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PubMed1982

Lipid A, the lipid component of bacterial lipopolysaccharides: relation of chemical structure to biological activity.

Lipopolysaccharides are integral components of the outer membrane of Gram-negative bacteria and they participate in various membrane functions essential for bacterial growth and survival. Lipopolysaccharides also represent the endotoxins of Gram-negative bacteria and possibly play a role for the pathogenesis and manifestations of bacterial infections. These biological activities are mediated mainly by the lipid component of lipopolysaccharides, termed lipid A. Chemically, lipid A consists of a beta 1, 6-linked D-glucosamine disaccharide which carries substituted phosphoryl groups and a range of D-3-hydroxy and D-3-acyloxyacyl residues, the latter being arranged in a hexagonal dense packing. A number of experimental data allow the conclusion that the highly ordered and compact lipid A structure confers stability to the outer membrane, renders it less permeable to lipophilic molecules and by providing a proper fluidity stabilizes the conformation of biologically active membrane proteins. For endotoxic activities of lipid A the polar substituents of phosphate residues are dispensable. The presence of D-3-hydroxy (or acyloxy) acyl-groups, linked to the glucosamine disaccharide, however, seems to be of importance. Analyses of now available synthetic lipid A analogues are expected to allow a more precise characterization of substructures and conformations required for the expression of physiological functions and endotoxic activities of lipid A.

Chemical Phenomena↗
PubMed1987

Intake of different eicosapentaenoic acid-containing lipids and fatty acid pattern of plasma lipids in the rats.

The ethyl ester of eicosapentaenoic acid (EPA) is the only pure EPA-containing lipid available in bulk for oral administration. However, there is doubt as to whether EPA ethyl ester can efficiently increase the plasma levels of EPA in comparison with the ability of other kinds of EPA-containing lipids to do so. Therefore, two other kinds of EPA-containing lipids were prepared to study the efficiency of oral administration of those lipids for increasing the EPA content in plasma phospholipids and cholesteryl esters. EPA-containing lipids which were investigated were [A] 1,2,3-trieicosapentaenoyl-glycerol, [B] 2-eicosapentaenoyl-phosphatidylcholine and [C] ethyl ester of EPA. An adjusted amount of lipids [A], [B] and [C] was administered to rats through a gastric tube for 4 days (the first experiment) or for 10 days (the second experiment), and the fatty acid composition of plasma phospholipids and cholesteryl esters was determined. In the first experiment, there were no significant differences in the efficiency for increasing EPA levels in either phospholipids or cholesteryl esters among the lipids. In the second experiment, the EPA levels of both plasma phospholipids and cholesteryl esters of rats administered ethyl ester of EPA were significantly higher than those of rats administered 2-eicosapentaenoyl-phosphatidylcholine. The EPA levels of the rats administered 1,2,3-trieicosapentaenoyl-glycerol were between the levels of the two groups mentioned above, but the differences in the EPA levels were not significant.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2006

An acute and coincident increase in FABP expression and lymphatic lipid and drug transport occurs during intestinal infusion of lipid-based drug formulations to rats.

PURPOSE: To determine a) whether administration of lipid-based formulations can acutely up-regulate the intestinal expression of I-FABP and L-FABP and b) whether this occurs coincidentally with an increase in intestinal lymphatic lipid and drug transport. METHODS: The expression of I-FABP and L-FABP mRNA (using q-PCR) and protein (using immunohistochemistry and Western blotting) in enterocytes was compared with data describing transport of lipid and drug into intestinal lymph following infusion of a set of lipid-based formulations. RESULTS: Administration of relatively small amounts of oleic acid (5-20 mg/h) over a 5 h period to rats acutely up-regulated the expression, and altered the intracellular distribution of, I-FABP and L-FABP in the enterocytes of the small intestinal epithelia. The increase in expression of I-FABP and L-FABP correlated well with previous data describing the transport of lipid and drug into intestinal lymph following infusion of the same formulations. CONCLUSION: The expression and intracellular distribution of I-FABP and L-FABP are acutely influenced by lipid infusion over a time period relevant to feeding or the administration of pharmaceutical lipidic formulations, and these changes occur coincidentally with increased drug transport into the lymphatics.

Animals↗
PubMed1988

Lipid peroxidation and cell viability in isolated hepatocytes in a redesigned oxystat system: evaluation of the hypothesis that lipid peroxidation, preferentially induced at low oxygen partial pressures, is decisive for CCl4 liver cell injury.

