Cooperative calcium binding and calmodulin regulation in the calcium-dependent adenosine triphosphatase purified from the erythrocyte membrane

A mathematical model to quantify RYR Ca2+ leak and associated heat production in resting human skeletal muscle fibers

Cycling of Ca2+ between the sarcoplasmic reticulum (SR) and myoplasm is an important component of skeletal muscle resting metabolism. As part of this cycle, Ca2+ leaks from the SR into the myoplasm and is pumped back into the SR using ATP, which leads to the consumption of O2 and generation of heat. Ca2+ may leak through release channels or ryanodine receptors (RYRs). RYR Ca2+ leak can be monitored in a skinned fiber preparation in which leaked Ca2+ is pumped into the t-system and measured with a fluorescent dye. However, accurate quantification faces a number of hurdles. To overcome them, we developed a mathematical model of Ca2+ movement in these preparations. The model incorporated Ca2+ pumps that move Ca2+ from the myoplasm to the SR and from the junctional space (JS) to the t-system, Ca2+ buffering by EGTA in the JS and myoplasm and by buffers in the SR, and Ca2+ leaks from the SR into the JS and myoplasm and from the t-system into the myoplasm. The model accurately simulated Ca2+ uptake into the t-system, the relationship between myoplasmic [Ca2+] and steady-state t-system [Ca2+], and the effect of blocking RYR Ca2+ leak on t-system Ca2+ uptake. The magnitude of the leak through the RYRs would contribute ∼5% of the resting heat production of human muscle. In normal resting fibers, RYR Ca2+ leak makes a small contribution to resting metabolism. RYR-focused pathologies have the potential to increase RYR Ca2+ leak and the RYR leak component of resting metabolism.

Plasma membrane calcium ATPase activity is regulated by actin oligomers through direct interaction

As recently described by our group, plasma membrane calcium ATPase (PMCA) activity can be regulated by the actin cytoskeleton. In this study, we characterize the interaction of purified G-actin with isolated PMCA and examine the effect of G-actin during the first polymerization steps. As measured by surface plasmon resonance, G-actin directly interacts with PMCA with an apparent 1:1 stoichiometry in the presence of Ca(2+) with an apparent affinity in the micromolar range. As assessed by the photoactivatable probe 1-O-hexadecanoyl-2-O-[9-[[[2-[(125)I]iodo-4-(trifluoromethyl-3H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine, the association of PMCA to actin produced a shift in the distribution of the conformers of the pump toward a calmodulin-activated conformation. G-actin stimulates Ca(2+)-ATPase activity of the enzyme when incubated under polymerizing conditions, displaying a cooperative behavior. The increase in the Ca(2+)-ATPase activity was related to an increase in the apparent affinity for Ca(2+) and an increase in the phosphoenzyme levels at steady state. Although surface plasmon resonance experiments revealed only one binding site for G-actin, results clearly indicate that more than one molecule of G-actin was needed for a regulatory effect on the pump. Polymerization studies showed that the experimental conditions are compatible with the presence of actin in the first stages of assembly. Altogether, these observations suggest that the stimulatory effect is exerted by short oligomers of actin. The functional interaction between actin oligomers and PMCA represents a novel regulatory pathway by which the cortical actin cytoskeleton participates in the regulation of cytosolic Ca(2+) homeostasis.

Diacylglycerol regulates the plasma membrane calcium pump from human erythrocytes by direct interaction

The plasma membrane Ca(2+)-ATPase (PMCA) plays a key role in the regulation of the intracellular Ca(2+) concentration. Ethanol stimulates this Ca(2+) pump in an isoform-specific manner. On search for a physiological molecule that could mimic the effect of ethanol, we have previously demonstrated that some sphingolipids containing free "hydroxyl" groups, like ceramide, are able to stimulate the PMCA. Since diacylglycerol (DAG) structurally shares some characteristics with ceramide, we evaluate its effect on the PMCA. We demonstrated that DAG is a potent stimulator of this enzyme. The activation induced is additive to that produced by calmodulin, protein-kinase C and ethanol, which implies that DAG interacts with the PMCA through a different mechanism. Additionally, by different fluorescent approaches, we demonstrated a direct binding between PMCA and DAG. The results obtained in this work strongly suggest that DAG is a novel effector of the PMCA, acting by a direct interaction.

High sensibility to reactivation by acidic lipids of the recombinant human plasma membrane Ca2+-ATPase isoform 4xb purified from Saccharomyces cerevisiae

The human plasma membrane Ca2+ pump (isoform 4xb) was expressed in Saccharomyces cerevisiae and purified by calmodulin-affinity chromatography. Under optimal conditions the recombinant enzyme (yPMCA) hydrolyzed ATP in a Ca2+ dependent manner at a rate of 15 micromol/mg/min. The properties of yPMCA were compared to those of the PMCA purified from human red cells (ePMCA). The mobility of yPMCA in SDS-PAGE was the expected for the hPMCA4xb protein but slightly lower than that of ePMCA. Both enzymes achieved maximal activity when supplemented with acidic phospholipids. However, while ePMCA in mixed micelles of phosphatidylcholine-detergent had 30% of its maximal activity, the yPMCA enzyme was nearly inactive. Increasing the phosphatidylcholine content of the micelles did not increase the activity of yPMCA but the activity in the presence of phosphatidylcholine improved by partially removing the detergent. The reactivation of the detergent solubilized yPMCA required specifically acidic lipids and, as judged by the increase in the level of phosphoenzyme, it involved the increase in the amount of active enzyme. These results indicate that the function of yPMCA is highly sensitive to delipidation and the restitution of acidic lipids is needed for a functional enzyme.

