Lateral phase separations in membrane lipids and the mechanism of sugar transport in Escherichia coli

Salinity and hydraulic retention time induce membrane phospholipid acyl chain remodeling in Halanaerobium congolense WG10 and mixed cultures from hydraulically fractured shale wells

Bacteria remodel their plasma membrane lipidome to maintain key biophysical attributes in response to ecological disturbances. For Halanaerobium and other anaerobic halotolerant taxa that persist in hydraulically fractured deep subsurface shale reservoirs, salinity, and hydraulic retention time (HRT) are important perturbants of cell membrane structure, yet their effects remain poorly understood. Membrane-linked activities underlie in situ microbial growth kinetics and physiologies which drive biogeochemical reactions in engineered subsurface systems. Hence, we used gas chromatography-mass spectrometry (GC-MS) to investigate the effects of salinity and HRT on the phospholipid fatty acid composition of H. congolense WG10 and mixed enrichment cultures from hydraulically fractured shale wells. We also coupled acyl chain remodeling to membrane mechanics by measuring bilayer elasticity using atomic force microscopy (AFM). For these experiments, cultures were grown in a chemostat vessel operated in continuous flow mode under strict anoxia and constant stirring. Our findings show that salinity and HRT induce significant changes in membrane fatty acid chemistry of H. congolense WG10 in distinct and complementary ways. Notably, under nonoptimal salt concentrations (7% and 20% NaCl), H. congolense WG10 elevates the portion of polyunsaturated fatty acids (PUFAs) in its membrane, and this results in an apparent increase in fluidity (homeoviscous adaptation principle) and thickness. Double bond index (DBI) and mean chain length (MCL) were used as proxies for membrane fluidity and thickness, respectively. These results provide new insight into our understanding of how environmental and engineered factors might disrupt the physical and biogeochemical equilibria of fractured shale by inducing physiologically relevant changes in the membrane fatty acid chemistry of persistent microbial taxa. GRAPHICAL ABSTRACTSalinity significantly alters membrane bilayer fluidity and thickness in Halanaerobium congolense WG10.

Sharp, localized phase transitions in single neuronal cells

Phase transitions in materials are accompanied by drastic changes in their properties. Systems abruptly become softer, more conductive, have better heat storage, or support chemical reactions more efficiently. Since changes take place over small variations in external conditions (tension, temperature, pH, calcium), they appear like an on/off switch. Here, we provide experimental evidence that membrane patches of single living cells can go through a reversible phase transition. It is extremely “sharp” (highly nonlinear), and from a thermodynamic point of view we conclude it cannot only be triggered by temperature but also by pH changes (as produced by enzymes). The results strongly support the idea that phase transitions may be a tool for living systems to control their functions even specifically.

The origin of nonlinear responses in cells has been suggested to be crucial for various cell functions including the propagation of the nervous impulse. In physics, nonlinear behavior often originates from phase transitions. Evidence for such transitions on the single-cell level, however, has so far not been provided, leaving the field unattended by the biological community. Here, we demonstrate that single cells of a human neuronal cell line display all optical features of a sharp, highly nonlinear phase transition within their membrane. The transition is reversible and does not originate from protein denaturation. Triggered by temperature and modified by pH here, a thermodynamic approach strongly suggests that similar nonlinear state changes can be induced by other variables such as calcium or mechanical stress. At least in lipid membranes, such state changes are accompanied by significant changes in permeability, enzyme activity, elastic, and electrical properties.

Depth- and temperature-specific fatty acid adaptations in ctenophores from extreme habitats

Animals are known to regulate the composition of their cell membranes to maintain key biophysical properties in response to changes in temperature. For deep-sea marine organisms, high hydrostatic pressure represents an additional, yet much more poorly understood, perturbant of cell membrane structure. Previous studies in fish and marine microbes have reported correlations with temperature and depth of membrane-fluidizing lipid components, such as polyunsaturated fatty acids. Because little has been done to isolate the separate effects of temperature and pressure on the lipid pool, it is still not understood whether these two environmental factors elicit independent or overlapping biochemical adaptive responses. Here, we use the taxonomic and habitat diversity of the phylum Ctenophora to test whether distinct low-temperature and high-pressure signatures can be detected in fatty acid profiles. We measured the fatty acid composition of 105 individual ctenophores, representing 21 species, from deep and shallow Arctic, temperate, and tropical sampling locales (sea surface temperature, -2° to 28°C). In tropical and temperate regions, remotely operated submersibles (ROVs) enabled sampling down to 4000 m. We found that among specimens with body temperatures 7.5°C or colder, depth predicted fatty acid unsaturation levels. In contrast, in the upper 200 m of the water column, temperature predicted fatty acid chain lengths. Taken together, our findings suggest that lipid metabolism may be specialized with respect to multiple physical variables in diverse marine environments. Largely distinct modes of adaptation to depth and cold imply that polar marine invertebrates may not find a ready refugium from climate change in the deep.

