Proton magnetic resonance studies of cholinergic ligand binding to the acetylcholine receptor in its membrane environment.
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 microM). The temperature dependence of the parameters observed in the choline titrations gave an enthalpy of binding delta H less than 1.5 kcal/mol and allowed estimates for the dissociation rate constant of the receptor-choline complex (kdiss greater than 1.6 x 10(3) S(1) and the respective activation energy, Ea (Kdiss) approximately 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.
D000818:Animals; D002217:Carbachol / Q000378:metabolism; D002794:Choline / Q000378:metabolism; D005399:Fishes; D007700:Kinetics; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D009045:Motor Endplate / Q000378:metabolism; D011950:Receptors, Cholinergic / Q000378:metabolism*; D013696:Temperature
D008024:Ligands; D011950:Receptors, Cholinergic; D002217:Carbachol; D002794:Choline