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PubMed1982

Temperature-induced transitions of porcine intestinal brush border membranes.

Thermotropic transitions of the membrane components in porcine intestinal brush border membranes were studied by means of fluorimetry using a fluorogenic thiol reagent, N-[7-dimethylamino-4-methylcoumarinyl]maleimide (DACM), and a lipophilic fluorescent probe, 1,6-diphenyl-1,3,5-hexatriene (DPH). 1. The reactivity of the sulfhydryl groups of the membrane proteins with DACM was dependent on temperature, with a transition point at about 33 degrees C. A conspicuous transition was also observed in the relation between temperature and the fluorescence intensity of DACM-labeled membranes at 35 degrees C. 2. Temperature dependence profiles of the solubilization of DPH in the membranes and of the fluorescence polarization of DPH-membrane complex suggested that the phase transition of the lipid from gel to liquid-crystalline state occurs over a temperature range of 30 to 35 degrees C. 3. Efficient fluorescence energy transfer was observed from tryptophan residues of the membrane proteins to DPH located in the lipid phase of the membranes, and its efficiency was extremely enhanced, dependent on temperature, above 35 degrees C. The intensity of the tryptophan fluorescence of the membrane proteins decreased with increasing temperature and a discontinuity was observed at about 33 degrees C. Based on these results, it may be concluded that there are co-operative interactions between proteins and lipids in the membranes and that the temperature-induced conformational changes of the membrane proteins are closely related to the dynamics of the hydrocarbon cores of the lipid.

Animals↗
PubMed1994

A mechanochemical study of MgDNA fibers in ethanol-water solutions.

Highly oriented calf-thymus MgDNA fibers, prepared by a wet spinning method, were studied with a simple mechanochemical set-up. The relative fiber length, L/Lo, was measured with the fibers submerged in ethanol-water solutions. In one type of experiment L/Lo was measured as a function of ethanol concentration at room temperature. No substantial decrease in L/Lo with increasing ethanol concentration was observed, indicating that MgDNA fibers stay in the B form even when the water activity is very low. For low ethanol concentrations the fiber structure is stable and does not dissolve even at very high water activities. In a second type of experiment, the heat-induced helix-coil transition was manifested by a marked contraction of the fibers. The transition temperature decreases linearly with increasing ethanol concentration between 52 and 68% ethanol. At higher ethanol concentrations the helix-coil transition temperature increases due to strong aggregation within the DNA fibers, and above 77% ethanol the fibers do not contract at all, not even at the upper temperature limit of the experiments, approximately 80 degrees C. This behavior is discussed with reference to dried DNA and the P form of DNA. The helix-coil transition temperature of the MgDNA fibers in 70% ethanol does not show any dependence on the MgCl2 concentration. It is shown that the Poisson-Boltzmann cylindrical cell model can account qualitatively for this lack of salt dependence.

Animals↗
PubMed1982

Liposomal membranes. XI. A suggestion to structural characteristics of acido-thermophilic bacterial membranes.

To understand the role of omega-cyclohexyl fatty acid residue of lipids in acido-thermophilic bacterial membranes, three unusual phosphatidylcholines, 1, 2-di-11-cyclohexylundecanoyl-L-alpha-phosphatidylcholine (11CYPC), 1,2-di-13-cyclohexyltridecanoyl-L-alpha-phosphatidylcholine (13CYPc), and 1-13-cyclohexyltridecanoyl-2-11-cyclohexylundecanoyl-L-alpha- phosphatidylcholine (1-13CY-2-11CYPC) were prepared and the steady-state fluorescence anisotropy of 1, 6-diphenylhexatriene (DPH) in the hydrophobic domain of these liposomal bilayers was determined. Compared with the case of dipalmitoyl (DPPC) or dimyristoyl phosphatidylcholine (DMPC), introducing the omega-cyclohexyl moiety onto lecithins makes the bilayers fluid below the phase transition temperature, while immobilizes them above the phase transition temperatures. The properties of the unusual phosphatidylcholine liposomes suggested by the steady-state fluorescence anisotropy investigation were in good agreement with those obtained from the thermotropic and permeability investigations. Results obtained are discussed from the view point of the role and function of lipid membranes of acido-thermophilic bacteria which contain unusual fatty acids.

