Relation of nitrite concentration to mutagen formation in soy sauce

When soy sauce was mixed with nitrite solutions at pH 1.0 and 3.0, only mixtures containing nitrite concentrations above 250 ppm were mutagenic in the Salmonella/mammalian microsome test. In buffered aqueous solution (pH 3) the mutagen precursor, (-)-(1S,3S)-1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid [(1S,3S)-MTCA] and its stereoisomer ( - )-(1R,3S)-MTCA reacted with 10 ppm or more of sodium nitrite; but at least 250 ppm nitrite was required for them to react in soy sauce (pH 3). Similarly, another mutagen precursor, tyramine, did not react even with 2300 ppm nitrite in soy sauce (pH 1), although pure tyramine in aqueous solution (pH 1) reacted with as little as 50 ppm nitrite. Nitrite concentration in human saliva and gastric juice does not generally exceed 50 ppm. Therefore, the most probable source of mutagens--nitrosation of MTCAs and tyramine--is likely to be very restricted in vivo and soy sauce is unlikely to be significantly mutagenic.