Sperm motility inhibitor from human seminal plasma: association with semen coagulum

Human seminal plasma contains a sperm motility inhibitor (SPMI) originating from the seminal vesicles as a 52 kDa precursor form that is rapidly degraded by prostatic proteases after ejaculation. In this study, the distribution of SPMI biological activity and antigens was analysed in chemically induced, as well as naturally occurring, arrest of semen liquefaction. SPMI activity was detected exclusively in the coagulated semen fraction at 2200 +/- 560 IU, whereas total seminal plasma proteins separated more evenly between soluble and coagulated components (91 +/- 19 and 65 +/- 18 mg, respectively). An SPMI antiserum recognized different forms of SPMI precursors at 52, 38, 35, 33 and 20 kDa in the coagulum while the soluble protein fraction contained only one major immunoreactive band at 15 kDa. High levels of SPMI activity (1500 +/- 180 IU/ml) together with high molecular mass forms of SPMI precursor and low sperm motility (26%) were detected in semen samples that failed to liquefy spontaneously at room temperature. Addition of prostatic secretions to the non-liquefying samples caused a decrease of SPMI activity (330 +/- 17 IU/ml) and transformed the SPMI precursor into low molecular mass forms (14-22 kDa) with a concomitant increase in sperm motility to 49%. The results suggest that SPMI is highly associated with the seminal coagulum components as very active forms that may adversely affect sperm motility when not properly processed after ejaculation.