Datensatz

Zusammenfassung

Proton magnetic resonance has been 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 = 190 ± 65 µ 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 x 103 S1) and the respective activation energy, Ea (kdiss) approx 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