Tryptophan fluorescence of sarcoplasmic reticulum ATPase. A fluorescence quench study

The calcium-dependent change in the tryptophan fluorescence intensity of the sarcoplasmic reticulum Ca²⁺- and Mg²⁺-ATPase was investigated using different quenching reagents. It is demonstrated that only those compounds which are bound to the enzyme (i.e., 1-(9,10-dibromomyristoyl)-sn-2-glycerophosphorylcholine and 1-(9,10-dibromostearoyl)-sn-glycero-3-phosphorylcholine) are able to decrease the amplitude of the fluorescence decrement observed after removal of calcium ions. From the position of the bromine atom within the lysophosphatidylcholines, it is concluded that the tryptophan residues involved are located in the hydrophobic part of the ATPase molecule and are in contact with the hydrocarbon chains of the phospholipids.     

Arginyl residue modification of the sarcoplasmic reticulum ATPase protein.

Glucocerebrosidase, a membrane bound enzyme, can be solubilized by acetone precipitation and the resultant soluble enzyme activity demonstrated in the presence of acidic phospholipid, e.g. phosphatidylserine. This is the first report of the detergent-free solubilization of glucocerebrosidase.     

Solvent perturbation evidence for a two-state system regulated by calcium in sarcoplasmic reticulum ATPase

Perturbation of sarcoplasmic reticulum ATPase with the nonionic detergent C12E8 is modulated by the amount of free Ca2+ present in the solvent prior to the addition of detergent. CD measurements show that the enzyme exists in solution in two different conformations that react differently with the detergent. They probably represent the free enzyme, and its complex with Ca2+. On this assumption, titrations with increasing amounts of Ca2+ produced data superimposable on curves obtained measuring Ca2+ bound to sarcoplasmic reticulum vesicles.     

The stereochemical course of phosphoric residue transfer catalyzed by sarcoplasmic reticulum ATPase

The stereochemical course of the phosphoric residue transfer from ADP to water catalyzed by the (Mg2+ + Ca2+)-dependent ATPase of sarcoplasmic reticulum has been determined. For this determination, the preparation is described of ATP gamma S, stereospecifically labeled in the gamma-position with both 17O and 18O. After hydrolysis of this nucleotide, the analysis of the product inorganic [16O,17O,18O]thiophosphate showed that the reaction proceeded with retention of configuration at the gamma-phosphorus atom. This result is expected since a phosphoenzyme is well characterized for this ATPase and provides support for the hypothesis that each phosphate transfer step occurs with inversion. In this case, the formation and breakdown of the phosphoenzyme occur each with inversion leading to the retention observed for the whole reaction.     

Solvent accessibility of the adenosine 5'-triphosphate catalytic site of sarcoplasmic reticulum CaATPase

The CaATPase of rabbit skeletal sarcoplasmic reticulum was labeled at or near the ATP catalytic site with fluoresceinyl isothiocyanate (FITC), and the accessibility of the attached probe to the bulk solvent was determined by I- quenching of its fluorescence. The quenching of free FITC was also measured. In both cases, the quenching was of the Stern-Volmer type and collisional quenching rate constants were obtained over the pH range 5-8 in the presence of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and with added Ca2+, vanadate, or phosphate. The fluorescence intensity and susceptibility to quenching by I- of free FITC were insensitive to the added ligands. In all cases, the intensity decreased with pH, as predicted from the known properties of FITC mono- and dianions. The collisional quenching rate constants increased at lower pH, as expected for I- quenching of a molecule with decreasing negative charge due to protonation. When FITC was attached to the CaATPase, the FITC fluorescence intensity and I- collisional quenching rate constants were sensitive to ligand binding as well as pH. The changes in fluorescence intensity with acidity, when compared to the results for free FITC, indicated the pKa of the FITC was reduced 0.6 unit when it was attached to the CaATPase. Excited-state lifetime measurements indicated that ligand effects at constant pH were not due to protonation-induced changes in FITC quantum yield but to conformational changes of the CaATPase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cooperative proton and calcium binding by sarcoplasmic reticulum ATPase.