An oxystat system is described which is capable of maintaining steady-state oxygen partial pressures (PO2) at levels between 0.1 and 300 mm Hg for hours or even days in incubations of respiring cells. The system was used to study effects of the hepatotoxin carbon tetrachloride (CCl4) on lipid peroxidation and cell viability in isolated hepatocytes from phenobarbital-pretreated rats at various steady-state PO2. At PO2 below 35 mm Hg, with a maximum effect at 7 mm Hg, CCl4 induced an immediate lipid peroxidation, the rate of which slowed down during further incubation. AT PO2 between 35 and 70 mm Hg, CCl4 initially induced only slight lipid peroxidation, while there was a significant increase in lipid peroxidation after approximately 30 min. At PO2 above 100 mm Hg, no lipid peroxidation was induced by CCl4. At PO2 of 70 mm Hg and below, with the maximum effect at 3 mm Hg, CCl4 also induced marked losses of cell viability. Under anaerobic conditions and at PO2 greater than 70 mm Hg, CCl4 was without effect on the viability of the liver cells. Cells isolated from the pericentral area of the liver lobule showed more lipid peroxidation and loss of cell viability than cells from the periportal area of the lobule. These results provide further evidence for the decisive role of lipid peroxidation, preferentially induced at low PO2, in CCl4 liver injury.

Animals↗
PubMed1987

Radioprotective effects of lipid A, liposomes, and liposomes containing lipid A in mice.

Lipid A from Gram-negative bacterial lipopolysaccharide (endotoxin) was incorporated into liposomal membranes and examined as a prophylactic radioprotectant compound in lethally irradiated mice. Splenic hematopoietic activity, resulting in increased numbers of spleen cell colonies, was induced both by lipid A alone or more strongly by liposomal lipid A. Increased survival of lethally irradiated animals was induced to a slight extent by liposomes alone, to a greater extent by lipid A, and at the highest level by liposomes containing lipid A. Under conditions where 100% of untreated or saline-treated animals died of acute radiation syndrome after 20 days, more than 90% of the animals pretreated with liposomal lipid A were still alive 30 days after irradiation. We conclude that lipid A had substantial radioprotectant activity by itself, and the activity was enhanced by incorporation into liposomes. Liposomes alone also exhibited mild radioprotectant effects.

Animals↗
PubMed1995

Modulation of substrate selectivity in plasma lipid transfer protein reaction over structural variation of lipid particle.

The modulation of substrate selectivity of human plasma LTP reaction is the subject of the present investigation. The moderate selectivity by a factor of 5 to 6 was observed in the LTP-catalyzed transfer of cholesteryl ester over triacylglycerol between plasma lipoproteins. On the other hand, the transfer of cholesteryl ester by LTP was highly selective over the negligible transfer of triacylglycerol, by a factor of 60 to 500, between the microemulsions with LDL size, regardless of the activators such as human and pig apolipoprotein (apo) A-I, human apo C-III and apo E that bound to the surface of the emulsion in equilibrium. The presence of free cholesterol in these microemulsions reduced slightly the rate of cholesteryl ester transfer but had no effect on triacylglycerol transfer. Other surface-active reagents such as cholic acid, Triton X-100 and Tween-20, did not have an effect on the triacylglycerol transfer either. Triacylglycerol transfer by LTP became measurable between such lipid particles as prepared by co-sonication of lipid with pig apo A-I and isolated as the mixed-microemulsions in the density of LDL and HDL. In these conditions, the substrate selectivity for cholesteryl ester over triacylglycerol was a factor of 6 to 16 mimicking the ratio in plasma lipoproteins. The conformation of pig apo A-I estimated by circular dichroism showed that its apparent helical content was further more induced when apo A-I was integrated into the mixed-microemulsion by co-sonication than the lipid-bound apo A-I in equilibrium. Apo A-I, thus integrated into lipid particles, was highly resistant to the denaturation by guanidine hydrochloride while the lipid-bound apo A-I in equilibrium was denatured as readily as the lipid-free protein. Thus, triacylglycerol transfer by LTP was induced by structural modulation of substrate-carrying lipid particles such as higher integration of apolipoproteins.

Animals↗
PubMed1983

Interactions between the active metabolite of tryptophan pyrolysate mutagen, N-hydroxy-Trp-P-2, and lipids: the role of lipid peroxides in the conversion of N-hydroxy-Trp-P-2 to non-reactive forms.

