title: Sepp1(UF) forms are N-terminal selenoprotein P truncations that have peroxidase activity when coupled with thioredoxin reductase-1.
abstract: 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.
authors.affiliation: Division of Gastroenterology, Hepatology, and Nutrition, Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
authors.forename: Suguru
authors.initials: S
authors.lastname: Kurokawa
authors.affiliation: Division of Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-171 77 Stockholm, Sweden.
authors.forename: Sofi
authors.initials: S
authors.lastname: Eriksson
authors.affiliation: Vanderbilt Proteomics Laboratory in the Mass Spectrometry Research Center, Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
authors.forename: Kristie L
authors.initials: KL
authors.lastname: Rose
authors.affiliation: Department of Human Genetics, University of Utah, Salt Lake City, UT 84112, USA.
authors.forename: Sen
authors.initials: S
authors.lastname: Wu
authors.affiliation: Division of Gastroenterology, Hepatology, and Nutrition, Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
authors.forename: Amy K
authors.initials: AK
authors.lastname: Motley
authors.affiliation: Vanderbilt Proteomics Laboratory in the Mass Spectrometry Research Center, Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
authors.forename: Salisha
authors.initials: S
authors.lastname: Hill
authors.affiliation: Division of Gastroenterology, Hepatology, and Nutrition, Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
authors.forename: Virginia P
authors.initials: VP
authors.lastname: Winfrey
authors.affiliation: Vanderbilt Proteomics Laboratory in the Mass Spectrometry Research Center, Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
authors.forename: W Hayes
authors.initials: WH
authors.lastname: McDonald
authors.affiliation: Department of Human Genetics, University of Utah, Salt Lake City, UT 84112, USA.
authors.forename: Mario R
authors.initials: MR
authors.lastname: Capecchi
authors.affiliation: Department of Human Genetics, University of Utah, Salt Lake City, UT 84112, USA; Department of Biochemistry, University College Cork, Cork, Ireland.
authors.forename: John F
authors.initials: JF
authors.lastname: Atkins
authors.affiliation: Division of Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-171 77 Stockholm, Sweden.
authors.forename: Elias S J
authors.initials: ES
authors.lastname: Arnér
authors.affiliation: Division of Gastroenterology, Hepatology, and Nutrition, Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
authors.forename: Kristina E
authors.initials: KE
authors.lastname: Hill
authors.affiliation: Division of Gastroenterology, Hepatology, and Nutrition, Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN 37232, USA. Electronic address: raymond.burk@vanderbilt.edu.
authors.forename: Raymond F
authors.initials: RF
authors.lastname: Burk
mesh_terms: D000818:Animals; D001692:Biological Transport; D004705:Endocytosis; D005979:Glutathione Peroxidase / Q000096:biosynthesis*; D006861:Hydrogen Peroxide / Q000378:metabolism; D007678:Kidney Glomerulus / Q000166:cytology / Q000473:pathology; D007687:Kidney Tubules, Proximal / Q000166:cytology / Q000378:metabolism; D051379:Mice; D010084:Oxidation-Reduction; D017434:Protein Structure, Tertiary; D017279:Selenocysteine / Q000378:metabolism; D051140:Selenoproteins / Q000737:chemistry / Q000378:metabolism*; D054481:Thioredoxin Reductase 1 / Q000378:metabolism*
chemical_list: D051140:Selenoproteins; D017279:Selenocysteine; D006861:Hydrogen Peroxide; C116888:Gpx3 protein, mouse; D005979:Glutathione Peroxidase; D054481:Thioredoxin Reductase 1; C516125:Txnrd1 protein, mouse
keywords: Extracellular peroxidase activity; Free radicals; Megalin; Selenium homeostasis; Selenium-containing proteins; Selenoprotein P forms