Uptake of Ca2+ mediated by the (Ca2+ + Mg2+)-ATPase in reconstituted vesicles

The (Ca2+ + Mg2+)-ATPase was purified from skeletal muscle sarcoplasmic reticulum and reconstituted into sealed phospholipid vesicles by solution in cholate and deoxycholate followed by detergent removal on a column of Sephadex G-50. The level of Ca2+ accumulated by these vesicles, either in the presence or absence of phosphate within the vesicles, increased with increasing content of phosphatidylethanolamine in the phospholipid mixture used for the reconstitution. The levels of Ca2+ accumulated in the absence of phosphate were very low for vesicles reconstituted with egg yolk phosphatidylcholine alone at pH 7.4, but increased markedly with decreasing pH to 6.0. Uptake was also relatively low for vesicles reconstituted with dimyristoleoyl- or dinervonylphosphatidylcholine, and addition of cholesterol had little effect. The level of Ca2+ accumulated increased with increasing external K+ concentration, and was also increased by the ionophores FCCP and valinomycin. Vesicle sizes changed little with changing phosphatidylethanolamine content, and the sidedness of insertion of the ATPase was close to random at all phosphatidylethanolamine contents. It is suggested that the effect of phosphatidylethanolamine on the level of Ca2+ accumulation follows from an effect on the rate of Ca2+ efflux mediated by the ATPase.     

Cholesterol effect on enzyme activity of the sarcolemmal (Ca2+ + Mg2+)-ATPase from cardiac muscle

The effect of cholesterol incorporation and depletion of the cardiac sarcolemmal sacs on (Ca2+ + Mg2+)-ATPase activity was examined. Cholesterol incorporation to the sarcolemmal sacs was achieved utilizing an in vivo and an in vitro procedure. Cholesterol depleted membranes were obtained in vitro after incubation of the sarcolemmal sacs with inactivated plasma. Arrhenius plots of the (Ca2+ + Mg2+)-ATPase activity showed a triphasic curve when the assays were carried out using a temperature range between 0 and 40 degrees C. The sarcolemmal (Ca2+ + Mg2+)-ATPase activity was shown to be inversely proportional to the cholesterol concentration of the membranes, showing a low ATPase activity with a high cholesterol content and a high ATPase activity when the cholesterol concentration was low. Although the (Ca2+ + Mg2+)-ATPase activity was found to be inhibited in the cholesterol incorporated sarcolemmal sacs, the withdrawal of small amounts of cholesterol from the membranes produced an important stimulatory effect. Changes in (Ca2+ + Mg2+)-ATPase activity due to variation in the membrane cholesterol concentration were shown to be reversible. Our results indicate the possibility of a slow exchange of cholesterol between the tightly bound lipid surrounding the (Ca2+ + Mg2+)-ATPase and the bulk lipid of the sarcolemma.     

Regulation of cardiac sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase

Summary The two high affinity calcium binding sites of the cardiac (Ca²⁺ + Mg²⁺)-ATPase have been identified with the use of Eu³⁺. Eu³⁺ competes for the two high affinity calcium sites on the enzyme. With the use of laser-pulsed fluorescent spectroscopy, the environment of the two sites appear to be heterogeneous and contain different numbers of H₂O molecules coordinated to the ion. The ion appears to be occluded even further in the presence of ATP. Using non-radiative energy transfer studies, we were able to estimate the distance between the two Ca²⁺ sites to be between 9.4 to 10.2 A in the presence of ATP. Finally, from the assumption that the calcium site must contain four carboxylic side chains to provide the 6–8 ligands needed to coordinate calcium, and based on our recently published data, we predict the peptidic backbone of the two sites.     

A fast passive Ca2+ efflux mediated by the (Ca2+ + Mg2+)-ATPase in reconstituted vesicles

The (Ca2+ + Mg2+)-ATPase from skeletal muscle sarcoplasmic reticulum was reconstituted into phospholipid bilayers. The permeability of lipid bilayers to Co2+ and glucose was increased slightly by incorporation of the ATPase, and the permeability of mixed bilayers of phosphatidylethanolamine and phosphatidylcholine increased with increasing content of phosphatidylethanolamine both in the presence and absence of the ATPase. The presence of the ATPase, however, resulted in a marked increase in permeability to Ca2+, the permeability decreasing with increasing phosphatidylethanolamine content. Permeability to Ca2+ was found to be dependent on pH and the external concentrations of Mg2+ and Ca2+, was stimulated by adenine nucleotides but was unaffected by inositol trisphosphate. A kinetic model is presented for Ca2+ efflux mediated by the ATPase. It is shown that the kinetic parameters that describe Ca2+ efflux from vesicles of sarcoplasmic reticulum also describe efflux from the vesicles reconstituted from the purified ATPase and phosphatidylcholine. It is shown that the effects of phosphatidylethanolamine on efflux can be simulated in terms of changes in the rates of the transitions linking conformations of the ATPase with inward- and outward-facing Ca2+-binding sites, and that effects of phosphatidylethanolamine on the ATPase activity of the ATPase can also be simulated in terms of effects on the corresponding conformational transitions. We conclude that the ATPase can act as a specific pathway for Ca2+ efflux from sarcoplasmic reticulum.     

Protein-lipid interaction. Biophysical studies of (Ca2+ + Mg2+)-ATPase reconstituted systems

Differential scanning calorimetry, fluorescence spectroscopy and freeze-fracture electron microscopy have been applied to a study of the reconstituted Ca2+-ATPase proteins from sarcoplasmic reticulum when they are incorporated into pure lipid/water systems. The results obtained with these techniques have been used to examine the effects of this intrinsic protein upon the surrounding lipid at temperatures above and below the main lipid solid-fluid phase transition temperature (Tc). 1. Above this Tc value, the freeze-fracture data show that the proteins are randomly distributed within the plane of the bilayer. The fluorescence data show that as the protein content in the bilayer increases, so does the 'microviscosity'. 2. Below Tc the proteins occur in high protein to lipid patches, separate from the remaining crystalline lipid. The fluorescence data indicate that at these temperatures the presence of the protein causes a decrease in microviscosity, whilst the calorimetric data indicate a decrease in enthalpy of the main lipid transition. 3. A premelting of the high protein to lipid patches formed by phase separation within the lipid bilayers is indicated by the calorimetric and fluorescence data. This observation is used to rationalise the 'anomalous' properties of the dipalmitoyl phosphatidylcholine-ATPase of exhibiting activity at temperatures well below the lipid phase transition at 41 degrees C.     

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.     

Calmodulin-mediated regulation of calcium transport and (Ca2+ + Mg2+)-activated ATPase activity in isolated cardiac sarcoplasmic reticulum

A severalfold activation of calcium transport and (Ca2+ + Mg2+)-activated ATPase activity by micromolar concentrations of calmodulin was observed in sarcoplasmic reticulum vesicles obtained from canine ventricles. This activation was seen in the presence of 120 mM KCl. The ratio of moles of calcium transported per mol of ATP hydrolyzed remained at about 0.75 when calcium transport and (Ca2+ + Mg2+)-activated ATPase activity were measured in the presence and absence of calmodulin. Thus, the efficiency of the calcium transport process did not change. Stimulation of calcium transport by calmodulin involves the phosphorylation of one or more proteins. The major 32P-labeled protein, as determined by sodium dodecyl sulfate slab gel electrophoresis, was the 22,000-dalton protein called phospholamban. The Ca2+ concentration dependency of calmodulin-stimulated microsomal phosphorylation corresponded to that of calmodulin-stimulated (Ca2+ + Mg2+)-activated ATPase activity. Proteins of 11,000 and 6,000 daltons and other proteins were labeled to a lesser extent. A similar phosphorylation pattern was obtained when microsomes were incubated with cAMP-dependent protein kinase and ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Phosphorylation produced by added cAMP-dependent protein kinase and calmodulin was additive. These studies provided further evidence for Ca2+-dependent regulation of calcium transport by calmodulin in sarcoplasmic reticulum that could play a role in the beat-to-beat regulation of cardiac relaxation in the intact heart.     

Mechanisms of Ca2+ transport in plasma membrane vesicles prepared from cultured pituitary cells. II. (Ca2+ + Mg2+)-ATPase-dependent Ca2+ transport activity

