ACETYLCHOLINESTERASE AS POLYELECTROLYTE - INTERACTION WITH MULTIVALENT CATIONIC INHIBITORS

Influence of inorganic salts on the interaction of cobra venom acetylc holinesterase (EC 3.1.1.7) with hexamethonium and gallamine has been s tudied. The observed negative electrostatic salt effect in the dissoci ation constant of the enzyme-ligand complex, K(D), has been described by equation pK(D) = pK(D)-degrees - Z(L) 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 polyani onic enzyme. The Z(L)psi(+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 th e theory making use of psi+1 = 0.50 found earlier from the influence o f salts upon the hydrolysis of acetylcholine by the enzyme. Increase i n the number of negative charges in acetylcholinesterase by its modifi cation with pyromellitic, dianhydride resulted in an increase of psi+1 to 0.6. The data show that the influence of salts on the electrostati c contribution to the energy of binding of cationic substrates and inh ibitors by acetylcholinesterase can be quantitavely described proceedi ng 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.