title: ATP/ADP exchange activity of gastric (H+ +K+)-ATPase.
abstract: The ATP/ADP exchange is shown to be a partial reaction of the (H+ +K+)-ATPase by the absence of measurable nucleoside diphosphokinase activity and the insensitivity of the reaction to P1, P5-di(adenosine-5') pentaphosphate, a myokinase inhibitor. The exchange demonstrates an absolute requirement for Mg2+ and is optimal at an ADP/ATP ratio of 2. The high ATP concentration (K0.5=116 microM) required for maximal exchange is interpreted as evidence for the involvement of a low affinity form of nucleotide site. The ATP/ADP exchange is regarded as evidence for an ADP-sensitive form of the phosphoenzyme. In native enzyme, pre-steady state kinetics show that the formation of the phosphoenzyme is partially sensitive to ADP while modification of the enzyme by pretreatment with 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) in the absence of Mg2+ results in a steady-state phosphoenzyme population, a component of which is ADP sensitive. The ATP/ADP exchange reaction can be either stimulated or inhibited by the presence of K+ as a function of pH and Mg2+.
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D002414:Cations, Monovalent; D004228:Dithionitrobenzoic Acid / Q000494:pharmacology; D017506:H(+)-K(+)-Exchanging ATPase; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D000226:Mitochondrial ADP, ATP Translocases / Q000378:metabolism*; D009713:Nucleotidyltransferases / Q000378:metabolism*; D013270:Stomach / Q000201:enzymology*; D013552:Swine
chemical_list: D002414:Cations, Monovalent; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D000226:Mitochondrial ADP, ATP Translocases; D004228:Dithionitrobenzoic Acid; D009713:Nucleotidyltransferases; D000251:Adenosine Triphosphatases; D017506:H(+)-K(+)-Exchanging ATPase