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G Inesi

Publications and source records attributed to G Inesi.

At least 109 records · Page 6Linked to original sources
PubMed1985

Stoichiometric and electrostatic characterization of calcium binding to native and lipid-substituted adenosinetriphosphatase of sarcoplasmic reticulum.

The stoichiometry of calcium binding to specific sites (i.e., those producing enzyme activation) was found to be 8-10 nmol/mg protein in native sarcoplasmic reticulum vesicles, and 13.9-15.4 nmol/mg of ATPase purified by non-ionic detergent solubilization and anion exchange chromatography. Parallel measurements of phosphoenzyme yielded levels of 4.0-4.9 and 6.0-7.7 nmol/mg of protein in the two preparations, respectively, demonstrating that each 115 kDa ATPase chain includes one catalytic site and two calcium binding sites. The apparent association constant, K = (6 +/- 2) X 10(5) M-1, and the binding cooperativity, nH = 1.9, were unchanged when measurements were carried out with native sarcoplasmic reticulum vesicles and when the membrane surface charge was altered by lipid substitution with phosphatidylcholine or phosphatidylserine, at neutral pH in the presence of 10 mM MgCl2 and 80 mM KCl. On the other hand, the apparent association constant was increased in the absence of Mg2+ or, to a lesser extent, in the absence of monovalent cations. It was also observed that the cooperative character of the calcium binding isotherms was reduced in low ionic-strength media. Analysis of the electrostatic effects indicates that the calcium-binding domain is shielded from the membrane phospholipid surface charge by virtue of its location within the ATPase protein. The effects of various electrolytes are attributed to monovalent-cation binding in the calcium-binding domain. The apparent loss of cooperativity of the calcium binding isotherms at low ionic strength is attributed to a progressive displacement of the titration curve which is minimal at low degrees of saturation and becomes larger at higher degrees of saturation. This behavior is described quantitatively by the progressive effect of calcium binding on an electrostatic potential generated by localized protein charge densities within, or near, the calcium-binding domain.

Adenosine Triphosphatases↗
PubMed1985

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

Calcium binding to calcium-dependent ATPase purified from erythrocyte membrane was assessed by measurements of the ATPase intrinsic fluorescence. Calcium-binding isotherms obtained by fluorescence titration are identical to curves representing the Ca2+-concentration dependence of ATPase activity, and demonstrate that cooperativity is in fact a feature of the binding mechanism rather than an apparent effect of enzyme kinetics. Loss of cooperativity and a reduction of the ATPase affinity for calcium is observed at very low enzyme concentrations. This effect of enzyme dilution is prevented by calmodulin at 37 degrees C but not at 25 degrees C. It is suggested that calcium binding by erythrocyte-membrane ATPase is influenced by hydrophobic interactions of binding domains, exhibiting a dissociation constant between 10(-7) and 10(-8) M in the absence of calmodulin, at 37 degrees C and in a specific set of experimental conditions. The dissociation constant is decreased by calmodulin.

Calcium↗
PubMed1985

Intrinsic regulation of substrate fluxes and energy conservation in Ca2+-ATPase.

The Ca2+-ATPase of sarcoplasmic reticulum was utilized to demonstrate an intrinsic regulation of enzyme catalysis, whereby the ratio of forward and reverse flow is altered by the binding of Ca2+ and Pi to the enzyme. This is related to displacement of internal equilibria among intermediate enzyme ligand complexes, independent of the overall equilibrium of the ATP in equilibrium ADP + Pi transformation. A very high energy conservation with a velocity of reverse flow approaching that of forward flow, was obtained by increasing the enzyme affinity for Pi in the presence of Me2SO.

Adenosine Triphosphate↗
PubMed1985

Enzyme phosphorylation with inorganic phosphate causes Ca2+ dissociation from sarcoplasmic reticulum adenosinetriphosphatase.

