Concentration-dependent kinetics of acetylcholinesterase inhibition by the organophosphate paraoxon

For decades the interaction of the anticholinesterase organophosphorus compounds with acetylcholinesterase has been characterized as a straightforward phosphylation of the active site serine (Ser-203) which can be described kinetically by the inhibitory rate constant k(i). However, more recently certain kinetic complexities in the inhibition of acetylcholinesterase by organophosphates such as paraoxon (O,O-diethyl O-(p-nitrophenyl) phosphate) and chlorpyrifos oxon (O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphate) have raised questions regarding the adequacy of the kinetic scheme on which k(i) is based. The present article documents conditions in which the inhibitory capacity of paraoxon towards human recombinant acetylcholinesterase appears to change as a function of oxon concentration (as evidenced by a changing k(i)), with the inhibitory capacity of individual oxon molecules increasing at lower oxon concentrations. Optimization of a computer model based on an Ordered Uni Bi kinetic mechanism for phosphylation of acetylcholinesterse determined k(1) to be 0.5 nM(-1)h(-1), and k(-1) to be 169.5 h(-1). These values were used in a comparison of the Ordered Uni Bi model versus a k(i) model in order to assess the capacity of k(i) to describe accurately the inhibition of acetylcholinesterase by paraoxon. Interestingly, the k(i) model was accurate only at equilibrium (or near equilibrium), and when the inhibitor concentration was well below its K-d (pseudo first order conditions). Comparisons of the Ordered Uni Bi and k(i) models demonstrate the changing k(i) as a function of inhibitor concentrations is not an artifact resulting from inappropriate inhibitor concentrations.