Presence of 1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid, a precursor of a mutagenic nitroso compound, in soy sauce.

After treatment with nitrite, Japanese soy sauce was strongly mutagenic to Salmonella typhimurium TA100 without S9 mixture. Two precursors of the mutagen were isolated from Japanese soy sauce, and these were identified as (-)-(1S,3S)-1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid [(-)-(1S,3S)-MTCA] and its stereoisomer (-)-(1R,3S)-MTCA. After treatment with nitrite, 1-mg samples of these compounds induced 17,400 and 13,000 revertants of TA100, respectively, without S9 mixture. Quantitative analysis of various kinds of soy sauces produced in Japan showed the presence of 82-678 micrograms of MTCA per ml. The mutagenicities of these compounds with nitrite accounted for 16-61% of the total mutagenicity of soy sauce with nitrite. Most soy sauces produced in the United States were less mutagenic than those produced in Japan and little, if any, of these two precursors of the mutagen was found in them. A major reaction product of (-)-(1S,3S)-MTCA and nitrite was a compound having a nitroso substitution at position N-2, but this compound was not mutagenic. Thus, the mutagen(s) formed from (-)-(1S,3S)-MTCA and nitrite was a minor product(s), and its specific mutagenic activity must be very high.