Phase transitions of phospholipid bilayers from an unsaturated fatty acid auxotroph of Escherichia coli

Total phospholipids were extracted from cells of temperature sensitive unsaturated fatty acid auxotrophs of Escherichia coli (K-12 UFA^ts) grown at 28°C (PL28), and at 42°C in the presence of 2% KCl as an osmotic stabilizer (PL42 (KCl)). From the analysis of fatty acids, it was shown that the content of unsaturated fatty acids of PL42 (KCl) is only 9% of the total fatty acids, while that of PL28 is 54%. The thermal phase transitions of the bilayers prepared from the phospholipid fractions were studied by proton magnetic resonance. The line widths of the methylene signals and the sums of the methylene and methyl signal intensities were plotted against reciprocal values of absolute temperature 1/T or temperature itself. From the plots phase transitions were detected at about 19°C for PL28 and at 43°C for PL42 (KCl). In spite of its complex composition of fatty acids a highly cooperative transition was observed in the case of PL42 (KCl). It was also suggested that the phospholipids bilayers in the biomembranes of this strain at the growth temperature (42°C) are in the state where the gel and liquid crystalline phases coexist.     

Alteration in the contents of unsaturated fatty acids in dnaA mutants of Escherichia coli

DnaA protein, the initiator of chromosomal DNA replication in Escherichia coli , has a high affinity for acidic phospholipids containing unsaturated fatty acids. We have examined here the fatty acid composition of phospholipids in dnaA mutants. A temperature-sensitive dnaA46 mutant showed a lower level of unsaturation of fatty acids (ratio of unsaturated to saturated fatty acids) at 42°C (non-permissive temperature) and at 37°C (semi-permissive temperature), but not at 28°C (permissive temperature), compared with the wild-type strain. Plasmid complementation analysis revealed that the dnaA46 mutation is responsible for the phenotype. Other temperature-sensitive dnaA mutants showed similar results. On the other hand, a cold-sensitive dnaAcos mutant, in which overinitiation of DNA replication occurs at low temperature (28°C), showed a higher level of unsaturation of fatty acids at 28°C. Based on these observations, we discuss the role of phospholipids in the regulation of the activity of DnaA protein.     

An essential role of phosphatidylglycerol in the formation of the osmotically stable liposomes of Escherichia coli phospholipids

A temperature sensitive auxotroph of Escherichia coli K-12 requiring unsaturated fatty acids can grow normally at 28 degrees C, but requires an osmotic stabilizer such as a high amount of salt or sugar in the medium for the growth at 42 degrees C. Namely, the apparent osmotic stability of the cells at 28 degrees C and 42 degrees C is quite different. The osmotic properties of liposomes of the phospholipids extracted from these cells were investigated. The osmotically induced volume change of the multilamellar liposomes was examined by the turbidimetric method. The liposomes prepared from cells grown at 28 degrees C can swell and shrink under a wide range of hypo-and hypertonic conditions. However, those from cells grown at 42 degrees C could not swell under hypotonic conditions. These results exhibit a good correlation between the apparent osmotic stability of E. coli cells and the osmotic properties of the liposomes prepared from the extracted total phospholipids. To clarify the role of each phospholipid component, the osmotic properties of the liposomes reconstituted from the purified phospholipid species were further investigated. The results clearly showed that phosphatidylglycerol is the key factor that stabilizes the membranes of E. coli phospholipids against osmotic pressure.     

Thermal regulation of the fatty acid composition of lipopolysaccharides and phospholipids of Proteus mirabilis.

