Mechanism of binding of a benzomorphan opiate to the acetylcholine receptor from Torpedo electroplaque.

The mechanism of binding of the benzomorphan opiate, (-)-N-allylnormetazocine [(-)-ANMC], to Torpedo acetylcholine receptor (AcChR)-rich membranes was investigated. Using a centrifugation assay, two equilibrium binding affinities were observed with KD values of 0.4 and 2 microM. The KD and the apparent Bmax of the higher affinity component were decreased by cholinergic agonists and antagonists but not by alpha-bungarotoxin alone. The high affinity binding site (KD = 0.4 microM) was found to be distinct from the binding site for tetracaine, a noncompetitive blocker. The apparent association rate constant was essentially independent of receptor concentration both in the presence and absence of the cholinergic agonist, carbamoylcholine. When carbamoylcholine was equilibrated with the AcChR prior to (-)-[3H]ANMC addition, the association rate constant was 2- to 3-fold greater than in the absence of cholinergic effectors. When carbamoylcholine and (-)-[3H]ANMC were added simultaneously to AcChR-rich membranes, association was too rapid to resolve manually and binding measured at 5 sec was greater than the equilibrium level both in the presence and absence of carbamoylcholine. Binding decreased as a function of time, reaching its equilibrium level with a time constant of approximately 1 min. This effect appeared to be agonist specific since it was not observed when the antagonist, d-tubocurarine, replaced carbamoylcholine. In the absence of cholinergic ligands, dissociation of (-)-ANMC was biphasic (t1/2 values of less than 5 sec and approximately 2.5 min) and, in the presence of cholinergic ligands, was monophasic (t1/2 of 40 sec). The simultaneous addition of carbamoylcholine and (-)-[3H]ANMC to the membranes initially results in a biphasic dissociation (t1/2 of 5 and 30 sec) which becomes monophasic with increasing times of incubation. A mechanism is proposed involving an isomerization of the receptor-ligand complex which agrees quantitatively and qualitatively with the data.