Mutagenicity testing with Salmonella typhimurium strains. II. The effect of unusual phenotypes on the mutagenic response

Metabolic activation of the mutagen azide in biological systems

Inorganic azide (N3-) mutagenicity is mediated through a metabolically synthesized organic azide, L-azidoalanine (N3-CH2-CH(-NH2)-COOH). L-Azidoalanine appears to be formed by the action of O-acetylserine (thiol)-Lyase (EC 4.2.99.8) using O-acetylserine and azide as substrates. In both plants and bacteria tested, azide substitutes for the natural substrate sulfide (S2-) in this reaction. Azide (L-azidoalanine) mutagenesis is highly attenuated by a deficiency in the excision of UV-like DNA damage (uvr-). Thus a premutation lesion recognizable by the bacterial excision-repair enzymes must be formed. Mutagenesis appears to proceed from this by 'direct mispairing' pathway. Azide (L-azidoalanine) mutagenicity is highly specific and involves a stereoselective process, but the molecular nature of the specificity has not been determined.

Measurement of drug-metabolizing systems in Salmonella typhimurium strains G46, TA15135, TA100, TA1538 and TA98

Salmonella typhimurium strains which are commonly used in the Ames test for screening potential carcinogens were examined for a number of drug-metabolizing systems. Neither cytochrome P-450 itself nor two activities catalyzed by the cytochrome P-450 system in mammalian cells, i.e., benzpyrene monooxygenase and ethoxycoumarin O-deethylation, could be detected. Nor do these bacterial strains demonstrate any ability to detoxify epoxides by hydrating them or to conjugate p-nitrophenol with glucuronic acid. On the other hand, S. tryphimurium strains G46, TA1535, TA100, TA1538 and TA98 contain considerable amounts of acid-soluble thiols, approx. 5--10% of which is glutathione. These bacteria can also enzymatically conjugate glutathione with 1-chloro-2,4-dinitrobenzene (CDNB) and can reduce oxidized glutathione using NADPH as cofactor. Thus, enzymatic and non-enzymatic reaction of immediate carcinogens with thiol groups in s. typhimurium may have a significant effect on the outcome of the Ames test in certain cases.

Substrate-specificity of uvr excision repair

The substrate specificity of the uvr endonuclease, the product of the uvrA, uvrB, and uvrC genes is reviewed. It is suggested that the relatively well-defined substrate specificity of this repair enzyme is useful as a guide in determining the nature of the DNA-lesion caused by a given mutagen.

Mutagenicity testing with Salmonella typhimurium strains. I. Unusual phenotypes of the tester strains

Some of the Salmonella typhimurium strains in mutagenesis testing are atypical. Unlike the wild S. typhimurium these do not produce H-2-S, are not agglutinated by S. typhimurium typing sera and on fermentation of carbohydrates they do not produce gas. Thus these tester strains cannot readily be identified as S. typhimurium by standard laboratory procedures. This may cause a problem in the differentiation of tester strains from bacterial contaminants. The possibility exits that these phenotypic traits may affect the response of the strains to mutagens.