Structure and function of the calcium pump protein of sarcoplasmic reticulum

Biochemical properties of isolated transverse tubular membranes

This review addresses the major biochemical and structural characteristics of isolated transverse tubule (T-tubule) membranes, including methods of isolation and morphology of purified membranes, evaluation of attendant membrane activities, including ion pumps and channels, and structural and compositional analyses of functionally relevant components. Particular emphasis is placed on the Mg2+-ATPase, its localization in the T-system, its unusual kinetic properties, its possible functions, and its potential regulation by diacylglycerol and other biologically-relevant lipids. Conclusions are drawn with respect to the biochemical markers characteristic of T-tubule membranes and the criteria to be applied in the assessment of isolated T-tubule membrane purity.

New mutants of Paramecium tetraurelia defective in a calcium control mechanism: genetic and behavioral characterizations

The k-shy mutants of Paramecium tetraurelia are altered in several Ca2+-dependent functions which regulate ciliary motility. The isolation, genetics, and phenotypes of these mutants are described. Of six independent isolates, all contained recessive single-factor mutations and comprise two unlinked loci, ksA and ksB. All k-shy strains showed prolonged backward swimming responses to depolarizing stimuli, but gave infrequent responses to some stimuli. At least four k-shy strains displayed temperature sensitivity. Neither ksA nor ksB was allelic or linked to dancer, a mutation causing weak Ca2+ current inactivation and prolonged backward swimming. Analysis of ks+; Dn double mutants revealed synergism between the two mutations. The ksA mutant survived Ba2+ solutions longer than wild type, but was more sensitive to K+. Together with previous studies, these results are consistent with a defect in reducing intracellular Ca2+ causing both prolonged ciliary reversal and reduced Ca2+ channel activity due to more active Ca2+-dependent feedback mechanisms. The integration of the Ca2+-dependent stimulatory and inhibitory functions is therefore dependent on ks+ gene functions. The ksA mutant was rescued by microinjection of wild-type cytoplasm, suggesting a possible behavioral assay for factors related to the ksA+ gene product.

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.

Bafilomycins: a class of inhibitors of membrane ATPases from microorganisms, animal cells, and plant cells

Various membrane ATPases have been tested for their sensitivity to bafilomycin A1, a macrolide antibiotic. F1F0 ATPases from bacteria and mitochondria are not affected by this antibiotic. In contrast, E1E2 ATPases--e.g., the K+-dependent (Kdp) ATPase from Escherichia coli, the Na+,K+-ATPase from ox brain, and the Ca2+-ATPase from sarcoplasmic reticulum--are moderately sensitive to this inhibitor. Finally, membrane ATPases from Neurospora vacuoles, chromaffin granules, and plant vacuoles are extremely sensitive. From this we conclude that bafilomycin A1 is a valuable tool for distinguishing among the three different types of ATPases and represents the first relatively specific potent inhibitor of vacuolar ATPases.

Symbioramide, a novel Ca2+-ATPase activator from the cultured dinoflagellate Symbiodinium sp

A novel sphingosine derivative, symbioramide, has been isolated from the laboratory-cultured dinoflagellate Symbiodinium sp. as a sarcoplasmic reticulum (SR)Ca2+-ATPase activator, and its structure elucidated to be 1 on the basis of spectral and chemical means.

Effect of urea on the partial reactions and crystallization pattern of sarcoplasmic reticulum adenosine triphosphatase

Steady-state ATPase activity, calcium binding, formation of phosphorylated enzyme intermediate with ATP in the presence of Ca2+, or with Pi in the absence of Ca2+, and association of ATPase molecules into bidimensional crystals, were studied using vesicular fragments of sarcoplasmic reticulum. The vesicles were exposed to increasing concentrations of urea in order to produce stepwise perturbations of protein structure and to test the effect of such perturbations on the partial reactions and crystallization pattern of sarcoplasmic reticulum ATPase. It was found that low concentrations of urea produce specific inhibition of Pi binding and enzyme phosphorylation with Pi (but not with ATP). Intermediate concentrations of urea reduce calcium binding affinity and cooperativity, while the ability of the enzyme to be phosphorylated with ATP and to form dimeric arrays is retained. These observations demonstrate that the sarcoplasmic reticulum ATPase is sensitive to physical perturbations producing specific and reversible changes in the Pi and calcium binding domains. These changes interfere with enzyme turnover, indicating that conformational effects related to binding and dissociation of Pi and calcium are tightly coupled to catalysis and energy transduction. Higher concentrations of urea produce irreversible denaturation, accompanied by total inhibition of calcium binding, enzyme phosphorylation with ATP, and association of ATPase chains in bidimensional crystals. Under these conditions, protein unfolding is manifested by a sharp reduction in the fluorescence of intrinsic tryptophan residues and of a covalently bound probe. These observations suggest that dimeric association and a tendency to form bidimensional crystals correspond to a basic property of the enzyme, which is linked to its native structure and whose character may change in the presence of ligands and/or during the catalytic cycle. On the other hand, the decavanadate-induced crystallization pattern cannot be interpreted in terms of a mechanistic relationship of ATPase dimerization with one of the intermediate states of the catalytic cycle.

Identification of the Ca2+-release activity and ryanodine receptor in sarcoplasmic-reticulum membranes during cardiac myogenesis

Ca2+-induced Ca2+ release and pH-induced Ca2+ release activities were identified in sarcoplasmic-reticulum (SR) vesicles isolated from adult- and fetal-sheep hearts. Ca2+-induced Ca2+ release and pH-induced Ca2+ release appear to proceed via the same channels, since both phenomena are similarly inhibited by Ruthenium Red. Ca2+ release from fetal SR vesicles is inhibited by higher concentrations of Ruthenium Red than is that from adult membranes. Both fetal and adult SR vesicles bind ryanodine. Fetal SR shows higher ryanodine-binding capacity than adult SR vesicles. Scatchard analysis of ryanodine binding revealed only one high-affinity binding site (Kd 6.7 nM) in fetal SR vesicles compared with two distinct binding sites (Kd 6.6 and 81.5 nM) in the adult SR vesicles. SR vesicles isolated from fetal and adult hearts were separated on discontinuous sucrose gradients into light (free) and heavy (junctional) SR vesicles. Heavy SR vesicles isolated from adult hearts exhibited most of the Ca2+ release activities. In contrast, Ca2+-induced Ca2+ release, pH-induced Ca2+ release and ryanodine receptors were detected in both light and heavy fetal SR. These results suggest that fetal SR may not be morphologically and functionally as well differentiated as that of adult cardiac muscle and that it may contain a greater number of Ca2+-release channels than that present in adult SR membranes.

Specificity of the sarcoplasmic reticulum calcium ATPase at the hydrolysis step

The coupling of Ca2+ transport to ATP hydrolysis by the SR ATPase requires that the enzyme operate with considerable specificity, which is different at different steps. The limits of specificity of the calcium-free phosphorylated enzyme for transfer of its phosphoryl group to water have been examined. The rate of transfer of the phosphoryl group to the simple nucleophile methanol was compared to its transfer to water by following the formation of methyl phosphate from inorganic phosphate. The reverse reaction, hydrolysis of methyl phosphate, was compared to phosphate-water oxygen exchange. The reactions involving methanol as nucleophile or leaving group are at least 2-3 orders of magnitude slower than those involving water. This result indicates that the transition state for this reaction involves strong and specific interactions of the H2O molecule with the enzyme. These interactions may also involve the bound Mg2+ ion. The results also suggest that the difference in specificity between Ca2+ free and Ca2+ bound states of the enzyme involves significant differences in the structure of the catalytic site.

Caffeine interaction with the Ca-release channels of heavy sarcoplasmic reticulum. Evidence that 170 kD Ca-binding protein is a caffeine receptor of the Ca-channels

The study of Ca2+- and caffeine-induced Ca2+ release from heavy sarcoplasmic reticulum vesicles under the different conditions suggests that Ca2+ and caffeine can interact with the common receptor of the Ca-release channels. The reticulum membranes were solubilized using nonionic detergent polyoxyethylene 9-lauryl ether, and affinity chromatography on reactive red 120-agarose was carried out. The 170 kD Ca-binding protein which is eluted by caffeine is the most probable candidate for the caffeine receptor of the Ca-channels.

Ryanodine binding to sarcoplasmic reticulum membrane; comparison between cardiac and skeletal muscle

[3H]Ryanodine binding to skeletal muscle and cardiac sarcoplasmic reticulum (SR) vesicles was compared under experimental conditions known to inhibit or stimulate Ca2+ release. In the skeletal muscle SR, ryanodine binds to a single class of high-affinity sites (Kd of 11.3 nM). In cardiac SR vesicles, more than one class of binding sites is observed (Kd values of 3.6 and 28.1 nM). Ryanodine binding to skeletal muscle SR vesicles requires high concentrations of NaCl, whereas binding of the drug to cardiac SR is only slightly influenced by ionic strength. In the presence of 5'-adenylyl imidodiphosphate (p[NH]ppA), increased pH, and micromolar concentration of Ca2+ (which all induce Ca2+ release from SR) binding of ryanodine to SR is significantly increased in skeletal muscle, while being unchanged in cardiac muscle. Ryanodine binding to skeletal but not to cardiac muscle SR is inhibited in the presence of high Ca2+ or Mg2+ concentrations (all known to inhibit Ca2+ release from skeletal muscle SR). Ruthenium red or dicyclohexylcarbodiimide modification of cardiac and skeletal muscle SR inhibit Ca2+ release and ryanodine binding in both skeletal and cardiac membranes. These results indicate that significant differences exist in the properties of ryanodine binding to skeletal or cardiac muscle SR. Our data suggest that ryanodine binds preferably to site(s) which are accessible only when the Ca2+ release channel is in the open state.

