title: The sided action of Na+ and of K+ on reconstituted shark (Na+ + K+)-ATPase engaged in Na+-Na+ exchange accompanied by ATP hydrolysis. I. The ATP activation curve.
abstract: The ATP hydrolysis dependent Na+-Na+ exchange of reconstituted shark (Na+ + K+)-ATPase is electrogenic with a transport stoichiometry as for the Na+-K+ exchange, suggesting that translocation of extracellular Na+ is taking place via the same route as extracellular K+. The preparation thus offers an opportunity to compare the sided action of Na+ and K+ on the affinity for ATP in a reaction in which the intermediary steps in the overall reaction seems to be the same without and with K+. With Na+ but no K+ on the two sides of the enzyme, the ATP-activation curve is hyperbolic and the affinity for ATP is high. Extracellular K+ in concentrations of 50 microM (the lowest tested) and up gives biphasic ATP activation curves, with both a high- and a low-affinity component for ATP. Cytoplasmic K+ also gives biphasic ATP-activation curves, however, only when the K+ concentration is 50 mM or higher (Na+ + K+ = 130 mM). The different ATP-activation curves are explained from the Albers-Post scheme, in which there is an ATP-dependent and an ATP-independent deocclusion of E2(Na2+) and E2(K2+), respectively, and in which the dephosphorylation of E2-P is rate limiting in the presence of Na+ (but no K+) extracellular, whereas in the presence of extracellular K+ it is the deocclusion of E2(K2+) which is rate limiting.
authors.affiliation: Institute of Biophysics, University of Aarhus, Denmark.
authors.forename: F
authors.initials: F
authors.lastname: Cornelius
authors.forename: J C
authors.initials: JC
authors.lastname: Skou
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D002462:Cell Membrane / Q000201:enzymology; D004284:Dogfish; D007700:Kinetics; D008081:Liposomes; D008954:Models, Biological; D011188:Potassium / Q000494:pharmacology; D011510:Proteolipids / Q000378:metabolism; D012007:Rectum; D012627:Sebaceous Glands / Q000201:enzymology; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
chemical_list: D008081:Liposomes; D011510:Proteolipids; C020485:proteoliposomes; D000255:Adenosine Triphosphate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium