Phase separations in membranes of Anacystis nidulans grown at different temperatures

Freeze fracture electron microscopy studies were performed on samples of Anacystis nidulans quenched from different temperatures. Membrane lipid phase separations were observed to take place over the ranges 15–30°C, 5–25°C and –5–15°C for cultures grown at 38, 28 and 18°C, respectively. Differential scanning calorimetry heating curves showed endotherms which coincided with these temperature ranges. Variations of phase separation temperatures with growth temperature, and hysteresis effects in the calorimetric measurements, were related to changes in the fatty acid composition of membrane lipids.     

Two qualitatively different structuro-functional states of mitochondria

Low (120 mosM) tonicity of incubation media of mitochondria was found to be associated with anomalous phase transition at 19--26 degrees C. A rise in temperature caused a decrease in the pyrene excitation in border lipids of the mitochondrial membrane. Within this temperature range the quenching of intrinsic protein fluorescence by pyrene was sharply decreased. It may be inferred from these data that at 100mosM tonicity and temperatures below 19 degrees C, mitochondrial membrane proteins are in an aggregated state. At temperatures above phase transition protein deaggregation takes place. It was shown that a decrease in tonicity from 300 to 120 mosM at 15 degrees C or a rise in temperature from 15 degrees to 37 degrees C at 300 mosM tonicity increased the phosphorylation of the 52 kDa mitochondrial protein. It was assumed that swelling of mitochondria in hypotonic media simulates one of the steps of the hormone-induced signal transfer in mitochondria in vivo.     

Seasonal changes in thermotropic behavior of phosphatidylcholine and phosphatidylethanolamine in different organs of the ascidian Halocynthia aurantium

Differential scanning calorimetry and polarising microscopy were used to investigate the crystal-liquid crystal-isotropic melt phase transitions of phosphatidylcholine (PC), and phosphatidylethanolamine (PE), isolated from muscles, gill pouches, gonads and digestive glands of Halocynthia aurantium, collected in summer and winter. We also analyzed the fatty chain composition of these phospholipids. In summer, the crystalline to liquid crystalline phase transitions of PC and PE from different organs were more co-operative than in winter. Their peak maximum temperatures were close and temperature ranges overlapped for summer samples. Peak maximum temperatures of winter samples decreased sharply, by 18-27 degrees C for PC and by 10-44 degrees C for PE, respectively, depending on the organ. Total heat changes of transitions also decreased. Thermograms were completely located at temperatures below -1.7 degrees C (minimal temperature of seawater in winter). In contrast to summer samples, peak maximum temperatures for PC and PE in winter differed significantly, (by 14-30 degrees C depending on organ), while the temperature ranges of their transitions still showed considerable overlap. Simultaneously, the temperature ranges of the liquid crystalline to isotropic phase transitions decreased. The main reason for changes in thermotropic behavior of phospholipids seems to be the decrease of saturated/unsaturated ratios. The existence of stable and thermoadaptative labile phospholipid pools in the membrane structure is proposed. The relationship of these transitions to low- and high-temperature adaptation is discussed.     

Effects of growth at different temperatures on the physical state of lipids in native microsomal membranes from Tetrahymena

Fluorescence measurements of the probe 1,6-diphenyl-1,3,5-hexatriene in native Tetrahymena pyriformis microsomal membranes revealed characteristic "break points" in curves of polarization vs. temperature. In the 5--35 degree C range, membranes from cells grown at 39 degrees C exhibited two break points, one at 11.6 +/- 0.6 degrees C and another at 23.1 +/- 1.6 degrees C. Membranes from 15 degrees C grown cells also showed two break points, one at 8.0 +/- 1.7 degrees C and another at 17.7 +/- 1.7 degrees C. Complementary measurements of turbidity (absorbance at 360 nm) vs. temperature revealed break points at approximately the same temperatures as observed with the fluorescent probe, thus strengthening the likelihood that the break points signify the onset or termination of lipid phase separations or some other significant structural alteration of lipids. In general, break points measured in the native membrane samples occurred at slightly lower temperatures than did break points in lipids extracted from comparable membranes. This suggests two possible types of protein--lipid interaction. First, there may be a selective withdrawal of relatively highly saturated phospholipid molecular species from the bulk lipid phase and into protein annulus regions. Alternatively, the configuration of the hydrophobic core of certain key membrane proteins may be such that nonspecific interactions with the lipids stabilize the liquid-crystalline phase.     