Phosphoenzyme decomposition in dog cardiac sarcoplasmic reticulum Ca2+-ATPase

A five-syringe quench-flow apparatus was used in the transient-state kinetic study of intermediary phosphoenzyme (EP) decomposition in a Triton X-100 purified dog cardiac sarcoplasmic reticulum (SR) Ca2+-ATPase at 20 degrees C. Phosphorylation of the enzyme by ATP in the presence of 100 mM K+ for 116 ms gave 32% ADP-sensitive E1P, 52% ADP- and K+-reactive E2P, and 16% unreactive residual EPr. The EP underwent a monomeric, sequential E1P 17 s-1----E2P 10.5 s-1----E2 + Pi transformation and decomposition in the ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid quenched Ca2+-devoid medium. The calculated rate constant for the total EP (i.e., E1P + E2P) dephosphorylation was 7.8 s-1. The E1P had an affinity for ADP with an apparent Kd congruent to 100 microM. When the EP was formed in the absence of K+ for 116 ms, no appreciable amount of the ADP-sensitive E1P was detected. The EP comprised about 80% ADP- and K+-reactive E2P and 20% residual EPr, suggesting a rapid E1P----E2P transformation. Both the E2P's formed in the presence and absence of K+ decomposed with a rate constant of about 19.5 s-1 in the presence of 80 mM K+ and 2 mM ADP, showing an ADP enhancement of the E2P decomposition. The results demonstrate mechanistic differences in monomeric EP transformation and decomposition between the Triton X-100 purified cardiac SR Ca2+-ATPase and deoxycholate-purified skeletal enzyme [Wang, T. (1986) J. Biol. Chem. 261, 6307-6319].

Sodium-calcium exchange in sarcolemmal vesicles from tracheal smooth muscle

Sarcolemmal vesicles prepared by a new procedure from bovine tracheal smooth muscle were found to have a Na-Ca exchange activity that is significantly higher than that reported for different preparations from other types of smooth muscle. The exchange process system co-purified with 5'-nucleotidase, a plasma membrane marker enzyme, and was significantly enriched (over 100-fold) compared to mitochondria (cytochrome-c oxidase) but only slightly enriched (4-fold) compared to sarcoplasmic reticulum (NADPH-cytochrome-c reductase). The Na+ dependence of Ca2+ transport was demonstrated through both uptake and efflux procedures. The uptake profile with respect to Ca2+ was monotonic with a linear vo VS. vo.S-1 plot. The resultant Km of Ca2+ from the airway sarcolemmal vesicles (20 microM) was similar in magnitude to the Km of cardiac sarcolemmal vesicles (30 microM). Tracheal vesicles demonstrated a Vmax of 0.3-0.5 nmol.mg-1.s-1 which is significantly higher than that reported in preparations from other smooth muscle types. Furthermore, two processes found to stimulate cardiac Na-Ca exchange, pretreatment with either a mixture of dithiothreitol and Fe2+ or with chymotrypsin, were ineffective on the tracheal smooth muscle. Thus, the Na-Ca exchanger identified in tracheal smooth muscle appears to be different from that observed in cardiac muscle, implying that regulation of this activity may also be different.

Stimulation of Ca2+ efflux from sarcoplasmic reticulum by preincubation with ATP and inorganic phosphate

Preincubation of sarcoplasmic reticulum with 1 mM-ATP completely inhibits Ca2+ accumulation and stimulates ATPase activity by over 2-fold. This effect of ATP is obtained only when the preincubation is carried out in the presence of Pi, but not with arsenate, chloride or sulphate. The inhibition by ATP of Ca2+ accumulation is pH-dependent, increasing as the pH is increased above 7.5. Inhibition of Ca2+ accumulation is observed on preincubation with ATP, but not with CTP, UTP, GTP, ADP, adenosine 5'-[beta gamma-methylene]triphosphate or adenosine 5'-[beta gamma-imido]triphosphate. The presence of Ca2+, but not Mg2+, during the preincubation, prevents the effect of ATP + Pi on Ca2+ accumulation. The ATP + Pi inhibition of Ca2+ accumulation is not due to modification of the ATPase catalytic cycle, but rather to stimulation of a rapid Ca2+ efflux from actively or passively loaded vesicles. This Ca2+ efflux is inhibited by dicyclohexylcarbodi-imide. Photoaffinity labelling of sarcoplasmic-reticulum membranes with 8-azido-[alpha-32P]ATP resulted in specific labelling of two proteins, of approx. 160 and 44 kDa. These proteins were labelled in the presence of Pi, but not other anions.

Lack of effects of calcium X calmodulin-dependent phosphorylation on Ca2+ release from cardiac sarcoplasmic reticulum

Canine cardiac sarcoplasmic reticulum is phosphorylated by an endogenous calcium X calmodulin-dependent protein kinase and phosphorylation occurs mainly on a 27 kDa proteolipid, called phospholamban. To determine whether this phosphorylation has any effect on Ca2+ release, sarcoplasmic reticulum vesicles were phosphorylated by the calcium X calmodulin-dependent protein kinase, while non-phosphorylated vesicles were preincubated under identical conditions but in the absence of ATP to avoid phosphorylation. Both non-phosphorylated and phosphorylated vesicles were centrifuged to remove calmodulin, and subsequently used for Ca2+ release studies. Calcium loading was carried out either by the active calcium pump or by incubation with high (5 mM) calcium for longer periods. Phosphorylation of sarcoplasmic reticulum by calcium X calmodulin-dependent protein kinase had no appreciable effect on the initial rates of Ca2+ released from cardiac sarcoplasmic reticulum vesicles loaded under passive conditions and on the apparent 45Ca2+-40Ca2+ exchange from cardiac sarcoplasmic reticulum vesicles loaded under active conditions. Thus, it appears that calcium X calmodulin-dependent protein kinase mediated phosphorylation of cardiac sarcoplasmic reticulum is not involved in the regulation of Ca2+ release and 45Ca2+-40Ca2+ exchange.

