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T E Thompson

Publications and source records attributed to T E Thompson.

At least 91 records · Page 5Linked to original sources
PubMed1982

Photon correlation spectroscopic study of the size distribution of phospholipid vesicles.

The dependence of phospholipid vesicle size on lipid composition is investigated by photon correlation spectroscopy. For each lipid composition prolonged ultracentrifugation was used to isolate a nearly uniform population of minimum-sized vesicles. The residual size variations in the samples were sufficient to cause polydispersity that made comparisons between samples difficult. Analyses of the data by the method of cumulants and by a method for approximating the particle size distributions directly are presented. The latter method made possible unambiguous comparisons that revealed small but systematic dependences of vesicle size on composition in vesicles containing mixtures of egg phosphatidylcholine and phosphatidylethanolamine, egg phosphatidylcholine and beef brain sphingomyelin, and in single lipid vesicles of egg phosphatidylcholine, dioleylphosphatidylcholine, and beef brain sphingomyelin. These size dependences are quantified within the resolution limits of the technique and their implications are discussed.

Animals↗
PubMed1982

Ammonia assimilation and synthesis of alanine, aspartate, and glutamate in Methanosarcina barkeri and Methanobacterium thermoautotrophicum.

The mechanism of ammonia assimilation in Methanosarcina barkeri and Methanobacterium thermoautotrophicum was documented by analysis of enzyme activities, 13NH3 incorporation studies, and comparison of growth and enzyme activity levels in continuous culture. Glutamate accounted for 65 and 52% of the total amino acids in the soluble pools of M. barkeri and M. thermoautotrophicum. Both organisms contained significant activities of glutamine synthetase, glutamate synthase, glutamate oxaloacetate transaminase, and glutamate pyruvate transaminase. Hydrogen-reduced deazaflavin-factor 420 or flavin mononucleotide but not NAD, NADP, or ferredoxin was used as the electron donor for glutamate synthase in M. barkeri. Glutamate dehydrogenase activity was not detected in either organism, but alanine dehydrogenase activity was present in M. thermoautotrophicum. The in vivo activity of the glutamine synthetase was verified in M. thermoautotrophicum by analysis of 13NH3 incorporation into glutamine, glutamate, and alanine. Alanine dehydrogenase and glutamine synthetase activity varied in response to [NH4+] when M. thermoautotrophicum was cultured in a chemostat with cysteine as the sulfur source. Alanine dehydrogenase activity and growth yield (grams of cells/mole of methane) were highest when the organism was cultured with excess ammonia, whereas growth yield was lower and glutamine synthetase was maximal when ammonia was limiting.

Alanine↗
PubMed1981

Thermal behavior of stearoylsphingomyelin-cholesterol dispersions.

The thermotropic behavior of aqueous dispersions of stearoylsphingomyelin-cholesterol mixtures was examined by high-sensitivity differential scanning calorimetry. When less than 20 mol % cholesterol was mixed with the sphingomyelin and the samples were held at room temperature for 7-9 days before the initiation of calorimetric measurements, a sharp endotherm at 56-57 degrees C and a broad endotherm at 35-50 degrees C were observed. In addition, samples containing 15-20 mol % stearol exhibited a sharp endotherm at 43-45 degrees C. If samples were held at room temperature for less than 2 h before the initiation of calorimetric analysis, the 56-57 degrees C endotherm was usually not seen. Instead, a combination of broad and sharp endotherms over the range of 35-50 degrees C was observed. Occasionally, exotherms were also observed within this temperature range. These results, along with those from previous studies, imply that a cholesterol-rich phase coexists with a cholesterol-poor phase in which the sphingomyelin molecules may exist in two distinctly different gel states.

Calorimetry, Differential Scanning↗
PubMed1981

Transbilayer distribution in small unilamellar phosphatidylglycerol-phosphatidylcholine vesicles.

The transbilayer distribution of the phospholipids in small unilamellar vesicles comprised of egg phosphatidylglycerol (PG) and egg phosphatidylcholine (PC) was ascertained by 31P NMR. These vesicles, containing 10-75 mol % PG, were formed by sonication (pH 7.6) and fractionated by centrifugation. Data from spectra accumulated in the presence and absence of a paramagnetic shift reagent, Mn2+, indicated that the phospholipids are randomly arranged across the bilayer. The absence of compositional asymmetry, which contradicts earlier reports, is also exhibited by small unilamellar vesicles (50 mol % PG) prepared by the rapid ethanol injection method. Control experiments showed that Mn2+ did not induce fusion, permeate the vesicles, or cause the phospholipids to migrate across the bilayer. It has been proposed that the transbilayer distribution of charged phospholipids in membranes is a consequence of the different surface charge densities on the opposing sides of the membrane. Our results suggest that it is the difference in the effective polar headgroup volumes of the components rather than the net charge of one component that determines the packing constraints for mixtures of phospholipids with the same acyl chains, at least in highly curved bilayers.