The fatty acid composition of the lipid A moiety of the lipopolysaccharide and phospholipid fractions of Proteus mirabilis changed significantly on varying the growth temperature. A decrease in the growth temperature from 43 degrees C to 15 degrees C resulted in a decrease in the palmitic acid content of the lipopolysaccharide from 19.4% of total fatty acids at 43 degrees C to 1.4% at 15 degrees C, and by the appearance of an unsaturated fatty acid residue, hexadecenoic acid. Changes in the 3-hydroxy-myristic acid content of the lipid A were minimal. The decrease in the growth temperature also resulted in a decrease in the saturated fatty acid content of the phospholipid fraction, which was accompanied by an increase in their fluidity, as measured by the freedom of motion of spin-labeled fatty acids incorporated into dispersions made of the phospholipids. Nevertheless, the fluidity obtained with membrane phospholipids extracted from the cells grown at various temperatures were essentially the same when fluidity was determined at the growth temperature, supporting the hypothesis that variations in the fatty acid composition of membrane phospholipids serve to produce membranes having a constant fluidity at different temperatures of growth.     

Effects of ambient temperature on lipid and fatty acid composition in the oviparous lizards, Phrynocephalus przewalskii

This study was designed to assess the effect of ambient temperature on lipid content, lipid classes and fatty acid compositions of heart, liver, muscle and brain in oviparous lizards, Phrynocephalus przewalskii, caught in the desert area of China. Significant differences could be observed in the contents of the total lipid and fatty acid compositions among different temperatures (4, 25 and 38 degrees C). The study showed that liver and muscle were principal sites of lipid storage. Triacylglycerol (TAG) mainly deposited in the liver, while phospholipids (PL) was identified as the predominant lipid class in the muscle and brain. Palmitic and stearic acid generally occupied the higher proportion in saturated fatty acids (SFA), while monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) consisted mainly of 16:1n-7, 18:1n-9, 18:2n-6, 18:3n-3, 20:4n-6 and 22:6n-3 regardless of tissue and temperature. These predominant fatty acids proportion fluctuations caused by temperature affected directly the ratio of unsaturated to saturated fatty acids. There was a tendency to increase the degree of unsaturation in the fatty acids of TAG and PL as environmental temperature dropped from 38 to 4 degrees C, although the different extent in different tissues. These results suggested that lipid characteristics of P. przewalskii tissues examined were influenced by ambient temperature.     

Alteration of fatty acid composition of LM cells by lipid supplementation and temperature.

Alteration of the fatty acid composition of monolayer cultures of LM cells grown in chemically defined medium was achieved by supplementation with fatty acids complexed to bovine serum albumin. Phospholipids containing up to 40% linoleate were found in cells grown in medium containing 20 mu g of linoleate/ml. Incorporation of linoleate into phospholipids reached a plateau after 12-24 hr, and cells remained viable for at least 3-4 days. Although linoleic, linolenic, and arachidonic acids were incorporated into LM cells equally well, only the latter was elongated by these cells under these experimental conditions. Nonadecanoic acid was incorporated to a lesser extent than the polyunsaturated fatty acids. Phosphatidylcholine and phosphatidylethanolamine of LM cells had different fatty acid compositions; phosphatidylethanolamine contained more longer chain and unsaturated fatty acids. Cells were also grown in the absence of choline and presence of choline analogs such as N,N-dimethylethanolamine, N-methylethanolamine, 3-amino-1-propanol, and 1-2-amino-1-butanol. The analog phospholipids in these cells had fatty acid compositions which were intermediate between those of phosphatidylethanolamine and phosphatidylcholine of control cells grown in the presence of choline. Linoleate was found in both phosphatidylcholine and phosphatidylethanolamine of cells supplemented with linoleate. The sphingolipid fraction of these cells, however, did not contain significant amounts of linoleate. When linoleate was present in the phospholipids, compensatory decreases in the oleate and palmitoleate content of phospholipids were observed. Lowering of the growth temperature to 28 degrees produced an increase in unsaturate fatty acid content of the phospholipids. When linoleate was supplied to cells grown at 28 degrees, there was no further increase in the unsaturated fatty acid composition of the phospholipids. Using both fatty acid supplementation and lowered growth temperature, LM cell membranes can be produced which have phospholipids with vastly different fatty acid compositions.     