A Method for High-Throughput Measurements of Viscosity in Sub-micrometer-Sized Membrane Systems

To unravel the underlying principles of membrane adaptation in small systems like bacterial cells, robust approaches to characterize membrane fluidity are needed. Currently available relevant methods require advanced instrumentation and are not suitable for high-throughput settings needed to elucidate the biochemical pathways involved in adaptation. We developed a fast, robust, and financially accessible quantitative method to measure the microviscosity of lipid membranes in bulk suspension using a commercially available plate reader. Our approach, which is suitable for high-throughput screening, is based on the simultaneous measurements of absorbance and fluorescence emission of a viscosity-sensitive fluorescent dye, 9-(2,2-dicyanovinyl)julolidine (DCVJ), incorporated into a lipid membrane. We validated our method using artificial membranes with various lipid compositions over a range of temperatures and observed values that were in good agreement with previously published results. Using our approach, we were able to detect a lipid phase transition in the ruminant pathogen Mycoplasma mycoides.

Investigation of TEMPO partitioning in different skin models as measured by EPR spectroscopy - Insight into the stratum corneum

Electron paramagnetic resonance (EPR) spectroscopy represents an established tool to study properties of microenvironments, e.g. to investigate the structure and dynamics of biological and artificial membranes. In this study, the partitioning of the spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in ex vivo human abdominal and breast skin, ex vivo porcine abdominal and ear skin as well as normal and inflammatory in vitro skin equivalents was investigated by EPR spectroscopy. Furthermore, the stratum corneum (SC) lipid composition (as determined by high-performance thin-layer chromatography), SC lipid chain order (probed by infrared spectroscopy) and the SC thickness (investigated by histology) were determined in the skin models. X-band EPR measurements have shown that TEMPO partitions in the lipophilic and hydrophilic microenvironment in varying ratios in different ex vivo and in vitro skin models. Ex vivo human abdominal skin exhibited the highest amount of TEMPO in the lipophilic microenvironment. In contrast, the lowest amount of TEMPO in the lipophilic microenvironment was determined in ex vivo human breast skin and the inflammatory in vitro skin equivalents. Individual EPR spectra of epidermis including SC and dermis indicated that the lipophilic microenvironment of TEMPO mainly corresponds to the most lipophilic part of the epidermis, the SC. The amount of TEMPO in the lipophilic microenvironment was independent of the SC lipid composition and the SC lipid chain order but correlated with the SC thickness. In conclusion, EPR spectroscopy could be a novel technique to determine differences in the SC thickness, thus suitably complementing existing methods.

Influence of riboflavin on the oxidation kinetics of unsaturated fatty acids at the air/aqueous interface revealed by sum frequency generation vibrational spectroscopy

Riboflavin, a common nutrient also known as vitamin B2, is known to potentially play important roles in preventing lipid peroxidations. However, the detailed antioxidant mechanisms, especially the influence of riboflavin on lipid oxidations at biological interfaces, have not yet been fully explored. In the current study, the effect of riboflavin molecules on the oxidation kinetics of monounsaturated cis-11-eicosenoic acid (EA) at the air/water interface was systematically investigated using sum frequency generation vibrational spectroscopy (SFG-VS). It was discovered that the oxidation rates of the interfacial EA molecules can be reduced by about two to three times in the presence of riboflavin in the aqueous subphase. Further SFG-VS measurements under the protection of nitrogen purging gas showed that more tightly packed and ordered monolayer structures were formed by the surface adsorption of riboflavin molecules, making the C[double bond, length as m-dash]C bonds less accessible to the gas phase oxidative species. These results suggested that the antioxidant mechanism for riboflavin in the vicinity of biomembranes may not necessarily involve other reducing agents. They also show the great importance of interfacial molecular structures in biologically relevant chemical reactions.

Comparative Experimental and Computational Study of Monoalkyl Chain Phosphatidylcholine-Containing Thermoresponsive Liposomes

Liposomes containing lysophospholipids are intensively studied as drug delivery systems that are stable at normal body temperature but exhibit fast release of their drug load at slightly elevated temperatures. In this study, the stability and release properties of dipalmitoylglycerophosphocholine (DPPC)-based liposomes incorporating the commonly used lysophosphatidylocholine (lyso-PC), and a series of monoalkyl chain ether-linked phosphatidylcholine, i.e., the biologically relevant monoalkyl chain platelet activating factor (PAF) and its derivatives lyso-PAF and methyl-PAF, were investigated. To this end a series of PEGylated small unilamellar liposomes with DPPC:monoalkyl lipid compositions of 5% and 10% molar ratio were prepared and compared with regard to stability (37 °C) and release properties at elevated temperatures (38-43 °C). All systems were characterized with respect to size distribution, ζ-potential, and phase transition characteristics. The presence of ether-lipids endows liposomes with superior (∼10% increase) release properties at 5% incorporation compared to lyso-PC, while at 10% molar ratio the formulations do not differ significantly, the release being close to 90%. The findings are supported by atomistic molecular dynamics simulations that suggest a correlation between the enhanced permeability and increased penetration of water molecules within the bilayers with density fluctuations resulting from the increased area-per-lipid and the disorder of the lysolipids alkyl chains.

Influence of lipids on protein-mediated transmembrane transport

Transmembrane proteins are responsible for transporting ions and small molecules across the hydrophobic region of the cell membrane. We are reviewing the evidence for regulation of these transport processes by interactions with the lipids of the membrane. We focus on ion channels, including potassium channels, mechanosensitive and pentameric ligand gated ion channels, and active transporters, including pumps, sodium or proton driven secondary transporters and ABC transporters. For ion channels it has been convincingly shown that specific lipid-protein interactions can directly affect their function. In some cases, a combined approach of molecular and structural biology together with computer simulations has revealed the molecular mechanisms. There are also many transporters whose activity depends on lipids but understanding of the molecular mechanisms is only beginning.