The classic work on binding of calcium to CaATPase is analyzed by an objective non-linear least squares procedure of 74 data points over six pH values. Binding of two calciums to the basic form of the sites occurs with an equilibrium stability constant product of log K1K2 = 13.2. Owing to competition from protons, this value drops in acidic and neutral solutions, becoming, for example, 11.9 at pH 6.8. Binding of the two calciums is so strongly cooperative that its extent is difficult to estimate reliably; there is very little of the one calcium species. Two protons are also bound cooperatively to the calcium sites. In solutions of calcium free protein, at pH less than 7.6 the predominant species holds two protons at the calcium sites, while at greater pH the dominant species bears no protons; there is very little of the intermediate one proton species. The analysis also reveals the likely presence of a small, less than statistical, amount of a ternary complex bearing one calcium and one proton.     

Kinetics and regulation of sarcoplasmic reticulum ATPase.

The measurement of ATP binding to the sarcoplasmic reticulum membrane reveals that the calcium pump possesses one high affinity (Kd = 2--3 muM) site. Competition with substrate analogs show the high specifity of that site. At high ATP concentration another class of site can be detected with a much higher dissociation constant (Kd approximately 500 muM). This class of sites is of low specificity and ATP is easily displaced by other polyphosphates. The steady state rate of ATP cleavage is measured in the presence of ATP analogs. It is shown that the catalysis is due to the high affinity site. The activation of the hydrolysis rate at high substrate concentration may be related to the effect of binding of ATP to the weak sites. The effect of ATP analogs for various ATP concentration supports this hypothesis.     

Assembly of ATPase protein in sarcoplasmic reticulum membranes.

Three specimen preparation techniques for electron microscopy were used to investigate the incorporation of the ATPase polypeptide chains in the membranes of fragmented sarcoplasmic reticulum (SR) obtained from rabbit skeletal muscle. Observations were made of both normal vesicles and vesicles exposed to trypsin, which is known to cleave the ATPase protein and to alter the ultrastructure of the vesicles in predictable ways. Freeze-fracture replicas reveal the typical 90-A particles on the concave (PF) faces with a density of 5,730 +/- 520/mum2. On the other hand both negatively stained and deeply etched preparations display outer projections, which are absent on trypsin-incubated vesicles. The etched specimens afford for the first time top views of the vesicles in the absence of any stain. These views reveal outer projections on the PS surface with a density of 21,000 +/- 3,900/mum2, a value nearly approximating the density of the ATPase polypeptide chains (106,000 mol wt) calculated on the basis of protein and membrane area determinations. On the other hand, this value is three to four times higher than that found for the density of the 90-A particles on the concave fracture faces. Since both outer projections and 90-A particles are identified with the ATPase protein, it is suggested that the ATPase polypeptide chains are amphiphilic molecules, with polar ends protruding individually as outer projections on the surface of the vesicles, and hydrophobic ends appearing as 90-A particles on the concave fracture faces. The discrepancy between the densities of the outer projections and the 90-A particles may be attributed either to variable penetration of the polypeptide chains into the membrane bilayer, or to formation of oligomers containing three or four hydrophobic ends and appearing as single 90-A particles. Each ATPase chain forms a complex with 20-30 phospholipid molecules. The remaining phospholipids (approximately 70% of the total SR phospholipids) account for less than half the membrane volume. It is proposed that the outer leaflet of the SR membrane is prevalently composed of the ATPase lipoprotein complex, and the inner leaflet is mostly a phospholipid monolayer.     

Cross-linking of the sarcoplasmic reticulum ATPase protein.

