The amino acid sequence of toxin D isolated from the venom of Indian cobra (Naja naja).

A neurotoxin, designated toxin D, was isolated from the venom of Naja naja by gel-filtration on Sephadex G-50 followed by CM-cellulose chromatography. The amino acid sequence of toxin D was determined by analyzing tryptic and chymotryptic peptides of the reduced and S-carboxymethyl derivative. The venom contains at least four long (toxins A, B, C and D) and two short neurotoxins (toxins I and II) that account for about 16% of the lyophilized crude venom by weight. Toxin D consists of 71 amino acid residues in a single peptide chain cross-linked by five intramolecular disulfide bridges. The lethal toxicity of toxin D is found to be about half those of toxins A, B and C in the venom. Analysis of the amino acid sequence reveals that toxin D differs from toxin A in having -Arg-31, -Glu-35 and -Lys-49 in place of -Ser-31, -Lys-35 and -Arg-49 residues.