Calcium ion-dependent dephosphorylation of the Ca2+-ATPase of human red-cells by ADP

Calcium-stimulated ATPase activity in plasma membrane vesicles from pancreatic acinar cells

Plasma membrane vesicles were prepared from rat pancreatic acinar cells using nitrogen cavitation and magnesium precipitation. The vesicles exhibited ATPase activity that was stimulated by submicromolar concentrations of free calcium and was dependent upon the presence of magnesium. This enzyme activity was localized to the cytoplasmic surface of the plasma membrane by two criteria. First, no activity was observed when intact cells replaced the membrane vesicles in the assay. Second, right side-out and inside-out vesicles were separated using concanavalin A sepharose-B. The calcium-stimulated, magnesium-dependent ATPase activity per mg protein in the inside-out fraction was 60% greater than that occurring in the mixed vesicle preparation. These results indicate that the plasma membrane of pancreatic acinar cells has a calcium-stimulated, magnesium-dependent ATPase located on its cytoplasmic surface and that this enzyme is stimulated by submicromolar concentrations of free calcium.

Differential effects of compound 48/80 on the ATPase and phosphatase activities of the Ca2+ pump of red cells

The calmodulin antagonist compound 48/80 inhibits the phosphatase activity of the Ca2+-ATPase lowering its maximum velocity and leaving unaltered its apparent affinity for the substrate regardless on whether phosphatase activity is elicited by Ca2+ plus ATP or by calmodulin. Compound 48/80 has no effect on the Ki for ATP as inhibitor of the phosphatase. These results contrast sharply with the large increase that compound 48/80 induces in the apparent affinity of the regulatory site for the nucleotide of the Ca2+-ATPase and suggest that the active site for phosphatase activity is different from the regulatory site for ATP of the Ca2+-ATPase.

Effects of Ca2+, Mg2+ and calmodulin on the formation and decomposition of the phosphorylated intermediate of the erythrocyte Ca2+-stimulated ATPase

Formation of the phosphorylated intermediate (ECaP) of the human erythrocyte Ca2+-stimulated ATPase (Ca2+-ATPase) was more rapid and reached steady state sooner at 400 microM-Ca2+ than at 1 microM-Ca2+. Calmodulin increased the apparent rate of ECaP formation at 1 microM-Ca2+, whereas at 400 microM-Ca2+, calmodulin decreased the steady-state level of the ECaP without affecting its apparent rate of formation. Removal of endogenous Mg2+ with trans-1,2-diaminocyclohexane-NNN'N'-tetra-acetic acid, which decreased both the velocity and Ca2+-sensitivity of the Ca2+-ATPase, did not alter the Ca2+-sensitivity or the apparent rate of formation of ECaP. ECaP formation at high Ca2+ concentrations was not affected by Mg2+ concentrations as high as 1 mM, and the ECaP could be dephosphorylated by ADP and ATP along either the forward or reverse pathways. The results suggest that high Ca2+ concentrations inhibit Ca2+-ATPase activity by preventing dephosphorylation of the E2P complex, rather than by inhibition of the transformation from E1CaP ('high-Ca2+-affinity' ECaP) to E2CaP ('lower-energy' ECaP).

Fast reversal of the initial reaction steps of the plasma membrane (Ca2+ + Mg2+)-ATPase

Calmodulin-depleted red cell membranes catalyse a Ca2+, Mg2+-dependent ATP-[3H]ADP exchange at 37 degrees C. The Ca2+, Mg2+-dependent exchange, measured at 20 microM CaCl2, 1.5 mM MgCl2, 1.5 mM ADP and 1.5 mM ATP, is comparable to the (Ca2+ + Mg2+)-ATPase activity, between 0.3 and 0.8 mmol/litre original cells per h. EDTA-washed membranes present a Ca2+-dependent ATP-ADP exchange whose rate is not more than 7% of that found in a Mg2+-containing medium, while their Ca2+-dependent ATPase is essentially zero. Addition of 1.5 mM MgCl2 to the medium restores both activities to the levels found with membranes not treated with EDTA. Calmodulin (16 micrograms/ml) produces an eight-fold stimulation of the Ca2+-dependent ATP-ADP exchange, slightly less than it stimulates the Ca2+-dependent ATP hydrolysis. The effect of 1.5 mM MgCl2 on the exchange is greater in the presence than in the absence of calmodulin. It is proposed that the reversal of the initial phosphorylation of the Ca2+ pump, occurring at a fast rate at 37 degrees C, involves a conformational change in the phosphoenzyme. Thus, it would be an ADP-liganded phosphoenzyme of the form EP(ADP) that would experience the fast conformational transition at 37 degrees C. The great difficulty in producing an overall reversal of the Ca2+ pump should then be due to one or more reaction steps later than and including Ca2+ release and dephosphorylation.

