Mutagenicity of cytochrome P450 2E1 substrates in the Ames test with the metabolic competent S. typhimurium strain YG7108pin3ERb 5

Abstract

Poor metabolic competence of in vitro systems was proposed to be one of their major shortcomings accounting for false negative results in genotoxicity testing. For several “low molecular weight cancer suspects” this was specifically attributed to the lack of cytochrome P450 2E1 (CYP2E1) in conventional in vitro metabolising systems. One promising attempt to overcome this problem is the transfection of “methyltransferase-deficient” S. typhimurium strains with the plasmid pin3ERb 5. This plasmid contains DNA encoding for a complete electron transport chain, comprising P450 reductase, cytochrome b 5 and cytochrome P450 2E1. In order to answer the question if CYP2E1 substrates that yield negative or inconclusive results in the Ames test can be activated by metabolic competent bacterial strains, we used YG7108pin3ERb 5 to investigate the following compounds: acetamide, acrylamide, acrylonitrile, allyl chloride, ethyl acrylate, ethyl carbamate, methyl-methacrylate, vinyl acetate, N-nitrosopyrrolidine, trichloroethylene and tetrachloroethylene. N-Nitrosodiethylamine served as a positive control. In addition to these known or proposed CYP2E1 substrates, we investigated the polycyclic aromatic hydrocarbon benzo[α]pyrene and the heterocyclic aromatic amines 2-aminofluorene and 2-aminoanthracene. Results The extensive metabolic competence of the transformed strain is underlined by results showing strong mutagenicity between 10 and 500 μg N-nitrosopyrrolidine per plate. Unexpectedly, 2-aminoanthracene was mutagenic at a concentration range between 25 and 250 μg per plate using YG7108pin3ERb 5. Moreover, we demonstrate for the first time a clear response of sufficiently characterised allyl chloride in the Ames test at a reasonably low concentration range between 300 and 1500 μg per plate. We achieved similar results in the parent strain YG7108 with conventional metabolic activation. Without metabolic activation less pronounced mutagenicity occurred, suggesting a contribution of a direct alkylating effect. Propylene oxide is usually contained in allyl chloride as stabilizer at amounts up to 0.09%. Though YG7108 revealed to be very sensitive towards propylene oxide, allyl chloride dissolved in water was not mutagenic, showing that no water soluble compounds contribute to its mutagenicity. None of the remaining compounds showed mutagenic effects using YG7108pin3ERb 5. Conclusion YG7108pin3ERb 5 and its parent strain YG7108 are sensitive for compounds which are negative in conventional tester strains including N-nitrosodiethylamine, N-nitrosopyrrolidine, propylene oxide and allyl chloride.