Properties and biosynthesis of cyclopropane fatty acids in Escherichia coli.

The lipid phase transition of Escherichia coli phospholipids containing cyclopropane fatty acids was compared with the otherwise homologous phospholipids lacking cyclopropane fatty acids. The phase transitions (determined by scanning calorimetry) of the two preparations were essentially identical. Infection of E. coli with phage T3 inhibited cyclopropane fatty acid formation over 98%, whereas infection with mutants which lack the phage coded S-adenosylmethionine cleavage enzyme had no effect on cyclopropane fatty acid synthesis. These data indicate that S-adenosylmethionine is the methylene in cyclopropane fatty acid synthesis.     

The effects of temperature on the composition and physical properties of the lipids of Pseudomonas fluorescens

1. Pseudomonas fluorescens was grown at various temperatures between 5 degrees C and 33 degrees C. The extractable lipids from organisms at various stages of growth and grown at different temperatures were examined. 2. The extractable lipids contained phosphatidylethanolamine, diphosphatidylglycerol, phosphatidylglycerol, phosphatidylcholine, and an ornithine-containing lipid. The relative amounts of these lipids did not vary significantly during growth or with the changes in growth temperature. 3. The major fatty acids were hexadecanoic, hexadecenoic and octadecenoic acids and the cyclopropane acids methylene-hexadecanoic and methylene-octadecanoic acids. The relative amount of unsaturated acids (including cyclopropane acids) did not change significantly during growth, but increased with decreasing temperature. 4. Phosphatidylethanolamines with different degrees of unsaturation and containing different amounts of cyclopropane acids were isolated from organisms grown at 5 degrees C and 22 degrees C and their surface and phase behaviour in water was investigated. Thermodynamic parameters for fusion and monolayer results for cyclopropane and other fatty acids were examined. 5. The surface pressure-area isotherms of phosphatidylethanolamines containing different amounts of unsaturated fatty acids show small differences but the individual isotherms remain essentially unchanged over the temperature range 5-22 degrees C. X-ray-diffraction methods show that the structures (lamellar+hexagonal) formed in water by phosphatidylethanolamine, isolated from organisms grown at 5 degrees C and 22 degrees C, are identical when compared at the respective growth temperatures. This points to a control mechanism of the physical state of the lipids that is sensitive to the operating temperature of the organism. 6. The molecular packing of cyclopropane acids is intermediate between that of the corresponding cis- and trans-monoenoic acids. However, substitution of a cyclopropane acid for a cis-unsaturated acid has insignificant effects on the molecular packing of phospholipids containing these acids.     

The impact of abiotic factors (temperature and glucose) on physicochemical properties of lipids from Yersinia pseudotuberculosis

The impact of the availability of glucose in nutrition medium and growth temperature on the composition and thermotropic behavior of lipids from Yersinia pseudotuberculosis (Enterobacteriaceae) was studied. Y. pseudotuberculosis was grown in nutrition broth (NB) with/without glucose at 8 and 37 degrees C, corresponding to the temperatures of saprophytic and parasitic phases of this bacterium life. The decrease of phosphatidylethanolamine, phosphatidylglycerol and unsaturated fatty acids and the parallel increase of lysophosphatidylethanolamine and diphosphatidylglycerol and saturated and cyclopropane acids were the most significant changes with temperature in bacterial phospholipid (PL) classes and fatty acids, respectively. Glucose did not effect the direction of temperature-induced changes in the contents of PLs, fatty acids, however it enhanced (for PLs) or diminished (for fatty acids) intensity of these changes. The thermally induced transitions of lipids were studied by differential scanning calorimetry (DSC). It was revealed that the addition of glucose to NB induced a sharp shift of DSC thermograms to lower temperatures in the "warm" variants of bacteria. The peak maximum temperature (Tmax) of thermal transitions dropped from 50 to 26 degrees C that is the optimal growth temperature of Y. pseudotuberculosis. Tmax of total lipids of the cells grown at 8 degrees C without glucose in NB was equal to growth temperature that corresponded to the classical mechanism of homeoviscous adaptation of bacteria. An addition of glucose to NB at this growth temperature caused the subsequent reduction of Tmax to -8 degrees C, while the temperature ranges of thermograms were not substantially changed. So, not only the temperature growth of bacteria, but also the presence of glucose in NB can modify the physical state of lipids from Y. pseudotuberculosis. In this case, both factors affect additively. It is suggested that glucose influences some membrane-associated proteins and then the fluidity of lipid matrix through temperature-inducible genes.     

