Ca2+-stimulated, Mg2+-dependent ATPase in bovine thyroid plasma membranes

An isolated plasma membrane fraction from bovine thyroid glands contained a Ca2+-stimulated, Mg2+-dependent adenosine triphosphatase ((Ca2+ + Mg2+)-ATPase) activity which was purified in parallel to (Na+ + K+)-ATPase and adenylate cyclase. The (Ca2+ + Mg2+)-ATPase activity was maximally stimulated by approx. 200 microM added calcium in the presence of approx. 200 microM EGTA (69.7 +/- 5.2 nmol/mg protein per min). In EGTA-washed membranes, the enzyme was stimulated by calmodulin and inhibited by trifluoperazine.     

Micromolar Ca2+ concentrations are essential for Mg2+-dependent binding of collagen by the integrin alpha 2beta 1 in human platelets

Integrin receptor alpha(2)beta(1) requires micromolar Ca(2+) to bind to collagen and to the peptide GPC(GPP)(5)GFOGER(GPP)(5)GPC (denoted GFOGER-GPP, where O represents hydroxyproline), which contains the minimum recognition sequence for the collagen-binding alpha(2) I-domain (Knight, C. G., Morton, L. F., Peachey, A. R., Tuckwell, D. S., Farndale, R. W., and Barnes, M. J. (2000) J. Biol. Chem. 275, 35-40). Platelet adhesion to these ligands is completely dependent on alpha(2)beta(1) in the presence of 2 mm Mg(2+). However, we show here that this interaction was abolished in the presence of 25 microm EGTA. Adhesion of Glanzmann's thrombasthenic platelets, which lack the fibrinogen receptor alpha(IIb)beta(3), was also inhibited by micromolar EGTA. Mg(2+)-dependent adhesion of platelets was restored by the addition of 10 microm Ca(2+), but millimolar Ca(2+) was inhibitory. Binding of isolated alpha(2)beta(1) to GFOGER-GPP was 70% inhibited by 50 microm EGTA but, as with intact platelets, was fully restored by the addition of micromolar Ca(2+). 2 mm Ca(2+) did not inhibit binding of isolated alpha(2)beta(1) to collagen or to GFOGER-GPP. Binding of recombinant alpha(2) I-domain was not inhibited by EGTA, nor did millimolar Ca(2+) inhibit binding. Our data suggest that high affinity Ca(2+) binding to alpha(2)beta(1), outside the I-domain, is essential for adhesion to collagen. This is the first demonstration of a Ca(2+) requirement in alpha(2)beta(1) function.     

Calcium requirement for efficient phagocytosis by Dictyostelium discoideum

Extracellular EDTA suppressed in a dose-dependent manner the phagocytosis of yeast particles by Dictyostelium discoideum cells. Activity was restored fully by the addition of Ca(2+), and partially by the addition of Mn(2+)or Zn(2+), but Mg(2+)was ineffective. The pH-sensitive, Ca(2+)-specific chelator EGTA also inhibited phagocytosis at pH 7.5, but not at pH 5, and Ca(2+)restored the inhibited phagocytosis. In contrast, pinocytosis was unaffected by EDTA. Consistent with the idea that Ca(2+)was required for phagocytosis, D. discoideum growth on bacteria was inhibited by EDTA, which was then restored by the addition of Ca(2+). It is concluded that Ca(2+)was needed for efficient phagocytosis by D. discoideum amoebae. A search for Ca(2+)-dependent membrane proteins enriched in phagosomes revealed the presence of p24, a Ca(2+)-dependent cell-cell adhesion molecule-1 (DdCAD-1) that could be the target of the observed EDTA and EGTA inhibition. DdCAD-1-minus cells, however, had normal phagocytic activity. Furthermore, phagocytosis was inhibited by EDTA and rescued by Ca(2+)in the mutant just as in wild type. Thus, DdCAD-1 was not responsible for the observed Ca(2+)-dependence of phagocytosis, indicating that one or more different Ca(2+)-dependent molecule(s) was involved in the process.     