Electrophysiological evidence suggests a defective Ca2+ control mechanism in a new Paramecium mutant

A new mutant of Paramecium tetraurelia, k-shyA, was characterized behaviorally and electrophysiologically. The mutant cell exhibited prolonged backward swimming episodes in response to depolarizing conditions. Electrophysiological comparison of k-shyA with wild type cells under voltage clamp revealed that the properties of three Ca2+-regulated currents were altered in the mutant. (i) The voltage-dependent Ca2+ current recovered from Ca2+-dependent inactivation two- to 10-fold more slowly than wild type. Ca2+ current amplitudes were also reduced in the mutant, but could be restored by EGTA injection. (ii) The decay of the Ca2+-dependent K+ tail current was slower in the mutant. (iii) The decay of the Ca2+-dependent Na+ tail current was also slower in the mutant. All other membrane properties studied, including the resting membrane potential and resistance and the voltage-sensitive K+ currents, were normal in k-shyA. Considered together, these observations are consistent with a defect in the ability of k-shyA to reduce the free intracellular Ca2+ concentration following stimulation. The possible targets of the genetic lesion and alternative explanations are discussed. The k-shy mutants may provide a useful tool for molecular and physiological analyses of the regulation of Ca2+ metabolism in Paramecium.

Phospholamban stoichiometry in canine cardiac muscle sarcoplasmic reticulum

Treatment of cardiac sarcoplasmic reticulum with the crosslinking reagent dithiobis (succinimidyl propionate) in the presence of 125I-calmodulin, resulted in the formation of a 40,000-dalton affinity labeled component, consisting of a 1:1, phospholamban: 125I-calmodulin complex. In parallel experiments, sarcoplasmic reticulum was phosphorylated in the presence of calmodulin and [gamma-32P]ATP, and then treated with the crosslinking reagent to produce an affinity labeled component consisting of a 1:1, calmodulin: 32P-phospholamban complex. These experiments permitted determination of the amount of 125I and 32P incorporated into the 40,000-dalton complexes, as well as the amount of 32P incorporated into the 23,000-dalton form of phospholamban. If 1 mol of Ca2+-dependent ATPase phosphoprotein represents 1 mol of 100,000-dalton Ca2+-dependent ATPase monomer, then there are 4.88 +/- 1.33 mol Ca2+-dependent ATPase/mol of phospholamban. If there are 2 mol of Ca2+-dependent ATPase phosphoprotein/mol of 100,000-dalton Ca2+-dependent ATPase monomer, then there are 9.76 +/- 2.66 mol Ca2+-dependent ATPase/mol phospholamban.

Gingerol, a novel cardiotonic agent, activates the Ca2+-pumping ATPase in skeletal and cardiac sarcoplasmic reticulum

Gingerol, isolated as a potent cardiotonic agent from the rhizome of ginger, stimulated the Ca2+-pumping activity of fragmented sarcoplasmic reticulum (SR) prepared from rabbit skeletal and dog cardiac muscles. The extravesicular Ca2+ concentrations of the heavy fraction of the fragmented SR (HSR) were measured directly with a Ca2+ electrode to examine the effect of gingerol on the SR. Gingerol (3-30 microM) accelerated the Ca2+-pumping rate of skeletal and cardiac SR in a concentration-dependent manner. The rate of 45Ca2+ uptake of HSR was also increased markedly by 30 microM gingerol without affecting the 45Ca2+ efflux from HSR. Furthermore, gingerol activated Ca2+-ATPase activities of skeletal and cardiac SR (EC50, 4 microM). The activation of SR Ca2+-ATPase activity by gingerol (30 microM) was completely reversed by 100-fold dilution with the fresh saline solution. Kinetic analysis of activating effects of gingerol suggests that the activation of SR Ca2+-ATPase is uncompetitive and competitive with respect to Mg . ATP at concentrations of 0.2-0.5 mM and above 1 mM, respectively. Kinetic analysis also suggests that the activation by gingerol is mixed-type with respect to free Ca2+ and this enzyme is activated probably due to the acceleration of enzyme-substrate complex breakdown. Gingerol had no significant effect on sarcolemmal Ca2+-ATPase, myosin Ca2+-ATPase, actin-activated myosin ATPase and cAMP-phosphodiesterase activities, indicating that the effect of gingerol is rather specific to SR Ca2+-ATPase activity. Gingerol may provide a valuable chemical tool for studies aimed at clarifying the regulatory mechanisms of SR Ca2+-pumping systems and the causal relationship between the Ca2+-pumping activity of SR and muscle contractility.

