Sepp1UF forms are N-terminal selenoprotein P truncations that have peroxidase activity when coupled with thioredoxin reductase-1

• Published in: Free radical biology & medicine 2014-04, Vol.69, p.67-76
• 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: Article
• Language: English
• ISSN: 0891-5849; 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 1 selenocysteine (U) as residue 40 in a proposed redox-active motif (-UYLC-) and a C-terminal domain (residues 240-361) that contains 9 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. It was therefore designated Sepp1 UF . Because the N-terminal domain of Sepp1 has a thioredoxin fold, Sepp1 UF was 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 H 2 O 2 or tert- butyl hydroperoxide 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.