Amino acid sequence of beta 2-bungarotoxin from Bungarus multicinctus venom. The amino acid substitutions in the B chains.

beta 2-Bungarotoxin (beta 2-toxin) was isolated from the venom of Bungarus multicinctus by means of CM-Sephadex C-25 column chromatography, Sephadex G-75 gel filtration and CM-Sephadex C-25 column rechromatography. beta 2-Toxin consisted of two dissimilar polypeptides, a (120 amino acid residues) and B (60 amino acid residues) chains, crosslinked by an interchain disulfide bond. The neurotoxicity (LD50) and phospholipase activity of beta 2-toxin were 0.029 micrograms/g of mouse and 48.9 units/mg of toxin, respectively, and both the activities were slightly weaker than those (0.019 micrograms/g and 60.9 units/mg) of beta 1-bungarotoxin (beta 1-toxin). beta 2-Toxin was reduced and carboxymethylated and then its RCM-A and -B chains were separated. Each RCM-chain was maleylated and then digested with TPCK-trypsin. The tryptic peptides were sequences by manual Edman degradation or the dansyl-Edman method, and the total alignment of the tryptic peptides from each RCM-chain was deduced based on the amino acid sequences of the A and B chains of beta 1-toxin. The amino acid sequence of the B chain of beta 2-toxin differed from that of the B chain of beta 1-toxin by 22 amino acid substitutions, while those of their A chains were identical. We concluded that the variation in the amino acid sequence of the B chains did not significantly affect the neurotoxicity of the beta-toxins. The amino acid sequences of the B chains of the two beta-toxins were homologous to those of proteinase inhibitors from snake venoms and mammalian pancreas, but no inhibitory activity of the two beta-toxins on proteinases was observed.