Sarcoplasmic reticulum ATPase is phosphorylated by ATP in the presence of calcium, with a consequent reduction of the affinity of the binding sites for calcium and dissociation of the divalent cation from the enzyme. ATPase phosphorylation with Pi, on the other hand, requires prior removal of calcium from the enzyme, indicating that the energy requirement for phosphorylation of the enzyme-calcium complex can be met by ATP but not by Pi. We find that when the energy yield of the Pi reaction with the enzyme is increased by the addition of dimethyl sulfoxide to the medium, ATPase phosphorylation with Pi occurs even in the presence of calcium, and the binding sites undergo a reduction in affinity with consequent dissociation of Ca2+ from the enzyme, in analogy to the effect of ATP. It is thereby demonstrated experimentally that an essential step in the coupling of catalytic and transport activities is an interdependence and mutual ligand exclusion of the phosphorylation and calcium sites, in which ATP does not play a direct role. An important difference between the effects of ATP and Pi is that the former produces dissociation of Ca2+ inside the vesicles as the result of advancement of the catalytic cycle in the forward direction, while Pi produces dissociation of calcium into the outer medium as a consequence of equilibration of enzyme states producing a shift in the reverse direction of the enzyme cycle. These observations demonstrate how equilibration of intermediate enzyme states determines extent and direction of overall reaction flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Specific dicyclohexylcarbodiimide inhibition of the E-P + H2O equilibrium E + Pi reaction and ATP equilibrium Pi exchange in sarcoplasmic reticulum adenosinetriphosphatase.

Treatment of sarcoplasmic reticulum adenosinetriphosphatase (ATPase) with N,N'-dicyclohexylcarbodiimide is known to produce total inhibition of calcium binding and enzyme activity. However, we now find that treatment with lower reagent:protein ratios produces selective inhibition of hydrolytic Pi cleavage, enzyme phosphorylation with Pi, and ATP in equilibrium Pi exchange, while calcium binding and enzyme phosphorylation with ATP remain largely unaffected. This specific inhibition is attributed to derivatization of residues which are normally involved in acid-base-assisted catalysis of the hydrolytic reaction and its reversal, but are not involved in calcium binding or in the mechanism of phosphoryl transfer from ATP to the enzyme. This specific inhibition is prevented by the presence of micromolar calcium during the incubation with the inhibitor, evidently through an allosteric effect of calcium binding on the catalytic site. We also find that the initial adducts formed between ATPase residues and N,N'-dicyclo[14C]carbodiimide undergo further degradation with release of radioactive product into the medium, while the protein residues remain inactivated probably by linkage with neighboring residues. Therefore, the stoichiometry of radioactive labeling underestimates the actual number of inactivated residues.

Adenosine Triphosphatases↗
PubMed1984

A comparative study of calcium transients by isotopic tracer, metallochromic indicator, and intrinsic fluorescence in sarcoplasmic reticulum ATPase.

Comparative measurements were carried out in order to evaluate the significance of intrinsic fluorescence transients with respect to various steps of the catalytic and transport cycle of sarcoplasmic reticulum ATPase. The enzyme can acquire three levels of intrinsic fluorescence. Level 1 (lowest fluorescence) is observed in the absence of Ca2+ and ATP. Level 2 (highest fluorescence) is induced by Ca2+ through a sequential mechanism including two binding steps interspaced by an isomerization step. This transition occurs more rapidly in the presence of ATP and produces enzyme activation. Level 3 (slightly higher fluorescence than level 1) is observed immediately upon ATP binding (or phosphorylation with Pi) in the absence of Ca2+. When ATP is added to the enzyme X calcium complex, the enzyme is rapidly phosphorylated and the bound calcium is translocated to a position which is protected from La3+ added to the medium. This initial phenomenon is followed by a slow isomerization of the phosphoenzyme which is revealed by a decrease of fluorescence intensity and produces calcium release inside the vesicles before hydrolytic cleavage of the phosphoenzyme. A reaction cycle is considered and subjected to analysis, based on three main enzyme states: E, in the absence of Ca2+; E', in the presence of Ca2+; and *E, subsequent to phosphorylation.

Adenosine Triphosphatases↗
PubMed1984

Adenosinetriphosphatase site stoichiometry in sarcoplasmic reticulum vesicles and purified enzyme.

The stoichiometry of phosphorylation (catalytic) sites in sarcoplasmic reticulum vesicles ( SRV ) and SR ATPase purified by differential solubilization with deoxycholate was found to be 4.77 +/- 0.4 and 6.05 +/- 0.18 nmol/mg of protein, respectively, when phosphorylation was carried out under conditions permitting 32P labeling of nearly all sites. Assuming that each site corresponds to a single 115K ATPase chain, the observed site stoichiometry accounts only for 55% and 70% of the total protein. Failure to obtain higher phosphorylation levels was due to the presence of nonspecific protein contaminants in SRV or to the presence of inactive aggregates in the ATPase purified with deoxycholate. This was demonstrated by dissolving SRV and purified ATPase with lithium dodecyl sulfate, subjecting them to molecular sieve HPLC, and collecting the elution fractions for determination of protein, measurement of 32P-labeled sites, and electrophoretic analysis. In fact, in the specific elution peak containing the 115K ATPase chains, phosphorylation levels were 6.62 +/- 0.33 and 7.03 +/- 0.18 in SRV and purified ATPase, corresponding to 68% and 86% of the protein in the specific elution peak. An alternate purification method was then developed, based on solubilization of SRV with dodecyl octaethylene glycol monoether ( C12E8 ), separation of delipidated ATPase by anion-exchange chromatography, and enzyme reactivation with phosphatidylcholine. This preparation yields 7.3 +/- 0.44 nmol of phosphorylation site/mg of protein of the SRV fraction before HPLC.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphatases↗
PubMed1984

Studies of the interactions of 2',3'-O-(2,4,6-trinitrocyclohexyldienylidine)adenosine nucleotides with the sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase active site.

The fluorescence of TNP-nucleotides bound to sarcoplasmic reticulum ATPase is enhanced upon formation of phosphorylated enzyme intermediate either with ATP in the presence of Ca2+ or, to a greater extent, with Pi in the absence of Ca2+. Binding of the TNP-nucleotides does not occur if the ATPase is labeled at the active site with fluorescein isothiocyanate. Addition of ADP to the TNP-nucleotide X enzyme complex phosphorylated with Pi causes dissociation of TNP-nucleotide and a proportional reduction in fluorescence. These and other kinetic observations indicate that the TNP-nucleotide exchanges with ADP following enzyme phosphorylation with ATP or occupies the ADP portion of the catalytic site following enzyme phosphorylation with Pi. This interaction with the phosphorylated site results in fluorescence enhancement of the TNP-nucleotide. Comparison of the TNP-nucleotide fluorescence features in different solvents with that of the TNP-nucleotide bound to sarcoplasmic reticulum ATPase indicates that, following phosphorylation, the binding domain excludes solvent molecules and confers restricted mobility to the TNP-nucleotide. Solvent exclusion and substrate immobilization accompany, to a greater extent, phosphorylation of the active site with Pi in the absence of Ca2+. TNP-nucleotides bound to the catalytic sites were also found to be acceptors of resonance energy transfer from enzyme tryptophan in the extramembranous domain of the ATPase which also contains the catalytic site.

Adenosine Diphosphate↗
PubMed1984

Interdependence of H+, Ca2+, and Pi (or vanadate) sites in sarcoplasmic reticulum ATPase.

