Properties of (Mg2 + Ca2+)-ATPase of erythrocyte membranes prepared by different procedures: influence of Mg2+, Ca2+, ATP, and protein activator

Blood cells: an historical account of the roles of purinergic signalling

The involvement of purinergic signalling in the physiology of erythrocytes, platelets and leukocytes was recognised early. The release of ATP and the expression of purinoceptors and ectonucleotidases on erythrocytes in health and disease are reviewed. The release of ATP and ADP from platelets and the expression and roles of P1, P2Y(1), P2Y(12) and P2X1 receptors on platelets are described. P2Y(1) and P2X(1) receptors mediate changes in platelet shape, while P2Y(12) receptors mediate platelet aggregation. The changes in the role of purinergic signalling in a variety of disease conditions are considered. The successful use of P2Y(12) receptor antagonists, such as clopidogrel and ticagrelor, for the treatment of thrombosis, myocardial infarction and stroke is discussed.

Kinetic properties of the purified Ca2+-translocating ATPase from human erythrocyte plasma membrane

The basic kinetic properties of the solubilized and purified Ca2+-translocating ATPase from human erythrocyte membranes were studied. A complex interaction between the major ligands (i.e., Ca2+, Mg2+, H+, calmodulin and ATP) and the enzyme was found. The apparent affinity of the enzyme for Ca2+ was inversely proportional to the concentration of free Mg2+ and H+, both in the presence or absence of calmodulin. In addition, the apparent affinity of the enzyme for Ca2+ was significantly increased by the presence of calmodulin at high concentrations of MgCl2 (5 mM), while it was hardly affected at low concentrations of MgCl2 (2 mM or less). In addition, the ATPase activity was inhibited by free Mg2+ in the millimolar concentration range. Evidence for a high degree of positive cooperativity for Ca2+ activation of the enzyme (Hill coefficient near to 4) was found in the presence of calmodulin in the slightly alkaline pH range. The degree of cooperativity induced by Ca2+ in the presence of calmodulin was decreased strongly as the pH decreased to acid values (Hill coefficient below 2). In the absence of calmodulin, the Hill coefficient was 2 or slightly below over the whole pH range tested. Two binding affinities of the enzyme for ATP were found. The apparent affinity of the enzyme for calmodulin was around 6 nM and independent of the Mg2+ concentration. The degree of stimulation of the ATPase activity by calmodulin was dependent on the concentrations of both Ca2+ and Mg2+ in the assay system.

Inhibition of Ca++-ATPase of rat erythrocyte membranes by k-opioid agonists

The effect of various opioid agonists on Ca++, Mg++-ATPase activity of rat erythrocyte membranes was studied. The Ca++-stimulated component of this enzyme (Ca++-ATPase) showed properties similar to those of the Ca++-pumping ATPase of human erythrocyte membranes, that is, high affinity for Ca++, potentiation by calmodulin, and insensitiveness to Na+. Ethylketocyclazocine (EKC) dose-dependently inhibited this Ca++-ATPase activity at a concentration less than one nM without changing basal Mg++-ATPase activity and this action was reversed by the antagonist Win 44,441. Other opioid agonists mimicked this EKC effect and the rank order of potency was dynorphin (1-13) = greater than EKC greater than levorphanol = morphine = B-endorphin = dihydromorphine greater than leu-enkephalin greater than (D-Ala)2-(D-leu)5-enkephalin (DADL) = morphiceptin. It is concluded that rat erythrocyte membranes possess k-type opioid receptors through which the Ca++-pump is inhibited.

Characterization of the Ca2+- and Mg2+-dependent ATPases in Electrophorus electroplax microsomes

Electrophorus electroplax microsomes were examined for Ca2+- and Mg2+-dependent ATPase activity. In addition to the previously reported low-affinity ATPase, a high-affinity (Ca2+,Mg2+)-ATPase was found. At low ATP and Mg2+ concentrations (200 microM or less), the high-affinity (Ca2+,Mg2+)-ATPase exhibits an activity of 18 nmol Pi mg-1 min-1 with 0.58 microM Ca2+. At higher ATP concentrations (3 mM), the low-affinity Ca2+-ATPase predominates, with an activity of 28 nmol Pi mg-1 min-1 with 1 mM Ca2+. In addition, Mg2+ can also activate the low-affinity ATPase (18 nmol Pi mg-1 min-1). The high-affinity ATPase hydrolyzes ATP at a greater rate than it does GTP, ITP, or UTP and is insensitive to ouabain, oligomycin, or dicyclohexylcarbodiimide inhibition. The high-affinity enzyme is inhibited by vanadate, trifluoperazine, and N-ethylmaleimide. Added calmodulin does not significantly stimulate enzyme activity; rinsing the microsomes with EGTA does not confer calmodulin sensitivity. Thus the high-affinity ATPase from electroplax microsomes is similar to the (Ca2+,Mg2+)-ATPase reported to be associated with Ca2+ transport, based on its affinity for calcium and its response to inhibitors. The low-affinity enzyme hydrolyzes all tested nucleoside triphosphates, as well as diphosphates, but not AMP. Vanadate and N-ethylmaleimide do not inhibit the low-affinity enzymes. The low-affinity enzyme reflects a nonspecific nucleoside triphosphatase, probably an ectoenzyme.