Calcium signal transmission in chick sensory neurones is diffusion based

In many cells, the cytosol is an excitable medium through which calcium waves propagate by calcium induced calcium release (CICR). Many labs. have reported CICR in neurones subsequent to calcium influx through voltage gated channels. However, these have used long depolarizations. We have imaged calcium within chick sensory neurones following 50 ms depolarizations. Calcium signals travelled rapidly throughout the cell, such that changes at the cell centre were delayed by 24 ms compared to regions 3 microm from the plasma membrane. The nuclear envelope imposed a delay of 9 ms. A simple diffusion model with few unknowns gave good fits to the measured data, indicating that passive diffusion is responsible for signal transmission in these neurones. Simulations run without indicator dye did not reveal markedly different spatiotemporal dynamics, although concentration changes were larger. Simulations of calcium changes during action potentials revealed that large calcium transients occurring in the cytosol close to the nucleus are significantly attenuated by the nuclear envelope. Our results indicate that for the brief depolarisations that neurones will experience during normal signal processing calcium signals are transmitted by passive diffusion only. Diffusion is perfectly capable of transmitting the calcium signal into the interior of nerve cell bodies, and into the nucleoplasm.

Role of matrix metalloprotease-2 in oxidant activation of Ca2+ ATPase by hydrogen peroxide in pulmonary vascular smooth muscle plasma membrane

Exposure of bovine pulmonary artery smooth muscle plasma membrane suspension with the oxidant H2O2 (1 mM) stimulated Ca2+ATPase activity. We sought to determine the role of matrix metalloprotease-2 (MMP-2) in stimulating Ca2+ATPase activity by H2O2 in the smooth muscle plasma membrane. The smooth muscle membrane possesses a Ca2+-dependent protease activity in the gelatin containing zymogram having an apparent molecular mass of 72 kDa. The 72 kDa protease activity was found to be inhibited by EGTA, 1 : 10-phenanthroline, a2-macroglobulin and tissue inhibitor of metalloprotease-2 (TIMP-2) indicating that the Ca2+-dependent 72 kDa protease is the MMP-2. Western immunoblot studies of the membrane suspension with polyclonal antibodies of MMP-2 and TIMP-2 revealed that MMP-2 and TIMP-2, respectively, are the ambient matrix metalloprotease and the corresponding tissue inhibitor of metalloprotease in the membrane. In addition to increasing the Ca2+ATPase activity, H2O2 also enhanced the activity of the smooth muscle plasma membrane associated protease activity as evidenced by its ability to degrade 14C-gelatin. The protease activity and the Ca2+ATPase activity were prevented by the antioxidant, vitamin E, indicating that the effect produced by H2O2 was due to reactive oxidant species(es). Both basal and H2O2 stimulated MMP-2 activity and Ca2+ATPase activity were inhibited by the general inhibitors of matrix metalloproteases: EGTA, 1 : 10-phenanthroline, a2-macroglobulin and also by TIMP-2 (the specific inhibitor of MMP-2) indicating that H2O2 increased MMP-2 activity and that subsequently stimulated Ca2+ATPase activity in the plasma membrane. This was further confirmed by the following observations: (i) adding low doses of MMP-2 or H2O2 to the smooth muscle membrane suspension caused submaximal increase in Ca2+ATPase activity, and pretreatment with TIMP-2 prevents the increase in Ca2+ATPase activity; (ii) combined treatment of the membrane with low doses of MMP-2 and H2O2 augments further the Ca2+ATPase activity caused by the respective low doses of either H2O2 or MMP-2; and (iii) pretreatment with TIMP-2 prevents the increase in Ca2+ATPase activity in the membrane caused by the combined treatment of MMP-2 and H2O2.

Pre-steady-state phosphorylation and dephosphorylation of detergent-purified plasma-membrane Ca2+-ATPase

Pre-steady-state phosphorylation and dephosphorylation of purified and phospholipid-depleted plasma-membrane Ca(2+)-ATPase (PMCA) solubilized in the detergent polyoxyethylene 10 lauryl ether were studied at 25 degrees C. The time course of phosphorylation with ATP of the enzyme associated with Ca(2+), probably the true phosphorylation reaction, showed a fast phase (k(app) near 400 s(-1)) followed by a slow phase (k(app)=23 s(-1)). With asolectin or acidic phosphatidylinositol, the concentration of phosphoenzyme (EP) increased at as high a rate as before, passed through a maximum at 4 ms and stabilized at a steady level that was approx. half that without lipids. Calmodulin (CaM) did not change the rate of the fast phase, accelerated the slow phase (k(app)=93 s(-1)) and increased [EP] with small changes in the shape of the time course. Dephosphorylation was slow (k(app)=30 s(-1)) and insensitive to CaM. Asolectin accelerated dephosphorylation, which followed biexponential kinetics with fast (k(app)=220 s(-1)) and slow (k(app)=20 s(-1)) components. CaM stimulated the fast component by nearly 50%. The results show that the behaviour of the PMCA is complex, and suggest that acidic phospholipids and CaM activate PMCA through different mechanisms. Acceleration of dephosphorylation seems relevant during activation of the PMCA by acidic phospholipids.

Tricyclic antidepressants inhibit the Ca(2+)-dependent ATPase activity from plasma membrane

Tricyclic antidepressants are moderately potent inhibitors of the plasma membrane Ca(2+)-ATPase activity measured in erythrocyte ghosts. For the calmodulin-activated activity, half-maximal inhibition was observed in the presence of 0.25 mM clomipramine. Desipramine, imipramine, and trimipramine show half-maximal inhibition in the range of 0.8 to 1 mM. The inhibition dependence on clomipramine concentration is the same whether the enzyme is activated by exogenous calmodulin or by tryptic digestion. A similar behavior was observed for desipramine. The inhibition mechanisms utilized by clomipramine and desipramine are different. The clomipramine effect is associated with the Ca(2+)-bound enzyme conformation and can be attributed to a decrease in the rate of phosphorylation by ATP. The desipramine effect appears more related to the Ca(2+)-free conformation, since the partial reaction involved in the release of inorganic phosphate is perturbed by this drug. There is also little or no effect of tricyclics on the enzyme's affinity for ligand (Ca(2+) or ATP) binding.

