N-nitrosothiazolidine-4-carboxylic Acid Research Articles

Occurrence in human urine of new sulphur-containing N-nitrosamino acids N-nitrosothiazolidine 4-carboxylic acid and its 2-methyl derivative, and their formation.

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.

Mutagenicity of 2-hydroxyalkyl- N-nitrosothiazolidines

The mutagenicity of 2-hydroxyalkyl- N-nitrosothiazolidines was tested using Salmonella typhimurium strains TA98 and TA100. The N-nitrosothiazolidines tested were unsubstituted N-nitrosothiazolidine (NT), N-nitrosothiazolidine-4-carboxylic acid (NTC), 2-hydroxymethyl- N-nitrosothiazolidine (HMNT), 2-(1,2,3,4-tetrahydroxybutyl)- N-nitrosothiazolidine, 2-(1,2,3,4-tetrahydroxypentyl)- N-nitrosothiazolidine, 2-(1,2,3,4,5-pentahydroxypentyl)- N-nitrosothiazolidine (PHPNT) and 2-(1,2,3,4,5-pentahydroxypentyl)- N-nitrosothiazolidine-4-carboxylic acid. Among the N-nitrosothiazolidines tested, only HMNT and PHPNT exhibited clear dose-response mutagenicity toward strain TA100 with or without metabolic activation. None of the 2-hydroxyalkyl- N-nitrosothiazolidines were mutagenic to strain TA98. NT exhibited much stronger mutagenicity than either HMNT or PHPNT. Mutagenic activities of NT and PHPNT were eliminated by carboxyl substitution in the position α to the N-nitroso group.

Presence in human urine of a new N-nitroso compound, N-nitrosothiazolidine 4-carboxylic acid

Urine samples collected in several countries from human subjects showed the presence of a number of N-nitroso compounds not previously identified. By several separative procedures and by comparison with authentic material, the major unknown N-nitroso compound was shown to be N-nitrosothiazolidine 4-carboxylic acid (NTCA). Although its origin in human urine is unknown, thiazolidine 4-carboxylic acid, the easily nitrosatable amine precursor, can be formed by reaction of formaldehyde with cysteine in vivo and in vitro. Thus measuring NTCA excreted in the urine may allow monitoring exposure of human subjects to precursors like formaldehyde and NO 3 − NO 2 − .