The stoichiometry of the Ca2+ pump in human erythrocyte vesicles: modulation by Ca2+, Mg2+ and calmodulin

Active Ca2+ uptake and the associated (Ca2+ + Mg2+)-ATPase activity were studied under the same conditions in an inside-out vesicle preparation of human red blood cells made essentially by the procedure of Quist and Roufogalis (Journal of Supramolecular Structure 6, 375-381, 1977). Some preparations were treated with 1 mM EDTA at 30 degrees to further deplete them of endogenous levels of calmodulin. As the Ca2+ taken up by the EDTA-treated inside-out vesicles, as well as the non-EDTA treated vesicles, was maintained after addition of 4.1 mM EGTA, the vesicles were shown to be impermeable to the passive leak of Ca2+ over the time course of the experiments. In the absence of added calmodulin, both active Ca2+ uptake and (Ca2+ + Mg2+)-ATPase were sensitive to free Ca2+ over a four log unit concentration range (0.7 microM to 300 microM Ca2+) at 6.4 mM MgCl2. Below 24 microM Ca2+ the stoichiometry of calcium transported per phosphate liberated was close to 2:1, both in EDTA and non-EDTA treated vesicles. Above 50 microM Ca2+ the stoichiometry approached 1:1. When MgCl2 was reduced from 6.4 mM to 1.0 mM, the stoichiometry remained close to 2:1 over the whole range of Ca2+ concentrations examined. In contrast to the results at 6.4 mM MgCl2, the Ca2+ pump was maximally activated at about 2 microM free Ca2+ and significantly inhibited above this concentration at 1 mM MgCl2. Calmodulin (0.5-2.0 microgram/ml) had little effect on the stoichiometry in any of the conditions examined. The possible significance of a variable stoichiometry of the Ca2+ pump in the red blood cell is discussed.

Calmodulin regulation of Ca2+ transport in human erythrocytes

Inside-out vesicles of human erythrocytes took up Ca2+ against an electrochemical gradient. This Ca2+ uptake was dependent on ATP and was stimulated by calmodulin. Treatment of vesicles with 1 mM-EDTA exposed an apparent low-CA2+-affinity Ca2+-transport component with Kd of about 100 microM-Ca2+ or more. This was converted into a single high-Ca2+-affinity transport activity of Kd about 2.5 microM-Ca2+ in the presence of 2 micrograms of calmodulin/ml, showing that the decrease in transport activity after EDTA treatment was reversible. Vesicles not extracted with EDTA showed mainly apparent high-Ca2+-affinity kinetics even in the absence of added calmodulin. Trifluoperazine (30 microM) and calmodulin-binding protein (20 micrograms/ml) inhibited about 50% of the high-affinity Ca2+ uptake and (Ca2+ + Mg2+)-ATPase (Ca2+-activated, Mg2+-dependent ATPase) activity of these vesicles, indicating that the vesicles isolated by the procedure used retained some calmodulin from the erythrocytes. Comparison of Ca2+ transport and (Ca2+ + Mg2+)-ATPase activities in inside-out vesicles yielded a variable Ca2+/P1 stoichiometric ratio. At low free Ca2+ concentrations (below 20 micro-Ca2+), a Ca2+/P1 ration of about 2 was found, whereas at higher Ca2+ concentrations the stoichiometry was approx. 1. The stoichiometry was not significantly altered by calmodulin.

Influence of EGTA on the apparent Ca2+ affinity of Mg2+-dependent, Ca2+-stimulated ATPase in the human erythrocyte membrane

The apparent Ca2+ affinity of Mg2+-dependent, Ca2+-stimulated ATPase (Mg2+ + Ca2+)-ATPase) in human erythrocyte membranes increased with increasing concentrations of EGTA used to buffer free Ca2+. The shift in apparent Ca2+ affinity was seen in membranes prepared by hypotonic hemolysis and in membranes depleted of endogenous activators by EDTA treatment. The effect of EGTA differed from that of calmodulin, as it increased Ca2+ affinity without increasing V. EGTA also increased the apparent Ca2+ affinity when calmodulin was present in the assay medium. ATP-stimulated calcium binding to membranes was greater at 1 mM EGTA than at 0.1 mM EGTA. Similarly to ATPase activation, whereas binding decreased as Ca2+ was raised above 35 microM at 1.0 mM EGTA, binding progressively increased up to 100 microM or more free Ca2+ at 0.1 mM EGTA. EGTA also increased the Ca2+ affinity of Triton X-100-solubilized (Mg2+ + Ca2+)-ATPase, indicating that its effect did not depend on an intact membrane. Analysis of the kinetic data by a computerized nonlinear curve fitting procedure showed that a low Ca2+ affinity state of the enzyme was converted to a high Ca2+ affinity state in the presence of EGTA. The species associated with the enzyme interconversion appeared to be [CaEGTA]2-.

