Hexanol and lidocaine affect the oligomeric state of the Ca-ATPase of sarcoplasmic reticulum

Characterization of a Biomimetic Mesophase Composed of Nonionic Surfactants and an Aqueous Solvent

We have investigated the physical and biomimetic properties of a sponge (L₃) phase composed of pentaethylene glycol monododecyl ether (C₁₂E₅), a nonionic surfactant, an aqueous solvent, and a cosurfactant. The following cosurfactants, commonly used for solubilizing membrane proteins, were incorporated: n-octyl-β-d-glucopyranoside (β-OG), n-dodecyl-β-d-maltopyranoside (DDM), 4-cyclohexyl-1-butyl-β-d-maltoside (CYMAL-4), and 5-cyclohexyl-1-pentyl-β-d-maltoside (CYMAL-5). Partial phase diagrams of these systems were created. The L₃ phase was characterized using crossed polarizers, diffusion of a fluorescent probe by fluorescence recovery after pattern photobleaching (FRAPP), and freeze fracture electron microscopy (FFEM). By varying the hydration of the phase, we were able to tune the distance between adjacent bilayers. The characteristic distance (d_b) of the phase was obtained from small angle scattering (SAXS/SANS) as well as from FFEM, which yielded complementary d_b values. These d_b values were neither affected by the nature of the cosurfactant nor by the addition of membrane proteins. These findings illustrate that a biomimetic surfactant sponge phase can be created in the presence of several common membrane protein-solubilizing detergents, thus making it a versatile medium for membrane protein studies.

Mechanism of allosteric effects of ATP on the kinetics of P-type ATPases

The roles of allosteric effects of ATP and protein oligomerisation in the mechanisms of P-type ATPases belong to the most controversial and least well understood topics in the field. Recent crystal structural and kinetic data, however, now allow certain hypotheses to be definitely excluded and consistent hypotheses to be developed. The aim of this review is to critically discuss recent results and, in the light of them, to present a set of conclusions which could form the basis of future research. The major conclusions are: (1) at saturating ATP concentrations P-type ATPases function as monomeric enzymes, (2) the catalytic units of P-type ATPases only possess a single ATP binding site, (3) at non-saturating ATP concentrations P-type ATPases exist as diprotomeric (or higher oligomeric) complexes, (4) protein-protein interactions within a diprotomeric complex enhances the enzymes' ATP binding affinity, (5) ATP binding to both protomers within a diprotomeric complex causes it to dissociate into two separate monomers. The physiological role of protein-protein interactions within a diprotomer may be to enhance ATP binding affinity so as to scavenge ATP and maximize the ion pumping rate under hypoxic or anoxic conditions. For the first time a structural basis for the well-known ATP allosteric acceleration of the E2 --> E1 transition is presented. This is considered to be due to a minimization of steric hindrance between neighbouring protomers because of the ability of ATP to induce a compact conformation of the enzymes' cytoplasmic domains.

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.

The amide class herbicide 3,4-dichloropropionanilide (DCPA) alters the mobility of hydrocarbon chains in T-lymphocyte but not macrophage membranes

Previous studies in our laboratory have demonstrated that the lipophilic herbicide 3,4-dicholoropropionanilide (DCPA) adversely affects cytokine production by activated macrophages and T lymphocytes. The purpose of this study was to test the hypothesis that DCPA alters the mobility of plasma membrane lipid hydrocarbon chains, which interferes with normal T-lymphocyte activation and macrophage function. Electron spin reasonance (ESR) spectroscopy of stearic acid spin labels incorporated into each cell type was used to test the effects of DCPA on lipid hydrocarbon chain mobility in the absence and presence of specific agents that activate each cell type. The results indicated that DCPA treatment had no significant effect on hydrocarbon chain mobility in either cell type per se. However, for T lymphocytes, but not macrophages, DCPA treatment increased a small population of lipid molecules that exhibited reduced hydrocarbon chain mobility near the bilayer hydrocarbon core following cell stimulation. In contrast, there were no significant effects of DCPA on hydrocarbon chain mobility near the head group region of the bilayer for either cell type. The identity of this subpopulation of lipids and its motional properties could not be elucidated from these studies. Nevertheless, data show that DCPA alters the distribution of lipids in distinct motional environments in the membrane of activated T lymphocytes.

Inhibition of the Na,K-ATPase of canine renal medulla by several local anesthetics

The ATPase activity of Na,K-ATPase-enriched membranes from canine renal medulla was determined in the absence of local anesthetic and in the presence of procaine, chloroprocaine, bupivacaine, mepivacaine, lidocaine, and two quaternary derivatives of lidocaine (QX-222 and QX-314) at 37( composite function)C. Chloroprocaine (IC(50)= 13 mM) had slightly greater potency than procaine (IC(50)= 17.7 mM). Bupivacaine (IC(50)= 6.7 mM) was more potent than its congener mepivacaine (IC(50)> 10 mM, the solubility limit). QX-222 (IC(50)> 600 mM) and QX-314 (IC(50)= 132 mM) had less potency than lidocaine (IC(50)= 30.4 mM). This study supports the interpretation that the uncharged forms of local anesthetics are much more potent inhibitors of Na,K-ATPase activity than the cationic forms.

Effects of local anaesthetics on the activity of the Na,K-ATPase of canine renal medulla

The purpose of this study is to characterize the effects of local anaesthetics on Na,K-ATPase activity. The ATPase activity of Na, K-ATPase-enriched membranes from canine renal medulla was determined in the absence and in the presence of lidocaine, procaine, tetracaine, benzocaine, bupivacaine, prilocaine, and procainamide at 37 and 25 degrees C. All of these local anaesthetics, except benzocaine, inhibit the activity of the Na,K-ATPase of canine renal medulla at both 25 and 37 degrees C. Benzocaine inhibits Na,K-ATPase activity at 37 degrees C, but stimulates activity at 25 degrees C. The influence of lidocaine on stimulation of Na,K-ATPase activity by Na(+) and K(+) was investigated. Lidocaine increases the apparent K(0.5) of the Na,K-ATPase for both Na(+) and K(+) and decreases the V(max) values for both ions. IC(50) values for lidocaine increase with increasing concentrations of both Na(+) and K(+). The data indicate that lidocaine diminishes the affinity of the Na,K-ATPase for Na(+) and K(+) and that binding of Na(+) or K(+) decreases the potency of lidocaine as an inhibitor of the Na,K-ATPase. Lidocaine markedly decreases the affinity of the Na,K-ATPase for ouabain, but only slightly diminishes the maximum amount of ouabain bound. Unprotonated lidocaine is apparently a more potent inhibitor than is the protonated form.

Effects of general anaesthetics on the activity of the Na,K-ATPase of canine renal medulla

Several previous studies have reported inhibition of Na,K-ATPase activity by chlorpromazine, phenobarbital and pentobarbital, thiopental, and monoketones. The purpose of this study is to investigate the influences of other general anaesthetics on Na,K-ATPase activity. The ATPase activity of Na,K-ATPase-enriched membranes from canine renal medulla was determined at 37 degrees C in the absence and in the presence of hexanol, diethylether, halothane, and propofol. The influence of hexanol on stimulation of Na,K-ATPase activity by Na+ and K+ was investigated. Hexanol, diethylether, halothane, and propofol inhibited the activity at 37 degrees C of the Na,K-ATPase of canine renal medulla. The IC50 values at 37 degrees C were: hexanol, 12.3 mM; diethylether, 170 mM; halothane, 7.35 mM; propofol, 0.127 mM. Hexanol increased the K0.5 of the Na,K-ATPase for K+ at 37 degrees C, but did not affect the K0.5 for Na+. At lower [K+] hexanol was a more potent inhibitor than at higher [K+].

An autoinhibitory peptide from the erythrocyte Ca-ATPase aggregates and inhibits both muscle Ca-ATPase isoforms

We have studied the effects of C28R2, a basic peptide derived from the autoinhibitory domain of the plasma membrane Ca-ATPase, on enzyme activity, oligomeric state, and E1-E2 conformational equilibrium of the Ca-ATPase from skeletal and cardiac sarcoplasmic reticulum (SR). Time-resolved phosphorescence anisotropy (TPA) was used to determine changes in the distribution of Ca-ATPase among its different oligomeric species in SR. C28R2, at a concentration of 1-10 microM, inhibits the Ca-ATPase activity of both skeletal and cardiac SR (CSR). In skeletal SR, this inhibition by C28R2 is much greater at low (0.15 microM) than at high (10 microM) Ca2+, whereas in CSR the inhibition is the same at low and high Ca2+. The effects of the peptide on the rotational mobility of the Ca-ATPase correlated well with function, indicating that C28R2-induced protein aggregation and Ca-ATPase inhibition are much more Ca-dependent in skeletal than in CSR. In CSR at low Ca2+, phospholamban (PLB) antibody (functionally equivalent to PLB phosphorylation) increased the inhibitory effect of C28R2 slightly. Fluorescence of fluorescein 5-isothiocyanate-labeled SR suggests that C28R2 stabilizes the E1 conformation of the Ca-ATPase in skeletal SR, whereas in CSR it stabilizes E2. After the addition of PLB antibody, C28R2 still stabilizes the E2 conformational state of CSR. Therefore, we conclude that C28R2 affects Ca-ATPase activity, conformation, and self-association differently in cardiac and skeletal SR and that PLB is probably not responsible for the differences.

Direct spectroscopic detection of molecular dynamics and interactions of the calcium pump and phospholamban

In order to test molecular models of cardiac calcium transport regulation, we have used spectroscopy to probe the structures, dynamics, and interactions of the Ca pump (Ca-ATPase) and phospholamban (PLB) in cardiac sarcoplasmic reticulum (SR) and in reconstituted membranes. Electron paramagnetic resonance (EPR) and phosphorescence of probes bound to the Ca pump show that the activity of the pump is quite sensitive to its oligomeric interactions. In cardiac SR, PLB aggregates and inhibits the pump, and both effects are reversed by PLB phosphorylation. Previous analyses of PLB's oligomeric state were only in detergent solutions, so we used EPR and fluorescence to determine the oligomeric structure of PLB in its native state in lipid bilayers. Wild-type PLB is primarily oligomeric in the membrane, while the mutant L37A-PLB is monomeric. For both proteins, phosphorylation shifts the dynamic monomer-oligomer equilibrium toward oligomers, and induces a similar structural change, as indicated by tyrosine fluorescence; yet L37A-PLB is more effective than wild-type PLB in inhibiting and aggregating the pump. Fluorescence energy transfer shows that the Ca pump increases the fraction of monomeric PLB, indicating that the pump preferentially binds monomeric PLB. These results support a reciprocal aggregation model for Ca pump regulation, in which the Ca pump is aggregated and inhibited by association with PLB monomers, and phosphorylation of PLB reverses these effects while decreasing the concentration of PLB monomers. To investigate the structure of the PLB pentamer in more detail, we measured the reactivities of cysteine residues in the transmembrane domain of PLB, and recorded EPR spectra of spin labels attached to these sites. These results support an atomic structural model, based on molecular dynamics simulations and mutagenesis studies, in which the PLB pentamer is stabilized by a leucine-isoleucine zipper within the transmembrane domain.

Effect of intrathecally administered local anesthetics on protein phosphorylation in the spinal cord

To elucidate the biochemical mechanisms of spinal anesthesia, we studied the effects of procaine and tetracaine on protein phosphorylation in the mouse spinal cord. Mice were injected intrathecally with either procaine, tetracaine (67 mM/approximately 2%, 10 microL, N = 5/drug), or saline (N = 4/group). Five minutes after injection, animals were killed with a guillotine, and the spinal cord was removed. The caudal 3-cm cord segment was homogenized and centrifuged, and an aliquot of the supernatant was used for phosphorylation assays. Calcium-dependent phosphorylation was initiated by incubating the samples in buffer containing [gamma-32P]ATP at 37 degrees for 30 min. The proteins were electrophoresed using slab gel and two-dimensional electrophoresis, and phosphorylated proteins were visualized by autoradiography. The data demonstrated that spinal anesthesia changes the phosphorylation state of five endogenous substrate proteins with apparent molecular masses of 130 (protein-a), 105 (protein-b), 55 (protein-c), 47 (protein-d), and 33 (protein-e) kDa. In two-dimensional electrophoresis, protein-a resolved into two proteins (a1 and a2). Analysis of variance of the densitometric data suggested a significant effect for the treatment (F(2,16) 735, P < 0.00005). Post hoc comparisons with the saline-treated controls, using the Newman-Keuls test, indicated that local anesthetics significantly affected phosphoproteins (P < 0.05) except for protein-al in the tetracaine-treated group. Further characterization of these phosphoproteins should aid in determining their role in the signal transduction cascade affected by spinal anesthesia.

Dissociation of phospholamban regulation of cardiac sarcoplasmic reticulum Ca2+ATPase by quercetin

Quercetin had a biphasic effect on Ca2+ uptake and calcium-stimulated ATP hydrolysis in isolated cardiac sarcoplasmic reticulum (SR). Stimulation of Ca2+ATPase was observed at low quercetin concentrations (<25 microM) followed by inhibition at higher concentrations. The effects were dependent upon the SR protein concentration, the MgATP concentration, and intact phospholamban regulation of cardiac Ca2+ATPase. Only the inhibitory effects at higher quercetin concentrations were observed in skeletal muscle SR which lacks phospholamban and in cardiac SR treated to remove phospholamban regulation. Stimulation was additive with monoclonal antibody 1D11 (directed against phospholamban) at submaximal antibody concentrations; however, the maximal antibody and quercetin stimulation were identical. Quercetin increased the calcium sensitivity of the Ca2+ATPase like that observed with phosphorylation of phospholamban or treatment with monoclonal antibody 1D11. In addition, low concentrations of quercetin increased the steady-state formation of phosphoenzyme from ATP or Pi, but higher quercetin decreased phosphoenzyme levels. Quercetin, even under stimulatory conditions, was a competitive inhibitor of ATP, but appears to relieve the Ca2+ATPase from phospholamban inhibition, thereby, producing an activation. The subsequent inhibitory action of higher quercetin concentrations results from competition of quercetin with the nucleotide binding site of the Ca2+ATPase. The data suggest that quercetin interacts with the nucleotide binding site to mask phospholamban's inhibition of the SR Ca2+ATPase and suggests that phospholamban may interact at or near the nucleotide binding site.

Structure-function relationships in the Ca(2+)-ATPase of sarcoplasmic reticulum: facts, speculations and questions for the future

Structural data on the Ca(2+)-ATPase of sarcoplasmic reticulum are integrated with kinetic data on Ca2+ transport. The emphasis is upon ATPase-ATPase interactions, the requirement for phospholipids, and the mechanism of Ca2+ translocation. The possible role of cytoplasmic [Ca2+] in the regulation of the synthesis of Ca(2+)-ATPase is discussed.

Interaction of alkanols and local anesthetics with spin-labeled Ca(2+)-ATPase of sarcoplasmic reticulum vesicles

Alkanols and tertiary amine derivative local anesthetics modify the activity of Ca(2+)-ATPase. In order to investigate the primary binding sites, associated to the functional changes, sarcoplasmic reticulum (SR) Ca(2+)-ATPase was labeled with maleimide derivative spin labels which bind covalently to SH groups of cysteine residues and allow to probe the regions of the protein close to those residues. The EPR measurements showed motional constraints induced by drug-treatment which indicate changes in the enzyme dynamics and structure. n-Alkanols are shown to affect some of the protein-bound labels by restricting their motion. There is, however, no correlation between the functional effects and the observed motional restriction, in the sense that concentrations of the different alcohols leading to the same functional effects do not induce the same degree of restriction. Dibucaine and tetracaine at functional relevant concentrations also restrict the movement of protein bound labels. But, in this case, correlation between spectral changes and functional effects is observed.

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.

