Binding sites on acetylcholinesterase for reversible ligands and phosphorylating agents: A theoretical model tested on haloxon and phosphostigmine

The reaction of acetylcholinesterase (EC 3.1.1.7; human erythrocytes) with phosphostigmine, haloxon and VX was studied, and the effect of three reversible ligands (TMA, edrophonium, coumarin) and of acetylthiocholine upon the time-dependent and time-independent (reversible) inhibition by the organophosphates was evaluated. The three ligands and acetylthiocholine decreased the second-order rate constant of phosphorylation by a factor proportional to the enzyme-ligand dissociation constant, or to both K_m and K_ss (Michaelis constant and the substrate-inhibition constant for acetylthiocholine) irrespective of the organophosphate. However, the time-independent inhibitions by phosphostigmine and haloxon were differently affected. Acetylthiocholine affected the time-independent inhibition by phosphostigmine by a factor proportional to K_m, and that by haloxon by a factor proportional to K_ss. Coumarin had no effect on the time-independent inhibition by phosphostigmine, while TMA and edrophonium displaced phosphostigmine from its complex. Coumarin displaced haloxon from its complex with the enzyme, while TMA and edrophonium had no effect. We conclude that phosphostigmine and haloxon bind reversibly to different sites on the enzyme and the experiments agree with a theoretical model that haloxon binds reversibly to a peripheral site on acetylcholinesterase, and phosphostigmine to the catalytic site.