Search

Compounds of various chemical classes were comparatively assayed in the Ames reversion test with his- S. typhimurium strains TA1535, TA157 , TA1538, TA98, TA100, and, in part, TA97 , and in a DNA-repair test with trp- E. coli strains WP2 (repair-proficient), WP67 (uvrA- polA-) and CM871 (uvrA- recA- lexA-). A liquid micromethod procedure for the assessment of the minimal inhibitory concentration (MIC) of test compounds, using the same reagents as the Ames test, was set up and calibrated in its technical details. Other techniques (spot test and treat-and-plate method) were applied to a number of compounds in order to obtain more complete information on their DNA-damaging activity in E. coli. From a qualitative standpoint, the results obtained in the reversion test and in the DNA-repair test (liquid micromethod) were overlapping for 96 (59 positive and 37 negative) out of 135 compounds (71.1%). There was disagreement for 39 compounds (28.9%), 9 of which were positive only in the reversion test (8 requiring metabolic activation and 5 genotoxic in the treat-and-plate method). 30 compounds were positive only in the lethality test, showing a direct DNA-damaging activity, which in half of the cases was completely eliminated by S9 mix. Although the experimental protocol intentionally included several compounds already reported as nonmutagenic carcinogens or as noncarcinogenic mutagens, the overall accuracy was 64.5% for the reversion test and 72.4% for the DNA-repair test, as evaluated for 75 compounds classified according to their carcinogenic activity. Quantitation of results was obtained in the Ames test by relating the net number of revertants to nmoles of compound and in the DNA-repair test by means of a formula relating the difference and ratio of MICs in repair-proficient and -deficient bacteria to nmoles of compound. Following these criteria, the genotoxic potency varied over a 4.5 X 10(7)-fold range among compounds positive in the reversion test and over a 6 X 10(9)-fold range among compounds damaging E. coli DNA. The genotoxic potencies in the two bacterial systems were correlated within the majority of the chemical classes under scrutiny.

Preparations of Chinese hamster ovary (CHO) cells decreased the genotoxicity of 3 ICR compounds (ICR 191, ICR 191-OH and ICR 170-OH), while they did not affect the genotoxicity of ICR 170 in the Salmonella reversion test nor in a DNA-repair test in E. coli. These data may contribute towards the explanation of the lack of activity of the two hydroxylated compounds in the CHO/HGPRT forward mutation system, as well as the different rank of mutagenicity of the two chloroethyl compounds in bacteria (ICR 191 greater than ICR 170), compared to cultured mammalian cells and in general to eukaryotic cells (ICR 170 greater than ICR 191).

Samples of gastric juice from variously treated subjects efficiently reduced hexavalent chromium and decreased its mutagenicity. Chromium reduction was due to thermostable components of gastric secretions and was favoured by the acidity of the intragastric environment. The circadian monitoring of pH and of chromium reduction, as assessed by colorimetric analysis at hourly intervals, showed a basal activity (less than 10 micrograms/ml gastric juice) during the night and interdigestive periods, and peaks (tens of micrograms/ml) during the 3-4-h periods after each meal. Assays in the Ames reversion test confirmed that the decrease in mutagenicity of sodium dichromate produced by gastric juice was significantly enhanced after meals. This physiological mechanism is expected to provide an important protective barrier against the oral toxicity of this metal, and may explain its lack of oral carcinogenicity.