Modulation of the sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase by pentobarbital

Influence of N-dodecyl-N,N-dimethylamine N-oxide on the activity of sarcoplasmic reticulum Ca2+-transporting ATPase reconstituted into diacylphosphatidylcholine vesicles: Effects of bilayer physical parameters

Sarcoplasmic reticulum Ca-transporting ATPase (EC 3.6.1.38) was isolated from rabbit white muscle, purified and reconstituted into vesicles of synthetic diacylphosphatidylcholines with monounsaturated acyl chains using the cholate dilution method. In fluid bilayers at 37 degrees C, the specific activity of ATPase displays a maximum (31.5+/-0.8 IU/mg) for dioleoylphosphatidylcholine (diC18:1PC) and decreases progressively for both shorter and longer acyl chain lengths. Besides the hydrophobic mismatch between protein and lipid bilayer, changes in the bilayer hydration and lateral interactions detected by small angle neutron scattering (SANS) can contribute to this acyl chain length dependence. When reconstituted into dierucoylphosphatidylcholine (diC22:1PC), the zwitterionic surfactant N-dodecyl-N,N-dimethylamine N-oxide (C12NO) stimulates the ATPase activity from 14.2+/-0.6 to 32.5+/-0.8 IU/mg in the range of molar ratios C12NO:diC22:1PC=0/1.2. In dilauroylphosphatidylcholines (diC12:0PC) and diC18:1PC, the effect of C12NO is twofold-the ATPase activity is stimulated at low and inhibited at high C12NO concentrations. In diC18:1PC, it is observed an increase of activity induced by C12NO in the range of molar ratios C12NO:diC18:1PC< or =1.3 in bilayers, where the bilayer thickness estimated by SANS decreases by 0.4+/-0.1 nm. In this range, the 31P-NMR chemical shift anisotropy increases indicating an effect of C12NO on the orientation of the phosphatidylcholine dipole N(+)-P- accompanied by a variation of the local membrane dipole potential. A decrease of the ATPase activity is observed in the range of molar ratios C12NO:diC18:1PC=1.3/2.5, where mixed tubular micelles are detected by SANS in C12NO+diC18:1PC mixtures. It is concluded that besides hydrophobic thickness changes, the changes in dipole potential and curvature frustration of the bilayer could contribute as well to C12NO effects on Ca(2+)-ATPase activity.

Partial ischemia reduces the efficiency of sarcoplasmic reticulum Ca2+ transport in rat EDL

To investigate the hypothesis that prolonged partial ischemia would result in a depression in homogenate sarcoplasmic reticulum (SR) Ca2+-sequestering and mechanical properties in muscle, a cuff was placed around the hindlimb of 8 adult Sprague-Dawley rats (267+/-5.8 g; x +/- S.E.) and partially inflated (315 mm Hg) for 2 h. Following occlusion, the EDL was sampled both from the ischemic (I) and contralateral control (C) leg and SR properties compared with the EDL muscles extracted from rats (n = 8) immediately following anaesthetization (CC). Ischemia was indicated by a lower (p < 0.05) concentration (mmol.kg dry wt(-1)) of ATP (19.0+/-0.7 vs. 16.7+/-0.7) and phosphocreatine (58.1+/-5.7 vs. 35.0+/-4.6) in I compared to C. Although Ca2+-ATPase activity (micromol x g protein(-1) x sec(-1)), both maximal and submaximal, was not different between C and I (19.7+/-0.4 vs. 18.5+/-1.3), reductions (p < 0.05) in Ca2+-uptake (mmol x g protein(-1) x sec(-1)) of between 18.2 and 24.7% across a range of submaximal free Ca2+-levels were observed in I compared to C. Lower submaximal Ca2+-ATPase activity and Ca2+-uptake were also observed in the EDL in C compared to CC animals. Time dependent reductions (p < 0.05) were found in peak twitch and maximal tetanic tension in EDL from I but not C. It is concluded that partial ischemia, resulting in modest reductions in energy state in EDL, induces a reduction in Ca2+-uptake independent of changes in Ca2+-ATPase activity. These changes reduce the coupling ratio and the efficiency of Ca2+-transport by SR.

pH and ligand binding modulate the strength of protein-protein interactions in the Ca(2+)-ATPase from sarcoplasmic reticulum membranes

