High-Affinity Nitrosamine Dealkylase System in Rat Liver Microsomes and Its Induction by Fasting¹

In order to elucidate the enzymic basis of nitrosamine metabolism, the in vitro metabolism of nitrosamines by rat liver microsomes and the effects of fasting on the microsomal enzymes have been studied. Fasting for 1 to 3 days causes a 2- to 3-fold enhancement of the reduced nicotinamide adenine dinucleotide phosphate-dependent nitrosodimethylamine demethylase (NDMAD) activity. The cytochrome P-450 content and the activities of reduced nicotinamide adenine dinucleotide phosphate-cytochrome P-450 reductase and benzphetamine demethylase, however, are only modestly increased. Gel electrophoretic analysis reveals the induction of a 50,000-dalton protein band during fasting. The induction of this protein band as well as the enhancement of NDMAD activity are inhibited by CoCI₂ and inhibitors of protein and RNA biosynthesis. The involvement of cytochrome P-450 in the NDMAD is supported by the fact that microsomal reduced nicotinamide adenine dinucleotide phosphate-cytochrome P-450 reductase is required for the demethylase activity. Kinetic analysis indicates that a low-K_m form of NDMAD (apparent K_m, 0.07 mm) is markedly induced by fasting. With microsomes of control rats, there are at least three apparent K_m values (0.07, 0.38, and 38.6 mm) for NDMAD; but with microsomes of fasting rats, the low-K_m (0.07 mm) form is predominant. These results suggest that rat liver microsomes contain a cytochrome P-450 isozyme which has high affinity for nitrosodimethylamine, and this isozyme is induced by fasting. In addition to nitrosodimethylamine, the oxidative demethylation of N-nitroso-N-methylethylamine, N-nitroso-N-methylbutylamine, N-nitroso-N-methylaniline, and N-nitroso-N-methylbenzylamine is also enhanced by fasting. The extent of enhancement and substrate dependency of these reactions, however, is different from that of NDMAD.