Identification and characterization of "basal" Ca2+-independent Mg2+-dependent ATP-hydrolase enzymatic activity in smooth muscle cell plasma membrane fractions

It is shown, that for correct definition of "basal" Ca(2+)-independent Mg(2+)-dependent ATPase ac-activity (10-13 mmol Pi/hour on 1 mg of protein) in a fraction of uterus smooth muscle cell plasma membranes is necessary to use in medium without calcium of an incubation not only EGTA and digitonin--of the factor of infringement in activity by this subcellular structure, but inhibitors of others Mg(2+)-dependent ATP-hydrolyse enzymatic systems localized as in plasma membrane (Na+, K(+)-ATPase) and in others subcellular frames, first of all, in mitochondria (Mg(2+)-ATPase) and endoplasmic reticulum (transport Ca2+, Mg(2+)-ATPase). In the case of a sacolemal fraction of a smooth muscle the contribution of others Mg(2+)-dependent ATP-hydrolyse systems in a common enzymatic hydrolysis ATP, which unconnected to functioning "basal" Ca(2+)-independent Mg(2+)-dependent ATPase, is very appreciable and achieves 35%. The researches, carried out in the frameworks of definition of initial velocity of enzymatic reaction, have enabled to define its some properties--cationic and anionic specificity, and also sensitivity to action of some inhibitors. It has appeared, that the "basal" Ca(2+)-independent Mg(2+)-dependent ATP-hydrolyse reaction is nonspecific rather both in relation to cations of divalent metals Me2+, and cations of monovalent metals and anions, which were utilized for support of ionic strength. The cations La--antagonist of cations Ca--practically did not influence enzymatic activity. The non-specific inhibitors transport of ATPases--p-chloromercuribenzoate, o-vanadate and eosine Y with a various degree of efficiency inhibited "basal" Ca(2+)-independent Mg(2+)-dependent ATP-hydrolyse reaction. On the basis of the analysis of the own and literary data the conclusion is made that "basal" Ca(2+)-independent Mg(2+)-dependent ATPase of a smooth muscle cell plasma membrane is considerably less sensitive to action of nonspecific inhibitors of the Ca(2+)-transporting systems, than these systems.     

K(+)- and Mg2(+)-dependent hydrolysis of acetyl phosphate catalyzed by the (Ca2+ + Mg2+)-ATPase of sarcoplasmic reticulum

The (Ca2+ + Mg2+)-ATPase of sarcoplasmic reticulum catalyzes the hydrolysis of acetyl phosphate in the presence of Mg2+ and EGTA and is stimulated by Ca2+. The Mg2(+)-dependent hydrolysis of acetyl phosphate measured in the presence of 6 mM acetyl phosphate, 5 mM MgCl2, and 2 mM EGTA is increased 2-fold by 20% dimethyl sulfoxide. This activity is further stimulated 1.6-fold by the addition of 30 mM KCl. In this condition addition of Ca2+ causes no further increase in the rate of hydrolysis and Ca2+ uptake is reduced to a low level. In leaky vesicles, hydrolysis continues to be back-inhibited by Ca2+ in the millimolar range. Unlike ATP, acetyl phosphate does not inhibit phosphorylation by Pi unless dimethyl sulfoxide is present. The presence of dimethyl sulfoxide also makes it possible to detect Pi inhibition of the Mg2(+)-dependent acetyl phosphate hydrolysis. These results suggest that dimethyl sulfoxide stabilizes a Pi-reactive form of the enzyme in a conformation that exhibits comparable affinities for acetyl phosphate and Pi. In this conformation the enzyme is transformed from a Ca2(+)- and Mg2(+)-dependent ATPase into a (K+ + Mg2+)-ATPase.     