Phase Transition of Thylakoid Membranes Modulates Photoinhibition in the Cyanobacterium Anabaena siamensis

A shift of the growth temperature from 40 degrees C to 18 degrees C promoted an increase in the degree of fatty acids unsaturation and a decrease, from 26 degrees C to 0 degrees C, of the phase transition temperature of thylakoid membranes in Anabaena siamensis. The pattern of photoinhibition of photosynthesis at distinct temperatures varied as a function of the phase transition temperature. In the absence of streptomycin, a pronounced photoinhibition at temperatures near the phase transition (26 degrees C) was observed in cells grown at 40 degrees C, while protection from photodamage was observed at chilling temperatures (15 degrees C to 5 degrees C). In this same range of temperature, such a protection was not verified if cells were grown at 18 degrees C. In both types of cells, however, the rate of photoinactivation in the presence of streptomycin was progressively decreased by lowering the temperature of photoinhibition. When recovery from photoinhibition was followed at the respective temperature in which cells were grown, the restoration profile of the photosynthetic O(2) evolution to initial levels was essentially the same in both types of cells. The protective effect of low temperatures against photoinhibition was attributed to a decreased solubility and diffusion of oxygen in the thylakoid membranes due to an increase of the membrane viscosity that would avoid the photogeneration of reactive oxygen species around PS II.     

Cell separator operation within temperature ranges to minimize effects on Chinese hamster ovary cell perfusion culture

A cell retention device that provides reliable high-separation efficiency with minimal negative effects on the cell culture is essential for robust perfusion culture processes. External separation devices generally expose cells to periodic variations in temperature, most commonly temperatures below 37 degrees C, while the cells are outside the bioreactor. To examine this phenomenon, aliquots of approximately 5% of a CHO cell culture were exposed to 60 s cyclic variations of temperature simulating an acoustic separator environment. It was found that, for average exposure temperatures between 31.5 and 38.5 degrees C, there were no significant impacts on the rates of growth, glucose consumption, or t-PA production, defining an acceptable range of operating temperatures. These results were subsequently confirmed in perfusion culture experiments for average exposure temperatures between 31.6 and 38.1 degrees C. A 2(5-1) central composite factorial design experiment was then performed to systematically evaluate the effects of different operating variables on the inlet and outlet temperatures of a 10L acoustic separator. The power input, ambient temperature, as well as the perfusion and recycle flow rates significantly influenced the temperature, while the cell concentration did not. An empirical model was developed that predicted the temperature changes between the inlet and the outlet of the acoustic separator within +/-0.5 degrees C. A series of perfusion experiments determined the ranges of the significant operational settings that maintained the acoustic separator inlet and outlet temperatures within the acceptable range. For example, these objectives were always met by using the manufacturer-recommended operational settings as long as the recirculation flow rate was maintained above 15 L day(-1) and the ambient temperature was near 22 degrees C.     

Fluorescence depolarization study on non-bilayer phases of phosphatidylethanolamine and phosphatidylcholine lipid mixtures

The orientational order and rotational dynamics of 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3,5- hexatrienyl)phenyl]ethyl] carbon yl]-3-sn-phosphatidylcholine (DPH-PC) in dilinoleoylphosphatidylethanolamine (DLPE) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) binary lipid mixtures were investigated. A previous study (Biochim. Biophys. Acta 731 (1983) 177) indicated that the empirical phase diagram of POPC/DLPE can roughly be divided into three zones. They are the lamellar (15% PC and higher), intermediate (5-15% PC) and inverted hexagonal (0-5% PC) phases. As the lipids changed from the lamellar to intermediate phase, the order parameter increased at all temperatures (1-50 degrees C). On the contrary, the rotational diffusion decreased at high temperatures (20-50 degrees C) but increased at low temperatures (1-10 degrees C). These results indicate that the intermediate phase is in a stressed state at high temperatures but in a highly mobile amorphous state at low temperatures. As the lipid progressed from the intermediate toward hexagonal phase, the order parameter decreased abruptly at all temperatures. The ratio of order parameter in the intermediate phase to that in the hexagonal phase was calculated. This ratio was found to increase linearly with temperature, indicating that a distinct change in the packing symmetry of lipids occurred as temperature increased. From the intermediate to hexagonal phase, the rotational diffusion increased slightly at high temperatures but declined abruptly at low temperatures. These results further agreed with the stressed and amorphous natures of the intermediate phases as described above.     

Influence of white light on production of aflatoxins and anthraquinones in Aspergillus parasiticus.

The effect of continuous light and continuous darkness on the growth of Aspergillus parasiticus and on the production of aflatoxin, averufin, versicolorin A, and versicolorin C by Aspergillus parasiticus were determined at six different temperatures with six replicates for each experiment. No growth was observed at 15 degrees C in the light, although slight growth was observed at this temperature in the dark. No aflatoxins or anthraquinones were produced in the light or dark at 35 and 40 degrees C, although growth was good at these temperatures. Differences in aflatoxins and anthraquinones for cultures grown in light and in dark were consistent at each temperature. Higher mean quantities of these secondary metabolites were produced in the light at 20 and 25 degrees C; lower mean quantities were produced in the light at 30 degrees C. The ranges of values overlapped considerably, but in all cases the differences between temperatures were significant.     

The action of caffeine on X-irradiated HeLa cells. IX. Hypothermic effects

Hypothermic enhancement of the lethal effect of 3.5 Gy of 220-kV X rays in the absence of caffeine as well as in its presence (4 mM) was examined at temperatures between 10 and 34 degrees C in monolayer cultures in the G1 phase of the cell cycle. Correction has been made for the toxicity of low temperatures, and of caffeine at low temperatures, by concomitantly measuring cell killing in unirradiated cells. In the absence of caffeine, incubation of irradiated cells for up to 34 h at temperatures in the range 15 to 30 degrees C (or possibly 34 degrees C) enhances killing compared to that observed at 38 degrees C; the amount of enhancement is about the same throughout this range, but is nil at 10 degrees C. The enhanced killing induced by caffeine at 38 degrees C decreases as the temperature is lowered to 15 degrees C; there is no enhancement at 10 degrees C. Less killing is manifested in the range 15 to 25 degrees C in the presence of caffeine than in its absence. Recovery (loss of sensitivity to caffeine) and fixation of potentially lethal damage were studied in late-S/G2-phase cells at reduced temperatures by delaying treatment with caffeine for increasing times after irradiation. As the temperature is progressively lowered to 20 degrees C, less recovery is manifested after 5 h of incubation; no recovery is detected in the range 10 to 20 degrees C. Despite extensive recovery at 34 degrees C, no fixation is observed at that (or any lower) temperature in G2-phase cells: the cells are able to recover essentially fully when returned to 38 degrees C. In addition, responses of unirradiated control series to incubation at low temperatures appear to differ from those reported by others for longer treatment times of different cell systems.     

Temperature dependence of the apparent affinity and the maximum velocity of the membrane-bound monosaccharide transport system in the yeast Rhodotorula gracilis

Analysis of the temperature dependence of the monosaccharide transport system in the yeast Rhodotorula gracilis (ATCC 26194, CBS 6681), as tested with D-xylose, revealed that the apparent affinity of the transport system, measured as the reciprocal of the half-saturation constant KT, increased when transport velocity was stimulated by temperature (15--30 degrees C) and decreased when the rate of uptake was reduced at temperatures aboce 30 degrees C. Breaks in Arrhenius plots were accompanied by corresponding breaks in van't Hoff plots. Whereas untreated cells exhibited in the van't Hoff plot a discontinuity at 28--30 degrees C this was not observed in heat-treated cells (at either 37 or 45 degrees C). In heat-treated cells the maximum transport velocity was always lower and the apparent affinity higher than in untreated cells at the same temperature; the optimum temperature for both transport velocity and apparent affinity was shifted to higher values. The data are interpreted in terms of a reversible phase transition of membrane lipids effecting an irreversible alteration of membrane structure. The temperature-induced reversible alkalinization of unbuffered yeast suspensions supports this interpretation.     

