The ip LD50s of the insecticides chlorpyrifos [O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphorothioate] and methyl chlorpyrifos [O,O-dimethyl O-(3,5,6-trichloro-2-pyridyl) phosphorothioate] were determined to be 192 and 2325 mg/kg, respectively, in male mice. In an attempt to explain this 12-fold difference in toxicity, the extent of glutathione (GSH)-dependent detoxification of chlorpyrifos, methyl chlorpyrifos, and their oxygen analogs was examined. Incubation of 1000 nmol of insecticides with GSH-fortified mouse liver cytosol resulted in the disappearance of 458 and 819 nmol of chlorpyrifos oxon and methyl chlorpyrifos, respectively. However, chlorpyrifos and methyl chlorpyrifos oxon were not substrates for GSH-dependent biotransformation in vitro. Pretreatment of mice with diethyl maleate resulted in a 2.0- and 8.5-fold increase in the acute toxicities of chlorpyrifos and methyl chlorpyrifos, respectively. Administration of methyl chlorpyrifos (1000 mg/kg) to mice produced a marked, prolonged depletion of hepatic GSH, while administration of chlorpyrifos (70 mg/kg) resulted in a moderate, transient decrease in hepatic GSH content. Both doses inhibited brain and plasma cholinesterase, and brain and liver nonspecific esterase activities to a similar degree. HPLC analyses of brain concentrations of methyl chlorpyrifos and chlorpyrifos revealed that brain levels of methyl chlorpyrifos 46 times greater than those of chlorpyrifos were required to achieve the same degree of brain cholinesterase inhibition. Furthermore, the concentration of methyl chlorpyrifos oxon needed to produce 50% inhibition of bovine red blood cell or mouse brain cholinesterase was 480 times greater than that required for chlorpyrifos oxon. These data suggest that differences in the extent of GSH-mediated detoxification can account for only a portion of the observed differences in acute toxicity between chlorpyrifos and methyl chlorpyrifos. © 1982.
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