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Methyl-Donor Deficiency Due to Chemically Induced Glutathione Depletion¹

Departments of Pharmacology and Toxicology [K. L.] and Pathology [J. M. S.], The University of Texas Medical Branch, Galveston, Texas 77555-1031, and Center for Experimental Therapeutics, Baylor College of Medicine, Houston, Texas 77030-3498 [R. S. O.]

Dietary deficiency of methionine (Met) is known to deplete cellular Met and cause DNA hypomethylation, but depletion of Met and impairment in methylation due to chemically induced glutathione (GSH) depletion has escaped recognition. In this study, the effect of GSH depletion on the Met pool and methylation capability was examined after bromobenzene (BB), a model GSH-depleting hepatotoxin, was administered to the Syrian hamster. An i.p. dose of BB (800 mg/kg) caused a rapid and extensive depletion of liver GSH; approximately 68% of the initial concentration was depleted during the first hour. The lowest level of GSH, only 4% of the control, was detected at 5 h. GSH depletion was accompanied by a prompt increase in liver Met during the first hour. This initial increase was followed by an extensive depletion during the next 4 h. At 5 h after BB, liver Met was 12% below the control value, and it remained around this concentration throughout the 24-h experiment. To further confirm these results, the endogenous Met pool was labeled with deuterated Met. The administration of L-Met-methyl-d₃ to the Syrian hamster after GSH had been depleted by BB resulted in a significant protection of the liver against necrosis. The protection was accompanied by a marked incorporation of deuterated Met into the liver Met pool. The incorporation, which was determined by gas chromatography-mass spectrometry, shows BB dose dependence. Approximately 53% of the liver Met was labeled when a toxic BB dose (800 mg/kg) was used, while only 25% incorporation was found for the nontoxic dose (100 mg/kg). These results were different from the controls, where only 15% incorporation was found. The differences in the incorporation indicate that there are differences in the degree of utilization and/or depletion of Met in these hamsters, and these differences apparently are dependent upon the degree of toxicity and GSH depletion. The marked incorporation of deuterated Met in the high-dose group was accompanied with a striking increase in the methylation capability. Urinary excretion of the O- and S-methylated 4- and 5-bromo-2-hydroxythiophenols and S-methylated 4- and 5-bromo-2-hydroxy-1,2-dihydrobenzenethiols was significantly increased when compared with the BB treated alone. Approximately 40–45% of the methyl groups in these methylated BB metabolites were methyl-d₃. These results provide direct evidence that depletion of GSH leads to Met depletion and also impairs the methylation processes.

¹ Supported in part by NIH Grant ES-04857 (to K. L.).

² To whom requests for reprints should be addressed, at Department of Pharmacology and Toxicology, The University of Texas Medical Branch, 301 University Boulevard, Galveston, TX 77555-1031.

Received 8/17/95. Accepted 1/ 3/96.

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