Abstract
To quantitate endogenous nitrosation reactions in man, the quantity of N-nitrosoproline (NPRO) excreted in the urine after ingestion of proline and/or nitrate was estimated. When this monitoring method (NPRO test) was applied in clinical and field studies, several hitherto unidentified N-nitroso compounds were frequently detected. These were recently identified as sulphur-containing N-nitrosamino acids, N-nitrosothiazolidine 4-carboxylic acid (NTCA), and trans- and cis-isomers of N-nitroso-2-methylthiazolidine 4-carboxylic acid (NMTCA). NTCA and NMTCA were readily formed in vitro following nitrosation at acidic pH of the respective precursor, thiazolidine 4-carboxylic acid (TCA) or of 2-methylthiazolidine 4-carboxylic acid (MTCA). As the latter compounds can be formed by reaction of L-cysteine with formaldehyde or acetaldehyde, respectively, NTCA and NMTCA were also formed by reacting L-cysteine with the respective aldehyde and with nitrite at optimal pH (2.5 for NTCA and 4.5 for NMTCA). Up to 95% of NTCA and NMTCA given orally to fasted rats was recovered as such in urine and faeces within 2 days. Administration of TCA or MTCA, together with nitrite increased the urinary excretion of NTCA and NMTCA, as did co-administration of L-cysteine, nitrite, and the respective aldehyde. NTCA and NMTCA were also detected in the 24-h urine of human volunteers, and smokers tended to excrete higher levels than nonsmokers. Daily excretion levels varied, however, and a diet supplemented with ascorbic acid significantly decreased the total amount of nitrosamino acids. NTCA and NMTCA may occur in human urine as a result of (i) intake of preformed N-nitroso compounds; (ii) intake of thiazolidine 4-carboxylic acid or its 2-methyl derivative and subsequent nitrosation in vivo; (iii) endogenous two-step synthesis by the reaction of L-cysteine with the respective aldehyde and a nitrosating agent. Thus, measurement of NTCA and NMTCA together with NPRO in urine may provide an index for the exposure of human subjects to nitrosamines or their precursors, i.e., nitrosating agents, certain aldehydes, or aldehyde-generating compounds. Our data demonstrate unequivocally that N-nitroso compounds are formed in the human body, as suggested previously by Druckrey. Their relevance to human cancer at specific sites should now be investigated.