Bacillus↗
PubMed1988

Phase characteristics of positional isomers of 1,2-di(heptacosadiynoyl)-sn-glycero-3-phosphocholine; tubule-forming phosphatidylcholines.

We have examined the phase behavior of positional isomers of a polymerizable diacetylenic phospholipid, 1,2-di(heptacosadiynoyl)-sn-glycero-3-phosphocholine which has the diacetylene in varying position along the acyl chains. Upon cooling multilamellar vesicles (MLVs) through the liquid-crystalline to gel phase transition, all isomers examined spontaneously formed hollow, cylindrical microstructures (or tubules). Differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) have been used to characterize positional isomers of this lipid in an effort to understand the effect of diacetylenic position on the molecular characteristics of tubule formation. Calorimetric results indicate that moving the position of the diacetylene along the acyl chain results in the alternation of the exotherm observed for the hydrated transition temperature associated with tubule formation, with higher transition temperatures (Tm) observed from isomers with an even number of methylenes between the diacetylene groups and the glycerol backbone. As the diacetylene is moved toward either end of the acyl chain, even with the observed alternation, the Tm was observed to increase. Calorimetric results of dry members of this series reveal an exotherm during cooling, the same temperature at which fully hydrated samples form tubules. This suggests that there is little difference in the phase behavior observed upon cooling the hydrated tubules and the dry diacetylenic material. FTIR results support the high degree of conformational order observed in tubules of this isomer series as a very strong CH2 wagging progression is observed between 1375 and 1200 cm-1. In addition, the C-H stretch region (3000 cm-1 to 2800 cm-1) indicates tight acyl chain packing with many all-trans segments. These results provide further evidence that tubules are uniquely crystalline microstructures and that this inherent crystallinity, and the formation of tubules is not affected by diacetylenic position.

Calorimetry, Differential Scanning↗
PubMed2004

Effects of non-steroid anti-inflammatory drugs in membrane bilayers.

The thermal effects of non-steroidal anti-inflammatory drugs (NSAIDs) meloxicam, tenoxicam, piroxicam and lornoxicam have been studied in dipalmitoylphosphatidylcholine (DPPC) membrane bilayers using neutral and acidic environments (pH 2.5). The strength of the perturbing effect of the drugs is summarized to a lowering of the main phase transition temperature and a broadening of the phase transition temperature as well as broadening or abolishment of the pretransition of DPPC bilayers. The thermal profiles in the two environments were very similar. Among the NSAIDs studied meloxicam showed the least perturbing effect. The differential scanning calorimetry results (DSC) in combination with molecular modeling studies point out that NSAIDs are characterized by amphoteric interactions and are extended between the polar and hydrophobic segments of lipid bilayers. The effects of NSAIDs in membrane bilayers were also investigated using Raman spectroscopy. Meloxicam showed a gauche:trans profile similar to DPPC bilayers while the other NSAIDs increased significantly the gauche:trans ratio. In conclusion, both techniques show that in spite of the close structural similarity of the NSAIDs studied, meloxicam appears to have the lowest membrane perturbing effects probably attributed to its highest lipophilicity.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1993

Prolongation of the circulation time of doxorubicin encapsulated in liposomes containing a polyethylene glycol-derivatized phospholipid: pharmacokinetic studies in rodents and dogs.

The pharmacokinetics of doxorubicin (DOX) encapsulated in liposomes containing polyethylene glycol-derivatized distearoylphosphatidylethanolamine (PEG/DSPE) were investigated in rodents and dogs. The plasma levels of DOX obtained with PEG/DSPE-containing liposomes were consistently higher than those without PEG/DSPE or when PEG/DSPE was replaced with hydrogenated phosphatidylinositol (HPI). Despite the inclusion of PEG/DSPE in liposomes, there was a significant drop in the plasma levels of DOX when the main phospholipid component, hydrogenated phosphatidylcholine, was replaced with lipids of lower phase transition temperature (dipalmitoylphosphatidylcholine, egg phosphatidylcholine), indicating that phase transition temperature affects the pharmacokinetics of liposome-encapsulated DOX. In beagle dogs, clearance was significantly slower for DOX encapsulated in PEG/DSPE-containing liposomes than in HPI-containing liposomes, with distribution half-lives of 29 and 13 hr, respectively. In both instances, almost 100% of the drug measured in plasma was liposome-associated. The apparent volume of distribution was only slightly above the estimated plasma volume of the dogs, indicating that drug leakage from circulating liposomes is insignificant and that the distribution of liposomal drug is limited mostly to the intravascular compartment in healthy animals.

