Reversal of the calcium pump in human red cells

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).

Synthesis of ATP catalyzed by the (Ca2+ + Mg2+)-ATPase from erythrocyte ghosts. Energy conservation in plasma membranes

The (Ca2+ + Mg2+)-ATPase from erythrocyte ghosts catalyzed the hydrolysis of ATP together with the synthesis of ATP or ATP in equilibrium 'Pi exchange. The modulation of the ATPase reaction cycle was controlled by high- and low-affinity calcium-binding sites asymmetrically located on the enzyme. Calmodulin accelerated the reaction cycle in both directions, stimulating the overall turnover of the enzyme. Calcium transport was achieved utilizing optimal conditions for the expression of the ATP in equilibrium Pi exchange system.

ATP synthesis catalyzed by the purified erythrocyte Ca-ATPase in the absence of calcium gradients

The Ca2+-transporting ATPase of erythrocytes was isolated by calmodulin affinity chromatography. The backward reaction of the ATPase was investigated. The phosphorylation of the solubilized enzyme by Pi required Mg and was inhibited by Ca and vanadate in the micromolar concentration range. Significant amounts of phosphoenzyme could be obtained only in a medium containing high dimethyl sulfoxide concentrations (greater than 25%) in order to diminish water activity at the phosphorylation site. The phosphoenzyme formed in this way could not phosphorylate ADP. However, upon addition of Ca2+ ions and dilution of dimethyl sulfoxide in the phosphorylated preparation (water activity jump), a highly reactive phosphoenzyme species was obtained which could transfer phosphate in nearly stoichiometric amounts to ADP to form ATP.

Vanadate inhibition of active Ca2+ transport across human red cell membranes

(1) Vanadate (pentavalent vanadium) inhibits with high affinity (K0.5 = 3 microM) the ATP-dependent Ca2+ efflux in reconstituted ghosts from human red cell. (2) To inhibit Ca2+ efflux vanadate has to have access to the inner surface of the cell membrane (3) The inhibitory effect of vanadate is potentiated by intracellular Mg2+ and by intracellular K+. (4) Ca2+ in the external medium antagonizes the inhibitory effect of vanadate.

Oxygen consumption and phosphate efflux in mammalian non-myelinated nerve fibres

1. A comparison has been made between the efflux of labelled phosphate from the non-myelinated fibres of the desheathed rabbit vagus nerve at 37 degrees C and the corresponding O2 consumption at rest and during activity, and during a variety of experimental interventions. 2. The resting rate constant of phosphate efflux was 2.61 X 10(-3) min-1: electrical stimulation (10 sec-1, 3 min) produced an extra fractional loss of 6.75 X 10(-6) impulse-1. 3. The corresponding resting O2 consumption was 0.484 m-mole x kg-1 impulse-1. 4. Ouabain (100 microM) produced a sustained depression (of about 40%) of the resting O2 consumption, accompanied by a transient fall (of about 14%) in the rate constant of phosphate efflux. 5. Na salicylate (10 mM) or Na arsenate (1 mM) produced a much larger increase in phosphate efflux than in resting O2 consumption. 6. Changing the external phosphate concentration (between 0.02 and 2 mM), addition of acetylcholine (1.7 mM), and addition of lanthanum (20 microM)--all of which are known to affect markedly the phosphate efflux in rabbit non-myelinated fibres--had little or no effect on the resting O2 consumption or, where tested, on the extra O2 consumption with electrical stimulation. 7. Changing the external Ca concentration (between 0.09 and 9 mM) had only minor effects on the O2 consumption (resting and stimulated) and on the rate constant of resting phosphate efflux. 8. It is concluded that although changes in metabolism of the nerve produce changes in the phosphate efflux expected on the basis of the concomitant changes in the internal concentration of inorganic phosphate, the converse is not true; and increases and decreases in the rate constant of phosphate efflux do not necessarily signal the corresponding metabolic changes.

Net ATP synthesis by running the red cell calcium pump backwards

Ca2+ loaded inside-out vesicles from human red blood cells, yielding C2+ into a Ca2+ free medium with 4 mM EGTA, 2 mM ADP and 10 mM phosphate, produced an excess of 14.9 pmoles . min-1 . (mg protein)-1 of ATP compared to controls in which the transmembrane Ca2+ gradient was abolished by the ionophore A 23 187.