The evaluation of the role of endogenous Ca-dependent regulators and protein kinases in activating and inhibiting ion-transport ATPases]

The data on hormonal regulation of ATP-driving ion pumps are contradictory depending on the object used: whether native cells or isolated membranes. To eliminate this contrariety, we studied the ion transporting ATPases in saponin-permeabilized cells in the presence of all endogenous regulators. In permeabilized erythrocytes we obtained the presence of Ca(2+)-dependent activation of Ca(2+)-ATPase by factor(s) not affected by calmodulin antagonist R24571. We obtained also Ca(2+)-dependent activation and inhibition of Na+,K(+)-ATPase. At a concentration of Mg(2+)-ions corresponding to the intracellular level (370 microM), the 0.5-0.7 microM Ca(2+)-activated Na+,K(+)-ATPase (up to 3-fold), whereas the 1-5 microM Ca2+ inhibited it. The cyclic AMP (10(-5) M) inhibited or eliminated Ca(2+)-dependent activation. The decrease in Mg(2+)-ion concentration to 50 microM eliminated the activation and strengthened the inhibition, which reached 100% at the 1-2 microM Ca2+ concentration. The washing of membranes with EGTA eliminated Ca2+ effects on Na+,K(+)-ATPase. These data suggest that the ion-transporting ATPases are activated or inhibited by Ca(2+)-dependent regulators whose activities may be changed by protein kinase catalysed phosphorylation.     

Regulation of reverse uniport activity in mitochondria by extramitochondrial divalent cations. Dependence on a soluble intermembrane space component.

We previously reported that uncoupling Ca2(+)-loaded mitochondria in the presence of [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) produces a partial expression of the permeability transition. From this and related observations, it was proposed that the absence of external free Ca2+ is inhibitory to reverse activity of the Ca2+ uniporter (Igbavboa, U., and Pfeiffer, D.R. (1988) J. Biol. Chem. 263, 1405-1412). By using Sr2(+)-instead of Ca2(+)-loaded mitochondria, the transition is avoided upon treatment with EGTA plus uncoupler, and inhibition of reverse uniport activity can be observed directly. In the presence of physiological Mg2+ concentrations, reverse uniport of Sr2+ is eliminated by external EGTA following a brief period of rapid activity. It is proposed that binding of Mg2+ rather than Sr2+ (Ca2+) at an external site is responsible for the inhibition. Regulation at the external site is modified by the size of the Sr2+ load. EGTA, in the presence of Mg2+, does not inhibit the reverse uniport-dependent release of Sr2+ from mitoplasts. The inhibitory effect can be recovered by adding back the soluble components obtained as the intermembrane space fraction following removal of the outer membrane. The soluble factor could be a regulatory subunit which contains the external cation binding site. Adjustments to uniporter activity due to regulation by the binding site and/or the soluble factor may be slow and may be significant in determining how mitochondria respond to rapid Ca2+ transients in vivo.     

Human NK cells lyse organ-specific endothelial cells: analysis of adhesion and cytotoxic mechanisms

Human organ-specific microvascular endothelial cells (ECs) were established and used in the present study to investigate their susceptibility to natural killer cell line (NKL)-induced lysis. Our data indicate that although IL-2-stimulated NKL (NKL2) cells adhered to the human peripheral (HPLNEC.B3), mesenteric lymph node (HMLNEC), brain (HBrMEC), and lung (HLMEC) and skin (HSkMEC.2) ECs, they significantly killed these cells quite differently. A more pronounced lysis of OSECs was also observed when IL-2-stimulated, purified peripheral blood NK cells were used as effector cells. In line with the correlation observed between adhesion pattern and the susceptibility to NKL2-mediated killing, we demonstrated using different chelators that the necessary adhesion step was governed by an Mg(2+)-dependent, but Ca(2+)-independent, mechanism as opposed to the subsequent Ca(2+)-dependent killing. To identify the cytotoxic pathway used by NKL2 cells, the involvement of the classical and alternate pathways was examined. Blocking of the Ca(2+)-dependent cytotoxicity pathway by EGTA/MgCl(2) significantly inhibited endothelial target cell killing, suggesting a predominant role for the perforin/granzyme pathway. Furthermore, using confocal microscopy, we demonstrated that the interaction between NKL2 effectors and ECs induced cytochrome c release and Bid translocation in target cells, indicating an involvement of the mitochondrial pathway in NKL2-induced EC death. In addition, although all tested cells were sensitive to the cytotoxic action of TNF, no susceptibility to TRAIL or anti-Fas mAb was observed. The present studies emphasize that human NK cell cytotoxicity toward ECs may be a potential target to block vascular injury.     

