Influence of ascorbic acid on the endogenous (intragastral) formation of N-nitroso compounds.

N-Nitroso compounds are not only environmentally occurring carcinogens, but can also be generated in vivo from their precursors. Nitrosation rates depend on the chemical structures of the compounds to be nitrosated, their basicity, the pH value of the reaction medium and the availability of nitrosating agents. Yields of N-nitroso compounds can be enhanced by catalysts or decreased by inhibitors. One of the most potent inhibitors of N-nitrosation is ascorbic acid, which has been shown to block formation of N-nitroso compounds very effectively in vitro and also in animal experiments at high dosages; alpha-tocopherol seems to be of similar effectiveness. In the human situation, ascorbic acid can be applied to prevent nitrosamine formation in the gastrointestinal tract. However, the pharmacokinetics of the nitrosable compounds also play an important role: recirculation of drugs with long half-lives through the bloodstream via the salivary glands would need a continuous intake of ascorbic acid as long as the drug is present in the plasma, since ascorbic acid practically does not follow the blood-saliva circulation process.