The interactions between lipids and the mutagenic active metabolite of 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) and 3-hydroxyamino-1-methyl-5H-pyrido[4,3-b]indole (N-hydroxy-Trp-P-2), were studied. Oleic acid showed an inhibitory effect on the formation of this active metabolite mainly by inhibition of hepatic microsomal oxidation systems. On the other hand, microsomal lipids from rat liver and commercial pig liver lecithin diminished the amount of N-hydroxy-Trp-P-2 without inhibiting the metabolism of Trp-P-2. The direct reaction of these lipids with N-hydroxy-Trp-P-2 was disclosed by experiments using N-hydroxy-Trp-P-2 and lipids without microsomes. Furthermore, the participation of lipid peroxides in this reaction was suggested by a linear relationship between the concentrations of the conjugated diene of lipids and the disappearance of N-hydroxy-Trp-P-2. When [3H]N-hydroxy-Trp-P-2 was incubated in the presence of pig liver lecithin, the polar products which were not formed in the incubation without lipids were newly detected by thin-layer chromatography (TLC) analysis.

Animals↗
PubMed1981

Lipid composition of metacestodes of Taenia taeniaeformis and lipid changes during growth.

A lipid analysis was performed on developing metacestodes of Taenia taeniaeformis removed from the livers of rats at times varying from 3 to 35 weeks post infection. Lipid accounted for 7-21% of the dry weight of the parasites. The highest proportions were found at the earlier stages. The distribution was as follows; neutral lipid 27-45%; glycolipid 5-11%; and phospholipid 50-61%. The major neutral lipid was cholesterol, and minor neutral lipids were sterol esters, triglycerides, diglycerides and monoglycerides. Hydrocarbons were present throughout development, but in the highest amounts at the earlier stages. Five different glycolipids were found, all of which were identified as glycosphingolipids. An increase in the proportion of more complex glycolipids was noted as parasites grew older. Ten different phospholipids were identified, with the major components being phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine. Other phospholipids were: lysophosphatides, phosphatidylinositol, phosphatidic acid, diphosphatidylglycerol, sphingomyelin, and an unknown phospholipid component. Changes in the relative amounts of the two major phospholipids were found when the early and late stages were compared. Two lipids found throughout development were identified as glycosylated dolichol phosphates, and they comprised between 1 and 3% of the total phospholipid fraction. Nineteen fatty acids were detected, and the fatty acid distribution for each lipid class at each stage was determined. Seven major fatty acids were common to each. These were: hexadecanoic, octadecanoic, oleic, linoleic, arachidonic, docosanoic, and docosahexaenoic.

Animals↗
PubMed1995

Tracking of serum lipids in children; association with the absolute lipid level--the cardiovascular risk in young Finns study.

We studied tracking of serum lipids in children and young adults (n = 248) during a 12-year follow-up. A novel method was developed for assessing tracking. It was based on the calculation of the absolute width of the individual track consisting of 5 serum lipid determinations. The total cholesterol (TC) track width tended to be broader in females compared to males (1.33 versus 1.24 standard deviation (SD) units). Approximately 35, 39, 24 and 13% of subjects had all 5 measurements within 1 SD (of the cohort mean) from serum TC, low density lipoprotein cholesterol, high density lipoprotein cholesterol and triglycerides, respectively. However, variations in excess of 2.5 SDs were not rare. We also assessed the dependence of tracking on the absolute lipid level by calculating the SD of the standardized 6-year change in a cohort of 2236 children. In each serum lipid variable studied, lowest degree of tracking was found in highest lipid values. The mean changes were also largest in extreme values due to regression toward mean phenomenon. Our findings indicate that the width of longitudinal track of serum lipids may be two wide to reliably detect high risk children. Further, the track width was even broader in subjects with extreme serum lipid values.

Adolescent↗
PubMed1995

Lipid A and the lipid A analogue anti-tumour compound ONO-4007 induce nitric oxide synthase in vitro and in vivo.