Heat and cold shock protein synthesis in arctic and temperate strains of rhizobia.

We compared heat shock proteins (HSPs) and cold shock proteins (CSPs) produced by different species of Rhizobium having different growth temperature ranges. Several HSPs and CSPs were induced when cells of three arctic (psychrotrophic) and three temperate (mesophilic) strains of rhizobia were shifted from their optimal growth temperatures (arctic, 25 degrees C; temperate, 30 degrees C) to shock temperatures outside their growth temperature ranges. At heat shock temperatures, three major HSPs of high molecular weight (106,900, 83,100, and 59,500) were present in all strains for all shock treatments (29, 32, 36.4, 38.4, 40.7, 41.4, and 46.4 degrees C), with the exception of temperate strains exposed to 46.4 degrees C, in which no protein synthesis was detected. Cell survival of arctic and temperate strains decreased markedly with the increase of shock temperature and was only 1% at 46.4 degrees C. Under cold shock conditions, five proteins (52.0, 38.0, 23.4, 22.7, and 11.1 kDa) were always present for all treatments (-2, -5, and -10 degrees C) in arctic strains. Among temperate strains, five CSPs (56.1, 37.1, 34.4, 17.3, and 11.1 kDa) were present at temperatures down to 0 degrees C. The 34.4- and the 11.1-kDa components were present in all temperate strains at -5 degrees C and in one strain at -10 degrees C. Survival of all strains decreased with cold shock temperatures but was always higher than 50%. These results show that rhizobia can synthesize proteins at temperatures not permissive for growth. In all shock treatments, no correspondence between the number of HSPs or CSPs produced and rhizobial survival was found.(ABSTRACT TRUNCATED AT 250 WORDS)     

Heat and cold shock protein synthesis in arctic and temperate strains of rhizobia.

We compared heat shock proteins (HSPs) and cold shock proteins (CSPs) produced by different species of Rhizobium having different growth temperature ranges. Several HSPs and CSPs were induced when cells of three arctic (psychrotrophic) and three temperate (mesophilic) strains of rhizobia were shifted from their optimal growth temperatures (arctic, 25 degrees C; temperate, 30 degrees C) to shock temperatures outside their growth temperature ranges. At heat shock temperatures, three major HSPs of high molecular weight (106,900, 83,100, and 59,500) were present in all strains for all shock treatments (29, 32, 36.4, 38.4, 40.7, 41.4, and 46.4 degrees C), with the exception of temperate strains exposed to 46.4 degrees C, in which no protein synthesis was detected. Cell survival of arctic and temperate strains decreased markedly with the increase of shock temperature and was only 1% at 46.4 degrees C. Under cold shock conditions, five proteins (52.0, 38.0, 23.4, 22.7, and 11.1 kDa) were always present for all treatments (-2, -5, and -10 degrees C) in arctic strains. Among temperate strains, five CSPs (56.1, 37.1, 34.4, 17.3, and 11.1 kDa) were present at temperatures down to 0 degrees C. The 34.4- and the 11.1-kDa components were present in all temperate strains at -5 degrees C and in one strain at -10 degrees C. Survival of all strains decreased with cold shock temperatures but was always higher than 50%. These results show that rhizobia can synthesize proteins at temperatures not permissive for growth. In all shock treatments, no correspondence between the number of HSPs or CSPs produced and rhizobial survival was found.(ABSTRACT TRUNCATED AT 250 WORDS)     

Viability loss of neem (Azadirachta indica) seeds associated with membrane phase behaviour.

