Thiol-enhanced decomposition of MNNG, ENNG, and nitrosocimetidine: relationship to mutagenicity in V79 Chinese hamster cells.

The nitrosated form of cimetidine (Tagamet), nitrosocimetidine (NC), belongs to a group of nitrosoamidines in which the mutagenic and carcinogenic properties of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) have been studied in detail. The common mechanism of action of these compounds is that nucleophilic atoms can attack their iminocarbon, thereby leading to the formation of alkyldiazohydroxide and, subsequently of an alkylating and mutagenic diazonium ion. A competitive, non-mutagenic pathway involves denitrosation, which is strongly dependent on pH and can be enhanced by glutathione transferase. The influence of different thiols (e.g. glutathione and the L- and D-forms of N-acetylcysteine (L-NAC and D-NAC respectively] at different extra- and intracellular concentrations on the mutagenicity of these nitrosoamidines in V79 cells has been studied in the present investigation. The results demonstrate that the mutagenicity of MNNG and ENNG is highly dependent on where their reaction with thiols takes place. Thus, an increase in the intracellular glutathione level in combination with treatment with MNNG (or ENNG) in thiol-free medium elevated the mutagenicity, whereas treatment with thiols in the medium reduced mutagenicity. The mutagenicity of NC was, on the other hand, only slightly affected by increasing extra- or intracellular thiol levels. The dependence of NC-induced mutagenicity on thiols was indicated, however, by the finding that depletion of intracellular glutathione reduced this mutagenicity almost completely. The effects of treatments with thiols alone or in combination with glutathione transferases suggest that, under our assay conditions (e.g. physiological pH and thiol levels, in combination with low levels of the nitrosoamidines), no denitrosation occurs. On the contrary, our results indicate that intracellular thiols, and possibly glutathione transferases, potentiate the production of mutagenic species from these nitrosamidines.