title: Loss of rat liver microsomal cytochrome P-450 during methimazole metabolism. Role of flavin-containing monooxygenase.
abstract: The role of flavin-containing monooxygenase (FMO) in the decrease in cytochrome P-450 content during the microsomal metabolism of methimazole (N-methyl-2-mercaptoimidazole) was investigated by heat inactivation of FMO. Incubation of liver microsomes from untreated Fischer 344 rats with NADPH and methimazole resulted in a 25% loss of cytochrome P-450 detectable as its ferrous-carbon monoxide complex. The same extent of cytochrome P-450 loss was observed with 1 and 20 mM methimazole, suggesting saturation of the process. There was no significant loss of cytochrome P-450 when microsomal FMO was heat-inactivated prior to incubation with NADPH and methimazole. Heat pretreatment of the microsomes did not affect cytochrome P-450 concentrations and cytochrome P-420 was not observed. These results indicate that FMO-catalyzed metabolism of methimazole is necessary for the loss of cytochrome P-450 in microsomes from untreated rats. Sulfite and N-methylimidazole, the ultimate products of methimazole metabolism, did not cause a significant loss of cytochrome P-450. There was no loss of cytochrome P-450 when glutathione was included in the incubation with methimazole, suggesting that cytochrome P-450 loss was due to an interaction with oxygenated metabolites of methimazole formed by FMO. Losses of cytochrome P-450 were also observed after incubation of microsomes from phenobarbital- (31%) of beta-naphthoflavone-pretreated rats (44%) with NADPH and methimazole. In contrast to microsomes from untreated rats, heat inactivation of FMO did not prevent the loss of cytochrome P-450 in microsomes from the pretreated rats. These results indicate that both phenobarbital and beta-naphthoflavone induce isozymes of cytochrome P-450 capable of directly activating methimazole.
authors.forename: G L
authors.initials: GL
authors.lastname: Kedderis
authors.forename: D E
authors.initials: DE
authors.lastname: Rickert
mesh_terms: D000818:Animals; D001571:Benzoflavones / Q000494:pharmacology; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004790:Enzyme Induction / Q000187:drug effects; D005260:Female; D005415:Flavins / Q000378:metabolism*; D066298:In Vitro Techniques; D008297:Male; D008713:Methimazole / Q000378:metabolism*; D008862:Microsomes, Liver / Q000201:enzymology*; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D009249:NADP / Q000378:metabolism; D010634:Phenobarbital / Q000494:pharmacology; D051381:Rats; D011916:Rats, Inbred F344; D019324:beta-Naphthoflavone
chemical_list: D001571:Benzoflavones; D005415:Flavins; D009249:NADP; D008713:Methimazole; D019324:beta-Naphthoflavone; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D010634:Phenobarbital