Treatment of rabbit sarcoplasmic reticulum vesicles with the cross-linking agent, cupric phenanthroline, causes production of high-molecular weight bands on SDS-gel electrophoresis. A plot of log mol wt $\text{vs}$ mobility indicates that the main band produced from the ATPase (mol wt = 10⁵) has a mol wt of 4 × 10⁵ and thus suggests formation of a tetramer. Notably, bands corresponding to dimers, trimers, pentamers, etc., are absent. The bands attributable to calsequestrin and calcium binding protein are unchanged by cupric phenanthroline. With extended treatment, the tetramer itself is polymerized (mol wt>10⁶). Partial disruption of the membranes with deoxycholate or Triton X-100 before cross-linking favors tetramer formation; the presence of sodium dodecyl sulfate, on the other hand, prevents intermolecular cross-linking. Our results suggest that the ATPase is at least partially associated within the membrane as a tetramer.     

Two functional states of sarcoplasmic reticulum ATPase.

The "total" ATPase activity of rabbit sarcoplasmic reticulum (SR) vesicles includes a Ca2+-independent component ("basic") and Ca2+-dependent component ("extra"). Only the "extra" ATPase is coupled to Ca2+ transport. These activities can be measured under conditions in which the observed rates approximate maximal velocities. The "basic" ATPase is predominant in one of the various SR fractions obtained by prolonged density-gradient centrifugation of SR preparations already purified by repeated differential centrifugations and extractions at high ionic strength. This fraction (low dnesity, high cholesterol) has a protein composition nearly identical with that of other SR fractions in which the "extra" ATPase is predominant. In these other fractions the ratio of "extra" to "basic" ATPase activities is temperature dependent, being approximately 9.0 at 40 degrees C and 0.5 at 4 degrees C. In all the fractions and at all temperatures studied, similar steady-state levels of phosphorylated SR protein are obtained in the presence of ATP and Ca2+. Furthermore, in all cases the "basic" (Ca2+-independent) ATPase acquires total Ca2+ dependence upon addition of the nonionic detergent Triton X-100. This detergent also transforms the complex substrate dependence of the SRATPase into a simple dependence, displaying a single value for the apparent Km. The experimental findings indicate that the ATPase of rabbit SR exists in two distinct functional states (E1 and E2), only one of which (E2) is coupled to Ca2+ transport. The E1 in equilibrium E2 equilibrium is temperature-dependent and entropy-driven, indicative of its relation to the physical state of the ATPase protein in its membrane environment. Thenonlinearity of Arrhenius plots of Ca2+-dependent ("extra") ATPase activity and Ca2+ transport is explained in terms of simultaneous contribtuions from both the free energy of activation of enzyme catalysis and the free energy of conversion of E1 to E2. Thermal equilibrium between the two functional states is drastically altered by factors which affect membrane structure and local viscosity.     

Direct fluorescence measurements of Mg2+ binding to sarcoplasmic reticulum ATPase

In the absence of calcium, interaction of magnesium with SR-ATPase induced a blue shift in intrinsic fluorescence emission. This Mg2+-induced fluorescence change was pH-dependent and an apparent Mg dissociation constant of 5 mM was found at pH 7. Equilibrium studies showed that magnesium competes for the high affinity Ca2+ binding sites and stopped flow measurements of the transient kinetics indicated a multistep interaction between magnesium and the calcium pump. These results suggest that magnesium drives the sarcoplasmic reticulum atpase toward an E.Mg species which might be a dead-end complex.     

Localization of ATPase protein in sarcoplasmic reticulum membrane.