The Ca(2+)-ATPase from sarcoplasmic reticulum (SR) membranes couples the Ca(2+) transport to ATP hydrolysis through phosphorylation in its cytoplasmic catalytic domain. Interactions between protein domains and the role of monomer-monomer interactions remain unclear. Here, we report a differential scanning calorimetric study of the thermal unfolding of this protein. In the pH range 6-8, thermal unfolding of the Ca(2+)-ATPase in glycogen phosphorylase-free SR membranes shows a major endothermic peak with a critical temperature midpoint ranging between 51 and 55 degrees C, depending on pH, Ca(2+), Mg(2+)-ADP and KCl concentrations. The enthalpy change of the overall unfolding process ranged between 250 and 300 kcal/mol of Ca(2+)-ATPase monomer. Thermal denaturation of the Ca(2+)-ATPase in SR membranes is well fitted to an irreversible process that can be rationalized in terms of a non-two state process, N (native)right harpoon over left harpoon I (intermediate)-->D (denatured). Thermodynamic analysis show that this protein has a compact structure, implying a tight structural interconnection between catalytic and Ca(2+) transport domains. The apparent cooperative unit, defined by the van 't Hoff enthalpy to the overall unfolding enthalpy ratio, increased from 1.1 at pH 6 to 1.8 at pH 8, showing that monomer-monomer interactions are stronger at weakly basic pH than at weakly acidic pH. While micromolar Ca(2+) concentrations had only a weak effect on the cooperativity of the unfolding process, this is clearly increased by millimolar Mg(2+)-ADP. In addition, high ionic strength lowered the apparent cooperative unit to approximately 1.0 in the pH range 6-8. Taken together, these results suggest that protein-protein interactions are altered by variables that modulate the catalytic activity of this enzyme.

Interaction of local anesthetic heptacaine homologs with phosphatidylcholine bilayers: spin label ESR study

Local anesthetic monohydrochlorides of [2-(alkoxy)phenyl]-2-(1-piperidinyl)ethyl esters of carbamic acid (CnA, n = 2, 3, 4, 6, 8, 10, 12 is the number of carbon atoms in the alkyloxy substituent) increase the probability of formation of gauche isomers p(g) and decrease the effective energy difference between gauche and trans conformation E(g) in egg yolk phosphatidylcholine (EYPC) acyl chains, as determined by electron spin resonance spectroscopy using dipalmitoylphosphatidylcholines labeled with the paramagnetic dimethyloxazolidinyl group on the 12-th or 16-th carbon atoms of their sn-2 acyl chain, and oriented EYPC bilayers hydrated at 81% relative water vapour pressure. CnAs also increase the hydration of EYPC in non-oriented bilayers at the same relative water vapour pressure. At the molar ratio of CnA:EYPC = 0.4:1, the maximum effect on p(g), E(g) and hydration has been observed for intermediate alkyloxy chain lengths n approximately 4/6.

Modulation of the Ca2+,Mg(2+)-ATPase of sarcoplasmic reticulum by the hypothalamic hypophyseal inhibitory factor

We have studied the effect of the endogenous inhibitor of the Na+ and Ca2+ pumps, HHIF, on sarcoplasmic reticulum (SR) vesicles. The effect of HHIF on the SR Ca2+,Mg(2+)-ATPase activity shows a biphasic pattern. Low HHIF concentrations activate the Ca2+,Mg(2+)-ATPase by dissipation of Ca2+ gradient across the SR membrane. Higher concentrations irreversibly inhibit this activity following a slow kinetic process both in intact SR membranes and in purified Ca2+,Mg(2+)-ATPase. Differential scanning calorimetry shows that the Ca2+,Mg(2+)-ATPase is denatured after incubation with HHIF concentrations which produced full inhibition of its activity. Micromolar Ca2+ and millimolar Mg2+ ADP protect against the irreversible inhibition of the Ca2+,Mg(2+)-ATPase by HHIF. The concentration of HHIF which produces 50% inhibition depends upon SR membrane concentration and upon the lipid:protein ratio in purified Ca2+,Mg(2+)-ATPase. From this we have obtained a partition coefficient for binding of HHIF to SR membranes of 0.6 (microgram SR protein/ml)-1.

Interaction between glycogen phosphorylase and sarcoplasmic reticulum membranes and its functional implications

Skeletal muscle glycogen phosphorylase b binds to sarcoplasmic reticulum (SR) membranes with a dissociation constant of 1.7 +/- 0.6 mg of phosphorylase/ml at 25 degrees C at physiological pH and ionic strength. Raising the temperature to 37 degrees C produced a 2-3-fold decrease in the dissociation constant. The SR membranes could bind up to 1.1 +/- 0.1 mg of glycogen phosphorylase b/mg of SR protein, whereas liposomes prepared with endogenous SR lipids and reconstituted Ca(2+)-ATPase were unable to bind glycogen phosphorylase. Binding of glycogen phosphorylase b to SR membranes is accompanied by inhibition of its activity in the presence of AMP. The Vmax for glycogen phosphorylase b associated with SR membranes is 40 +/- 5% of that for purified glycogen phosphorylase and shows a decreased affinity for its allosteric activators, AMP and IMP. These kinetic effects are also observed with purified glycogen phosphorylase b when starch or alpha-amylose is used as substrate instead of glycogen. Treatment of SR membranes with alpha-amylase produced dissociation of glycogen phosphorylase b from the SR membranes. Thus, linear polysaccharide fragments of glycogen bound to the SR membranes are likely mediating the binding of glycogen phosphorylase b to these membranes.

