While selenoprotein P was believed to play a role as an antioxidant and to transport selenium throughout the body,  its physiological function was unknown.

Over 20 selenoproteins have been identified, including key enzymes for oxidant defense such as the glutathione peroxidases and the thioredoxin reductases. Selenoproteins are usually enzymes that contain a single selenocysteine as a constituent of the active site. Dietary selenium is essential for normal sperm development and male fertility. Selenoprotein P (SEPP1) is a unique family member that is an extracellular glycoprotein and depending on the species, contains 10-17 selenocysteine residues in its primary structure, thereby carrying about 60 percent of the selenium in blood plasma.

To understand the physiological function of SEPP1 in the testes and epididymis of mammals, a team of scientists at Vanderbilt University in Nashville studied male mice that lack the gene to produce SEPP1. These genetically altered males have levels of selenium in the testis that are less than 10 percent of the normal concentration.

The research team, headed by Dr. Gary E. Olson, found that the mutant male mice lacking SEPP1 were generally infertile because of  sperm development with defective tails, similar to the sperm produced by unaltered male mice fed a low-selenium diet. Even further, the infertility of the mutant mice could not be cured by prolonged feeding on a diet supplemented with high levels of selenium.

According to Olson and his colleagues, these findings strongly indicate that SEPP1 is the source of the selenium needed for development of normal sperm and for male mice to maintain their fertility.

The origanl paper:

Biology of Reproduction, Papers in Press. Published on March 2, 2005 as DOI:10.1095/biolreprod.105.040360