PURIFICATION AND CHARACTERIZATION OF THE ACTIVE PRECURSOR OF A HUMAN SPERM MOTILITY INHIBITOR SECRETED BY THE SEMINAL-VESICLES - IDENTITY WITH SEMENOGELIN

Human seminal plasma contains a sperm motility inhibitor that originat es from seminal vesicles as a precursor form. This precursor is degrad ed into smaller peptides by prostatic proteases shortly after ejaculat ion. The seminal plasma sperm motility inhibitor (SPMI) precursor was purified by a combination of cation-exchange chromatography on S-Sepha rose followed by C4 reverse-phase high-performance liquid chromatograp hy directly from seminal vesicle fluid or washed seminal coagulum. The purification procedure yielded a protein of apparent homogeneity, wit h a molecular mass of 52 kDa by SDS-PACE. It migrated as a 105-kDa pro tein by molecular sieving under denaturing conditions. The purified SP MI precursor was digested by the prostatic protease prostate-specific antigen (PSA), causing a 76 +/- 4% drop in biological activity and tra nsformation into low molecular mass SPMI polypeptides (5-20 kDa) simil ar to those observed in liquefied semen. The N-terminal amino acid seq uences of three degradation peptides were obtained by Edman degradatio n and found to correspond to residues 4550, 85-90, and 137-143 of seme nogelin, a protein characterized as the major structural component of semen coagulum. The amino acid composition of SPMI precursor was found to be almost identical to that of semenogelin. Moreover, the mass of the precursor was estimated at 49 620 daltons by electrospray-ionizati on mass spectrometry, a value in close agreement with the expected mas s of semenogelin according to its cDNA sequence. The SPMI precursor wa s found to inhibit sperm motility in a dose-dependent manner, with com plete immobilization at 500 U/ml of SPMI. The motility of completely i mmobilized spermatozoa was partially recovered after washing of the ce lls. The results suggest that SPMI precursor is the major component of the seminal vesicle secretions and seminal coagulum. It can be degrad ed by PSA in a manner reminiscent of its processing in whole semen. Ta ken together these results indicate that the SPMI precursor is semenog elin and that intact semenogelin can immobilize spermatozoa.