Uncoupling of Ca2+ transport from ATP hydrolysis activity of sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase

Summary In reconstituted rabbit skeletal muscle (Ca²⁺ + Mg²⁺)-ATPase proteoliposomes, Ca²⁺-uptake is decreased by more than 90% with T₂ cleavage (Arg-198). However, no difference in the ATP dependence of hydrolysis activity is seen between SR and trypsin-treated SR. A large decrease in E-P formation and hydrolysis activity of the enzyme appear only at T₃ cleavage, which represents the cleavage of A₁ fragment to A_1a + A_1b forms. The disappearance of hydrolysis activity due to digestion is prior to the disappearance of E-P formation. No significant difference is found in the passive Ca²⁺ efflux between control SR and tryptically digested SR in the absence of Mg⁺ ruthenium red or in the presence of ATP. However, the passive Ca²⁺ efflux rate for tryptically digested SR is much larger than control SR in the presence of Mg²⁺ + ruthenium red. These results show that the Ca²⁺ channel cannot be closed after trypsin digestion of SR membranes by the presence of the Ca²⁺ channel inhibitors, Mg²⁺ and ruthenium red. In the reconstituted ATPase proteoliposomes, the Ca²⁺ efflux rates are the same regardless of digestion (T₂); also, efflux is not affected by the presence or absence of Mg²⁺ + ruthenium red. These results indicate that T₂ cleavage causes uncoupling of the Ca²⁺-pump from ATP hydrolytic activity.A theoretical model is developed in order to fit the extent of tryptic digestion of the A fragment of the (Ca²⁺ + Mg²⁺)-ATPase polypeptide with the loss of Ca²⁺-transport. Fits of the theoretical equations to the data are consistent with that Ca²⁺-transport system appears to require a dimer of the polypeptide (Ca²⁺ + Mg²⁺)-ATPase.     

Oxygen radical-mediated lipid peroxidation and inhibition of Ca2+-ATPase activity of cardiac sarcoplasmic reticulum

Oxygen radicals have been implicated as important mediators of myocardial ischemic and reperfusion injury. A major product of oxygen radical formation is the highly reactive hydroxyl radical via a biological Fenton reaction. The sarcoplasmic reticulum is one of the major target organelles injured by this process. Using a oxygen radical generating system consisting of dihydroxyfumarate and Fe3+-ADP, we studied lipid peroxidation and Ca2+-ATPase of cardiac sarcoplasmic reticulum. Incubation of sarcoplasmic reticulum with dihydroxyfumarate plus Fe3+-ADP significantly inhibited enzyme activity. Addition of superoxide dismutase, superoxide dismutase plus catalase (15 micrograms/ml) or iron chelator, deferoxamine (1.25-1000 microM) protected Ca2+-ATPase activity. Time course studies showed that this system inhibited enzyme activity in 7.5 to 10 min. Similar exposure of sarcoplasmic reticulum to dihydroxyfumarate plus Fe3+-ADP stimulated malondialdehyde formation. This effect was inhibited by superoxide dismutase, catalase, singlet oxygen, and hydroxyl radical scavengers. EPR spin-trapping with 5,5-dimethyl-1-pyrroline-N-oxide verified production of the hydroxyl radical. The combination of dihydroxyfumarate and Fe3+-ADP resulted in a spectrum of hydroxyl radical spin trap adduct, which was abolished by ethanol, catalase, mannitol, and superoxide dismutase. The results demonstrate the role of oxygen radicals in causing inactivation of Ca2+-ATPase and inhibition of lipid peroxidation of the sarcoplasmic reticulum which could possibly be one of the important mechanisms of oxygen radical-mediated myocardial injury.     

