Abstract
The cytochrome P450 (P450) and flavin-containing monooxygenase (FMO) enzymes are the major oxidative enzymes in phase I metabolism. Many organophosphate and carbamate thioether compounds are excellent substrates for these enzymes. Stereoselective sulfoxidation of fenthion and methiocarb by human liver, kidney, and microsomes was investigated. A high level of stereoselectivity in the formation of fenthion (+)-sulfoxide was observed in kidney and intestinal microsomes. This activity was not inhibited by the P450 inhibitor 1-aminobenzotriazole but was dramatically reduced following mild heat treatment. In liver, fenthion was metabolized to its sulfoxide in a nonstereoselective manner, and the activity was sensitive to both 1-aminobenzotriazole and heat treatment. The carbamate pesticide methiocarb also was sulfoxidated with a high degree of stereoselectivity in human kidney microsomes. Human liver microsomes formed both stereoisomers in equal amounts. Sulfoxide formation in kidney was not inhibited by 1-aminobenzotriazole but was abolished in liver microsomes. Formation of methiocarb sulfoxides was not observed in intestinal microsomes. The relative contribution of FMO1 and FMO3 to the sulfoxidation of carbophenothion, demeton-O, ethiofencarb, fonofos, and methiocarb also was investigated by using baculovirus-expressed recombinant proteins. FMO1 showed the highest catalytic activity for all pesticides. This study indicates that FMO1 may have a bigger role in extrahepatic metabolism than previously thought.
Footnotes
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This work was funded through the University of California, Riverside U.S. Department of Agriculture/Agricultural Experiment Station.
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Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
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doi:10.1124/dmd.104.000984.
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ABBREVIATIONS: P450, cytochrome P450; FMO, flavin-containing monooxygenase; HPLC, high-performance liquid chromatography; 1-ABT, 1-aminobenzotriazole.
- Received June 13, 2004.
- Accepted November 10, 2004.
- The American Society for Pharmacology and Experimental Therapeutics
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