The role of glutathione and cytochrome P-450 in the metabolism of methyl chloride

Abstract

Rat liver microsomes metabolized methyl chloride to formaldehyde at a rate 15-fold less than the rate of benzamphetamine demethylation. The reaction rate was stimulated approximately 2-fold in microsomes from phenobarbital-pretreated rats and was inhibited by addition of SKF-525A, carbon monoxide, metyrapone, and hexobarbital to the microsomal suspension, indicating dependence on cytochrome P-450. The in vivo incorporation of 14CH 3Cl into liver macromolecules, previously shown to reflect metabolism to CH 3Cl to formate, was not significantly altered by SKF-525A, Aroclor 1254, or 3-methylcholanthrene pretreatment of rats, although pretreatment with phenobarbital produced a 35 and 28% increase in 14CH 3Cl uptake into liver lipid and acid-insoluble material, respectively. Pretreatment with phenobarbital increased the in vivo metabolism of 14CH 3Cl to 14CO 2 (also derived from a formate intermediate) by 19%, but had no effect on urinary metabolites derived from 14CH 3Cl. SKF-525A inhibited 14CO 2 production from 14CH 3Cl by 30% and also had no effect on urinary excretion of 14C. In contrast, pretreatment with diethylmaleate inhibited 14CH 3Cl incorporation into liver macromolecules by 70 to 85%, and lowered 14CO 2 expiration and urinary 14C excretion by 52 and 60%, respectively. S-Methylcysteine pretreatment produced a similar inhibition of 14CH 3Cl incorporation and metabolism to 14CO 2; urinary excretion of 14C, however, was approximately doubled. 14CH 3Cl uptake into liver was also stimulated by cysteine pretreatment. These results indicate a strong dependence of CH 3Cl metabolism on tissue nonprotein sulfhydryl content and suggest a possible role for cytochrome P-450 in the in vivo metabolism of CH 3Cl. A scheme for the metabolism of CH 3Cl is postulated which involves initial reaction with glutathione, and sequential metabolism of the conjugate to S-methylcysteine, methanethiol, and formaldehyde.