Discrepancy of Metabolite Genotoxicity Test Results by Rat Liver S9 Fractions Treated with Different Inducers

<p>In in vitro genotoxicity assays such as the Ames test and the micronucleus test, rat liver microsomes (S9 fractions) is commonly used to assess the genotoxic potential of metabolites. When obtaining S9 fraction, Aroclor 1254, a polychlorinated biphenyls (PCB) isoform, is administered to induce the expression of cytochrome P 450 in the US and Europe, but due to the restriction on the use of PCBs in Japan, Phenobarbital + Beta-Naphthoflavone (PB/BNF) was administered to induce the enzyme. We investigated whether different inducers lead to differences in genotoxicity test results possibly due to the formation of different metabolites in terms of both quality and/or quantity by S9 fractions. In this study, to determine the genotoxicity, micro Ames test was used with S9 induced by Aroclor 1254 or PB/BNF, respectively, and we observed an increase in the number of colonies when various compounds were examined. Criteria for positive results were a 2-fold increase in the number of colonies in strain TA 100 and a 3-fold increase in strain TA 98 compared to controls. In the group treated with compounds commonly used as positive controls in the Ames test such as 2-aminoanthracene, there was no difference in the negative/positive judgment depending on the type of inducer, but different results were observed for some nitrosamine compounds. The S9 fraction induced by Aroclor 1254 tended to show higher CYP activity than that induced by PB/BNF. These results suggest that different inducers may have formed qualitatively and quantitatively different metabolites probably through different CYP activity, which may have caused differences in the genotoxicity test results.</p>