Proteins and cholesterol lipid vesicles are mediators of drug release from thermosensitive liposomes

Thermosensitive liposomes (TSL) are a promising tool for triggered drug delivery in combination with local hyperthermia. Objective of this study was to investigate the influence of serum on TSL in more detail and to identify serum components which are responsible for increasing drug release. Four different formulations were investigated: DPPC/DSPC/1,2-dipalmitoyl-sn-glycero-3-phosphodiglycerol (DPPG(2)) 50/20/30 (mol/mol) (DPPG(2)-TSL); DPPC/DSPC/DPPG(2)/DSPE-PEG2000 50/15/30/5 (mol/mol) (DPPG(2)/PEG-TSL), DPPC/P-Lyso-PC/DSPE-PEG2000 90/10/4 (mol/mol) (PEG/Lyso-TSL), and DPPC/DSPC/DSPE-PEG2000 80/15/5 (mol/mol) (PEG-TSL). DPPG(2)-TSL was the only formulation which was unaffected by osmotic stress. All formulations tested were influenced by serum components but the susceptibility was depended on the lipid composition of the vesicle. Presence of albumin (HSA) or cholesterol-containing lipid vesicles (DPPC/Chol-LLV) increased the membrane permeability for all tested formulations at temperatures around and above T(m) in a concentration based manner. PEGylation was not able to prevent the observed effect. PEG-TSL and PEG/Lyso-TSL were more susceptible to DPPC/Chol-LLV than DPPG(2)-containing TSL. In contrast, immunoglobulin type G (IgG) affected only anionic formulations. The membrane of DPPG(2)-TSL and DPPG(2)/PEG-TSL was more susceptible toward IgG as compared to HSA. DPPG(2)-TSL and PEG/Lyso-TSL were differentially influenced by fetal calf serum (FCS). As DPPG(2)-TSL was stabilized by pre-incubation with FCS at 37°C, this was the opposite for PEG/Lyso-TSL which were destabilized under these conditions. Individual serum components were unable to mimic the complex situation in full serum. Hence, the use of plasma or serum is still inevitable to investigate stability and release properties of novel TSL formulations until all serum components have been identified that alter TSL integrity.

Adventures in physical chemistry

My research has included chemical physics, electron and NMR spectroscopy, membrane biophysics, and immunology. This research was curiosity driven as well as problem and technique oriented. A theoretical equation was developed for relating nuclear hyperfine splittings to electron spin distributions in free radicals. Another equation was developed to relate NMR spectra to chemical reaction rates. Early evidence for the liquid-like properties of cell membranes was obtained through the use of paramagnetic probes (spin labels). Spin labels were used in measurements of lateral as well as transverse diffusion of phospholipids in bilayer membranes. Liquid-liquid phase separations were discovered in monolayer membranes containing phospholipids and cholesterol. In the area of immunology, it was shown that antigenic peptides bind to reconstituted class II MHC molecules in membranes and trigger specific T-helper cells.

Lipid bilayer regulation of membrane protein function: gramicidin channels as molecular force probes

Membrane protein function is regulated by the host lipid bilayer composition. This regulation may depend on specific chemical interactions between proteins and individual molecules in the bilayer, as well as on non-specific interactions between proteins and the bilayer behaving as a physical entity with collective physical properties (e.g. thickness, intrinsic monolayer curvature or elastic moduli). Studies in physico-chemical model systems have demonstrated that changes in bilayer physical properties can regulate membrane protein function by altering the energetic cost of the bilayer deformation associated with a protein conformational change. This type of regulation is well characterized, and its mechanistic elucidation is an interdisciplinary field bordering on physics, chemistry and biology. Changes in lipid composition that alter bilayer physical properties (including cholesterol, polyunsaturated fatty acids, other lipid metabolites and amphiphiles) regulate a wide range of membrane proteins in a seemingly non-specific manner. The commonality of the changes in protein function suggests an underlying physical mechanism, and recent studies show that at least some of the changes are caused by altered bilayer physical properties. This advance is because of the introduction of new tools for studying lipid bilayer regulation of protein function. The present review provides an introduction to the regulation of membrane protein function by the bilayer physical properties. We further describe the use of gramicidin channels as molecular force probes for studying this mechanism, with a unique ability to discriminate between consequences of changes in monolayer curvature and bilayer elastic moduli.

