In vitro steady-state levels of hydrogen peroxide after exposure of male F344 rats and female B6C3F1 mice to hepatic peroxisome proliferators

Abstract

The hypothesis that hepatocarcinogenesis resulting from treatment of rats and mice with peroxisome proliferators is linked to increased cellular levels of hydrogen peroxide from peroxisomal β-oxidation was investigated. Male F344 rats and female B6C3F2 ₁ mice were treated for 14 days with di(2-ethylhexyl)phthalate (DEHP) or di(2-ethylhexyl)adipate (DEHA), industrial plasticizers, or nafenopin, a hypolipi-demic drug. Activities of enzymes responsible for the production [peroxisomal palmitoyl CoA oxidase (PCO)] and degradation [catalase (Cat) and glutathione peroxidase (GSHPx)] of H ₂ O ₂ were assayed in liver homogenates prepared from treated animals. The activities of the peroxisomal enzymes PCO and Cat were enhanced 5- to 25-fold and 1.5-to 3-fold respectively by treatment with the peroxisome proliferators. The activity of GSHP _x , a cytoplasmk enzyme, was decreased 40–60% in liver homogenates prepared from treated pnimalx compared to control animals. A kinetic treatment of the rates of formation of hydrogen peroxide by PCO, and of degradation of hydrogen peroxide by catalase was used to estimate steady-state hydrogen peroxide concentrations ([H ₂ O ₂ ]) during peroxisomal oxidation of palmitoyl CoA. Increases in peroxisomal steady-state [H ₂ O ₂ ] for the F344 rat liver homogenates correlated well with the carcinogenic potential of these chemicals, determined in previous carcinogenicity studies. Increases in the steady-state [H ₂ O2] were also calculated for liver homogenates prepared from mice treated with these compounds. Decreases in liver lipid peroxidation were observed after treatment with each chemical in both species. The results of these studies are consistent with an involvement of increased peroxisomal hydrogen peroxide in the hepatocarcinogenesis of these compounds.