Purified plasma membrane vesicles from GH3 rat anterior pituitary cells exhibit a Mg2+-ATP-dependent Ca2+ transport activity. Concentrative uptake of Ca2+ is abolished by exclusion of either Mg2+ or ATP or by inclusion of the Ca2+ ionophore A23187. Furthermore, addition of A23187 to vesicles which have reached a steady state of ATP-supported Ca2+ accumulation rapidly and completely discharges accumulated cation. Ca2+ uptake is unaffected by treatment of vesicles with oligomycin, the uncoupler CCCP, or valinomycin and is greatly reduced in non-plasma membrane fractions. Likewise, Ca2+ accumulation is not stimulated by oxalate, consistent with the plasma membrane origin of this transport system. (Na+, K+)-ATPase participation in the Ca2+ transport process (i.e. via coupled Na+/Ca2+ exchange) was eliminated by omitting Na+ and including ouabain in the reaction medium. Ca2+ transport activity in GH3 vesicles has a similar pH dependence as that seen in a number of other plasma membrane systems and is inhibited by orthovanadate in the micromolar range. Inhibition is enhanced if the membranes are preincubated with vanadate for a short time. A kinetic analysis of transport indicates that the apparent Km for free Ca2+ and ATP are 0.7 and 125 microM, respectively. The average Vmax is 3.6 nmol of Ca2+/min/mg of protein at 37 degrees C. Addition of exogenous calmodulin or calmodulin antagonists had no significant effect on these kinetic properties. GH3 plasma membranes also contain a Na+/Ca2+ exchange system. The apparent Km for Ca2+ is almost 10-fold higher in this system than that for ATP-driven Ca2+ uptake. When both processes are compared under similar conditions, the Vmax of the exchanger is approximately 2-3 times that of ATP-dependent Ca2+ accumulation. Similar results are obtained when purified plasma membranes from bovine anterior pituitary glands were investigated. It is suggested that both Na+/Ca2+ exchange and the (Ca2+ + Mg2+)-ATPase are important in controlling intracellular levels of Ca2+ in anterior pituitary cells.     

Cross-linking of the (Ca2+ + Mg2+)-ATPase protein.

The addition of cupric-1,10,-phenanthroline, a cross-linking catalyst, to sarcoplasmic reticulum membranes caused protein sulfhydryl groups to form disulfide bridges. Following a short exposure to the catalyst (15 s, 22 degrees C) most of the protein was in a dimeric form (Mr = 248 000). Longer exposure times resulted in the formation of trimers, tetramers and other oligomers too large to enter the gel. At low temperatures (4 degrees C) dimer formation predominates even for exposure times as long as 5 min. Cross-linking in the presence of 7.5 mM Triton X-100 (a concentration that resulted in clearing of the membrane suspension and thus solubilization of the membrane components) showed the appearance of a considerable dimer fraction, however, most of the (Ca2+ + Mg2+)-ATPase protein appeared as a monomer. Following 1 min of cross-linking at 22 degrees C, freeze-etched membranes showed no alteration in the number or appearance of 80 A intramembranous particles. Thus extensive cross-linking of the (Ca2+ + Mg2+)-ATPase protein can occur without disruption of the normal position of the intramembrane portion of the molecule.     

Characterization of Mg2+- and (Ca2+ + Mg2+)-ATPase activity in adipocyte endoplasmic reticulum

The presence of an energy-dependent calcium uptake system in adipocyte endoplasmic reticulum (D. E. Bruns, J. M. McDonald, and L. Jarett, 1976, J. Biol. Chem.251, 7191–7197) suggested that this organelle might possess a calcium-stimulated transport ATPase. This report describes two types of ATPase activity in isolated microsomal vesicles: a nonspecific, divalent cation-stimulated ATPase (Mg²⁺-ATPase) of high specific activity, and a specific, calcium-dependent ATPase (Ca²⁺ + Mg²⁺-ATPase) of relatively low activity. Mg²⁺-ATPase activity was present in preparations of mitochondria and plasma membranes as well as microsomes, whereas the (Ca²⁺ + Mg²⁺)-ATPase activity appeared to be localized in the endoplasmic reticulum component of the microsomal fraction. Characterization of microsomal Mg²⁺-ATPase activity revealed apparent K_m values of 115 μm for ATP, 333 μm for magnesium, and 200 μm for calcium. Maximum Mg²⁺-ATPase activity was obtained with no added calcium and 1 mm magnesium. Potassium was found to inhibit Mg²⁺-ATPase activity at concentrations greater than 100 mm. The energy of activation was calculated from Arrhenius plots to be 8.6 kcal/mol. Maximum activity of microsomal (Ca²⁺ + Mg²⁺)-ATPase was 13.7 nmol ³²P/mg/min, which represented only 7% of the total ATPase activity. The enzyme was partially purified by treatment of the microsomes with 0.09% deoxycholic acid in 0.15 m KCl which increased the specific activity to 37.7 nmol ³²P/mg/min. Characterization of (Ca²⁺ + Mg²⁺)-ATPase activity in this preparation revealed a biphasic dependence on ATP with a Hill coefficient of 0.80. The apparent K_m^s for magnesium and calcium were 125 and 0.6–1.2 μm, respectively. (Ca²⁺ + Mg²⁺)-ATPase activity was stimulated by potassium with an apparent K_m of 10 mm and maximum activity reached at 100 mm potassium. The energy of activation was 21.5 kcal/mol. The kinetics and ionic requirements of (Ca²⁺ + Mg²⁺)-ATPase are similar to those of the (Ca²⁺ + Mg²⁺)-ATPase in sarcoplasmic reticulum. These results suggest that the (Ca²⁺ + Mg²⁺)-ATPase of adipocyte endoplasmic reticulum functions as a calcium transport enzyme.     

