Bacterial photomutagenicity testing: distinction between direct, enzyme-mediated and light-induced events

A bacterial photomutagenicity test system has been established according to a predetermined protocol using a light source emitting multiple wave lengths, including UV-A and UV-B, and the tester strains used for standard bacterial mutagenicity testing. 8-Methoxypsoralen (8-MOP) was used as a photomutagenic positive control showing no mutagenicity in the absence of light and borderline (Salmonella typhimurium TA1537) or clear (Salmonella typhimurium TA102 and Escherichia coli WP2) mutagenic effects in the presence of light. Using the same experimental conditions, the UV filters p-aminobenzoic acid (PABA), octyl dimethyl PABA (Eusolex 6007), and 4-methylbenzylidene camphor (Eusolex 6300) were non-mutagenic (in the absence of light) and non-photomutagenic (in the presence of light). Dihydroxyacetone (DHA), used as an active ingredient in self-tanning lotions, slightly increased the number of revertants in Salmonella typhimurium TA100 and TA102 in the absence of light. However, the relevance of these effects is equivocal, since they occurred at very high, cytotoxic concentrations (5000 micrograms/plate). Furthermore, these increases were not potentiated by light, thus demonstrating the non-photomutagenicity of DHA. In contrast, 7,12-dimethylbenz[a]anthracene (DMBA), which is non-mutagenic per se and is activated by external metabolizing systems (e.g., S9-mix) in the absence of light, was clearly mutagenic in Salmonella typhimurium TA98 and TA1537 in the presence of light (and the absence of S9-mix). Although the photomutagenic potency of DMBA, on a molar basis, was certainly lower than that of 8-MOP, the absolute mutagenic effects of DMBA in the respective bacterial strains were in a similar range under either S9-mix or photoactivation conditions. The strain specificity of the mutagenic effects were, however, different for either enzyme- or light-mediated mutagenicity. This indicates that different reactive intermediates are responsible for the mutagenicity in the tests using the two different activation systems. These results further suggest to use DMBA as a positive photomutagenic control compound alternatively to 8-MOP, since the latter is non- or only weakly photomutagenic in Salmonella typhimurium TA98 and TA1537. Furthermore, the usefulness and application of this photomutagenicity test system could be demonstrated for the testing of different photoabsorbing chemicals and cosmetic ingredients.