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Epitope Analysis of a Prostate-specific Antigen (PSA) C-Terminal-specific Monoclonal Antibody and New Aspects for the Discrepancy between Equimolar and Skewed PSA Assays

¹ Department of Medical Science, Cosmo Research Institute for Biomedical Research, 1134-2 Gongendo, Satte, Saitama 340-0193, Japan.

² Laboratory of Hormonal Bioregulation, Centre de L'Université Lavel, Sainte-Foy, Quebec G1V 4G2, Canada.

³ Research & Development Department, International Reagents Corporation, Kobe 651-2241, Japan.
^a Author for correspondence. Fax 81-480-42-3790; e-mail CSM01013@nifty.ne.jp or hiroshi_nagasaki{at}cosmo-oil.co.jp

Background: Immunoassays to measure prostate-specific antigen (PSA) often give different values for the same patient samples, and the calibrators among commercial immunoassays are not interchangeable. We developed three novel assays to quantify the free and complexed forms of PSA in serum.

Methods: We synthesized 46 peptides, which encompassed the entire PSA molecule, and determined the interactions between selected monoclonal antibodies (MAbs) and those peptides or the intact PSA molecule.

Results: MAb PA313 did not cross-react with human glandular kallikrein (hK2), which has 78% amino acid homology to PSA. This MAb bound with K_D = 40 nmol/L to the C-terminal peptide of PSA and distinguished between a synthetic peptide derived from PSA (PSA46A: NH₂-C-R²²⁶KWIKDTIVANP²³⁷-COOH) that differed from one derived from hK2 (PSA46B: NH₂-C-R²²⁶KWIKDTAANP²³⁷-COOH) by a single amino acid. Only the MAb combination of PA313/PA121 showed equimolar reactivity with PSA and with PSA complexed with ₁-antichymotrypsin (PSA-ACT). The free form of PSA (F-PSA) was determined by MAbs PA313/FPA503, and the amount of complexed PSA (C-PSA) in PSA-ACT was determined by ACT/PA313. The total PSA (T-PSA) measured by either of the equimolar assays (PA313/PA121 or Tandem-R) was consistent with the sum of F-PSA and C-PSA. In contrast, T-PSA by a skewed assay (IMx) was higher than F-PSA + C-PSA when the ratio of F-PSA to T-PSA (F/T) was >0.15. T-PSA measured by IMx was nearly equal to F-PSA/0.55 + C-PSA. The coefficient 0.55 reflected different reactivities of the IMx assay with PSA-ACT and PSA.

Conclusion: The discrepancy between the values measured by equimolar and skewed assays depends on the ratio of free to total PSA in the sample.© 1999 American Association for Clinical Chemistry

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

				S. Wesseling, C. Stephan, A. Semjonow, M. Lein, B. Brux, P. Sinha, S. A. Loening, and K. Jung Determination of Non-{alpha}1-Antichymotrypsin-complexed Prostate-specific Antigen as an Indirect Measurement of Free Prostate-specific Antigen: Analytical Performance and Diagnostic Accuracy Clin. Chem., June 1, 2003; 49(6): 887 - 894. [Abstract] [Full Text] [PDF]

				J. Peter, C. Unverzagt, and W. Hoesel Analysis of Free Prostate-specific Antigen (PSA) after Chemical Release from the Complex with {alpha}1-Antichymotrypsin (PSA-ACT) Clin. Chem., April 1, 2000; 46(4): 474 - 482. [Abstract] [Full Text] [PDF]