Animals↗
PubMed1983

Temperature-dependent transitions in isometric contractions of rat muscle.

The effect of temperature on tetanic tension development was examined in extensor digitorum longus (fast-twitch) and soleus (slow-twitch) muscles of the rat, in vitro and with direct stimulation. The temperature range was from 35 to 10 degrees C. 2. The maximum tetanic tension decreased slightly on cooling from 35 to 25 degrees C. Cooling below 20 degrees C resulted in a marked depression of tetanic tension. The results were similar in the two muscles. 3. Analysis (in the form of Arrhenius plots) of the rate of tetanic tension development and relaxation clearly showed the occurrence of two phases in their temperature dependence, due to an increased temperature sensitivity below about 25 degrees C. Arrhenius activation energy estimates for temperatures lower than 21 degrees C were around twice as high as those for temperatures higher than 24 degrees C in both muscles.

Animals↗
PubMed1984

Biochemical changes in mouse brain myelin during experimental primary amoebic meningo-encephalitis.

Myelin membranes were purified from normal as well as experimental primary amoebic meningo-encephalitis-afflicted mouse brains. Infected myelin fractions were observed to form a floating fraction in 0.32 M sucrose. Total proteins, total lipids, phospholipids, cholesterol and galactolipids decreased markedly due to amoebic infection. Phosphatidyl choline, phosphatidyl ethanolamine plus phosphatidyl serine and sphingomyelin were decreased with a concomitant increase in lysophosphatidyl choline, lysophosphatidyl ethanolamine plus lysophosphatidyl serine and phosphatidic acid. SDS-polyacrylamide gel electrophoresis of myelin proteins exhibited six major protein fractions. Marked depletion in proteolipid protein was observed; 5'-nucleotidase activity increased while Na,K-ATPase and Mg-ATPase activities decreased. Infected myelin Na,K-ATPase had lower Vmax and higher Km values than normal. Arrhenius plots of normal myelin Na,K-ATPase exhibited transition temperature at 22 degrees C while no transition temperature could be observed in case of infected myelin Na,K-ATPase.

Amebiasis↗
PubMed1989

Regulation of adenylate cyclase by cannabinoid drugs. Insights based on thermodynamic studies.

The abilities of lipophilic cannabinoid drugs to regulate adenylate cyclase activity in neuroblastoma cell membranes were analyzed by thermodynamic studies. Arrhenius plots of hormone-stimulated adenylate cyclase activity exhibited a break point at 20 degrees. The break point was reduced to 14 degrees by benzyl alcohol, consistent with results from other laboratories that have correlated this response with the increase in membrane fluidity induced by benzyl alcohol. Because cannabinoid drugs partition into membrane lipids and alter membrane fluidity parameters in a number of model systems, it was of interest to examine the influence of delta 9-tetrahydrocannabinol and cannabidiol on enzyme activity analyzed by the Arrhenius plot. delta 9-Tetrahydrocannabinol, known to inhibit adenylate cyclase, failed to decrease the transition temperature either at 1 microM or at concentrations exceeding its aqueous solubility (30 microM), suggesting that delta 9-tetrahydrocannabinol could not mimic the effects observed with benzyl alcohol. In contrast, 30 microM cannabidiol, which stimulated enzyme activity slightly, decreased the Arrhenius plot break point to 17.5 degrees. The decrease in the transition temperature in response to benzyl alcohol or cannabidiol was not accompanied by a change in activation energies above or below the transition temperature. delta 9-Tetrahydrocannabinol inhibits adenylate cyclase activity via Gi as does the muscarinic agonist carbachol (Howlett et al., Mol Pharmacol 29: 307-313, 1986). Both carbachol and delta 9-tetrahydrocannabinol decreased the enthalpy and entropy of activation. The net free energy of activation at 37 degrees was increased in the presence of both of these inhibitory agonists. These data suggest that, for the entropy-driven hormone-stimulated adenylate cyclase enzyme, less disorder of the system occurs in the presence of regulators that inhibit the enzyme via Gi. In summary, thermodynamic data suggest that cannabidiol can influence adenylate cyclase by increasing membrane fluidity, but that the inhibition of adenylate cyclase by delta 9-tetrahydrocannabinol is not related to membrane fluidization.

