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

Abstract

The kinetics of the specific irreversible reaction of a snake neurotoxin, .alpha.-bungarotoxin, with the acetylcholine receptor of electroplax [from Electrophorus electricus] membrane preparations were investigated. The effects of activators (decamethonium carbamylcholine) and inhibitors (.alpha.-bungarotoxin, d-tubocurarine) of neural transmission on this reaction were measured. The irreversible reaction was preceded by the reversible formation of toxin-receptor complexes. Two types of receptor binding site existed. d-Tubocuraraine directly competed with the toxin for 1 type of binding site. Decamethonium and carbamylcholine were noncompetitive inhibitors of the toxin reaction. The date were inconsistent with binding sites on separate and distinct molecules or with preexisting nonequivalent binding sites. A simple model was proposed to explain the kinetic data and equilibrium measurements which indicated that activators and inhibitors of neural transmission compete for only 1/2 of the receptor compounds investigated, the binding sites of activators do not overlap with those of inhibitors and that 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 valid for many well-characterized regulatory enzymes.