The phosphorylation of sarcoplasmic reticulum ATPase with Pi in the absence of Ca2+ was studied by equilibrium and kinetic experimentation. The combination of these measurements was then subjected to analysis without assumptions on the stoichiometry of the reactive sites. The analysis indicates that the species undergoing covalent interaction is the tertiary complex E X Pi X Mg formed by independent interaction of the two ligands with the enzyme. The binding constant of Pi or Mg2+ to either free or partially associated enzyme is approximately equal to 10(2) M-1, and no significant synergistic effect is produced by one ligand on the binding of the other; the equilibrium constant (Keq) for the covalent reaction E X Pi X Mg E-P X Mg is approximately equal to 16, with kphosph = 53 s-1, and khyd = 3-4 s-1 (25 degrees C, pH 6.0, no K+). The phosphorylation reaction of sarcoplasmic reticulum ATPase with Pi is highly H+ dependent. Such a pH dependence involves the affinity of enzyme for different ionization states of Pi, as well as protonation of two protein residues per enzyme unit in order to obtain optimal phosphorylation. The experimental data can then be fitted satisfactorily assuming pK values of 5.7 and 8.5 for the two residues in the nonphosphorylated enzyme (changing to 7.7 for one of the two residues, following phosphorylation) and values of 50.0 and 0.58 for the equilibrium constants of the H2(E X HPO4) in equilibrium with H(E-PO3) + H2O and H(E X HPO4) in equilibrium with E-PO3 + H2O reactions, respectively. In addition to the interdependence of H+ and phosphorylation sites, an interdependence of Ca2+ and phosphorylation sites is revealed by total inhibition of the Pi reaction when two high affinity calcium sites per enzyme unit are occupied by calcium. Conversely, occupancy of the phosphate site by vanadate (a stable transition state analogue of phosphate) inhibits high affinity calcium binding. The known binding competition between the two cations and their opposite effects on the phosphorylation reaction suggest that interdependence of phosphorylation site, H+ sites, and Ca2+ sites is a basic mechanistic feature of enzyme catalysis and cation transport.

Adenosine Triphosphatases↗
PubMed1983

Functional characterization of reconstituted sarcoplasmic reticulum vesicles.

A detailed functional characterization of reconstituted sarcoplasmic reticulum (SR) vesicles with similar lipid content as normal SR was obtained by studies of ATPase activity and calcium transport in transient state, steady state, and equilibrium conditions. For this purpose, enzyme phosphorylation with ATP, hydrolytic activity, calcium transport, phosphorylation with Pi, and ATP synthesis by reversal of the pump were measured, and utilized to demonstrate function and orientation of catalytic sites. The preparations used in these studies displayed the highest activity reported for reconstituted sarcoplasmic reticulum systems. The rates of phosphoenzyme formation from ATP and hydrolysis as well as steady state levels matched the values obtained with normal SR vesicles. Calcium transport and repeated cycles of ATP synthesis by reversal of the pump were also obtained. However, the efficiency of transport and ATP synthesis from a Ca2+ gradient was approximately three times lower than in native vesicles. This deficiency could not be attributed to passive calcium leak from the reconstituted vesicles but, in part, can be explained by the bidirectional alignment of the calcium pump in reconstituted SR. It is suggested that vectorial transport requires a more complex level of protein structure than that for sustaining simple ATPase activity. Time resolution of the phosphorylation reaction by rapid quench methods can be used to estimate the orientation of the calcium pump in the membrane. Such studies indicate that the calcium pump protein is largely bidirectionally oriented in reconstituted SR vesicles.

Adenosine Triphosphatases↗
PubMed1983

Calmodulin stimulates both adenosine 5'-triphosphate hydrolysis and synthesis catalyzed by a cardiac calcium ion dependent adenosinetriphosphatase.

A Ca2+-dependent ATPase purified from a rabbit heart membrane preparation was compared to the Ca2+-dependent ATPase purified from skeletal muscle sarcoplasmic reticulum. The two ATPases display an identical electrophoretic pattern and an identical Ca2+-concentration dependence. However, only the cardiac preparation exhibits a 2-3-fold activation by calmodulin. This effect is best observed when the molar concentrations of calmodulin and ATPase are equivalent and in the presence of high Ca2+ (approximately 10(-5) M) and ATP (approximately 10(-3) M) concentrations. It is demonstrated for the first time that calmodulin stimulates the rate of ATP synthesis, as revealed by an increased production of Pi and a faster ATP in equilibrium Pi exchange, as well as the rate of ATP hydrolysis. It is also demonstrated that calmodulin activation is expressed with purified and detergent-solubilized enzyme in addition to membrane-bound systems. These findings indicate that the effect of calmodulin is an acceleration of the enzyme turnover, due to direct interaction of calmodulin with the enzyme.

Adenosine Triphosphate↗
PubMed1983

The effect of trifluoroperazine on the sarcoplasmic reticulum membrane.