The E1----E2 transition of Ca2+-transporting ATPase in sarcoplasmic reticulum occurs without major changes in secondary structure. A circular-dichroism study

C.d. spectroscopy was used to investigate the structures of Ca2+-ATPase (Ca2+-transporting ATPase) in the E1 and E2 states in native, in fluorescein isothiocyanate (FITC)-labelled and in solubilized sarcoplasmic reticulum (SR) preparations. The E1 state was stabilized by 100 microM-Ca2+ and the E2 state by 0.5 mM-Na3 VO4 and 0.1 mM-EGTA. There were no significant differences detected in the c.d. spectra and the calculated secondary structures between the E1 and E2 states in any of the three types of preparations. The FITC-labelled SR did show the characteristic changes in FITC fluorescence on addition of Ca2+ or vanadate, indicating that the preparation was competent for E1----E2 transitions. Therefore the absence of changes in the c.d. spectra implies that the E1----E2 transition in the Ca2+-ATPase does not involve a major net rearrangement of the polypeptide backbone conformation.

Gel to liquid crystalline phase transition promotes a conformational reorganization of Ca2+, Mg2+-ATPase from sarcoplasmic reticulum in dimyristoylphosphatidylcholine reconstituted systems

Sarcoplasmic reticulum Ca2+, Mg2+-ATPase has been reconstituted in membranes highly enriched in dimyristoylphosphatidylcholine. According to electron microscopy data these membranes form vesicles of an average diameter of 1000 +/- 200 A. These reconstituted membranes show hysteretic behavior in some physical-chemical properties, such as light scattering and fluorescence when labeled with iodoacetamidofluorescein and with N-iodoacetyl-N'-(5-sulfo-1-naphthyl) ethylenediamine. Hysteretic behavior in catalytic activity can also be inferred from the kinetic data presented in this paper, because the temperature dependence of the Ca2+, Mg2+-ATPase activity is altered by a mild thermal pretreatment of the samples. Furthermore, it was noticed that the Ca2+-dependent ATPase activity of these complexes, when assayed above the phase transition temperature (Tc) of the lipid matrix, showed a lag phase in the minute time scale range. On the basis of these findings, it is suggested that the gel-to-liquid crystalline phase transition of the lipid is able to shift the conformational equilibrium E----E* of Ca2+, Mg2+-ATPase. The fact that the -SH reactivity against 5,5'-dithio-bis-nitrobenzoic acid of these complexes is also altered by preincubation above Tc for several minutes also supports that lipid melting induces a conformational change in Ca2+, Mg2+-ATPase.

Distances between the functional sites of sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase and the lipid/water interface

Measurements of fluorescence energy transfer have been performed to determine the distance between the lipid-water interface and the ATP-binding site in the (Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum. The calculated distance between the donor, FITC bound to the protein (nucleotide binding-site marker), and the acceptor, rhodamine-5'-isothiocyanyldipalmitoylphosphatidylethanolamine (RITC-DPPE) incorporated in the membrane, was in the range of 34-42 A. In addition the distance between the high affinity Ca2+-binding sites and the lipid/water interface has been calculated by luminescence energy transfer from Tb3+ bound to the Ca2+ sites to RITC-DPPE included in the membrane, and it was approx. 10 A.

Oxidation of thiols in the Ca2+-ATPase of sarcoplasmic reticulum microsomes

We recently showed that oxidative stress impairs the function of the sarcoplasmic reticulum to transport and retain calcium. Inhibition results primarily from oxidation of one or more thiol groups in the Ca2+-ATPase. We now report that thiol oxidation does not result in disulfide formation. Oxidative inhibition of Ca2+-ATPase activity was not reversed by dithiothreitol. Also, arsenite, which crosslinks dithiols, only mildly inhibited Ca2+-ATPase activity and protected against inhibition by peroxydisulfate. These data suggest the thiols susceptible to oxidation are not spatially close enough to form a disulfide. Furthermore, these thiols appear to be involved in some aspect of phosphoenzyme formation. ATP, in the presence of calcium and magnesium, protected against inhibition of Ca2+-ATPase activity by both oxidants and thiol-binding agents. Both inhibitors also decreased binding of the nucleotide analogue TNP-AMP after phosphorylation by Pi. Dithiothreitol and arsenite were protective. In conclusion, reversible redox regulation of the Ca2+-ATPase of sarcoplasmic reticulum by thiol-disulfide exchange does not occur. However, some other mechanism of redox regulation may operate because the enzyme is sensitive to oxidants, thiol-binding agents and activity can be enhanced by prolonged exposure to dithiothreitol.

Difference in activation by Tris(hydroxymethyl)aminomethane of Ca,Mg-ATPase activity between young and old rat skeletal muscles

Fresh weight of rat skeletal muscles (M. tibialis anterior) was decreased with age. Specific activity of myosin-ATPase in the homogenate was decreased significantly at later stages of age, but not Ca,Mg-ATPase activity. The activity of Ca,Mg-ATPase was activated by high concentration (more than 75 mM) of Tris(hydroxymethyl)aminomethane. The degree of the activation was observed to be an age-related change; the activation of Ca,Mg-ATPase activity in old rats was lower than that of young rats.

