A Gap Between Total Prostate-specific Antigen and the Sum of Free Prostate-specific Antigen Plus α1-Antichymotrypsin-Prostate-specific Antigen in Patients with Prostate Carcinoma but not in Those with Benign Prostate Hyperplasia

Aproximately 70–90% of the total serum prostate-specific antigen (t-PSA) in serum is complexed to α1-antichymotrypsin (ACT), whereas 10–30% of t-PSA is not bound to serum proteins and is called free PSA (f-PSA). The determination of f-PSA and the calculation of the ratio of free to total PSA has proven to be a promising tool for differentiating between prostate cancer (PCa) and benign prostate hyperplasia (BPH), because the ratio is lower in PCa than in BPH (1)(2). Although the determination of ACT-PSA would have the analytical advantage of measuring the major and not the minimal portion and the clinical advantage of measuring the portion of serum PSA apparently directly related to PCa (3)(4)(5), overrecovery problems related to interferences with the ACT-cathepsin G complex for ACT-PSA assays are obstructive to accurate ACT-PSA measurement (6). Thus, it is generally assumed that the sum of f-PSA plus ACT-PSA yields t-PSA, whereas the small amounts of PSA being bound to other proteins such as α1-antitrypsin and protein C may be neglected. However, there is no information concerning whether the occurrence of minor forms of PSA is different between patients with PCa and those with BPH. We have now studied this problem by using a reliable ACT-PSA assay. t-PSA, f-PSA, and ACT-PSA were measured on an ES immunoanalyzer (Boehringer Mannheim GmbH). The principle of the tests is based on the one-step (for t-PSA; cat. no. 1555332) and/or two-step (for f-PSA, cat. no. 1776444; for ACT-PSA, a research assay) sandwich technique. For the determination of t-PSA, samples (calibrators, patient samples, and control materials) are incubated together with biotinylated mouse monoclonal anti-t-PSA antibody (mAb M10) solution and horseradish peroxidase-labeled mouse anti-PSA conjugate in streptavidin-coated plastic tubes. For the determination of f-PSA and ACT-PSA, the newly developed assay, samples …