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Immunopeptidometric Assay for Enzymatically Active Prostate-Specific Antigen

Ping Wu^a,1, Lei Zhu¹, Ulf-Hkan Stenman¹ and Jari Leinonen¹

¹ Department of Clinical Chemistry, Helsinki University Central Hospital, FIN-00029 Helsinki, Finland.

^aAddress correspondence to this author at: Department of Clinical Chemistry, Biomedicum, A417a, Helsinki University Central Hospital, Haartmaninkatu 8, PB 700, FIN-00029 Helsinki, Finland. Fax 358-9-47171731; e-mail ping.wu{at}helsinki.fi.

Background: Determinations of certain forms of prostate-specific antigen (PSA) have been shown to increase the specificity for prostate cancer (PCa). One such variant, proteolytically active PSA, is a potentially useful tumor marker, but it is not specifically recognized by antibodies. Using phage display libraries, we previously identified a "family" of peptides that bind specifically to active PSA. We used these to develop an immunopeptidometric assay (IPMA) that specifically detects this form of PSA.

Methods: Microtitration plates coated with a PSA antibody were used to capture PSA, and a PSA-binding glutathione S-transferase (GST) fusion peptide was used as a tracer. Bound tracer was detected with an antibody to GST labeled with a europium chelate. PSA isoenzymes with high and low enzymatic activity were used to study binding specificity.

Results: The IPMA detected enzymatically active PSA but not internally cleaved PSA and pro-PSA, which are enzymatically inactive. The assay detected 1–10% of free PSA in serum from PCa patients.

Conclusions: Peptides identified by phage display can be used to develop assays with unique specificities for enzymatically active PSA. IPMA represents a new assay principle with wide potential utility.

The following articles in journals at HighWire Press have cited this article:

				L. Zhu, H. Koistinen, U. Landegren, and U.-H. Stenman Proximity Ligation Measurement of the Complex between Prostate Specific Antigen and {alpha}1-Protease Inhibitor Clin. Chem., September 1, 2009; 55(9): 1665 - 1671. [Abstract] [Full Text] [PDF]

				C. Hekim, J. Leinonen, A. Narvanen, H. Koistinen, L. Zhu, E. Koivunen, V. Vaisanen, and U.-H. Stenman Novel Peptide Inhibitors of Human Kallikrein 2 J. Biol. Chem., May 5, 2006; 281(18): 12555 - 12560. [Abstract] [Full Text] [PDF]

				U.-H. Stenman Improving Immunoassay Performance by Antibody Engineering Clin. Chem., May 1, 2005; 51(5): 801 - 802. [Full Text] [PDF]