Disulfide isomers of alpha-neurotoxins from king cobra (Ophiophagus hannah) venom

Two novel Lu-neurotoxins, Oh-6A and Oh-6B, isolated from the king cobra (Op hiophagus hannah) venom, consist of 70 amino acid residues with 10 cysteine residues and share the same amino acid sequences as determined by Edman de gradation on the peptide fragments generated from the proteolytic hydrolysa tes. Their sequences share 46-53% homology with Oh-4, Oh-5, Toxin a, and To xin b from the same venom. The finding that Oh-6A and Oh-6B had different r etention times in the reversed-phase column suggested that the two toxin mo lecules should not have the same conformation. Selective reduction on the d isulfide bond, Cys26-Cys30, at the tip of their loop II structures resulted in the production of the partially reduced derivatives eluted at the same position, Under redox conditions, the partially reduced Oh-6A and 6B exclus ively converted into native Oh-6A as evidenced by HPLC analyses. This sugge sts that Oh-6A and Oh-6B are disulfide isomers which probably arise from ci s-trans isomerization of the Cys26-Cys30 disulfide bond. Alternatively, the two toxins exhibited binding activity toward nAChR and lethal toxicity equ ally. It reflects that the diversity around the extra loop at the loop II s tructure does not exert a significant effect on the manifestation of the ne urotoxicity of Oh-6A and Oh-6B. (C) 1999 Academic Press.