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  • 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.
    Author: Sørensen PG.
    Journal: Comp Biochem Physiol B; 1983; 75(1):153-61. PubMed ID: 6133679.
    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.
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