Effect of graded levels of selenium supplementation as selenite on expression of selenosugars, selenocysteine, and other selenometabolites in rat liver

Using high pressure liquid chromatography (HPLC) coupled with selenium-specific inductively coupled plasma mass spectrometry (ICP-MS) and molecule specific (Orbitrap MS/MS) detection, we previously found that far more selenium (Se) is present as selenosugar (seleno-N-acetyl galactosamine) in Se-adequate turkey liver than is present as selenocysteine (Sec) in true selenoproteins, and that selenosugars account for half of the Se in high-Se turkey liver. To expand these observations to mammals, we studied Se metabolism in rats fed graded levels of selenite from 0 to 5 mu g Se/g for 4 wk. In Se-adequate (0.24 mu g Se/g) rats, 43% of liver Se was present as Sec, 32% was present as selenosugars, and 22% as inorganic Se bound to protein. In liver of rats fed 5 mu g Se/g as selenite, the quantity of Sec remained at the Se-adequate plateau (11% of total Se), 22% was present as low molecular weight (LMW) selenosugars with substantial additional selenosugars linked to protein, but 64% was present as inorganic Se bound to protein. No selenomethionine was found at any level of selenite supplementation. Below the Se requirement, Se is preferentially incorporated into Sec-selenoproteins. Above the dietary Se requirement, selenosugars become by far the major LMW water soluble Se species in liver, and levels of selenosugar-decorated proteins are far higher than Sec-selenoproteins, making these selenosugar-decorated proteins the major Se-containing protein species in liver with high Se supplementation. This accumulation of selenosugars linked to cysteines on proteins or the build-up of inorganic Se bound to protein may underlie Se toxicity at the molecular level. Graphical Abstract HPLC coupled with Se-specific (ICP MS) and molecule specific (Orbitrap MS/MS) analysis found that as selenium (Se) status increases in rats supplemented with inorganic Se as selenite, Se is preferentially incorporated as selenocysteine into true selenoproteins in rats fed Se deficient to adequate dietary Se (<= 0.24 mu g Se/g diet). At and above the Se requirement, dietary Se is increasingly metabolized into selenosugars (seleno-N-acetyl galactosamine, SeGalNac) in the liver, where it is found as both low molecular weight methyl- and glutathionyl-selenosugars, and as high molecular weight selenosugar-decorated proteins. With toxic Se supplementation with selenite (5 mu g Se/g diet), inorganic Se species predominate, with limited selenoglutathione species. No selenomethionine was detected by Orbitrap MS/MS at any level of Se status, indicating that the monogastric rat does not metabolize selenite to selenomethionine.