The mode of inhibition by calcium of cell-membrane adenosine-triphosphatase activity

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Abstract
The mechanism of the inhibition of Na+-plus-K+-activated adenosine triphosphatase (ATPase) by Ca was investigated with an enzyme preparation from rabbit kidney cortex and with membranes of human erythrocytes. CaATP, rather than ionic Ca2+, acts as a competitive inhibitor, competing with MgATP in the Na+-plus-K+ activated ATPase reaction. There appears to be no competition between Ca and Na+ for the activation of ATPase. The inhibition of NA+-plus-K+-activated ATPase of cell membranes by low concentrations of CaATP and the consequent need of intact cells to keep the cytoplasmic concentration of Ca low relative to that of magnesium suggests a raison d''etre for the mitochondrial Ca pump.