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On the presence of prostatic secretion protein in rat seminal fluid

The copulating plug collected from the tip of the penis from rats immediately after decapitation contains a protein very similar and probably identical to PSP (prostatic secretion protein); this protein has earlier been purified from rat prostatic cytosol and characterized. The protein present in the copulating plug interacts with [3H]estramustine and binds to the antibody raised against rat PSP. The concentration of the protein in the copulating plug is 400 ng/mg of total protein, when measured using the radio immunoassay technique developed earlier for measurement of PSP in rat prostate. The [3H]estramustine–protein complex formed in a preparation of the copulating plug has an apparent molecular weight of about 50,000 and a sedimentation coefficient of about 3S when analyzed using sucrose density gradient centrifugation. The complex was retained on Concanavallin‐A Sepharose indicating that the protein is a glycoprotein. Binding of the complex was also observed on hydroxylapatite and DEAE‐Sephadex columns, from which it ws eluted at 0.18 M KC1. Light microscope autoradiograms of rat sperms incubated with 125I‐labeled PSP indicated that PSP is bound to all parts of the sperms. A macromolecule interacting with the PSP‐antibodies is also present in human seminal fluid but at a concentration considerably lower than in rat seminal fluid. The present study shows that a macromolecule probably identical to prostatic secretion protein is present in the copulating plug from the rat. The biological role of this protein in normal male fertility is discussed.