The inhibition of erythrocyte cholinesterase by tri-esters of phosphoric acid. 2. Diethyl p-nitrophenyl thionphosphate (E 605) and analogues

Abstract

Pure E605 (parathion) has a low in vitro activity against cholinesterase. A specimen of E605 previously used (Aldridge, Biochem. Jour. 46: 451. 1950) owed its high activity to the presence of E600 (diethyl p-nitrophenyl phosphate). The S-phenyl isomer of E605 is a highly active inhibitor and the inhibition follows first order kinetics. The specimen of S-ethyl isomer contains approx. 4% E600. It was observed (Aldridge and Davison, ibid. 51: 62. 1952) that in a series of diethyl phenylphosphates substituted in the aromatic ring, the more stable to hydrolysis the inhibitor, the less effective it is as an inhibitor. This relationship was extended to include E605, Q2 (O, O-diethylO-8-quinolylthiophosphate) and the S-phenyl isomer of E605. A "hydrolysis technic" was developed and kinetic measurements were utilized for the detection and detn. of impurities in organo-phosphorus inhibitors.