Monitoring endogenous nitrosamine formation in man

Results from animal experiments and studies in human subjects indicated that the amount of nitrosoproline (NPRO) excreted in the 24-h urine, following ingestion of precursors, is an index for the rate of endogenous nitrosation; this method was found to be sensitive, reproducible and could be satisfactorily applied to human subjects in clinical and field studies. N-Nitrosothiazolidine 4-carboxylic acid (NTCA) and its 2-methyl derivative (NMTCA) were also identified in the urine of human subjects. As the respective amino precursors (thiazolidine 4-carboxylic acids) can be formed by reaction of formaldehyde or acetaldehyde with cysteine, measurement of NTCA and NMTCA in urine may provide a further index for endogenous nitrosation in the human body and may also allow monitoring of exposure of human subjects to aldehydes, nitrate and nitrite. The yield of nitroso compounds formed endogenously in the human body was shown to be linked to the intake of precursors, but several inhibitors and catalysts, either as pure substances or occurring in complex mixtures, were shown to modify the nitrosation reaction in vivo. In particular, ingestion of ascorbic acid after nitrate-rich meals was efficient in lowering human exposure to endogenously formed N-nitroso compounds. A dose-response relationship was established for the formation of NPRO in rats in vivo, after concurrent administration of various concentrations of the precursors, L-proline and sodium nitrite. The logarithm of the amount of NPRO formed was found to be proportional to the logarithm of the product of the proline dose and the square of the nitrite dose. On the basis of these results, a kinetic model was formulated allowing the estimation of the daily precursor dose quantity, ([amine][nitrite]2), required to give 50% tumour incidence in rats after two years of feeding. The potential application of this model, for the estimation of carcinogenic risk from endogenously formed N-nitrosamines in humans, is discussed. Our results demonstrate unequivocally the endogenous formation of N-nitroso compounds in the human body, the significance of which in human carcinogenesis remains to be established.