Phosphorylation of seminal vesicle protein IV on Ser58 enhances its peroxidase-stimulating activity

In this study we show that SV-IV, a major immunomodulatory, anti-inflammato ry, and sperm immunoprotective protein secreted from the rat seminal vesicl e epithelium, acts in vitro as a substrate of protein kinase C (PKC) compet ing efficiently with H1 histone, a very well known PKC substrate. Electrosp ray mass spectrometry (ES-MS) analysis demonstrated that approximately 10% of the native SV-TV molecules were phosphorylated by PKC and that such a mo dification involved only a single serine residue (Ser58) out of the 22 occu rring in the protein. Interestingly this modification produced a substantia l enhancement (approximately 50%) of the native SV-IV's ability to stimulat e the activity of both horseradish peroxidase (POD) and selenium-dependent glutathione peroxidase (GPX), an enzyme that is known to protect the mammal ian spermatozoa from oxidative stress and loss of motility in the female ge nital tract following ejaculation. In contrast, the phosphorylation of SV-I V on Ser58 did not produce any effect on the anti-inflammatory properties o f SV-IV, as measured by its ability to inhibit the phospholipase A(2).