Domain structure of bi-functional selenoprotein P

Human selenoprotein P (SeP), a selenium-rich plasma glycoprotein, is presumed to contain ten selenocysteine residues; one of which is located at the 40th residue in the N-terminal region and the remaining nine localized in the C-terminal third part. We have shown that SeP not only catalyses the reduction of phosphatidylcholine hydroperoxide by glutathione [Saito, Hayashi, Tanaka, Watanabe, Suzuki, Saito and Takahashi (1999) J. Biol. Chem. 274, 2866-2871], but also supplies its selenium to proliferating cells [Saito and Takahashi (2002) Eur. J. Biochem. 269, 5746-5751]. Treatment of SeP with plasma kallikrein resulted in a sequential limited proteolysis (Arg-235-Gln-236 and Arg-242-Asp-243). The N-terminal (residues 1-235) and C-terminal (residues 243-361) fragments exhibited enzyme activity and selenium-supply activity respectively. These results confirm that SeP is a bifunctional protein and suggest that the first selenocysteine residue is the active site of the enzyme and the remaining nine residues function as a selenium supplier.