Background. Angiotensin-converting enzyme (ACE) is expressed by all epithelial cells of the human body. Although the main proportion of ACE is synthesized by the lungs, in men, ACE is also secreted by the testes (testicular form), seminal vesicles and the prostate. In semen, the level of ACE is up to 50 times higher than in blood plasma. The substitution of highly specific epithelial cells of the prostate by tumor cells causes a dramatic decrease in ACE production by the prostate cells.
Aim. To assess the possibility of using prostatic ACE as a new marker of prostate cancer (PCa).
Materials and methods. ACE phenotyping in prostate of patients with PCa and benign prostatic hyperplasia (BPH) included measurement of the activity of two ACE substrates (HHL and ZPHL); calculation of the ratio of their hydrolysis rates (ZPHL/HHL ratio); quantitative assessment of the ACE immunoreactive protein, the ratio of the immunoreactive protein to the ACE activity, as well as the conformation of ACE using a panel of monoclonal antibodies (mAb) to different epitopes of ACE.
Results. ACE activity in tumor cells was markedly reduced and the ratio of immunoreactive ACE to its activity increased. The ratio of the hydrolysis rates of two substrates (ZPHL/HHL ratio) in patients with PCa increased compared to control group, while it was not observed in the vast majority of patients with BPH. There were several tissue samples with a histological diagnosis of BPH, but ACE phenotype was typical for PCa.
Discussion. Since a decrease in ACE activity was found in all patients with PCa, we suggest that it may serve as a reliable and early marker of the tumor development. Changes in the ACE phenotype, which are typical for PCa, but found in patients with BPH, may indicate earlier malignant changes in prostate cells, which are not visible on routine prostate biopsy.
Conclusions. ACE activity and its conformation in prostatic biopsies has the potential to be an early biomarker or a differential criterion for PCa. In PCa, the ACE activity in the prostate is significantly reduced, and the ZPHL/HHL ratio is markedly increased in comparison to control group. However, there were no such changes in patients with BPH. In hyperplastic processes of the prostate (BPH, PCa), there is a change in ACE sialylation, which is accompanied by an increase in the binding of ACE to mAb 3F10 compared to the control group. Patients with negative biopsy result, but properties of prostate ACE, which are typical for PCa, require close follow-up, since they may have an increased risk of subsequent developing PCa. However, due to a small sample of patients, the diagnostic potential of prostate ACE for PCa and BPH requires to be validated in a larger number of patients to confirm its predictive accuracy.

Keywords: prostate cancer, BPH, diagnosis, ACE, ACE phenotyping, ZPHL/HHL ratio