The inhibitory effect of trifluoroperazine (25-200 microM) on the sarcoplasmic reticulum calcium pump was studied in sarcoplasmic reticulum vesicles isolated from skeletal muscle. It was found that the lowest effective concentrations of trifluoroperazine (10 microM) displaces the Ca2+ dependence of sarcoplasmic reticulum ATPase to higher Ca2+ concentrations. Higher trifluoroperazine concentrations (100 microM) inhibit the enzyme even at saturating Ca2+. If trifluoroperazine is added to vesicles filled with calcium in the presence of ATP, inhibition of the catalytic cycle is accompanied by rapid release of accumulated calcium. ATPase inhibition and calcium release are produced by identical concentrations of trifluoroperazine and, most likely, by the same enzyme perturbation. These effects are related to partition of trifluoroperazine ino the sarcoplasmic reticulum membrane, and consequent alteration of the enzyme assembly within the membrane structure, and of the bilayer surface properties. The effect of trifluoroperazine was also studied on dissociated ('chemically skinned') cardiac cells undergoing phasic contractile activity which is totally dependent on calcium uptake and release by sarcoplasmic reticulum, and is not influenced by inhibitors of slow calcium channels. It was found that trifluoroperazine interferes with calcium transport by sarcoplasmic reticulum in situ, as well as with the role of sarcoplasmic reticulum in contractile activation.

Animals↗
PubMed1983

Effect of magnesium on the calcium-dependent transient kinetics of sarcoplasmic reticulum ATPase, studied by stopped flow fluorescence and phosphorylation.

At pH 7 and 20 degrees C and in the absence of potassium and magnesium, the intrinsic fluorescence rise after addition of calcium to a calcium-deprived enzyme was monoexponential. On the other hand, when the calcium-deprived enzyme was preincubated with magnesium, this fluorescence rise was clearly biphasic at high calcium concentrations. For a constant magnesium concentration (higher than millimolar), the rate constant of the slow phase and the amplitude of the fast phase rose for the same range of calcium concentrations, between pCa 5 and 4. At pH 6, fluorescence signals were monophasic even with 20 mM Mg2+. We also found that at pH 7, phosphorylation of the enzyme after simultaneous addition of calcium, magnesium, and ATP was faster when sarcoplasmic reticulum was originally calcium-deprived in the presence of magnesium. At pH 6, on the contrary, preincubation with magnesium did not influence the phosphorylation time course. It is concluded that (a) magnesium produces an enzyme conformation which is able to react with calcium and ATP as required by the kinetics of the ATPase reaction; (b) the magnesium-enzyme complex presents one readily accessible site of relatively low affinity (Kd congruent to 25 microM) for calcium; (c) occupancy of this site by calcium triggers a relatively slow (4-6 s-1) conformational change unmasking a second high affinity site; (d) interaction between and occupancy of these two calcium sites occur with positive cooperativity and yield enzyme activation; and (e) the kinetic difference between the low time constant of the second component of fluorescence rise following addition of calcium (in the absence of ATP) and the relatively fast phosphorylation obtained in the same conditions (but in the presence of ATP) is attributed to an Mg2+-dependent accelerating effect of ATP on enzyme isomerization.

Animals↗
PubMed1983

Calcium and proton dependence of sarcoplasmic reticulum ATPase.

The influence of Ca2+ and H+ concentrations on the sequential reactions of the ATPase cycle was studied by a series of pre-steady state and steady state experiments with sarcoplasmic reticulum vesicles. It is shown that H+ competition with calcium binding results in a reduced population of activated enzyme, which is manifested by a lower level of phosphorylated enzyme intermediate following addition of ATP. Further effects of Ca2+ and H+ are demonstrated on the progression of the phosphoenzyme through the reaction cycle and on the final hydrolytic cleavage of Pi. The overall dependence of steady state ATP flux on Ca2+ and H+ concentrations in leaky vesicles is expressed by a series of curves showing that as the H+ concentration is raised higher Ca2+ concentrations are required to obtain half-maximal ATP fluxes. At saturating Ca2+, maximal ATP fluxes are observed at an intermediate H+ concentration (pH 7.2), while lower levels are obtained as the H+ concentration is reduced (to pH 8) or increased (to pH 6). A preliminary model is then proposed based on the presence of two interacting domains permitting competitive binding of Ca2+ or H+, per each catalytic site undergoing phosphorylation by ATP. The model considers three main states and thirteen substates (depending on the occupancy of the binding sites in each state by Ca2+, H+, or neither) in the progression of the ATP cycle, coupled to transport of Ca2+ and counter transport of H+ in leaky vesicles. Considering the preliminary nature of the model and the experimental scatter, a rather satisfactory agreement is noted between a family of curves generated by theoretical analysis and the ATP flux curves obtained experimentally.