Inhibition of erythrocyte Ca2+-ATPase by activated oxygen through thiol- and lipid-dependent mechanisms

We have studied erythrocyte Ca2+-ATPase as a model target for elucidating effects of activated oxygen on the erythrocyte membrane. Either intracellular or extracellular generation of activated oxygen causes parallel decrements in Ca2+-ATPase activity and cytoplasmic GSH, oxidation of membrane protein thiols, and lipid peroxidation. Subsequent incubation with either dithiothreitol or glucose allows only partial recovery of Ca2+-ATPase, indicating both reversible and irreversible components which are modeled herein using diamide and t-butyl hydroperoxide. The reversible component reflects thiol oxidation, and its recovery depends upon GSH restoration. The irreversible component is largely due to lipid peroxidation, which appears to act through mechanisms involving neither malondialdehyde nor secondary thiol oxidation. However, some portion of the irreversible component could also reflect oxidation of thiols which are inaccessible for reduction by GSH, since we demonstrate existence of different classes of thiols relevant to Ca2+-ATPase activity. Activated oxygen has an exaggerated effect on Ca2+-ATPase of GSH-depleted cells. Sickle erythrocytes treated with dithiothreitol show a heterogeneous response of Ca2+-ATPase activity. These findings are potentially relevant to oxidant-induced hemolysis. They also may be pertinent to oxidative alteration of functional or structural membrane components in general, since many components share with Ca2+-ATPase both free thiols and close proximity to unsaturated lipid.

Distinguishing between functional monomeric and oligomeric complexes of the Ca,Mg-ATPase in sarcoplasmic reticulum

Techniques are described for using blocking agents to distinguish between enzymes which are functional monomers and oligomers. To achieve this distinction the blocking agent must react exclusively at the active site with a stoichiometry of one mole of site per mole enzyme. The effect of the blocking agent on enzymatic activity in oligomers of n = 2 and 4 are described and the optimal degree of blocking is considered for tests of enzyme activity at saturating and less than saturating substrate concentrations. For saturating concentrations and a dimer the distinction between dimer and monomer is best observed with 50 per cent of sites blocked. For a tetramer the distinction is best made at higher degrees of blockade. The use of saturating substrate concentrations is thus limited to small oligomers. If nonsaturating substrate concentrations are used and normalized double reciprocal plots of the dependence of enzyme activity on substrate concentrations are made then the distinction between monomer and oligomer can readily be made for dimers, tetramers, and higher n-mers. The principles developed to distinguished monomeric from oligomeric enzymes are applied to published data obtained with the Ca Mg-ATPase of sarcoplasmic reticulum. Fluorescein isothiocyanate is the blocking agent. Plots of the published data support both the monomeric and tetrameric models for allosteric regulation with the preponderance of the data supporting the monomeric model.

Phenolic antioxidants: potent inhibitors of the (Ca2+ + Mg2+)-ATPase of sarcoplasmic reticulum

Bis(2-hydroxy-3-tert-butyl-5-methylphenyl)methane (bis-phenol) is the most potent inhibitor of the (Ca2+ + Mg2+)-ATPase of skeletal muscle sarcoplasmic reticulum yet identified. The compound behaves as a reversible, tight-binding inhibitor with apparent Ki = 0.3 microM. Butylated hydroxytoluene, butylated hydroxyanisole, and 4-nonylphenol are also effective inhibitors. These observations are of particular interest in light of the widespread use of such phenolic antioxidants and stabilizers in the food industry and in the manufacture of rubbers and plastics and the ease with which the compounds are extracted into organic solvents.

Myopathy caused by a deficiency of Ca2+-adenosine triphosphatase in sarcoplasmic reticulum (Brody's disease)

Four male patients from two families were first seen with impaired skeletal muscle relaxation that rapidly worsened during exercise. Muscle biopsies from 2 patients were examined by appropriate biochemical and microscopic immunocytochemical techniques. The adenosine triphosphate (ATP)-dependent Ca2+ transport rate was extremely low in a particulate membrane fraction of skeletal muscle, and there was also a marked reduction of the concentration of 100-kD phosphoprotein, corresponding to Ca2+-ATPase of sarcoplasmic reticulum, in muscle microsomes. The concentration of immunoreactive Ca2+-ATPase of sarcoplasmic reticulum was markedly reduced on immunoblots. Evaluation by microscopic immunocytochemical techniques, using one polyclonal and two monoclonal antibodies against sarcoplasmic reticulum Ca2+ transport protein, revealed that the severe reduction of immunoreactive Ca2+-ATPase was limited to the histochemical type 2 fibers. The deficiency of the Ca2+ transport protein in the sarcoplasmic reticulum of type 2 fibers, which may be the primary expression of a presumed gene defect, can explain the impaired muscle relaxation of the patients. This disease appears to be a clinically, electromyographically, and biochemically distinct metabolic myopathy.

