Homocysteine-Dependent Demethylation of Trimethylselenonium Ion and Selenobetaine with Methionine Formation

In the presence of rat liver cytosol and homocysteine, trimethylselenonium ion (TMSe+) underwent time-dependent demethylation to dimethylselenide with the concurrent formation of methionine. Convenient methods were developed for assay of this activity using either radioactive methods based on the gamma emitting isotope 75Se or nonradioactive HPLC assay of methionine. The rate of demethylation was linear with protein concentration and dependent on homocysteine, which could not be replaced by cysteine, glutathione, or dithiothreitol. The TMSe+ demethylation rate was inhibited by the addition of betaine, sulfobetaine (dimethylthetin), or dimethylglycine. The Km for TMSe+ was 8 mM compared to 0.04 mM for betaine, but the rate of TMSe+ demethylation was approximately 50-fold that of betaine when both were assayed at 25 mM. Methionine was also produced from selenobetaine, selenobetaine methylester, and sulfobetaine. The selenium analogues of betaine inhibited the demethylation of TMSe+ with only minor decreases in methionine production, indicating substrate competition. In preliminary studies aimed at the partial purification of the TMSe+:homocysteine methyltransferase activity, the enzyme was found to have chromatographic and heat stability characteristics similar to betaine:homocysteine methyltransferase. The data indicate that betaine:homocysteine methyltransferase, or a very similar enzyme, is involved in the demethylation of TMSe+ and show that TMSe, an in vivo urinary selenium metabolite of many selenium compounds, is not biologically inert.