Effect of length of exposure to malathion on xenobiotic biotransformation in male rat liver.

The effect of exposure to malathion on several parameters of hepatic xenobiotic biotransformation was studied in male Sprague-Dawley rats. Groups of rats dosed i.p. daily for 1 or 2 weeks with 40 or 200 mg/kg malathion showed an increase in epoxide hydrolase activity (1 week, 200 mg/kg) and glutathione S-transferase activity (1 week, 200 mg/kg; 2 weeks 40 and 200 mg/kg). Aldrin epoxidation was decreased after 1 week of exposure to 200 mg/kg and by both dosage regimens after 2 weeks. After 9 weeks exposure to 40 mg/kg malathion administered i.p. 3 times per week, however, no changes in hepatic xenobiotic biotransformation were noted. The results demonstrate that only continuous exposure to high doses of malathion results in an induction of epoxide hydrolase and glutathione S-transferase activities. Inductive effects on hepatic cytochrome P-450 monooxygenase activity were not observed irrespective of whether exposure was short- or medium-term.