Calcium ion-dependent phosphorylation of human erythrocyte membranes

The plasma membrane Ca2+ pump catalyzes the hydrolysis of ATP at low rate in the absence of Ca2+

The plasma membrane Ca2+ ATPase catalyzed the hydrolysis of ATP in the presence of millimolar concentrations of EGTA and no added Ca2+ at a rate near 1.5% of that attained at saturating concentrations of Ca2+. Like the Ca-dependent ATPase, the Ca-independent activity was lower when the enzyme was autoinhibited, and increased when the enzyme was activated by acidic lipids or partial proteolysis. The ATP concentration dependence of the Ca2+-independent ATPase was consistent with ATP binding to the low affinity modulatory site. In this condition a small amount of hydroxylamine-sensitive phosphoenzyme was formed and rapidly decayed when chased with cold ATP. We propose that the Ca2+-independent ATP hydrolysis reflects the well known phosphatase activity which is maximal in the absence of Ca2+ and is catalyzed by E(2)-like forms of the enzyme. In agreement with this idea pNPP, a classic phosphatase substrate was a very effective inhibitor of the ATP hydrolysis.

On the mechanism of activation of the plasma membrane Ca2+-ATPase by ATP and acidic phospholipids

The activation of purified and phospholipid-depleted plasma membrane Ca2+-ATPase by phospholipids and ATP was studied. Enzyme activity increased with [ATP] along biphasic curves representing the sum of two Michaelis-Menten equations. Acidic phospholipids (phosphatidylinositol (PI) and phosphatidylserine (PS)) increased Vmax without affecting apparent affinities of the ATP sites. In the presence of 20 microm ATP, phosphorylation of the enzyme preincubated with Ca2+ (CaE1) was very fast (kapp congruent with 400 s-1). vo of phosphorylation of CaE1 increased with [ATP] along a Michaelis-Menten curve (Km of 15 microm) and was phospholipid-independent. Without Ca2+ preincubation (E1 + E2), vo of phosphorylation was also phospholipid-independent, but was slower and increased with [ATP] along biphasic curves. The high affinity component reflected rapid phosphorylation of CaE1, the low affinity component the E2 --> E1 shift, which accelerated to a rate higher than that of the ATPase activity when ATP was bound to the regulatory site. Dephosphorylation of EP did not occur without ATP. Dephosphorylation increased along a biphasic curve with increasing [ATP], showing that ATP accelerated dephosphorylation independently of phospholipid. PI, but not phosphatidylethanolamine (PE), accelerated dephosphorylation even in the absence of ATP. kapp for dephosphorylation was 57 s-1 at 0 microM ATP; that rate was further increased by ATP. Steady-state [EP] x kapp for dephosphorylation varied with [ATP], and matched the Ca2+-ATPase activity measured under the same conditions. Apparently, the catalytic cycle is rate-limited by dephosphorylation. Acidic phospholipids stimulate Ca2+-ATPase activity by accelerating dephosphorylation, while ATP accelerates both dephosphorylation and the conformational change from E2 to E1, further stimulating the ATPase activity.

Quantitation of plasma membrane calcium pump phosphorylated intermediates by electrophoresis

P-ATPases are characterized by the formation of acid-stable phosphorylated intermediates (EP) during their reaction cycle. We have developed a microscale method to determine EP that involves the phosphorylation of the enzyme using [gamma-(32)P]ATP and precipitation with TCA; separation of the sample by SDS-PAGE, and measurement of the enzyme protein and (32)P-labeled EP by digital analysis of both the stained gel and its autoradiogram, respectively. The principal advantages of this method over typical procedures (filtration and centrifugation) are the low amount of enzyme required and the substantial decrease in the blank values and data scattering produced by unspecific phosphorylation and nonquantitative recovering of the enzyme. Application of this new method to a purified preparation of the plasma membrane calcium ATPase (PMCA) results in overcoming the difficulties of measuring EP at high ATP concentrations. A biphasic behavior of the substrate curve for EP was observed when the study was extended to ATP levels within the physiological range. Since, in principle, the method does not require the use of highly purified preparations, it could be helpful for the study of phosphorylated intermediates especially under conditions in which small amounts of protein are available, e.g., mutated variants of P-ATPases.

