Cytotoxicity induced by N-methyl-N'-nitro-N-nitrosoguanidine may involve S-nitrosyl glutathione and nitric oxide.

1. The mutagenic/carcinogenic activity of N-methyl-N'-nitro-N- nitrosoguanidine (MNNG) is generally thought to involve direct methylation of DNA guanine by methyldiazohydroxide, an alkylating hydrolysis product. The molecular cytotoxic mechanism of MNNG was studied in order to determine if and how MNNG is metabolically activated. 2. MNNG was rapidly metabolized by glutathione (GSH) and GSH transferase in rat hepatocyte to form S-nitrosylglutathione (GNSO). After GSH depletion, mitochondrial respiration inhibition, ATP depletion and lipid peroxidation ensued before cell death occurred. However, depleting hepatocyte GSH beforehand prevented MNNG cytotoxicity, lipid peroxidation and the inhibition of mitochondrial respiration, suggesting that GSNO initiated the cytotoxic process. 3. The iron chelator desferoxamine or various antioxidants prevented both cytotoxicity and lipid peroxidation, even when added after MNNG metabolism, suggesting a free radical-mediated mechanism of cytotoxicity. The P450 inhibitors phenylimidazole, metyrapone and imidazole also prevented MNNG cytotoxicity. 4. Similar results were previously obtained for butyl nitrite induced hepatocyte cytotoxicity, which suggest that MNNG cytotoxicity can be attributed to metabolic activation to GSNO rather than direct methylation of macromolecules.