Mechanism of inhibition of Ca2+-transport activity of sarcoplasmic reticulum Ca2+-ATPase by anisodamine

The mechanism of inhibition of Ca2+-transport activity of rabbit sarcoplasmic reticulum Ca 2+-ATPase (SERCA) by anisodamine (a drug isolated from a medicinal herb Hyoscyamuns niger L) was investigated by using ANS (1-anilino-8-naphthalenesulfonate) fluorescence probe, intrinsic fluorescence quenching and Ca 2+-transport activity assays. The number of ANS binding sites for apo Ca2+-ATPase was determined as 8, using a multiple-identical binding site model. Both anisodamine and Ca2+ at millimolar level enhanced the ANS binding fluorescence intensities. Only anisodamine increased the number of ANS molecules bound by SERCA from 8 to 14. The dissociation constants of ANS to the enzyme without any ligand, with 30 mM anisodamine and with 15 mM Ca 2 were found to be 53.0 microM, 85.0 microM and 50.1 microM, respectively. Both anisodamine and Ca2+ enhanced the ANS binding fluorescenc with apparent dissociation constants of 7.6 mM and 2.3 mM, respectively, at a constant concentration of the enzyme. Binding of anisodamine significantly decreased the binding capacity of Ca2+ with the dissociation constant of 9.5 mM, but binding of Ca2+ had no obvious effect on binding of anisodamine. Intrinsic fluorescence quenching and Ca2+-transport activity assays gave the dissociation constants of anisodamine to SERCA as 9.7 and 5.4 mM, respectively, which were consistent with those obtained from ANS-binding fluorescence changes during titration of SERCA with anisodamine and anisodamine + 15 mM Ca2+, respectively. The results suggest that anisodamine regulates Ca2+-transport activity of the enzyme, by stabilizing the trans-membrane domain in an expanded, inactive conformation, at least at its annular ring region.     

Change in the passive influx of Ca2+ into sarcoplasmic reticulum vesicles as a result of chemical modification of surface amino groups

Treatment of sarcoplasmic reticulum (SR) vesicles with trinitrobenzene (TNBS) and 1-ethyl-3(3-dimethylaminopropyl)carbodiimide (EDC) stimulates the initial rate of passive influx of Ca2+ into SR vesicles, but does not affect either the binding or the maximal passive loading of SR vesicles with Ca2+. The changes in the kinetics of KCl-stimulated passive influx of Ca2+ depend on the reagent used. It is supposed that stimulation of passive influx of Ca2+ into SR vesicles and the changes in the reaction kinetics may be caused by modification of the Ca2+ channel gating behaviour as a result of binding of surface amino groups.     

Regulation of Ca2+ transport by sarcoplasmic reticulum Ca2+-ATPase at limiting [Ca2+

The factors regulating Ca2+ transport by isolated sarcoplasmic reticulum (SR) vesicles have been studied using the fluorescent indicator Fluo-3 to monitor extravesicular free [Ca2+]. ATP, in the presence of 5 mM oxalate, which clamps intravesicular [Ca2+] at approximately 10 microM, induced a rapid decline in Fluo-3 fluorescence to reach a limiting steady state level. This corresponds to a residual medium [Ca2+] of 100 to 200 nM, and has been defined as [Ca2+]lim, whilst thermodynamic considerations predict a level of less than 1 nM. This value is similar to that measured in intact muscle with Ca2+ fluophores, where it is presumed that sarcoplasmic free [Ca2+] is a balance between pump and leaks. Fluorescence of Fluo-3 at [Ca2+]lim was decreased 70% to 80% by histidine, imidazole and cysteine. The K0.5 value for histidine was 3 mM, suggesting that residual [Ca2+]lim fluorescence is due to Zn2+. The level of Zn2+ in preparations of SR vesicles, measured by atomic absorption, was 0.47+/-0.04 nmol/mg, corresponding to 0.1 mol per mol Ca-ATPase. This is in agreement with findings of Papp et al. (Arch. Biochem. Biophys., 243 (1985) 254-263). Histidine, 20 mM, included in the buffer, gave a corrected value for [Ca2+]lim of 49+/-1.8 nM, which is still higher than predicted on thermodynamic grounds. A possible 'pump/leak' mechanism was tested by the effects of varying active Ca2+ transport 1 to 2 orders with temperature and pH. [Ca2+]lim remained relatively constant under these conditions. Alternate substrates acetyl phosphate and p-NPP gave similar [Ca2+]lim levels even though the latter substrate supported transport 500-fold slower than with ATP. In fact, [Ca2+]lim was lower with 10 mM p-NPP than with 5 mM ATP. The magnitude of passive efflux from Ca-oxalate loaded SR during the steady state of [Ca2+]lim was estimated by the unidirectional flux of 45Ca2+, and directly, following depletion of ATP, by measuring release of 40Ca2+, and was 0.02% of Vmax. Constant infusion of CaCl2 at [Ca2+]lim resulted in a new steady state, in which active transport into SR vesicles balances the infusion rate. Varying infusion rates allows determination of [Ca2+]-dependence of transport in the absence of chelating agents. Parameters of non-linear regression were Vmax=853 nmol/min per mg, K0.5(Ca)=279 nM, and nH(Ca)=1.89. Since conditions employed in this study are similar to those in the sarcoplasm of relaxed muscle, it is suggested that histidine, added to media in studies of intracellular Ca2+ transients, and in the relaxed state, will minimise contribution of Zn2+ to fluophore fluorescence, since it occurs at levels predicted in this study to cause significant overestimation of cytoplasmic free [Ca2+] in the relaxed state. Similar precautions may apply to non-muscle cells as well. This study also suggests that [Ca2+]lim in the resting state is a characteristic feature of Ca2+ pump function, rather than a balance between active transport and passive leakage pathways.     

