Association of (Ca+Mg)‐ATPase activity with ATP‐dependent Ca uptake in vesicles prepared from human erythrocytes

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Abstract
Ghost membranes prepared from human erythrocytes exhibit 2 distinct (Ca+Mg)-ATPase1 activities (Quist and Roufogalis, Arch Biochem Biophys 168:240, 1975). (Ca+Mg)-ATPase activity dependent on a water soluble protein fraction is selectively lost from ghost membranes during preparation of vesicles under low ionic strength, slightly alkaline conditions. In this study, the Ca2+ dependence of the remaining membrane bound (Ca+Mg)-ATPase activity and ATP-dependent Ca uptake in vesicles were compared. The C2+ activation curves for (Ca+Mg)-ATPase activity and Ca uptake into vesicles were parallel over a Ca2+ range of 0.3–330 μM, and both curves have 2 apparent KA values for Ca2+ of 0.45 and 100 μM. Addition of a concentrated soluble protein fraction containing predomintly spectrin to the vesicles increased (Ca+Mg)-ATPase activity over twofold but did not affect the rate of Ca uptake. These findings suggest that the (Ca+Mg)-ATPase activity remaining in vesicles after extraction of the water soluble proteins is associated with the Ca pump whereas (Ca+Mg)-ATPase activity dependent on the soluble protein fraction is associated with some other function.

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