Adenylyl Cyclase Inhibitors↗
PubMed1999

Formulation of proteins in vacuum-dried glasses. II. Process and storage stability in sugar-free amino acid systems.

The purpose of this research was to investigate the freeze- and vacuum-drying behavior of L-amino acids of current/potential use as adjuvants for formulating proteins. The analytical methods used were wide-angle x-ray diffraction, differential scanning calorimetry, and scanning electron microscopy. Protein analysis was performed either as an activity assay (lactate dehydrogenase [LDH]) or by size-exclusion chromatography (granulocyte colony-stimulating factor [rhG-CSF]). After samples were freeze-dried, only the four basic amino acids (arginine, lysine, histidine, and citrulline) formed amorphous solids, which, however, were partially crystalline. The remaining amino acids all formed fully crystalline solids. After samples were vacuum-dried, (20 degrees C, 0.1 mbar, 1 ml fill volume in 2-ml vials) fully crystalline solids were formed by all of the amino acids. For arginine, the addition of either HCl, H3PO4, or H2SO4 sufficient to form the respective salt produced amorphous solids after vacuum-drying, but they had high residual water contents and low glass transition temperatures (Tg). Addition of phenylalanine to arginine base inhibited crystallization of the latter at low concentrations during vacuum-drying procedure, leading to formation of a pure rubbery solid. At higher concentrations the phenylalanine crystallized, producing dry products with glass transition temperatures of > 60 degrees C. The process and storage stability of LDH and rhG-CSF in the vacuum-dried phenylalanine/arginine glasses was greatly improved at temperatures up to 40 degrees C compared with the unprotected proteins. Uptake of moisture during storage was, however, a complicating factor, reducing Tg, promoting crystallization, and leading to decreased protein stability. The PO4 salt of arginine produced especially high glass transition temperatures after it was vacuum-dried. These sugar-free amino acid formulations thus are potential stabilizes for proteins.

Amino Acids↗
PubMed1990

Interaction of antimycobacterial and anti-pneumocystis drugs with phospholipid membranes.

Liposomes can be used as carriers of drugs in the treatment of viral, bacterial and protozoal infections. The potential for liposome-mediated therapy of Mycobacterium avium-intracellulare complex infections, one of the most common opportunistic infections in AIDS, is currently under study. Here, we have investigated the effect of the lipid-soluble antimycobacterial drugs ansamycin, clofazimine and CGP7040 on the thermotropic behavior of liposomes composed of dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylglycerol (DPPG) using differential scanning calorimetry (DSC). In the presence of ansamycin (rifabutine), the peak gel-liquid crystalline phase transition temperature (Tm) of DPPG was reduced, as was the sub-transition temperature (Ts), whereas the Tm of DPPC was reduced only slightly. The temperature of the pre-transition (Tp) of DPPC was lowered, while the pre-transition of DPPG was abolished. Ansamycin also caused the broadening of the transition endotherm of both lipids. Equilibration of the drug/lipid complex for 1 or 5 days produced different thermotropic behavior. In the presence of clofazimine, the cooperativity of the phase transition of DPPG decreased. Above 10 mol% clofazimine formed two complexes with DPPG, as indicated by two distinguishable peaks in DSC thermograms. The Tm of both peaks were lowered as the mole fraction increased. Clofazimine had minimal interaction with DPPC. In contrast, CGP7040 interacted more effectively with DPPC than with DPPG, causing a reduction of the size of the cooperative unit of DPPC even at 2 mol%. The main transition of DPPC split into 3 peaks at 5 mol% drug. The pre-transition was abolished at all drug concentrations and the sub-transition disappeared at 10 mol% CGP7040. These studies suggest that maximal encapsulation of clofazimine in liposomes would require a highly negatively charged membrane, while that of CGP7040 would necessitate a zwitterionic membrane. We have also investigated the interaction of the water-soluble antibiotic pentamidine, which has been used against Pneumocystis carinii, the most lethal of AIDS-related opportunistic pathogens. Aerosol administration of this drug leads to long-term sequestration of the drug in the lungs. The DPPG/pentamidine complex exhibited a pre-transition at 3.5 degrees C, an endothermic peak at 42 degrees C, and an exothermic peak at 44.5 degrees C, followed by another endothermic peak at 55 degrees C. The exotherm depended on the history of the sample, requiring pre-incubation for several minutes below the 42 degrees C transition. These observations suggest that upon melting of the DPPG chains at 42 degrees C, the DPPG crystallizes as a DPPG/pentamidine complex that melts at 55 degrees C.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1982

Organization of the glycosphingolipid asialo-GM1 in phosphatidylcholine bilayers.

