Specific binding to isolated acetylcholine receptor of a synthetic peptide duplicating the sequence of the presumed active center of a lethal toxin from snake venom.

To verify the existence of a lethal "active center" in snake venom neurotoxins and to assess its delineation within their polypeptide sequences, a tritriacontapeptide matching residues 16-48 of the natural "major" toxin of Naja naja philippinensis (Hauert, J., Maire, M., Sussmann, A., and Bargetzi, J. P. (1974) Int. J. Pept. Protein Research 6, 201-222) has been synthesized by solid-phase technology (Juillerat, M. A., and Bargetzi, J. P. (1980), results presented at the 16th European Peptide Symposium, Helsingor, Denmark, September, (1980). After deblocking, cyclization by reoxidation, and purification, one of the resulting peptides exhibiting the correct chemical and physical characteristics was found to be highly "active" in binding isolated, purified, and standardized acetylcholine receptor protein. A new assay procedure had been developed using 3H-labeled alpha-bungarotoxin as nonreversible back-titrant. It has the advantage of measuring only specific binding of an unknown ligand competing for the same receptor protein. The observed KD was 2.2 x 10(-7) M, a value attesting to a higher affinity than acetylcholine itself, 2.5 x 10(-6) M, as well as curare and the small organic cholinergic ligands, albeit somewhat lower than the affinity of the parent native toxin, as expected from differences in molecular size.