Magnesium-ions accelerate the formation of the phosphoenzyme of the (Ca2+ + Mg2+)-activated ATPase from plasma membranes by acting on the phosphorylation reaction

Magnesium ions in the reaction medium at 37 degrees C increased up to 222 s-1 the kapp for phosphorylation by ATP of the Ca2(+)-ATPase of pig red cell membranes. This effect was observed after partial proteolysis with trypsin which makes the enzyme behave like the E1 conformer during phosphorylation. These findings lead to the conclusion that Mg2+ increased the rate of phosphorylation of the Ca2(+)-ATPase by acting directly on this reaction. The apparent dissociation constant of Mg2+ for this effect was 44 microM whereas the apparent dissociation constant for Mg2+ to accelerate the shift E2----E1 between conformers measured on the intact enzyme was 50 microM. This suggests that Mg2+ accelerated both reactions from a single class of site.

The calmodulin-stimulated ATPase of maize coleoptiles forms a phosphorylated intermediate

The calmodulin-stimulated ATPase of maize coleoptiles is a 140,000 Mr polypeptide. In the present study, formation of a phosphorylated intermediate by the enzyme is demonstrated. Phosphorylation is sensitive to chasing with unlabelled ATP and to hydroxylamine; lanthanum enhances its intensity while calmodulin enhances phosphorylation in the presence of lanthanum but not in its absence. Ethyleneglycol-bis-(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) inhibits phosphorylation of the purified enzyme, but microsome preparations give a band of phosphorylation of 153,000 Mr in its presence. This latter phosphorylated band was not abolished after a variety of permeabilising treatments in the presence of Triton X-100; phosphorylation of the enzyme was absent when sodium deoxycholate was used as the solubilising detergent. The identity of the 153,000 Mr band is discussed.

Asymmetric effects of divalent cations and protons on active Ca2+ efflux and Ca2+-ATPase in intact red blood cells

The influence of the asymmetric addition of various divalent cations and protons on the properties of active Ca2+ transport have been examined in intact human red blood cells. Active Ca2+ efflux was determined from the initial rate of 45Ca2+ loss after CoCl2 was added to block Ca2+ loading via the ionophore A23187. Ca2+-ATPase activity was measured as phosphate production over 5 min in cells equilibrated with EGTA-buffered free Ca2+ in the presence of A23187. The apparent Ca affinity of active Ca2+ efflux (K0.5 = 30-40 mumol/liter cells) was significantly lower than that measured by the Ca2+-ATPase assay (K0.5 = 0.4 microM). Possible reasons for this apparent difference are considered. Both active Ca2+ efflux and Ca2+-ATPase activity were reduced to less than 5% of maximal levels (20 mmol/liter cells.hr) in Mg2+-depleted cells, and completely restored by reintroduction of intracellular Mg2+. Active Ca2+ efflux was inhibited almost completely by raising external CaCl2 (but not MgCl2) to 20 mM, probably by interaction of Ca2+ at the externally oriented E2P conformation of the pump. Cd2+ was more potent than Ca2+ in this inhibition, while Mn2+ was less potent and 10 mM Ba2+ was without effect. A Ca2+: proton exchange mechanism for active Ca2+ efflux was supported by the results, as external protons (pH 6-6.5) stimulated active Ca2+ efflux at least twofold above the efflux rate at pH 7.8 Ca2+ transport was not affected by decreasing the membrane potential across the red cell.

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.

The reaction of Mg2+ with the Ca2+-ATPase from human red cell membranes and its modification by Ca2+