The relationships between growth temperature,fatty acid composition and the physical state and fluidity of membrane lipids in Yersinia enterocolitica

The relationship between membrane lipid composition and membrane lipid phase transitions was investigated in Yersinia enterocolitica cells grown at 5, 22 and 37°C. The total phospholipid concentrations were 9.4, 7.3 and 6.3% of the cell dry weight for cells grown at 5, 22 and 37°C, respectively. The relative concentrations of the three major phospholipids, phosphatidylethanolamine (73–76%), phosphatidylglycerol (9–11%) and cardiolipin (11–13%) were essentially the same at all three growth temperatures. The ratios of unsaturated to saturated fatty acids were 2.2, 1.1 and 0.4 for cells grown at 5, 22 and 37°C, respectively. This change in the fatty acid composition in response to temperature changes is similar to the patterns reported for other organisms. Reversible thermotropic phase transitions were detected by calorimetric analysis in both pure lipid preparations and membrane preparations. The mid-points of the thermotropic phase transitions were at ?13, ?9 and 1°C for membranes from cells grown at 5, 22 and 37°C, respectively. The phase transitions of the membranes from cells grown at the three different temperatures occurred below the lowest growth temperature (5°C). The alternations in the fatty acid composition in Y. enterocolitica did not, therefore, appear to be required to adjust membrane fluidity but might rather be required for some other membrane function.     

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.     

Seasonal changes in oleosomic lipids and fatty acids of perennial root nodules of beach pea

Seasonal changes in the fatty acid composition of phospholipids (PL), monoglycerides (MG), diglycerides (DG), free fatty acids (FA) and triglycerides (TG) separated from oleosomes (lipid bodies) of perennial root nodules of beach pea (Lathyrus maritimus) were analysed. Thin layer chromatography (TLC) revealed that PL and MG are the major lipids in nodule oleosomes. The fatty acid profile and overall double bond index (DBI) varied among lipid classes depending upon the season. High DBI in PL and MG found during late winter and early spring indicated that they may play a major role in winter survival and regeneration of perennial nodules. The DBI of DG was high at the end of the fall season and the DBI of FA and TG was high in summer months. The dominant fatty acids are C16:0 followed by C18:0 and C18:1. The levels of many unsaturated fatty acids such as C18:1, C18:2 and C18:3 increased while saturated fatty acid C18:0 decreased during winter. These unsaturated fatty acids possibly play an important role in the protection of nodule cells from cold stress. Nodules seem to retain some fatty acids and selectively utilize specific fatty acids to survive the winter and regenerate in spring.     

Chlorinated fatty acid distribution in Mycobacterium convolutum phospholipids after growth on 1-chlorohexadecane

The composition of phospholipids from Mycobacterium convolutum R22 was determined after growth at two temperatures (20 and 30 degrees C) with 1-chlorohexadecane as the substrate. Comparisons were made with the phospholipids of cells grown on n-hexadecane. Phosphatidylinositolmannosides and phosphatidylethanolamine (PE) were the major phospholipids in n-hexadecane-grown cells. In 1-chlorohexadecane-grown cells, phosphatidylinositolmannosides were approximately half of the total phospholipids, with lesser amounts of PE and cardiolipin (CL). The relative level of PE was greater at 20 degrees C (versus that at 30 degrees C) after growth on either substrate. A determination was made of structure and positional distribution of constituent fatty acid in both CL and PE. The relative amount of unsaturated fatty acid was higher at 20 degrees C. There were two C16:1 fatty acids (C16:1 delta 9 and C16:1 delta 11), and these had positional preferences in both CL and PE. The positional sites of chlorinated fatty acids differed in both CL and PE at the two temperatures. The results confirm that microorganisms can specifically distribute chlorinated fatty acids into cellular phospholipids.     

