Regulation of Selenocysteine Content of Human Selenoprotein P by Dietary Selenium and Insertion of Cysteine in Place of Selenocysteine

Abstract

Selenoproteins are a unique group of proteins that contain selenium in the form of selenocysteine (Sec) co-translationally inserted in response to a UGA codon with the help of cis- and trans-acting factors. Mammalian selenoproteins contain single Sec residues, with the exception of selenoprotein P (SelP) that has 7–15 Sec residues depending on species. Assessing an individual’s selenium status is important under various pathological conditions, which requires a reliable selenium biomarker. Due to a key role in organismal selenium homeostasis, high Sec content, regulation by dietary selenium, and availability of robust assays in human plasma, SelP has emerged as a major biomarker of selenium status. Here, we found that Cys is present in various Sec positions in human SelP. Treatment of cells expressing SelP with thiophosphate, an analog of the selenium donor for Sec synthesis, led to a nearly complete replacement of Sec with Cys, whereas supplementation of cells with selenium supported Sec insertion. SelP isolated directly from human plasma had up to 8% Cys inserted in place of Sec, depending on the Sec position. These findings suggest that a change in selenium status may be reflected in both SelP concentration and its Sec content, and that availability of the SelP-derived selenium for selenoprotein synthesis may be overestimated under conditions of low selenium status due to replacement of Sec with Cys.

Sex-specific and inter-individual differences in biomarkers of selenium status identified by a calibrated ELISA for selenoprotein P

“…Our data show that the theoretical predictions should not be taken for granted, as done in cases when genetic information is extrapolated to the proteins [54]. Recently, we have shown that not all theoretical positions are fully occupied by Sec in SELENOP isolated from healthy human subjects [20]. This can be considered as translational relaxation or translational errors, which have been shown to be of relevance for the function of selenoenzymes, e.g.…”

“…However, there are indications that the circulating SELENOP is not a homogenous protein. Variants of different molecular weight have been described [13], [18], in relation to genotype, premature translational termination, limited posttranslational proteolysis or partial replacement of Sec by Cys [19], [20], [21], [22].…”

Facile Recoding of Selenocysteine in Nature

“…A similar search of 92 mammalian genomes (215 Gbp) and of the Drosophila melanogaster genome (139 Mbp) showed no exception to the use of UGA as the Sec codon. Whether this is related to the necessity of selenoproteins in high-level redox signaling pathways [21] or due to the sophisticated backup systems [22] remains to be investigated. However, in the lower eukaryote Euplotes crassus UGA serves both as a Cys and also as a SECIS-dependent Sec codon [23] .…”

Selenoprotein Expression in Macrophages Is Critical for Optimal Clearance of Parasitic Helminth Nippostrongylus brasiliensis

“…Targeted deletion of the floxed Trsp allele by a tissue/cell-specific promoter-driven Cre recombinase markedly diminished expression of all selenoproteins (12). Substitution of Sec residue with Cys in some selenoproteins has been observed during selenium deficiency, which also markedly reduces their enzymatic activity (13,14).…”