The substrate specificity of an extensively purified flavanone synthase from light-induced cell suspension cultures of Petroselinum hortense was investigated. p-Coumaroyl-CoA was found to be the only efficient substrate for flavanone synthesis, producing naringenin (5,7,4′-trihydroxyflavanone). Besides 4-hydroxy-6[4-hydroxystyryl]2-pyrone (F. Kreuzaler and K. Hahlbrock (1975) Arch. Biochem. Biophys.169, 84–90) two further release products of the synthase reaction in vitro were identified as 4-hydroxy-5,6-dihydro-6(4-hydroxyphenyl)2-pyrone and p-hydroxybenzalacetone. The apparent K_m values for malonyl-CoA and p-coumaroyl-CoA in the reaction leading to naringenin, and for p-coumaroyl-CoA in the reaction leading to the styrylpyrone derivative were 35, 1.6, and 2.6 μm, respectively. With caffeoyl-CoA as substrate only a very small amount of eriodictyol (5,7,3′,4′-tetrahydroxyflavanone) was formed besides relatively large amounts of the corresponding styrylpyrone, dihydropyrone, and benzalacetone derivatives. No flavanone formation was observed with feruloyl-CoA as substrate, but again appreciable amounts of the three types of short-chain release products were formed. No reaction at all took place with cinnamoyl-CoA, p-methoxycinnamoyl-CoA, isoferuloyl-CoA, or p-hydroxybenzoyl-CoA.None of the styrylpyrone, dihydropyrone, and benzalacetone derivatives has been detected in the cell cultures in vivo. The present results suggest that naringenin is the only natural product of the synthase reaction and that further substitution in the B-ring of the flavonoids occurs in parsley at or after the flavanone stage. The nature of the smaller release products is consistent with the assumption of a stepwise addition of acetate units from malonyl-CoA to the acyl moiety of the starter molecule, p-coumaroyl-CoA.     

The influence of sodium trichloroacetate on the tryptophan fluorescence of sarcoplasmic reticulum ATPase

The chaotropic anion trichloroacetate quenches the tryptophan fluorescence of the sarcoplasmic reticulum calcium transport ATPase. Half-maximum quenching was observed at 50 mM trichloroacetate. In contrast to native preparations, in trichloroacetate-treated sarcoplasmic reticulum vesicles a decrease of the tryptophan fluorescence is observed on addition of millimolar concentrations of calcium. It is concluded that trichloroacetate renders the tryptophan fluorescence of the ATPase sensitive to the occupancy of its low-affinity sites.     

Structural properties of sarcoplasmic reticulum Ca2+-ATPase as studied by intrinsic protein fluorescence

1. From the intrinsic fluorescence spectral properties and fluorescence quenching experiments done with acrylamide and iodide, using native sarcoplasmic reticulum vesicles, purified ATPase and ATPase solubilized with 1% Triton X-100, it is deduced that practically all the fluorescent tryptophanyl residues of this protein belong to a single population showing similar hydrophobic microenvironments. 2. Both acrylamide and iodide seem to be able to penetrate through the sarcoplasmic reticulum membrane. 3. The intrinsic fluorescence of the Ca2+-ATPase due to tryptophan residues probably buried inside the membrane is used as a tool to follow thermotropic changes in membrane fluidity of reconstituted systems.     

A direct fluorescence study of the transient steps induced by calcium binding to sarcoplasmic reticulum ATPase

The sarcoplasmic reticulum intrinsic fluorescence level was closely correlated with the ATPase functional state, from pH 5.5 to 8.5. The fluorescence signal was used in stopped flow measurements for direct study of transient pump kinetics after calcium binding or removal. The signal change time course, which depends solely on the free calcium concentration in the observation chamber, was analyzed as a single exponential. Rate constants (kobs) were relatively slow (5 to 20 s-1), indicating multistep interaction between calcium and the transport protein. At pH 7 and 20 degrees C, and in the presence of 100 mM potassium and 1 to 20 mM MgCl2, kobs first decreased, and then increased as the calcium concentration rose. Similar experiments were performed at pH 6. Data were analyzed according to a scheme in which sarcoplasmic reticulum . calcium complex formation is controlled by a slow isomerization step occurring either before or after the rapid calcium binding to the high affinity site. The results are discussed with reference to published rapid quenching experiments. Under our conditions, i.e. in the absence of a calcium gradient across the membrane, the calcium pump cycle step in which reorientation of the calcium binding sites occurs cannot be identified with the isomerization step mentioned above.     

