Potentiation of the hepatotoxicity of N-nitrosodimethylamine by fasting, diabetes, acetone, and isopropanol.

Previous studies indicate that pretreatment with acetone or isopropanol, fasting, and streptozotocin-induced diabetes enhance hepatic microsomal nitroso-dimethylamine (NDMA) demethylase in rats. This study demonstrates that these same treatments also potentiate the hepatotoxicity of NDMA as indicated by plasma glutamic pyruvate transaminase (GPT) levels and histologic data. Pretreatment with acetone or isopropanol (2.5 ml/kg) and 2 days of fasting caused a 2-fold potentiation of NDMA-induced plasma GPT elevation, whereas streptozotocin-induced diabetes caused a 4.6-fold potentiation. The centrilobular necrosis produced by NDMA was more severe after pretreatment with the inducers. NDMA treatment also decreased hepatic microsomal demethylase activity. These results lend support to the concept that a NDMA demethylase is responsible for the activation of NDMA in vivo to a toxic intermediate, and induction of this enzyme activity potentiates NDMA hepatotoxicity.

Alterations of microsomal monooxygenase system and carcinogen metabolism by streptozotocin-induced diabetes in rats.

The effects of diabetes on the liver microsomal monooxygenase enzymes and carcinogen metabolism have been studied in rats. Treatment with streptozotocin causes a marked enhancement in microsomal N-nitrosodimethylamine (NDMA) demethylase activity. The enhancement is due mainly to the induction of a high affinity NDMA demethylase (Km, approximately 0.05 mM) which is accompanied by the induction of a protein species with mol. wt. of 50,000. The treatment also induces aniline hydroxylase whose activity is in parallel with NDMA demethylase. Streptozotocin-induced diabetes also increases the metabolism of N-nitrosomethylethylamine but not that of N-nitrosomethylaniline or N-nitrosomethylbenzylamine. On the other hand, diabetes decreases the metabolism of benzo[a]pyrene, benzphetamine, and ethylmorphine. The result suggest that diabetes causes an alteration of the composition of cytochrome P-450 isozymes; the forms efficient in metabolizing NDMA are increased while certain other forms of cytochrome P-450 are decreased.

The nature of nitrosamine denitrosation by rat liver microsomes.

The nature of the denitrosation of nitrosamines by rat liver microsomes was investigated. The rates of NADPH-dependent nitrosamine demethylation and denitrosation were compared in the same incubation mixture using several types of microsomes and inhibitors. Pretreatment with isopropanol, pyrazole, phenobarbital, and 3-methylcholanthrene had parallel effects on the microsomal demethylation and denitrosation reactions. Nitrite was produced with N-nitrosodimethylamine, N-nitroso-N-methylethylamine, N-nitroso-N-methylbutylamine, N-nitroso-methylbenzylamine, or N-nitroso-N-methylaniline as a substrate. With control microsomes, the rate of the denitrosation reaction was 9-39% that of demethylation depending on the type and concentration of nitrosamines used. Using nitrosodimethylamine as the substrate, the Km of denitrosation was about twice that of the demethylation reaction. Several polar organic solvents such as ethanol and isopropanol inhibited the denitrosation and demethylation reactions and each solvent inhibited both reactions to about the same extent. In the presence of cumene hydroperoxide, microsomes can catalyze the denitrosation of nitrosodimethylaime which is also accompanied by demethylation. Studies with a reconstituted system and with inhibitors indicate that the denitrosation reaction requires the presence of both cytochrome P-450 and NADPH-cytochrome-P-450 reductase. The results suggest that the denitrosation is closely linked to the demethylation reaction.