Voltage- and calcium-dependent inactivation of calcium channels in Lymnaea neurons.

Ca(2+) channel inactivation in the neurons of the freshwater snail, Lymnaea stagnalis, was studied using patch-clamp techniques. In the presence of a high concentration of intracellular Ca(2+) buffer (5 mM EGTA), the inactivation of these Ca(2+) channels is entirely voltage dependent; it is not influenced by the identity of the permeant divalent ions or the amount of extracellular Ca(2+) influx, or reduced by higher levels of intracellular Ca(2+) buffering. Inactivation measured under these conditions, despite being independent of Ca(2+) influx, has a bell-shaped voltage dependence, which has often been considered a hallmark of Ca(2+)-dependent inactivation. Ca(2+)-dependent inactivation does occur in Lymnaea neurons, when the concentration of the intracellular Ca(2+) buffer is lowered to 0.1 mM EGTA. However, the magnitude of Ca(2+)-dependent inactivation does not increase linearly with Ca(2+) influx, but saturates for relatively small amounts of Ca(2+) influx. Recovery from inactivation at negative potentials is biexponential and has the same time constants in the presence of different intracellular concentrations of EGTA. However, the amplitude of the slow component is selectively enhanced by a decrease in intracellular EGTA, thus slowing the overall rate of recovery. The ability of 5 mM EGTA to completely suppress Ca(2+)-dependent inactivation suggests that the Ca(2+) binding site is at some distance from the channel protein itself. No evidence was found of a role for serine/threonine phosphorylation in Ca(2+) channel inactivation. Cytochalasin B, a microfilament disrupter, was found to greatly enhance the amount of Ca(2+) channel inactivation, but the involvement of actin filaments in this effect of cytochalasin B on Ca(2+) channel inactivation could not be verified using other pharmacological compounds. Thus, the mechanism of Ca(2+)-dependent inactivation in these neurons remains unknown, but appears to differ from those proposed for mammalian L-type Ca(2+) channels.     

Phosphatidylserine-dependent adhesion of T cells to endothelial cells

Phosphatidylserine (PS) was exposed at the surface of human umbilical vein endothelial cells (HUVECs) and cultured cell lines by agonists that increase cytosolic Ca(2+), and factors governing the adhesion of T cells to the treated cells were investigated. Thrombin, ionophore A23187 and the Ca(2+)-ATPase inhibitor 2, 5-di-tert-butyl-1,4-benzohydroquinone each induced a PS-dependent adhesion of Jurkat T cells. A23187, which was the most effective agonist in releasing PS-bearing microvesicles, was the least effective in inducing the PS-dependent adhesion of Jurkat cells. Treatment of ECV304 and EA.hy926 cells with EGTA, followed by a return to normal medium, resulted in an influx of Ca(2+) and an increase in adhering Jurkat cells. Oxidised low-density lipoprotein induced a procoagulant response in cultured ECV304 cells and increased the number of adhering Jurkat cells, but adhesion was not inhibited by pretreating ECV304 cells with annexin V. PS was not significantly exposed on untreated Jurkat cells, as determined by flow cytometry with annexin V-FITC. However, after adhesion to thrombin-treated ECV304 cells for 10 min followed by detachment in 1 mM EDTA, there was a marked exposure of PS on the Jurkat cells. Binding of annexin V-FITC to the detached cells was inhibited by pretreating them with unlabelled annexin V. Contact with thrombin-treated ECV304 cells thus induced the exposure of PS on Jurkat cells and, as Jurkat cells were unable to adhere to thrombin-treated ECV304 cells in the presence of EGTA, the adhesion of the two cell types may involve a Ca(2+) bridge between PS on both cell surfaces. The number of T cells from normal, human peripheral blood that adhered to ECV304 cells was not increased by treating the latter with thrombin. However, findings made with several T cell lines were generally, but not completely, consistent with the possibility that adhesion to surface PS on endothelial cells may be a feature of T cells that express both CD4(+) and CD8(+) antigens. Possible implications for PS-dependent adhesion of T cells to endothelial cells in metastasis, and early in atherogenesis, are discussed.     

