Allosteric interactions between the membrane-bound acetylcholine receptor and chemical mediators. Kinetic studies

The kinetics of the specific irreversible reaction of a snake neurotoxin, alpha-bungarotoxin, with the acetylcholine receptor of electroplax membrane preparations have been investigated. The effects of activators (decamethonium, carbamylcholine) and inhibitors (alpha-bungarotoxin, d-tubocurarine) of neural transmission on this reaction have been measured and the following new information obtained. (1) The irreversible reaction is preceded by the reversible formation of toxin-receptor complexes. (2) Two types of receptor binding site exist. d-Tubocurarine directly competes with the toxin for one type of binding site. Decamethonium and carbamylcholine are noncompetitive inhibitors of the toxin reaction. (3) The data are inconsistent with binding sites on separate and distinct molecules or with preexisting nonequivalent binding sites. A simple model is proposed to explain both the kinetic data and equilibrium measurements which indicated that activators and inhibitors of neural transmission compete for only one-half of the receptor sites available to them. The model proposes that for the compounds investigated the binding sites of activators do not overlap with those of inhibitors and the ligand-induced conformational changes of the receptor result in changes in the affinities of the binding sites. The model is simple and is based on mechanisms which have been found to be valid for many well-characterized regulatory enzymes.