Prostate specific antigen in benign prostatic hyperplasia: purification and characterization.

OBJECTIVE

The ratio of free-to-total prostate specific antigen (PSA) in serum is greater in patients with benign prostatic hyperplasia (BPH) than in those with prostate cancer, and it provides a means of partially discriminating these 2 diseases. To understand the molecular mechanism of why the free-to-total PSA ratio is greater in BPH than in prostate cancer we analyzed PSA obtained directly from nodules of BPH tissue.

METHODS

PSA from BPH nodule fluids was first purified by gel filtration and ion exchange chromatography. Purified BPH PSA was characterized by gel electrophoresis, enzyme assay and N-terminal sequence analysis of the amino acids.

RESULTS

A single band at 30 kDa. on sodium dodecyl sulfate polyacrylamide gel electrophoresis under nonreducing conditions was identical to that of seminal fluid PSA. Under reducing conditions most BPH PSA was degraded, whereas most seminal fluid PSA existed as an intact molecule. BPH PSA had multiple internal cleavages in addition to the common cleavage site between lysines 145 and 146 of seminal fluid PSA. Cleavage sites at C-terminals of histidine 54 and phenylalanine 57 were also detected. Enzymatic activity studies with different substrates showed that PSA from seminal fluid and BPH nodules had similar specific trypsin-like activity. However, BPH PSA had much lower specific chymotrypsin-like activity than seminal fluid PSA. N-terminal sequence analysis showed that BPH PSA was neither in the pre-proenzyme form (261 amino acids) nor the zymogen proenzyme form (244 amino acids) of PSA, both of which are known precursors of mature PSA (237 amino acids).

CONCLUSIONS

Most PSA in BPH nodules is in the nicked form with low chymotrypsin-like activity. When it leaks into the circulation it will form fewer PSA-antichymotrypsin complexes and more will remain in the free form. We predict that a protease with trypsin-like activity in BPH nodule fluid is probably responsible for the nicked form of BPH PSA. These findings suggest that antibodies produced against PSA in BPH nodules may be useful in discriminating prostate cancer from BPH.