The membrane glycoprotein Ia-IIa (VLA-2) complex mediates the Mg++- dependent adhesion of platelets to collagen

We have purified the platelet membrane glycoprotein Ia-IIa complex by detergent solubilization and sequential affinity chromatography on Concanavalin A-Sepharose and collagen-Sepharose. The complex, which is identical to the VLA-2 complex of lymphocytes and other cells and contains subunits of 160 and 130 kD on SDS-PAGE, was labeled with 125I and incorporated into phosphatidyl choline liposomes. The liposomes, like intact platelets, adhered to collagenous substrates in an Mg++-dependent manner with a K'a(Mg++) of 3.5 mM. Little adhesion of the liposomes to collagen occurred when Mg++ was replaced by Ca++ or EDTA. Calcium ions inhibited the Mg++-dependent adhesion with a K'i(Ca++) of 5.5 mM. Liposomes containing the Ia-IIa complex adhered to substrates composed of types I, II, III, and IV collagen, but did not effectively adhere to substrates composed of type V collagen or gelatin. Adhesion to collagen was specific. The liposomes did not adhere to fibronectin, vitronectin, laminin, thrombospondin, fibrinogen, or von Willebrand factor substrates. The monoclonal antibody P1H5, which specifically immunoprecipitated the Ia-IIa complex, also specifically inhibited the Mg++-dependent adhesion of both platelets and Ia-IIa-containing liposomes to collagen substrates. These findings provide additional evidence that the platelet membrane Ia-IIa complex is the mediator of Mg++-dependent platelet adhesion to collagen and suggest that the VLA-2 complex may also function as an Mg++-dependent collagen receptor in other cells.     

Extracellular Ca2+ modulates leukocyte function-associated antigen-1 cell surface distribution on T lymphocytes and consequently affects cell adhesion

Transition of leukocyte function-associated antigen-1 (LFA-1), from an inactive into an activate state depends on the presence of extracellular Mg2+ and/or Ca2+ ions. Although Mg2+ is directly involved in ligand binding, the role of Ca2+ in LFA-1 mediated adhesion remained obscure. We now demonstrate that binding of Ca2+, but not Mg2+, directly correlates with clustering of LFA-1 molecules at the cell surface of T cells, thereby facilitating LFA-1-ligand interaction. Using a reporter antibody (NKI-L16) that recognizes a Ca(2+)-dependent epitope on LFA-1, we found that Ca2+ can be bound by LFA-1 with different strength. We noticed that weak binding of Ca2+ is associated with a dispersed LFA-1 surface distribution on T cells and with non- responsiveness of these cells to stimuli known to activate LFA-1. In contrast, stable binding of Ca2+ by LFA-1 correlates with a patch-like surface distribution and vivid ligand binding after activation of LFA- 1. Mg(2+)-dependent ligand binding does not affect binding of Ca2+ by LFA-1 as measured by NKI-L16 expression, suggesting that Mg2+ binds to a distinct site, and that both cations are important to mediate adhesion. Only Sr2+ ions can replace Ca2+ to express the L16 epitope, and to induce clustering of LFA-1 at the cell surface. We conclude that Ca2+ is involved in avidity regulation of LFA-1 by clustering of LFA-1 molecules at the cell surface, whereas Mg2+ is important in regulation of the affinity of LFA-1 for its ligands.     

