Acetylcholinesterase as polyelectrolyte: interaction with multivalent cationic inhibitors.

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, KD, has been described by equation pKD = pKD degrees-ZL psi +Z log[Me+Z] following from Manning's polyelectrolyte theory, where psi +Z is the fraction of condensed counterions Me+Z per one negative charge of the polyanionic enzyme. The ZL psi+Z values for the complex formation between native acetylcholinesterase and hexamethonium (ZL = +2) or gallamine (ZL = +3) were in quantitative agreement with those predicted by the theory making use of psi+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 psi+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 psi+1 for a given subtype or modified form of the enzyme.