The effect of calmodulin on the phosphoprotein intermediate of Mg2+-dependent Ca2+-stimulated adenosine triphosphatase in human erythrocyte membranes

The effect of calmodulin on the formation and decomposition of the Ca2+-dependent phosphoprotein intermediate of the (Mg2+ + Ca2+)-dependent ATPase in erythrocyte membranes was investigated. In the presence of 60 microM-Ca2+ and 25 microM-MgCl2, calmodulin (0.5-1.5 microgram) did not alter the steady-state concentration of the phosphoprotein, but increased its rate of decomposition. Higher calmodulin concentrations significantly decreased the steady-state concentration of phosphoprotein. Calmodulin (0.5-1.7 microgram) increased Ca2+-transport ATPase activity by increasing the turnover rate of its phosphoprotein intermediate. Increasing the MgCl2 concentration from 25 microM to 250 microM increased the (Mg2+ + Ca2+)-dependent ATPase activity, but decreased the concentration of the phosphoprotein intermediate. Similarly to calmodulin, MgCl2 increased the turnover rate of the Ca2+-transport ATPase complex (about 3-fold). At the higher MgCl2 concentration calmodulin did not further affect the decomposition of the phosphoprotein intermediate. It was concluded that both calmodulin and MgCl2 increase the turnover of the Ca2+-pump by enhancing the decomposition of the Ca2+-dependent phosphoprotein intermediate.

Characterization of calmodulin effects on calcium transport in cardiac microsomes enriched in sarcoplasmic reticulum

Calmodulin prepared from red cell hemolysates was found to significantly increase Ca2+ uptake into cardiac microsomal preparations enriched in sarcoplasic reticulum in a dose-dependent manner. The stimulation of calcium uptake by calmodulin was additive to that stimulation produced by maximal stimulatory concentrations of adenosine cyclic 3',5'-phosphate (cAMP) dependent protein kinase and cAMP, indicating separate mechanisms of action and potentially different modulatory roles for these two systems in the control of calcium transport. K+ significantly decreased calmodulin stimulation of calcium uptake, while in the absence of calmodulin, K+ increased Ca2+ uptake. In the absence of K+, calmodulin increased Ca2+ uptake to levels observed at maximal K+ concentrations without calmodulin present. Na+ produced effects similar to those of K+ in this preparation both in the presence and absence of calmodulin. The effect of calmodulin on the intermediate steps of the (Mg2+,Ca2+)ATPase in cardiac sarcoplasmic reticulum was also investigated. Calmodulin was found to reduce the steady-state level of the Ca2+-dependent phosphoprotein (ECaP) and increase the (Mg2+,Ca2+)ATPase activity of this preparation. Dephosphorylation of ECaP in the presence of Tris-ATP (0.5 mM) was significantly stimulated by calmodulin. These studies indicate that calmodulin stimulates Ca2+ transport in cardiac sarcoplasmic reticulum by increasing the turnover rate of the transport process.

Activator-associated Ca2+-ATPase in erythrocyte membranes from cystic fibrosis patients

Erythrocyte membranes were prepared by a method which should ensure binding of an activator protein (calmodulin) to the calcium dependent membrane ATPase involved in calcium transport. The level of enzyme activity, assayed at optimum conditions, was 5-400 times higher than that found in previous investigations on cystic fibrosis patients. The Ca2+-ATPase activity of the cystic fibrosis patients was reduced by 15% compared to control subjects, whereas patients suffering from chronic pulmonary diseases did not deviate from controls. Even if a reduction of Ca2+ pumping activity occurs in other cells, a 15% decrease could hardly be the only cause of the changed calcium concentrations in secretions from cystic fibrosis patients.

