The selenium-rich C-terminal domain of mouse selenoprotein P is necessary for the supply of selenium to brain and testis but not for the maintenance of whole body selenium

Selenoprotein P (Sepp1) has two domains with respect to selenium content: the N-terminal, selenium-poor domain and the C-terminal, selenium-rich domain. To assess domain function, mice with deletion of the C-terminal domain have been produced and compared with Sepp1-/- and Sepp1+/+ mice. All mice studied were males fed a semipurified diet with defined selenium content. The Sepp1 protein in the plasma of mice with the C-terminal domain deleted was determined by mass spectrometry to terminate after serine 239 and thus was designated Sepp1Delta240-361. Plasma Sepp1 and selenium concentrations as well as glutathione peroxidase activity were determined in the three types of mice. Glutathione peroxidase and Sepp1Delta240-361 accounted for over 90% of the selenium in the plasma of Sepp1Delta240-361 mice. Calculations using results from Sepp1+/+ mice revealed that Sepp1, with a potential for containing 10 selenocysteine residues, contained an average of 5 selenium atoms per molecule, indicating that shortened and/or selenium-depleted forms of the protein were present in these wild-type mice. Sepp1Delta240-361 mice had low brain and testis selenium concentrations that were similar to those in Sepp1-/- mice but they better maintained their whole body selenium. Sepp1Delta240-361 mice had depressed fertility, even when they were fed a high selenium diet, and their spermatozoa were defective and morphologically indistinguishable from those of selenium-deficient mice. Neurological dysfunction and death occurred when Sepp1Delta240-361 mice were fed selenium-deficient diet. These phenotypes were similar to those of Sepp1-/- mice but had later onset or were less severe. The results of this study demonstrate that the C terminus of Sepp1 is critical for the maintenance of selenium in brain and testis but not for the maintenance of whole body selenium.