Virotoxins: Actin-Binding Cyclic Peptides of Amanita Virosa Mushrooms

Virotoxins are toxic peptides singularly found in Amanita virosa mushrooms. After purification and resolution by high-pressure liquid chromatography, the main component, viroisin, was selectively cleaved and submitted to Edman degradation. The structure could be completely elucidated and was in part found to be the same as in phallotoxins. Differing from the phallotoxins, however, virotoxins are monocyclic peptides and contain D-serine instead of L-cysteine. in addition, two amino acids were detected in virotoxins which thus far have not been found in nature: 2, 3-trans-3, 4-di-hydroxy-L-proline and 2'-(methylsulfonyl)-L-tryptophan. The biological activity of viroisin is comparable to that of the phallotoxins: e.g., with 2.5 mg of viroisin per kg (white mouse), 50% of the animals die within 2-5 h by hemorrhagia of the liver. Also, on the molecular level, the virotoxins behave similar to the phallotoxins. Thus, viroisin binds to rabbit muscle actin as proved by difference UV spectroscopy. With an apparent equilibrium dissociation constant K_D ~ 2 × 10^-8 M, the affinity of viroisin is very similar to that of phalloidin. However, the flexibility of the monocyclic structure and the presence of two additional hydroxy groups in the virotoxins suggest a different mode of interaction with actin. While there is proof that the bicyclic phallotoxins possess a rigid binding site, the virotoxins may adopt the biologically active conformation by an induced-fit mechanism upon contact with actin.