Cholesterol depletion induces solid-like regions in the plasma membrane

Glycosylphosphatidylinositol-linked and transmembrane major histocompatibility complex (MHC) class II I-E(k) proteins, as well as N-(6-tetramethylrhodaminethiocarbamoyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (Tritc-DHPE), are used as probes to determine the effect of cholesterol concentration on the organization of the plasma membrane at temperatures in the range 22 degrees C-42 degrees C. Cholesterol depletion caused a decrease in the diffusion coefficients for the MHC II proteins and also for a slow fraction of the Tritc-DHPE population. At 37 degrees C, reduction of the total cell cholesterol concentration results in a smaller suppression of the translational diffusion for I-E(k) proteins (twofold) than was observed in earlier work at 22 degrees C (five sevenfold) Vrljic, M., S. Y. Nishimura, W. E. Moerner, and H. M. McConnell. 2005. Biophys. J. 88:334-347. At 37 degrees C, the diffusion of both I-E(k) proteins is Brownian (0.9 < alpha-parameter < 1.1). More than 99% of the protein population diffuses homogeneously when imaged at 65 frames per s. As the temperature is raised from 22 degrees C to 42 degrees C, a change in activation energy is seen at approximately 35 degrees C in the Arrhenius plots. Cytoskeletal effects appear to be minimal. These results are consistent with a previously described model of solid-like domain formation in the plasma membrane.

Molecular dynamics and interactions for creation of stimulation-induced stabilized rafts from small unstable steady-state rafts

We have evaluated the sizes and lifetimes of rafts in the plasma membrane from the existing literature, with a special attention paid to their intrinsically broad distributions and the limited time and space scales that are covered by the observation methods used for these studies. Distinguishing the rafts in the steady state (reserve rafts) from those after stimulation or unintentional crosslinking of raft molecules (stabilized receptor-cluster rafts) is critically important. In resting cells, the rafts appear small and unstable, and the consensus now is that their sizes are smaller than the optical diffraction limit (250 nm). Upon stimulation, the raft-preferring receptors are clustered, inducing larger, stabilized rafts, probably by coalescing small, unstable rafts or cholesterol-glycosphingolipid complexes in the receptor clusters. This receptor-cluster-induced conversion of raft types may be caused by suppression of alkyl chain isomerization and the lipid lateral diffusion in the cluster, with the aid of exclusion of cholesterol from the bulk domain and the boundary region of the majority of transmembrane proteins. We critically inspected the possible analogy to the boundary lipid concept. Finally, we propose a hypothesis for the coupling of GPI-anchored receptor signals with lipid-anchored signaling molecules in the inner-leaflet raft.

Phosphatidylethanolamine and phosphatidylglycerol are segregated into different domains in bacterial membrane. A study with pyrene-labelled phospholipids

To detect and characterize membrane domains that have been proposed to exist in bacteria, two kinds of pyrene-labelled phospholipids, 2-pyrene-decanoyl-phosphatidylethanolamine (PY-PE) and 2-pyrene-decanoyl-phosphatidylglycerol (PY-PG) were inserted into Escherichia coli or Bacillus subtilis membrane. The excimerization rate coefficient, calculated from the excimer-to-monomer ratio dependencies on the probe concentration, was two times higher for PY-PE than for PY-PG at 37 degrees C. This was ascribed to different local concentrations rather than to differences in mobility. The extent of mixing between the two fluorescent phospholipids, estimated by formation of their heteroexcimer, was found very low both in E. coli and B. subtilis, in contrast to model membranes. In addition, these two pyrene derivatives exhibited different temperature phase transitions and different detergent extractability, indicating that the surroundings of these phospholipids in bacterial membrane differ in organization and order. Inhibition of protein synthesis, leading to condensation of nucleoid and presumably to dissipation of membrane domains, indeed resulted in increased formation of heteroexcimers, broadening of phase transitions and equal detergent extractability of both probes. It is proposed that in bacterial membranes these phospholipids are segregated into distinct domains that differ in composition, proteo-lipid interaction and degree of order; the proteo-lipid domain being enriched by PE.

Coexistence of domains with distinct order and polarity in fluid bacterial membranes

In this study we sought the detection and characterization of bacterial membrane domains. Fluorescence generalized polarization (GP) spectra of laurdan-labeled Escherichia coli and temperature dependencies of both laurdan's GP and fluorescence anisotropy of 1,3-diphenyl-1,3,5-hexatriene (DPH) (rDPH) affirmed that at physiological temperatures, the E. coli membrane is in a liquid-crystalline phase. However, the strong excitation wavelength dependence of rlaurdan at 37 degrees C reflects membrane heterogeneity. Time-resolved fluorescence emission spectra, which display distinct biphasic redshift kinetics, verified the coexistence of two subpopulations of laurdan. In the initial phase, <50 ps, the redshift in the spectral mass center is much faster for laurdan excited at the blue edge (350 nm), whereas at longer time intervals, similar kinetics is observed upon excitation at either blue or red edge (400 nm). Excitation in the blue region selects laurdan molecules presumably located in a lipid domain in which fast intramolecular relaxation and low anisotropy characterize laurdan's emission. In the proteo-lipid domain, laurdan motion and conformation are restricted as exhibited by a slower relaxation rate, higher anisotropy and a lower GP value. Triple-Gaussian decomposition of laurdan emission spectra showed a sharp phase transition in the temperature dependence of individual components when excited in the blue but not in the red region. At least two kinds of domains of distinct polarity and order are suggested to coexist in the liquid-crystalline bacterial membrane: a lipid-enriched and a proteolipid domain. In bacteria with chloramphenicol (Cam)-inhibited protein synthesis, laurdan showed reduced polarity and restoration of an isoemissive point in the temperature-dependent spectra. These results suggest a decrease in membrane heterogeneity caused by Cam-induced domain dissipation.

Transport physiology of the urinary bladder in teleosts: a suitable model for renal urea handling?

The transport physiology of the urinary bladder of both the freshwater rainbow trout (Oncorhychus mykiss) and the marine gulf toadfish (Opsanus beta) was characterized with respect to urea, and the suitability of the urinary bladder as a model for renal urea handling was investigated. Through the use of the in vitro urinary bladder sac preparation urea handling was characterized under control conditions and in the presence of pharmacological agents traditionally used to characterize urea transport such as urea analogues (thiourea, acetamide), urea transport blockers (phloretin, amiloride), and hormonal stimulation (arginine vasotocin; AVT). Na(+)-dependence and temperature sensitivity were also investigated. Under control conditions, the in vitro trout bladder behaved as in vivo, demonstrating significant net reabsorption of Na(+), Cl(-), water, glucose, and urea. Bladder urea reabsorption was not affected by pharmacological agents and, in contrast to renal urea reabsorption, was not correlated to Na(+). However, the trout bladder showed a threefold greater urea permeability compared to artificial lipid bilayers, a prolonged phase transition with a lowered E(a) between 5 degrees C and 14 degrees C, and differential handling of urea and analogues, all suggesting the presence of a urea transport mechanism. The in vitro toadfish bladder did not behave as in vivo, showing significant net reabsorption of Na(+) but not of Cl(-), urea, or water. As in the trout bladder, pharmacological agents were ineffective. The toadfish bladder showed no differential transport of urea and analogues, consistent with a low permeability storage organ and intermittent urination. Our results, therefore, suggest the possibility of a urea transport mechanism in the urinary bladder of the rainbow trout but not the gulf toadfish. While the bladders may not be suitable models for renal urea handling, the habit of intermittent urination by ureotelic tetrapods and toadfish seems to have selected for a low permeability storage function in the urinary bladder.

On the regulatory role of dipeptidyl peptidase IV (=CD=adenosine deaminase complexing protein) on adenosine deaminase activity

The molecular mechanism controlling the variable activity of the malignancy marker adenosine deaminase (ADA) is enigmatic. ADA activity was found to be modulated by the membrane-bound adenosine deaminase complexing protein (CP=DPPIV=CD26). The role of lipid-protein interactions in this modulation was sought. While direct solubilization of ADA in vesicles resulted in loss of ADA activity, the binding of ADA to CP reconstituted in vesicles restored the specific activity. The activity of ADA, free or bound to CP in solution, resulted in continuous linear Arrhenius plots. However, ADA bound to reconstituted CP exhibited two breaks associated with approximately 30% increased activity, at 25 and 13 degrees C, yielding three lines with similar apparent activation energies (E(a)). Continuum solvent model calculations of the free energy of transfer of the transmembrane helix of CP from the aqueous phase into membranes of various widths show that the most favorable orientations of the helix above and below the main phase transition may be different. We suggest that the 20% change in the thickness of the bilayer below and above the main phase transition may modify the orientation of CP in the membrane, thereby affecting substrate accessibility of ADA. This could account for ADA's reduced activity associated with increased membrane fluidity in transformed vs. normal fibroblasts.

Bioenergetic responses of Synechocystis 6803 fatty acid desaturase mutants at low temperatures

Fatty acid composition of the membrane lipids in the mesophilic cyanobacterium Synechocystis sp. PCC 6803 was altered in earlier work by targeted mutagenesis of genes for fatty acid desaturases. In this work, cells of several mutant strains, depleted in the unsaturated fatty acids in membrane lipids, were grown at 34 degrees C. Spheroplasts (permeabilized cells) were prepared by lysozyme digestion of the cell wall followed by gentle osmotic shock. The bioenergetic parameters ATP formation, electron transport, and H+ uptake were measured at various temperatures. All three bioenergetic parameters for spheroplasts from wild-type cells (which had abundant polyunsaturated fatty acids) were active down to the lowest temperatures used (1 degrees - 2 degrees C). In two strains, which lacked the capacity to desaturate fatty acids at the A 12 position and at the A 12 and A6 positions (designated as desA- and desA-/desD-, respectively), the spheroplasts lost the capacity to form ATP (measured as phenazine methosulfate cyclic phosphorylation) at about 5 degrees C but retained electron transport (water oxidation-dependent ferricyanide reduction) and H+ uptake linked to phenazine methosulfate cyclic electron transport. It appears that the absence of the unsaturation of fatty acids in the A 12 and A6 positions blocks the ability of the photosynthetic membranes to couple a bioenergetically competent proton-motive force to the ATP formation mechanism at temperatures below 5 degrees C. It remains to be determined whether the loss of ATP formation in the mutant strains is the failure of available protons to properly flow into the CF0CF1-ATP synthase or a failure in the CF1 part of the complex in coupling the dissipative H+ flow to the enzyme mechanism of the synthase.

