Immunochemical studies on the metabolism of nitrosamines by ethanol-inducible cytochrome P-450

The ethanol-induced rabbit liver microsomal cytochrome P-450, P-450_LM3a, has been shown previously to efficiently catalyze the demethylation of N-nitrosodimethylamine (NDMA) with a K_m of 2.9 mM. Since the predominant K_m in hepatic microsomes from ethanol-treated rabbits is 0.07 mM, the role of P-450_LM3a in the activation of this carcinogen has been uncertain. In the present study, antibodies to P-450_LM3a were shown to almost completely inhibit NDMA demethylation by the purified P-450 in a reconstituted system as well as the low-K_m activity of liver microsomes from control or ethanol-treated rabbits. In contrast, the antibody did not inhibit the high-K_m NDMA demethylase activity in the microsomes. These results indicate that P-450_LM3a is the major P-450 responsible for the low-K_m NDMA demethylase activity. In addition, evidence is provided for the existence of a cytochrome immunochemically similar to P-450_LM3a in liver microsomes from rats, mice, and guinea pigs that effectively catalyzes the demethylation of NDMA.