Media prepared with CDTA and low concentrations of Ca2+, as judged by the lack of Na+-dependent phosphorylation and ATPase activity of (Na+ +K+)-ATPase preparations are free of contaminant Mg2+. In these media, the Ca2+-ATPase from human red cell membranes is phosphorylated by ATP, and a low Ca2+-ATPase activity is present. In the absence of Mg2+ the rate of phosphorylation in the presence of 1 microM Ca2+ is very low but it approaches the rate measured in Mg2+-containing media if the concentration of Ca2+ is increased to 5 mM. The KCa for phosphorylation is 2 microM in the presence and 60 microM in the absence of Mg2+. Results are consistent with the idea that for catalysis of phosphorylation the Ca2+-ATPase needs Ca2+ at the transport site and Mg2+ at an activating site and that Ca2+ replaces Mg2+ at this site. Under conditions in which it increases the rate of phosphorylation, Ca2+ is without effect on the Ca2+-ATPase activity in the absence of Mg2+ suggesting that to stimulate ATP hydrolysis Mg2+ accelerates a reaction other than phosphorylation. Activation of the E1P----E2P reaction by Mg2+ is prevented by Ca2+ after but not before the synthesis of E1P from E1 and ATP, suggesting that Mg2+ stabilizes E1 in a state from which Mg2+ cannot be removed by Ca2+ and that Ca2+ stabilizes E1P in a state insensitive to Mg2+. The response of the Ca2+-ATPase activity to Mg2+ concentration is biphasic, activation with a KMg = 88 microM is followed by inhibition with a Ki = 9.2 mM. Ca2+ at concentration up to 1 mM acts as a dead-end inhibitor of the activation by Mg2+, and Mg2+ at concentrations up to 0.5 mM acts as a dead-end inhibitor of the effects of Ca2+ at the transport site of the Ca2+-ATPase.

Investigation of (Ca2+ + Mg2+)-ATPase phosphoprotein formation in erythrocyte membranes of patients with cystic fibrosis

The (Ca2+ + Mg2+)-ATPase present per mg of protein in erythrocyte membranes of controls and patients with cystic fibrosis (CF) was determined by estimation of the levels of its phosphoprotein. In the presence of 10 mM free Ca2+, which inhibits phosphoprotein decomposition, significantly less phosphoprotein intermediate, ECaP, was found in erythrocyte membranes from CF patients than in age- and sex-matched controls; this correlated with a significant decrease in (Ca2+ + Mg2+)-ATPase activity. These observations indicate a decrease in the number of functional (Ca2+ + Mg2+)-ATPase molecules in erythrocyte membranes from CF patients or an alteration in either the structure of the pump protein or the composition of its environment.

Transmembrane calcium movement in 20,25-diazacholesterol myotonia

An abnormality in myoplasmic Ca2+ regulation has frequently been proposed in 20,25-diazacholesterol (20,25-D) myotonia. We report here the results of several studies of transmembrane Ca2+ movement in this animal model. (i) Physiologic Ca2+ release by intact sarcoplasmic reticulum (SR) was examined in chemically skinned single muscle fibers preloaded in EGTA-buffered Ca2+ solutions (pCa2+7.0 to 6.4). Isometric tension development and Ca2+ release thresholds in response to Cl- or caffeine showed no differences between control and 20,25-D fibers at any pCa2+. (ii) The kinetics of energy-dependent Ca2+ accumulation in purified SR vesicles were followed spectrophotometrically using Ca2+-sensitive dyes. The apparent rate for ATP-dependent Ca2+ uptake and Ca2+ sequestering capacity were unchanged in SR from 20,25-D animals vs. controls. (iii) Surface membrane Ca2+ATPase activity was measured in red blood cell ghosts and sarcolemma. Enzyme Vmax was decreased by 25 to 50% in both membranes in the 20,25-D-treated animals with a compensatory increase in the number of Ca2+ATPase molecules. In general, the SR handling of Ca2+ appears normal in 20,25-D myotonia, although the activity of Ca2+ATPase in membranes with high sterol content may be altered in response to changes in the lipid environment in this model.