Characterization of hyaluronan-binding proteins on guinea pig polymorphonuclear leukocytes: possible involvement of complement receptor type 3 (CR3, CD11b/CD18) in the hyaluronan-leukocyte interaction

Hyaluronan (HA), a high-molecular-weight glycosaminoglycan ubiquitously present in the extracellular matrices (ECMs) of animals, plays important roles in ECM organization and cell behavior through binding to hyaluronan-binding proteins (HABPs). We previously reported that HA has anti-inflammatory effects on guinea pig phagocytes, although the nature of guinea pig HABPs was unknown. In this study, we characterized guinea pig HABPs on peritoneal polymorphonuclear leukocytes (PMNs) and blood neutrophils by flow cytometry and affinity chromatography. It was found that PMNs express diverse HABPs with different molecular weights. These HABPs maximally bound with HA over a wide pH range (6-8), and recognized HAs as small as the pentadisaccharide units of d-glucuronic acid and N-acetyl-d-glucosamine. Furthermore, they could be divided into Mg(2+)-dependent and Ca(2+)/Mg(2+)-independent groups. Interestingly, two proteins in the Mg(2+)-dependent group were found to be the two subunits of complement receptor type 3 (CR3, CD11b/CD18). Unlike PMNs, blood neutrophils expressed several functionally inactive HABPs. Among these inactive HABPs, Mg(2+)-dependent proteins including CR3 but not Ca(2+)/Mg(2+)-independent proteins were activated on phorbol ester-stimulation. These results show the existence of diverse HABPs on guinea pig neutrophils and the cell activation-dependent activation of HABPs. It is also suggested that the CR3-HA interaction is possibly involved in the regulation of neutrophil function.     

Expanding the range of free calcium regulation in biological solutions

Many biological systems use ethylene glycol bis (beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) to regulate the free calcium concentration ([Ca(2+)](free)) in the presence of physiological levels of free Mg(2+) ([Mg(2+)](free)). Frequently, it is necessary to work at [Ca(2+)](free) beyond EGTA's buffering capabilities. Therefore, we have developed methods to extend the buffering range by adding nitrilotriacetic acid (NTA) to solutions containing EGTA. This extension results from NTA having a lower K'(dCa) than EGTA. Such equilibria are solved by pCa Calculator, a computer program designed to aid in the study of Ca(2+)-dependent physiological processes while accounting for the effects of pH, temperature, and ionic strength. With multiple chelators and pH buffers from which to choose, pCa Calculator calculates the total concentration of each species required to achieve specified free concentrations of Ca(2+), ATP, and Mg(2+). The program is intuitive, user-friendly, and flexible enough to fix or vary the [Mg-ATP(2-)] and ionic strength. Moreover, it can account for increases in experimental volume from calcium addition. A comparative analysis is reported for testing solutions in the presence and absence of NTA by measuring the calcium binding affinity of fluorescent cardiac troponin C. These findings demonstrate that EGTA, when used in conjunction with NTA, improves and expands the regulation of free calcium in solution.     

Identification of the Ca(2+)-dependent calmodulin-binding region of chromogranin A.

Chromogranin A (CGA), the most abundant protein in bovine adrenal chromaffin granules, is a high-capacity, low-affinity Ca(2+)-binding protein found in most neuroendocrine cells, and binds calmodulin (CaM) in a Ca(2+)-dependent manner. The binding of chromogranin A to calmodulin was determined by measuring the intrinsic tryptophan fluorescence of chromogranin A in the presence and absence of Ca2+. Binding was specifically Ca(2+)-dependent; neither Mg2+ nor Mn2+ could substitute for Ca2+. Chelation of Ca2+ by EGTA completely eliminated the chromogranin A-calmodulin interaction. CaM binding was demonstrated by a synthetic CGA peptide representing residues 40-65. When the CGA peptide and CaM were mixed in the presence of 15 mM CaCl2, the intrinsic tryptophan fluorescence emission underwent a substantial blue-shift, shifting from 350 to 330 nm. Like the intact CGA, the peptide-CaM binding was specifically Ca(2+)-dependent, and neither Mg2+ nor Mn2+ could induce the binding. Calmodulin bound both to CGA and to the synthetic CGA peptide with a stoichiometry of one to one. The dissociation constants (Kd) determined by fluorometric titration were 13 nM for the peptide-CaM binding and 17 nM for intact CGA-CaM binding. The Kd values are comparable to those (approximately 10(-9) M) of other CaM-binding proteins and peptides, demonstrating a tight binding of CaM by CGA. The CaM-binding CGA residues 40-65 are 100% conserved among all the sequenced CGAs in contrast to 50-60% conservation found in the entire sequence, implying essential roles of this region.     

