methimazole and 2-4-dichloro-6-phenylphenoxyethylamine

Studies were carried out, using antibodies to specific cytochrome P-450 isozymes, to identify the isozymes involved in the NADPH-dependent activation of 3,3'-dichlorobenzidine (DCB) by rat hepatic microsomes to mutagens in the Ames test. DCB activation was not affected by a monoclonal antibody specific for P-450c or by a monoclonal antibody specific for P-450b, but was inhibited 69% by a polyclonal antibody made against P-450d. DCB activation was also inhibited 46% by antibody specific for NADPH-cytochrome P-450 reductase. Further, addition of methimazole, a high affinity substrate for the flavin-containing monooxygenase, reduced the residual mutagenicity in the systems containing antibody to P-450d and cytochrome P-450 reductase to 9% and 19%, respectively, of the appropriate control values. It is concluded that P-450d contributes to a majority of the P-450-dependent activation of DCB in hepatic microsomes. The results also suggest that the flavin-containing monooxygenase may contribute to the microsomal activation of DCB.

Topics: 3,3'-Dichlorobenzidine; Animals; Benzidines; Biotransformation; Cytochrome P-450 Enzyme System; Enzyme Induction; Isoenzymes; Male; Methimazole; Microsomes, Liver; Mutagens; NADPH-Ferrihemoprotein Reductase; Oxygenases; Polychlorinated Biphenyls; Rats; Rats, Inbred Strains

Methapyrilene ([14C]MPH) was found to bind to calf thymus DNA only after activation by both rat liver microsomes and NADPH. The cytochrome P-450 inhibitors 2,4-dichloro-6-phenylphenoxyethylamine, 2-diethylaminoethyl-2,2-diphenylvalerate and metyrapone inhibited binding, but methimazole, a flavin-dependent monooxygenase inhibitor, had no effect. However, 1,2-epoxy-3,3,3-trichloropropane, an epoxide hydrolase inhibitor, decreased binding by 30%. Pre-treatment of rats with isosafrole, pregnenolone-16 alpha-carbonitrile or phenobarbital had little or no effect on binding while 3-methylcholanthrene pretreatment decreased binding by 37%. Incubations in the presence of either N-acetylcysteine, glutathione, catalase or glutathione-peroxidase decreased binding to DNA while superoxide dismutase had no effect. These data suggest that MPH is metabolically activated to a species which binds to DNA and that this activation may be mediated by cytochrome P-450 isozymes.

Topics: Acetylcysteine; Aminopyridines; Animals; Biotransformation; Cattle; Cytochrome P-450 Enzyme Inhibitors; Cytochrome P-450 Enzyme System; DNA; Methapyrilene; Methimazole; Metyrapone; Microsomes, Liver; NADP; Phenobarbital; Polychlorinated Biphenyls; Pregnenolone Carbonitrile; Proadifen; Rats; Safrole

Topics: Amines; Aminophenols; Aniline Compounds; Animals; Biotransformation; Carbon Monoxide; Guinea Pigs; Kinetics; Male; Methimazole; Microsomes, Liver; Polychlorinated Biphenyls; Proadifen; Structure-Activity Relationship; Superoxide Dismutase