Oxygen diffusion-concentration in erythrocyte plasma membranes studied by the fluorescence quenching of anionic and cationic pyrene derivatives

Fluorescence quenching by oxygen of cationic [pyrene-(CH₂) _n N(CH₃) ₃ ⁺ ;n=1, 4, and 11] and anionic [pyrene-(CH₂) _n CO ₂ ^– ,n=3, 8, 11, and 15] probes was investigated in erythrocyte plasma membranes (leaky) in order to assess the ability of oxygen molecules to interact with solutes located at different positions in the membrane. The pseudounimolecular quenching rate constants measured increase, both for cationic and anionic probes, whenn increases. These results are interpreted in terms of an increased oxygen solubility toward the center of the membrane interior, and imply that lateral diffusion contributes more than transverse diffusion to total oxygen mobility. For all of the probes considered, quenching rates increase whenn-alkanols are added. The effect observed increases whenn decreases and when the size of then-alkanol alkyl chain increases. Arrhenius-type plots for the quenching rate constants show noticeable downward curvatures. Average (0–40°C) activation energies are 6 kcal/mol.Abbreviations EPM erythrocyte plasma membrane - PMTMA (1-pyrenyl)methyltrimethyl-ammonium - PBTMA 4-(1-pyrenyl)butyltrimethylammonium - PUTMA 11-(1-pyrenyl)-undecyltrimethylammonium - PB 4-(1-pyrenyl)butanoate - PN 9-(1-pyrenyl)nonanoate - PD 12-(1-pyrenyl)dodecanoate - PHD 16-(1-pyrenyl)hexadecnoate     

Inhibition of mitochondrial F1 ATPase and sarcoplasmic reticulum ATPase by hydrophobic molecules

The hydrophobic nature of the active site of two energy-transducing ATPases was explored by comparing interactions between Pi and each of three hydrophobic drugs in the absence and presence of organic solvents. The drugs tested were the Fe . bathophenanthroline complex and the anticalmodulin drugs, calmidazolium and trifluoperazine. All inhibit the Pi in equilibrium with ATP exchange reaction catalyzed by submitochondrial particles and the ATPase activity of both submitochondrial particles and soluble F1 ATPase. The inhibition by the three drugs is reversed by either raising the Pi concentration or by adding organic solvent (dimethylsulfoxide, ethyleneglycol or methanol) to the medium. The inhibition of the Pi in equilibrium with ATP exchange by trifluoperazine becomes more pronounced when the electrochemical proton gradient formed across the membrane of the submitochondrial particles is decreased by the addition to the medium of the proton ionophore carbonylcyanide p-trifluoromethoxyphenylhydrazone. The ATPase activity and the Ca2+ uptake by sarcoplasmic reticulum vesicles are inhibited by the Fe . bathophenanthroline complex, calmidazolium and trifluoperazine. Phosphorylation of the ATPases by Pi, synthesis of ATP from ADP and Pi and the fast efflux of Ca2+ observed during reversal of the Ca2+ pump are inhibited by the three drugs. The inhibition is reversed by raising the concentration of Pi or dimethylsulfoxide. The three drugs tested appear to compete with Pi for a common binding site on the Ca2+-ATPase. The data presented are interpreted according to the proposal that the catalytic site of an enzyme involved in energy transduction undergoes a hydrophobic-hydrophilic transition during the catalytic cycle.     

The reaction of N-(1-pyrene)maleimide with sarcoplasmic reticulum.

The excimer fluorescence of the adduct of N-(1-pyrene)maleimide (PMI) with the Ca2+-ATPase was proposed as a probe of ATPase-ATPase interactions in sarcoplasmic reticulum (Lüdi and Hasselbach, Eur. J. Biochem., 1983, 130:5-8). We tested this proposition by analyzing the spectral properties and stoichiometry of the adducts of pyrenemaleimide with sarcoplasmic reticulum and with dithiothreitol and by comparing the effects of various detergents on the excimer fluorescence of the two adducts, with their influence on the sedimentation characteristics, ATPase activity, and light scattering of the pyrenemaleimide-labeled sarcoplasmic reticulum. These studies indicate that pyrenemaleimide reacts nearly randomly with several SH groups on the Ca2+-ATPase, and suggest that the observed excimer fluorescence of pyrenemaleimide-labeled sarcoplasmic reticulum may reflect intramolecular phenomena rather than ATPase-ATPase interactions. Further work is required to establish the relative contribution of intra- and intermolecular mechanisms to the excimer fluorescence.