title: S-adenosyl-L-methionine:thioether S-methyltransferase, a new enzyme in sulfur and selenium metabolism.
abstract: The final urinary excretion product of selenium detoxification is trimethylselenonium ion. An assay has been developed for the enzyme, S-adenosylmethionine:thioether S-methyltransferase, responsible for this final methylation reaction. This assay employed high pressure liquid chromatography separation and quantitation of the trimethylselenonium ion produced by thioether methyltransferase acting on S-adenosylmethionine and dimethyl selenide. The enzyme was shown to reside primarily in the cytosol of mouse lung (30 pmol/mg protein/min) and liver (7 pmol/mg protein/min). Purification from mouse lung to a preparation that exhibited a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was achieved by DEAE, gel filtration, and chromatofocusing chromatographies. Thioether methyltransferase is monomeric with a molecular weight of 28,000 and has a pI of 5.3. The pH optimum was 6.3, and Km values for dimethyl selenide and S-adenosylmethionine were 0.4 and 1.0 microM, respectively. The enzyme was inhibited 50% by 25 microM sinefungin, an analog of S-adenosylmethionine, or 40 microM S-adenosylhomocysteine, the reaction product. Pure thioether methyltransferase methylated selenium in dimethyl selenide, tellurium in dimethyl telluride, and S in dimethyl sulfide and many other thioethers. These data suggest a general role for this novel enzyme in the synthesis of onium compounds with increased aqueous solubility helpful in their excretion.
authors.affiliation: Department of Biochemistry, University of Louisville, Kentucky 40292.
authors.forename: N M
authors.initials: NM
authors.lastname: Mozier
authors.forename: K P
authors.initials: KP
authors.lastname: McConnell
authors.forename: J L
authors.initials: JL
authors.lastname: Hoffman
mesh_terms: D000818:Animals; D005260:Female; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008780:Methyltransferases / Q000302:isolation & purification / Q000378:metabolism*; D051379:Mice; D009928:Organ Specificity; D012643:Selenium / Q000378:metabolism*; D013347:Subcellular Fractions / Q000201:enzymology; D013379:Substrate Specificity; D013455:Sulfur / Q000378:metabolism*
chemical_list: D013455:Sulfur; D008780:Methyltransferases; C055340:S-adenosyl methionine thioether S-methyltransferase; D012643:Selenium