Substrate specificity of the erythrocyte Ca2+-ATPase

In the absence of Mg2+, the observed activity of the erythrocyte plasma membrane Ca2+-ATPase is due to the hydrolysis of CaATP at a low rate. In the presence of Mg2+, the activity of the enzyme is much higher, but it is inhibited by high levels of free Mg2+. This inhibition appears to be due to competition of Mg2+ and Ca2+ for a site on the enzyme, rather than for ATP.

Regulatory interaction between calmodulin and ATP on the red cell Ca2+ pump

The interactions between calmodulin, ATP and Ca2+ on the red cell Ca2+ pump have been studied in membranes stripped of native calmodulin or rebound with purified red cell calmodulin. Calmodulin stimulates the maximal rate of (Ca2+ + Mg2+)-ATPase by 5-10-fold and the rate of Ca2+-dependent phosphorylation by at least 10-fold. In calmodulin-bound membranes ATP activates (Ca2+ + Mg2+)-ATPase along a biphasic concentration curve (Km1 approximately 1.4 microM, Km2 approximately 330 microM), but in stripped membranes the curve is essentially hyperbolic (Km approximately 7 microM). In calmodulin-bound membranes Ca2+ activates (Ca2+ + Mg2+)-ATPase at low concentrations (Km less than 0.28 microM) in stripped membranes the apparent Ca2+ affinities are at least 10-fold lower. The results suggest that calmodulin (and perhaps ATP) affect a conformational equilibrium between E2 and E1 forms of the Ca2+ pump protein.

Structure and evolution of calcium-modulated proteins

This review suggests that the intracellular functions of calcium are best understood in terms of calcium's functioning as a second messenger. Further, when functioning as a second messenger, calcium completes its mission not by transferring charge nor by binding to lipid but by binding to specific targets, calcium-modulated proteins. This concept is broadly interpreted to include proteins involved in calcium transport. There is strong evidence that many, if not all, of these calcium-modulated proteins are homologs. Their structures and properties are contrasted to those of extracellular calcium-binding proteins which are not homologous to one another or to the intracellular calcium-modulated proteins. Finally, this line of thought leads to a suggestion of the evolutionary reason for the choice of calcium as the sole inorganic second messenger.

Reversal of the calcium pump in human red cells

Human red cells containing low ATP and high Pi concentrations were suspended in media with and without 2 mM Ca2+, and the incorporation of (32P)Pi into ATP was measured. There was some incorporation whatever the medium, but in every experiment there was an extra incorporation when the cells were in the Ca2+-containing medium. This extra incorporation was abolished by the ionophore A23187, which collapses the Ca2+ concentration gradient across the membranes, or by LaCl3, which blocks the Ca2+ pump. Starved and phosphate-loaded cells also show an uptake of Ca2+ which is not apparent in fresh cells. Results are consistent with the idea that Ca2+-dependent incorporation of Pi into ATP is catalyzed by the Ca2+ pump using energy derived from the Ca2+ concentration gradient.

Activation of partial reactions of the Ca2+-ATPase from human red cells by Mg2+ and ATP

(1) At ATP concentrations up to 30 micrometer addition of 0.5 mM MgCl2 in the reaction mixture increases both the rate of formation and the steady-state level of the phosphoenzyme of the Ca2+-ATPase from human red cell membranes. Under these conditions Mg2+ has no effect on the rate of dephosphorylation, which remains slow. (2) In the presence of Mg2+ the rate of dephosphorylation is increased 5 to 10 times by high (1 mM) concentrations of ATP. (3) Provided Mg2+ has reacted with the phosphoenzyme, acceleration of dephosphorylation by ATP takes place in the absence of Mg2+. This suggests that the role of Mg2+ on dephosphorylation is to convert the phosphoenzyme into a form that, after combination with ATP, reacts rapidly with water. (4) The results are consistent with the idea that combination of ATP at a non-catalytic site is needed for rapid dephosphorylation of the Ca2+-ATPase.