SelP is the only known eukaryotic selenoprotein that contains multiple selenocysteine (Sec) residues, and accounts for more than 50% of the selenium content of rat and human plasma. It is thought to be glycosylated. SelP may have antioxidant properties. It can attach to epithelial cells, and may protect vascular endothelial cells against peroxynitrite toxicity. The high selenium content of SelP suggests that it may be involved in selenium intercellular transport or storage. The promoter structure of bovine SelP suggest that it may be involved in countering heavy metal intoxication, and may also have a developmental function. The N-terminal region of SelP can exist independently of the C terminal region. Zebrafish selenoprotein Pb lacks the C terminal Sec-rich region, and a protein encoded by the rat SelP gene and lacking this region has also been reported. N-terminal region contains a conserved SecxxCys motif, which is similar to the CysxxCys found in thioredoxins. It is speculated that the N terminal region may adopt a thioredoxin fold and catalyze redox reactions. The N-terminal region also contains a His-rich region, which is thought to mediate heparin binding. Binding to heparan proteoglycans could account for the membrane binding properties of SelP. The function of the bacterial members of this family is uncharacterized.