Acetylcholinesterase as polyelectrolyte: interaction with multivalent cationic inhibitors

Abstract

Influence of inorganic salts on the interaction of cobra venom acetylcholinesterase (EC 3.1.1.7) with hexamethonium and gallamine has been studied. The observed negative electrostatic salt effect in the dissociation constant of the enzyme-ligand complex, K D, has been described by equation p K D = p K D° − Z L Ψ +Z log[Me +Z] following from Manning's polyelectrolyte theory, where Ψ +Z is the fraction of condensed counterions Me +Z per one negative charge of the polyanionic enzyme. The Z L Ψ +Z values for the complex formation between native acetylcholinesterase and hexamethonium ( Z L = + 2) or gallamine ( Z L = + 3) were in quantitative agreement with those predicted by the theory making use of Ψ +1 = 0.50 found earlier from the influence of salts upon the hydrolysis of acetylcholine by the enzyme. Increase in the number of negative charges in acetylcholinesterase by its modification with pyromellitic dianhydride resulted in an increase of Ψ +1 to 0.6. The data show that the influence of salts on the electrostatic contribution to the energy of binding of cationic substrates and inhibitors by acetylcholinesterase can be quantitatively described proceeding from the counterion condensation model of Manning by using only one empirical parameter Ψ +1 for a given subtype or modified form of the enzyme.