title: (Na+ + K+)-adenosine triphosphatase of mammalian brain. Catalytic and regulatory K+ sites distinguishable by selectivity for Li+.
abstract: Li+, K+, and Rb+ are compared as activators of the hydrolysis of p-nitrophenylphosphate by beef brain (Na+ + K+)-ATPase. Previous experiments have established two classes of K+ binding sites that are involved in this reaction: "catalytic sites" have the higher affinity, their occupation is essential for catalytic activity, and they appear to correspond to the extracellular binding sites for active K+ transport; regulatory sites appear to have an allosteric function to "unmask" the catalytic sites. A separate set of Na+-binding regulatory sites bring about a similar unmasking of catalytic sites under phosphorylating conditions. Rb+ can activate p-nitrophenylphosphate hydrolysis both in the presence and absence of Na+ and, thus, can interact effectively with both K+ regulatory and catalytic sites. Li+ does not activate p-nitrophenylphosphate hydrolysis at 25 degrees C in the absence of other monovalent ligands. Li+ does activate when the catalytic sites are exposed by Na+ + ATP. Thus, K+ regulatory and catalytic sites differ in their cation selectivity. At temperatures less than 25 degrees C Li+ is able to activate the phosphatase reaction in the absence of other monovalent ligands: maximum activity occurs at 10-12 degrees C. A plot of the ratio, Li+ activation/K+ activation, as a function of temperature shows that the allosteric transition that unmasks catalytic sites occurs spontaneously with decreasing temperatures.
mesh_terms: D009597:4-Nitrophenylphosphatase / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000201:enzymology*; D002417:Cattle; D007700:Kinetics; D008094:Lithium / Q000494:pharmacology*; D008274:Magnesium / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology*; D011485:Protein Binding; D012413:Rubidium / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013696:Temperature
chemical_list: D008094:Lithium; D009597:4-Nitrophenylphosphatase; D000254:Sodium-Potassium-Exchanging ATPase; D008274:Magnesium; D012413:Rubidium; D011188:Potassium