Purification and characterization of the highly toxic lectin modeccin.

Modeccin was extracted from the root of Adenia digitata and purified by gel filtration on Sephadex G-100, ion exchange chromatography on DEAE-cellulose, and affinity chromatography on a column containing immobilized, neuraminidase-treated fetuin. The toxin was eluted from the affinity column with lactose. Polyacrylamide gel electrophoresis of affinity-purified modeccin in the presence of sodium dodecyl sulfate showed one main protein (Mr = about 63,000) and two minor bands. After treatment with 2-mercaptoethanol the material moved as two bands corresponding to molecular weights of 28,000 and 38,000. Upon treatment of modeccin with 2-mercaptoethanol it lost its toxic activity. A considerable part of this activity was recovered after dialysis to remove %-mercaptoethanol and allow reoxidation to take place. Sucrose gradient centrifugation of ‘2SI-labeled, affinity-purified modeccin showed that the toxic material moved at the same rate as the major peak of radioactivity. Furthermore, the major part of the radioactivity, as well as the toxic material, were bound to a column containing immobilized concanavalin A and could be eluted with methyl-a-n-mannoside. It thus appears that the toxic material is identical with the major protein eluted with lactose from the affinity column containing immobilized neuraminidase-treated fetuin. Isoelectric focusing of affinity purified modeccin revealed the presence of at least four different toxins, most likely isotoxins. Immunodiffusion tests and experiments, where the ability of antitoxins to protect HeLa cells against different toxins was measured, showed that modeccin did not cross-react immunologically with abrin and only weakly with ricin. Abrin and &in showed weak crossreactions. Similarities and differences between modeccin, abrin and ricin are discussed.