Proton magnetic resonance studies of cholinergic ligand binding to the acetylcholine receptor in its membrane environment.

Abstract

PMR was used to monitor binding of choline, a known partial agonist, to acetylcholine receptor-enriched membrane preparations from Torpedo californica electroplax. The interaction between choline and receptor led to a broadening of the resonance of the choline methyl groups and this effect was reversed by .alpha.-bungarotoxin, a quasi-irreversible antagonist of the acetylcholine receptor. From the concentration dependence of line broadening the equilibrium dissociation constant for choline was obtained (Kd = 196 .+-. 65 .mu.M). The temperature dependence of the parameters observed in the choline titrations gave an enthalpy of binding .DELTA.H < 1.5 kcal/mol and allowed estimates for the dissociation rate constant of the receptor-choline complex (kdiss > 1.6 .times. 103 S1) and the respective activation energy, Ea (kdiss) .apprxeq. 5.5 kcal/mol. The association of other ligands with the membrane-bound receptor could also be studied by observing effects of varying concentrations of such ligands on the choline methyl group linewidth at a constant choline concentration.