Solution structures of the adhesion molecule DdCAD-1 reveal new insights into Ca(2+)-dependent cell-cell adhesion

DdCAD-1 is a novel Ca(2+)-dependent cell adhesion molecule that lacks a hydrophobic signal peptide and a transmembrane domain. DdCAD-1 is expressed by the social amoeba Dictyostelium discoideum at the onset of development. It is synthesized as a soluble protein and then transported to the plasma membrane by contractile vacuoles. Here we describe the novel features of the solution structures of Ca(2+)-free and Ca(2+)-bound monomeric DdCAD-1. DdCAD-1 contains two beta-sandwich domains, belonging to the betagamma-crystallin and immunoglobulin fold classes, respectively. Whereas the N-terminal domain has a major role in homophilic binding, the C-terminal domain tethers the protein to the cell membrane. From structural and mutational analyses, we propose a model for the Ca(2+)-bound DdCAD-1 dimer as a basis for understanding DdCAD-1-mediated cell-cell adhesion at the molecular level. Our results provide new insights into Ca(2+)-dependent mechanisms for cell-cell adhesion.     

Effects of calcium and soluble cytoplasmic activator protein (calmodulin) on various states of (Ca2+ + Mg2+)-ATPase activity in isolated membranes of human red cells

(Ca2+ + Mg2+)-ATPase activity of red cells and their isolated membranes was investigated in the presence of various Ca2+ concentrations and cytoplasmic activator protein. Red cell ATPase activity was high at low Ca2+ concentrations, and low at moderate and high concentrations of Ca2+. In the case of isolated membranes, both low and moderate ca2+ concentrations produced higher (Ca2+ + Mg2+)-ATPase activity than high Ca2+ concentration. Membrane-free hemolysate containing soluble activator of (Ca2+ + Mg2+)-ATPase produced a significant increase in (Ca2+ + Mg2+)-ATPase activity only at low ca2+ concentration. Regardless of Ca2+ and activator concentrations, the enzyme activity in the membrane was lower than lysed red cells. The low level of (Ca2+ + Mg2+)-ATPase activity seen at high Ca2+ concentration can be augmented by lowering the Ca2+ concentration of EGTA in the assay medium. However, once the membrane was exposed to a high Ca2+ concentration, the activator could no longer exert it maximum stimulation at the low Ca2+ concentration brought about by addition of EGTA. This loss of activation was not attributable to the Ca2+-induced denaturation of activator protein but rather related to the alteration of (Ca2+ + Mg2+)-ATPase states in the membrane. On the basis of these data, it is suggested that only a small portion of (Ca2+ + Mg2+)-ATPase activity of isolated membranes can be stimulated by the soluble activator and that (ca2+ + Mg2+)ATPase most likely exists in various states depending upon ca2+ concentration and the presence of activator. The enzyme state exhibiting the high degree of stimulation by activator may undergo irreversible damage in the presence of high Ca2+ concentrations.     

Inhibitory antibodies to plasmalemmal Ca2+-transporting ATPases. Their use in subcellular localization of (Ca2+ + Mg2+)-dependent ATPase activity in smooth muscle.

