PMID: 21656Aug 1, 1977

Kinetic properties of a magnesium ion- and calcium ion-stimulated adenosine triphosphatase from the outer-membrane fraction of rat spleen mitochondria

The Biochemical Journal
E K Vijayakumar, M J Weidemann


1. Isolated outer membranes from rat spleen mitochondria can be stored in liquid N(2) for several weeks without significant loss of ATPase (adenosine triphosphatase) activity. 2. The ATPase reaction has a broad pH optimum centering on neutral pH, with little significant activity above pH9.0 or below pH5.5. 3. A sigmoidal response of the ATPase activity to temperature is observed between 0 and 55 degrees C, with complete inactivation at 60 degrees C. The Arrhenius plot shows that the activation energy above the transition temperature (22 degrees C) (E(a)=144kJ/mol) is one-third of that calculated for below the transition temperature (E'(a)=408kJ/mol). 4. The outer-membrane ATPase (K(m) for MgATP=50mum) is inactive unless Mg(2+) is added, whereas the inner-membrane ATPase (K(m) for ATP=11mum) is active without added Mg(2+) unless the mitochondria have been depleted of all endogenous Mg(2+) (by using ionophore A23187). 5. The substrate for the outer-membrane ATPase is a bivalent metal ion-nucleoside triphosphate complex in which Mg(2+) (K(m)=50mum) can be replaced effectively by Ca(2+) (K(m)=6.7mum) or Mn(2+), and ATP by ITP. Cu(2+), Co(2+), Sr(2+), Ba(2+), Ni(2+), Cd(2+) and Zn(2+) support very little ATP hydrolysis. 6. Univalent...Continue Reading

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