title: Ca2+,Mg2+-ATPase of microsomal membranes from bovine aortic smooth muscle. Identification and characterization of an acid-stable phosphorylated intermediate of the Ca2+,Mg2+-ATPase.
abstract: An acid-stable phosphoprotein was formed in a microsomal membrane fraction isolated from bovine aortic smooth muscle in the presence of Mg2+ + ATP and Ca2+. The microsomes also showed Ca2+ uptake activity. The Ca2+ dependence of phosphoprotein formation and of Ca2+ uptake occurred over the same range of Ca2+ concentration (1-10 microM), and resembled similar findings from rabbit skeletal microsomes. The molecular weight of the phosphorylated protein, estimated by SDS-gel electrophoresis, was approximately 105,000. The phosphoprotein was labile at alkaline pH, and its decomposition was accelerated by hydroxylamine. Half-maximum incorporation of 32P in the presence of 10 microM Ca2+ occurred at 60 nM ATP. The calcium-dependent phosphoprotein formation was not affected by 5 mM NaN3, but was inhibited in a dose-dependent fashion by ADP with a 50% inhibition occurring at 180 microM. Fifty mM MgCl2 was required for the maximal phosphorylation. The rate of phosphoprotein decomposition after adding 2 mM EGTA was accelerated by varying the Mg2+ concentration from 10 microM to 3 mM. Alkaline pH (9.0) slowed the rate of phosphoprotein decay. Optimal Ca2+-dependent phosphoprotein occurred at 15 degrees C over a broad pH range (6.4 to 9.0). The activation energy of EGTA-induced phosphoprotein decomposition was 25.6 kcal/mol between 0 and 16 degrees C and 14.6 kcal/mol between 16 and 30 degrees C. The phosphoprotein formed by aortic microsomes was thus quite similar to the acid-stable phosphorylated intermediate of the Ca2+-transport ATPase of sarcoplasmic reticulum from skeletal and cardiac muscle. These data suggest that the Ca2+-dependent phosphoprotein is a reaction intermediate of the Ca2+,Mg2+-ATPase of the aortic microsomes.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000502:physiology; D000818:Animals; D001011:Aorta / Q000201:enzymology; D001693:Biological Transport, Active; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000302:isolation & purification*; D002417:Cattle; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D006898:Hydroxylamines / Q000494:pharmacology; D007425:Intracellular Membranes / Q000201:enzymology; D008274:Magnesium / Q000494:pharmacology; D015636:Magnesium Chloride; D008861:Microsomes / Q000201:enzymology / Q000378:metabolism; D009131:Muscle, Smooth, Vascular / Q000201:enzymology* / Q000378:metabolism; D010750:Phosphoproteins / Q000302:isolation & purification / Q000378:metabolism; D013696:Temperature
chemical_list: D006898:Hydroxylamines; D010750:Phosphoproteins; D015636:Magnesium Chloride; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium