The role of hepatic biotransformation in mediating the acute toxicity of the phosphorothionate insecticide chlorpyrifos.

The apparent (app) Km and app Vmax for mouse hepatic microsomal oxidative detoxification of chlorpyrifos to 3,5,6-tricholoro-2-pyridinol were 16.10 +/- 6.8 microM and 263.2 +/- 22.5 nmol/liver/min, respectively. The app Km and app Vmax for the oxidative activation of chlorpyrifos to chlorpyrifos oxon were 20.0 +/- 6.5 microM and 126.1 +/- 14.6 nmol/liver/min, respectively, whereas the app Km and app Vmax for hepatic microsomal hydrolysis of chlorpyrifos oxon to 3,5,6-trichloro-2-pyridinol were 1.87 +/- 0.36 mM and 89,450.7 +/- 12,087.3 nmol/liver/min, respectively. Under first-order conditions the capacity of mouse hepatic microsomes to detoxify chlorpyrifos oxon exceeded their capacity to generate this potent cholinesterase inhibitor from chlorpyrifos by a factor of 7.6. Pretreatment of mice with phenobarbital (70 mg/kg daily for 4 days) resulted in a 2.5-fold increase in the app Vmax's for oxidative activation and detoxification of chlorpyrifos, and a 1.6-fold increase in the app Vmax for hydrolysis of chlorpyrifos oxon. The app Km's were not altered by phenobarbital pretreatment. Administration of beta-naphthoflavone (80 mg/kg/daily for 2 days) to mice resulted in a slight decrease in the app Vmax's for oxidative activation and detoxification of chlorpyrifos, without altering the app Km's of the same reactions, or the hydrolysis of chlorpyrifos oxon. Phenobarbital and beta-naphthoflavone increased and decreased, respectively, the predicted hepatic clearance of chlorpyrifos. The acute toxicity of chlorpyrifos was slightly antagonized by phenobarbital pretreatment, but was potentiated by beta-naphthoflavone administration. These pretreatments did not affect cholinesterase, nonspecific esterase, or plasma A-esterase activities. Collectively, these results suggest that chlorpyrifos oxon formed within the liver does not escape hydrolysis by the liver, and that extrahepatic sites of activation are important in directly mediating the acute toxicity of chlorpyrifos.