Egg Yolk↗
PubMed1981

Vesicle- vesicle interactions in sonicated dispersions of dipalmitoylphosphatidylcholine.

The time course of the size transformation of sonicated small unilamellar dipalmitoylphosphatidylcholine vesicles at 23 degrees C has been followed with (31)P and (1)H nuclear magnetic resonance (NMR) spectroscopy. Comparison of these results with turbidity measurements indicates that vesicle aggregation, monitored by turbidity, and size transformation, followed by NMR, occur on distinctly different time scales. For lipid concentrations in the 5-50 mM range, aggregation takes place on a time scale of minutes, whereas size transformation takes many hours. Aggregation, unlike size transformation, can be reversed by increasing the temperature above the phospholipid phase transition temperature. Analysis of the (31)P NMR line shapes provides evidence for a model in which the small vesicles transform into the product vesicles [characterized by Schullery, S.E., Schmidt, C.F., Felgner, P., Tillack, T.W., & Thompson, T.E. (1980) Biochemistry 19, 3919] without producing significant amounts of intermediate-size vesicles. Kinetic analysis indicates that the size transformation is apparently second order. (1)H NMR data indicate that the rate of transformation is decreased if trivalent ions are added to the dispersions and also if the temperature is periodically increased above the transition temperature. Analysis of the latter experiment provides some evidence that vesicle aggregation is a necessary precursor to size transformation. It was also found that increasing the average vesicle size decreases the extent of transformation.

Kinetics↗
PubMed1981

Effect of surface curvature on stability, thermodynamic behavior, and osmotic activity of dipalmitoylphosphatidylcholine single lamellar vesicles.

The size and surface curvature dependence of the properties and stability of single lamellar vesicles have been investigated by using a variety of physicochemical techniques. Dipalmitoylphosphatidylcholine single lamellar vesicles of sizes ranging between 200 and 900 A in diameter have been prepared by the French press method and characterized with respect to their size distribution, stability, and thermotropic behavior by negative stain electron microscopy, molecular sieve chromatography, nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. Vesicles with a diameter smaller than 400 A are unstable below their transition temperature and fuse spontaneously to form larger single lamellar vesicles. Correlation analysis of experimentally obtained size distributions and calorimetric phase transitions profiles allowed estimation of the size dependence of the transition temperature. The phase transition temperature depends on the vesicle size in a sigmoidal fashion. Throughout the entire 200-700 A diamter range, the phase transition parameters are sensitive to size; however, the size dependence is especially pronounced around 400 A in diameter. The anomalous size dependence of the transition temperature for vesicles smaller than 400 A in diameter has been attributed to a decrease in the effective bilayer curvature due to packing rearrangements of the lipid molecules. Changes in the fractional degree of self-quenching of trapped 6-carboxyfluorescein induced by osmotic stress indicate that large single lamellar vesicles are not spherical under isoosmotic conditions. These vesicles are relatively flexible and can sustain almost a 2-fold increase in their internal aqueous volume without any leakage of the internal content.

Drug Stability↗
PubMed1981

Transbilayer distribution of phosphatidylethanolamine in large and small unilamellar vesicles.

There is much evidence which strongly suggests that most constituents of biological membranes display a transbilayer compositional asymmetry. The tendency of binary mixtures of phospholipids to form compositionally asymmetric bilayers spontaneously has been studied extensively. In small unilamellar vesicles, most mixtures of phospholipids with different head groups have been reported to be nonrandomly arranged across the bilayer. In this study, the influence of the radius of curvature on the transbilayer phospholipid distribution has been investigated. The distribution of egg phosphatidylethanolamine in large unilamellar vesicles comprised of egg phosphatidylethanolamine and egg was determined by 2,4,6-trinitrobenzenesulfonic acid labeling. These large vesicles were obtained by modifying the ethanol injection procedure originally described by Batzri & Korn (1973) [Batzri, S., & Korn, E. D. (1973) Biochim. Biophys. Acta 298, 1015] by using a slow injection rate. After injection, the ethanol was removed by molecular sieve chromatography and the vesicle dispersion centrifuged. This results in a population of large, homogeneous, and unilamellar vesicles as determined by molecular sieve chromatography, 32P NMR, and electron microscopy. The phosphatidylethanolamine component in unilamellar vesicles of this type is equally distributed between the two monolayers. In contrast, phosphatidylethanolamine in small unilamellar vesicles is known to be preferentially localized in the outer monolayer at low phosphatidylethanolamine concentrations and in the inner monolayer at high phosphatidylethanolamine concentrations. These results suggest that while phospholipids may form asymmetric bilayers spontaneously in highly curved regions of biological membranes, other factors must be responsible for the generalized phospholipid asymmetry seen in these systems.

