The maximal velocity and the calcium affinity of the red cell calcium pump may be regulated independently

The kinetics of active Ca2+ transport in inside-out red cell membrane vesicles and the Ca2+-ATPase activity of the purified Ca2+ pump were studied and the effects of calmodulin, acidic phospholipids, and controlled trypsinization were compared. In the presence of calmodulin the maximal rate and the apparent affinity of the pump for Ca2+ were greatly increased in both preparations. The lowest value of Km(Ca) was between 0.5 and 0.7 microM depending on the concentration of calmodulin and on the enzyme preparation. Positive cooperativity for Ca2+ activation with a Hill coefficient of 1.6-1.7 was observed in all cases. When acidic phospholipids (phosphatidylinositol 4-phosphate was routinely used) were added to the inside-out vesicles or to the purified enzyme, maximal transport rates equal to those obtained with calmodulin were measured but the Km(Ca) decreased to 0.25 microM and the positive cooperativity disappeared (the Hill coefficient approached 1). Highly active, calmodulin-independent proteolytic fragments of molecular mass of 81 and 76 kDa were produced with controlled trypsinization. When the trypsin treatment was directed to obtain primarily the 81-kDa fragment, the preparation showed characteristics similar to those of the intact Ca2+ pump in the presence of calmodulin; that is, the same Vmax was obtained, the Km(Ca2+) was 0.5-0.6 microM, and the Hill coefficient was about 1.6. Addition of phosphatidylinositol 4-phosphate or allowing further proteolysis to produce the 76-kDa fragment, shifted the Km(Ca) to 0.25 and reduced the Hill coefficient to 1, without changes in the maximal rate. Based on these results it is suggested that the maximal velocity and the Ca2+ affinity on the erythrocyte Ca2+ pump may be regulated independently and that independent polypeptide regions of the enzyme are involved in the regulations.