REVERSIBLE SHIFT BETWEEN 2 STATES OF CA2+-ATPASE IN HUMAN ERYTHROCYTES MEDIATED BY CA2+ AND A MEMBRANE-BOUND ACTIVATOR

The (Ca2+ + Mg2+)-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) from human erythrocytes occurred in two different states, A-state and B-state, depending on the membrane preparation. The A-state showed low maximum activity (V) and the Ca2+ activation was characterized by a Hill coefficient, nH, of about 1 and a Michaelis constant, KCa, about 30 μM. The B-state showed high V, a nH above 1, which indicates positive cooper-activity of Ca2+ activation, and a KCa of about 1 μM. With varying ATP concentrations, both the A-state and the B-state showed negative cooperativity and slightly different values of Km. The B-state was shifted to the A-state when the membranes were exposed to low Ca2+ concentrations. The shift reached 50% at approx. 0.5 μM Ca2+. At the low Ca2+ concentrations an activator was released from the membranes. The A-state was shifted to the B-state when the membranes were exposed to Ca2+ in the presence of the activator. The shift reached 50% at about 30 μM Ca2+. The recovery of high V was time dependent and lasted several minutes. Increasing concentrations of Ca2+ and activator accelerated the recovery. It is suggested that the A-state and the B-state correspond to enzyme free of activator and enzyme associated with activator, respectively. Furthermore, the two states may represent a resting and an active state, respectively, of the calcium pump. © 1978.