Phosphatidylcholine makes specific activity of the purified Ca(2+)-ATPase from plasma membranes independent of enzyme concentration

Ca(2+)-ATPase of plasma membranes (PMCA) was isolated from either human or pig red cells by calmodulin-affinity chromatography and supplemented with phosphatidylcholine (PC). The specific activity of the purified PMCA diluted in media with detergent (C(12)E(10)) was very low, and increased with the concentration of the enzyme along a curve that reached the maximum at 8 microg/ml with K(0.5)=1.2-2.5 microg/ml. Such behavior has been described and attributed to self-association of the enzyme (D. Kosk-Kosicka and T. Bzdega, J. Biol. Chem. 263 (1988) 18184-18189). After heat-inactivation, the PMCA was as effective an activator as the intact enzyme, increasing, to the maximum, the specific activity of diluted enzyme with K(0. 5)=2.2 microg/ml. The inactivated PMCA failed to increase the activity of concentrated enzyme, suggesting that activation did not depend on interaction of intact with denatured enzyme molecules. When enough PC was added to the reaction medium to make its final concentration 16-33 microg/ml, the specific activity of the PMCA was maximum and independent of enzyme concentration. Under these conditions, activation by calmodulin lowered to 10%. As a function of the concentration of pure PC, maximum specific activity was reached along a curve with K(0.5)=4 microg/ml. This curve was identical to that of activation at increasing enzyme concentration, suggesting that, in the latter case, activation could have depended on PC contributed to the assay medium by the enzyme. The results show that PC made the purified PMCA solubilized in detergent reach maximum activity at any concentration of the enzyme.

Novel components and enzymatic activities of the human erythrocyte plasma membrane calcium pump

The plasma membrane Ca2+ pump is essential for the maintenance of cystolic calcium ion concentration levels in eukaryotes. Here we show that the Ca2+-ATPase, purified from human erythrocytes, contains two homopolymers, poly(3-hydroxybutyrate) (PHB) and inorganic polyphosphate (polyP), which form voltage-activated calcium channels in the plasma membranes of Escherichia coli and other bacteria. Furthermore, we demonstrate that the plasma membrane Ca2+-ATPase may function as a polyphosphate kinase, i.e. it exhibits ATP-polyphosphate transferase and polyphosphate-ADP transferase activities. These findings suggest a novel supramolecular structure for the functional Ca2+-ATPase, and a new mechanism of uphill Ca2+ extrusion coupled to ATP hydrolysis.

Pseudosubstrate hydrolysis by the erythrocyte plasma membrane Ca(2+)-ATPase: kinetic evidence for a modified E1 conformation in dimethylsulfoxide

The purified Ca(2+)-ATPase of pig red cells displays a phosphatase activity towards p-nitrophenylphosphate which is inhibited by Ca2+ in the absence of solvents, and activated by calmodulin. This activity has been attributed to the E2 conformation of the enzyme. Here we show that the pNPPase activity in the absence of Ca2+ is stimulated 10-25-fold by the presence of the organic solvent dimethylsulfoxide (Me2SO). This is an activation that surpasses by severalfold that induced by calmodulin in the absence of the solvent. At 30% Me2SO, activation by calmodulin disappears. In the absence of calmodulin and at pH 7.2, the Ca2+ concentration needed for half-maximal inhibition of the pNPPase activity (K1) increases from 130 microM in the absence of Me2SO to 860 microM at 30% Me2SO. This effect of Me2SO is enhanced at pH 8.0: the K for Ca2+ increases from 2.7 microM in the absence of the solvent to 2.0 mM in its presence. However, the K0.5 for Ca2+ activation of the ATPase activity decreases from 8.3 to 2.6 microM following addition of the same Me2SO concentration. This indicates that, even in the presence of Me2SO, microM Ca2+ concentrations shift the equilibrium towards E1 but the decrease in activity that would be expected if pNPP hydrolysis were catalysed exclusively by the E2 conformation is not observed. The affinity for pNPP as a substrate increases from 2.6 mM in the absence of Me2SO to 1.6 mM in the presence of 20% Me2SO. These results suggest that Me2SO induces multiple effects in the Ca(2+)-ATPase that (i) increase the reactivity of E2 towards substrate: (ii) surpass the activation by calmodulin and, (iii) allow the enzyme to hydrolyze pNPP even when Ca2+ is bound to the high-affinity sites of the enzyme. The change in reactivity is attributed to an increase on substrate catalysis rather than on pNPP binding.

Role of an aprotinin-sensitive protease in the activation of Ca(2+)-ATPase by superoxide radical (O2-.) in microsomes of pulmonary vascular smooth muscle

