Prostate-specific antigen (PSA) is a glycoprotein belonging to the kallikrein family of neutral serine proteases.1 Produced by normal, benign, and malignant prostatic epithelial cells, PSA is present in seminal fluid, serum, and urine. Hara et al.4 provided the first description of PSA in 1971, referring to a prominent protein in human seminal plasma as ?-seminoprotein. Shortly thereafter, Li and Beling2 isolated and purified a molecule of Mr 31,000, which they referred to as E1 from human seminal plasma. A more detailed biochemical and electrophoretic analysis was subsequently provided in 1978, when Sensabaugh3 reported that this molecule was glycosylated, was highly immunogenic, and had a molecular mass of 30,000 Da, from whence came the name p30. Wang et al.1 established the unique association of this molecule with the prostate, naming the molecule PSA. Shortly thereafter, Papsidero et al.5, from the same research team, detected PSA in serum and proposed that PSA might be used as a tumor marker. Since then, more than 300 reports have been published on this subject, many of which are discussed in a recent and thorough review by Oesterling6.
The monitoring of PSA concentrations in serum has become indispensable in the management of samples with primary or recurrent prostate cancer7-12. PSA, being a constituent of normal prostatic secretions, is found in the sera of normal samples at concentrations generally <4 ?g/L, as quantified by the Hybritech Tandem-R PSA immunoassay13. This cited �normal range� is not absolute, being influenced by differences among calibrators in the various PSA immunoassays, by the inclusion criteria used to select the subjects, and by population demographics14. Also, increases in PSA to 4-10 ?g/L are not uncommon among samples with benign prostatic hyperplasia (BPH) or prostatitis. Nevertheless, Catalona et al.15 and other scientists16 demonstrated an increase in the cancer detection rate when PSA values were acquired as one aspect of a sample screening protocol.
Measurement of serum PSA concentrations can be an important tool in monitoring samples with prostatic cancer and in determining the potential and actual effectiveness of surgery or other therapies. Recent studies also indicate that PSA measurements can enhance early prostate cancer detection when combined with digital rectal examination (DRE)17.
Enzyme Immunoassay for the Quantitative Determination of Prostate Specific Antigen (PSA) in Human Serum
Principle of the assay: PSA ELISA test is based on the principle of a solid phase enzyme-linked immunosorbent assay.18-20 The assay system utilizes a goat anti-PSA antibody directed against intact PSA for solid phase immobilization (on the microtiter wells). A monoclonal anti-PSA antibody conjugated to horseradish peroxidase (HRP) is in the antibody-enzyme conjugate solution. The test sample is allowed to react first with the immobilized goat antibody at room temperature for 60 minutes. The wells are washed to remove any unbound antigen. The monoclonal anti-PSA-HRP conjugate is then reacted with the immobilized antigen for 60 minutes at room temperature resulting in the PSA molecules being sandwiched between the solid phase and enzyme-linked antibodies. The wells are washed with water to remove unbound-labeled antibodies. A solution of TMB is added and incubated at room temperature for 20 minutes, resulting in the development of a blue color. The color development is stopped with the addition of 1N HCl changing the color to yellow. The concentration of PSA is directly proportional to the color intensity of the test sample. Absorbance is measured spectrophotometrically at 450 nm.
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