Protein C inhibitor (PCI), a serine‐proteinase inhibitor first purified from human blood plasma, occurs at high concentrations (3–4 μM) in seminal fluid in both a high‐molecular‐mass and low‐molecular‐mass form. Immunochemical data have previously suggested that PCI in seminal plasma forms complexes with the most abundant serine proteinase in semen, prostate‐specific antigen (PSA). To provide a structural characterization of the PCI target, immunodetected as PSA, a procedure was developed to isolate low‐molecular‐mass and high‐molecular‐mass‐forms of PCI from seminal fluid. The high‐molecular‐mass form of PCI, recognized by monoclonal antibodies against PSA, was dissociated by alkaline treatment into the low‐molecular‐mass form of PCI and a 33‐kDa protein identified as PSA by 25 conclusive steps of N‐terminal sequence analysis. We developed a sensitive immunofluorometric assay (IFMA) to measure PCI‐PSA complexes in body fluids and investigated the rate at which purified PSA may form complexes with purified PCI. Formation of complexes detected by this IFMA and the appearance of SDS‐stable approximately 90‐kDa complexes paralleled loss of PSA activity recorded with chromogenic substrates. The rate of complex formation was slow compared to that reported for PCI and activated protein C, but was enhanced up to sixfold in the presence of heparin. Less than 10% of the initial PSA activity remained after 3 h incubation with a sevenfold molar excess of PCI and in the presence of heparin.

In freshly collected ejaculates, the rate of PCI‐PSA complex formation measured by IFMA was similar to that observed between the purified proteins, and paralleled the appearance of SDS‐stable complexes by immunoblotting. During gel dissolution in freshly collected ejaculates, approximately 40% of immunodetected PCI becomes complexed to PSA. Although PCI is a slow inhibitor of PSA, complexes between PCI and PSA are detected at levels that correspond to an inactivation of up to 5% of the PSA activity in the ejaculate.