Abstract Introduction: The toxic effects of some organophosphate pesticides (OPs) which are capable to produce free radicals and induce disturbance in body antioxidant systems. Paraoxon is an OP that is the active form parathion. Objective: To evaluate the effect of paraoxon on liver antioxidant system of rat. Materials and Methods: Wister rats were randomly divided into four groups including: control (paraoxon solvent), and three groups receiving different doses of paraoxon (0.3, 0.7 and 1 mg/kg) by intraperitoneal injection. Animal were anesthetized and liver tissue removed 24 hours after injection. After hemogenation of liver tissue Superoxide Dismutase (SOD) and Catalase (CAT), Lactate Dehydrogenese (LDH) and Glutathione S- Transferase (GST) activities, Glutathione (GSH) and malondialdehyde (MDA) levels were determined by biochemical methods. Results: Paraoxon increased CAT, LDH and SOD activities and MDA level at doses higher than 0.3 mg/kg, while GSH level was decreased significantly, as compare with the control group. GST activity was increased significantly at 0.3 and 0.7 mg/kg concentration (P<0.05), but at 1 mg/kg concentration was decreased as compare with the control group. Conclusion: Probably Paraoxon induced the production of free radicals and oxidative stress in a dose- dependent manner. The enhanced activity of antioxidant enzymes of liver is due to increase the detoxification capacity. Decrease of tissue GSH content indicatives of oxidative tissue injury and the increase of MDA level indicatives per oxidation that occurs in membranes of liver.

Keywords: Antioxidants, Lipid per oxidation, Liver, Paraoxon, Rats

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