Separation of isoforms of serum PSA by isoelectric focusing


  • G.R. Stone

    1. Senior Biomedical Scientist and A.M. Fielding, Principal Clinical Biochemist, Department of Chemical Pathology, Swansea NHS Trust, Morriston Hospital, Swansea, UK
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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.