Localization of tryptophan residues in Ca2+-ATPase from sarcoplasmic reticulum

By the methods of spectroscopy, fluorimetry and chemical modification of tryptophane residues with N-bromsuccinimide, the sarcoplasmic reticulum of rabbit sceletal muscle was shown to contain 18 +/- 1 tryptophane residues per Ca2+-ATPase molecule, 6 of which were, probably, inside the protein globule, in its hydrophobic region, and thus unavailable for modifier, while the rest 12 +/- 1 were easily transformed to the 6-oxyindole chromophore being the main source of the intrinsic fluorescence of the enzyme. The quantum yield for the rest four residues was equal to 0.015. Four tryptophane residues are located at the distance of less than 14 A from the ATP-binding site of the enzyme. The quantum yields of fluorescence for 8 of the tryptophane residues of Ca2+-ATPase were similar and equal to 0.03.     

Ca2+-ATPase from sarcoplasmic reticulum: acylphosphatase activity

Fractionation of sarcoplasmic reticulum vesicles from rabbit skeletal muscle was performed by solubilization of the vesicles in the presence of deoxycholate, followed by sucrose density gradient centrifugation and gel filtration chromatography. This procedure permitted the isolation of essentially pure Ca²⁺-ATPase; this enzyme showed ATPase as well as acylphosphatase activity, both activities being clearly enhanced by deoxycholate. The acylphosphatase activity of the purified Ca²⁺-ATPase was characterized with regard to some kinetic properties, such as pH, Mg²⁺, Ca²⁺, and deoxycholate dependence, and substrate affinity, determined in the presence of acetylphosphate, succinylphosphate, carbamylphosphate, and benzoylphosphate; in addition, the stability of both activities was checked in time-course experiments. The main similarities between the two activities, such as the Mg²⁺ requirement, the deoxycholate activation, and the pH dependence, together with the competitive inhibition of the benzoylphosphatase activity by ATP, the inhibition of both activities by tris(bathophenanthroline)-Fe²⁺, and the relief of this inhibitory effect by carbonylcyanide-4-trifluoromethoxyphenyl hydrazone support the hypothesis that acylphosphatase and ATPase activities of sarcoplasmic reticulum vesicles reside in the same active site of the enzyme. With regard to possible relationships between acylphosphatase activity of the purified Ca²⁺-ATPase and “soluble” acylphosphatase present in the 100,000g supernatant fraction, comparison of some kinetic and structural parameters indicate that these two activities are supported by quite different enzymes.     