An affinity purified monovalent ferritin conjugate of Ricinus communis agglutinin (RCA 60) is used with freeze-etch electron microscopy to study the ultrastructural localization of the glycosphingolipid asialo-GM1 in multilamellar phosphatidylcholine liposomes. Dimyristoylphosphatidylcholine (DMPC) liposomes containing up to 20 mol% asialo-GM1 and quenched below the main transition temperature show a striking linear localization of ferritin-RCA 60 between phospholipid ridges. The glycosphingolipid localization is similar to that postulated for up to 20 mol% cholesterol in pure phosphatidylcholine bilayers by Copeland, B.R. and McConnell, H.M. (Biochim. Biophys. Acta, 599, 95-109 (1980)). Above the main phase transition temperature, asialo-GM1 appears to be organized into clusters, especially in palmitoyloleoylphosphatidylcholine (POPC) liposomes. This clustered distribution of glycosphingolipids seen above the phase transition temperature suggests that this type of lipid may exhibit compositional domain structure in biological membranes.

Chemical Phenomena↗
PubMed1989

Dependence of the bilayer to hexagonal phase transition on amphiphile chain length.

Several series of amphiphiles of increasing chain length were tested for their abilities to modify the L alpha-HII transition of dielaidoylphosphatidylethanolamine using differential scanning calorimetry. Acylcarnitines, alkyl sulfates, alkylsulfobetaines, and phosphatidylcholines, with chain lengths between about 6 and 12 carbon atoms, show an increasing capacity to raise the L alpha-HII phase transition temperature of phosphatidylethanolamine. This is ascribed to increased partitioning of the added amphiphile from water into the membrane as the chain length increases. Alkyl sulfates and alkyltrimethylammonium bromides have diminished capacities to raise the L alpha-HII transition temperature as the chain length is increased from 12 to 16. This is caused by an increase in the hydrophobic portion of the amphiphile leading to a change in the intrinsic radius of curvature and a decrease in the hydrocarbon packing constraints in the HII phase relative to the shorter chain amphiphiles. The L alpha-HII transition temperature of phosphatidylethanolamine with acylcarnitines of chain length 14-20 carbon atoms, alkylsulfobetaines above 14 carbon atoms, and phosphatidylcholines with acyl groups having above 10 carbon atoms is relatively insensitive to chain length. We suggest that this is caused by a balance between increasing hydrocarbon volume promoting the HII phase through decreased intrinsic radius of curvature and greater relief of hydrocarbon packing constraints vs greater intermolecular interactions favoring the more condensed L alpha phase. This latter effect is more important for amphiphiles with large headgroups which can pack more efficiently in the L alpha phase. The phosphatidylcholines show a gradual decrease in bilayer stabilization between 10 and 22 carbon atoms.(ABSTRACT TRUNCATED AT 250 WORDS)

Calorimetry, Differential Scanning↗
PubMed2002

Superconductivity in compressed lithium at 20 K.

Superconductivity at high temperatures is expected in elements with low atomic numbers, based in part on conventional BCS (Bardeen-Cooper-Schrieffer) theory. For example, it has been predicted that when hydrogen is compressed to its dense metallic phase (at pressures exceeding 400 GPa), it will become superconducting with a transition temperature above room temperature. Such pressures are difficult to produce in a laboratory setting, so the predictions are not easily confirmed. Under normal conditions lithium is the lightest metal of all the elements, and may become superconducting at lower pressures; a tentative observation of a superconducting transition in Li has been previously reported. Here we show that Li becomes superconducting at pressures greater than 30 GPa, with a pressure-dependent transition temperature (T(c)) of 20 K at 48 GPa. This is the highest observed T(c) of any element; it confirms the expectation that elements with low atomic numbers will have high transition temperatures, and suggests that metallic hydrogen will have a very high T(c). Our results confirm that the earlier tentative claim of superconductivity in Li was correct.

