Selenoprotein P (Sepp1) is a plasma and extracellular protein that is rich in selenium. Deletion of Sepp1 results in sharp decreases of selenium levels in the brain and testis with dysfunction of those organs. Deletion of Sepp1 also causes increased urinary selenium excretion, leading to moderate depletion of whole-body selenium. The lipoprotein receptor apolipoprotein E receptor-2 (apoER2) binds Sepp1 and facilitates its uptake by Sertoli cells, thus providing selenium for spermatogenesis. Experiments were performed to assess the effect of apoER2 on the concentration and function of selenium in the brain and on whole-body selenium. ApoER2 Ϫ/Ϫ and apoER2 ϩ/ϩ male mice were fed a semipurified diet with selenite added as the source of selenium. ApoER2 Ϫ/Ϫ mice had depressed brain and testis selenium, but normal levels in liver, kidney, muscle, and the whole body. Feeding a selenium-deficient diet to apoER2 Ϫ/Ϫ mice led to neurological dysfunction and death, with some of the characteristics exhibited by Sepp1 Ϫ/Ϫ mice fed the same diet. Thus, although it does not affect whole-body selenium, apoER2 is necessary for maintenance of brain selenium and for prevention of neurological dysfunction and death under conditions of selenium deficiency, suggesting an interaction of apoER2 with Sepp1 in the brain.

Glutathione peroxidase-3 (Gpx3), also known as plasma or extracellular glutathione peroxidase, is a selenoprotein secreted primarily by kidney proximal convoluted tubule cells. In this study Gpx3(-/-) mice have been produced and immunocytochemical techniques have been developed to investigate Gpx3 metabolism. Gpx3(-/-) mice maintained the same whole-body content and urinary excretion of selenium as did Gpx3(+/+) mice. They tolerated selenium deficiency without observable ill effects. The simultaneous knockout of Gpx3 and selenoprotein P revealed that these two selenoproteins account for >97% of plasma selenium. Immunocytochemistry experiments demonstrated that Gpx3 binds selectively, both in vivo and in vitro, to basement membranes of renal cortical proximal and distal convoluted tubules. Based on calculations using selenium content, the kidney pool of Gpx3 is over twice as large as the plasma pool. These data indicate that Gpx3 does not serve in the regulation of selenium metabolism. The specific binding of a large pool of Gpx3 to basement membranes in the kidney cortex strongly suggests a need for glutathione peroxidase activity in the cortical peritubular space.

Selenoprotein P (Sepp1) has two domains with respect to selenium content: the N-terminal, selenium-poor domain and the C-terminal, selenium-rich domain. To assess domain function, mice with deletion of the C-terminal domain have been produced and compared with Sepp1 ؊/؊ and Sepp1 ؉/؉ mice. All mice studied were males fed a semipurified diet with defined selenium content. The Sepp1 protein in the plasma of mice with the C-terminal domain deleted was determined by mass spectrometry to terminate after serine 239 and thus was designated Sepp1 ⌬240 -361 . Plasma Sepp1 and selenium concentrations as well as glutathione peroxidase activity were determined in the three types of mice. Glutathione peroxidase and Sepp1 ⌬240 -361 accounted for over 90% of the selenium in the plasma of Sepp1 ⌬240 -361 mice. Calculations using results from Sepp1 ؉/؉ mice revealed that Sepp1, with a potential for containing 10 selenocysteine residues, contained an average of 5 selenium atoms per molecule, indicating that shortened and/or selenium-depleted forms of the protein were present in these wild-type mice. Sepp1 ⌬240 -361 mice had low brain and testis selenium concentrations that were similar to those in Sepp1 ؊/؊ mice but they better maintained their whole body selenium. Sepp1 ⌬240 -361 mice had depressed fertility, even when they were fed a high selenium diet, and their spermatozoa were defective and morphologically indistinguishable from those of selenium-deficient mice. Neurological dysfunction and death occurred when Sepp1 ⌬240 -361 mice were fed selenium-deficient diet. These phenotypes were similar to those of Sepp1 ؊/؊ mice but had later onset or were less severe. The results of this study demonstrate that the C terminus of Sepp1 is critical for the maintenance of selenium in brain and testis but not for the maintenance of whole body selenium.Selenoprotein P (Sepp1) 3 contains most of the selenium in plasma (1). Rat Sepp1 cDNA codes for 366 amino acid residues, 10 of which are selenocysteines and 17 cysteines (2). Two domains with respect to selenium content are discernable. The N-terminal 244 residues include 1 selenocysteine and 7 cysteines. are present in this domain, allowing the prediction that it has enzymatic properties (3). The smaller C-terminal domain comprises 122 amino acid residues: 9 of them are selenocysteines and 10 are cysteines. Thus, the C-terminal one-third of the protein contains 90% of its selenium, raising the possibility that this domain plays a role in selenium transport. Each domain, then, has been postulated to have a distinct function. Sepp1 was purified from rat plasma using a monoclonal antibody to the N-terminal domain. After applying the purified protein to a heparin column, we eluted 4 isoforms using a pH gradient (4). Mass spectrometry analysis identified the isoforms as sharing the same amino acid sequence but terminating at positions corresponding to UGAs (which code for selenocysteines) in the open reading frame (5). In addition to the fulllength protein, isoforms terminating at the second, t...

The brain and testis retain selenium better than other tissues during selenium deficiency. Studies of mice with selenoprotein P (Sepp1) deleted (Sepp1(-/-) mice) showed that brain and testis selenium levels are largely dependent on Sepp1. Therefore, we examined tissue selenium in mice fed varying amounts of selenium and in Sepp1(-/-) mice to characterize better the role(s) of Sepp1. Mice were fed a selenium-deficient diet for 8 wk supplemented with selenium as selenite from none to 0.25 mg/kg diet and tissue selenium was measured. Brain and testis maintained their selenium better than did liver, kidney, and muscle when dietary selenium was limiting but testis selenium fell sharply in the group fed the deficient diet. Brain retained its selenium well, even in the group fed the deficient diet. After intravenous injection of (75)Se-Sepp1 into Sepp1(-/-) and Sepp1(+/+) mice, qualitative differences between brain and testis (75)Se uptake were noted, further suggesting differences in their uptake of selenium from Sepp1. Finally, selenium was measured in brain regions of Sepp1(-/-) and Sepp1(+/+) mice fed the diet supplemented with 1 mg selenium/kg and Sepp1(+/+) mice fed the deficient diet. Deletion of Sepp1 and selenium deficiency each lowered selenium a similar amount in cortex, midbrain, brainstem, and cerebellum. Selenium in the hippocampus was lowered by deletion of Sepp1 but not by selenium deficiency. These results suggest that Sepp1 is more important for maintaining selenium in the hippocampus than in other brain regions. They also confirm the position of the brain at the apex of the organ selenium hierarchy.

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