Role of aldehydes in the toxic and mutagenic effects of nitrosamines.

α-Hydroxynitrosamine metabolites of nitrosamines decompose to a reactive diazohydroxide and an aldehyde. To test the hypothesis that the aldehydes contribute to the harmful effects of nitrosamines, the toxic and mutagenic activities of three model methylating agents were compared in Chinese hamster ovary cells expressing or not expressing human O6-alkylguanine DNA alkyltransferase (AGT). N-Nitrosomethylurethane (NMUr), acetoxymethylmethylnitrosamine (AMMN), and 4-(methylnitrosamino)-4-acetoxy-1-(3-pyridyl)-1-butanone (NNK-4-OAc) are all activated by ester hydrolysis to methanediazohydroxide. NMUr does not form an aldehyde, whereas AMMN generates formaldehyde, and NNK-4-OAc produces 4-oxo-1-(3-pyridyl)-1-butanone (OPB). Since these compounds were likely to alkylate DNA to different extents, the toxic and mutagenic activities of these compounds were normalized to the levels of the most cytotoxic and mutagenic DNA adduct, O6-mG, to assess if the aldehydes contributed to the toxicological properties of these methylating agents. Levels of 7-mG indicated that the differences in cytotoxic and mutagenic effects of these compounds resulted from differences in their ability to methylate DNA. When normalized against the levels of O6-mG, there was no difference between these three compounds in cells that lacked AGT. However, AMMN and NNK-4-OAc were more toxic than NMUr in cells expressing AGT when normalized against O6-mG levels. In addition, AMMN was more mutagenic than NNK-4-OAc and MNUr in these cells. These findings demonstrate that the aldehyde decomposition products of nitrosamines can contribute to the cytotoxic and/or mutagenic activity of methylating nitrosamines.

Studies of a deuterium isotope effect in carcinogenesis by N-nitroso-N-alkylurethanes in rats.

Nitroso-N-methylurethane and nitroso-N-ethylurethane were administered to groups of 20 female F344 rats in corn oil solution by gavage. Each rat received once a week 0.2 ml of solution containing 7 mg/ml or 1.75 mg/ml of the methyl compound or 8 mg/ml or 2 mg/ml of the ethyl compound for 20 weeks. In parallel with these treatments additional groups of rats were given equimolar dose of nitrosomethylurethane and nitrosoethylurethane fully labeled with deuterium in the N-alkyl groups. All animals were allowed to die naturally. The total doses received by the rats were 0.2 mmol at the higher concentration and 0.05 mmol at the lower concentration. Almost all of the treated rats died with papillomas and carcinomas of the forestomach (non-glandular stomach), and the other induced tumors of significance were carcinomas and papillomas of the esophagus in rats given the 7 mg/ml dose of nitrosomethylurethane, both labeled and unlabeled. There was no significant difference in tumor incidence or rate of mortality from tumors that would indicate a difference in carcinogenic effectiveness between the nitrosoalkylurethanes and their deuterium-labeled counterparts. Apart from the esophageal tumors induced only by nitrosomethylurethane at the higher dose, there was no significant difference in carcinogenic effectiveness between the methyl and ethyl nitrosourethanes.