The structure and interactions of Ca(2+)-ATPase

Electron crystallographic studies on membrane crystals of Ca(2+)-ATPase reveal different patterns of ATPase-ATPase interactions depending on enzyme conformation. Physiologically relevant changes in Ca2+ concentration and membrane potential affect these interactions. Ca2+ induced difference FTIR spectra of Ca(2+)-ATPase triggered by photolysis of caged Ca2+ are consistent with changes in secondary structure and carboxylate groups upon Ca2+ binding; the changes are reversed during ATP hydrolysis suggesting that a phosphorylated enzyme form of low Ca2+ affinity is the dominant intermediate during Ca2+ transport. A two-channel model of Ca2+ translocation is proposed involving the membrane-spanning helices M2-M5 and M4, M5, M6 and M8 respectively, with separate but interacting Ca2+ binding sites.

Anesthetics alter the physical and functional properties of the Ca-ATPase in cardiac sarcoplasmic reticulum

We have studied the effects of the local anesthetic lidocaine, and the general anesthetic halothane, on the function and oligomeric state of the CA-ATPase in cardiac sarcoplasmic reticulum (SR). Oligomeric changes were detected by time-resolved phosphorescence anisotropy (TPA). Lidocaine inhibited and aggregated the Ca-ATPase in cardiac SR. Micromolar calcium or 0.5 M lithium chloride protected against lidocaine-induced inhibition, indicating that electrostatic interactions are essential to lidocaine inhibition of the Ca-ATPase. The phospholamban (PLB) antibody 2D12, which mimics PLB phosphorylation, had no effect on lidocaine inhibition of the Ca-ATPase in cardiac SR. Inhibition and aggregation of the Ca-ATPase in cardiac SR occurred at lower concentrations of lidocaine than necessary to inhibit and aggregate the Ca-ATPase in skeletal SR, suggesting that the cardiac isoform of the enzyme has a higher affinity for lidocaine. Halothane inhibited and aggregated the Ca-ATPase in cardiac SR. Both inhibition and aggregation of the Ca-ATPase by halothane were much greater in the presence of PLB antibody or when PLB was phosphorylated, indicating a protective effect of PLB on halothane-induced inhibition and aggregation. The effects of halothane on cardiac SR are opposite from the effects of halothane observed in skeletal SR, where halothane activates and dissociates the Ca-ATPase. These results underscore the crucial role of protein-protein interactions on Ca-ATPase regulation and anesthetic perturbation of cardiac SR.

Self-association accompanies inhibition of Ca-ATPase by thapsigargin

Recent studies have demonstrated a relationship between the activity of the Ca-ATPase of sarcoplasmic reticulum and its state of self-association. In the present study, the effects of thapsigargin (TG), a toxin that specifically inhibits the Ca-ATPase of rabbit skeletal muscle sarcoplasmic reticulum membrane, were studied by detecting the time-resolved phosphorescence anisotropy (TPA) decay of the Ca-ATPase that had been labeled with the phosphorescent probe erythrosin-isothiocyanate (ErITC). Anisotropy decays were fit to a function that consisted of three exponential decays plus a constant background, as well as to a function describing explicitly the uniaxial rotation of proteins in a membrane. In the absence of TG, the anisotropy was best-fit by a model representing the rotation of three populations, corresponding to different-sized oligomeric species in the membrane. The addition of stoichiometric amounts of TG to the Ca-ATPase promptly decreased the overall apparent rate of decay, indicating decreased rotational mobility. A detailed analysis showed that the principal change was not in the rates of rotation but rather in the population distribution of the Ca-ATPase molecules among the different-sized oligomers. TG decreased the proportion of small oligomers and increased the proportion of large ones. Preincubation of the ErITC-SR in 1 mM Ca2+, which stabilizes the E1 conformation relative to E2, was found to protect partially against the changes in the TPA associated with the presence of the inhibitor. These results are consistent with the hypothesis that TG inhibits the Ca-ATPase by stabilizing it in an E2-like conformation, which promotes the formation of larger aggregates of the enzyme. When combined with the effects of other inhibitors on the Ca-ATPase, these results support a general model for the coupling of enzyme conformation and self-association in this system.