The ability of lipid A and the antitumour compound, ONO-4007 (sodium2-deoxy-2-[3S-(9-phenylnonanoyloxy)tetradecanoyl] amino-3-O-(9phenylnonanoyl)-D-glucopyranose 4-sulphate) to induce nitric oxide (NO) synthase was investigated in vitro and in vivo, in comparison to the effects of lipopolysaccharide and di- and monophosphoryl lipid A. In J744.2 macrophages, lipopolysaccharide, di-and monophosphoryl lipid A and ONO-4007 (10(-9) - 10(-5) g/ml) alone, or in combination with interferon-gamma, induced NO synthase (order of potency: lipopolysaccharide > diphosphoryl lipid A > monophosphoryl lipid A > ONO-4007). ONO-4007 increased the activity of the inducible NO synthase in the lung of anesthetised rats (20% of the increased caused by bacterial lipopolysaccharide). Thus, ONO-4007 is a weak inducer of the inducible isoform of NO synthase in vitro and in vivo. The finding that di- and monophosphoryl lipid A also induce NO synthase indicates that the lipid A moiety of lipopolysaccharide contributes to the induction of NO synthase by lipopolysaccharide. The induction of NO synthase by ONO-4007, resulting in the formation of cytotoxic NO may contribute to the antitumour activity of the compound.

Adult↗
PubMed2003

Triacsin C inhibits the formation of 1H NMR-visible mobile lipids and lipid bodies in HuT 78 apoptotic cells.

Nuclear magnetic resonance-visible mobile lipids (ML) have been reported to accumulate during cell apoptosis in vitro and in vivo. The biogenesis, biochemical nature and structure of these lipids are still under debate. In this study, a human lymphoblastoid cell line, HuT 78, was induced to apoptosis by exposure to anti-Fas monoclonal antibodies (alpha-Fas mAb) followed by incubation for different time intervals (1-24 h, hypodiploid cell fraction, H, varying from 1% to over 60%) either in the presence or in the absence of 5.0 microM Triacsin C (TRC), specific inhibitor of long-chain acyl-CoA synthetase (ACS). The increase of ML in apoptotic cells correlated linearly with H and was associated with: (a) accumulation of intracellular lipid bodies, detected by confocal laser scanning microscopy in lipophilic dye-stained cells; (b) increases, detected by thin-layer chromatography in total lipid extracts, in the relative abundance of triacylglycerides (TAG) and cholesteryl esters (CE), with corresponding decreases of phospholipids (PL). TRC completely abolished both ML and lipid body formation in anti-Fas-treated apoptotic cells, with concomitant reversion of TAG, CE and PL to control levels, but did not alter cell viability nor did it inhibit apoptosis. ML signals detected during anti-Fas-induced apoptosis therefore appear to originate from neutral lipids assembled in intracellular lipid bodies, synthesised from cellular acyl-CoA pools.

Antibodies, Monoclonal↗
PubMed2004

Investigations on the structure of solid lipid nanoparticles (SLN) and oil-loaded solid lipid nanoparticles by photon correlation spectroscopy, field-flow fractionation and transmission electron microscopy.

Recently, colloidal dispersions made from mixtures of solid and liquid lipids were described to combine controlled release characteristics with higher drug loading capacities than solid lipid nanoparticles (SLN). It has been proposed that these nanostructured lipid carriers (NLC) are composed of oily droplets which solubilize the drug and which are embedded in a solid lipid matrix. The structures of SLN and NLC based on glyceryl behenate and medium chain triglycerides were characterized by photon correlation spectroscopy (PCS) and laser diffraction (LD), field-flow fractionation (FFF) with multi-angle light scattering detection (MALS), and cryo transmission electron microscopy (cryo TEM). PCS indicates that SLN and NLC differ from a nanoemulsion with respect to Brownian motion due to asymmetric particle shapes. Non-spherical particles, in case of SLN and NLC, lead to higher polydispersity indices compared to the nanoemulsion. In FFF, the nanodroplets elute much earlier than SLN- and NLC-platelets although their PCS and LD data show similar particle sizes. In TEM platelet (for SLN), oil loaded platelet ("nanospoons"; for NLC) and droplet (for nanoemulsion) structures were observed. In contrast to literature reports, the investigated SLN appear as thin platelets. NLC are found to be lipid platelets with oil spots sticking on the surface. Very short diffusion pathways in platelets, increased water-lipid interfaces and low drug incorporation in crystalline lipids are the drawback of SLN and NLC compared to conventional nanoemulsions.

Chemical Phenomena↗
PubMed2004

Changes in serum lipoprotein lipids and their fatty acid compositions and lipid peroxidation in growing rats fed soybean protein versus casein with or without cholesterol.

