title: A high affinity calcium-stimulated magnesium-dependent ATPase in rat liver plasma membranes. Dependence of an endogenous protein activator distinct from calmodulin.
abstract: A Mg-dependent adenosine triphosphatase (ATPase) activated by submicromolar free Ca2+ was identified in detergent-dispersed rat liver plasma membranes after fractionation by concanavalin A-Ultrogel chromatography. Further resolution by DE-52 chromatography resulted in the separation of an activator from the enzyme. The activator, although sensitive to trypsin hydrolysis, was distinct from calmodulin for it was degraded by boiling for 2 min, and its action was not sensitive to trifluoperazine; in addition, calmodulin at concentrations ranging from 0.25 ng-25 micrograms/assay had no effect on enzyme activity. Ca2+ activation followed a cooperative mechanism (nH = 1.4), half-maximal activation occurring at 13 +/- 5 nM free Ca2+. ATP, ITP, GTP, CTP, UPT, and ADP displayed similar affinities for the enzyme; K0.5 for ATP was 21+/- 9 microM. However, the highest hydrolysis rate (20 mumol of Pi/mg of protein/10 min) was observed at 0.25 mM ATP. For all the substrates tested kinetic studies indicated that two interacting catalytic sites were involved. Half-maximal activity of the enzyme required less than 12 microM total Mg2+. This low requirement for Mg2+ of the high affinity (Ca2+-Mg2+)ATPase was probably the major kinetic difference between this activity and the nonspecific (Ca2+ or Mg2+)ATPase. In fact, definition of new assay conditions, i.e. a low ATP concentration (0.25 mM) and the absence of added Mg2+, allowed us to reveal the (Ca2+-Mg2+)ATPase activity in native rat liver plasma membranes. This enzyme belongs to the class of plasma membrane (Ca2+-Mg2+)ATPases dependent on submicromolar free Ca2+ probably responsible for extrusion of intracellular Ca2+.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000494:pharmacology*; D002135:Calcium-Binding Proteins / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002147:Calmodulin / Q000494:pharmacology*; D002462:Cell Membrane / Q000201:enzymology; D004533:Egtazic Acid / Q000494:pharmacology; D004789:Enzyme Activation; D005260:Female; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D011506:Proteins / Q000502:physiology*; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D002135:Calcium-Binding Proteins; D002147:Calmodulin; D011506:Proteins; D004533:Egtazic Acid; D000251:Adenosine Triphosphatases; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium