The dynamics of membrane structure

Characterization of multiple lysophosphatidic acid acyltransferases in the plant pathogen Xanthomonas campestris

Phosphatidic acid (PA) is the precursor of most phospholipids like phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin. In bacteria, its biosynthesis begins with the acylation of glycerol-3-phosphate to lysophosphatidic acid (LPA), which is further acylated to PA by the PlsC enzyme. Some bacteria, like the plant pathogen Xanthomonas campestris, use a similar pathway to acylate lysophosphatidylcholine to phosphatidylcholine (PC). Previous studies assigned two acyltransferases to PC formation. Here, we set out to study their activity and found a second much more prominent function of these enzymes in LPA to PA conversion. This PlsC-like activity was supported by the functional complementation of a temperature-sensitive plsC-deficient Escherichia coli strain. Biocomputational analysis revealed two further PlsC homologs in X. campestris. The cellular levels of the four PlsC-like proteins varied with respect to growth phase and growth temperature. To address the question whether these enzymes have redundant or specific functions, we purified two recombinant, detergent-solubilized enzymes in their active form, which enabled the first direct biochemical comparison of PlsC isoenzymes from the same organism. Overlapping but not identical acyl acceptor and acyl donor preferences suggest redundant and specialized functions of the X. campestris PlsC enzymes. The altered fatty acid composition in plsC mutant strains further supports the functional differentiation of these enzymes.

Free Energy Cost of Interdigitation of Lamellar Bilayers of Fatty Alcohols with Cationic Surfactants from Molecular Dynamics Simulations

Cationic surfactant mixed with fatty alcohol as cosurfactant in excess water can form stable emulsions, known as "lamellar gel networks," that contain extended and interconnected networks of swollen bilayers, including ones with in-plane liquidlike disorder (L_α phase) and solidlike order (L_β phase). To study their structure and thermodynamics, molecular dynamics (MD) simulations with lateral pressure and temperature scans along reversible pathways were used to drive reversible phase changes, including formation at negative lateral pressure of the L_βI phase with interdigitated tails of opposing leaflets. Thermodynamic integration, with extrapolations to infinitely slow scans, yielded a free energy difference between the interdigitated L_βI and non-interdigitated L_β phases of 2.4 ± 0.5 kJ/mol, which is consistent with the spontaneous formation of the L_β phase under atmospheric pressure in simulation. Thermodynamic cycles involving temperature and lateral pressure for which the free energy difference is identically zero were constructed as negative controls to verify the method. Using lateral pressure, including negative lateral pressure, helps avoid kinetic bottlenecks that occur when temperature alone is used as the control variable. The method, using negative lateral pressure, should be widely applicable to other bilayers to identify molecular properties that control interdigitation and other bilayer properties.

The Implications for Cells of the Lipid Switches Driven by Protein-Membrane Interactions and the Development of Membrane Lipid Therapy

The cell membrane contains a variety of receptors that interact with signaling molecules. However, agonist-receptor interactions not always activate a signaling cascade. Amphitropic membrane proteins are required for signal propagation upon ligand-induced receptor activation. These proteins localize to the plasma membrane or internal compartments; however, they are only activated by ligand-receptor complexes when both come into physical contact in membranes. These interactions enable signal propagation. Thus, signals may not propagate into the cell if peripheral proteins do not co-localize with receptors even in the presence of messengers. As the translocation of an amphitropic protein greatly depends on the membrane's lipid composition, regulation of the lipid bilayer emerges as a novel therapeutic strategy. Some of the signals controlled by proteins non-permanently bound to membranes produce dramatic changes in the cell's physiology. Indeed, changes in membrane lipids induce translocation of dozens of peripheral signaling proteins from or to the plasma membrane, which controls how cells behave. We called these changes "lipid switches", as they alter the cell's status (e.g., proliferation, differentiation, death, etc.) in response to the modulation of membrane lipids. Indeed, this discovery enables therapeutic interventions that modify the bilayer's lipids, an approach known as membrane-lipid therapy (MLT) or melitherapy.

Laurdan monitors different lipids content in eukaryotic membrane during embryonic neural development

We describe a method based on fluorescence-lifetime imaging microscopy (FLIM) to assess the fluidity of various membranes in neuronal cells at different stages of development [day 12 (E12) and day 16 (E16) of gestation]. For the FLIM measurements, we use the Laurdan probe which is commonly used to assess membrane water penetration in model and in biological membranes using spectral information. Using the FLIM approach, we build a fluidity scale based on calibration with model systems of different lipid compositions. In neuronal cells, we found a marked difference in fluidity between the internal membranes and the plasma membrane, being the plasma membrane the less fluid. However, we found no significant differences between the two cell groups, E12 and E16. Comparison with NIH3T3 cells shows that the plasma membranes of E12 and E16 cells are significantly more fluid than the plasma membrane of the cancer cells.

Recent Progress in Advanced Nanobiological Materials for Energy and Environmental Applications

In this review, we briefly introduce our efforts to reconstruct cellular life processes by mimicking natural systems and the applications of these systems to energy and environmental problems. Functional units of in vitro cellular life processes are based on the fabrication of artificial organelles using protein-incorporated polymersomes and the creation of bioreactors. This concept of an artificial organelle originates from the first synthesis of poly(siloxane)-poly(alkyloxazoline) block copolymers three decades ago and the first demonstration of protein activity in the polymer membrane a decade ago. The increased value of biomimetic polymers results from many research efforts to find new applications such as functionally active membranes and a biochemical-producing polymersome. At the same time, foam research has advanced to the point that biomolecules can be efficiently produced in the aqueous channels of foam. Ongoing research includes replication of complex biological processes, such as an artificial Calvin cycle for application in biofuel and specialty chemical production, and carbon dioxide sequestration. We believe that the development of optimally designed biomimetic polymers and stable/biocompatible bioreactors would contribute to the realization of the benefits of biomimetic systems. Thus, this paper seeks to review previous research efforts, examine current knowledge/key technical parameters, and identify technical challenges ahead.

Luminescence spectroscopy of singlet oxygen enables monitoring of oxygen consumption in biological systems consisting of fatty acids

The interaction of singlet oxygen ((1)O2) generated in a photosensitized process with well-known reference photosensitizers Perinaphthenone (PN) and TMPyP is investigated in a model system consisting of fatty acids and the respective exogenous photosensitizer (PS) in solution by direct detection of the luminescence photons of (1)O2 at 1270 nm. Such a model system is a first approach to mimic the complex environment of (1)O2 in a biological cell which consists mainly of water, proteins, sugars and lipids. Firstly, the important issue of oxygen consumption is evaluated which has to be considered during luminescence detection of (1)O2. It is known that the luminescence signal of (1)O2 is dependent on the oxygen concentration of the environment. Cellular components such as lipids represent oxygen consumers due to peroxidation of their unsaturated double bonds. Secondly, the experimental conditions for this model system regarding oxygen consumption are optimized to estimate the rates and rate constants of the coupled system. Thirdly, the triplet decay of the PS can provide more precise information about the actual oxygen concentration close to the PS and can be used, therefore, as a more precise method to determine the oxygen concentration in more complex systems such as a biological cell. The aim is to get a better understanding of photosensitized reactions of (1)O2 with cellular components to further improve methodologies, in particular at a cellular level using luminescence spectroscopy. In conclusion, luminescence detection might be a helpful tool to monitor precisely and promptly changes in oxygen concentration in a complex environment.

Introduction to membrane proteins

Membrane proteins (MPs) mediate important physiological processes for the cell via extracellular and intracellular interactions. To better understand the biochemical and structural bases of these interactions, well-characterized preparations of purified MPs are required. This introduction reviews common problems encountered in MP preparation.

Lateral diffusion of phospholipids in the plasma membrane of soybean protoplasts: Evidence for membrane lipid domains

Fluorescent lipid and phospholipid probes were incorporated at 4 degrees C into soybean protoplasts prepared from cultured soybean (SB-1) cells. Fluorescence microscopy showed that the plasma membrane as well as the nucleus were labeled. Fluorescence redistribution after photobleaching (FRAP) analysis was performed on these cells at 18 degrees C to monitor the lateral mobility of the incorporated probes. After labeling at low concentrations (40 mug/ml) of phosphatidyl-N-(4-nitrobenzo-2-oxa-1,3-diazolyl)ethanolamine (NBD-PtdEtn), a single mobile component was observed with a diffusion coefficient (D) of approximately 3 x 10(-9) cm(2)/sec. After labeling at higher probe concentrations (>/=100 mug/ml), two diffusing species were observed, with diffusion coefficients of approximately 3 x 10(-9) cm(2)/sec ("fast") and approximately 5 x 10(-10) cm(2)/sec ("slow"). Similar results were observed with fluorescent derivatives of phosphatidylcholine and fatty acids. In contrast to these results, parallel analysis of 3T3 fibroblasts, using the same probes and conditions, yielded only a single diffusion component. These results suggest that the soybean plasma membrane may contain two distinct lipid domains in terms of lipid mobility. Consistent with this idea, experiments with soybean protoplasts yielded a single diffusion component under the following conditions: (i) labeling with NBD-PtdEtn (100 mug/ml), FRAP analysis at 37 degrees C (D = 1.1 x 10(-8) cm(2)/sec); (ii) labeling with NBD-PtdEtn (100 mug/ml), FRAP analysis at 18 degrees C in the presence of 2 mM EGTA (D = 4.2 x 10(-9) cm(2)/sec); (iii) labeling with 5-(N-dodecanoyl)aminofluorescein (a short-chain lipid probe), FRAP analysis at 18 degrees C or 37 degrees C (D = 2.5 x 10(-8) cm(2)/sec). These results suggest that the plasma membrane of soybean cells may contain stable immiscible domains of fluid and gel-like lipids.

Thermal Tolerance of Zymomonas mobilis: Temperature-Induced Changes in Membrane Composition

The membrane composition of Zymomonas mobilis changed dramatically in response to growth temperature. With increasing temperature, the proportion of vaccenic acid declined with an increase in myristic acid, the proportion of phosphatidylcholine and cardiolipin increased with decreases in phosphatidylethanolamine and phosphatidylglycerol, and the phospholipid/protein ratio of the membrane declined. These changes in membrane composition were correlated with changes in thermal tolerance and with changes in membrane fluidity. Cells grown at 20 degrees C were more sensitive to inactivation at 45 degrees C than were cells grown at 30 degrees C, as expected. However, cells grown at 41 degrees C (near the maximal growth temperature for Z. mobilis) were hypersensitive to thermal inactivation, suggesting that cells may be damaged during growth at this temperature. When cells were held at 45 degrees C, soluble proteins from cells grown at 41 degrees C were rapidly lost into the surrounding buffer in contrast to cells grown at lower temperatures. The synthesis of phospholipid-deficient membranes during growth at 41 degrees C was proposed as being responsible for this increased thermal sensitivity.

Cholesterol and anionic phospholipids increase the binding of amyloidogenic transthyretin to lipid membranes

Deposition of transthyretin (TTR) amyloid is a pathological hallmark of familial amyloidotic polyneuropathy (FAP). Recently we showed that TTR binds to membrane lipids via electrostatic interactions and that membrane binding is correlated with the cytotoxicity induced by amyloidogenic TTR. In the present study, we examined the role of lipid composition in membrane binding of TTR by a surface plasmon resonance (SPR) approach. TTR bound to lipid bilayers through both high- and low-affinity interactions. Increasing the mole fraction of cholesterol in the bilayer led to an increase in the amount of high-affinity binding of an amyloidogenic mutant (L55P) TTR. In addition, a greater amount of L55P TTR bound with high affinity to membranes made from anionic phospholipids, phosphatidylglycerol (PG) and phosphatidylserine (PS), than to membranes made from zwitterionic phospholipid phosphatidylcholine (PC). The anionic phospholipids (PS and PG) promoted the aggregation of L55P TTR by accelerating the nucleation phase of aggregation, whereas the zwitterionic phospholipid PC had little effect. These results suggest that cholesterol and anionic phospholipids may be important for TTR aggregation and TTR-induced cytotoxicity.

Effects of fermentation temperature and Saccharomyces species on the cell fatty acid composition and presence of volatile compounds in wine

Low temperature alcoholic fermentations are becoming more frequent due to the wish to produce wines with more pronounced aromatic profiles. However, their biggest drawback is the high risk of stuck and sluggish fermentations. Changes in the plasma membrane composition may be an adaptive response to low temperature fermentations. The production of volatile compounds and the changes in the membrane fatty acids were determined by GC to show the degree of cell adaptation and performance at low temperatures (13 degrees C) taking 25 degrees C as reference. The tests were done in two strains of Saccharomyces cerevisiae and one strain of Saccharomyces bayanus. Low temperatures restricted yeast growth and lengthened the fermentations. The analysis of plasma membrane fatty acids showed that dry yeasts had similar levels of unsaturation, between 70% and 80%, with no medium-chain fatty acids (MCFA). Long-chain saturated fatty acids (SFA) were the most frequent membrane fatty acids throughout the fermentations. Lipid composition changed with the growth temperature. The optimal membrane fluidity at low temperatures was modulated by changes in the unsaturation degree in S. cerevisiae strains. In S. bayanus, however, this change in the unsaturated fatty acid (UFA) percentage was not observed at different growth temperatures but the concentration of MCFA at low fermentation temperatures was higher. Concentrations of volatile compounds were higher in wines produced at lower temperatures and depended on the strain.

Preparation of beta-artemether liposomes, their HPLC-UV evaluation and relevance for clearing recrudescent parasitaemia in Plasmodium chabaudi malaria-infected mice

Egg phosphatidylcholine-cholesterol liposome formulations containing the antimalarial drug beta-artemether have been prepared and analyzed for their encapsulating capacity, chemical stability, leakage, in vitro release and their therapeutic efficiency against Plasmodium chabaudi infection. A HPLC-UV analysis of beta-artemether liposomes without derivatisation was achieved. A good linearity of y=4437.7 x+469.01 (R(2)=0.9999) with a detection limit of 2 microg ml(-1) was reached. Prior to this, liposomal formulations composed of different molar ratios of EPC-CHOL were prepared to select beta-artemether crystal-free liposome preparations. The formulation corresponding to 4:3 and a total concentration of 300 mg lipids ml(-1) buffer (pH 7.2), which could incorporate as much as 1.5 mg beta-artemether was selected for therapy. A trapping efficiency of nearly 100% was reached, the drug being located in the lipid bilayers. A dialysis test demonstrated that the drug could be reversibly released from the liposomes, reaching equilibrium within 24 h. After 3 months storage at 4 degrees C, no leakage of beta-artemether had occurred indicating a high stability of the liposomes. These liposomes were used to treat mice infected with the virulent rodent malaria parasite Plasmodium chabaudi chabaudi, with a 100% cure by clearing the recrudescent parasitaemia.

Optimization of cationic lipid/DNA complexes for systemic gene transfer to tumor lesions

Intravenous (i.v.) administration of cationic lipid N-[( 1-(2-3-dioleyloxy)propyl)]-N-N-N-trimethylammonium chloride (DOTMA)-based transfection complexes in mice with subcutaneous squamous cell tumors yielded plasmid delivery and expression in tumor lesions. The efficiency of gene transfer in tumors was significantly lower than in the lung. This was consistent with low plasmid levels associated with the tumor, suggesting that plasmid delivery to the tumor site was a limiting factor. Lowering the lipid/DNA charge ratio from 5:1 to 0.8:1 (+/-) did not change DNA levels in tumor but significantly reduced DNA levels in lung. However, expression levels were significantly reduced in both tissues at lower lipid/DNA charge ratios. Complexes prepared from small unilamellar liposomes gave significantly lower expression levels in the lungs but similar expression levels in tumors when compared to complexes prepared from larger unilamellar liposomes. The small liposome complexes were better tolerated than large liposome complexes. Varying the cationic lipid to colipid (cholesterol or DOPE) molar ratio from 4: 1 to 1: 1 significantly reduced expression levels in both tumor and lung. Cationic lipid substitution, using a cholesterol cationic lipid, diethyldiamino-carbamyl-cholesterol instead of DOTMA, produced reduced expression in all other tissues except tumor. Incorporation of PEG into preformed transfection complexes reduced DNA delivery to lung, increased circulation half-life, and enhanced DNA delivery to tumor. In a lung metastatic mouse tumor model, where the accessibility of the i.v. administered transfection complexes to tumor lesions should be less challenging, DOTMA: CHOL complexes (4: 1 lipid to colipid molar ratio, 3: 1 +/- lipid to plasmid charge ratio) were preferentially localized in tumor lesions. These data demonstrate that systemic gene transfer to distal tumor sites by lipid/ DNA complexes may be limited by low plasmid delivery. Modifying the chemical surface properties of transfection complexes enhanced both DNA delivery and expression in tumor and is one approach that may overcome limitations.

Effect of cold shock on lipid A biosynthesis in Escherichia coli. Induction At 12 degrees C of an acyltransferase specific for palmitoleoyl-acyl carrier protein

Palmitoleate is not present in lipid A isolated from Escherichia coli grown at 30 degrees C or higher, but it comprises approximately 11% of the fatty acyl chains of lipid A in cells grown at 12 degrees C. The appearance of palmitoleate at 12 degrees C is accompanied by a decline in laurate from approximately 18% to approximately 5.5%. We now report that wild-type E. coli shifted from 30 degrees C to 12 degrees C acquire a novel palmitoleoyl-acyl carrier protein (ACP)-dependent acyltransferase that acts on the key lipid A precursor Kdo2-lipid IVA. The palmitoleoyl transferase is induced more than 30-fold upon cold shock, as judged by assaying extracts of cells shifted to 12 degrees C. The induced activity is maximal after 2 h of cold shock, and then gradually declines but does not disappear. Strains harboring an insertion mutation in the lpxL(htrB) gene, which encodes the enzyme that normally transfers laurate from lauroyl-ACP to Kdo2-lipid IVA (Clementz, T., Bednarski, J. J., and Raetz, C. R. H. (1996) J. Biol. Chem. 271, 12095-12102) are not defective in the cold-induced palmitoleoyl transferase. Recently, a gene displaying 54% identity and 73% similarity at the protein level to lpxL was found in the genome of E. coli. This lpxL homologue, designated lpxP, encodes the cold shock-induced palmitoleoyl transferase. Extracts of cells containing lpxP on the multicopy plasmid pSK57 exhibit a 10-fold increase in the specific activity of the cold-induced palmitoleoyl transferase compared with cells lacking the plasmid. The elevated specific activity of the palmitoleoyl transferase under conditions of cold shock is attributed to greatly increased levels of lpxP mRNA. The replacement of laurate with palmitoleate in lipid A may reflect the desirability of maintaining the optimal outer membrane fluidity at 12 degrees C.

Saccharomyces cerevisiae cell fatty acid composition and release during fermentation without aeration and in absence of exogenous lipids

Medium-chain fatty-acids (MCFA) are among the main aroma compounds of fermented beverages. High concentrations of MCFA have been found in sluggish and stuck fermentations. It has been suggested that they arrest cell growth, as they may be toxic, but the causes of sluggish and stuck fermentations are still unclear. The aim of this work was to see whether the production of MCFA is related to fatty acid synthesis in the absence of exogenous lipids and aeration, and whether their increase can be regarded as a consequence, instead of the cause, of sluggish and stuck fermentations. Two possibilities were considered: (i) MCFA are produced to replace unsaturated fatty acids (UFA) for cell membrane fluidity when the lack of oxygen makes desaturation of saturated fatty acids (SFA) impossible; or (ii) MCFA are produced following the release of medium-chain acyl-CoA from the fatty acid synthetase complex (FAS) due to the accumulation of SFA, and their hydrolysis to recycle CoA-SH. In the first hypothesis, MCFA should be active in cell metabolism and be found in cell structures; in the second, MCFA should be a discard and prevalently found outside the cell. We carried out a Saccharomyces cerevisiae fermentation experiment in a synthetic, lipid-free medium without aeration. We measured the fatty acid composition of yeast cells and the amounts of MCFA and their ethyl esters in the medium throughout the fermentation. Cell growth and the oxygen content of the medium were also monitored. We found that MCFA are not immobilized in cell structures, but mainly released into the medium. Cell growth is arrested because fatty acid biosynthesis is prevented by the lack of oxygen. The higher MCFA concentrations found in sluggish and stuck fermentations can be thus regarded as an effect, and not the cause, of this arrest. Some suggestions for the prevention of these events are proposed.

The effect of sterols on structures formed in the gel/subgel phase state of dipalmitoylphosphatidylcholine bilayers

The effect of cholesterol and lanosterol on the formation of structures in the gel/subgel phase of 1,2-dipalmitoylphosphatidylcholine was investigated using freeze-fracture electron microscopy and X-ray diffraction. Mixtures with up to 25 mol% sterol were analysed after annealing for between several days and some months at 4 degrees C. Bilayers of DPPC with 5 or 10 mol% sterol showed a domain structure in the gel state. There are rounded or lens-like and often inclined plates within less smooth either plane or striped bilayer areas. The stripes are formed by parallel lines separated by a distance of 30-60 nm. Parallel lines can be induced also in the less smooth but plane areas by warming up to 25 degrees C. X-ray diffraction showed two lamellar repeat spacings at 6.45 and 8.3 nm in both 5 and 10 mol% samples. The plates are interpreted as domains of (nearly) pure DPPC within the sterol containing bilayer. Stripes are present if the concentration of cholesterol is below a critical value (approx. 15 mol%). With time of incubation at 4 degrees C curved deformations appear in parts of the bilayers. Two main types are formed. The small type has a repeat distance of about 100 nm and the large type of about 400 nm. The curved deformations were progressively flattened by warming up to 25-32 degrees C with an accompanying reappearance of stripes but no plates. After prolonged annealing at 4 degrees C there is also the formation of regular ripples. It is concluded that in presence of 5 and 10 mol% sterol in bilayers of 1,2-dipalmitoy-phosphatidylcholine the immiscibility of gel phase and subgel phase changes during prolonged annealing at 4 degrees C. We assume a rearrangement of the molecules into a homogeneous phase state with liquid-ordered properties.

Effect of changing the size of lipid headgroup on peptide insertion into membranes

Adsorption of amphiphilic peptides to the headgroup region of a lipid bilayer is a common mode of protein-membrane interactions. Previous studies have shown that adsorption causes membrane thinning. The degree of the thinning depends on the degree of the lateral expansion caused by the peptide adsorption. If this simple molecular mechanism is correct, the degree of lateral expansion and consequently the membrane thinning should depend on the size of the headgroup relative to the cross section of the hydrocarbon chains. Previously we have established the connection between the alamethicin insertion transition and the membrane thinning effect. In this paper we use oriented circular dichroism to study the effect of varying the size of the headgroup, while maintaining a constant cross section of the lipid chains, on the insertion transition. A simple quantitative prediction agrees very well with the experiment.

Effect of lipid ozonation products on liposomal membranes detected by Laurdan fluorescence

We report here the influence of the lipid ozonation products, 1-palmitoyl-2-(9-oxononanoyl)-sn-glycero-3-phosphocholine (PC-aldehyde) and 1-palmitoyl-2[8-(5-octyl-1, 2, 4,-trioxolan-3-yl)- octanoyl]-sn-glycero-3-phosphocholine (PC-Criegee ozonide), on the phase domains of small unilamellar vesicles. (See Scheme 1 for structures.) 6-Lauroyl-2-dimethylaminonaphtalene (Laurdan) fluorescence excitation and emission spectra and generalized polarization measurements allowed us to study how lipid ozonation products affect the phase components of phospholipid membranes. A shift of excitation and emission spectra and a decrease in generalized polarization reveal the presence of a more polar environment surrounding the probe. We find that when either PC-aldehyde or PC-Criegee ozonide are incorporated into a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membrane, or when the POPC membrane is directly ozonated, a change in polarity of the phospholipid environment occurs that changes the properties of the bilayer. The introduction of more oxygenated and more polar phospholipids creates a more polar environment allowing the deeper penetration of water molecules into the membrane. Water penetration also is facilitated by the membrane disorder-producing effect of the ozonation products. The presence of an increased number of water molecules in the membrane effects the bilayer, altering packing order and cooperatively among fatty acyl chains as well as enhancing membrane fluidity.

Free radical reactions of MTDQ and its effect on biological membrane

The effect of 6,6'-methylene-bis-2,2,4-trimethyl-1,2-dihydroquinoline (MTDQ) and its water soluble species (MTDQ-DA) was studied in biological membrane and model systems using EPR spectroscopy for detecting molecular motion and radical formation. Both compounds influenced the rotational mobility of maleimide spin labels attached to proteins of the nerve membrane: the addition of MTDQ or MTDQ-DA induced an increase of the rigidity of the membrane in the environment of the attaching sites. The reaction of MTDQ-DA with hydroxyl and superoxide free radicals showed that this compound was also a competitive OH and superoxide free radical scavenger. The reaction rate constant of the formation of MTDQ-DA free radical was k = (4.0 +/- 0.5) x 10(9) M-1 s-1 in the hydroxyl free radical generating system. Simulation of EPR spectra supported that MTDQ-DA free radical was very likely a stable nitroxide free radical.

Phase separation of biomolecules in polyoxyethylene glycol nonionic detergents

The advantage of aqueous two-phase systems based on polyoxyethylene detergents over other liquid-liquid two-phase systems lies in their capacity to fractionate membrane proteins simply by heating the solution over a biocompatible range of temperatures (20 to 37 degrees C). This permits the peripheral membrane proteins to be effectively separated from the integral membrane proteins, which remain in the detergent-rich phase due to the interaction of their hydrophobic domains with detergent micelles. Since the first reports of this special characteristic of polyoxyethylene glycol detergents in 1981, numerous reports have consolidated this procedure as a fundamental technique in membrane biochemistry and molecular biology. As examples of their use in these two fields, this review summarizes the studies carried out on the topology, diversity, and anomalous behavior of transmembrane proteins on the distribution of glycosyl-phosphatidylinositol-anchored membrane proteins, and on a mechanism to describe the pH-induced translocation of viruses, bacterial endotoxins, and soluble cytoplasmic proteins related to membrane fusion. In addition, the phase separation capacity of these polyoxyethylene glycol detergents has been used to develop quick fractionation methods with high recoveries, on both a micro- and macroscale, and to speed up or increase the efficiency of bioanalytical assays.

Arachidonic acid regulates unsaturated fatty acid synthesis in lymphocytes by inhibiting stearoyl-CoA desaturase gene expression

This work was based upon the observation that a reduction in the level of serum, provided to murine lymphocytes in culture, augmented endogenous unsaturated fatty acid (UFA) synthesis. Since the phospholipids of BW5147 cells grown in 1% serum were especially deficient in arachidonic acid (20:4), and given the findings of previous workers, we suspected that the availability of exogenous 20:4 in serum might correlate with the squelching of UFA synthesis. Indeed, after a 5 h exposure to 4-28 microM 20:4, the 20:4 content of BW5147 cell phospholipids increased from 1% to 15% of the total fatty acids with a coincident reduction in 18:1 synthesis to approx. 30% of starting values. Subsequent studies were done to define the mechanism by which 20:4 down-regulates 18:1 synthesis. The results indicated that 20:4 inhibited endogenous 18:1 synthesis by reducing stearoyl-CoA desaturase (SCD) enzyme activity. Moreover, as determined by Northern blot analyses, the inhibitory effect of 20:4 on stearoyl-CoA desaturase activity coincided with decreased stearoyl-CoA desaturase mRNA levels. Exposure of BW5147 cells to either 20:4, actinomycin D, or both, resulted in a temporal decay of stearoyl-CoA desaturase mRNAs with half-lives ranging from 4.0 h to 4.4 h. Such a similarity in decay times implied that 20:4 regulates stearoyl-CoA desaturase expression by inhibiting transcription. This was confirmed by nuclear run-on studies in which 20:4 was found to inhibit transcription of nascent stearoyl-CoA desaturase mRNA. Collectively, these findings implicate 20:4 as an important regulator of stearoyl-CoA desaturase gene expression, and hence UFA synthesis, in lymphoid cells.

Vitamin E distribution and modulation of the physical state and function of pulmonary endothelial cell membranes

Vitamin E, a dietary antioxidant, is thought to incorporate into the lipid bilayer of biological membranes. We evaluated the lipid composition and distribution of [3H]-vitamin E in various membranes of pulmonary endothelial cells and determined whether vitamin E incorporation caused alterations in membrane structure and function in these cells. Following 6-, 12-, 18-, 24-, and 48-h incubation periods, vitamin E incorporation values were 3.0, 5.7, 6.9, 7.2, and 6.8 nmol/mg protein or 3.8, 7.3, 8.8, 9.2, and 8.7 nmol/mg phospholipid in mitochondrial membranes and 2.0, 4.4, 5.2, 5.3, and 5.0 nmol/mg protein or 3.5, 7.7, 9.1, 9.3, and 8.8 nmol/mg phospholipid in microsomal membranes, respectively. Vitamin E incorporation into the plasma membranes was greater than in mitochondrial and microsomal membranes after 12-, 24-, and 48-h incubations (18.9, 20.8, and 19.6 nmol/mg protein, respectively [P less than .001] versus mitochondria and microsomes or 12.2, 13.4, and 12.6 nmol/mg phospholipid, respectively [P less than .05] versus mitochondria and microsomes). The total phospholipid content, as well as the unsaturation index of the fatty acid content of these membranes, were in the same order, (i.e., plasma membrane greater than mitochondrial membranes and microsomal membranes). The physical state of the intact plasma membrane and the mitochondrial and microsomal membranes were measured by monitoring fluorescence anisotropies (rs) of the molecular probes, diphenylhexatriene (DPH) and trimethylamino-DPH (TMA-DPH). Vitamin E incorporation caused significant increases in rs for DPH (P less than .01) and TMA-DPH (P less than .01) in all three membranes compared to controls. Similar increases in rs values for DPH and TMA-DPH were observed in lipid vesicles prepared from these membranes. Following vitamin E incorporation, 5-hydroxytryptamine (5-HT) transport was measured as an index of plasma membrane function. Vitamin E incorporation resulted in an 18% reduction (P less than .05) in 5-HT uptake. These results indicate that vitamin E was distributed nonuniformly in endothelial cell membranes but resulted in comparable decreases in fluidity in all three membranes. In addition to its role as an antioxidant, vitamin E may alter the membrane physical state and modulate a variety of endothelial cell functions, including 5-HT transport.

Effect of polyunsaturated fatty acids and phospholipids on [3H]-vitamin E incorporation into pulmonary artery endothelial cell membranes

Vitamin E, a dietary antioxidant, is presumed to be incorporated into the lipid bilayer of biological membranes to an extent proportional to the amount of polyunsaturated fatty acids or phospholipids in the membrane. In the present study we evaluated the distribution of incorporated polyunsaturated fatty acids (PUFA) and phosphatidylethanolamine (PE) in various membranes of pulmonary artery endothelial cells. We also studied whether incorporation of PUFA or PE is responsible for increased incorporation of [3H]-vitamin E into the membranes of these cells. Following a 24-hr incubation with linoleic acid (18:2), 18:2 was increased by 6.9-, 9.2-, and 13.2-fold in plasma, mitochondrial, and microsomal membranes, respectively. Incorporation of 18:2 caused significant increases in the unsaturation indexes of mitochondrial and microsomal polyunsaturated fatty acyl chains (P less than .01 versus control in both membranes). Incubation with arachidonic acid (20:4) for 24 hr resulted in 1.5-, 2.3-, and 2.4-fold increases in 20:4 in plasma, mitochondrial, and microsomal membranes, respectively. The unsaturation indexes of polyunsaturated fatty acyl chains of mitochondrial and microsomal membranes also increased (P less than .01 versus control in both membranes). Although incubations with 18:2 or 20:4 resulted in several-fold increases in membrane 18:2 or 20:4 fatty acids, incorporation of [3H]-vitamin E into these membranes was similar to that in controls. Following a 24-hr incubation with PE, membrane PE content was significantly increased, and [3H]-vitamin E incorporation was also increased to a comparable degree, i.e., plasma membrane greater than mitochondria greater than microsomes. Endogenous vitamin E content of the cells was not altered because of increased incorporation of PE and [3H]-vitamin E. When [3H]-vitamin E was incorporated into lipid vesicles prepared from the total lipid extracts of endothelial cells and varying amounts of exogenous PE, vitamin E content was directly related to PE content. These results demonstrate that PUFA and PE distribute in all pulmonary artery endothelial cell membranes. However, only increases in PE were associated with increased incorporation of [3H]-vitamin E in membranes of these cells.

Erythrocyte rheology

Two main subjects of erythrocyte rheology, deformation and aggregation, are discussed in detail, on the basis of biochemical structure. The close relationship between the life span (or cell aging) and the rheology of individual erythrocytes is also briefly described. A currently important problem is emphasized, that is, the molecular aspect of the dynamic cytoskeletal structure and the mechanism of its regulation. This concerns not only the rheological function and the survival of circulating erythrocytes, but also the pathophysiology of abnormal erythrocytes.

The significance of membrane changes in the safe and effective use of therapeutic and diagnostic ultrasound

The cellular changes, such as alterations in motility and the stimulation of synthesis and secretion, induced by relatively low intensities of therapeutic ultrasound (e.g. 500 mW cm-2, SAPA; 100 mW cm-2 SATA) are primarily non-thermal in origin. They appear to be associated with changes in the permeability of the cell (plasma) membrane and in the transport of ions and molecules across it, effects which have been demonstrated in cells irradiated in suspension. In epithelial tissues, both in vitro and in vivo, it has been demonstrated that not only the cellular membrane transport pathways but also the paracellular or intercellular pathways are affected. Although membrane-mediated effects can be of value therapeutically, they could produce adverse effects if they were to occur during development, for the reception and transmission by the membrane of environmental signals are involved in determination of the fate of each cell. Determination is followed by selective gene expression and differentiation, that is, by the progressive increase in structural complexity brought about by the acquisition of specialised characteristics by various cell groups. Most cells of early embryos are ionically coupled via gap junctions which provide an intercellular pathway for electrochemical signalling and the maintenance of the concentration gradients which provide the cells with positional information. Differentiation of the cells varies according to their location with respect to these gradients. Increase in the intracellular concentration of calcium ions, which has been shown to occur after exposure to therapeutic levels of ultrasound, can decrease the permeability of gap junctions and uncouple cells, in the manner which occurs when they differentiate. Ultrasonically induced increases in calcium ion concentration are thus of considerable clinical significance, since they could affect differentiation and consequently histogenesis. Modification of plasma membrane permeability and transport properties, resulting in changes in the availability and activity of second messengers such as free calcium ions, can have profound effects on cell behaviour. Calcium channels appear to be the first channels to develop in the cell membranes of embryos, and internal calcium ion concentration is known to affect the synthesis of fetal proteins. Although generally reversible at intensities of less than 500 mW cm-2, changes in membrane permeability, particularly to calcium ions, could, if prolonged, have undesirable side effects not only on embryogenesis but on late prenatal and postnatal development. It is therefore recommended that the environmental conditions, thresholds, and mechanisms involved in the production of such changes be determined, so that they can be avoided when ultrasound is used diagnostically on sensitive targets such as embryos and fetuses.

X-ray diffraction studies of lipid phase transitions in hydrated mixtures of cholesterol and diacylphosphatidylcholines and their relevance to the structure of biological membranes

Previous X-ray diffraction data on the effects of temperature on hydrated cholesterol/dimyristoylphosphatidylcholine mixtures have been confirmed and equivalent new data on cholesterol/stearolyoleoylphosphatidylcholine obtained. Molecular interpretations are discussed and related to previous studies of cholesterol/dioleoylphosphatidylcholine and of cholesterol-rich biological membranes.

Theory of protein-induced lateral phase separation in lipid membranes

An account is given of the current status of theoretical modeling of the phase equilibria in lipid membranes with intrinsic proteins. Special attention is paid to the description of lateral phase separation, which is important for membrane function since it may lead to biologically differentiated regions. We discuss in particular the mattress-model approach by Mouritsen and Bloom, who take matching between protein and lipid hydrophobic thicknesses as a determining factor for the phase behavior. The model has been developed in the framework of phenomenological thermodynamic solution theory. The predictions of the theory are compared to a variety of experimental measurements, including those of membrane recombinants of the protein content of the reaction center and antenna protein of the bacterial photosynthetic apparatus as well as the erythrocyte band 3 protein. The physical effects of lipid-protein interactions are contrasted to those of lipid-cholesterol interactions. The concept of hydrophobic matching is then used as a basis for discussing a possible relationship between membrane thickness and physiological function.

Morphological and immunocytochemical studies of fibronectin-coated, plasma membrane-limited vesicles in the early chicken embryo

Vesticles with a mean outside diameter of 32.8 nm have been observed in the early chicken embryo after fixation with a mixture of glutaraldehyde and tannic acid. Densitometric tracing has revealed that the vesicles are limited by a unit membrane. The presence of complex carbohydrates is suggested by the increased electron density of the vesticles after addition of tannic acid to the fixative. Immunocytochemical staining with a monoclonal antibody directed against chicken cellular fibronectin demonstrated the presence of this glycoprotein along the surface of the vesicles. These results suggest a cellular origin of the vesicles, since their surface shares morphological and biochemical similarities with the cell surfaces of the embryonic tissue layers. Recycling of plasma-membrane vesicles may occur, as vesicles were found in the vicinity of coated vesicles. We postulate that extracellular materials of the cell surface, which may affect cell and tissue interactions, are shed in the environment together with plasma-membrane vesicles. The difficulties encountered in observing the vesicles stems from the facts that an adequate visualization method is necessary and that they are few in number. The latter reason suggests their transient nature. The vesicles probably rapidly disintegrate in the extracellular milieu or are recycled by the cell surface.

Interaction between oxygen and cell membranes: modification of membrane lipids to enhance pulmonary artery endothelial cell tolerance to hypoxia

Because high partial pressures of oxygen (O2) can cause peroxidative cleavage of membrane lipids, it is plausible to hypothesize that hyperoxia alters the physical state and composition of lipids in the membranes of pulmonary endothelial cells and that manipulation of the lipid profile may modify endothelial cell tolerance to hyperoxic injury. To test this, porcine pulmonary artery endothelial cells were exposed to 95% O2 at 1 atmosphere absolute (ATA) in the presence or absence of cis vaccenic acid (CVA), a monounsaturated fatty acid (C18:1#11). Plasma membrane fluidity was assessed by fluorescence spectroscopy, plasma membrane lipid composition was quantitated using thin layer and gas chromatography, and cytotoxicity was monitored by measuring release of lactate dehydrogenase (LDH). Hyperoxia caused peroxidation of membrane lipids and decreased fluidity in three distinct lipid domains within the plasma membrane. Incubation with CVA was associated with a reduction in the degree of unsaturation of the constituent fatty acids within all plasma membrane lipid subclasses except monoglycerides. CVA-treated cells were also more resistant to hyperoxic injury as judged by LDH release. These results support the hypothesis that cells with membranes in which the fatty acyl chains are more resistant to the disordering effects of high O2 tensions may be more resistant to O2 toxicity.

X-ray diffraction studies of lipid phase transitions in cholesterol-rich membranes at sub-zero temperatures

In X-ray diffraction studies of hydrated (greater than 60%) cholesterol/dioleoylphosphatidylcholine mixtures the lipid packing band showed an abrupt transition from liquid crystal-type to gel-type position and definition at a temperature which decreased progressively to almost -50 degrees C as the proportion of cholesterol was increased to a saturation level of about 50 mol%. Plots of transition temperature against composition (mol% cholesterol) and of peak position against composition provided evidence of a significant change in phospholipid configuration at about 20 mol% cholesterol. However, the data overall suggested a uniform dispersion of the cholesterol molecules in the phospholipid bilayer at all concentrations up to the saturation point. Parallel studies of hydrated lipid extract of erythrocyte membranes and of several cholesterol-rich membrane preparations showed a similar overall change from liquid crystal-type packing at +20 degrees C to a gel-type packing at -30 degrees C to -40 degrees C but without displaying a defined transition temperature.

The phase behaviour of dispersions of Bis-Azo PC: photoregulation of bilayer dynamics via lipid photochromism

A phospholipid, 1,2-bis(4-(n-butyl)phenylazo-4'-phenylbutyroyl)phosphatidylcholine (Bis-Azo PC), has been synthesised and shown to form stable bilayer vesicles. Light-scattering measurements and differential scanning calorimetry show that a dispersion of the lipid has a cooperative phase transition at a similar temperature to that of dipalmitoylphosphatidylcholine, which Bis-Azo PC resembles in overall size. The phase behaviour of Bis-Azo PC has been investigated by fluorescence spectroscopy and using a series of spin-labelled fatty acid probes. Fluorescence measurements using chlorophyll a as probe sense the onset of the cooperative phase transition, but this is not clearly revealed by any of the spin probes tested. Hysteresis in the phase transition is detected both by light scattering measurements and by fluorescence spectroscopy. No transition is observed for a lipid analogue having a palmitic acid chain and a single azo-containing substituent. Bis-Azo PC is reversibly photochromic, isomerising on exposure to ultraviolet light to a photostationary state mixture where cis isomer predominates. Electron microscopy shows that photoisomerisation decreases average vesicle size, and light scattering and calorimetry demonstrate that the cooperative phase transition is abolished. Illumination with visible light establishes a new photostationary state where trans isomer predominates, and the phase transition is restored. The ability to modulate bilayer phase behaviour reversibly has possible application to relaxation studies of bilayer membrane function, and to drug delivery research.

P0 myelin protein produces experimental allergic neuritis in Lewis rats

P0 protein was prepared from bovine spinal root myelin. The purity was shown by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunostaining with affinity purified antisera. P0 in the presence of lysophosphatidylcholine induced paralysis and histological lesions resembling experimental allergic neuritis in Lewis rats. Lysophosphatidylcholine also enhanced the ability of P2 to produce neuritis. The conformation of these proteins may be important in determining their ability to induce experimental allergic neuritis. P0 deserves consideration as an antigen relevant to Guillain-Barré syndrome.