Chlorinated fatty acid distribution in Mycobacterium convolutum phospholipids after growth on 1-chlorohexadecane.

The composition of phospholipids from Mycobacterium convolutum R22 was determined after growth at two temperatures (20 and 30 degrees C) with 1-chlorohexadecane as the substrate. Comparisons were made with the phospholipids of cells grown on n-hexadecane. Phosphatidylinositolmannosides and phosphatidylethanolamine (PE) were the major phospholipids in n-hexadecane-grown cells. In 1-chlorohexadecane-grown cells, phosphatidylinositolmannosides were approximately half of the total phospholipids, with lesser amounts of PE and cardiolipin (CL). The relative level of PE was greater at 20 degrees C (versus that at 30 degrees C) after growth on either substrate. A determination was made of structure and positional distribution of constituent fatty acid in both CL and PE. The relative amount of unsaturated fatty acid was higher at 20 degrees C. There were two C16:1 fatty acids (C16:1 delta 9 and C16:1 delta 11), and these had positional preferences in both CL and PE. The positional sites of chlorinated fatty acids differed in both CL and PE at the two temperatures. The results confirm that microorganisms can specifically distribute chlorinated fatty acids into cellular phospholipids.     

Independence of deoxyribonucleic acid replication and initiation from membrane fluidity and the supply of unsaturated fatty acids in Escherichia coli.

Mutant derivatives of the unsaturated fatty acid auxotroph K1062 were employed to investigate whether the supposedly membrane-bound bacterial replication machinery requires for its replicatory functions a fluid membrane environment as is known for several membrane-associated protein functions. Temperatures Tt for fluid reversible nonfluid phase transitions of membrane phospholipids are raised from below 18 to 38 degrees C when mutant cells are supplemented with elaidate instead of with oleate. In this experimental system current or synchroneously initiated new rounds of DNA replication are shown in vivo to continue 8 degrees below Tt, provided appropriate corrections for the concurrent cellular metabolic breakdown are considered. Temperature rate profiles for in vitro deoxyribonucleic acid replication rates measured in lysates of either oleate- or elaidate-supplemented cells yield congruent Arrhenius plots without discontinuities at corresponding Tt positions. We conclude that neither the start nor the propagation of replication forks depends on a fluid membrane. The capacity for the assembly of new replication complexes was studied in replication-aligned cells either shifted from oleate to elaidate (at temperatures below Tt for newly synthesized phospholipids) or starved for oleate. Regardless of whether unsaturated fatty acids are exchanged or completely withheld, new replication complexes can be normally assembled and initiated. These results do not support the conclusions reached by Fralick and Lark (1973) that the availability of unsaturated fatty acids is a prerequisite for the assembly of a functional replication complex.     

The effect of growth temperature on the thermotropic behavior of the membranes of a thermophilic Bacillus. Composition-structure-function relationships

