Covalent structure of the insect toxin of the North African scorpion Androctonus australis Hector.

The complete covalent structure of the insect toxin purified from the venom of the North-African scorpion Androctonus australis Hector was described. Its amino acid sequence was established by phenylisothiocyanate degradation of several protein derivatives and proteolytic fragments in a liquid protein sequencer using either a "protein" or a "peptide" program. The position of the four disulfide bridges were deduced by analysis of proteolytic peptides before and after diperformic oxidation, and by partial labeling of the half cystine residues with [14C]-iodoacetic acid and determining the specific radioactivities of the S-[14C]-carboxymethylated phenylthiohydantoin cysteines. The sequences of the insect and mammal toxins from scorpions can be aligned with homology with the positions of seven half-cystine residues as registers. The mammal and insect toxins have three disulfide bridges at homologous positions. The mammal and insect toxins have three disulfide bridges at homologous positions. The fourth bridge is different in that Cys12 in mammal toxin II is replaced by Cys38 in the insect toxin. It is likely that the position of the disulfide bridges is the same for all scorpion neurotoxins active on mammals. We believe that the shift of one half-cystine residue in the insect toxin may induce a conformational change in the structure of the protein, which, in turn, may partially account for the total specificity of this toxin for insect nervous system.