Abstract
The hydrolysis of malathion [S-(1,2-dicarbethoxyethyl) O, O-dimethyl phosphorodithioate] is catalysed by two types of enzymes in the mouse liver. The first is malathion B-esterase which hydrolyses one of the carboxyethyl ester groups giving malathion mono-acid. This is a B type esterase, being sensitive to low concentrations of DFP (diisopropyl phosphorofluoridate), paraoxon (diethyl p-nitrophenyl phosphate) and EPNO the oxygen analogue of EPN ( O-ethyl O- p-nitrophenyl phenylphosphorothionate). It is predominently in the microsomes, acts at pH 7.4–7.6 and is probably the same as the non-specific carboxylesterases (EC 3.1.1.1). The second enzyme is malathion A-esterase which is insensitive to DFP and other organophosphates even at high concentrations. It is predominently in the cell-sap, has an optimum pH of 8.8, requires 2-mercaptoethanol, reduced glutathione or similar SH compounds for activation, and is inhibited by p-chloromercuribenzoate and heavy metals. The A esterase degrades malathion at the P-S linkage giving O, O-dimethyl phosphorothioate.
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