title: The (Na + K+)-dependent ATPase. Mode of inhibition of ADP/ATP exchange activity by MgC12.
abstract: Na+-dependent ADP/ATP exchange activity, of a (Na++K+)-dependent ATPase preparation from eel electric organ, was measured in terms of the incorporation of 14C into ATP during incubations with labeled ATP and [14C]ADP. Estimates of initial rates of exchange were possible by keeping changes in nucleotide concentrations, from both exchange and extraneous hydrolytic processes, to less than 10%. Under these conditions, increases in MgC12 concentration, from 0.2 to 3 mM, generally inhibited this exchange activity. The concentrations of free Mg2+, Mg-ATP, and Mg-adp present, with a range of MgC12, ATP, and ADP concentrations, were calculated from measured dissociation constants. Inhibition was associated with Mg-ATP as well as with Mg2+, at concentrations from 0.4 to 1 mM (Mg-ADP, in the same concentration range, probably inhibited also). The affinity of the enzyme for these inhibitors is in fair correspondence with demonstrated affinties for Mg2+, Mg-atp, and Mg-ADP at low affinity substrate sites, measured kinetically. These observations are considered in terms of a dimeric enzyme with high and low affinity substrates sites: ADP/ATP exchange being catalyzed at the high affinity sites, with inhibition occurring through occupancy by Mg2+, Mg-ATP, or Mg-ADP, of the low affinity sites, thereby pulling the reaction process away from those steps involved in exchange.
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism* / Q000494:pharmacology; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D004524:Eels; D004557:Electric Organ / Q000187:drug effects / Q000378:metabolism*; D004789:Enzyme Activation; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D011188:Potassium / Q000494:pharmacology*; D012964:Sodium / Q000494:pharmacology*
chemical_list: D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D012964:Sodium; D000251:Adenosine Triphosphatases; D008274:Magnesium; D011188:Potassium