Thermal unfolding of monomeric Ca(II), Mg(II)-ATPase from sarcoplasmic reticulum of rabbit skeletal muscle

The thermal unfolding of monomeric and delipidated Ca(2+)-ATPase, solubilized in C12E8, can be appropriately described as a non-two-state irreversible denaturation, with only one endothermic peak. In the Ca2+ concentration range (0-0.5 mM) which stimulates the ATPase activity of solubilized monomeric ATPase, Ca2+ shifts the critical temperature midpoint of the denaturation process (Tm) from 42 to 50 degrees C without segregation of the endothermic peak into two separate components. Because 20 mM Mg2+ only shifts the Tm from 42 to 44 degrees C, we conclude that the effect of Ca2+ upon the Tm is likely to be due to binding to the high affinity Ca2+ sites in the ATPase. The effect of Ca2+ upon the enthalpy of denaturation is biphasic, suggesting the presence of low affinity Ca2+ sites (K0.5 in the millimolar range) in monomeric and solubilized ATPase.

Kinetic characterization of the normal and procaine-perturbed reaction cycles of the sarcoplasmic reticulum calcium pump

We investigated the effect of the local anesthetic procaine on the activity of the calcium pump protein of sarcoplasmic reticulum (SR) vesicles. Procaine slowed down the rate of calcium uptake by SR vesicles without enhancing the vesicles' passive permeability. This slowing of the unidirectional pumping rate was reflected by the inhibition of the maximal rate of the transport-coupled Ca(2+)-ATPase activity. The inhibition was dependent on Mg2+ concentration; at optimal (i.e. low) concentrations of magnesium, half-maximal inhibition occurred with procaine concentrations close to 15-20 mM. Inhibition of ATPase was not mediated by a change in the properties of the bulk lipid phase. Procaine moderately reduced the true affinity of ATPase for ATP, whereas equilibrium binding of calcium to ATPase in the absence of ATP was virtually not modified by procaine. In fast-kinetics studies, we explored the various intermediate steps in the ATPase catalytic cycle, in order to determine which of them were targets for inhibition by procaine. We found that procaine slowed down ATPase dephosphorylation, an effect which is at least partly responsible for the observed inhibition of overall ATPase activity. In contrast, procaine accelerated the calcium-induced transconformation of unphosphorylated ATPase in the absence of ATP, and altered neither the rate of the Ca(2+)-dependent phosphorylation of ATPase, nor the rate of the dissociation of Ca2+ from phosphorylated ATPase towards the SR lumen, a critical step, the rate of which was measured by a novel fast-filtration method. These results are discussed with respect to the possible site(s) of binding of this amphiphile on the ATPase, and in relation to the contribution of individual steps in the catalytic cycle to the rate limitation of unperturbed SR ATPase activity.

Modulation by phosphorylation of glycogen phosphorylase-sarcoplasmic reticulum interaction

Glycogen phosphorylase b at concentrations close to those found in skeletal muscle interacts with sarcoplasmic reticulum membranes, but not with liposomes made of lipids extracted from these membranes, and is inhibited upon binding to the membrane. The interaction of glycogen phosphorylase with the sarcoplasmic reticulum membrane is modulated by phosphorylation, for the a form of this enzyme shows a K0.5 of interaction about 10-fold lower than the b form. Upon association to the membrane the fluorescence properties of the coenzyme of glycogen phosphorylase, pyridoxal-5'-phosphate, are strongly altered, for the fluorescence at 535 nm is partially quenched and the fluorescence at 415-420 nm increases. Using fluorescein labeled sarcoplasmic reticulum membranes we have found that the average conformation of the Ca2+ + Mg(2+)-ATPase is also altered on binding of phosphorylase b. In conclusion, the results reported in this paper suggest that glycogen phosphorylase and Ca2+ + Mg(2+)-ATPase directly interact under experimental conditions similar to those found in the sarcoplasm, and that this interaction is modulated by phosphorylation of the phosphorylase.