Characterization of calcium-ion-activated adenosine triphosphatase in the plasma membrane of rat mast cells

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Abstract
The enzyme was activated by Ca2+, Mg2+ and to a lesser extent by Mn2+ and Co2+. Ca2+ alone was necessary for full activity and the further addition of Mg2+ did not have any effect. The chelating agents EGTA [ethanedioxybis(ethylamine)tetra-acetate] and EDTA completely inhibited the reaction. The pH optimum was 7.8. Reduced glutathione, cysteine, dithiothreitol, N-ethylmaleimide, urea, ADP, NaF, increasing ionic strength and Triton X-100 all inhibited the reaction. On subcellular fractionation of mast-cell homogenates by density-gradient centrifugation, the distribution of Ca2+-activated ATPase resembled that of 5''-nucleotidase, but differed from that of the other markers used, suggesting localization in the plasma membrane. Further experiments indicated that the enzyme was present on the external surface of the plasma membrane.