Mechanism of N-acetylcysteine (NAC) and other thiols as both positive and negative modifiers of MNNG-induced mutagenicity in V79 Chinese hamster cells.

Carcinogenic xenobiotics can be detoxified by nucleophilic thiols, which interact directly or through enzyme catalyzed reactions with electrophilic metabolites/compounds or metabolically produced oxidants. Formation of such conjugates is assumed to be a dominating competitive pathway reducing the mutagenic and carcinogenic effects of several known carcinogens. In the case of the potent carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) the situation is different since this carcinogen is transformed to reactive intermediates by nucleophilic agents such as thiols. As a consequence of this, the modulating effect of thiols has to be discussed in relation to the reaction kinetics of this nitroso compound. In this report we present data on the mutagenicity of MNNG which was enhanced by intracellular excess of glutathione, achieved by pre-treatment with N-acetylcysteine, cysteine and glutathione. Depletion of GSH by treatment with diethylmaleate or buthioninesulfoximine resulted in a decreased mutagenic effect of MNNG. A decreased effect of MNNG was also obtained by extracellular excess of thiols. The modification of MNNG-mutagenicity was compared to the effect of thiols on the mutagenicity of N-methyl-N'-nitrosourea (MNU) in V79 Chinese hamster cells. No effect of the thiols could be detected on the mutagenicity of MNU, indicating that the intermediates of MNU and MNNG react, in agreement with the reaction kinetics, in favour of water and thiols become less important. The results indicate that the activation of MNNG to mutagenic intermediates is occurring within the cells. This activation is mediated by reaction of MNNG with thiols (and amines) and subsequent release of short-lived alkylating intermediates. The mutagenic effect of MNNG can be reduced or enhanced respectively by decreasing or increasing the intracellular thiol levels. As demonstrated with MNU, intracellular thiol concentrations (in milli/molar range), which is high enough to bring about the activation of MNNG, are not sufficiently high to protect DNA from damage by the alkylating intermediate. Extracellular levels of thiols 'protect' the cells simply by increasing the decomposition of MNNG in the treatment solution.