Inhibition of sarcoplasmic reticulum Ca2+-ATPase by miconazole

The inhibition of sarcoplasmic reticulumCa²⁺-ATPase activity by miconazole was dependent on theconcentration of ATP and membrane protein. Half-maximal inhibition wasobserved at 12 µM miconazole when the ATP concentration was 50 µMand the membrane protein was 0.05 mg/ml. When ATP was 1 mM, a lowmicromolar concentration of miconazole activated the enzyme, whereashigher concentrations inhibited it. A qualitatively similar responsewas observed when Ca²⁺ transport was measured. Likewise,the half-maximal inhibition value was higher when the membraneconcentration was raised. Phosphorylation studies carried out aftersample preequilibration in different experimental settings shed lighton key partial reactions such as Ca²⁺ binding and ATPphosphorylation. The miconazole effect on Ca²⁺-ATPaseactivity can be attributed to stabilization of theCa²⁺-free enzyme conformation giving rise to a decrease inthe rate of the Ca²⁺ binding transition. The phosphoryltransfer reaction was not affected by miconazole.

     

Affinity labeling of the active site of the Ca2+-ATPase of sarcoplasmic reticulum

The inactivation of sarcoplasmic reticulum ATPase by fluorescein isothiocyanate (FITC) was shown to have a hyperbolic dependence on the concentration of FITC. The results were quantitatively accounted for by a model in which the reagent first binds reversibly (Kf = 70 microM) to the ATPase and then reacts irreversibly (kmax = 0.8 and 2 min-1 in the absence and presence of 1 mM Mg2+, respectively) to form inactive enzyme. Comparison with the rate constant for the reaction of the model compound alpha-acetyllysine with FITC showed that the FITC-reactive lysyl side-chain of the ATPase is not unusually reactive, indicating that the specificity of the reaction is due to affinity labeling behavior of the reagent. This was supported by protection experiments using ATP, ADP, AdoPP[NH]P, ITP, and TNP-ATP, all of which displayed protection constants similar to their known binding constants to the active site of the ATPase. Both inorganic phosphate and orthovanadate were effective in preventing inactivation by FITC, and calcium only partially reversed the effect of these anions, implying the existence of a ternary complex such as Ca2.E.Pi. Since all ligands (ATP, ADP and Pi) which bind or react at the catalytic site protect it, only the unliganded form appears to bind and react with FITC. Addition of calcium to the MgATP complex of the ATPase caused an increase in the FITC inactivation rate, implying that during turnover there is a larger fraction of unliganded enzyme present, i.e., substrate binding is weaker (Ks is larger). Protection was also observed with fluorescein and two related dyes, eosin and erythrosin. Like FITC, the isothiocyanates of these dyes were effective inactivators. In separate experiments, these two dyes were shown to promote photoinactivation of the ATPase. ATP exerted a protective effect with a concentration dependence consistent with high-affinity active-site binding.     

Inhibition of sarcoplasmic reticulum Ca2+-ATPase by 2-mercaptoethanol

The Ca²⁺-dependent ATPase activity of sarcoplasmic reticulum was inhibited when membrane vesicles were incubated at 0°C in presence of thiols. 2-mercaptoethanol was the most effective inhibitor from the thiols tested. The effect of 2-mercaptoethanol on the ATPase activity was biphasic; enzyme inhibition originally increased and then decreased with increasing thiol concentration. The inhibitory action of this thiol was significantly higher at low membrane concentrations and the rate of inactivation at 22°C was considerably lower than that at 0°C. Ca²⁺-ATPase previously inhibited by 2-mercaptoethanol was partially reactivated by incubation with periodate.     

The dimeric form of Ca2+-ATPase is involved in Ca2+ transport in the sarcoplasmic reticulum

To identify the functional unit of Ca(2+)-ATPase in the sarcoplasmic reticulum, we assessed Ca(2+)-transport activities occurring on sarcoplasmic reticulum membranes with different combinations of active and inactive Ca(2+)-ATPase molecules. We prepared heterodimers, consisting of a native Ca(2+)-ATPase molecule and a Ca(2+)-ATPase molecule inactivated by FITC labelling, by fusing vesicles loaded with each type of Ca(2+)-ATPase. The heterodimers exhibited neither Ca(2+) transport nor ATP hydrolysis, suggesting that Ca(2+) transport by the Ca(2+)-ATPase requires an interaction between functional Ca(2+)-ATPase monomers. This finding implies that the functional unit of the Ca(2+)-ATPase is a dimer.     