Storage of neem (Azadirachta indica) seeds is difficult because of their sensitivity to chilling stress at moisture contents (MC) > or =10% or imbibitional stress below 10% MC. The hypothesis was tested that an elevated gel-to-liquid crystalline phase transition temperature (Tm) of membranes is responsible for this storage behaviour. To this end a spin probe technique, Fourier transform infrared microspectroscopy, and electron microscopy were used. The in situ Tm of hydrated membranes was between 10 degrees C and 15 degrees C, coinciding with the critical minimum temperature for germination. During storage, viability of fresh embryos was lost within two weeks at 5 degrees C, but remained high at 25 degrees C. The loss of viability coincided with an increased leakage of K+ from the embryos upon imbibition and with an increased proportion of cells with injured plasma membranes. Freeze-fracture replicas of plasma membranes from chilled, hydrated axes showed lateral phase separation and signs of the inverted hexagonal phase. Dehydrated embryos were sensitive to soaking in water, particularly at low temperatures, but fresh embryos were not. After soaking dry embryos at 5 degrees C (4 h) plus 1 d of further incubation at 25 degrees C, the axis cells were structurally disorganized and did not become turgid. In contrast, cells had a healthy appearance and were turgid after soaking at 35 degrees C. Imbibitional stress was associated with the loss of plasma membrane integrity in a limited number of cells, which expanded during further incubation of the embryos at 25 degrees C. It is suggested that the injuries brought about by storage or imbibition at sub-optimal temperatures in tropical seeds whose membranes have a high intrinsic Tm (10-15 degrees C), are caused by gel phase formation.     

Explicit formula of finite difference method to estimate human peripheral tissue temperatures during exposure to severe cold stress

During cold exposure, peripheral tissues undergo vasoconstriction to minimize heat loss to preserve the maintenance of a normal core temperature. However, vasoconstricted tissues exposed to cold temperatures are susceptible to freezing and frostbite-related tissue damage. Therefore, it is imperative to establish a mathematical model for the estimation of tissue necrosis due to cold stress. To this end, an explicit formula of finite difference method has been used to obtain the solution of Pennes' bio-heat equation with appropriate boundary conditions to estimate the temperature profiles of dermal and subdermal layers when exposed to severe cold temperatures. The discrete values of nodal temperature were calculated at the interfaces of skin and subcutaneous tissues with respect to the atmospheric temperatures of 25 °C, 20 °C, 15 °C, 5 °C, −5 °C and −10 °C. The results obtained were used to identify the scenarios under which various degrees of frostbite occur on the surface of skin as well as the dermal and subdermal areas. The explicit formula of finite difference method proposed in this model provides more accurate predictions as compared to other numerical methods. This model of predicting tissue temperatures provides researchers with a more accurate prediction of peripheral tissue temperature and, hence, the susceptibility to frostbite during severe cold exposure.     

Factors affecting steroid and prostaglandin secretion by reproductive tissues of cycling and pregnant sows in vitro

Mitochondrial, microsomal and pellicular membranes were isolated from Tetrahymena cells grown at 39 degrees C or 15 degrees C, and phospholipids, in turn, were separated from total lipids extracted from these membranes. The effect of growth temperature on their solid-to-fluid phase transition temperature was examined by wide-angle X-ray diffraction. The transition temperatures of phospholipids from mitochondria, microsomes and pellicles were 21, 19 and 26 degrees C for cells grown at 39 degrees C and -8, -3 and 6 degrees C for cells grown at 15 degrees C, respectively. All phospholipids were found in a completely fluid state at these growth temperatures. From a comparison between the phospholipids and total lipids from pellicles of cells grown at 39 degrees C, a triterpenoid alcohol, tetrahymanol, caused the transition temperature to increase. The alignment of tetrahymanol in membranes was examined with pellicle'a total lipid oriented in a sample holder.     