We have investigated the role of an aprotinin-sensitive protease in regulating Ca(2+)-ATPase activity and Ca2+ uptake (ATP-dependent and Na(+)-dependent) in microsomes of bovine pulmonary vascular smooth muscle during treatment with the O2(-.)-generating system hypoxanthine plus xanthine oxidase. Treatment of the smooth muscle microsomes with the O2(-.)-generating system produced a protease in a gelatin-containing zymogram with an apparent molecular mass of 16 kDa. This 16 kDa proteolytic protein was found to be inhibited by superoxide dismutase (SOD) and aprotinin but not by PMSF. Using polyclonal antiserum to aprotinin, we found that it is an ambient antiprotease of the smooth muscle microsomes. Treatment of the microsomes with the O2(-.)-generating system stimulated protease activity tested with a synthetic substrate N-benzoyl-DL-arginine p-nitroanilide and also enhanced Ca(2+)-ATPase activity. It also stimulated ATP-dependent Ca2+ uptake. In contrast, Na(+)-dependent Ca2+ uptake was found to be inhibited by the O2(-.)-generating system. Pretreatment of the microsomes with SOD and aprotinin preserved the increase in protease activity, Ca(2+)-ATPase activity and ATP-dependent Ca2+ uptake. In addition, O2(-.)-caused inhibition of the Na(+)-dependent Ca2+ uptake which was reversed by SOD and aprotinin. Pretreatment with PMSF did not cause any discernible alteration in the protease activity, Ca(2+)-ATPase activity. ATP-dependent Ca2+ uptake and Na(+)-dependent Ca2+ uptake in the microsomes caused by the O2(-.)-generating system. These results suggest that an aprotinin-sensitive protease plays a pivotal role in regulating Ca(2+)-ATPase and Ca(2+)-uptake activities in microsomes of pulmonary vascular smooth muscle under oxidant O2(-.)-triggered conditions.

Analysis of phosphorylation and mutation of tyrosine residues of calmodulin on its activation of the erythrocyte Ca(2+)-transporting ATPase

The role played by the phosphorylation sites of calmodulin on its ability to activate the human erythrocyte Ca(2+)-transporting ATPase (Ca(2+)-ATPase) was evaluated. Phosphorylation of mammalian calmodulin on serine/threonine residues by casein kinase II decreased its affinity for Ca(2+)-ATPase by twofold. In contrast, tyrosine phosphorylation of mammalian calmodulin by the insulin-receptor kinase did not significantly alter calmodulin-stimulated Ca(2+)-ATPase activity. Two variant calmodulins, each containing only one tyrosine residue (the second Tyr is replaced by Phe) were also examined: [F138]calmodulin, a mutant containing tyrosine at position 99, and wheat germ calmodulin which has tyrosine at position 139. The concentrations of [F138]calmodulin and wheat germ calmodulin required for half-maximal activation of Ca(2+)-ATPase were tenfold and fourfold higher, respectively, than mammalian calmodulin. Phosphorylation at Tyr99 of [F138]calmodulin shifted its affinity for Ca(2+)-ATPase towards that of mammalian calmodulin. However, phosphorylation at Tyr139 of wheat germ calmodulin had essentially no effect on its interaction with Ca(2+)-ATPase. Thus, all of the observed effects of both phosphorylation and substitution of residues of calmodulin are on its affinity for Ca(2+)-ATPase, not on Vmax. The effects are dependent on the site of phosphate incorporation. Replacement of tyrosine with phenylalanine has a larger effect than phosphorylation of tyrosine, suggesting that the observed functional alterations reflect a secondary conformational change in the C-terminal half of calmodulin, the region that is important in its activation of Ca(2+)-ATPase.

Intrinsic fluorescence as a probe of structure-function relationships in Ca(2+)-transport ATPases

Applications of intrinsic fluorescence measurements in the study of Ca(2+)-transport ATPases are reviewed. Since the initial reports showing that the fluorescence emission was sensitive to Ca2+ binding, a substantial amount of work has focused on the use of both steady-state and time-resolved fluorescence spectroscopy to investigate structure-function relationships in sarcoplasmic reticulum and plasma membrane Ca(2+)-ATPases. These studies have revealed ligand-induced conformational changes, as well as provided information on protein-protein, protein-solvent and/or protein-lipid interactions in different functional states of these proteins. The main results of these studies, as well as possible future prospects are discussed.

Mechanism of inhibition of the plasma membrane Ca(2+)-ATPase by barbiturates

We have demonstrated that sodium pentobarbital inhibited the activation of the human red blood cell plasma membrane Ca(2+)-ATPase produced by dimerization of enzyme monomers or by calmodulin binding to enzyme monomers. The effects of the barbiturate were dose-dependent. Both Vmax and Ca2+ affinity were reduced. The Ca(2+)-ATPase activity of the dimeric enzyme was distinctly less sensitive with respect to the effective inhibitory concentrations of pentobarbital and to the rate of onset of inhibition than was the calmodulin-dependent activation of enzyme monomers. Temperature dependence of the inhibition was in agreement with direct, nonpolar interactions of pentobarbital with a water-exposed nonpolar patch on the surface of this transmembrane protein. The barbiturate prevented the increase of intrinsic tryptophan fluorescence associated with substrate Ca2+ binding to the enzyme dimer. On the basis of the barbiturate effects we propose a model for the action of detergent-like compounds on the enzyme. They inhibit Ca(2+)-ATPase activity by binding to a nonpolar patch on the water-exposed dimerization surface of the enzyme monomer, part of which is also the binding site for calmodulin. The model assumes that their binding to the nonpolar patch on the monomer interferes with dimerization and weakens but does not prohibit calmodulin binding, whose activation of the enzyme is then submaximal. The model should be applicable to other proteins as the two activation pathways studied have been demonstrated for various enzymes.

How do volatile anesthetics inhibit Ca(2+)-ATPases?

