Abstract
Ghost membranes prepared from human erythrocytes exhibit 2 distinct (Ca+Mg)‐ATPase^1^ 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 Ca^2+^ dependence of the remaining membrane bound (Ca+Mg)‐ATPase activity and ATP‐dependent Ca uptake in vesicles were compared. The C^2+^ activation curves for (Ca+Mg)‐ATPase activity and Ca uptake into vesicles were parallel over a Ca^2+^ range of 0.3–330 μM, and both curves have 2 apparent K~A~ values for Ca^2+^ 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.