Identification of novel prostate-specific antigen-binding peptides modulating its enzyme activity

Prostate-specific antigen (PSA) is a serine protease with highly prostate-s pecific expression. Measurement of PSA in serum is widely used for diagnosi s and monitoring of prostate cancer. PSA dissolves the seminal gel forming after ejaculation. It has been suggested to mediate invasion and metastasis of prostate cancer but also to exert antiangiogenic activity. We have iden tified peptides specific for PSA by screening cyclic phage display peptide libraries. PSA-binding peptides were isolated from four different libraries and produced as a fusion protein with glutathione S-transferase (GST). The phage and fusion proteins were shown to bind to PSA specifically as indica ted by lack of binding to other serine proteinases. A peptide with four cys teines showed the highest affinity for PSA. Zn2+, an inhibitor of PSA activ ity, increased the affinity of the peptides to PSA. The binding specificity was characterized by cross-inhibition using monoclonal anti-PSA antibodies of known epitope specificities. The peptides bound to the same region as m Abs specific for free PSA indicating that they bind close to the active sit e of the enzyme. The peptides enhanced the enzyme activity of PSA against a chromogenic substrate. These results show that peptides binding to PSA and modulating its enzyme activity can be developed by phage display technique . The peptides have the potential to be used for identification of PSA vari ants and for imaging and targeting of prostatic tumors.