Metabolism of phosphorus-containing compounds by pig liver microsomal FAD-containing monooxygenase

Oxidative desulfuration of phosphonate insecticides such as fonofos (S-phenyl ethyl ethylphosphonodithioate) and its analogs is catalyzed by pig liver microsomal FAD-containing monooxygenase, although desulfuration of phosphorodithioates, such as parathion, is not. Substitution of an alkyl group for the remaining alkoxy group, as in S-phenyl diethylphosphinodithioate, did not increase its oxidation rate. Diethylphenylphosphine sulfide, containing 3 phosphorus-carbon bonds, was actually a poorer substrate than fonofos. Replacement of the S-phenyl group of fonofos with an O-phenyl group increased the Km value above the solubility limit. Trivalent phosphorus-containing compounds were also excellent substrates for this enzyme. Diethylphenylphosphine had a Km value lower than 2.5 microM. Diethyl phenylphosphonite also appeared to be an excellent substrate but its rapid nonenzymatic hydrolysis and/or oxidation precluded accurate Km determinations. Stoichiometry studies with diethylphenylphosphine and its sulfide showed that O2 and NADPH consumption were approximately equal to the substrate consumed. The major metabolite of both diethylphenylphosphine and its sulfide was the phosphine oxide. These results show that microsomal FAD-containing monooxygenase of pig liver has activity as a phosphorus-oxidase, in addition to its well characterized nitrogen- and sulfur-oxidase roles.