CHARACTERIZATION OF P-S BOND HYDROLYSIS IN ORGANOPHOSPHOROTHIOATE PESTICIDES BY ORGANOPHOSPHORUS HYDROLASE

The extensive use of organophosphorothioate insecticides in agricultur e has resulted in the risk of environmental contamination with a varie ty of broadly based neurotoxins that inhibit the acetylcholinesterases of many different animal species. Organophosphorus hydrolase (OPH, EC 3.1.8.1) is a broad-spectrum phosphotriesterase that is capable of de toxifying a variety of organophosphorus neurotoxins by hydrolyzing var ious phosphorus-ester bonds (P-O, P-F, P-CN, and PS) between the phosp horus center and an electrophilic leaving group. OPH is capable of hyd rolyzing the P-X bond of various organophosphorus compounds at quite d ifferent catalytic rates: P-O bonds (k(cat) = 67-5000 s(-1)), P-F bond s (k(cat) = 0.01-500 s(-1)), and P-S bonds s (k(cat) = 0.0067 to 167 s (-1)). P-S bond cleavage was readily demonstrated and characterized in these studies by quantifying the released free thiol groups using 5,5 '-dithio-bis-2-nitrobenzoic acid or by monitoring an upheld shift of a pproximately 31 ppm by P-31 NMR. A decrease in the toxicity of hydroly zed products was demonstrated by directly quantifying the loss of inhi bition of acetylcholinesterase activity. Phosphorothiolate esters, suc h as demeton-S, provided noncompetitive inhibition for paraoxon (a P-O triester) hydrolysis, suggesting that the binding of these two differ ent classes of substrates was not identical. (C) 1995 Academic Press, Inc.