Might be responsible for some of the extracellular antioxidant defense properties of selenium or might be involved in the transport of selenium. May supply selenium to tissues such as brain and testis. Passes from plasma into the glomerular filtrate where it is removed by endocytosis mediated by LRP2 in the proximal tubule epithelium. Widely expressed, mainly by the liver. Secreted in plasma. The C-terminus is not required for endocytic uptake in the proximal tubule epithelium. Isoform Se-P1 contains several disulfide bridges and a selenide-sulfide bond between Sec-59 and Cys-62. These bonds are speculated to serve as redox-active pairs. Phosphorylation sites are present in the extracellular medium. Plasma contains 4 isoforms, which are named isoforms Se-P10, Se-P6, Se-P2 and Se-P1, according to the number of selenocysteines they contain. All isoforms arise from the same mRNA. The 3 shortened isoforms terminated at the opal stop codons at positions 264, 282, 371, when selenocysteine has not been inserted. Belongs to the selenoprotein P family.