain was non-toxic, the converse hybrid, modeccin A-chain/ricin B-chain, was even more toxic than were the parent toxins, native ricin and modeccin.