title: Evidence for a carbocation intermediate during conversion of bipinnatin-A and -C into irreversible inhibitors of nicotinic acetylcholine receptors.
abstract: 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.
mesh_terms: D001386:Azides / Q000737:chemistry / Q000494:pharmacology; D001665:Binding Sites; D007202:Indicators and Reagents; D007700:Kinetics; D015394:Molecular Structure; D009498:Neurotoxins; D018733:Nicotinic Antagonists / Q000737:chemistry*; D011355:Prodrugs / Q000737:chemistry*; D011978:Receptors, Nicotinic / Q000187:drug effects*; D012717:Sesquiterpenes / Q000737:chemistry*; D019810:Sodium Azide; D012965:Sodium Chloride / Q000494:pharmacology; D012997:Solvents / Q000737:chemistry
chemical_list: D001386:Azides; D007202:Indicators and Reagents; D009498:Neurotoxins; D018733:Nicotinic Antagonists; D011355:Prodrugs; D011978:Receptors, Nicotinic; D012717:Sesquiterpenes; D012997:Solvents; C086377:bipinnatin C; D012965:Sodium Chloride; D019810:Sodium Azide; C086378:bipinnatin A