Disulfide isomerization within the C-terminus of cobrotoxin decelerates by thiol compounds and trinitrophenylation, but accelerates by modification of carboxyl groups.

A disulfide isomerization at the C-terminus of cobrotoxin occurred spontaneously by dissolving in alkali buffer. Irreversible conversion of cobrotoxin into its isomers was completely achieved within 4 days. The isomerization reaction was decelerated by thiol compounds including GSSG, GSH, cystamine, and cysteamine in a pseudo-first-order kinetic, and GSSG was the most effective one among the thiol compounds used. Moreover, the oxidized thiol compounds were always superior to reduced ones in decreasing the rate of disulfide interchange. To further assess the intrinsic elements essential for the occurrence of disulfide isomerization of cobrotoxin, the toxin molecule was subjected to modification on its Arg, Lys, Trp, Tyr, and carboxyl groups. In sharp contrast to other modified derivatives, the isomerization reaction was decelerated by trinitrophenylation on Lys-26, Lys-27, and Lys-47, whereas it was rapidly completed after modification of carboxyl groups. Neither chemical modification nor the toxin's conformation affected the irreversibility of isomerization reaction. Thus, the observed change in the rate of disulfide isomerization reflects the involvement of Lys residues and carboxyl groups in this reaction. Although thiol compounds further decelerated the conversion of trinitrophenylated cobrotoxin into its isomers, they did not exert a notable effect on the isomerization of carboxyl groups-modified derivative. These results clearly indicate that disulfide isomerization of cobrotoxin is, in part, driven by the positively charged Lys residues at positions 26, 27, and 47 of the toxin molecule, and that the thiol compounds are coordinated with the negatively charged groups of cobrotoxin to exert their inhibitory action.