Volatile anesthetics at concentrations that are used in clinical practice to induce anesthesia selectively inhibit activity of the plasma membrane Ca(2+)-transport ATPase (Kosk-Kosicka, D., and Roszczynska, G. (1993) Anesthesiology 79, 774-780). We have investigated the mechanism of the inhibitory action of several anesthetics on the purified erythrocyte Ca(2+)-ATPase by employing fluorescence spectroscopy measurements that report changes in the environment of intrinsic tryptophans and of an extrinsic probe attached in the active site of the enzyme. We have shown that the observed inhibition of the Ca(2+)-dependent activation of the enzyme correlates well with the elimination of the Ca(2+)-induced conformation change that is important for the proper function of the enzyme. Analysis of the anesthetics effects on the total tryptophan fluorescence indicates a significant effect on enzyme conformation. Similar changes have been observed in the sarcoplasmic reticulum Ca(2+)-ATPase. We propose that volatile anesthetics inhibit Ca(2+)-ATPase by interacting with nonpolar sites in protein interior, in analogy to the binding demonstrated for myoglobin, hemoglobin, and adenylate kinase (Schoenborn, B. P., and Featherstone, R. M. (1967) Adv. Pharmacol. 5, 1-17; Tilton, R. F., Kuntz, I. D., and Petsko, G. A. (1984) Biochemistry 23, 2849-2857). Such binding is expected to modify conformational substate(s) of the enzyme and perturb its function. We view this process as an example of a general phenomena of interaction of small molecules with internal sites in proteins.

Neutral organic solute effects on the activity of the plasma membrane Ca(2+)-ATPase

We have compared effects of dimethylsulfoxide (Me2SO) and two polyols on the Ca(2+)-ATPase purified from human erythrocytes. As studied under steady-state conditions over a broad solute concentration range and temperature, Me2SO, glycerol, and xylitol do not inhibit the Ca(2+)-ATPase activity; this is in contrast to numerous other organic solutes that we have investigated. Under specific experimental conditions, Me2SO (but not glycerol) substantially increases Ca(2+)-ATPase activity, suggesting a possible facilitation of enzyme oligomerization. The activation is more pronounced at low Ca2+ concentrations. In contrast to glycerol, Me2SO shows no protective effect on enzyme structure as assessed by determining residual Ca(2+)-ATPase activity after exposing the enzyme to thermal denaturation at 45 degrees C. Under these conditions several other organic solutes strongly enhance the denaturating effect of temperature. Because of the temperature dependence of its effect on the Ca(2+)-ATPase activity we believe that Me2SO activates the Ca(2+)-ATPase by indirect water-mediated interactions.

Stabilizing and destabilizing effects on plasma membrane Ca(2+)-ATPase activity

We have examined the temperature-dependent effects of several organic compounds on the activity of the purified Ca(2+)-ATPase of erythrocytes. The monomeric enzyme was activated either by interaction with calmodulin or by oligomerization in the absence of calmodulin. Of the four homologous solute series studied including polyols, alkanols, aprotic solvents, and N-methyl derivatives of formamide and acetamide only polyols stabilized the enzyme over a broad range of concentration and temperature. Similarity of Ca(2+)-ATPase activity patterns at 25 and 37 degrees C and in the presence of glycerol is in agreement with indirect, stabilizing interactions. Glycerol also protected the Ca(2+)-ATPase from thermal denaturation at 45 degrees C. Within each homologous series, inhibitory effects increased with increasing solute concentration and with increasing structural similarity to detergents, indicating that direct destabilizing interactions are responsible for the observed inhibition. These were comparable to the destabilizing effect of urea. Oligomers were more resistant to all inhibitory solutes as compared to calmodulin-activated monomers suggesting that the nonpolar patches of the oligomerized enzyme are less accessible to solutes.

Irreversible effects of calcium ions on the plasma membrane calcium pump

The calcium pump of human red cells can be irreversibly activated by preincubation of the membranes in the presence of calcium ions, with a pattern reminiscent of that produced by controlled trypsin attack. With 1 mM Ca2+, the activity of the basal enzyme increases three to fourfold over 30 to 60 min, to levels about half those obtained in the presence of calmodulin. On the whole, the effect occurs slowly, with a very low Ca2+ affinity at 37 degrees C and is unaffected by serine-protease inhibitors. The activation caused by 1 mM Ca2+ is little affected by leupeptin (a thiol-protease inhibitor) and that obtained at 10 microM Ca2+ is not inhibited. Preincubations at 0 degrees C also lead to activation, to a level up to half that seen at 37 degrees C, and the effect is not affected by leupeptin or antipain. No activation is observed by preincubating soluble purified Ca,Mg-ATPase in Ca(2+)-containing solutions at 37 degrees C. Instead, calcium ions protect the detergent-solubilized enzyme from thermal inactivation, the effect being half-maximal between 10 and 20 microM Ca2+. We conclude that the activation of the membrane-bound Ca,Mg-ATPase by Ca2+ should result from an irreversible conformational change in the enzyme and not from attack by a membrane-bound protease, and that this change presumably arises from the release of inhibitory particles existing in the original membrane preparations.

Synergistic activation of the human red cell calcium ATPase by magnesium and vanadate

The Ca(2+)-ATPase activity of human red cells was studied on calmodulin-free membrane fragments after previous incubation with Mg2+ and vanadate. In the presence of EGTA (5 mM), the activity was slightly affected by either ion alone. However, when added together, both Ca2+ affinity and Vmax were increased up to levels found with calmodulin (0.3 microM). This synergistic activation was not abolished by proteinase inhibitors (iodoacetamide, 10 mM; leupeptin, 200 microM; pepstatin A, 100 microM; phenylmethanesulfonyl fluoride, 100 microM), neomycin (200 microM), washing with EDTA (5 mM) or by both incubating and washing with delipidized serum albumin (1 mg/ml). During preincubation under optimal Mg2+ and vanadate conditions, the replacement of K+ by Na+ or Li+ was without effect. Co2+ or Zn2+ (10 mM) could not substitute for Mg2+, whereas Mn2+ almost replaced it at equimolar amounts. By contrast, addition of ATPMg (2 mM) decreased the activation by about one-half. Like calmodulin, pretreatment with Mg2+ plus vanadate also increased the affinity for ATP and elicited appearance of a second (low) affinity site (apparent Km = 120 microM). The fluorescence depolarization of 1,6-diphenyl- and 1-(4-trimethylammonium phenyl)-6-phenyl 1,3,5-hexatriene incorporated into membrane fragments was not affected after preincubating with Mg2+, vanadate or Mg2+ plus vanadate. The results show that Mg2+ and vanadate are acting neither via proteolysis or fatty acid production nor by facilitating phospholipid metabolism or altering membrane fluidity. They may be enhancing the Ca(2+)-ATPase activity by stabilizing the E1 conformer or promoting an enzyme conformation which facilitates the E2-E1 transition.

