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Review
.2009 Nov;1790(11):1441-7.
doi: 10.1016/j.bbagen.2009.03.026. Epub 2009 Apr 1.

Selenoprotein P-expression, functions, and roles in mammals

Affiliations
Review

Selenoprotein P-expression, functions, and roles in mammals

Raymond F Burk et al. Biochim Biophys Acta.2009 Nov.

Abstract

Selenoprotein P (Sepp1) is a secreted protein that is made up of 2 domains. The larger N-terminal domain contains 1 selenocysteine residue in a redox motif and the smaller C-terminal domain contains the other 9 selenocysteines. Sepp1 isoforms of varying lengths occur but quantitation of them has not been achieved. Hepatic synthesis of Sepp1 affects whole-body selenium content and the liver is the source of most plasma Sepp1. ApoER2, a member of the lipoprotein receptor family, binds Sepp1 and facilitates its uptake into the testis and retention of its selenium by the brain. Megalin, another lipoprotein receptor, facilitates uptake of filtered Sepp1 into proximal tubule cells of the kidney. Thus, Sepp1 serves in homeostasis and distribution of selenium. Mice with deletion of Sepp1 suffer greater morbidity and mortality from infection with Trypanosoma congolense than do wild-type mice. Mice that express only the N-terminal domain of Sepp1 have the same severity of illness as wild-type mice, indicating that the protective function of Sepp1 against the infection resides in the N-terminal (redox) domain. Thus, Sepp1 has several functions. In addition, plasma Sepp1 concentration falls in selenium deficiency and, therefore, it can be used as an index of selenium nutritional status.

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Figures

Figure 1
Figure 1
Domains of rat Sepp1. Panel A depicts the N-terminal domain (residues 1–244) that contains one selenocysteine residue (U) in a thioredoxin-like motif (U-x-x-C). A typical heparin-binding site (HBS) and 2 histidine-rich stretches (H-rich) are also present in this domain as are 3 occupied N-glycosylation sites. Panel B depicts the C-terminal selenium-rich domain (residues 245–366) containing 9 selenocysteine residues. It has a single occupied O-glycosylation site. Sepp1 isoforms terminate before the selenocysteine residues at positions 245, 263, and 352 as indicated by ▼
Figure 2
Figure 2
Messenger RNA of rat Sepp1. The open reading frame (ORF) has 10 UGAs that code for selenocysteine residues. The 3′utr has 2 stem loops designated SECIS 1 and 2. SECIS 2 is postulated to decode the lone UGA in the N-terminal region while SECIS 1 decodes the other 9 UGAs [22].
Figure 3
Figure 3
Sepp1 in selenium homeostasis and transport to testis, brain, and kidney. Whole-body selenium is controlled by selenium excretion in the urine. Sepp1 and selenium excretory metabolites compete for metabolically available selenium in the liver, determining urinary selenium excretion. The lipoprotein receptor apoER2 binds Sepp1 and facilitates its uptake into the testis where selenium is incorporated into spermatozoa. ApoER2 also maintains brain selenium. Sepp1 is filtered by the kidney into the glomerular filtrate and binds to megalin on the brush border of PCT cells. Those cells endocytose the Sepp1 bound to megalin and presumably use its selenium to synthesize Gpx3.
Figure 4
Figure 4
Comparison of the effects of Sepp1 deletion, selenium deficiency, and apoER2 deletion on tissue and whole-body selenium concentrations. TheSepp1−/− andSepp1+/+ mice (panel A) and theapoER2−/− andapoER2+/+ mice (panel C) were fedTorula yeast-based diet supplemented with 0.25 mg selenium/kg [38]. They were studied 4 weeks after weaning. The selenium-deficient and control mice (panel B) were fed theTorula yeast-based diet in selenium-deficient form or supplemented with 0.25 mg selenium/kg (control), respectively, for 18 weeks beginning at weaning. The mice were exsanguinated and tissues were removed and weighed. Selenium was determined in tissues, blood, and carcass [38]. Selenium values were added to obtain the whole-body selenium. Values are means ± S.D., n=5. Values in pairs that were different from each other by the Studentt-test have the percentage of difference indicated. Panel C is a modification of results presented in [41], presented here with permission.
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References

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