Studies on the antimutagenic activity of ascorbic acid in vitro and in vivo.

The possibility that ascorbic acid, as a nucleophile, may inhibit mutagenicity induced by electrophilic metabolites of N-nitroso compounds was examined. In vitro data are presented to show that ascorbic acid does not decrease the mutagenicity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in a modified Ames bacterial mutagenicity system if deionized water is used to prepare the incubation medium. However, ascorbic acid prevents the mutagenicity of MNNG in vitro if added to bacteria in a medium prepared with either sterile tap water or deionized water and Cu2+ ions and that this antimutagenic response is blocked by EDTA. Additional in vitro experiments suggest that when ascorbic acid and Cu2+ ions are mixed in aqueous solution, H2O2 and free radicals derived from H2O2 are formed and these compounds may deactivate N-nitroso compounds. In vivo data are presented to show that ascorbic acid supplementation to guinea pigs (2000 mg/kg body weight/day) has no effect on the mutagenicity of N-nitrosodimethylamine, MNNG, N-methylnitrosourea and streptozotocin using the intrahepatic host-mediated bacterial mutagenicity assay. Additional in vivo studies demonstrate that simultaneous oral administration of ascorbic acid prevents the mutagenicity that follows the intragastric nitrosation of aminopyrine by nitrite while dietary pre-treatment with ascorbic acid does not. These findings suggest that ascorbic acid can block the intragastric formation of mutagenic N-nitroso compounds but that ascorbic acid has no effect on mutagenicity of N-nitroso compounds once they are formed.