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Sir,

Serum total PSA concentrations do not discriminate between benign and malignant prostatic disease. PSA exists in serum as a mixture of protein-bound and ‘free’ (unbound) forms, the detection of which is the basis for newer discriminatory tests such as complexed PSA and free PSA to total PSA ratios. In 1995 Huber et al.[1] showed that isoforms of PSA were detectable in serum using column chromatofocusing. Isoforms of PSA in sera from patients with BPH were differentiated from isoforms in the sera of patients with prostatic cancer. Both Stone [2] and Reeves and Martin [3] using gel isoelectric focusing (IEF), confirmed the presence of distinctive isoforms in sera from prostatic cancer, at least 10 bands being seen with a pI of < 6.5.

Some problems remain in its use; with the substrate Opti-4CN, with a streptavidin amplified stage, Stone [2] found that the sensitivity of the method was 100 µg/L of serum total PSA, i.e. too high for diagnostic use. However, Reeves and Martin [3], using an enhanced chemiluminescent detection probe, were able to detect isoform patterns in sera from patients with no prostatic disease. They also claimed that there was no distinction in pattern between BPH and prostatic cancer sera, although visual examination of their gel patterns shows differences in the intensity of staining of the isoform bands with a pI of < 5.2.

Charrier et al.[4], using two-dimensional IEF, proposed that sera from patients with BPH may contain more cleaved forms of PSA and that more basic forms were also present which could be the precursor zymogen form. Both precursor zymogen forms of PSA [5] and inactive variants of free PSA [6] have been detected in sera from patients with prostate cancer. A ‘clipped’ or cleaved form of PSA, ‘BPSA’, has been suggested as a marker for BPH [7]. These isoforms have now been examined as potential markers to identify or differentiate prostate cancer from benign disease (precursor [8]; intact free PSA [9]; cleaved form [10]). We suggest that IEF is an under-used method which has potential for examining the isoforms of PSA in serum in prostatic disease. We think that its use will provide an insight into PSA metabolism and enable the development of further diagnostic tests.

Acknowledgement: This work was supported by a grant awarded by the Swansea NHS Trust Urology Research Fund.

  • 1
    Huber PR, Schmid H-P, Mattarelli G, Strittmatter B, Van Steenbrugge GJ, Maurer A. Serum free prostate antigen. isoenzymes in benign hyperplasia and cancer of the prostate. Prostate 1995; 27: 2129
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    Stone GR. Development of an Isoelectric Focusing Technique for the Separation and Identification of Serum PSA Isoenzymes. MSc Thesis. University of the West of England, UK, 1999
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    Reeves C, Martin J. Isoelectric focusing of serum psa isoforms in prostatic disease. Proceedings of the Pathology 2000 Meeting, Birmingham UK 2000: 110
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    Charrier J-P, Tournel C, Michel S, Dalbon P, Jolivet M. Two-dimensional electrophoresis of prostate-specific antigen in sera of men with prostate cancer or benign prostate hyperplasia. Electrophoresis 1999; 20: 107581
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    Peter J, Unverzagt C, Krogh TN, Vorm O, Hoesel W. Identification of precursor forms of free prostate-specific antigen in serum of prostate cancer patients by immunosorption and mass spectrometry. Cancer Res 2001; 61: 95762
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    Nurmikko P, Pettersson K, Piironen T, Hugosson J, Lilja H. Discrimination of prostate cancer from benign disease by plasma measurement of intact, free prostate-specific antigen lacking an internal cleavage site at Lys145-Lys146. Clin Chem 2001; 47: 141523
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    Mikolajczyk SD, Millar LS, Wang TJ et al. ‘BPSA’, a specific molecular form of free prostate-specific antigen, is found predominantly in the transition zone of patients with nodular benign prostatic hyperplasia. Urology 2000; 55: 415
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    Sokoll LJ, Chan DW, Mikolajczyk SD et al. Proenzyme PSA for the early detection of prostate cancer in the 2.5–4.0 ng/ml range: preliminary analysis. Urology 2003; 61: 2746
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    Mikolajczyk SD, Marks LS, Partin AW, Rittenhouse HG. Free prostate-specific antigen in serum is becoming more complex. Urology 2002; 59: 797802
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    Linton HJ, Marks LS, Millar LS, Knott CL, Rittenhouse HG, Mikolajczyk SD. Benign prostate- specific antigen (BPSA) in serum is increased in benign prostatic disease. Clin Chem 2003; 49: 2539