Erythrocyte cytosolic free Ca2+ and plasma membrane Ca2+-ATPase activity in cystic fibrosis

The properties of the Ca2+, Mg2+-ATPase of erythrocyte membranes from patients with cystic fibrosis (CF) were extensively compared to that of healthy controls. Following removal of an endogenous membrane inhibitor of the ATPase, activation of the enzyme by Ca2+, calmodulin, limited tryptic digestion or oleic acid, as well as inhibition by trifluoperazine, were studied. The only properties found to be significantly different (CF cells vs controls) were calmodulin-stimulated peak activity (90 vs 101, P less than 0.02) and trypsin-activated peak activity (92 vs 102, P less than 0.02). No significant difference could be measured in the steady-state Ca2+-dependent phosphorylation of CF and control erythrocyte membranes indicating similar numbers of enzyme molecules per cell. The functional state of Ca2+ homeostasis in intact erythrocytes was investigated by measuring the resting cytosolic free Ca2+ levels using quin-2. Both CF and control erythrocytes maintained cytosolic free Ca2+ between 20 to 30 nM. Addition of 50 uM trifluoperazine resulted in an increase in erythrocyte cytosolic free Ca2+ to about 50 nM in both CF and control cells. Estimates of erythrocyte membrane permeability using the steady-state uptake of 45Ca into intact erythrocytes revealed no differences between CF and control cells. These results confirm that there is a small decrease in the calmodulin-stimulated activity of the erythrocyte Ca2+, Mg2+-ATPase in CF. However, this deficit is apparently not large enough to impair the ability of the CF erythrocyte to maintain normal resting levels of cytosolic free Ca2+.

Limited proteolysis of human erythrocyte Ca2+-ATPase in membrane-bound form. Identification of calmodulin-binding fragments

Water-soluble and membrane-bound calmodulin-binding polypeptides formed upon limited proteolysis of erythrocyte ghosts were isolated by means of affinity chromatography. Immune blotting revealed that all isolated fragments originated from Ca2+-ATPase. Among the fragments obtained those having formed an acylphosphate intermediate were identified. The N-terminal residue of purified intact Ca2+-ATPase was shown to be blocked (probably acylated).

Calcium-induced phosphorylations and [125I]calmodulin binding in renal membrane preparations

Calcium-induced phosphorylated intermediates and calmodulin-binding proteins in membrane preparations from the renal cortex were analyzed by SDS-polyacrylamide gel electrophoresis at low pH, protein electroblotting and [125I]calmodulin overlay. Two calcium-induced phosphoproteins were found, with a molecular mass of 135 and 115 kDa, respectively. By comparing different preparations characterized by marker enzymes, it was shown that the 135 kDa phosphoprotein is localized in the basal-lateral fragment of the plasma membrane, whereas the 115 kDa phosphoprotein is more pronounced in preparations containing a high proportion of endoplasmic reticulum. A prominent calmodulin-binding protein comigrated with the 135 kDa phosphoprotein; there was no calmodulin binding to polypeptides in the molecular mass range of the 115 kDa phosphoprotein. Partial proteolysis by trypsin and the effect of 20 microM La2+ on the formation of phosphoproteins before and after trypsinization support the conclusion that the 135 kDa protein can be identified with the plasma membrane calcium pump, whereas the 115 kDa phosphoprotein is the phosphorylated intermediate of a different type of calcium pump probably originating from the endoplasmic reticulum. Calmodulin binding in renal membrane preparations analyzed on Laemmli-type slab gels revealed that there are many calmodulin-binding proteins in our preparations. We have identified one band with the renal calcium pump localized in the basal-lateral membrane. Another calmodulin-binding protein migrating at 108 kDa, is not localized in the basal-lateral membrane and could be one of the calmodulin-binding proteins originating from the cytoskeleton.

The site of action of La3+ in the reaction cycle of the human red cell membrane Ca2+-pump ATPase

Lanthanum (La3+) inhibits the Ca-pump of the red cell by arresting the protein in a phosphorylated form (PI). Similar La3+ concentrations are required to increase the amount of PI and to stop PI-decay. In the presence of La3+ phosphorylation becomes insensitive to Mg2+. PI made in the presence of Mg2+ is not prevented from decaying by subsequent addition of La3+, whereas that made in the absence of Mg2+ is. Taken together, these findings seem to indicate that La3+ blocks the transition between a 1st and a 2nd form of PI.