Journal Article↗
PubMed1997

Fever, temperature, and the immune response.

Fever's ability to manipulate the character and extent of physiological temperature gradients correlates with the unusual influence different physiological temperatures have upon model immune responses in vitro. This relationship may help to explain the remarkable evolutionary conservation of the febrile response to infection. A very restricted range of the upper physiological temperatures supports the activation of resting lymphocytes for proliferation and effector formation in the two major limbs of the immune system, cell-mediated immunity and humoral immunity. In contrast, once effectors are formed they can function in a fashion which is nearly independent of physiological temperature. This suggests that physiological temperature change acts to regulate the emergence of new immune responses but does not restrict the activity of existing effector mechanisms once they have been formed. The differential sensitivity of these processes to different physiological temperatures suggests that fever's biological purpose with respect to the immune system is the elimination of lower peripheral tissue temperatures rather than the elevation of core temperatures. However, further studies may reveal that some functions are amplified by the core temperature transitions while other functions are selectively regulated by peripheral tissue temperature transitions. The critical cell for the temperature dependence of immune responses seems to be the Th since its ability to produce cytokines is highly temperature dependent. In contrast, macrophages produce cytokines equally well at all temperatures except those of the febrile core, a feature which may serve to downregulate the production of endogenous pyrogens.

Body Temperature↗
PubMed1991

Studies on the effect of dietary fish oil on the physical and chemical properties of low density lipoproteins in cynomolgus monkeys.

To determine the effect of isocaloric substitution of dietary fish oil for lard on the physical and chemical properties of plasma low density lipoproteins (LDL), ten adult male cynomolgus monkeys were fed diets containing 11% (by weight) fish oil or lard in a crossover study consisting of two 15-week periods with a 6-week washout period in between. The atherogenic diets contained 40% of calories as fat with 0.26 mg cholesterol/kcal. Periodic measurements of plasma lipids were made throughout the study and a large blood sample was taken near the end of each 15-week period for LDL isolation and characterization, and for quantification of plasma apolipoproteins. Values for both studies were combined (mean +/- SE; n = 10) by diet. Significantly lower high density lipoprotein (HDL) cholesterol (28 +/- 2 vs. 57 +/- 8 mg/dl), apoA-I (53 +/- 11 vs. 88 +/- 7 mg/dl), and apoE (4.2 +/- 0.9 vs. 8.2 +/- 1.5 mg/dl) concentrations were found when the animals were consuming the fish oil versus the lard diet, respectively, but total plasma cholesterol (408 +/- 35 vs. 416 +/- 14 mg/dl), LDL cholesterol (356 +/- 34 vs. 331 +/- 17 mg/dl), and apoB (227 +/- 35 vs. 205 +/- 23 mg/dl) levels were not affected. LDL size was smaller during fish oil feeding (4.2 +/- 0.1 vs. 4.9 +/- 0.1 g/mumol) and LDL particle concentration was greater (2.3 +/- 0.2 vs. 1.8 +/- 0.1 microM). During fish oil feeding LDL cholesteryl esters (CE) and phospholipids (PL) were enriched in n-3 fatty acids and were relatively poor in 18:1 and 18:2 LDL CE transition temperature was about 11 degrees C lower during fish oil feeding (32 +/- 1 vs. 44 +/- 0.5 degrees C) and was positively correlated with the number of saturated, monoun-saturated, and n-6 polyunsaturated CE molecules per LDL. The results suggested that the range of transition temperatures among individual animal LDL was primarily determined by the number of monounsaturated CE, and the accumulation of n-3 polyunsaturated CE in LDL during fish oil feeding uniformly lowered the transition temperature of the LDL particle. There was a significant decrease in the percentage of LDL phosphatidylcholine (59 +/- 1 vs. 72 +/- 1%) and an increase in lysophosphatidylcholine (13 +/- 1 vs. 5 +/- 1%) and sphingomyelin (22 +/- 1 vs. 17 +/- 1%) during fish oil feeding relative to that of lard.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2002

Sphingomyelinase cleavage of sphingomyelin in pure and mixed lipid membranes. Influence of the physical state of the sphingolipid.

