Inhibition of rat synaptic membrane Na+/K+-ATPase and ecto-nucleoside triphosphate diphosphohydrolases by 12-tungstosilicic and 12-tungstophosphoric acid

The in vitro influence of Keggin structure polyoxotungstates, 12-tungstosilicic acid, H4SiW12O40 (WSiA) and 12-tungstophosphoric acid, H3PW12O40 (WPA), and monomer Na2WO4 × 2H2O on rat synaptic plasma membrane (SPM) Na+/K+-ATPase and E-NTPDase activity was studied, whereas the commercial porcine cerebral cortex Na+/K+-ATPase served as a reference. Dose-dependent Na+/K+-ATPase inhibition was obtained for all investigated compounds. Calculated IC50 (10 min) values, in mol/l, for SPM/commercial Na+/K+-ATPase, were: 3.4 × 10−6/4.3 × 10−6, 2.9 × 10−6/3.1 × 10−6 and 1.3 × 10−3/1.5 × 10−3 for WSiA, WPA and Na2WO4 × 2H2O, respectively. In the case of E-NTPDase, increasing concentrations of WSiA and WPA induced its activity reduction, while Na2WO4 × 2H2O did not noticeably affect the enzyme activity at all investigated concentrations (up to 1 × 10−3 mol/l). IC50 (10 min) values, obtained from the inhibition curves, were (in mol/l): 4.1 × 10−6 for WSiA and 1.6 × 10−6 for WPA. Monolacunary Keggin anion was found as the main active molecular species present under physiological conditions (in the enzyme assays, pH 7.4), for the both polyoxotungstates solutions (1 mmol/l), using Fourier transform infrared (FT-IR) and micro-Raman spectroscopy. Additionally, commercial porcine cerebral cortex Na+/K+-ATPase was exposed to the mixture of Na2WO4 × 2H2O and WSiA at different concentrations. Additive inhibition effect was achieved for lower concentrations of Na2WO4 × 2H2O/WSiA (⩽1 × 10−3/4 × 10−6 mol/l), while antagonistic effect was obtained for all higher concentrations of the inhibitors.