title: Mg2+ and ATP effects on K+ activation of the Ca2+-transport ATPase of cardiac sarcoplasmic reticulum.
abstract: ATP and the divalent cations Mg2+ and Ca2+ regulated K+ stimulation of the Ca2+-transport ATPase of cardiac sarcoplasmic reticulum vesicles. Millimolar concentrations of total ATP increased the K+-stimulated ATPase activity of the Ca2+ pump by two mechanisms. First, ATP chelated free Mg2+ and, at low ionized Mg2+ concentrations, K+ was shown to be a potent activator of ATP hydrolysis. In the absence of K+ ionized Mg2+ activated the enzyme half-maximally at approximately 1 mM, whereas in the presence of K+ the concentration of ionized Mg2+ required for half-maximal activation was reduced at least 20-fold. Second MgATP apparently interacted directly with the enzyme at a low affinity nucleotide site to facilitate K+-stimulation. With a saturating concentration of ionized Mg2+, stimulation by K+ was 2-fold, but only when the MgATP concentration was greater than 2 mM. Hill plots showed that K+ increased the concentration of MgATP required for half-maximal enzymic activation approx. 3-fold. Activation of K+-stimulated ATPase activity by Ca2+ was maximal at an ionized Ca2+ concentration of approx. 1 microM. At very high concentrations of either Ca2+ or Mg2+, basal Ca2+-dependent ATPase activity persisted, but the enzymic response to K+ was completely inhibited. The results provide further evidence that the Ca2+-transport ATPase of cardiac sarcoplasmic reticulum has distinct sites for monovalent cations, which in turn interact allosterically with other regulatory sites on the enzyme.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D004285:Dogs; D004789:Enzyme Activation / Q000187:drug effects; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology*; D008861:Microsomes / Q000201:enzymology; D009206:Myocardium / Q000201:enzymology*; D011188:Potassium / Q000494:pharmacology*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
chemical_list: D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D011188:Potassium; D002118:Calcium