Synthesis and characterization of a peptide segment of (Ca2+ + Mg2+)-ATPase. A candidate for calcium transport site

The second tryptic digestion (TD2) of the (Ca2+ + Mg2+)-ATPase results in the decrease of Ca2+ transport due to uncoupling and the alteration of one of the two high affinity sites to a low affinity site. The eight amino acids adjacent to the tryptic digestion site form a torus with two carboxylic side chains of one aspartic and one glutamic acid for the fast twitch skeletal ATPase and two aspartic acids for the slow twitch/cardiac ATPase toward the inside. The eight amino acid peptides were synthesized for both forms of the ATPase and their binding characteristics were studied with luminescent Eu3+ as a Ca2+ analogue. The data indicate that the peptide binds Eu3+ with 1.0 Eu3+/peptide and strips off two water molecules. The peptide region is a candidate for the Ca2+ transport site of the (Ca2+ + Mg2+)-ATPase.     

Effect of amphiphilic surfactant LDAO on the solubilization of DOPC vesicles and on the activity of Ca(2+)-ATPase reconstituted in DOPC vesicles

Solubilization of large unilamellar 1,2-dioleoylphosphatidylcholine (DOPC) vesicles by N-dodecyl-N,N-dimethylamine-N-oxide (LDAO) was studied using turbidimetry. From turbidity data, the LDAO partition coefficient between the aqueous phase and DOPC bilayers was obtained. Using this partition coefficient, the LDAO:DOPC molar ratio in the bilayer was calculated and effects of LDAO on the bilayer stability, bilayer thickness and on the phosphohydrolase activity of sarcoplasmic reticulum Ca(2+) transporting ATPase (SERCA) reconstituted into DOPC were compared at the same LDAO:DOPC molar ratios in the bilayer. The sequence "bilayers in vesicles - bilayer fragments (flat mixed micelles) - tubular mixed micelles - globular mixed micelles" was suggested for the solubilization mechanism of DOPC vesicles from the combined turbidimetric and small-angle neutron scattering (SANS) results. The effective molecular packing parameter delta = 0.5, corresponding to the mixed bilayer - mixed tubular micelle transition, was calculated from fragmental DOPC and LDAO volumes at the molar ratio LDAO:DOPC = 2.00 in bilayers, in the middle of transition region observed earlier experimentally by small-angle neutron scattering (SANS). The bilayer thickness decrease induced by LDAO in DOPC observed by SANS did not result in the SERCA phosphohydrolase activity decrease and this indicates that some other factors compensated this bilayer effect of LDAO. The ATPase activity decrease at higher LDAO concentrations was caused by the bilayer deformation. This deformation resulted in the formation of non-bilayer aggregates in LDAO+DOPC system.     

ATP-dependent calcium transport in cardiac sarcolemmal membrane vesicles

Cardiac sarcolemmal (SL) membrane vesicles accumulated Ca in the presence of ATP. The accumulated Ca was released by osmotic shock and by the Ca ionophore A23187, indicating that the Ca had been transported into the vesicle interior. Ca uptake by the SL vesicles was not inhibited by ruthenium red, 2,4-dinitrophenol, carbonyl cyanide $\text{m}$-chlorophenyl hydrazone, of NaN₃, agents that are known to inhibit mitochondrial Ca transport activity. In contrast to the behavior of cardiac sarcoplasmic reticulum, Ca accumulation by the SL vesicles was not stimulated by oxalate and could not driven by p-nitrophenylphosphate hydrolysis. NaCl inhibited ATP-dependent Ca uptake by the SL vesicles. This effect was shown to be due to a stimulation of Ca efflux by Na, mediated by the sarcolemmal NaCa exchange system. The results provide conclusive evidence for the presence of an ATP-dependent Ca “pump” in the cardiac SL membrane.     

