Hydrolysis of tetriso by an enzyme derived from Pseudomonas diminuta as a model for the detoxication of O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothiolate (VX)

Abstract

An enzyme termed organophosphorus hydrolase (OPH), derived from Pseudomonas diminuta, had been found previously to hydrolyze the powerful acetylcholinesterase (AChE) inhibitor O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothiolate (VX). This enzyme has now been shown to be correlated with the loss of AChE inhibitory potency (detoxication). OPH also hydrolyzed and detoxified the VX analogue, O, O-diisopropyl S-(2-diisopropylaminoethyl) phosphorothiolate (Tetriso), also a potent AChE inhibitor, about five times faster than VX. The K m for the hydrolysis of the Pz.sbnd;S bond of Tetriso was 6.7 × 10 −3M. OPH also hydrolyzed diisopropylphosphorofluoridate (DFP) 50–60 times faster than Tetriso, and 1,2,2-trimethylpropyl methylphosphonofluoridate (Soman) about seven times faster than Tetriso. DFP was a non-competitive inhibitor of Tetriso hydrolysis, K i = 8.7 × 10 −4M. The DFP hydrolysis product, diisopropyl phosphate, was a competitive inhibitor, K i = 2.3 × 10 −4M. The rate of detoxication of Tetriso compared with the rate of hydrolysis suggests that OPH may not be totally specific for P-S bond cleavage. OPH was inhibited completely by 1.5 × 10 −4M 8-hydroxyquinoline-5-sulfonate or 1,10-phenanthroline, both transition element chelators, but inhibited only partially by EDTA, a much more potent chelator.