Relation between hepatic microsomal metabolism of N-nitrosamines and cytochrome P-450 species.

Effects of SKF 525A (0.1 mM), metyrapone (0.1 mM), alpha-naphthoflavone (ANF) (0.5 mM) and pyrazole (1.0 mM) on N-nitrosodimethylamine (NDMA), N-nitrosomethylbutylamine (NMBuA) and N-nitrosomethylbenzylamine (NMBeA) metabolism by hepatic microsomes from rats pretreated with inducers were investigated. NDMA demethylation was weakly increased by phenobarbital (PB) treatment. The demethylation was inhibited by SKF 525A and enhanced by metyrapone in non-treated and PB-treated microsomes, and weakly inhibited by ANF in 3-methylcholanthrene(MC)-treated microsomes. NMBuA demethylation was increased by PB treatment and inhibited by SKF 525A in all microsomes. Metyrapone inhibited the demethylation in PB-treated microsomes. NMBuA debutylation was increased by PB and MC treatments, and inhibited by metyrapone in all microsomes. The strongest inhibition by metyrapone was observed in PB-treated microsomes. The debutylation was inhibited by SKF 525A in non-treated and PB-treated microsomes and by ANF in MC-treated microsomes. NMBeA demethylation was decreased by MC treatment and weakly inhibited by SKF 525A in all microsomes. The effects of the inducers and inhibitors on NMBeA debenzylation were almost the same as those on NMBuA debutylation except that the increasing effect of MC was small. Pyrazole was a relatively selective inhibitor of NDMA demethylation. These results suggest the following: NDMA demethylation is catalyzed by PB-induced cytochrome P-450 species (P450-PB) and MC-induced cytochrome P-450 species (P448-MC). But their specific activity is low and the other cytochrome P-450 species demethylate NDMA. NMBuA demethylation is catalyzed by P450-PB. But the specific activity is not high and the other cytochrome P-450 species also demethylate NMBuA. NMBuA debutylation is catalyzed by P450-PB and P448-MC. Almost all of NMBeA demethylation is catalyzed by cytochrome P-450 species other than P450-PB and P448-MC. NMBeA debenzylation is catalyzed by P450-PB and P448-MC, but the specific activity of P448-MC is not high.