Sepp1(UF) forms are N-terminal selenoprotein P truncations that have peroxidase activity when coupled with thioredoxin reductase-1

• Published in: Free radical biology & medicine Vol. 69; p. 67
• Main Authors: Kurokawa, Suguru, Eriksson, Sofi, Rose, Kristie L, Wu, Sen, Motley, Amy K, Hill, Salisha, Winfrey, Virginia P, McDonald, W Hayes, Capecchi, Mario R, Atkins, John F, Arnér, Elias S J, Hill, Kristina E, Burk, Raymond F
• Format: Journal Article
• Language: English
• Published: United States 01-04-2014
• Subjects: Animals; Biological Transport; Endocytosis; Glutathione Peroxidase - biosynthesis; Hydrogen Peroxide - metabolism; Kidney Glomerulus - cytology; Kidney Glomerulus - pathology; Kidney Tubules, Proximal - cytology; Kidney Tubules, Proximal - metabolism; Medicin och hälsovetenskap; Mice; Oxidation-Reduction; Protein Structure, Tertiary; Selenocysteine - metabolism; Selenoproteins - chemistry; Selenoproteins - metabolism; Thioredoxin Reductase 1 - metabolism; Selenoprotein P forms; Free radicals; Extracellular peroxidase activity; Selenium-containing proteins; Megalin; Selenium homeostasis
• ISSN: 1873-4596; 1873-4596
• DOI: 10.1016/j.freeradbiomed.2014.01.010

Mouse selenoprotein P (Sepp1) consists of an N-terminal domain (residues 1-239) that contains one selenocysteine (U) as residue 40 in a proposed redox-active motif (-UYLC-) and a C-terminal domain (residues 240-361) that contains nine selenocysteines. Sepp1 transports selenium from the liver to other tissues by receptor-mediated endocytosis. It also reduces oxidative stress in vivo by an unknown mechanism. A previously uncharacterized plasma form of Sepp1 is filtered in the glomerulus and taken up by renal proximal convoluted tubule (PCT) cells via megalin-mediated endocytosis. We purified Sepp1 forms from the urine of megalin(-/-) mice using a monoclonal antibody to the N-terminal domain. Mass spectrometry revealed that the purified urinary Sepp1 consisted of N-terminal fragments terminating at 11 sites between residues 183 and 208. They were therefore designated Sepp1(UF). Because the N-terminal domain of Sepp1 has a thioredoxin fold, Sepp1(UF) were compared with full-length Sepp1, Sepp1(Δ240-361), and Sepp1(U40S) as a substrate of thioredoxin reductase-1 (TrxR1). All forms of Sepp1 except Sepp1(U40S), which contains serine in place of the selenocysteine, were TrxR1 substrates, catalyzing NADPH oxidation when coupled with H2O2 or tert-butylhydroperoxide as the terminal electron acceptor. These results are compatible with proteolytic cleavage freeing Sepp1(UF) from full-length Sepp1, the form that has the role of selenium transport, allowing Sepp1(UF) to function by itself as a peroxidase. Ultimately, plasma Sepp1(UF) and small selenium-containing proteins are filtered by the glomerulus and taken up by PCT cells via megalin-mediated endocytosis, preventing loss of selenium in the urine and providing selenium for the synthesis of glutathione peroxidase-3.