OBJECTIVE: We compared the effects of diets based on soybean protein and casein supplemented or not supplemented with 0.1% cholesterol on plasma lipoprotein lipid amounts and their fatty acid compositions, lecithin:cholesterol acyl-transferase activity, and lipid peroxidation. METHODS: The composition and concentration of lipid and apolipoprotein in different lipoprotein classes, plasma LCAT activity, and lipid peroxidation were determined in rats fed 20% highly purified soybean protein or casein with or without 0.1% cholesterol for 2 mo. RESULTS: Soybean protein and casein diets with or without cholesterol had similar plasma total cholesterol concentrations. Soybean protein consumption diminished very low-density lipoprotein particle number, as measured by diminished contents of very low-density lipoprotein triacylglycerol, phospholipid, and apolipoprotein-B100. Lecithin:cholesterol acyl-transferase activity was not significantly modified by either protein. The soybean protein diet decreased the linoleate desaturation index (20:4[omega-6]/18:2[omega-6]) in liver and high-density lipoprotein fraction 2-3-phospholipids but enhanced red blood cell resistance against free radical attack. Addition of cholesterol to both protein diets decreased concentrations of high-density lipoprotein fraction 2-3 cholesterol. Lecithin:cholesterol acyl-transferase activity tended to be greater after cholesterol feeding, likely due to the enhanced high-density lipoprotein fraction 2-3 apolipoprotein-AI, a cofactor activator for lecithin:cholesterol acyl-transferase. Regardless of dietary protein source, cholesterol supplementation decreased the linoleate desaturation index in liver and plasma lipoprotein lipids and red blood cell resistance to free radical attack. CONCLUSIONS: Our results suggest that the dietary protein origin affects lipid peroxidation and polyunsaturated fatty acid biosynthesis and distribution among liver and different lipoprotein lipid classes, but plays only a minor role in the regulation of plasma and lipoprotein cholesterol concentrations. Providing dietary cholesterol (0.1%) with casein or soybean protein attenuates the effects of these proteins, with the exception of plasma cholesterol.

Animals↗
PubMed2003

Role of cholesterol in lipid raft formation: lessons from lipid model systems.

Biochemical and cell-biological experiments have identified cholesterol as an important component of lipid 'rafts' and related structures (e.g., caveolae) in mammalian cell membranes, and membrane cholesterol levels as a key factor in determining raft stability and organization. Studies using cholesterol-containing bilayers as model systems have provided important insights into the roles that cholesterol plays in determining lipid raft behavior. This review will discuss recent progress in understanding two aspects of lipid-cholesterol interactions that are particularly relevant to understanding the formation and properties of lipid rafts. First, we will consider evidence that cholesterol interacts differentially with different membrane lipids, associating particularly strongly with saturated, high-melting phospho- and sphingolipids and particularly weakly with highly unsaturated lipid species. Second, we will review recent progress in reconstituting and directly observing segregated raft-like (liquid-ordered) domains in model membranes that mimic the lipid compositions of natural membranes incorporating raft domains.

Animals↗
PubMed2003

Lipid-protein interactions in Escherichia coli membranes over-expressing the sugar-H(+) symporter, GalP EPR of spin-labelled lipids.

The D-galactose-H(+) symport protein (GalP) of Escherichia coli is a homologue of the human glucose transport protein, GLUT1. After amplified expression of the GalP transporter in E. coli, lipid-protein interactions were studied in gradient-purified inner membranes by using spin-label electron paramagnetic resonance (EPR) spectroscopy. Phosphatidylethanolamine, -glycerol, -choline and -serine, in addition to phosphatidic and stearic acids, were spin-labelled at the 14 C-atom of the sn-2 chain. EPR spectra of these spin labels at probe amounts in GalP membranes consist of two components. One component corresponds to a lipid population whose motion is restricted by direct interaction with the transmembrane sections of the integral protein. The other component corresponds to a lipid population with greater chain mobility, and is similar to the single-component EPR spectrum of the spin-labelled lipids in membranes of E. coli lipid extract. Quantitation of the protein-interacting spin-label component allows determination of the stoichiometry and selectivity of lipid-protein interactions. On average, approximately 20 mol of lipid are motionally restricted per 52 kDa of protein in GalP membranes. At the pH of the transport assay, there is relatively little selectivity between the different phospholipids tested. Only stearic acid displays a stronger preferential interaction with this protein.

Calcium-Binding Proteins↗
PubMed1997

Combination of antitumor ether lipid with lipids of complementary molecular shape reduces its hemolytic activity.