Calcium ion homeostasis in smooth muscle

Ca2+ plays an important role in the regulation of smooth-muscle contraction. In this review, we will focus on the various Ca(2+)-transport processes that contribute to the cytosolic Ca2+ concentration. Mainly the functional aspects will be covered. The smooth-muscle inositol 1,4,5-trisphosphate receptor and ryanodine receptor will be extensively discussed. Smooth-muscle contraction also depends on extracellular Ca2+ and both voltage- and Ca(2+)-release-activated plasma-membrane Ca2+ channels will be reviewed. We will finally discuss some functional properties of the Ca2+ pumps that remove Ca2+ from the cytoplasm and of the Ca2+ regulation of the nucleus.

The Ca(2+)-transport ATPases from the plasma membrane

The initial studies on the plasma membrane (PM) Ca(2+)-transport ATPases were made in the erythrocyte, a structure that can not be taken as representing a typical eukaryotic cell. In other cell types however, the study of the PM Ca(2+)-transport ATPase is complicated by the simultaneous expression of related Ca(2+)-pumps in intracellular stores. Whereas there are as yet no known specific inhibitors for the PM Ca(2+)-transport ATPase, a number of selective inhibitors for the endo(sarco)plasmic reticulum Ca2+ pumps have been described: thapsigargin, cyclopiazonic acid and 2,5-di-(tert-butyl)-1,4-benzohydroquinone. With the recent introduction of the molecular biological approach, it became quickly obvious that a family of at least 5 different PM Ca(2+)-transport ATPase genes govern the tissue-dependent expression of PM Ca2+ pumps. Moreover alternative splicing of the primary gene transcripts was found to further enhance the number of pump variants. The PM Ca(2+)-transport ATPase are subject to modulatory control by calmodulin, by acidic phospholipids, and by the known families of protein kinases. Each of the ensuing effects are mutually related and interdependent. The wide variety PM Ca2+ pump isoforms and their regulation by such an intricate modulatory network allows the distinct tissues to adapt most adequately to the prevailing tissue and stimulus specific requirements.

Time-resolved fluorescence of erythrocyte plasma membrane Ca2(+)-ATPase in different functional states

The fluorescence decay of the plasma membrane calmodulin-activated Ca2(+)-ATPase from the erythrocyte was measured for the first time. The availability of a novel procedure for on-line blank subtraction in frequency-domain lifetime data acquisition (G.G. Reinhart, B. Feddersen, D. Jameson and E. Gratton, Biophys. J. 57 (1990) 189a) permitted the elimination of background interference from detergent-solubilized purified plasma membrane ATPase samples. The fluorescence decay of the erythrocyte Ca2(+)-ATPase was measured in the absence of Ca2+, or in the presence of Ca2+ or Ca2+ plus calmodulin. In the three different experimental conditions the fluorescence decay was very heterogeneous and could be best described by Lorentzian distributions of lifetime values. In the absence of Ca2+ the decay was described by a broad lifetime distribution centered at 4.4 ns with a width of 3.2 ns, indicating heterogeneity of tryptophan microenvironments in the ATPase. Calcium ion binding promoted an 11% increase in the center and a 27% decrease in the width of the distribution. By contrast, addition of calmodulin in the presence of Ca2+ caused a 15% decrease in the center of the distribution, revealing structural difference between calmodulin-activated and Ca2(+)-activated states of the ATPase. These results indicate the usefulness of on-line blank subtraction in frequency-domain lifetime measurements to investigate conformational changes in detergent-solubilized membrane protein samples.

Long-term stabilization and crystallization of (Ca2+ + Mg2+)-ATPase of detergent-solubilized erythrocyte plasma membrane

Conditions which were optimal for the stabilization of Ca2(+)-transporting ATPase in solubilized sarcoplasmic reticulum membranes (Pikułla, S., Mullner, N., Dux, L. and Martonosi, A. (1988) J. Biol. Chem. 263, 5277-5286) were also found conducive for preservation of (Ca2+ + Mg2+)-ATPase activity in detergent-solubilized erythrocyte plasma membrane for up to 60 days. Of particular importance for the stabilization of calmodulin-stimulated Ca2(+)-dependent activity of (Ca2+ + Mg2+)-ATPase of solubilized erythrocyte plasma membrane was the presence of Ca2+ (10-20 mM), glycerol, anti-oxidants, proteinase inhibitors and appropriate detergents. Among eight detergents tested octaethylene glycol dodecyl ether, polyoxyethylene glycol(10) lauryl alcohol and polydocanol were found to be promotive in long-term preservation of the enzyme activity. Under these conditions (Ca2+ + Mg2+)-ATPase of erythrocyte ghosts became highly stable and developed microcrystalline arrays after storage for 35 days. Electron micrographs of the negatively stained and thin sectioned material indicated that crystals of purified, detergent-solubilized, lipid-stabilized erythrocyte (Ca2+ + Mg2+)-ATPase differ from those of Ca2(+)-ATPase of detergent-solubilized sarcoplasmic reticulum microsomes.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

Similarities between the effects of dimethyl sulfoxide and calmodulin on the red blood cell Ca2(+)-ATPase

