Thiol S-methyltransferase and thioether S-methyltransferase in sulfur and selenium metabolism

Intermediary metabolism of selenium in animals characteristically involves reduction by glutathione reductase (GR) and methylation by a series of enzymes resulting in the excretion of urinary and respiratory metabolites that include dimethyl selenide (DMSe) and trimethyl selenonium ion (TMSe). The present work defines the enzymology and regulation of selenium methylation, detoxification, and demethylation by testing the hypothesis that GR, thiol S-methyltransferase (TLMTase), and thioether methyltransferase (TEMTase) are necessary and sufficient enzymes for the biosynthesis of DMSe and TMSe starting from selenite. TLMTase, a microsomal enzyme responsible for the S-adenosylmethionine-dependent methylation of various thiols, was purified from mouse liver. Enzyme purification was made possible by the initial solubilization of NaCl-washed microsomal membranes by CHAPS followed by a series of chromatographic steps utilizing gel filtration, ion exchange, and hydroxylapatite supports. Pure TLMTase methylated various sulfhydryl compounds such as 2-mercaptoethanol (2-ME), dithiotheitol, and mercaptoethylamine. The naturally occurring hydrophilic thiols, glutathione and cysteine, acted neither as substrates nor as inhibitors. The biochemical properties of TLMTase suggest a physiological role in sulfur-xenobiotic detoxification by synthesizing products suitable for excretion or for further methylation by TEMTase, an enzyme responsible for the final methylation of thioethers and selenoethers.; TLMTase and TEMTase were found to act sequentially to convert 2-ME to its respective dimethyl sulfonium ion (DMTE) in vivo and in vitro. They also were demonstrated to be common to both the sulfur and selenium pathways in vitro. Arsenite (1-5 {dollar}mu{dollar}M) inhibited DMSe synthesis which secondarily lowered TMSe synthesis (TEMTase is insensitive to arsenite). Arsenite injected in mice at various concentrations inhibited in parallel the biosynthesis of {dollar}rmlbracksp{lcub}75{rcub}Serbrack DMSe{dollar} from {dollar}rmsp{lcub}75{rcub}SeOsb3sp{lcub}2-{rcub}{dollar} and of (1,2-{dollar}sp{lcub}14{rcub}{dollar}C) DMTE from (1,2-{dollar}sp{lcub}14{rcub}{dollar}C) 2-ME. The synthesis of DMSe by pure TLMTase and the parallel inhibition by arsenite of thiol and Se methylation both in vivo and in vitro strongly suggest that TLMTase methylates HSe{dollar}sp-{dollar} to CH{dollar}sb3{dollar}SeH and then to DMSe which is known to be converted to TMSe by TEMTase. Lastly, it was demonstrated that a partially purified preparation of mouse liver cytosolic thetin:homocysteine methyltransferase (HcyMtase) catalyzes the demethylation of trimethyl sulfonium ion, DMTE, and TMSe. Therefore, a methylation/demethylation cycle may exist between TLMTase, TEMTase, and HcyMtase.