The inhibition of cholinesterase by diethyl phosphorochloridate

Abstract

A method is described which enables the measurement of the rate constant for the inhibition of enzymes by covalent inhibitors that hydrolyze rapidly. The method(s) also yields the rate constant for the hydrolysis of the inhibitor. It was found that diethyl phosphorochloridate, depending on the buffer composition, hydrolyzes with a first-order rate constant of 1.0 min −1 in phosphate buffer ( I = 0.1) at pH 7.0 and 25 °C and inhibits acetylcholinesterase (eel) (acetylcholine hydrolase, EC 3.1.1.7) with a second-order rate constant of 1.5·10 6 M −1·min −1. Cholinesterase from bovine erythrocytes and horse serum were also studied. In any case, the phosphorochloridate was not found to be much more potent as an inhibitor than the corresponding phosphorofluoridate, even though the rate of hydrolysis of the latter is 2000 times slower.