Respiratory response to temperature and hypoxia in the zebra mussel Dreissena polymorpha

The effects of temperature acclimation, acute temperature variation and progressive hypoxia on oxygen consumption rates (VO2) were determined for the zebra mussel Dreissena polymorpha. In the first experiment, after acclimation to 5, 15 or 25 degrees C for at least 2 weeks, VO2 was determined at 5 degrees C increments from 5 to 45 degrees C. VO2 increased in all three acclimation groups from 5 to 30 degrees C, corresponding to the normal ambient temperature range for this species. Mussels displayed imperfect temperature compensation at temperatures above 15 degrees C, but exhibited little acclimatory ability below 15 degrees C. In the hypoxia experiment, VO2 was determined over the course of progressive hypoxia, from full saturation (oxygen tension [PO2]=160 Torr [21.3 kPa]) to a PO2 at which oxygen uptake ceased (<10 Torr [1.3 kPa]). Mussels were acclimated to either 5, 15 or 25 degrees C for at least 2 weeks and their respiratory response to progressive hypoxia was measured at three test temperatures (5, 15 and 25 degrees C). The degree of oxygen regulation increased with increasing test temperature, particularly from 5 to 15 degrees C, but decreased with increasing acclimation temperature. The decreased metabolic rate observed for warm-acclimated animals, particularly in the upper portion of the temperature range of the zebra mussel, may allow for conservation of organic energy stores during warm summer months. Compared to other freshwater bivalves, D. polymorpha is a relatively poor oxygen regulator, corresponding with its preference for well-oxygenated aquatic habitats. In addition, a new quantitative method for determining the degree of oxygen regulation is presented.     

The polymorphic phase behaviour of phosphatidylethanolamines of natural and synthetic origin. A 31P NMR study.

1. The polymorphic phase behaviour of aqueous dispersions of phosphatidylethanolamines isolated from human erythrocytes, hen egg yolk and Escherichia coli have been investigated employing 31P NMR techniques. All species exhibit well defined, reversible bilayer to hexagonal (H11) phase transitions as the temperature is increased. The temperatures at which these transition take place (10, 25--30 and 55--60 degrees C for erythrocyte, egg yolk and E. coli phosphatidylethanolamine, respectively) are sensitive to the fatty acid composition, occurring at a temperature up to 10 degrees C above the high temperature end of the hydrocarbon phase transition as detected by differential scanning calorimetry. In some cases the bilayer to hexagonal (H11) transitions may also be detected employing calorimetric techniques. 2. The addition of equimolar concentrations of cholesterol to these naturally occurring phosphatidylethanolamines does not dramatically affect the bilayer-hexagonal (H11) transition temperature, producing changes of up to 10 degrees C. 3. 18 : 1t/18 : 1t phosphatidylethanolamine undergoes the bilayer to hexagonal (H11) phase transition as the temperature is increased through the interval 50--55 degrees C. Alternatively, hydrated 12 : 0/12 : 0 phosphatidylethanolamine remains in the bilayer phase at temperatures up to 90 degrees C (50 degrees C above the hydrocarbon phase transition temperature). 4. The presence of 100 mM NaCl or 10 mM CaCl2 in aqueous dispersions of egg yolk phosphatidylethanolamine does not alter the temperature-dependent polymorphic phase behaviour significantly. However, at 40 degrees C, increasing the p2H above 8.0 results in progressive inhibition of the hexagonal (H11) phase and the appearance of a phase possibly of cubic structure at p2H 9.0. At p2H 10.0 the bilayer phase is preferred. 5. It is suggested that in biomembranes containing phosphatidylethanolamine as a majority species (such as that of E. coli) the fatty acid composition may primarily reflect the need to maintain bilayer structure. Alternatively, it is pointed out that in mammalian membranes such as that of the erythrocyte, phosphatidylethanolamine tends to destabilize bilayer structure. The resulting possibility that transitory non-bilayer lipid configurations may occur may be directly related to many important properties of biological membranes.     

Factors affecting carbohydrate and free amino acid content in overwintering larvae of Enosima leucotaeniella