Membrane defects in Duchenne dystrophy: protease affecting sarcoplasmic reticulum

Human muscle sarcoplasmic reticulum (SR) yields three major protein bands. The percent distribution of the mean values of the bands from 15 normal human muscles was 55.4, 14.6, and 30.0 for the 100, 55, and 45-kDa mass proteins, respectively. A mean distribution similar to that in normal muscle SR was found in preparations from 7 patients with polymyositis and from 7 patients with myotonic dystrophy. In 12 preparations from patients with Duchenne dystrophy, the protein distribution differed from that of preparations from normal muscle. The 100-kDa mass protein band was decreased, whereas the 55- and 45-kDa mass bands were increased. Protease inhibitors pepstatin A, antipain, and leupeptin, as well as ethyleneglycol-bis(aminoethyl ether)-N,N,N',N'-tetraacetic acid or ethylenediaminetetraacetic acid, significantly reduced this change. However, some of the changes cannot be prevented by the addition of inhibitors and must be expressed in vivo. Neither protease inhibitors nor chelators affected SR preparations from normal muscle. We found a five- to ten-fold increase in calcium-activated neutral protease activity in Duchenne dystrophic muscles that degraded the calcium-adenosinetriphosphatase of SR. The active protease was identified as the cytoplasmic calpain II. The increased activity in Duchenne muscles may explain many reported abnormalities.

Role of lipids in sarcoplasmic reticulum: a higher lipid content is required to sustain phosphoenzyme decomposition than phosphoenzyme formation

Enzyme preparations with variable phospholipid contents were obtained by removing lipids from sarcoplasmic reticulum with deoxycholate. Preparations containing from 90 to 37 phospholipids per enzyme showed normal values of both Ca2+-ATPase activity and steady-state phosphoenzyme levels. Fractions containing 37 to 23 phospholipids per enzyme had a reduced ATPase activity but normal phosphoenzyme levels, showing that in this range of lipid content the ATPase reaction is inhibited in a reaction step subsequent to phosphoenzyme formation but prior to phosphoenzyme decomposition. Delipidation below 23 lipids per enzyme caused a marked reduction of the amount of phosphoenzyme formed, so that although both reactions require lipids, fewer lipids are required for phosphoenzyme formation than for decomposition. The effect of lipid removal could be completely reversed by readdition of lipids to fractions containing more than 11 lipids per enzyme. It is proposed that phosphoenzyme formation requires full occupancy of a boundary domain of 23 lipids per enzyme, and that the selective inhibition of phosphoenzyme decomposition at higher lipid contents is caused by a decrease in the rotational mobility of the enzyme.

Calcium-dependent non-equivalent characteristics of calcium binding sites of the sarcoplasmic reticulum Ca2+-ATPase

The Ca2+ binding sites of purified sarcoplasmic reticulum Ca2+-ATPase were filled with 0.1-20 microM Ca2+ to varying degrees of maximal Ca2+-binding capacity (1.9-2.3 mol/mol of the phosphorylatable enzyme) in the absence of ATP at pH 7.0 and 0 degree C. The exchange reaction of bound Ca2+ for unbound Ca2+ at equilibrium was studied by the nitrocellulose filtration method, and was compared with the dissociation reaction of the bound Ca2+ by EGTA. When about 90% of the Ca2+ sites were filled, half of the bound Ca2+ slowly exchanged at a rate about 20-30-times slower than the dissociation rate of about 0.3 s-1, i.e., the reaction was 'occluded'. The other half of the bound Ca2+ was not 'occluded', and rapidly exchanged at the same rate as the dissociation rate. The Ca2+ in both states was, however, equally dissociated by EGTA. The Ca2+ 'occlusion' was not as pronounced when 40-90% of the sites were filled. On the other hand, when less than 40% of the sites were filled, no Ca2+ 'occlusion' was observed, and the Ca2+ on the sites homogeneously exchanged. More than half of the unoccluded and exchangeable Ca2+ was converted to an 'occluded' state after saturation of the sites with Ca2+. Based on the fact that one Ca2+-ATPase polypeptide binds one Ca2+ (Verjovsky-Almeida, S. and Silva, J.L. (1981) J. Biol. Chem. 256, 2940-2944), these results suggest the existence of two types of interacting Ca2+-ATPase molecule which are kinetically distinguishable. It further suggests that the non-equivalence of the two sites, each of which is on one of the putative interacting molecules, is produced by Ca2+ saturation of the sites of the enzyme oligomer (2 X n-mer, n greater than 1) rather than by Ca2+ binding itself to the two putative sites. Ca2+ on one of the two sites may be 'occluded' in the enzyme. The Ca2+ 'occlusion' was relieved by ATP. The 'occluded' state seems to be involved in a process of the active Ca2+ transport in the sarcoplasmic reticulum.

