Measurement of endogenous n-nitrosation in rats and humans by monitoring urinary and faecal excretion of N-nitrosamino acids.

N-Nitrosamino acids [N-nitrosoproline (NPRO), N-nitrosohydroxyproline and N-nitrososarcosine] given orally to rats were found to be excreted unchanged in the urine and faeces almost quantitatively. Fasted rats were gavaged with solutions of a nitrosatable amino acid, followed by nitrite or N-nitrosodiphenylamine (NDPhA). The N-nitrosamino acid excreted in the urine and faeces during 24 h was monitored as an index of endogenous N-nitrosation. The results obtained by applying this procedure to the study of endogenous N-nitrosation in rats were all in good agreement with those of in vitro experiments. Endogenous transnitrosation by NDPhA to proline was shown to occur in rats; thiocyanate catalysed this transnitrosation reaction in vivo. Formation of NPRO in a human was demonstrated by monitoring the excretion of NPRO in the urine of one male volunteer who had ingested vegetable juice, as a source of nitrate, and proline. The amounts of NPRO excreted in the 24-h urine appeared to be proportional to the proline dose and increased exponentially with the dose of nitrate ingested. The simultaneous intake of ascorbic acid or alpha-tocopherol inhibited NPRO formation in the human subject. Monitoring of urinary NPRO would appear to be useful for the estimation of the extent of endogenous N-nitrosation in high-risk populations or individuals, in which endogenously formed N-nitroso compounds have been associated with increased risk of cancers, such as those of the stomach and oesophagus.