Effect of chemical modification on the crystallization of Ca2+-ATPase in sarcoplasmic reticulum

The influence of chemical modification on the morphology of crystalline ATPase aggregates was analyzed in sarcoplasmic reticulum (SR) vesicles. The Ca²⁺-ATPase forms monomer-type (P1) type crystals in the E₁ and dimer-type (P2) crystals in the E₂ conformation. The P1 type crystals are induced by Ca²⁺ or lanthanides; P2 type crystals are observed in Ca²⁺-free media in the presence of vanadate or inorganic phosphate. P1- and P2-type Ca²⁺-ATPase crystals do not coexist in significant amounts in native sarcoplasmic reticulum membrane. The crystallization of Ca²⁺-ATPase in the E₂ conformation is inhibited by guanidino-group reagents (2,3-butanedione and phenylglyoxal), SH-group reagents, phospholipases C or A₂, and detergents, together with inhibition of ATPase activity. Amino-group reagents (fluorescein 5′-isothiocyanate, pyridoxal phosphate and fluorescamine) inhibit ATPase activity but do not interfere with the crystallization of Ca²⁺-ATPase induced by vanadate. In fluorescamine-treated sarcoplasmic reticulum the vanadate-induced crystals contain significant P1-type regions in addition to the dominant P2 form.     

Protein rotation, enzyme activity and photooxidation of SH groups in sarcoplasmic reticulum Ca2+-ATPase

The binding of probe molecules such as fluorescein isothiocyanate, eosin isothiocyanate and erythrosin isothiocyanate to the Ca2+-ATPase of sarcoplasmic reticulum followed by illumination of the labelled protein causes substantial reductions of ATPase activity over a 1-h period. The degree of light-sensitivity induced by these probes is related to the triplet yield of these probe molecules. Consistent with this, the greatest effect is seen with erythrosin isothiocyanate and the least effect with fluorescein isothiocyanate. These reductions of ATPase activity associated with illumination are also associated with an aggregation of the protein molecules. This is indicated by laser flash photolysis measurements and also by polyacrylamide gel electrophoresis. A reduction in the number of thiol groups present on the ATPase molecule parallels the reduction of enzyme activity and changes in the protein mobility. The results are discussed in relation to the use of these probe molecules to study biological systems and also in terms of oxidative processes which may affect protein function in vivo.     

Vanadate inhibition of sarcoplasmic reticulum Ca2+-ATPase and other ATPases.

Vanadate is a potent inhibitor of the Ca²⁺-ATPase activity of sarcoplasmic reticulum in the presence of A-23187. The purified enzyme is sensitive to vanadate even in the absence of the ionophore. Ca²⁺ and norepinephrine protect the enzyme against inhibition of vanadate. The nonspecificity of vanadate is emphasized by the finding of inhibition of several other ATPases including the Ca²⁺Mg²⁺-ATPases of the ascites and human red cell plasma membranes, Mg²⁺-ATPase of the ascites plasma membrane, and the K⁺-ATPases of $\text{E.}\text{coli}$ and hog gastric mucosal cell membranes. The ascites plasma membrane Ca²⁺-ATPase (an ecto ATPase) and mitochondrial ATPase are not inhibited by vanadate.     

Variation of scallop sarcoplasmic reticulum Ca2+-ATPase activity with temperature

Methods for preparing native scallop sarcoplasmic reticulum vesicles, largely purified membranous scallop sarcoplasmic reticulum Ca2+-ATPase, and nonionic detergent-solubilized sarcoplasmic reticulum Ca2+-ATPase are described. The effect of a range of polyoxyethylene-based detergents on the solubilized Ca2+-ATPase was tested. Decaethylene glycol dodecyl ether (C12E10) supported the highest levels of activity, although C12E8 and C12E9 were more routinely used. Arrhenius plots of Ca2+-ATPase activity, where the assays were carried out with the same pH at all temperatures (7.4), showed a region of nonlinearity at 10 degrees C. A very similar plot was obtained when no compensation was made for pH variation with temperature. Both the break in the Arrhenius plot and the activation energies for the scallop sarcoplasmic reticulum above and below the break were very similar to those found for lobster sarcoplasmic reticulum (Madeira, V. M. C., Antunes-Madeira, M. C., and Carvalho, A. R. (1974) Biochem. Biophys. Res. Commun. 65, 997-1003). The Arrhenius plot of the scallop Ca2+-ATPase in C12E8 no longer showed the nonlinearity at 10-12 degrees C seen with the native sarcoplasmic reticulum, but instead a break now appeared at 20-21 degrees C. This is close to the Arrhenius break temperature of rabbit Ca2+-ATPase in C12E8 and of a perturbation in C12E8 (Dean, W. L. (1982) Biophys. J. 37, 56-57).     

