Selenoprotein P in seminal fluid is a novel biomarker of sperm quality

- The serum transporter SePP supplies selenium (Se) to testes for GPX4 biosynthesis.
- Testes and GPX4 are preferentially supplied making them selenium-independent.
- We find SePP in seminal plasma as a stable biomarker correlating to sperm vitality.
- Seminal SePP does not originate in testes but in Se-responsive accessory sex glands.
- Besides transport, SePP likely protects sperm as a powerful antioxidative enzyme.