Metabolomics of selenium: Se metabolites based on speciation studies

Selenium is a trace element essential for the normal function of the body. This metalloid is quite unique in its metabolism compared with typical essential metals such as copper and zinc. In the present communication, the metabolism of selenium in the body was reviewed from the viewpoint of metabolomics based on speciation studies. Both inorganic and organic forms of slenium can be the nutritional source, and they are transformed to the common intermediate, selenide or its equivalent. Selenite and selenate are reduced simply to selenide for further utilization and/or excretion. On the other hand, organic selenocysteine is directly lysed to selenide, while selenomethionine is transformed to selenocysteine (trans- selenation pathway), similarly to the trans-sulfuration pathway for methionine to cysteine, and then lysed to selenide. Selenide is known to be transformed to selenocysteine on tRNA, and the selenocysteinyl residue is incorporated into selenoprotein sequences by the codon specific to selenocysteine, UGA. Diverse selenium chemicals in foods seem to be recognized as selenium species and transformed to selenide, and then utilized for the synthesis of selenoproteins. Surplus selenium is methylated stepwise to methylated selenium metabolites from the common intermediate selenide. The major urinary metabolite is 1 beta-methylseleno-N-acetyl-D-glactosamine (selenosugar). Trimethylselenonium has been recognized as the urinary metabolite excreted in re- sponse to excessive doses and as a biological marker for excessive doses. However, recent results contradicted this.