Molecular heterogeneity of free PSA in sera of patients with benign and malignant prostate tumors

Objective: To analyze free prostate-specific antigen (f-PSA) in sera from p atients with prostate cancer (PCa) and benign prostatic hyperplasia (BPH), and to detect possible differences in subtypes as potential diagnostic para meters. Materials and Methods: PSA was purified from sera by an immunoaffin ity procedure developed on the basis of oriented antibody immobilization, a nd subjected to size exclusion chromatography (SEC), Western blotting, and N-terminal amino acid sequencing. Results: The novel procedure allowed the purification of PSA with high yield from sera containing PSA <10 ng/ml. SEC under nonreducing conditions as well as Western blots demonstrated the pre sence of several molecular forms of f-PSA. Th ree of the smaller polypeptid es exhibited the N-terminal sequence of PSA while one represented the C-ter minal fragment Lys(146)-Pro(237). Shortening of some polypeptides by the N- terminal amino acid Ile(1) suggestive of aminopeptidase action was also obs erved. No propeptide sequence could be detected, and none of the bands from patient sera reacted with antibodies raised against propeptide antigens. B PH sera expressed higher proportions of smaller PSA fragments per unit p33, and contained significant amounts of fragments <14,000 which appeared to b e very low or absent from most PCa sera. Conclusions: f-PSA as obtained fro m BPH and PCa sera represents a heterogeneous fraction. The major component (p33) is not in the nicked form and does not contain proPSA. Diagnostic po tential could arise from the quantitative differences of the smaller PSA de rivatives seen between PCa and BPH sera.