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

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 Ca2+ activation curves for (Ca + Mg)-ATPase activity and Ca uptake into vesicles were parallel over a Ca2+ range of 0.3-330 micrometer, and both curves have 2 apparent KA values for Ca2+ of 0.45 and 100 micrometer. Addition of a concentrated soluble protein fraction containing predominantly 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.