Biosynthesis of dimethyl selenide from sodium selenite in rat liver and kidney cell-free systems.

A pathway for the synthesis of dimethyl selenide from sodium selenite was studied in rat liver and kidney fractions under anaerobic conditions in the presence of GSH, a NADPH-generating system, and S-adenosylmethionine. Chromatography of liver or kidney soluble fraction on Sephadex G-75 yielded a Fraction C (30,000 molecular weight) which synthesized dimethyl selenide, but at a low rate. Addition of proteins eluting at the void volume (Fraction A) to Fraction C restored full activity. Fractionation of Fraction A on DEAE-cellulose revealed that its ability to stimulate Fraction C was associated with two fractions, one containing glutathione reductase and the other a NADPH-dependent disulfide reductase. It was concluded that Fraction C contains a methyltransferase acting on small amounts of hydrogen selenide produced non-enzymically by the reaction of selenite with GSH, and that stimulation by Fraction A results partly from the NADPH-linked formation of hydrogen selenide catalyzed by glutathione reductase present in Fraction A. Washed liver microsomal fraction incubated with selenite plus 20 mM GSH also synthesized dimethyl selenide, but addition of soluble fraction stimulated activity. A synergistic effect was obtained when liver soluble fraction was added to microsomal fraction in the presence of a physiological level of GSH (2 mM), whereas at 20 mM GSH the effect was merely additive. The microsomal component of the liver system was labile, had maximal activity around pH 7.5, and was exceedingly sensitive to NaAsO2 (93% inhibition by 10(-6) M arsenite in the presence of a 20,000-fold excess of GSH). The microsomal activity apparently results from a Se-methyltransferase, possibly a dithiol protein, that methylates hydrogen selenide produced enzymically by the soluble fraction or non-enzymically when a sufficiently high concentration of GSH is used.