Regulation of (Ca2+, Mg2+)-ATPase in human erythrocytes dependent on calcium and calmodulin

The enzymatic basis for active Ca2+ transport in erythrocytes, and probably in many other cells, is a Mg2+-dependent Ca2+-ATPase located in the plasma membrane. The Ca2+-ATPase can be prepared in two different forms, a calmodulin-deficient (A-state) and a calmodulin-saturated (B-state). The ATPase is regulated by Ca2+-, K+, and ATP, and these effectors interact in a cooperative way, especially in the B-state. Both A-state and B-state can be solubilized by treatment with Triton X-100. The detergent activates the A-state but reduces the cooperative interactions in both states. The membrane-bound A and B state contain 5 and 25 nmoles calmodulin/g membrane protein, respectively, and the amount of Ca2+-ATPase was estimated at 10--20 nmoles/g membrane protein. It is discussed whether both A and B state represent natives states of the pump-enzymes.     

Effect of nitric oxides and hydrogen peroxide on Ca2+, Mg(2+)-ATPase and Mg(2+)-ATPase activity in myometrium sarcolemma

The action of sodium nitroprusside, nitrite-anions and hydrogen peroxide on Ca2+, Mg(2+)-ATPase and Mg(2+)-ATPase (Ca(2+)-independent) enzymatic activity in myometrium sarcolemma fraction is investigated. It is established, that 0.1 mM sodium nitroprusside and 10(-8)-10(-5) M nitrite-anions essentially reduce Ca2+, Mg(2+)-ATPase activity whereas Mg(2+)-ATPase proved to be absolutely resistant to them. At rather high concentration of nitrite-anions (0.1 mM) appreciable stimulation of Ca2+, Mg(2+)-ATPase was observed. Hydrogen peroxide (10(-8)-10(-4)), depending on the concentration suppressed both enzymes activity. However, Ca2+, Mg(2+)-ATPase proved to be more sensitive to the action of H2O2 (seeming K(i) = 0.42 +/- 0.1 microM), than Mg(2+)-ATPase (seeming K(i) = 3.1 +/- 0.9 microM). At presence of 1 mM ditiothreitole (a reducer of SH groups of the membrane surface) action of investigated substances considerably decreased. Reagents on carboxic- (dicyclogexilcarbodiimid) and amino- groups of the membrane (trinitrobenzolsulfonic acid) inhibited both Ca2+, Mg(2+)-ATPase, and Mg(2+)-ATPase activity in membrane fractions. In the presence of noted reagents sodium nitroprusside and nitrite-anions action was not almost shown. Hence, nitrogen oxide, nitrite-anions and hydrogen peroxide suppress Ca2+, Mg(2+)-ATPase and Mg(2+)-ATPase (only hydrogen peroxide) activity in the plasmatic membrane of myometrium cells, and this action can be connected with direct updating of superficial chemical groups of the membrane.     

Role of Mg2+ in the Ca2+-Ca2+ exchange mediated by the membrane-bound (Ca2+, Mg2+)-ATPase of sarcoplasmic reticulum vesicles

Sarcoplasmic reticulum vesicles were preloaded with either 45Ca2+ or unlabeled Ca2+. The unidirectional Ca2+ efflux and influx, together with Ca2+-dependent ATP hydrolysis and phosphorylation of the membrane-bound (Ca2+, Mg2+)-ATPase, were determined in the presence of ATP and ADP. The Ca2+ efflux depended on ATP (or ADP or both). It also required the external Ca2+. The Ca2+ concentration dependence of the efflux was similar to the Ca2+ concentration dependences of Ca2+ influx, Ca2+-dependent ATP hydrolysis, and phosphoenzyme formation. The rate of the efflux was approximately in proportion to the concentration of the phosphoenzyme up to 10 microM Ca2+. These results and other findings indicate that the Ca2+ efflux represents the Ca2+-Ca2+ exchange (between the external medium and the internal medium) mediated by the phosphoenzyme. In the range of 0.6-5.2 microM Mg2+, no appreciable Ca2+-Ca2+ exchange was detected although phosphoenzyme formation occurred to a large extent. Elevation of Mg2+ in the range 5.2 microM-4.8 mM caused a remarkable activation of the exchange, whereas the amount of the phosphoenzyme only approximately doubled. The kinetic analysis shows that this activation results largely from the Mg2+-induced acceleration of an exchange between the bound Ca2+ of the phosphoenzyme and the free Ca2+ in the internal medium. It is concluded that Mg2+ is essential for the exposure of the bound Ca2+ of the phosphoenzyme to the internal medium.