The Ca2(+)-ATPase of the erythrocyte plasma membrane can be activated by calmodulin, acidic phospholipids, limited proteolysis and self-association. Recently, it has been shown that different organic solvents increase both the Vmax and the Ca2+ affinity of the enzyme (Benaim, G. and De Meis, L. (1989) FEBS Lett. 244, 484-486). In this report the effects of calmodulin and dimethyl sulfoxide (20%, v/v) on the Ca2(+)-ATPase are compared. Dimethyl sulfoxide also elicits the appearance of the low-affinity binding site, which in this enzyme is strictly dependent on calmodulin. Dimethyl sulfoxide increases the Ca2+ affinity of the enzyme in a manner similar to that observed with the use of calmodulin and of acidic phospholipids. This was tested using both native and partially trypsinized ATPase. When activated by calmodulin the enzyme is inhibited by compound 48/80, trifluoperazine and calmidazolium. When activated by dimethyl sulfoxide the enzyme is still inhibited by calmidazolium but is no longer inhibited by either compound 48/80 or trifluoperazine. Activation of the ATPase promoted by either calmodulin or dimethyl sulfoxide is abolished when the Ca2+ concentration is raised from 10 microM to 2 mM. The effect of dimethyl sulfoxide is also abolished by 20 mM Pi. In the presence of 1 to 10 mM Ca2+ the ATPase catalyzes an ATP in equilibrium Pi exchange. The rate of exchange increases several fold when dimethyl sulfoxide is included in the assay medium.

Fluorescence studies on calmodulin binding to erythrocyte Ca2(+)-ATPase in different oligomerization states

The fluorescent spinach calmodulin derivative 2-(4-maleimidoanilino)naphthalene-6-sulfonic acid-calmodulin (MIANS-CaM) was used to investigate calmodulin interaction with the purified, detergent-solubilized erythrocyte Ca2(+)-ATPase. Previous studies have shown that the Ca2(+)-ATPase exists in equilibria between monomeric and oligomeric forms. We report here that MIANS-CaM binds to both enzyme forms in a Ca2(+)-dependent manner, with a approximately 50% fluorescence enhancement. These findings confirm our previous observation that enzyme oligomers retain their ability to bind calmodulin, even though they are fully activated in the absence of calmodulin. The Ca2+ dependence of MIANS-CaM binding to monomeric Ca2(+)-ATPase is of higher affinity (K 1/2 = 0.09 microM Ca2+) and less cooperative (nH = 1.1) than the Ca2+ dependence of enzyme activation by MIANS-CaM (K 1/2 = 0.26 microM Ca2+, nH = 2.8). These Ca2+ dependences and the order of events, in which calmodulin binding precedes enzyme activation, demonstrate that calmodulin indeed could be a physiological activator of the monomeric enzyme. The calcium dependence of calmodulin binding to oligomeric Ca2(+)-ATPase occurs at even lower levels of Ca2+ (K 1/2 = 0.04 microM Ca2+), in a highly cooperative fashion (nH = 2.3), and essentially in parallel with enzyme activation (K 1/2 = 0.05 microM Ca2+, nH = 2.9). The observed differences between monomers and oligomers suggest that the oligomerized Ca2(+)-ATPase is in a conformation necessary for efficient, cooperative calcium binding at nanomolar Ca2+, which the monomeric enzyme acquires only upon interaction with calmodulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Erythrocyte Ca2(+)-ATPase: activation by enzyme oligomerization versus by calmodulin

The subject of our studies is the mechanism of activation of the erythrocyte Ca2(+)-ATPase. Using purified, detergent solubilized enzyme it was found that equivalent maximal Ca2(+)-ATPase activity is obtained either upon addition of calmodulin or upon increase of enzyme concentration. Three independent methods, including Ca2(+)-ATPase activity, polarization of the enzyme modified with an external fluorescent probe, and efficiency of fluorescence resonance energy transfer between enzyme molecules have established that the concentration dependent activation is due to enzyme oligomerization. The oligomers bind calmodulin with a lower stoichiometry (0.5 mol calmodulin/mol Ca2(+)-ATPase), higher Ca2+ affinity (KCa = pCa 7.4), and higher cooperativity for Ca2+ (nH = 2.6) than the monomeric form (stoichiometry = 1 mol calmodulin/mol Ca2(+)-ATPase, KCa = pCa 7.0, nH = 1.1). The Ca2+ dependence of calmodulin binding and activation of monomers indicates that calmodulin binds before the Ca2(+)-ATPase activity is exhibited, demonstrating that the activation of this enzyme form is totally dependent on calmodulin. In contrast, oligomers reveal very similar Ca2+ dependence for calmodulin binding and for Ca2(+)-ATPase activity as well as for Ca2+ binding (assessed by tryptophan fluorescence), and for the oligomerization process (assessed by fluorescence energy transfer). The calmodulin antagonist drug 48/80 inhibits the calmodulin dependent activity of the monomers (I50 = 1.4 micrograms/ml) but has no effect on the activity of oligomers, confirming that calmodulin plays no role in the activation of the oligomeric enzyme. Our studies indicate that the erythrocyte Ca2(+)-ATPase can be activated by its high affinity, Ca2+ dependent binding of calmodulin or by a Ca2+ dependent oligomerization process which may involve calmodulin binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium binding capacity of erythrocyte membrane in human hypertension