Interaction of Gd(III) MRI contrast agents with membranes: a review of recent EPR studies

Rational development of new selective paramagnetic contrast agents (PCAs) requires a detailed understanding of their interactions with biological macromolecules. This report shows how some of these interactions can be studied with electron paramagnetic resonance (EPR) through examples of Gd3+ complexes interactions with model phospholipid membranes. It is shown that the spin label EPR method can be used to detect: (i) presence and possible location of lipophilic contrast agents in the model membranes, (ii) changes and distortions in membrane organization upon interaction with the PCAs, and (iii) changes in the local polarity of the bilayer and its phase behavior due to addition of Gd3+ complexes. This work demonstrates that interaction of Gd3+ complexes with phospholipid bilayers can be observed directly from changes in their continuous wave (CW) EPR spectra obtained at frequencies higher than X-band (9.5 GHz), where signals arising from aqueous and lipid-bound Gd3+ complexes become resolved. Analysis of frequency dependence of the effective g-factors of the EPR signal provides estimates of zero-field splitting (ZFS) parameter for these complexes at physiological conditions and information on how this parameter is affected by interaction with lipids. Multifrequency EPR experiments at high magnetic fields are also useful in providing data on the frequency dispersion of electronic relaxation caused mainly by a modulation of the electron-electron dipolar interaction (ZFS) of these high spin ions.

Hydrophobic properties of NAD glycohydrolase from neurospora crassa conidia and interaction with dioxane

NAD glycohydrolase (NADase, EC 3.2.2.5) from Neurospora crassa conidia shows marked hydrophobic properties which are related to the self inhibition of the enzyme. Both aliphatic amines and carboxylic acids are able to inhibit noncompetitively the catalytic activity of the enzyme and the inhibition depends on the non-polar moiety of the substances. Also dioxane is an inhibitor of NAD glycohydrolase even though it apparently increases the specific activity of the enzyme. This effect can be explained by the fact that NADase is present as a dimer when the enzyme is concentrated or at high temperature, and dioxane binds the enzyme breaking the hydrophobic bonds in the dimeric enzyme and yielding the most active monomeric form which is only slightly inhibited by the organic solvent.

Temperature-dependent block of capacitative Ca2+ influx in the human leukemic cell line KU-812

The mechanism by which depletion of intracellular Ca2+ stores activates Ca2+ influx is not understood. We recently showed that primaquine, an inhibitor of vesicular transport, blocks the activation of the calcium release-activated calcium current (ICRAC) in rat megakaryocytes (Somasundaram, B., Norman, J. C., and Mahaut-Smith, M. P. (1995) Biochem. J. 309, 725-729). Since it is well established that vesicular transport is temperature-sensitive, we have investigated the effect of temperature on both the activation and maintenance of store-mediated Ca2+ and Mn2+ influx in the human leukemic cell line KU-812 using a combination of whole cell ICRAC recordings and measurements of Mn2+ photoquench of fura-2. Activation of ICRAC was temperature-sensitive, showing a nonlinear reduction when the temperature was lowered from 27 to 17 degrees C with an abrupt change at 21-22 degrees C and complete inhibition at 17 degrees C. Once activated, ICRAC also displayed an abrupt reduction at 21-22 degrees C but was not completely blocked even when the temperature was reduced to 14 degrees C, suggesting that at least one of the temperature-sensitive components is exclusively involved in ICRAC activation. Activation of store-mediated Mn2+ influx also showed similar nonlinear temperature sensitivity and complete inhibition at 19 degrees C. However, in contrast to ICRAC measurements, lowering the temperature following maximal activation of the influx pathway at 37 degrees C did not result in any detectable residual Mn2+ entry below 19 degrees C. We conclude that the mechanism of store-mediated Ca2+ influx involves temperature-dependent steps in both its maintenance and activation, suggesting dependence on a lipid membrane environment.

Inhibitory effect of NaCl on hog kidney mitochondrial membrane-bound monoamine oxidase: pH and temperature dependences

For a further understanding of the inhibitory effect of NaCl on hog kidney mitochondrial monoamine oxidase (MAO), the activity for benzylamine as substrate was assayed spectrophotometrically in the absence and presence of NaCl for mitochondrial outer membrane preparations as well as whole mitochondria. The effect of CaCl2 was also examined for comparison. The inhibition by NaCl but not CaCl2 was strongly pH dependent. The pH dependence of the inhibitory effect of NaCl in phosphate buffer was parallel to the pH dependence of the MAO activity itself. The point at which the slope of the Arrhenius plot in the absence of NaCl decreases with increasing temperature was to be 32.3 degrees C at pH 7.0 and 30.4 degrees C at pH 7.5 in phosphate buffer, while the Arrhenius plot in the presence of NaCl exhibited discontinuities without change in the slope in small temperature ranges, 39.2 degrees C-40.0 degrees C and 33.0 degrees C-34.2 degrees C. It was estimated that the inhibitory effect of NaCl was due to a pH and temperature sensitive cooperative state change involving MAO protein and boundary lipids, while the effect of CaCl2 could be induced by specific Ca2+ binding to acidic phospholipids.