Adenosine Triphosphatases↗
PubMed1982

The use of 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate for studies of nucleotide interaction with sarcoplasmic reticulum vesicles.

The ATP analogue 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) was used to study nucleotide site stoichiometry and interactions in sarcoplasmic reticulum (SR) vesicles. The TNP absorption spectrum in the visible region undergoes a specific change upon binding of the nucleotide to SR ATPase. Equilibrium binding was therefore measured by differential spectrophotometry. In parallel experiments, TNP-[gamma-32P]ATP binding was measured directly by radioisotope distribution. The maximum number of nucleotide sites was estimated to be 8 nmol/mg of protein in SR vesicles. These binding sites can be separated into two distinct groups of different affinity. Accordingly, 10-100 microM ATP displaces a maximum of only 4 nmol of TNP-ATP/mg of protein from high affinity sites which are considered to be specific for enzyme catalysis. Free TNP-ATP in aqueous solution yields a weak fluorescence signal which is slightly increased upon binding of the analogue to SR ATPase. However, a pronounced fluorescence enhancement and a spectral change are observed when ATP is added in concentrations permitting partial occupancy of the specific sites by TNP-ATP. This effect is strictly dependent on ATP utilization by the SR ATPase, inasmuch as it requires Ca2+, and it is not produced by adenyl-5'-yl-imidodiphosphate. The fluorescence enhancement is reversible upon exhaustion of added ATP. It is concluded that TNP-ATP acts as a reporter of an ATPase conformational change following enzyme phosphorylation of the catalytic site by ATP, and that the observed conformational change is operative in the mechanism of calcium site translocation for active transport. Furthermore, the sensitivity of the bound analogue to the phosphorylation reaction is likely to be related to nucleotide regulation of enzyme turnover.

Adenosine Triphosphatases↗
PubMed1982

A serial study of muscle microsomes during the early growth of genetically dystrophic chickens.

Two lines of genetically involved and control chickens were compared with regard to the onset of muscle dystrophy during the early stages of growth ex ovo. Definite structural and functional involvement of pectoralis muscle developed within the first 4-5 weeks. In parallel experiments, microsomal membranes were obtained weekly from pectoralis muscle during the first 14 weeks ex ovo. The microsomes were studied with respect to ultrastructural features, protein composition, Ca2+ uptake and ATPase activity. Microsomal preparations obtained from all newborn chickens contain two types of vesicles: one type reveals an asymmetric distribution and 'high density' of particles on freeze-fracture faces which is characteristic of sarcoplasmic reticulum (SR) membrane; the other type reveals a symmetric distribution and 'low density' of particles. The yield of 'low density' microsomes from muscle of normal birds is very much reduced as the chicks grow from 1 to 4-5 weeks ex ovo. On the contrary, it remains high in chicks developing muscle dystrophy. Ca2+ uptake and coupled ATPase activity are found to be of nearly identical specific activity in control and genetically involved newborn chicks. The specific activity of the control birds, however, increases as the chicks grow from 1 to 4-5 weeks of age, while the specific activity of the dystrophic birds remains low. Such a difference appears to be related to the relative representation of sarcoplasmic reticulum and 'low density' vesicles in the microsomal preparations. It is concluded that failure to obtain a normal differentiation of muscle cell membranes is a basic defect noted in the early growth of genetically involved chickens. This defect appears along with the earliest signs of the dystrophic process.

Adenosine Triphosphatases↗