¹Medizinische Klinik I, Universitäst-Klinik Marienhospital, Ruhr-Universität Bochum, Herne, Germany

Correspondence: Dr H. Schlüter, Univ.-Klinik Marienhospital der Ruhr-Universität Bochum, Hölkeskampring 40, 44625 Herne, Germany.

Abstract

Characterization of p-hydroxy-hippuric acid as an inhibitor of Ca²⁺-ATPase in end-stage renal failure. In patients with end-stage renal failure (ESRF), disturbances of Ca²⁺ metabolism are common, Besides hormonal changes, inhibition of cellular Ca²⁺-ATPase was postulated to contribute to uremic toxicity. We purified a potent inhibitor of the Ca²⁺-ATPase from the ultrafiltrate of patients with ESRF by multiple steps of high-performance liquid chromatography to homogeneity, and identified the isolated inhibitor by mass spectrometric methods as p-hydroxy-hippuric acid. The enzyme used for the Ca²⁺-ATPase assay system was isolated from red blood cells by crossflow filtration. The activity of the Ca²⁺-ATPase was measured spectrophotometrically as the difference in hydrolysis of adenosine 5'-triphosphate (ATP) in the presence and absence of Ca²⁺ with different concentrations of ATP and p-hydroxyhippuric acid. The Ca²⁺-ATPase was found to be inhibited by p-hydroxy-dippuric acid at a concentration above 11.7 p.mol/L. p-Hydroxyhippuric acid inhibited the erythrocyte Ca²⁺-ATPase by reducing V_max and increasing the K_m value. The EC₅₀ (log mol/L; mean SEM) for p-hydroxy-hippuric acid was calculated as 4.82 0.14. In conclusion, p-hydroxy-hippuric acid may play a role in disturbed Ca²⁺ metabolism in end-stage renal failure.