Permeability of dimyristoyl phosphatidylcholine/dipalmitoyl phosphatidylcholine bilayer membranes with coexisting gel and liquid-crystalline phases

The passive permeation of glucose and a small zwitterionic molecule, methyl-phosphoethanolamine, across two-component phospholipid bilayers (dimyristoyl phosphatidylcholine (DMPC)/dipalmitoyl phosphatidylcholine (DPPC) mixtures) exhibit a maximum when gel domains and fluid domains coexist. The permeability data of the two-phase bilayers cannot be fitted to single-rate kinetics, but are consistent with a Gaussian distribution of rate constants. In pure DMPC and DPPC as well as in their mixtures, at the temperature of the maximum excess heat capacity, the logarithm of the average permeability rate constants are linearly correlated with the mole fraction of DPPC in the total system. In addition, in the 50:50 mixture, the excess heat capacity values as well as the apparent fractions of interfacial lipid correlate with the logarithm of the excess permeabilities in the two-phase region. These results suggest that small polar molecules can cross the membrane at the interface between gel and fluid domains at a much faster rate than through the homogeneous phases; the acyl chains located at the domain interface experience lateral density fluctuations that are inversely proportional to their average length, and large enough to allow rapid transmembrane diffusion of the solute molecules. The distribution of the permeability rate constants may reflect temporal and spatial fluctuations of the lipid composition at the phase boundaries.

Very high frequency electron paramagnetic resonance of 2,2,6,6-tetramethyl-1-piperidinyloxy in 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine liposomes: partitioning and molecular dynamics

Partitioning and molecular dynamics of 2,2,6,6,-tetramethylpiperedine-1-oxyl (TEMPO) nitroxide radicals in large unilamellar liposomes (LUV) composed from 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine were investigated by using very high frequency electron paramagnetic resonance (EPR) spectroscopy. Experiments carried out at a microwave frequency of 94.3 GHz completely resolved the TEMPO EPR spectrum in the aqueous and hydrocarbon phases. An accurate computer simulation method combined with Levenberg-Marquardt optimization was used to analyze the TEMPO EPR spectra in both phases. Spectral parameters extracted from the simulations gave the actual partitioning of the TEMPO probe between the LUV hydrocarbon and aqueous phases and allowed analysis of picosecond rotational dynamics of the probe in the LUV hydrocarbon phase. In very high frequency EPR experiments, phase transitions in the LUV-TEMPO system were observed as sharp changes in both partitioning and rotational correlation times of the TEMPO probe. The phase transition temperatures (40.5 +/- 0.2 and 32.7 +/- 0.5 degrees C) are in agreement with previously reported differential scanning microcalorimetry data. Spectral line widths were analyzed by using existing theoretical expressions for motionally narrowed nitroxide spectra. It was found that the motion of the small, nearly spherical, TEMPO probe can be well described by anisotropic Brownian diffusion in isotropic media and is not restricted by the much larger hydrocarbon chains existing in ripple structure (P beta') or fluid bilayer structure (L alpha) phases.

Effect of electric field gradients on lipid monolayer membranes

Externally applied nonuniform electric fields can strongly affect thermodynamic phases in a lipid monolayer when applied under conditions of temperature, pressure, and composition that are near phase boundaries. Under such conditions nonuniform applied fields can produce or suppress phase separations. Field-induced phase-separated domains have sizes that are in good agreement with calculations. Field gradients can also produce large concentration gradients in binary mixtures just above their critical points. The present work elaborates our earlier studies of these field effects using thermodynamic models of the phase behavior of two-component liquid mixtures. The calculations are of interest in connection with biological membranes that, at the growth temperature, are in a liquid state close to a phase boundary.

Effects of growth with ethanol on fermentation and membrane fluidity of Saccharomyces cerevisiae

Saccharomyces cerevisiae HSc was grown with ethanol at concentrations up to 10% (v/v). The immediate effects of additions of externally added ethanol on CO2 production and O2 consumption of washed organisms were studied by stopped-flow membrane inlet quadrupole mass spectrometry. Fermentative activities of organisms grown with ethanol (0-5% v/v) showed similar sensitivities to inhibition by ethanol, whereas those grown with 10% (v/v) ethanol had become protected and were markedly less sensitive. The fluidity of subcellular membrane fractions was measured by determination of the temperature dependence of the rotational order parameter of the spin label 5-doxyl stearic acid (free radical) by electron spin resonance. Mitochondria prepared from yeasts grown with 0, 7, and 9% (v/v) ethanol showed similar overall fluidity, although differences in temperature-dependent behaviour indicate altered lipid composition or lateral phase separations. On the other hand the microsomal fraction from organisms grown with 9% ethanol showed a remarkable increase in fluidity. These data suggest that the protective effects of growth with ethanol near the limit of tolerance on fermentative activities may arise from altered plasma membrane fluidity properties.

Correlation between modification of membrane phospholipids and some biological activity of lymphocytes, neutrophils and macrophages

Our study considered the possibility of modifying the functional response of human neutrophils, of mouse lymphocytes and macrophages treated with phospholipids having different polar groups, different isomerisms with saturated and unsaturated fatty acids from C12 to C20 carbon atoms. The results are as follows. a) Most of the phospholipids containing fatty acids from C12 to C20 cause inhibition of the blastogenic capacity of the polyclonal activators tested. b) The phospholipids tested cause a decrease in adherence of polymorphonuclear leukocytes with the exception of the phosphatidyl-choline containing saturated and unsaturated fatty acids. c) A decrease in polymorphonuclear leukocytes migrational capacity almost always occurs. d) The cells treated with L-phosphatidyl-ethanolamine having fatty acids from C14 to C17 show an increase in chemiluminescence; those treated with phosphatidyl-choline and L-phosphatidyl-glycerol show a decrease of the chemiluminescence; L-phosphatidic acid and L-phosphatidyl-ethanolamine having Microbial fatty acids (FAs) at C16 cause a decrease in the formation of phagolisosomes in the macrophages tested.

