Characterization and immunological determination of the complex between prostate-specific antigen and alpha2-macroglobulin.

Prostate-specific antigen (PSA) rapidly forms a complex with alpha2-macroglobulin (A2M) in vitro; however, PSA complexed with A2M (PSA-A2M) is not detected by conventional immunoassays for PSA because it is encapsulated by the A2M. In this study, we show that denaturation of PSA-A2M at high pH renders PSA immunoreactive. Part of the complexed PSA is released in free form and part remains bound to denatured A2M. These forms can be measured by a conventional immunoassay for PSA. This finding enabled us to design a dissociation assay for the detection of PSA-A2M, which was based on the removal of immunoreactive PSA in serum by immunoadsorption, denaturation of PSA-A2M at high pH, and measurement of the released PSA immunoreactivity by a conventional PSA immunoassay. This PSA-A2M assay was calibrated with PSA-A2M formed in vitro. The detection limit of the assay was 0.14 microg/L. Inter- and intraassay coefficients variation were 4-9% and 8-14%, respectively. When purified PSA was incubated with A2M, the loss of PSA immunoreactivity was highly correlated with the PSA-A2M formed, as measured by the dissociation assay for PSA-A2M (r = 0.99; P <0.0001). The concentration of PSA-A2M in serum correlated with that of total PSA both in prostate cancer (PCa) and benign prostatic hyperplasia (BPH); however, the ratio of PSA-A2M in relation to total PSA was significantly higher in BPH than in PCa (P <0.0003). ROC curve analysis suggested that measurement of the ratio of PSA-A2M to total PSA in serum improves the diagnostic accuracy for PCa compared with assays for total PSA only.