Heterologous expression of sarcoplasmic reticulum Ca2+-ATPase

In recent years, expression of rabbit sarcoplasmic reticulum (SR) Ca²⁺-ATPase in heterologous systems has been a widely used strategy to study altered enzymes generated by site-directed mutagenesis. Various eukaryotic expression systems have been tested, all of them yielding comparable amounts of recombinant protein. However, the relatively low yield of recombinant protein obtained so far suggests that novel purification techniques will be required to allow further characterization of this enzyme based on direct ligand-binding measurements.     

Occlusion of Ca2+ in soluble monomeric sarcoplasmic reticulum Ca2+-ATPase

Sarcoplasmic reticulum Ca2+-ATPase solubilized in monomeric form by nonionic detergent was reacted with CrATP in the presence of 45Ca2+. A Ca2+-occluded complex formed, which was stable during high performance liquid chromatography in the presence of excess non-radioactive Ca2+. The elution position corresponded to monomeric Ca2+-ATPase. It is concluded that a single Ca2+-ATPase polypeptide chain provides the full structural basis for Ca2+ occlusion.     

Crystallization of Ca2+-ATPase in detergent-solubilized sarcoplasmic reticulum

Microcrystalline arrays of Ca2+-transporting ATPase (EC 3.6.1.38) develop in detergent-solubilized sarcoplasmic reticulum upon exposure to 10-20 mM CaCl2 at pH 6.0 for several weeks at 2 degrees C, in a crystallization medium that preserves the ATPase activity for several months. Of 48 detergents tested, optimal crystallization was obtained with Brij 36T, Brij 56, and Brij 96 at a detergent:protein weight ratio of 4:1 and with octaethylene glycol dodecyl ether at a ratio of 2:1. Similar Ca2+-induced crystalline arrays were obtained with the purified or delipidated Ca2+-ATPase of sarcoplasmic reticulum but at lower detergent:protein ratios. The crystals are stabilized by fixation with glutaraldehyde and persist even after the removal of phospholipids by treatment with phospholipases A or C and by extraction with organic solvents. The crystals obtained so far can be used only for electron microscopy, but ongoing experiments suggest that under similar conditions large ordered arrays may develop that are suitable for x-ray diffraction analysis.     

Measurement of sarcoplasmic reticulum Ca2+-ATPase activity and E-type Mg2+-ATPase activity in rat heart homogenates

The presence of a high and nonlinear Ca2+-independent (or basal) ATPase activity in rat heart preparations makes difficult the reliable measurement of sarcoplasmic reticulum (SR) Ca2+-ATPase activity by usual methods. A spectrophotometric assay for the accurate determination of SR Ca2+-ATPase activity in unfractionated homogenates from rat heart is described. The procedure is based on that reported by Simonides and van Hardeveld (1990, Anal. Biochem. 191, 321-331) for skeletal muscle homogenates. To avoid overestimation of the Ca2+-ATPase activity of cardiac homogenates that occurs when sequential measurements of total and basal ATPase activities are performed, two parallel and independent assays are required: one with low (micromolar) and other high (millimolar) calcium concentration. Addition of thapsigargin (0.2 microM) blocked totally the activity considered as Ca2+-ATPase activity. Using this method, the rat heart homogenate Ca2+-ATPase activity was 10.5 +/- 2.0 micromol. min-1 x g-1 tissue wet weight (n = 8). Likewise, a spectrophotometric assay for measuring E-type Mg2+-ATPase activity in cardiac total homogenates has been developed, comparing the following characteristics of the enzymatic activity in homogenate and a membrane-enriched fraction: first-order rate constant for ATP-dependent inactivation, Km for ATP, and effects of concanavalin A, Triton X-100, and specific inhibitors.