Stereospecificity in the oxidation of phorate and phorate sulphoxide by purified FAD-containing mono-oxygenase and cytochrome P-450 isozymes.
1. Both the cytochrome P-450-dependent mono-oxygenase system and the FAD-containing mono-oxygenase catalyse the sulphoxidation of thioether-containing organophosphate insecticides. Using purified FAD-containing mono-oxygenase and purified cytochrome P-450 isozymes isolated from mouse liver microsomes, the stereospecificity of the oxidation of phorate to (+)-and (-)-phorate sulphoxide and the further oxidations of the (+)-and (-)-phorate sulphoxides to the sulphone, the oxon sulphoxide and the oxon sulphone were examined. 2. The FAD-containing mono-oxygenase catalysed the formation of (-)-phorate sulphoxide, while two cytochrome P-450 isozymes (cytochrome P-450-B2, a constitutive form, and cytochrome P-450-PB, the principal form induced by phenobarbital) produced (+)-phorate sulphoxide. The other three constitutive cytochrome P-450 isozymes examined yielded racemic mixtures. 3. The FAD-containing mono-oxygenase had the lowest Km for the sulphoxidation reaction, 32 microM, while the Km values for the cytochrome P-450 isozymes ranged from 67 microM to 250 microM. No additional oxidation of phorate sulphoxide by the FAD-containing monooxygenase was detected using either (+)-phorate sulphoxide or (-)-phorate sulphoxide as substrates. 4. In contrast, all five cytochrome P-450 isozymes tested formed additional oxidation products; the (+)-phorate sulphoxide was the preferred substrate for all cytochrome P-450 forms. 5. The final oxidation product, phorate oxon sulphone, was derived by desulphuration of phorate sulphone, with the formation of the oxon sulphoxide being a terminal pathway.
Levi
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PE
Toxicology Program, North Carolina State University, Raleigh 27695-7633.
Hodgson
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D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D005260:Female; D005182:Flavin-Adenine Dinucleotide / Q000378:metabolism; D007306:Insecticides / Q000378:metabolism*; D007527:Isoenzymes / Q000378:metabolism*; D051379:Mice; D008815:Mice, Inbred Strains; D008862:Microsomes, Liver / Q000201:enzymology; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D009946:Organothiophosphorus Compounds / Q000378:metabolism*; D010105:Oxygenases / Q000378:metabolism*; D010702:Phorate / Q000378:metabolism*; D013237:Stereoisomerism; D013379:Substrate Specificity
D007306:Insecticides; D007527:Isoenzymes; D009946:Organothiophosphorus Compounds; D005182:Flavin-Adenine Dinucleotide; C055235:phorate sulfoxide; D010702:Phorate; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D010105:Oxygenases