Demonstration of the phosphorylated intermediates of the Ca2+-transport ATPase in a microsomal fraction and in a (Ca2+ + Mg2+)-ATPase purified from smooth muscle by means of calmodulin affinity chromatography

Ca2+ -dependent hydroxylamine-sensitive phosphorylated proteins can be demonstrated in a microsomal fraction of porcine antrum (stomach) smooth muscle and in a Ca2+ -transport ATPase ((Ca2+ + Mg2+)-ATPase) purified from this tissue by means of a calmodulin affinity technique. These phosphoproteins represent the phosphorylated intermediates of the (Ca2+ + Mg2+)-ATPases. In the (Ca2+ + Mg2+)-ATPase purified from smooth muscle the phosphorylated intermediate has an Mr of 130000 corresponding to the value found for erythrocyte (Ca2+ + Mg2+)-ATPase. In the smooth muscle microsomal fraction this 130 kDa phosphoprotein can also be seen, although its intensity is usually very low compared to a corresponding phosphorylation at Mr 100000. Including La3+ together with Ca2+ during phosphorylation of the microsomes increased selectively the steady state-level of the 130 kDa phosphoprotein over the value of the 100 kDa one. The 100 kDa Ca2+ -dependent phosphoprotein could either indicate the presence of a (Ca2+ + Mg2+)-ATPase of the same type of sarcoplasmic reticulum of skeletal muscle, or it could represent a proteolytic product of the 130 kDa phosphoprotein.

Substrate interactions with brain (Ca + Mg)-ATPase

ATP hydrolysis by a partially purified (Ca + Mg)-ATPase preparation from rat brain increased with substrate concentration in a biphasic fashion, with apparent Km values of 3 microM and 0.1 mM. Ca-dependent phosphorylation, however, had only a single Km value, 3 microM. KCl increased ATPase activity in both concentration ranges, but the K0.5 for KCl decreased from 7 mM to 0.3 mM as the ATP concentration was reduced from 1 mM to 10 microM. The K0.5 for MgCl2 decreased somewhat less, from 3 mM to 0.6 mM with ATP concentrations from 1 mM to 1 microM, but was far lower for steady-state phosphorylation, 0.03 mM. (Ca + Mg)-dependent hydrolysis was not demonstrable with other nucleotide triphosphates or p-nitrophenyl phosphate, and these substances, as well as a reaction product, Pi, were also inhibitors. On the other hand, ADP inhibited at both ATP concentration ranges, and also stimulated dephosphorylation. This pattern of responses to substrate and cations is reminiscent of that of well-characterized transport ATPases, suggesting similar roles and mechanisms.

Effects of divalent metal ions on the calcium pump and membrane phosphorylation in human red cells

In inside-out red cell membrane vesicles ATP-dependent calcium transport is activated by the divalent metal ions Mg2+, Mn2+, Co2+, Ni2+ and Fe2+. This activation is based on the formation of Me2+ -ATP complexes which can serve as energy-donor substrates for the calcium pump, and probably, satisfy the requirement for free Me2+ in this transport process. Higher Me2+ concentrations inhibit calcium transport with various efficiencies. Mn2+ directly competes with Ca2+ at the transport site, while other divalent metal ions investigated have no such effect. The formation of the hydroxylamine-sensitive phosphorylated intermediate (EP) of the red cell membrane calcium pump from [gamma-32P]ATP is induced by Ca2+ while rapid dephosphorylation requires the presence of Mg2+. At higher concentrations Mn2+ and Ni2+ inhibit predominantly the formation of EP, while Co2+ and Fe2+ block dephosphorylation. The possible sites and nature of the divalent metal interactions with the red cell calcium pump are discussed. Hydroxylamine-insensitive membrane phosphorylation in inside-out vesicles from [gamma-32P]ATP is significantly stimulated by Mn2+ and Co2+, as compared to that produced by Mg2+, Fe2+ and Ni2+. Part of this labelling is found in phospholipids, especially in phosphatidylinositol. The results presented for the metal dependency of protein and lipid phosphorylation in red cell membranes may help in the characterization of ATP consumptions directly related to the calcium pump and those involved in various regulatory processes.

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.

