title: The effects of several ligands on the potassium-vanadate interaction in the inhibition of the (Na+ + K+)-ATPase and the Na+, K+ pump.
abstract: Inhibition by vanadate of the K+-dependent p-nitrophenylphosphatase activity catalyzed by the (Na+ + K+)-ATPase partially purified from pig kidney showed competitive behavior with the substrate, K+ and Mg2+ acted as cofactors in promoting that inhibition. Ligands which inhibited the K+-dependent p-nitrophenyl phosphate hydrolysis (Na+, nucleotide polyphosphates, inorganic phosphate) protected against inhibition by vanadate. The magnitude of that protection was proportional to the inhibition produced in the absence of vanadate. In the presence of only p-nitrophenyl phosphate and Mg2+, or when the protective ligands were tested alone, the activation of p-nitrophenyl phosphate hydrolysis by K+ followed a sigmoid curve in the presence as well in the absence of vanadate. However, the combination of 100 mM NaCl and 3 mM ATP resulted in a biphasic effect of K+ on the p-nitrophenyl phosphate hydrolysis in the presence of vanadate. After an initial rise at low K+ concentration, the p-nitrophenylphosphatase activity declined at high K+ concentrations; this decline became more pronounced as the vanadate concentration was increased. This biphasic response was not seen when a nonphosphorylating ATP analog was combined with Na+ (which favors the nucleotide binding) or with inorganic phosphate (a requirement for K+ - K+ exchange). Experiments with inside-out resealed vesicles from human red cells showed that in the absence of Na+ plus ATP, K+ promoted vanadate inhibition of p-nitrophenylphosphatase activity in a nonbiphasic manner, acting at cytoplasmic sites. On the other hand, in the presence of Na+ plus ATP, the biphasic response of p-nitrophenyl phosphate hydrolysis is due to K+ acting on extracellular sites. In vanadate-poisoned intact red blood cells, the biphasic response of the ouabain-sensitive Rb+ influx as a function of the external Rb+ concentration failed to develop when there was no Na+ in the extracellular media. In addition, in the absence of extracellular Na+, external Rb+ did not influence the magnitude of inhibition. The present findings indicate that external K+ favors vanadate inhibition by displacing Na+ from unspecified extracellular membrane sites.
mesh_terms: D001693:Biological Transport, Active / Q000187:drug effects; D004910:Erythrocyte Membrane / Q000187:drug effects / Q000201:enzymology*; D004912:Erythrocytes / Q000201:enzymology*; D006801:Humans; D007700:Kinetics; D010710:Phosphates / Q000494:pharmacology; D011188:Potassium / Q000097:blood*; D012413:Rubidium / Q000494:pharmacology; D012964:Sodium / Q000097:blood*; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors* / Q000097:blood; D014638:Vanadates; D014639:Vanadium / Q000494:pharmacology*
chemical_list: D010710:Phosphates; D014639:Vanadium; D014638:Vanadates; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D012413:Rubidium; D011188:Potassium