Inhibitory effects of NADH/NADPH in S9 mix on photo-mutagenicity of thiabendazole following UVA-irradiation in E.coli

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Thiabendazole (TBZ), a post-harvest fungicide commonly used on imported citrus fruits, exhibited photo-mutagenicity following UVA-irradiation (320-400nm) in Trp^+ reverse mutation assay using Escherichia coli WP2uvrA/pKM101 strain. The photo-mutagenicity was not observed in the presence of S9 mix, a rat liver homogenate microsome fraction with co-factors for metabolic activation. We found that NADH and NADPH used as co-factor in the S9 mix efficiently suppressed the photo-mutagenicity of TBZ. This evidence strongly suggested that non-mutagenicity in the presence of S9 mix was not due to the metabolic detoxification of TBZ or the scavenging of UVA-activated TBZ by macromolecules in the S9 mix. Rather quenching effect of NADH and NADPH (λmax=338nm) may be more responsible for suppression of UVA-activation of TBZ, because oxidized forms of NAD^+ and NADP^+ did not show inhibitory effects. Mutagenicity of the UVA-irradiated photo-mutagens such as angelicin and chlorpromazine was also suppressed by the addition of NADH or NADPH. Our present results suggest the possible underestimation in risk evaluation for photo-mutagenic compounds when they are assayed in the presence of S9 mix.
日本環境変異原学会の論文
2005-04-15