Active calcium transport and calcium-dependent membrane phosphorylation in human peripheral blood lymphocytes

The characteristics of the calcium pump were investigated in intact human peripheral blood lymphocytes /PBL/ and in inside-out vesicles prepared from their plasma membranes. Intact PBL were loaded with calcium by a short exposure to A23187 ionophore. After the elimination of the ionophore, calcium-loaded PBL produced an ATP-dependent, external lanthanum sensitive, uphill calcium extrusion. Calcium pump in intact PBL was insensitive to ouabain and /until cellular ATP was provided/ to oligomycin and dinitrophenol. Maximum calcium extrusion rate and the alkali cation sensitivity of the process were similar to those in human red cells. Calcium was partially sequestered by PBL, and this calcium could be released by A23187 ionophore only. Inside-out plasma membrane vesicles prepared from hypotonically lysed PBL showed an ATP + Mg2+-dependent uphill calcium uptake. This calcium transport was insensitive to ouabain, oligomycin, or dinitrophenol, while blocked by lanthanum and quercetin. Calmodulin significantly stimulated calcium pumping in EDTA-washed vesicles. ATP-dependent and -independent calcium uptake rates, respectively, showed different calcium concentration dependences. When PBL membrane vesicles were phosphorylated by gamma 32P-ATP, a calcium-induced, hydroxylamine-sensitive incorporation of 32P was found in 120-150 000 molecular weight proteins. Depending on the way of membrane preparation, the molecular weight of the phosphoprotein was shifted. Similarly to that found in red cell membranes, sensitivity to calmodulin stimulation and partial proteolysis of the calcium pump molecule showed an inverse relationship.

Calcium ions, drug action and the red cell membrane

Intracellular calcium regulates a number of membrane functions in the erythrocyte, including control of shape, membrane lipid composition and cation permeability. Measurement of total red cell calcium has yielded values between 5 and 15 nmol/ml cells, and these low values in part reflect the absence of Ca2+ -containing organelles. Most intracellular Ca2+ is bound and the low cell ionized Ca2+ concentration (approximately 0.2 microM) is maintained by a combination of low membrane permeability and a powerful Ca2+ -pump. This pump has been identified with a (Ca2+ + Mg2+)-stimulated ATPase, and both Ca2+ transport and ATP splitting are stimulated by calmodulin, a low molecular weight protein which binds Ca2+ avidly and activates many Ca2+ -dependent enzymes. Both high and low affinity kinetics for Ca2+ pumping have been demonstrated, depending on the extent of binding of calmodulin to the pump. A stoichiometry of either 1 or 2 Ca2+ ions pumped per ATP molecule split has been shown, and the value may vary with the level of intracellular Ca2+. Phenothiazines, such as chlorpromazine inhibit the Ca2+ -pump by antagonizing the increment in activity produced by calmodulin. The passive inward leak of Ca2+ into erythrocytes can be quantitated by 45Ca2+ uptake into red cells whose Ca2+ -pump has been inhibited. Estimates of the Ca2+ permeability, based on unidirectional influx, yield values many orders of magnitude lower than for nucleated cells. Influx of Ca2+ into human erythrocytes occurs by a facilitated diffusion process, which can be inhibited by phenothiazines and the cinchona alkaloids. Calcium affects many membrane functions including cation permeability, lipid composition and some cytoskeletal interactions which may determine cell shape. Any rise in intracellular Ca2+ activates a specific K+ channel which normally makes little contribution to K+ fluxes. Kinetic studies of this process demonstrate either high or low affinity Ca2+ -activation of K+ efflux, with low affinity of the channel to Ca2+ being the probable state in vivo. Propranolol is the best known activator of Ca2+ -stimulated K+ efflux, although the mechanism of stimulation is unclear. Like other tissues, red cells possess a Ca2+ -activated phosphoinositol phosphodiesterase. Although it has been suggested that the echinocytic shape change induced by Ca2+ is due to the hydrolysis of polyphosphoinositides, it seems more likely that this shape change results from an effect of Ca2+ on the macromolecular interactions of the cytoskeleton. Abnormal Ca2+ permeability may contribute to red cell destruction in a variety of diseases. For example, in sickle cell anemia a large Ca2+ influx occurs when cells are sickled under deoxy conditions, and moreover, the ability of the Ca2+ -pump to extrude the increment of cell Ca2+ is impaired. Thus, red cell Ca2+ is increased 3-7-fold above normal and this may contribute to the short survival of sickle red cells...

