Identification of the cytochrome P450 isoenzymes involved in the metabolism of diazinon in the rat liver.
The metabolism of diazinon, an organo-phosphorothionate pesticide, to diazoxon and pyrimidinol has been studied in incubations with hepatic microsomes from control Sprague-Dawley (SD) rats or SD rats treated with different P450-specific inducers (phenobarbital, dexamethasone, beta-napthoflavone, and pyrazole). Results obtained indicate an involvement of CYP2C11, CYP3A2, and CYP2B1/2, whereas CYP2E1 and CYP1A1 do not contribute to the pesticide oxidative metabolism. Indeed, diazinon was metabolized by microsomes from control rats; among the inducers, phenobarbital and dexamethasone only increased the production of either metabolites, although to different extents. The production of the two metabolites is self-limiting, due to P450 inactivation; therefore, the inhibition of CYP-specific monooxygenase activities after diazinon preincubation has been used to selectively identify the competent CYPs in diazinon metabolism. Results indicate that, after diazinon preincubation, CYP3A2-catalyzed reactions (2beta- and 6beta-testosterone hydroxylation) are very efficiently inhibited; CYP2C11- and CYP2B1/2-catalyzed reactions (2alpha- and 16beta-testosterone hydroxylation, respectively) are weakly inhibited, while CYP2E1-, CYP2A1/2-, and CYP1A1/2-related activities were unaffected. Results obtained by using chemical inhibitors or antibodies selectively active against specific CYPs provide a direct evidence for the involvement of CYP2C11, CYP3A2, and CYP2B1/2, indicating that each of them contributed about 40-50% of the diazinon metabolism, in hepatic microsomes from untreated, phenobarbital-, and dexamethasone-treated rats, respectively. The higher diazoxon/pyrimidinol ratio observed after phenobarbital-treatment together with the significantly more effective inhibition toward diazoxon production exerted by metyrapone in microsomes from phenobarbital-treated rats supports the conclusion that CYP2B1/2 catalyze preferentially the production of diazoxon.
Fabrizi
L
L
Department of Comparative Toxicology and Ecotoxicology, Istituto Superiore di Sanità, Rome, Italy.
Gemma
S
S
Testai
E
E
Vittozzi
L
L
D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases; D019363:Cytochrome P-450 CYP1A1 / Q000378:metabolism; D019362:Cytochrome P-450 CYP2B1 / Q000378:metabolism; D019392:Cytochrome P-450 CYP2E1 / Q000378:metabolism; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D000072467:Cytochrome P450 Family 2; D003907:Dexamethasone / Q000494:pharmacology; D003976:Diazinon / Q000378:metabolism* / Q000493:pharmacokinetics; D007700:Kinetics; D008297:Male; D008565:Membrane Proteins; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D009943:Organophosphorus Compounds / Q000032:analysis; D010634:Phenobarbital / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D039181:Steroid 16-alpha-Hydroxylase; D013250:Steroid Hydroxylases / Q000378:metabolism; D013379:Substrate Specificity
D008565:Membrane Proteins; D009943:Organophosphorus Compounds; D003907:Dexamethasone; D003577:Cytochrome P-450 Enzyme System; C000912:diazoxon; D013250:Steroid Hydroxylases; D019392:Cytochrome P-450 CYP2E1; D001189:Aryl Hydrocarbon Hydroxylases; C450279:CYP2C11 protein, rat; C465260:Cyp3a2 protein, rat; D019363:Cytochrome P-450 CYP1A1; D019362:Cytochrome P-450 CYP2B1; D051544:Cytochrome P-450 CYP3A; D000072467:Cytochrome P450 Family 2; D039181:Steroid 16-alpha-Hydroxylase; C025619:steroid 16-beta-hydroxylase; C022657:steroid hormone 6-beta-hydroxylase; D010634:Phenobarbital; D003976:Diazinon