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Mutagenic Synergy of Organophosphorus Ester Insecticides and Dietary Mutagenss

Previously, our laboratory reported that ethyl paraoxon, a non-mutagenic active metabolite of the organophos-phorus ester (OP) insecticide ethyl parathion, interacted synergistically with several aromatic amines to generate enhanced mutagenic responses in Salmonella typhimurium. Though genotoxic synergistic responses are rarely incorporated in risk-assessment models, they are important in establishing the toxicological characteristics of agents. In this work, we investigated the mutagenic synergy between methyl parathion, another OP insecticide, and the dietary promutagens 2-amino-3-methylimidazo-[4,5-f]quinoline (IQ) and 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP). A microplate assay was used to determine the cytotoxicity to S. typhimurium YG1024 of methyl parathion and its main metabolite, methyl paraoxon with the dietary promutagens with and without a microsomal metabolizing mixture, S9. The microplate assay measured optical density to determine a range of concentrations for each compound that would not yield acute cytotoxicity. Then, the mutagenic properties of the compounds were investigated through the use of a S. typhimurium reversion assay. Strain YG1024 of S. typhimurium is unable to produce the essential amino acid histidine unless a specific reverse mutation occurs at the hisD3052 allele. YG1024 cells were treated with IQ or PhIP and varying concentrations of methyl parathion or methyl paraoxon. The bacteria were then plated on selective medium upon which only histidine revertant bacteria could grow into visible colonies. Methyl parathion after S9 metabolism induced a threefold increase in the mutagenicity of IQ, and a twofold increase in the mutagenicity of PhIP. Methyl paraoxon induced a twofold increase in the mutagenicity of IQ and a sixfold increase in the mutagenicity of PhIP. These data may have a profound impact on the risk assessment of OP insecticides.

This research was funded by C-FAR grant No. 00I-017-5 and an Environmental Council SURE grant.

Elizabeth Wagner, Principal Research Specialist
Department of Crop Sciences
(217) 244-9869; edwagner@illinois.edu

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