Significance of different molecular forms of serum PSA. The free, noncomplexed form of PSA versus that complexed to alpha 1-antichymotrypsin.

Prostate specific antigen is an abundant prostate-derived serine protease in the seminal fluid. Low concentrations of the protein are normally released into blood, but above normal concentrations are frequently detected in prostate disease. The PSA-ACT complex is the predominant molecular form of serum PSA (up to approximately 95%) although complex formation is slow between the purified proteins in vitro. A free, noncomplexed form of PSA constitutes a minor fraction of the serum PSA, although serum ACT occurs in large molar excess. The free, noncomplexed form of serum PSA is reported to constitute a significantly smaller proportion of the PSA in untreated prostate cancer than in BPH. The molecular basis for this finding is unclear, but measurements of the proportion of the free form of serum PSA or the proportion of serum PSA-ACT may facilitate discrimination between prostate cancer and BPH.