Sickle red cell calcium metabolism: studies on Ca2+-Mg2+ATPase and Ca-binding properties of sickle red cell membranes

Sickle (Hb SS) red cells, preloaded with 45Ca by reversal of hemolysis, exhibit an incomplete 45Ca extrusion, retaining approximately four times more 45Ca than normal cells. Studies indicated that neither the reduction in Hb SS cell Ca2+-Mg2+ ATPase activity (84% of normal) nor the activation of Ca2+-Mg2+ ATPase by calmodulin was sufficiently different from normal cells to attribute a major role to the calcium pump in 45Ca retention. These results suggested that 45Ca retention may reflect an alteration in the calcium-binding properties of Hb SS cell membranes. Low-affinity calcium-binding (freely dissociable) was similar in normal and Hb SS cell membranes. However, the total calcium bound with high-affinity (tightly bound) was four-to-five times greater in Hb SS cell membranes than in normal membranes. These results are compatible with the hypothesis that Hb SS cell 45Ca retention reflects an exchange of a fraction of the total 45Ca with a tightly bound calcium pool, larger in Hb SS cell membranes than in normal membranes. A comparable degree of red cell 45Ca retention, which did not correlate with the reticulocyte population, was observed in other chronic anemic states. These findings suggest that the increased high-affinity calcium binding by the membrane may be a consequence of cellular changes induced by the anemic condition.

A protein activator of Mg2+-dependent, Ca2+-stimulated ATPase in human erythrocyte membranes distinct from calmodulin

Treatment of extensively washed erythrocyte membranes with 0.1mm-EDTA decreased their Mg(2+)-dependent, Ca(2+)-stimulated ATPase [(Mg(2+)+Ca(2+))-ATPase] activity. An activator released by this treatment restored the (Mg(2+)+Ca(2+))-ATPase to its original value in a Ca(2+)-dependent manner. This activator was different from calmodulin, as determined by a number of criteria. It was retained on an Amicon XM-100 ultrafiltration membrane (molecular-weight cut-off 100000); it appeared in the void volume of Sephadex G-100 and G-75 columns; it was not retained on a DEAE-cellulose ion-exchange column at ionic strengths similar to those used to retain calmodulin; and it maximally activated (Mg(2+)+Ca(2+))-ATPase activity less than calmodulin and at a higher Ca(2+) concentration. Like calmodulin, the activator is heat-stable. The activator fraction isolated on a 2.5-15% sucrose gradient in 0.16m-KCl showed a single band of mol.wt. 63000 and no calmodulin on 10%-polyacrylamide/sodium dodecyl sulphate gels. A trace amount of calmodulin was detected in the activator fraction by radioimmunoassay (approx. 10pg/ml of ;ghosts'), but this amount was insufficient to account for the (Mg(2+)+Ca(2+))-ATPase activation. Furthermore, calmodulin-binding protein failed to inhibit (Mg(2+)+Ca(2+))-ATPase activity by more than 10-20% in the membrane preparations from which the activator was extracted. It was concluded that erythrocyte membranes contain a (Mg(2+)+Ca(2+))-ATPase activator that may attenuate the activation of the Ca(2+)-transport ATPase by calmodulin.

(Mg2+ + Ca2+)-ATPase activity in plasma membrane enriched preparations of human skin fibroblasts: decreased activity in fibroblasts derived from cystic fibrosis patients

Plasma membrane enriched preparations obtained from cultured human skin fibroblasts by differential centrifugation and sucrose density centrifugation techniques were found to contain a Mg2+-dependent Ca2+-stimulated ATPase activity. The specific activity of the (Mg2+ + Ca2+)-ATPase present was 4-5-fold higher than that present in crude membrane preparations and 80-100-fold higher than that present in homogenates. The (Mg2+ + Ca2+)-ATPase activity of both crude and plasma membrane enriched preparations of cultured fibroblasts from cystic fibrosis patients was significantly reduced compared to that activity observed in age-matched controls. This study corroborates our previous observations made in crude homogenate preparations of fibroblasts and indicates another cell type where (Mg2+ + Ca2+)-ATPase activity may be altered in cystic fibrosis.

Divalent cation dependent ATPase activities of red blood cell membranes: influence of the oxidation of membrane thiol groups close to each other

An Mg2+-dependent low ATPase activity can be detected in erythrocyte "white membranes," in addition to that of the well known (Ca2+ + Mg2+)-ATPase. The thiol oxidizing agent diamide affects both activities. The oxidation of neighboring thiols seems to leave the mechanism of the (Ca2+ + Mg2+)-ATPase amplification system evoked by Ca2+ largely unaffected. The perturbation caused by diamide in the membranes seems to affect primarily a step of the ATP hydrolysis mechanism that is common to both ATPase activities. The effectiveness of diamide seems to be the same when either Ca2+ and Mg2+, or Mg2+ alone are present during the reagent action. Reduction of disulfide bonds by DTE after diamide treatment restores the (Ca2+ + Mg2+)-ATPase activity but is unable to take the Mg2+-ATPase activity back to the original level. The hypothesis is discussed that the redox state of one (or more than one) couple of --SH close to each other and possibly connected to the active site, may be an important factor in optimizing the efficiency of Ca action on the (Ca2+ + Mg2+)-ATPase.