Development of novel peptide ligands modulating the enzyme activity of prostate-specific antigen

Prostate-specific antigen (PSA) is a serine proteinase produced mainly by e pithelial cells of the prostate. Measurement of PSA in serum is widely used for diagnosis and monitoring of prostate cancer. The major problem of the PSA determination in early diagnosis is the high false positive rate due to benign prostatic hyperplasia, but the clinical accuracy can be improved by determining the proportions of various molecular forms of PSA. The main bi ological function of PSA is liquefaction of the seminal gel formed after ej aculation, but PSA has also been suggested to regulate invasiveness and met astatic potential of prostatic tumors. Thus, agents binding to and affectin g the function of PSA have the potential to be used for diagnosis and thera py of prostate cancer. We have developed peptides specific for PSA by using cyclic phage display p eptide libraries. After deducing the amino acid sequence of the peptides by sequencing the relevant part of phage genome, the peptides were expressed as glutathione-S-transferase (GST) fusion proteins or produced by chemical synthesis. The peptides were shown to bind to PSA specifically as indicated by lack of binding to other related serine proteinases. The binding of the peptides with PSA was strongly inhibited by monoclonal antibodies specific for free PSA and they did not bind to PSA-inhibitor complexes indicating t hat they bind close to the active site of the enzyme. Most of the peptides enhanced the enzyme activity of PSA against a chromogenic substrate. The af finity of the peptides could be increased by including Zn2+ in the reaction mixture. These results show that peptides that bind to PSA and modulate it s enzyme activity can be developed by phage display techniques. These pepti des have the potential to be used for targeting of prostatic tumors and dia gnostics of prostate cancer.