Successful separation between benign prostatic hyperplasia and prostate cancer by measurement of free and complexed PSA

Prostate-specific antigen (PSA) is a serine protease belonging to the human glandular kallikrein gene family [1–3]. The expression of PSA is mainly androgen dependent, and the detection of very high expression levels is restricted to the prostate tissue, but extraprostatic production at much lower levels has been demonstrated in several other tissues such as normal and malignant breast epithelium, endometrium, and bulbourethral glands [4–10]. PSA is synthesized by the columnar epithelium in the glandular ducts and acini of the prostate, but not by any other cells in prostate tissue. It is secreted at high concentrations (0.2-5mg/mL) into seminal fluid [4–6,11]. PSA is synthesized as an inactive precursor [2,3,12]. Like other glandular kallikreins, the PSA-precursor is processed stepwise by release of a leader peptide followed by liberation of an activation peptide that results in conversion of the zymogen into enzymatically active PSA [12]. This process may occur in parallel with the secretory release from the prostate epithelium and most probably occurs prior to the ejaculatory mixing of secretions from the prostate, seminal vesicles, and epididymis, since PSA is active in ejaculates collected from subjects with defective seminal vesicles and deferent ducts [1]. The protease(s) responsible for processing of the PSA precursor have not been identified. The mature 237-amino-acid form of PSA is a single-chain serine protease with extensive structural similarity to the glandular kallikreins [1,12–14]. However, the substrate specificity is uniquely different from that of the trypsin-like glandular kallikreins and resembles that of chymotrypsin, since PSA catalyzes the hydrolysis of peptide bonds’ carboxy-terminal to residues of tyrosine and leucine [15–17]. Synthetic peptide substrates for chymotrypsin can be used to measure PSA activity, but they are hydrolyzed much less efficiently by PSA than by chymotrypsin and are therefore both nonspecific and insensitive in detecting PSA activity [16]