Evidence for a carbocation intermediate during conversion of bipinnatin-A and -C into irreversible inhibitors of nicotinic acetylcholine receptors.

The lophotoxins are naturally occurring antagonists of nicotinic acetylcholine receptors. These toxins are small diterpenes that irreversibly inhibit nicotinic receptors by specific covalent modification of Tyr190 in the alpha-subunits of the receptor. The naturally occurring lophotoxin analogs, bipinnatin-A and -C, are inactive protoxins. Activation of these toxins occurs spontaneously in buffer and involves replacement of the C2 acetate ester with a hydroxyl group. The mechanism involved in conversion of the inactive bipinnatins into their biologically active solvolysis products was investigated in this study. Solvolysis of bipinnatin-A in buffer containing [18O]water demonstrated that the C2 hydroxyl of the biologically active solvolysis product originated from the solvent. The rates of solvolysis of bipinnatins-A and -C were not affected by sodium azide. However, in the presence of azide, solvent products decreased and new azide-containing products appeared. Thus azide acted as a nucleophile after a rate-limiting step, such as the formation of a carbocation intermediate. The kaz/ks values for bipinnatin-A (2900 M-1) and bipinnatin-C (1450 M-1) suggest that the carbocation intermediates are relatively stable. Compounds capable of spontaneously generating carbocations may represent a novel new class of active-site-directed affinity reagents that can be applied to other receptors and enzymes.