title: The Mg2+-dependent ATPase from the erythrocyte plasma membrane of the flounder Platichthys flesus L. General properties and some observations on the steady state kinetics.
abstract: 1. The plasma membrane of the flounder erythrocyte contains a Mg2+-dependent ATPase which is insensitive to ouabain. Mg2+ is part of the substrate, Mg-ATP, and Mg2+ also functions as a nonessential activator. 2. Ca2+, Mn2+ and Co2+ can replace Mg2+ as an activator of ATP hydrolysis. Cu2+ and Zn2+ abolish the Mg-dependent activity. It is shown that Ca2+ and Mg2+ activate the same enzyme and that Mg-ATP and Ca-ATP are mutually competitive. 3. The hydrolysis of ATP obeys Michaelis-Menten kinetics whether or not the Mg2+-ATPase is fully activated by Mg2+. The KM values for Mg-ATP were found to be 0.13 and 0.43 mM respectively. 4. Free ATP acts as a competitive inhibitor towards Mg-ATP and the dissociation constant for the enzyme-ATP complex was determined to be about 0.55 mM. 5. The Mg2+ -ATPase has a low specificity and reacts with the common nucleoside triphosphates GTP, ITP, UTP and CTP. 6. The enzyme has a broad pH optimum ranging from 6.5 to 7.2 and an energy of activation of 13.5 kcal/mol between 0 and 30 degrees C. 7. The effect of some activators and inhibitors of membrane-bound ATPases are reported.
mesh_terms: D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors / Q000097:blood*; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002413:Cations, Divalent; D004789:Enzyme Activation; D004910:Erythrocyte Membrane / Q000201:enzymology*; D004912:Erythrocytes / Q000201:enzymology*; D005260:Female; D005399:Fishes / Q000097:blood*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008297:Male; D009711:Nucleotides / Q000378:metabolism; D013379:Substrate Specificity; D013816:Thermodynamics; D014357:Trypsin
chemical_list: D002413:Cations, Divalent; D009711:Nucleotides; D014357:Trypsin; D000251:Adenosine Triphosphatases; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases