Electrokinetic and hydrodynamic properties of sarcoplasmic reticulum vesicles: a study by laser Doppler electrophoresis and quasi-elastic light scattering

Electrophoretic mobility of sarcoplasmic reticulum vesicles - analytical model includes amino acid residues of A+P+N domain of Ca(2+)-ATPase and charged lipids

This work is an experimental and theoretical study of electrostatic and hydrodynamic properties of the surface of sarcoplasmic reticulum (SR) membrane using particle electrophoresis. The essential structural components of SR membrane include a lipid matrix and a dense layer of Ca(2+)-ATPases embedded in the matrix. The Ca(2+)-ATPase layer both drives and impedes vesicle mobility. To analyze the experimental mobility data, obtained at pH4.0, 4.7, 5.0, 6.0, 7.5, and 9.0 in 0.1M monovalent (1:1) electrolyte, an analytical solution for the vesicle mobility and electroosmotic flow velocity distribution was obtained by solving the Poisson-Boltzmann and the Navier-Stokes-Brinkman equations. The electrophoretic mobility model includes two sets of charges that represent: (a) charged lipids of the lipid matrix of the vesicle core, and (b) charged amino acid residues of APN domains of Ca(2+)-ATPases. APN domains are assumed to form a charged plane displaced from the surface of lipid matrix. The charged plane is embedded in a frictional layer that represents the surface layer of calcium pumps. Electrophoretic mobility is driven by the charged APN domain and by lipid matrix while the surface layer provides hydrodynamic friction. The charge of APN domain is determined by ionized amino acid residues obtained from the amino acid composition of SERCA1a Ca(2+)-ATPase. Agreement between the measured and the predicted mobility is evaluated by the weighted sum of mobility deviation squared. This model reproduces the experimental dependence of mobility on pH and predicts that APN domains are located in the upper half of the SR vesicle surface layer.

Partitioning of Tetrachlorophenol into Lipid Bilayers and Sarcoplasmic Reticulum: Effect of Length of Acyl Chains, Carbonyl Group of Lipids and Biomembrane Structure

We report results of a partitioning study of 2,3,4,6-tetrachlorophenol (TeCP). In the study we explored (1) the effect of the length of acyl chains of lipids (C16:1 - C24:1) and alkanes (C6-C16), (2) the role of the carbonyl group of lipids, and (3) the effect of molecular structure of the sarcoplasmic reticulum membrane on TeCP partitioning. Mole fraction partition coefficients have been measured using equilibrium dialysis for un-ionized (HA), and ionized (A) species, Kp(x) (HA), Kp(x) (A). Their values are concentration-dependent. Partition coefficients were analyzed in terms of a model that accounts for saturation of membrane associated with the finite area of partition site, and electrostatic interactions of (A-) species with charged membrane. Limiting values of partition coefficients, corresponding to infinite dilution of solute, Kp(x0) (HA), Kp(x0) (A) were obtained. Kp(x0) (HA) and Kp(x0) (A ) measure the strength of solute-membrane interactions. Studies were done with single-layered vesicles of lipids with variable chain length: 1,2-dipalmitoleoyl-sn-glycero-3-phosphocholine (C16:1), 1,2-dioleoyl-sn-glycero-3-phosphocholine (C18:1), 1,2-dierucoyl-sn-glycero-3-phosphocholine (C22:1), and 1 ,2-dinervonoyl-sn-glycero-3-phosphocholine (C24:1), and egg-PC. Kp(x0) for transfer of TeCP from water into lipid membranes was found to be independent of the length of acyl chains, whereas Kp(x0) for transfer from water into alkanes increased with the length of alkane. The effect of the carbonyl CO group of lipids on partitioning was measured using 1,2-di-o-octadecenyl-sn-glycero-3-phosphocholine (CO absent) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (CO present) liposomes. Carbonyl groups, known to change dipolar potential, had no effect on partitioning. Partition coefficients of un-ionized and ionized forms of TeCP were invariant to the presence of proteins and other membrane components of sarcoplasmic reticulum (SR) membrane.

