Chemical syntheses and biological activities of lactam analogues of alpha-conotoxin SI

Bicyclization represents an effective method for the introduction of confor mational constraints into small, biologically important peptides. Several s trategies have been developed for the preparation of-bicyclic lactam analog ues of alpha -conotoxin SI, a 13-residue peptide neurotoxin found in cone s nail venom. Four analogues of the natural regioisomer of alpha -conotoxin S I were designed and synthesized, each with one of the two paired cysteines of the parent peptide being replaced by a side-chain lactam bridged glutami c acid/lysine pair. Solid-phase lactamization was studied to determine rate s of formation of the two possible loops and to document the extent of dime rization and higher oligomerization. Radioligand binding assays were carrie d out on all synthesized peptides, including the naturally occurring two-di sulfide form, in order to determine their affinities for nicotinic acetylch oline receptors (nAChRs). Replacement of the Cys(2)-Cys(7) loop Of alpha -c onotoxin SI With a lactam bridge resulted in complete loss of activity, whe reas replacement of the Cys(3)-Cys(13) disulfide loop resulted in a similar to 60-fold reduction in affinity for one orientation and a similar to 70-f old increase in affinity for the other. The two active lactam analogues ret ain the selectivity exhibited by the naturally occurring peptide for the al pha/delta subunit of nAChRs, as judged by competition experiments with the curariform antagonist metocurine.