Glutathione-dependent biotransformation of methyl parathion by mouse liver in vitro

The present study was undertaken in an attempt to reconcile the seemingly conflicting observations that glutathione-dependent biotransformation of methyl parathion (O,O-dimethyl-O-(4-nitrophenyl)phosphorothionate) by hepatic supernatant or partially purified glutathione S-transferases occurs in vitro, but not to any significant degree in vivo in the mouse. While incubation of 20 microM methyl parathion with glutathione-fortified 100,000 x g hepatic supernatant resulted in biotransformation of this insecticide, addition of the carbon monoxide exposed microsomal fraction (without NADPH) to the supernatant abolished this metabolism. HPLC analyses of the distribution of methyl parathion between 100,000 x g supernatant and carbon monoxide-exposed microsomes revealed that little methyl parathion could be recovered in the 100,000 x g supernatant, and that the bulk of this insecticide was associated with the microsomal fraction. Increasing the concentration of methyl parathion to 1 mM resulted in a greater fraction of methyl parathion found in the supernatant compared to that with 20 microM, although the bulk of methyl parathion remained associated with the microsomal fraction. While this increase in the fraction of substrate located within the supernatant led to limited glutathione-dependent metabolism of methyl parathion, it must be emphasized that a liver concentration of 1 mM methyl parathion is far greater than that which could be achieved in vivo. In conclusion, the results of the present study support the hypothesis that in the mouse, glutathione-dependent metabolism of methyl parathion does not occur to a significant degree in vivo because of its limited access to the soluble glutathione S-transferases.