Because the therapeutic use of the antitumor ether lipid 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphorylcholine (ET-18-OCH3) is restricted by its hemolytic activity we explored the use of lipid packing parameters to reduce this toxicity by creating structurally optimized ET-18-OCH3 liposomes. We postulated that combination of ET-18-OCH3, which is similar in structure to lysophosphatidylcholine, with lipid molecules of complementary molecular shape (opposite headgroup/chain volume) would likely yield a stable lamellar phase from which ET-18-OCH3 exchange to red blood cell membranes would be curtailed. To quantitate the degree of shape complementarity, we used a Langmuir trough and measured the mean molecular area per molecule (MMAM) for monolayers comprised of ET-18-OCH3, the host lipids, and binary mixtures of varying mole percentage ET-18-OCH3. The degree of complementarity was taken as the reduction in MMAM from the value expected based on simple additivity of the individual components. The greatest degree of shape complementarity was observed with cholesterol: the order of complementarity for the ET-18-OCH3-lipid mixtures examined was cholesterol >> DOPE > POPC approximately DOPC. Phosphorus NMR and TLC analysis of aqueous suspensions of ET-18-OCH3 (40 mol%) with the host lipids revealed them to all be lamellar phase. For ET-18-OCH3 at 40 mol% in liposomes, the hemolytic activity followed the trend of the reduction in MMAM and was least for the ET-18-OCH3/cholesterol system (H50 = 661 microM ET-18-OCH3) followed by ET-18-OCH3/DOPE (H50 = 91 microM) and mixtures with POPC and DOPC which were comparable at H50 = 26 microM and 38 microM, respectively: the H50 concentration for free ET-18-OCH3 was 16 microM. This experimental strategy for designing optimized liposomes with a reduction in exchange, and hence toxicity, may be useful for other amphipathic/lipophilic drugs that are dimensionally compatible with lipid bilayers.

Antineoplastic Agents↗
PubMed2003

Lipid-gramicidin interactions: dynamic structure of the boundary lipid by 2D-ELDOR.

The use of 2D-electron-electron double resonance (2D-ELDOR) for the characterization of the boundary lipid in membrane vesicles of DPPC and gramicidin A' (GA) is reported. We show that 2D-ELDOR, with its enhanced spectral resolution to dynamic structure as compared with continuous-wave electron spin resonance, provides a reliable and useful way of studying lipid-protein interactions. The 2D-ELDOR spectra of the end-chain spin label 16-PC in DPPC/GA vesicles is composed of two components, which are assigned to the bulk lipids (with sharp auto peaks and crosspeaks) and to the boundary lipids (with broad auto peaks). Their distinction is clearest for higher temperatures and higher GA concentrations. The quantitative analysis of these spectra shows relatively faster motions and very low ordering for the end chain of the bulk lipids, whereas the boundary lipids show very high "y-ordering" and slower motions. The y-ordering represents a dynamic bending at the end of the boundary lipid acyl chain, which can then coat the GA molecules. These results are consistent with the previous studies by Ge and Freed (1999) using continuous-wave electron spin resonance, thereby supporting their model for GA aggregation and H(II) phase formation for high GA concentrations. Improved instrumental and simulation methods have been employed.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2003

Regulation of calcium channel activity by lipid domain formation in planar lipid bilayers.

The sarcoplasmic reticulum channel (ryanodine receptor) from cardiac myocytes was reconstituted into planar lipid bilayers consisting of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) in varying ratios. The channel activity parameters, i.e., open probability and average open time and its resolved short and long components, were determined as a function of POPE mole fraction (X(PE)) at 22.4 degrees C. Interestingly, all of these parameters exhibited a narrow and pronounced peak at X(PE) approximately 0.80. Differential scanning calorimetric measurements on POPE/POPC liposomes with increasing X(PE) indicated that the lipid bilayer enters a composition-driven transition from the liquid-crystalline state to the gel state at 22.4 degrees C when X(PE) approaches 0.80. Thus, the peaking of the reconstituted channel activity at X(PE) approximately 0.80 in the planar bilayer could result from the appearance of gel/liquid-crystalline domain boundaries at this POPE content. Lipid packing at domain boundaries is known to be looser as compared to the homogenous gel or liquid-crystalline state. We propose that the attractive potential of packing defects at lipid domain boundaries and entropic excluded-volume effects could result in the direct interactions of the transmembrane region of the channel protein with the lipid-packing defects at the lipid/protein interface, which could thus provide a favorable environment for the open state of the protein. The present findings indicate that the activity of the sarcoplasmic reticulum calcium channel could be modulated by lipid domain formation upon slight changes in membrane lipid composition in vivo.

Calcium Channels↗