Sphingomyelin hydrolysis by sphingomyelinase is essential in regulating membrane levels of ceramide, a well-known metabolic signal. Since natural sphingomyelins have a gel-to-fluid transition temperature in the range of the physiological temperatures of mammals and birds, it is important to understand the influence of the physical state of the lipid on the enzyme activity. With that aim, large unilamellar vesicles consisting of pure egg sphingomyelin (gel-to-fluid crystalline transition temperature ca. 39 degrees C) were treated with sphingomyelinase in the temperature range 10-70 degrees C. The vesicles were also examined by differential scanning calorimetry (DSC). Shingomyelinase was active on pure sphingomyelin bilayers, leading to concomitant lipid hydrolysis, vesicle aggregation, and leakage of aqueous liposomal contents. Enzyme activity was found to be much higher when the substrate was in the fluid than when it was in the gel state. Sphingomyelinase activity was found to exhibit lag times, followed by bursts of activity. Lag times decreased markedly when the substrate went from the gel to the fluid state. When egg phosphatidylcholine, or egg phosphatidylethanolamine were included in the bilayer composition together with sphingomyelin, sphingomyelinase activity at 37 degrees C, that was negligible for the pure sphingolipid bilayers, was seen to increase with the proportion of glycerophospholipid, while the latency times became progressively shorter. A DSC study of the mixed-lipid vesicles revealed that both phosphatidylcholine and phosphatidyletanolamine decreased in a dose-dependent way the transition temperature of sphingomyelin. Thus, as those glycerophospholipids were added to the membrane composition, the proportion of sphingomyelin in the fluid state at 37 degrees C increased accordingly, in this way becoming amenable to rapid hydrolysis by the enzyme. Thus sphingomyelinase requires the substrate in bilayer form to be in the fluid state, irrespective of whether this is achieved through a thermotropic transition or by modulating bilayer composition.

Bacillus cereus↗
PubMed1997

Permeability of acetic acid across gel and liquid-crystalline lipid bilayers conforms to free-surface-area theory.

Solubility-diffusion theory, which treats the lipid bilayer membrane as a bulk lipid solvent into which permeants must partition and diffuse across, fails to account for the effects of lipid bilayer chain order on the permeability coefficient of any given permeant. This study addresses the scaling factor that must be applied to predictions from solubility-diffusion theory to correct for chain ordering. The effects of bilayer chemical composition, temperature, and phase structure on the permeability coefficient (Pm) of acetic acid were investigated in large unilamellar vesicles by a combined method of NMR line broadening and dynamic light scattering. Permeability values were obtained in distearoylphosphatidylcholine, dipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine, and dilauroylphosphatidylcholine bilayers, and their mixtures with cholesterol, at various temperatures both above and below the gel-->liquid-crystalline phase transition temperatures (Tm). A new scaling factor, the permeability decrement f, is introduced to account for the decrease in permeability coefficient from that predicted by solubility-diffusion theory owing to chain ordering in lipid bilayers. Values of f were obtained by division of the observed Pm by the permeability coefficient predicted from a bulk solubility-diffusion model. In liquid-crystalline phases, a strong correlation (r = 0.94) between f and the normalized surface density sigma was obtained: in f = 5.3 - 10.6 sigma. Activation energies (Ea) for the permeability of acetic acid decreased with decreasing phospholipid chain length and correlated with the sensitivity of chain ordering to temperature, [symbol: see text] sigma/[symbol: see text](1/T), as chain length was varied. Pm values decreased abruptly at temperatures below the main phase transition temperatures in pure dipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine bilayers (30-60-fold) and below the pretransition in dipalmitoylphosphatidylcholine bilayers (8-fold), and the linear relationship between in f and sigma established for liquid-crystalline bilayers was no longer followed. However, in both gel and liquid-crystalline phases in f was found to exhibit an inverse correlation with free surface area (in f = -0.31 - 29.1/af, where af is the average free area (in square angstroms) per lipid molecule). Thus, the lipid bilayer permeability of acetic acid can be predicted from the relevant chain-packing properties in the bilayer (free surface area), regardless of whether chain ordering is varied by changes in temperature, lipid chain length, cholesterol concentration, or bilayer phase structure, provided that temperature effects on permeant dehydration and diffusion and the chain-length effects on bilayer barrier thickness are properly taken into account.

1,2-Dipalmitoylphosphatidylcholine↗