Methylation of Xenobiotic Thiols by Euglena gracilis: Characterization of a Cytoplasmic Thiol Methyltransferase

The green alga Euglena gracilis contains a thiol methyltransferasethat catalyzes the S-adenosylmethionine-dependent methylationof pentachlorobenzenethiol. The enzyme was localized in thecytoplasm and partially purified. The pH optimum for the enzymewas 6.5. The enzyme methylated a number of foreign thiols, butnot the cellular thiols, glutathione or cysteine. Phenols andanilines were not substrates. When pentachloro-benzenethiolwas the methyl acceptor the K_m was found to be 82 µM andthe corresponding K_m for S-adenosylmethionine was 140 µM.The molecular weight of the enzyme was 21,000, as determinedby gel filtration. A role for this enzyme in detoxifying xenobioticthiols is proposed. (Received September 28, 1984; Accepted April 25, 1985)