Adsorption of ruthenium red to phospholipid membranes

We have measured the distribution of the hexavalent ruthenium red cation (RuR) between water and phospholipid membranes, have shown the critical importance of membrane negative surface charge for RuR binding, and determined the association constant of RuR for different phospholipid bilayers. The studies were performed with liposomes made of mixtures of zwitterionic L-alpha-phosphatidylcholine (PC), and one of the negatively charged phospholipids: L-alpha-phosphatidylserine (PS), L-alpha-phosphatidylinositol (PI), or L-alpha-phosphatidylglycerol (PG). Lipid composition of PC:PX membranes was 1:0, 19:1, 9:1, and 4:1. Liposomes were processed using freeze-and-thaw treatment, and their size distribution was characterized by light scattering and electron microscopy. Experimental distribution isotherms of RuR obtained by ultracentrifugation and spectrophotometry can be reproduced with the Langmuir-Stern-Grahame model, assuming that RuR behaves in the diffuse double layer as an ion with effective valency < 6. In terms of this model, PC-PS, PC-PI, and PC-PG membranes were found to be electrostatically equivalent and the intrinsic association constants of RuR were obtained. RuR has highest affinity to PS-containing membranes; its association constant for PC-PI and PC-PG membranes is about 5 times smaller than that for PC-PS membranes. From the comparison of RuR binding to mixed negatively charged phospholipid membranes and RuR binding to sarcoplasmic reticulum (SR), we conclude that the low-affinity RuR binding sites may indeed be associated with the lipid bilayer of SR.

Lipid composition and structure of commercial parenteral emulsions

In order to study the influence of the phospholipid/triacylglycerol (PL/TG) ratio of parenteral emulsions on the distribution and the physico-chemical properties of their fat particles, commercial 10, 20 or 30% fat formulas were fractionated by centrifugation into an upper lipid cake (resuspended in aqueous glycerol) and a subnatant or mesophase, from which a PL-rich subfraction (d = 1.010-1.030 g/l) was purified by density gradient ultracentrifugation. Chemical and 31P-NMR analyses of these fractions indicated that at least two types of fat particles coexist in parenteral emulsions: (i) TG-rich particles (mean diameter: 330, 400, 470 nm in the 10, 20, 30% emulsion) which contain practically all the TG and esterified phytosterols of native emulsions, but only a fraction of their PL, unesterified cholesterol and phytosterols, and other minor lipids; (ii) PL-bilayer particles or liposomes (mean diameter: 80-100 nm) which are constituted with the remaining PL and relatively very small amounts of TG and other lipids. The higher the oil content of the emulsion, the lower the amount of these PL-rich particles, which represent the major particle population of the mesophase. Indeed, minute amounts of TG-rich particles (probably the smallest ones) are also present in the mesophase, even in the PL-rich subfraction which contains the bulk of liposomal PL. Since the PL-rich particles of the infused emulsion generate lipoprotein X-like particles, only the large TG-rich particles can be considered as true chylomicron counterparts.

Effects of intravenous infusions of commercial fat emulsions (Intralipid 10 or 20%) on rat plasma lipoproteins: phospholipids in excess are the main precursors of lipoprotein-X-like particles

Like most commercial parenteral emulsions, Intralipid contains the same amount of phospholipids (12 mg/ml) to stabilize 100 or 200 mg of soybean oil (10 or 20% formula, respectively). By centrifugation, 10 or 20% Intralipid was separated into a supernatant, fat particles containing the bulk of triacylglycerols stabilized by a fraction of phospholipids and an infranatant--called mesophase--consisting mainly of phospholipids used in excess as emulsifier. We observed that the initial triacylglycerol/phospholipid ratio of the emulsion (100/12 and 200/12, respectively) determines the size of the triacylglycerol-rich particles (260 and 350 nm) as well as the phospholipid content of the mesophase (6.02 and 4.67 mg/ml). To understand the mechanism of the lipoprotein-X (LPX) accumulation generally reported after intravenous fat infusions, plasma lipid levels and lipoprotein profiles were first compared in the rats after infusion (at a constant rate of 0.5 or 1 ml/h for 43 h) of Intralipid 10 or 20%. For the same intravenous triacylglycerol load (100 mg/h), rats infused with Intralipid 10% at 1 ml/h displayed higher triacylglycerol levels than rats infused with the 20% emulsion at 0.5 ml/h, suggesting that the size of exogenous fat particles modulated the catabolic rate of their triacylglycerols. The plasma levels of LPX varied according to the infusion rate of phospholipids not associated with triacylglycerol-rich particles of the emulsion. Moreover, an apo E and apo B enrichment of plasma and an elevation of the apo B48/apo B100 ratio was always observed after Intralipid infusions. In order to confirm that phospholipids of the mesophase are the main LPX precursors, lipoprotein profiles were then compared in the rats after intravenous infusion, at a constant rate of 1 ml/h, of either the mesophase or a suspension of triacylglycerol-rich particles isolated from Intralipid 20%. As expected, significant LPX amounts were only detected in rats infused with the pure mesophase of the emulsion. It was concluded that products of the lipolysis of exogenous fat particles play only a minor role in the formation of LPX. In fact these abnormal lipoproteins are generated by phospholipids of the mesophase which, like infused liposomes, actively mobilize endogenous free cholesterol. Consequently, in order to be considered as true chylomicron models for safe fat delivery in parenteral nutrition and in order to prevent some detrimental effects on cholesterol metabolism, commercial emulsions should be cleared of phospholipid excess.

Changes of some physical properties of isolated and purified plasma and thylakoid membrane vesicles from the freshwater cyanobacterium Synechococcus 6301 (Anacystis nidulans) during adaptation to salinity

Photoautotrophically growing cultures of the freshwater cyanobacterium Anacystis nidulans (Synechococcus sp.) became adapted to the presence of 0.4-0.5 M NaCl in the growth medium (about seawater level) with a lag phase of 2 days after which time the growth rate resumed at 80-90% of the control. Major changes in structure and function of the plasma membranes (and, to a much lesser extent, of the thylakoid membranes) were found to accompany the adaptation process. Plasma and thylakoid membranes were separated from crude cell-free extracts of French pressure cell-treated Anacystis by discontinuous sucrose density gradient centrifugation and purified by repeated recentrifugation on fresh gradients. Concentrations of copper, iron, calcium, and magnesium ions were determined by inductively coupled plasma atomic emission spectrometry with EDTA-washed and dialyzed membrane preparations; salt adaptation was found to increase (decrease) the concentration of membrane-bound calcium in plasma (thylakoid) membranes, qualitatively reciprocal results being obtained for magnesium. Levels of plasma membrane-bound copper and iron roughly tripled during the adaptation process; by contrast, corresponding effects on thylakoid membranes were negligible. The size of the membrane vesicles was measured by quasi-elastic laser light-scattering and the electric surface charge of the membranes was measured by laser Doppler velocimetry. Salt adaptation decreased the mean diameter of plasma membrane vesicles to a much higher extent than that of thylakoid membrane vesicles. Overall surface charge densities of resting vesicles were only slightly affected by the salt treatment as was also seen from titration of the electrophoretic mobility of the vesicles with electrolytes. Yet, induction of (photosynthetic or respiratory) electron transport provoked a charge separation across the membrane which was easily measurable in terms of electrophoretic mobility. The results will be discussed with particular emphasis on the stimulated cytochrome c oxidase activity of plasma (but not thylakoid) membranes from salt-adapted cells compared to control cells and also with respect to the decreased ion permeability of the plasma membrane of salt grown cells.

Isoelectric focusing studies of bacteriorhodopsin

Purified bacteriorhodopsin (BR) samples show a minimum of four isoelectric forms in immobilized pH gradient isoelectric focusing gels. The bands occur as doublets with isoelectric points (pI) centered at 5.20 (principal species) and 5.60. In typical preparations additional bands may be observed at 4.90, 5.07 and 5.50. Purple membrane (PM) was proteolyzed with papain to calibrate the pI shift produced by changing the number of charges on the protein. Asp-242 is removed during the first cleavage between residues 239 and 240 resulting in the loss of a single negative charge and a shift of the principal doublet by +0.35 pH units to pI 5.55. The second papain cleavage occurs between residues 231 and 232 which removes Glu-232, -234 and -237 and shifts the pI by +0.60 pH units to pI 6.10. The +0.60 pH shift upon the second papain cleavage is consistent with the loss of two negative charges and is supported by prior evidence that at least one of the three glutamate residues lost during the second proteolysis step is protonated and neutral in the intact protein. The native and proteolyzed products of BR retain the characteristic 550 nm absorption maxima for solubilized BR. A model for the structural origin of the pI heterogeneity of BR species in proteolyzed PM is presented.

Activation of Ca2+ release in isolated sarcoplasmic reticulum

The relationship between Ca2+ release from sarcoplasmic reticulum, induced by elevated pH, tetraphenylboron (TPB-) or chemical modification, and the change in the surface charge of the membranes as measured by the fluorescence intensity of anilinonaphthalene sulfonate (ANS) is examined. The simulated Ca2+ release is inhibited by dicyclohexylcarbodiimide and external Ca2+. TPB-, but not tetraphenylarsonium (TPA+), causes a decrease in ANS- fluorescence, with 50% decrease occurring at about 5 microM TPB-. The decrease in ANS- fluorescence as well as the inhibition of Ca2+ accumulation induced by TPB- are prevented by TPA+. A linear relationship between the decrease in membrane surface potential and the extent of the Ca2+ released by TPB- is obtained. Similar levels of [3H]TPB-bound to sarcoplasmic reticulum membranes were obtained regardless of whether or not the vesicles have taken up Ca2+. The inhibition of Ca2+ accumulation and the [3H]TPB- incorporation into the membranes were correlated. Ca2+ release from sarcoplasmic reticulum, by pH elevation, chemical modification or by addition of NaSCN (0.2 to 0.5 M) or the Ca2+ ionophore ionomycin, is also accompanied by a decrease in ANS- fluorescence intensity. However, chemical modification and elevated pH affects the surface potential much less than SCN- or TPB- do. These results suggest that the enhancement of Ca2+ release by these treatments is not due to a general effect on the membrane surface potential, but rather through the modification of a specific protein. They also suggest that membrane surface charges might play an important role in the control mechanism of Ca2+ release.

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.

Aggregation and proton release of purple and white membranes following cleavage of the carboxyl-terminal tail of bacteriorhodopsin

Our results indicate that the previously reported decrease in proton release by proteolyzed purple membrane sheets was due merely to the aggregation state of these preparations and not to the loss of the carboxyl-terminal tail. Changes in H+/M412 ratios obtained for purple and white membrane preparations correlate with the measured aggregation. White membrane preparations consistently exhibit H+/M412 ratios more than twice those measured for native purple membranes under the same conditions. Quasi-elastic light scattering was used to characterize the size of isolated purple and white membrane sheets before and after proteolysis. The results clearly show that native purple membrane preparations are larger in size than would be expected and that, following trypsin treatment, they are on average more than an order of magnitude larger. Negative staining electron microscopy showed that the purple membrane became aggregated in stacked arrays. Bleaching and reconstitution with retinal also affect aggregation, but iodination or nitration of purple membrane does not affect the measured size. The average size of white membranes is smaller; this is consistent with results of electron microscopy and the size increase is much less than that of purple membranes following trypsin treatment. No size change occurs with retinal reconstitution. In aggregated purple membrane preparations, protons and other cations are unable to exchange freely with the aqueous medium, explaining why proteolysis lowers the proton release from purple membrane sheets in suspension.

Study of the electrokinetic properties of reconstituted sarcoplasmic reticulum vesicles

A study of electrokinetic properties of reconstituted sarcoplasmic reticulum was undertaken to determine the nature of the groups bearing the negative charge of the membrane. After incorporation of phosphatidylcholine into the bilayer, it was found that the Ca2+-ATPase embedded in functional vesicles bore 3e- per mole. When the surface charge density of the hydrodynamic particles became more negatively charged by incorporation of phosphatidylserine molecules, the reconstituted vesicles had a tendency to build large structures resulting from vesicle-vesicle interaction and containing large amounts of divalent cations. These aggregated structures may partially explain the discrepancy observed between the expected value of the surface charge density and the data obtained by electrophoretic mobility measurements. This work emphasizes the importance of a renewal of the classical interpretation of electrophoretic mobility data in order to analyze the results obtained with biological material. To explain the energy transduction process which takes place in the sarcoplasmic reticulum membrane, it was of interest to determine whether or not variations of the surface electrical properties affect the calcium ion translocation upon ATP hydrolysis. Relatively significant modifications of the bilayer composition and surface charge density did not appreciably affect the calcium transport activity.

Charge changes in sarcoplasmic reticulum and Ca2+-ATPase induced by calcium binding and release: a study using lipophilic ions

Changes in the charge of sarcoplasmic reticulum (SR) vesicles are studied using lipophilic ions, which are adsorbed by the membrane phase. Upon addition of MgATP, phenyldicarbaundecaborane (PCB-) and tetraphenylboron (TPB-) are taken up by the SR vesicles, while tetraphenylphosphonium (TPP+) is released into the water phase. The PCB- uptake occurs as well under conditions when SR membrane is shunted by high Cl- concentration. MgATP induces minor additional binding of PCB- in the presence of oxalate and it is followed by release of the lipophilic anion from the vesicles. EGTA partly reverses the ATP effect, and calcium ionophore A23187 plus EGTA reverses it completely. Vesicles that were preliminarily loaded by Ca2+ demonstrated higher passive and lower ATP-dependent PCB- binding. Activation of isolated Ca2+-ATPase in the presence of 0.1 mM EGTA results in PCB- release into the medium and additional TPP+ binding to the enzyme. We suggest that the redistribution of the lipophilic ions between the water phase and SR membrane reflects charge changes in Ca2+-binding sites inside both SR vesicles and Ca2+-ATPase molecules in the course of Ca2+ translocation.

Surface charge of purple membranes measured by laser Doppler velocimetry

Laser Doppler velocimetry measurements on purple membrane suspensions from Halobacterium halobium showed a linear correlation between electrophoretic mobility and applied electric field, electrokinetic responses could be rapidly monitored. Native membranes are less charged than white membrane preparations (from the R1mW mutant). Chemical modification of carboxyl residues reduces surface charge, and nitrotyrosine modified membranes are more or less charged than native membranes at pH greater than or less than 6.5, respectively. Changes in surface charge are found upon actinic illumination and are greatest (Ca 5 X 10(-4)/A2) under conditions where decay of the M412 intermediate of the photoreaction cycle is inhibited, such as at high pH or after chemical modification.