Quantification of seminal plasma motility inhibitor/semenogelin in human seminal plasma

Semenogelin I and II (Sg I and II) are the major components of human semen coagulum. The protein is rapidly cleaved after ejaculation by the chymotrypsin-like protease prostate-specific antigen (PSA), which results in the liquefaction of the semen coagulum and the progressive release of motile spermatozoa. One of the cleavage products of the protein, a 14-kDa protein, is a sperm motility inhibitor (seminal plasma motility inhibitor [SPMI]). We developed a monoclonal antibody (mAb) that is specific to the fragment of Sgs, SPMI, and a sandwich enzyme-linked immunosorbent assay (ELISA) system for the quantification of Sgs using this mAb. Then, we measured SPMI/Sg levels in human seminal plasma from healthy male volunteers (n = 100, aged 18-24 years). The mean level of SPMI/Sg in seminal plasma was 19 +/- 13 mg/mL (range, 4-68 mg/mL). Log-transformed SPMI/Sg levels were negatively correlated with the sperm motility (r = -0.229, P =.0220) and positively correlated with the total protein concentration (r = 0.793, P <.0001). This result supports that SPMI, one of the fragments of Sg, has its inhibitory effect on ejaculated spermatozoa in liquefied semen under physiological conditions.