Sensitivity to radiations of Mg2+ and (Ca2+ + Mg2+) ATPase activities associated with erythrocyte membrane fragments

Treatment of disrupted erythrocyte membranes with ionizing radiation induces a partial oxidation of -SH groups (as expected from reported data) and a loss of membrane phospholipids as confirmed by the decrease of membrane amino groups. The resulting disturbance of the membrane assembly strongly affects the membrane bound divalent cation-dependent ATPase activities, possibly by causing the formation of a dead-end enzyme complex unable to complete the ATP splitting cycle.

Decrease of apparent calmodulin affinity of erythrocyte (Ca2+ + Mg2+)-ATPase at low Ca2+ concentrations

The calmodulin activation of the (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) in human erythrocyte membranes was studied in the range of 1 nM to 40 microM of purified calmodulin. The apparent calmodulin-affinity of the ATPase was strongly dependent on Ca2+ and decreased approx. 1000-times when the Ca2+ concentration was reduced from 112 to 0.5 microM. The data of calmodulin (Z) activation were analyzed by the aid of a kinetic enzyme model which suggests that 1 molecule of calmodulin binds per ATPase unit and that the affinities of the calcium-calmodulin complexes (CaiZ) decreases in the order of Ca3Z greater than Ca4Z greater than Ca2Z greater than or equal to CaZ. Furthermore, calmodulin dissociates from the calmodulin-saturated Ca2+-ATPase in the range of 10(-7)-10(-6) M Ca2+, even at a calmodulin concentration of 5 microM. The apparent concentration of calmodulin in the erythrocyte cytosol was determined to be 3 to 5 microM, corresponding to 50-80-times the cellular concentration of Ca2+-ATPase, estimated to be approx. 10 nmol/h membrane protein. We therefore conclude that most of the calmodulin is dissociated from the Ca2+-transport ATPase in erythrocytes at the prevailing Ca2+ concentration (probably 10(-7)-10(-8) M) in vivo, and that the calmodulin-binding and subsequent activation of the Ca2+-ATPase requires that the Ca2+ concentration rises to 10(-6)-10(-5) M.

Phosphorylation and dephosphorylation of the Ca2+ pump of human red cells in the presence of monovalent cations

(1) In the presence of calcium ions, K+ increases the rate and the steady state level of phosphorylation of human red cell membranes by [gamma-32P)ATP. The effect of K+ is mimicked by Rb+, NH4+ and Cs+. Electrophoresis experiments suggest that the phosphorus taken up by the membranes in the presence of K+ is bound to the phosphoenzyme of the Ca2+-ATPase. (2) (Ca2+ + K+)-dependent phosphorylation requires Ca2+ and ATP with the same apparent affinity as the phosphorylation of the Ca2+ pump and the effect of K+ on phosphorylation is exerted with the same apparent affinity as that for the activation of the Ca2+-ATPase by K+. (3) The rate of hydrolysis of phosphoenzyme made in the presence of K+ is higher than that made in its absence and K+ increases the ratio Ca2+-ATPase activity/Ca2+-dependent phosphoenzyme concentration. (4) Results suggest that monovalent cations activate the Ca2+ pump because they increase the level and the turnover of the phosphoenzyme of the Ca2+-ATPase.

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-.

Activation and deactivation kinetics of Ca transport in inside-out erythrocyte membrane vesicles

The kinetics of active Ca2+ transport in inside-out vesicles of human erythrocyte membranes has been studied. Hemolysate or purified calmodulin increased the apparent affinity of the Ca2+ transport system for Ca2+ and increased the apparent maximum velocity of Ca2+ transport. However, as Ca2+ concentration was increased above 90 microM in the presence of hemolysate or calmodulin, the extent of activation of Ca2+ fluxes decreased. This deactivation was less prominent if the Mg2+ concentration was increased. These results may be explained by postulating that calmodulin has a site which binds either Mg2+ or Ca2+ and if Mg2+ occupies this site calmodulin activates Ca2+ fluxes, while if Ca2+ occupies this site, calmodulin is unable to activate the transport system.