Egg Yolk↗
PubMed1981

Asymmetric incorporation of trisialoganglioside into dipalmitoylphosphatidylcholine vesicles.

Results are presented which demonstrate that purified trisialoganglioside spontaneously incorporates into performed phospholipid vesicles. Determinations of the extent of incorporation were made by separating large unilamellar dipalmitoylphosphatidylcholine vesicles containing incorporated ganglioside from micellar ganglioside on a Sepharose-2B column. Incorporation occurs without appreciably altering the vesicular character of the phospholipid bilayer as judged by the maintenance of an outside/inside ratio, determined by 31P NMR, comparable to that of the original vesicles. All of the incorporated ganglioside ias accessible to neuraminidase, indicating that incorporation occurs only on the outer face of the bilayer. The thermotropic behavior of these asymmetric dipalmitoylphosphatidylcholine-trisialoganglioside vesicles, examined by high sensitivity scanning calorimetry, strongly suggests that the incorporated ganglioside is intercalated into the outer monolayer of the vesicle bilayer. Calorimetric studies indicate that the ganglioside stabilizes these vesicular structures by inhibiting the fusion of small vesicles that occurs below the phase-transition temperature. These structures are a representative model system, which like the mammalian plasma membrane contain an asymmetric distribution of glycosphingolipid in the outer surface.

Calorimetry, Differential Scanning↗
PubMed1980

Fusion of dipalmitoylphosphatidylcholine vesicles.

Small unilamellar dipalmitoylphosphatidylcholine vesicles formed by sonication are shown to fuse spontaneously below the phase transition temperature. The ultimate fusion products are unilamellar vesicles about 700 A in diameter, which are stable and provide an intact ionic permeation barrier either above or below the phase transition. The fused vesicles have been characterized by gel chromatography, trapped volume, 31P nuclear magnetic resonance, and negative stain and freeze-fracture electron microscopy.

Chromatography, Gel↗
PubMed1980

Thermotropic behavior of dipalmitoylphosphatidylcholine vesicles reconstituted with the glycoprotein of vesicular stomatitis virus.

The vesicular stomatitis virus glycoprotein reconstituted into dipalmitoylphosphatidylcholine (DPPC) vesicles exerts a profound effect upon the DPPC gel to liquid-crystalline phase transition. The glycoprotein was reconstituted into DPPC vesicles by octyl glucoside dialysis. The gel to liquid-crystalline phase transition of these vesicles was monitored by differential scanning calorimetry. Vesicles formed in the absence of glycoprotein (600--2100-A diameter) underwent the phase transition at 41.0 degrees C and had an associated enthalpy change of 8.0 +/- 1.6 kcal/mol. Increasing the mole ratio of glycoprotein to DPPC in the vesicles to 0.15 mol % reduced both the transition temperature and the transition enthalpy change. The enthalpy change as a function of the mole percent glycoprotein could be fit to a straight line by a least-squares procedure. Extrapolation of the results to the glycoprotein concentration where the enthalpy change was zero indicated one glycoprotein molecule bound 270 +/- 150 molecules of DPPC.

Calorimetry, Differential Scanning↗
PubMed1980

Mechanism of the spontaneous transfer of phospholipids between bilayers.

A fluorescent phospholipid, 1-palmitoyl-2-pyrenedecanoylphosphatidylcholine, was used to study the mechanism of spontaneous phospholipid transfer between single-walled phospholipid vesicles. The half-time for transfer of this molecule between vesicles of dimyristoylphosphatidylcholine at 36 degrees C is 13 h if flip-flop is negligible or 24 h if flip-flop is faster than intervesicle exchange. The half-time is unaffected by the concentration of acceptor vesicles, which indicates that transfer of label takes place by diffusion of monomers or micelles through the aqueous phase rather than by collision of vesicles. These results are compared with previous studies of spontaneous lipid transfer.

Chemical Phenomena↗
PubMed1980

Evidence for metastability in stearoylsphingomyelin bilayers.

The physical properties of aqueous stearoylsphingomyelin dispersions have been investigated by differential scanning calorimetry, fluorescence spectroscopy, and X-ray diffraction. These studies indicate that the thermodynamically most stable gel state of this sphingomyelin is more highly ordered than the corresponding form of other phospholipids. This gel undergoes a transition to a liquid-crystalline conformation at 57 degrees C which is not rapidly reversible. Instead, the liquid-crystalline sphingomyelin supercools below 57 degrees C until a metastable gel is formed at 44 degrees C. The metastable gel slowly converts to the more highly ordered gel phase in an exothermic process at room temperature. The presence of various lipophilic substances enhances the stability of the metastable gel relative to the highly ordered form.

Calorimetry↗
PubMed1979

Thermal behavior of synthetic sphingomyelin-cholesterol dispersions.

The thermotropic behavior of aqueous dispersions of palmitoylsphingomyelin-cholesterol and lignoceryl-sphingomyelin-cholesterol mixtures has been examined by high-sensitivity differential scanning calorimetry. When less than 25 mol % cholesterol is mixed with either sphingomyelin, the calorimetric endotherm is composed of a sharp and a broad component. The sharp-component enthalpy change decreases as the mole percent cholesterol increases with the extrapolated zero enthalpy point being 25 to 30 mol %. With palmitoylsphingomyelin, the temperature of maximum heat capacity of the sharp component decreases monotonically with increasing cholesterol content, while the lignocerylsphingomyelin sharp-component maximum remains constant until more than 20 mol % sterol is present. The broad-component enthalpy change maximizes at 3--4 kcal/mol between 10 and 20 mol % cholesterol and decreases as the ratio of cholesterol is increased or decreased from this range for both sphingomyelins. The results are compared with those from a previous study on dipalmitoylphosphatidylcholine-cholesterol mixtures and are interpreted as evidence for the coexistence of cholesterol-rich and cholesterol-poor phases.

Animals↗
PubMed1979

Thermotropic behavior of monoglucocerebroside--dipalmitoylphosphatidylcholine multilamellar liposomes.

The thermotropic behavior of multilamellar liposomes prepared from mixtures of glucocerebroside and dipalmitoylphosphatidylcholine has been studied by high-sensitivity scanning calorimetry. It is shown that glucocerebroside has a marked effect on the gel--liquid crystalline transition of dipalmitoylphosphatidylcholine. The pretransition seen in pure samples of dipalmitoylphosphatidylcholine is undetectable at small mode fractions of glucocerebrosides (less than 10%). The main transition is shifted to higher temperatures and becomes broader and less cooperative in the presence of glucocerebroside. The enthalpy change of the main transition decreases with increasing the glucocerebroside content. However, this decrease is not linear with the glucocerebroside/phospholipid mole ratio. Glucocerebroside itself does not show a separate transition in the temperature range of these studies (10--75 degree C). The origin of these effects and their dependence on the glucocerebroside content suggest that the in-plane distribution of glucocerebroside molecules is affected by the physical state of the lipid bilayer and by the glucocerebroside/phospholipid mole ratio.

Calorimetry, Differential Scanning↗
PubMed1979

Energy transfer in lipid bilayers.

The quenching of fluorescence due to energy transfer between a dilute, random array of donor and acceptor chromophores in lipid bilayer was measured and compared to theoretical expressions developed to predict the decrease in emission intensity under these circumstances. The observed intensity was found to be the same function of quencher concentration in both planar, multilamellar dispersions and small, spherical vesicles. The degree of quenching was accurately predicted by a simple relation derived in this paper, as well as a more complex equation previously developed by Tweet, et al. The results suggest that significant quenching may be observed even when the average donor-acceptor separation exceeds the Förster critical distance by severalfold. Application of these results to problems of current interest in membrane research are discussed.

Energy Transfer↗
PubMed1978

Transmembrane movement and distribution of cholesterol in the membrane of vesicular stomatitis virus.

The transmembrane movement and distribution of cholesterol in the vesicular stomatitis virus membrane were studied by following the depletion of cholesterol from virions to interacting phospholipid vesicles and by exchange of radiolabeled cholesterol between virions and phospholipid-cholesterol vesicles. The kinetics of the cholesterol exchange or depletion reactions revealed the presence of two exponential rates: a rapid rate, dependent on the vesicle to virus ratio, and a slower rate, independent of the vesicle to virus ratio. The kinetics of cholesterol movement could be best interpreted by a model of the virion membrane considered as a two pool system in which approximately 30% of the cholesterol resides in the outer monolayer and approximately 70% in the inner monolayer. The half-time for equilibration of the two pools was calculated to be 4--6 h and was assumed to represent the time required for transmembrane movement of cholesterol across the bilayer. The initial rate of transfer of cholesterol from virus into vesicles increased when vesicle phospholipids contained more unsaturated and shorter chain fatty acids. Furthermore, the transfer of cholesterol appeared to occur by a collisional mechanism requiring membrane-membrane contact. Interaction with lipid vesicles did not significantly affect the integrity of the virion membrane as assessed by the relative inaccessibility of internal proteins to lactoperoxidase-catalyzed iodination and by the small loss of [3H]amino acid labeled protein from the virus.

Cell Line↗