The kinetics of reactivation, by oximes, of cholinesterase inhibited by organophosphorus compounds.

Abstract

The inhibition of cholinesterase (ChE) and related enzymes, by organophosphorus compounds, is generally believed to occur by direct phosphorylation of some group at the active centre of the enzyme, together with the liberation of an acid anion (Wilson, 1951; Balls & Jansen, 1952; Hartley & Kilby, 1952; Aldridge, 1954; Kilby & Youatt, 1954). Inhibition by organophosphorus compounds is generally not reversible by dialysis or dilution, but the inhibited enzyme can be reactivated by treatment with many chemical reagents with a nucleophilic affinity for phosphorus, presumably by direct chemical displacement of the enzyme from the phosphorus atom (Wilson, 1951, 1952). A survey of a wide range ofreactivating reagents has been given by Wilson (1955 b), who has shown that reactivators are particularly effective if they possess a group capable of forming a complex with the inhibited enzyme. In addition, the rate of reactivation was found to be markedly dependent upon pH, and showed an optimum depending upon the pKA of the reactivating agent. Few detailed studies have been reported upon the kinetics of reactivation. The rate of reactivation by choline of tetraethyl pyrophosphate (TEPP) inhibited ChE was found by Wilson (1952) to approach zero order with increasing concentration of the reactivator, i.e. it shows the phenomenon of saturation as found in normal enzyme-substrate action. This effect was not observed with hydroxylamine, but even so the rate did not show a simple dependence upon hydroxylamine concentration. Cunningham (1954) obtained similar results in the reactivation by hydroxylamine of diethyl pnitrophenyl phosphate (E 600) inhibited chymotrypsin. Wilson, Ginsberg & Meislich (1955) have reported that the rate of reactivation of inhibited true ChE by hydroxamic acids also shows the saturation effect found with choline, while a similar result was obtained with hydroxamic acids and the pseudo enzyme (Wilson, 1955a). In a previous paper (Childs, Davies, Green & Rutland, 1955) the reactivating power of a wide range of oximes was reported. The present paper