The cell membrane calcium binding capacity of genetically hypertensive rats is reduced when measured in the presence of the submicromolar calcium concentrations proper of intracellular environment. The present work, performed as an ancillary study to an epidemiological survey on an entire population, aimed to investigate the existence of a similar abnormality in human hypertension. Calcium binding to the erythrocyte membrane was measured in clinically healthy normotensive (n = 12) and hypertensive individuals (n = 24). For this purpose, a filtration technique was used, based on the determination of 45Ca bound to the erythrocyte membrane in the presence of free calcium concentrations (40 nmol/l and 1 mumol/l), which are similar to those of the intracellular environment. The intra-assay technical error was determined on 35 duplicate samples and, when expressed as percent of the mean, was 24.1 at the 40 nmol/l concentration and 16.8 at the 1 mumol/l concentration. Membranes of untreated hypertensive patients, at both calcium concentrations, bound significantly less calcium than the control group. Treated and untreated hypertensive individuals had comparable values of membrane calcium binding capacity. Membranes of the treated hypertensive group bound less calcium than those of the normotensive group at both calcium concentrations, but the difference was statistically significant only in the presence of 40 nmol/l free calcium. A significant positive correlation was observed between the calcium binding capacity at 40 nmol/l concentration and that at 1 mumol/l in the treated and untreated hypertensive groups (r = 0.73 and 0.75, respectively; 0.51 for the normotensive group). These findings support the hypothesis that a cell membrane abnormality is detectable in some hypertensive patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of the purified erythrocyte plasma membrane Ca2+- ATPase by organic solvents

In this report it is shown that organic solvents mimic the stimulatory effects of calmodulin and acidic phospholipids on the erythrocyte plasma membrane Ca2+-ATPase. The solvents used were dimethyl sulfoxide (20%, v/v), glycerol (20% v/v), ethylene glycol (20%, v/v) and polyethylene glycol (Mr 6000-8000) (10%, w/v). These solvents increased both the affinity for Ca2+ and the turnover number of the enzyme. The increase in Ca2+ affinity is additive to that achieved with calmodulin. The calcium cooperativity observed in the presence of calmodulin disappears after the addition of dimethyl sulfoxide to the medium. The present data support the proposal that activation of the erythrocyte plasma membrane Ca2+-ATPase is promoted by hydrophobic interactions along the enzyme molecule.

ATP-dependent Ca2+ transport in the rat parotid basolateral plasma membrane is regulated by calmodulin

Calmodulin regulation of ATP-dependent Ca2+ transport activity was assessed in inverted basolateral plasma membrane vesicles (BLMV) isolated from rat parotid glands. The initial rate of Ca2+ transport in media containing 100 nM Ca2+ was stimulated by approximately 60% at maximal concentrations (300 nM) of exogenously added calmodulin (CAM). Half-maximal activation was obtained at 50 and 175 nM CAM in KCl and mannitol containing assay media, respectively. In the KCl medium, addition of 300 nM CAM increased the affinity of the BLMV Ca2+ transport activity for Ca2+ from approximately 70 nM, in the absence of added CAM, to approximately 50 nM. Vmax was consistently increased by approximately 20% under these conditions. When BLMV were treated with ethylene glycol bis(beta-aminoethylether) N,N'-tetraacetic acid (EGTA) (200 microM), the affinity of the transporter for Ca2+ decreased by 50% to approximately 150 nM, with no change in Vmax. When CAM was added to the EGTA-treated membranes, Ca2+ transport activity was comparable to that obtained when CAM was added directly to control, untreated BLMV. The CAM antagonists, trifluoperazine (TFP), W-7, and calmidazolium, inhibited Ca2+ transport in the presence of CAM. Half-maximal inhibition of transport was achieved by 12 microM TFP and 20 microM W-7. Calmidazolium (1 microM) inhibited Ca2+ transport by 75%. The inhibitory effects on ATP-dependent Ca2+ transport exerted by these agents were not due to an increase in the passive permeability of the membranes to Ca2+. Furthermore, in the absence of added CAM, the inhibitory effects of these agents on initial Ca2+ transport rate was decreased. The data presented suggest that the Ca2+-dependent interaction of CAM with the ATP-dependent Ca2+ transporter in rat parotid BLMV modifies the kinetic properties of this Ca2+ transporting mechanism.

Cyclic GMP-dependent protein kinase stimulates the plasmalemmal Ca2+ pump of smooth muscle via phosphorylation of phosphatidylinositol

The effect of phosphorylation by cyclic GMP-dependent protein kinase (G-kinase) on the activity of the plasmalemmal Ca2+-transport ATPase was studied on isolated plasma membranes and on the ATPase purified from pig erythrocytes and from the smooth muscle of pig stomach and pig aorta. Incubation with G-kinase resulted, in both smooth-muscle preparations, but not in the erythrocyte ATPase, in a higher Ca2+ affinity and in an increase in the maximal rate of Ca2+ uptake. Cyclic AMP-dependent protein kinase (A-kinase) did not exert such an effect. The stimulation of the (Ca2+ + Mg2+)-dependent ATPase activity of the purified Ca2+ pump reconstituted in liposomes depended on the phospholipid used for reconstitution. The stimulation of the (Ca2+ + Mg2+)-ATPase activity by G-kinase was only observed in the presence of phosphatidylinositol (PI). G-kinase, but not A-kinase, stimulated the phosphorylation of PI to phosphatidylinositol phosphate (PIP) in a preparation of (Ca2+ + Mg2+)-ATPase obtained by calmodulin affinity chromatography from smooth muscle, but not in a similar preparation from erythrocytes. Adenosine inhibited both the phosphorylation of PI and the stimulation of the (Ca2+ + Mg2+)-ATPase by G-kinase. In the absence of G-kinase the (Ca2+ + Mg2+)-ATPase was stimulated by the addition of PIP, but not by PI. In contrast with previous results of Furukawa & Nakamura [(1987) J. Biochem (Tokyo) 101, 287-290], no convincing evidence for a phosphorylation of the (Ca2+ + Mg2+)-ATPase was found. Evidence is presented showing that the apparent phosphorylation occurs in a contaminant protein, possibly myosin light-chain kinase. It is proposed that G-kinase stimulates the plasmalemmal Ca2+ pump of smooth-muscle cells indirectly via the phosphorylation of an associated PI kinase.

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).