Prostate specific antigen predominantly forms a complex with alpha₁‐antichymotrypsin in blood. Implications for procedures to measure prostate specific antigen in serum

Background. Prostate specific antigen (PSA) is a zymogen of a 33‐kilodalton (kD) serine proteinase with extensive similarity to glandular kallikreins. The mechanism responsible for converting the zymogen into active proteinase has not been defined, but active PSA may be irreversibly inactivated in vitro by two of the major proteinase inhibitors in blood: alpha₁‐antichymotrypsin and alpha₂‐macroglobulin. Methods. Procedures have been designed to characterize the different molecular forms of PSA in serum. One assay detects PSA epitopes available on both PSA binding to serine proteinase inhibitors and PSA not binding to a proteinase inhibitor. A second assay only detects PSA in complex with alpha₁‐antichymotrypsin. A third assay mainly detects PSA not binding to a proteinase inhibitor. Results. In serum samples, an 80‐kD to 90‐kD species of PSA in complex with alpha₁‐antichymotrypsin is the predominant molecular form and a minor molecular form of serum PSA was an approximately 30‐kD fraction not binding to a proteinase inhibitor. Conclusions. The benefits of detecting different molecular forms of serum PSA should be investigated regarding possibilities to facilitate differential diagnosis of carcinoma of the prostate (CAP) and benign prostatic hyperplasia (BPH).