title: Effect of Na+ and nucleotide on the stability of solubilized Ca(2+)-free Ca-ATPase from scallop sarcoplasmic reticulum.
abstract: In membranous scallop sarcoplasmic reticulum, the alkali metal cations Na+ and K+ and nucleotide together promote dimer formation by the Ca(2+)-free Ca-ATPase and stabilize the enzyme activity [Kalabokis, V. N., Bozzola, J. J., Castellani, L., & Hardwicke, P. M. D. (1991) J. Biol. Chem. 266, 22044-22050]. The dependence of stabilization of the Ca(2+)-free membranous scallop Ca-ATPase on Na+ concentration does not show saturation and may involve several superimposed effects. In order to assess the contribution of dimer toward stabilization, i.e., determine the relative importance of intra- and intermolecular effects on stabilization, the influence of varying Na+ concentration and nucleotide on the decay of enzyme activity of the Ca(2+)-free detergent-solubilized Ca-ATPase was studied. Loss of enzyme activity on removal of Ca2+ with EGTA was associated with loss of capacity for phosphorylation by ATP, a Ca(2+)-dependent function. Stabilization of the soluble Ca(2+)-free enzyme by Na+ showed major differences from that seen with the membranous enzyme. The extent of stabilization of the Ca(2+)-free soluble enzyme by Na+ showed clear saturation with increasing Na+ concentration. In contrast to the Ca(2+)-free membranous enzyme, which is inactivated at pH 7.0 with biphasic first-order kinetics, loss of enzymatic function by the solubilized Ca-ATPase at pH 6.92, 0 degrees C, followed monophasic first-order kinetics.(ABSTRACT TRUNCATED AT 250 WORDS)
authors: Department of Medical Biochemistry, Southern Illinois University, Carbondale 62901-4409.
authors: V N
authors: VN
authors: Kalabokis
authors: M M
authors: MM
authors: Santoro
authors: P M
authors: PM
authors: Hardwicke
mesh_terms: D000255:Adenosine Triphosphate / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000737:chemistry / Q000378:metabolism*; D002942:Circular Dichroism; D004533:Egtazic Acid / Q000494:pharmacology; D004795:Enzyme Stability; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008433:Mathematics; D008962:Models, Theoretical; D008974:Mollusca; D009132:Muscles / Q000201:enzymology; D010766:Phosphorylation; D011487:Protein Conformation; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D012964:Sodium / Q000494:pharmacology*
chemical_list: C005147:5'-adenylyl (beta,gamma-methylene)diphosphonate; D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D012964:Sodium; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; C002630:alpha,beta-methyleneadenosine 5'-triphosphate; D002118:Calcium