Some properties of the purified (Ca2+ + Mg2+)-ATPase from human red cell membranes

The (Ca2+ + Mg2+)-ATPase from red cell membranes, purified by means of a calmodulin-containing affinity column according to the method of Gietzen et al. (Gietzen, K., Tejcka, M. and Wolf, H.U. (1980) Biochem. J. 189, 81-88) with either phosphatidylcholine or phosphatidylserine as phospholipid is characterized. The phosphatidylcholine preparation can be activated by calmodulin, while the phosphatidylserine preparation is fully activated without calmodulin. The enzyme shows a biphasic ATP dependence with two Km values of 3.5 and 120 microM. The enzyme is phosphorylated by ATP in the presence of Ca2+ only.

Vanadate inhibition of the Ca2+-ATPase from human red cell membranes

(1) VO3(-) combines with high affinity to the Ca2+-ATPase and fully inhibits Ca2+-ATPase and Ca2+-phosphatase activities. Inhibition is associated with a parallel decrease in the steady-state of the Ca2+-dependent phosphoenzyme. (2) VO3(-) blocks hydrolysis of ATP at the catalytic site. The sites for VO3(-) also exhibit negative interactions in affinity with the regulatory sites for ATP of the Ca2+-ATPase. (3) The sites for VO39-) show positive interaactions in affinity with sites for Mg2+ and K+. This accounts for the dependence on Mg2+ and K+ of the inhibition by VO3(-). Although, with less effectiveness, Na2+ and K+ substitutes for K+ whereas Li+ does not. The apparent affinites for Mg24 and K+ for inhibiton by VO3(-) seem to be less than those for activation of the Ca2+-ATPase. (4) Inhibition by VO3(-) is independent of Ca2+ at concentrations up to 50 microM. Higher concentrations of Ca2+ lead to a progressive release of the inhibitiory effect of VO3(-).

Characterization of the phosphorylated intermediate of the isolated high-affinity (Ca2+ + Mg2+)-ATPase of human erythrocyte membranes

Phosphorylation of solubilized and purified high-affinity (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) of human erythrocyte membranes shows no dependence on cyclic AMP concentration in the range 0.1--1000 microM. Ca2+-dependent phosphoprotein is sensitive to hydroxylamine and molybdate treatment. The phosphate linkage shows maximum stability at low pH values, which is progressively lost as the pH rises, with a shoulder around pH 6. SDS gel electrophoresis of the phosphorylated protein yields a peak which shows relative mobility corresponding to a molecular weight of 145 000 and sensitivity to MgATP-chase and hydroxylamine treatment. This indicates that the phosphoprotein represents the phosphorylated intermediate of the high-affinity (Ca2+ + Mg2+)-ATPase of human erythrocyte membranes.

Phosphorylation of the isolated high-affinity (Ca2+ + Mg2+) ATPase of the human erythrocyte membrane

Solubilized and purified high-affinity (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) of the human erythrocyte membrane (Wolf, H.U., Dieckvoss, G. and Lichtner, R. (1977) Acta Biol. Ger. 36, 847) has been phosphorylated and dephosphorylated under various conditions with respect to Ca2+ and Mg2+ concentrations. In the range, 0.001--100 mM, the rate of phosphorylation was dependent on Ca2+ concentration, showing a maximum at 10 mM. The phosphorylation rate was nearly independent of the Mg2+ concentration within the range 0.01-1 mM. This enzyme has at least three Ca2+ binding sites with different affinities and regulatory functions: (1) binding to the high-affinity site yields phosphorylation of the enzyme; (2) binding to a low-affinity site (Ca2+ concentrations higher than 40 microM) inhibits dephosphorylation or the conformational change which is necessary for dephosphorylation; (3) by binding to an additional low-affinity site, Ca2+ at concentrations higher than 1 mM abolishes negative cooperative behaviour (shown below 1 mM Ca2+) and causes weak positive cooperativity between at least two catalytic subunits in the phosphorylation reaction. The phosphoprotein obtained at Ca2+ concentrations above 1 mM dephosphorylates spontaneously after removal of the divalent metal ions. Addition of Mg2+ accelerates the dephosphorylation rate. Affinities of the inhibitory Ca2+ binding sites are reduced by the binding of substrate or K+.