Selenoprotein P (Sepp1) is a secreted protein that is made up of 2 domains. The 
larger N-terminal domain contains 1 selenocysteine residue in a redox motif and 
the smaller C-terminal domain contains the other 9 selenocysteines. Sepp1 
isoforms of varying lengths occur but quantitation of them has not been 
achieved. Hepatic synthesis of Sepp1 affects whole-body selenium content and the 
liver is the source of most plasma Sepp1. ApoER2, a member of the lipoprotein 
receptor family, binds Sepp1 and facilitates its uptake into the testis and 
retention of its selenium by the brain. Megalin, another lipoprotein receptor, 
facilitates uptake of filtered Sepp1 into proximal tubule cells of the kidney. 
Thus, Sepp1 serves in homeostasis and distribution of selenium. Mice with 
deletion of Sepp1 suffer greater morbidity and mortality from infection with 
Trypanosoma congolense than do wild-type mice. Mice that express only the 
N-terminal domain of Sepp1 have the same severity of illness as wild-type mice, 
indicating that the protective function of Sepp1 against the infection resides 
in the N-terminal (redox) domain. Thus, Sepp1 has several functions. In 
addition, plasma Sepp1 concentration falls in selenium deficiency and, 
therefore, it can be used as an index of selenium nutritional status.