Evidence that lipid lateral phase separation induces functionally significant structural changes in the Ca+2ATPase of the sarcoplasmic reticulum

We have studied lipid lateral phase separation (LPS) in the intact sarcoplasmic reticulum (SR) membrane and in bilayers of isolated SR membrane lipids as a function of temperature, [Mg+2], and degree of hydration. Lipid LPS was observed in both the intact membrane and in the bilayers of isolated SR lipids, and the LPS behavior of both systems was found to be qualitatively similar. Namely, lipid LPS occurs only at relatively low temperature and water content, independently of the [Mg+2], and the upper characteristic temperature (th) for lipid LPS for both the membrane and bilayers of its isolated lipids coincide to within a few degrees. However, at similar temperatures, isolated lipids show more LPS than the lipids in the intact membrane. Lipid LPS in the intact membrane and in bilayers of the isolated lipids is fully reversible, and more extensive for samples partially dehydrated at temperatures below th. Our previous x-ray diffraction studies established the existence of a temperature-induced transition in the profile structure of the sarcoplasmic reticulum Ca+2ATPase which occurs at a temperature corresponding to the [Mg+2]-dependent upper characteristic temperature for lipid LPS in the SR membrane. Furthermore, the functionality of the ATPase, and in particular the lifetime of the first phosphorylated enzyme conformation (E1 approximately P) in the Ca+2 transport cycle, were also found to be linked to the occurrence of this structural transition. The hysterisis observed in lipid LPS behavior as a function of temperature and water content provides a possible explanation for the more efficient transient trapping of the enzyme in the E1 approximately P conformation observed in SR membranes partially dehydrated at temperatures below th. The observation that LPS behavior for the intact SR membrane and bilayers of isolated SR lipids (no protein present) are qualitatively similar strongly suggests that the LPS behavior of the SR membrane lipids is responsible for the observed structural change in the Ca+2ATPase and the resulting significant increase in E1 approximately P lifetime for temperatures below th.

Internal dynamics of lactose permease

The transport protein lactose permease was reconstituted in vesicles of dimyristoylphosphatidylcholine, and the internal dynamics were studied by measuring the fluorescence anisotropy decay of the tryptophan residues and of a covalently bound pyrene label. For the tryptophans three relaxation processes and for the pyrene two relaxation processes with relaxation times in the nanosecond range were observed. The slowest process, of approximately 50 ns, is assigned to orientational fluctuations of membrane-spanning helices. When the temperature is decreased below the lipid-phase transition, this relaxation process is slowed down and restricted in amplitude. Because the transport rate is known to also decrease below the phase transition, this observation suggests a coupling between internal dynamics and transport. This coupling is analyzed on the basis of the Kramers relation for chemical reactions.

Spontaneous vesiculation of large multilamellar vesicles composed of saturated phosphatidylcholine and phosphatidylglycerol mixtures

The influence of temperature and ionic strength on the vesiculation properties of large multilamellar vesicles containing various proportions of dimyristoylphosphatidylglycerol has been investigated. It is shown that at low ionic strengths preformed large multilamellar vesicles composed of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol (7:3) on incubation at the gel to liquid-crystalline transition temperature (Tc approximately 23 degrees C) spontaneously vesiculate to form predominantly unilamellar systems with a mean diameter of 120 nm. Such vesiculation is not observed for incubations at temperatures appreciably above or below Tc, and is also inhibited by higher ionic strengths. Stable large multilamellar vesicles are formed, however, in systems containing the dioleoyl species of phosphatidylcholine or phosphatidylglycerol and also for dimyristoylphosphatidylcholine/dimyristoylphosphatidylserine mixtures. The vesiculation properties of dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol mixtures, therefore, appear to reflect an instability in the region of the Tc driven by surface potential effects which are specific for the glycerol headgroup.

Ca2+-induced lateral phase separation in phosphatidic acid/phosphatidylcholine monolayers as revealed by fluorescence microscopy

Phase separation in mixed monolayers of phosphatidylcholine (PC) and pyrene-labeled phosphatidic acid (PA) was observed by fluorescence microscopy on an air/water interface as a function of subphase Ca2+ concentration and lateral packing pressure of the film. Below 45 mN m-1 and in the absence of Ca2+ no indications of phase immiscibility were observed. Addition of 1 mM Ca2+ caused extensive phase separation, which was evident immediately after spreading of the film. Further increase in Ca2+ concentration up to 30 mM increased the pyrene excimer intensity of the separated phosphatidic acid enriched domains. In the presence of Ca2+ (1-30 mM) and at surface pressures below 10 mN m-1 phase separation was always evident. However, as surface pressure exceeded 10 mN m-1, mixing of PC and PA occurred. Upon decompression of the film, phase separation reappeared at surface pressures close to 10 mN m-1. The surface textures of the film before and after the compression and subsequent relaxation were different. Inclusion of 30 mol% cholesterol increased the number and decreased the size of the PA domains. In films containing 50 mol% cholesterol no phase separation could be detected at the resolution available.