Affi-gel blue treatment simplifies the protein composition of sarcoplasmic reticulum vesicles

Sarcoplasmic reticulum vesicles isolated by conventional techniques usually contain, in addition to the recognized sarcoplasmic reticulum components, several other proteins (phosphorylase, myosin, glyceraldehyde-3-phosphate dehydrogenase, etc.) in variable amounts; these proteins complicate the interpretation of chemical modification data. Incubation of sarcoplasmic reticulum vesicles with Affi-Gel blue particles for 1-4 h at 2 degrees C, followed by sedimentation of the Affi-Gel in a clinical centrifuge, simplifies the protein composition by selective adsorption of the accessory proteins, and improves the consistency of the preparations. The Affi-Gel blue treatment is recommended as part of the standard procedure for the isolation of sarcoplasmic reticulum vesicles.

Effect of alterations in lipid packing order by hydrophobic solutes on the association state of protein assemblies in model membranes

The effects of two hydrophobic solutes which perturb lipid packing order, permethrin and allethrin, on the aggregated state of a lipid membrane-incorporated protein, bacteriorhodopsin (BR), have been determined by resonance energy transfer measurements. As temperature is increased from well below the main gel-fluid phase transition temperature (Tc) of the lipid, patches of aggregated BR dissociate into monomers, a few degrees below the Tc (M.P. Heyn, A. Blume, M. Rehorek and N.A. Dencher, Biochemistry 20 (1981) 7109; M.P. Heyn, R.J. Cherry and N.A. Dencher, Biochemistry 20 (1981) 840). Permethrin and allethrin were found to cause a decrease in the temperature of BR disaggregation which was associated with a decrease in the Tc of the lipid. In gel phase dipalmitoylphosphatidylcholine at 25 degrees C, the pertubing effects of permethrin on lipid packing order were associated with a decrease in the average patch radius from 123 to 33 A. It is concluded that perturbation of lipid packing order by small hydrophobic molecules may alter the stability of protein assemblies in membranes.

Two Ca2+ ATPase genes: homologies and mechanistic implications of deduced amino acid sequences

Rabbit genomic DNA contains two genes that encode Ca2+ ATPases of fast twitch and of slow twitch (and cardiac) sarcoplasmic reticulum, respectively. The deduced amino acid sequences of the products of the two genes are highly conserved in putative Ca2+ binding regions, in sectors leading from cytoplasmic domains into transmembrane domains, and in transmembrane helices. A transport mechanism is proposed in which Ca2+ binds to negatively charged groups on amphipathic stalk sectors, becoming occluded during enzyme phosphorylation by bound ATP. Rotation of the stalk sectors is induced as the energy in the phosphorylated enzyme (E1P) is utilized in conformational changes leading to the low energy form, E2P. Rotation leads to disruption of high affinity Ca2+ binding sites and release of Ca2+ into a charge-lined membrane channel. Ca2+ then traverses the membrane by exchange diffusion.

Studies on the bivalent-cation-activated ATPase activities of highly purified human platelet surface and intracellular membranes

Membrane-bound Ca2+-ATPases are responsible for the energy-dependent transport of Ca2+ across membrane barriers against concentration gradients. Such enzymes have been identified in sarcoplasmic reticulum of muscle tissues and in non-muscle cells in both surface membranes and endoplasmic-reticulum-like intracellular membrane complexes. In a previous study using membrane fractionation by density-gradient and free-flow electrophoresis, we reported that the intracellular membranes of human blood platelets were a major storage site for Ca2+ and involved in maintaining low cytosol [Ca2+] in the unactivated cell. In the present report we demonstrated that the intracellular membranes also exhibit a high-affinity Ca2+-ATPase which appears to be kinetically associated with the Ca2+-sequestering process. We found that both the surface membrane and the intracellular membrane exhibited a basal Mg2+-ATPase activity, but Ca2+ activation of this enzyme was confined only to the intracellular membrane. Use of Ca2+-EGTA buffers to control the extravesicle [Ca2+] allowed a direct comparison of the Ca2+-ATPase and the Ca2+-uptake process over a Ca2+ range of 0.01 microM to 1.0 mM, and it was found that both properties were maximally expressed in the range of external [Ca2+] 1-50 microM, with concentrations greater than 100 microM showing substantial inhibition. Double-reciprocal plots for the Ca2+-ATPase activity and Ca2+ uptake gave apparent Km values for Ca2+ of 0.15 and 0.13 microM respectively. However, similar plots for ATP with the enzyme revealed a discontinuity (two affinity sites, with Km 20 and 145 microM), whereas plots for the Ca2+ uptake gave a single Km value for Ca2+, 1.1 microM. Phosphorylation studies during Ca2+ uptake using [gamma-32P]ATP revealed two components of 90 and 95 kDa phosphorylated at extravesicle [Ca2+] of 3 microM. The Ca2+-ATPase activity, Ca2+ uptake and phosphorylation were all almost completely inhibited in the presence of 500 microM-Ca2+. Similar studies using mixed membranes revealed four other phosphoproteins (50, 40, 20 and 18 kDa) formed in addition to the 90 and 95 kDa components. The findings are discussed in the context of platelet Ca2+ mobilization for function and the mechanisms whereby Ca2+ homoeostasis is controlled in the unactivated cell.