Amounts of several metabolites were measured in overwintering larvae of Enosima leucotaeniella acclimated to temperatures between -5 and 15 degrees C for 30days. In the diapausing stage, cold hardiness, as shown by the survival rate, began rising below 15 degrees C. Glycogen content decreased as the temperature decreased from 10 to 0 degrees C. Trehalose content rose as the temperature decreased from 15 to 5 degrees C, but remained unchanged as the temperature decreased from 5 and 0 degrees C. Twenty-eight free amino acids were detected in the haemolymph; levels of proline, glutamine and glutamic acid increased at high temperatures, but alanine increased at low temperatures, especially as temperature decreased from 5 to 0 degrees C. Lipid content was unchanged by the different acclimation temperatures. The effects of temperature, diapause and aerobic conditions on the levels of carbohydrates and amino acids in overwintering larvae were analyzed. Alanine levels rose at low temperature only when the larvae were in the diapausing stage. The level of trehalose rose at low temperature in both the diapausing and post-diapausing stages, although it was higher at aerobic conditions in the post-diapausing stage. These results suggest that efficient trehalose synthesis occurs under the combination of low temperature and aerobic conditions of the post-diapausing stage, so that cold hardiness in overwintering E. leucotaeniella larvae may rise to a high level in winter.     

Effects of temperature acclimation on Neurospora phospholipids. Fatty acid desaturation appears to be a key element in modifying phospholipid fluid properties

Experiments were conducted on the effect of growth temperature on phospholipids of Neurospora. Strains grown at high (37 degrees C) and low (15 degrees C) temperatures show large differences in the proportions of phospholipid fatty acid alpha-linolenate (18 : 3) which can vary by 10-fold over this temperature range. Changes in the phospholipid base composition are less dramatic; the most significant is an increase in phosphatidylethanolamines at low temperatures accompanied by a concomitant decrease in phosphatidylcholine. It appears that phospholipid fatty acid desaturation is closely regulated with respect to growth temperature. Over the 37 to 15 degrees C growth temperature range there appear to be at least two desaturase systems in Neurospora which are under different controls. Production of 18 : 1 and 18 : 2 species appears to occur at high levels over the entire temperature range, whereas the production of 18 : 3 seems to be inversely related to growth temperature. Shifting 37 degrees C-acclimated cultures to 15 degrees C produces a growth lag period of approximately 3 h, during which the level of 18 : 3 increases markedly. Differential scanning calorimetry of phospholipids from 37 degrees C cells shows a phase transition at -22 degrees C while lipids from 15 degrees C cultures exhibit a phase transition with reduced enthalpy at about -41 degrees C. The data are consistent with the idea that phospholipid composition in Neurospora is under strict control and suggest that membrane fluidity is regulated with respect to growth temperature through changes in membrane lipid composition.     

Inverted hexagonal and cubic phases induced by alpha-tocopherol in fully hydrated dispersions of dilauroylphosphatidylethanolamine

The effect of alpha-tocopherol on the thermotropic phase behaviour and structure of aqueous dispersions of 1,2-di-lauryl-sn-glycero-3-phosphoethanolamine was examined by synchrotron X-ray diffraction. The pure phospholipid exhibited a lamellar gel to liquid-crystal phase transition at 30 degrees C on heating at 3 degrees C min(-1) between 10 degrees C and 90 degrees C. The transition was reversible with a temperature hysteresis of 0.3 degrees C on cooling. At temperatures less than 10 degrees C only lamellar gel phase of the pure phospholipid was seen in co-dispersions of up to 20 mol % alpha-tocopherol. The presence of 2.5 mol % alpha-tocopherol caused the appearance of inverted hexagonal phase at temperatures just below the main phase transition temperature that co-existed with the lamellar gel phase. The intensity of scattering from the hexagonal-II phase increased with increasing proportion of alpha-tocopherol in the mixture and in proportions greater than 10 mol % it persisted at temperatures above the main transition and co-existed with the lamellar liquid-crystal phase of the pure phospholipid. At higher temperatures all co-dispersions containing up to 15 mol % alpha-tocopherol showed the presence of cubic phases. These phases indexed a Pn3m or Pn3 space grouping. When the proportion of alpha-tocopherol was increased to 20 mol % the only non-lamellar phase observed was inverted hexagonal phase. This phase co-existed with lamellar gel and liquid-crystal phases of the pure phospholipid, but was the only phase present at temperatures >60 degrees C. The X-ray diffraction data were used to construct a partial phase diagram of the lipid mixture in excess water between 10 degrees and 90 degrees C and up to 20 mol % alpha-tocopherol in phospholipid.