Antibodies directed against the purified calmodulin-binding (Ca2+ + Mg2+)-ATPase [(Ca2+ + Mg2+)-dependent ATPase] from pig erythrocytes and from smooth muscle of pig stomach (antral part) were raised in rabbits. Both the IgGs against the erythrocyte (Ca2+ + Mg2+)-ATPase and against the smooth-muscle (Ca2+ + Mg2+)-ATPase inhibited the activity of the purified calmodulin-binding (Ca2+ + Mg2+)-ATPase from smooth muscle. Up to 85% of the total (Ca2+ + Mg2+)-ATPase activity in a preparation of KCl-extracted smooth-muscle membranes was inhibited by these antibodies. The (Ca2+ + Mg2+)-ATPase activity and the Ca2+ uptake in a plasma-membrane-enriched fraction from this smooth muscle were inhibited to the same extent, whereas in an endoplasmic-reticulum-enriched membrane fraction the (Ca2+ + Mg2+)-ATPase activity was inhibited by only 25% and no effect was observed on the oxalate-stimulated Ca2+ uptake. This supports the hypothesis that, in pig stomach smooth muscle, two separate types of Ca2+-transport ATPase exist: a calmodulin-binding ATPase located in the plasma membrane and a calmodulin-independent one present in the endoplasmic reticulum. The antibodies did not affect the stimulation of the (Ca2+ + Mg2+)-ATPase activity by calmodulin.     

Plasma Membrane Ca-ATPase of Radish Seedlings : I. Biochemical Characterization Using ITP as a Substrate

In this work, we exploited the capability of the plasma membrane Ca-ATPase to utilize ITP as a substrate to study its characteristics in plasma membrane vesicles purified from radish (Raphanus sativus L.) seedlings. The majority of the ITPase activity of plasma membrane was Ca(2+)-dependent. The Ca(2+)-dependent ITPase activity was Mg(2+)-dependent and was stimulated by the calcium ionophore A23187. It was inhibited by erythrosin B (concentration giving 50% inhibition, 50 nanomolar) and by vanadate (concentration giving 50% inhibition, 3 micromolar) and displayed a broad pH optimum around pH 7.2 to 7.5. Both the hydrolytic and the transport activity of the plasma membrane Ca-ATPase were half-saturated by Ca(2+) in the micromolar concentration range. No major effect of EGTA on the saturation kinetics of the enzyme was observed. The affinity of the plasma membrane Ca-ATPase for Ca(2+) was about fourfold higher at pH 7.5 than at pH 6.9. The Ca(2+)-dependent ITPase activity was stimulated about twofold by polyoxyethylene 20 cetyl ether, although it was inhibited by Triton X-100 and by lysolecithin.     

Ca(2+)-dependent protein kinase from the halotolerant green alga Dunaliella tertiolecta: partial purification and Ca(2+)-dependent association of the enzyme to the microsomes.

Ca(2+)-dependent protein kinase (CDPK) was purified 900-fold from the soluble fraction of Dunaliella tertiolecta cells by ammonium sulfate precipitation, DEAE-Toyopearl, phenyl-Sepharose, and hydroxylapatite column chromatography. The CDPK was activated by micromolar concentration of Ca2+ and required neither calmodulin nor phospholipids for its activation. The enzyme phosphorylated casein, myosin light chain, and histone type III-S (histone H-1), but did not phosphorylate protamine and phosvitin. The Km values for ATP and casein were 11 microM and 300 micrograms/ml, respectively. Phosphorylation of casein was inhibited by calmodulin antagonists, calmidazolium, trifluoperazine, and compound 48/80, but not affected by calmodulin. CDPK bound to phenyl-Sepharose in the presence of Ca2+ and was eluted by ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid (EGTA). This suggests that hydrophobicity of the enzyme was increased by Ca2+. CDPK was also bound to the microsomes isolated from Dunaliella cells in the presence of micromolar concentration of Ca2+ and released in the presence of EGTA, suggesting the possibility of in vivo Ca(2+)-dependent association of the enzyme. The enzyme phosphorylated many proteins in the microsomes but few in the cytosol, if at all.