The following study was carried out with the aim of widening our understanding of the thermoadaptive mechanisms of the membrane of thermophiles, using Bacillus stearothermophilus var. nondiastaticus as test-organism. The phospholipids and their acyl chain composition of this Bacillus studied in relation to the physical properties of its membrane from bacteria grown at various temperatures. Phospholipids account for 68-75 weight% of the total lipid in cells grown at 45, 55 or 65 degrees C. Phosphatidylglycerol and diphosphatidylglycerol constitute up to 90% of the total phospholipids; no amino phospholipids were found. Increasing the growth temperatures from 45 degrees to 65 degrees C caused an approximately 4-fold decrease in the proportion of the branched-chain fatty acids and a 2-fold increase in the amount of the saturated acyl chains. The reduced proportion of the branched fatty acids was mainly due to a decrease in their anteiso forms. Unsaturated fatty acids were not produced by cells grown at 65 degrees C. In accordance with the fatty acid composition, the molecular packing of phospholipids in monolayers was more expanded with phospholipids from 45 degrees C grown cells as compared with cultures grown at 55 degrees C. The thermotropic gel to liquid-crystalline phase transition of the membrane lipids was monitored by differential scanning calorimetry and fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene. With increase of the growth temperature the phase transition was progressively shifted to higher but narrower range of temperatures. Completion of the lipid melting occurred always at temperatures below those employed for growth. A constructed phase diagram enabled to relate the growth temperature, the fatty acid composition and the lipid apparent microviscosity at temperatures not used in the present study for growth of the thermophile. The minimum temperature for growth and the upper boundary temperature of the least saturated lipid crystallization were extrapolated in this manner; they correspond to the experimentally determined minimal growth temperature. The apparent microviscosity, a measure of membrane order, decreased gradually and conspicuously as the growth temperature was elevated. The delimiting apparent microviscosity values, at the maximal (65 degrees C) and minimal (41 degrees C) growth temperatures were 0.8 and 1.8 poise, respectively. This lack of rigorous homeostatic control of the bulk lipid viscosity prompted reevaluation of the physiological significance of 'homeoviscous adaptation' in Bacillus stearothermophilus.     

The modulating influence of the fluidity of cell membrane on excision repair of DNA in UV-irradiated Escherichia coli

When the extent of liquid holding recovery (LHR) was measured as a function of the temperature at the time of liquid holding and the Arrhenius plot was made, two distinctive phases for the LHR were demonstrated in UV-irradiated RecA- derivative of E. coli ole28E1, which are unable to synthesize and degrade unsaturated fatty acids. The inflection temperatures were 17-18 degrees C, 23-24 degrees C and 28-30 degrees C for linoleate-, oleate- and elaidate-grown cells, respectively. These temperatures well corresponded to the phase transition temperatures of the cell membrane supplemented with the fatty acid. It is therefore concluded that at least a component involved in in vivo excision repair in E. coli is associated with cell membrane.     

Effect of growth temperature on cellular fatty acids in sulphate-reducing bacteria

The effect of growth temperature on the cellular fatty acid composition of sulphate-reducing bacteria (SRB) was studied in 12 species belonging to eight genera including psychrophiles and mesophiles. Most of these species were of marine origin. The investigated SRB with the exception of four Desulfobacter species exhibited only a minor increase in the proportion of cis-unsaturated fatty acids (by < or = 5% per 10 degrees C) when the growth temperature was decreased; psychrophiles maintained their typically high content of cis-unsaturated fatty acids (around 75% of total fatty acids) nearly constant. The four Desulfobacter species, however, increased the proportion of cis-unsaturated among total fatty acids significantly (by > or =14% per 10 degrees C; measured in late growth phase) with decreasing growth temperature. The ratio between unsaturated and saturated fatty acids in Desulfobacter species changed not only with the growth temperature, but also with the growth state in batch cultures at constant temperature. Changes of cellular fatty acids were studied in detail with D. hydrogenophilus, the most psychrotolerant (growth range 0-35 degrees C) among the mesophilic SRB examined. Desulfobacter hydrogenophilus also formed cis-9,10-methylenehexadecanoic acid (a cyclopropane fatty acid) and 10-methylhexadecanoic acid. At low growth temperature (12 degrees C), the relative amount of these fatty acids was at least threefold lower; this questions the usefulness of 10-methylhexadecanoic acid as a reliable biomarker of Desulfobacter in cold sediments.     

Temperature- and growth-phase-dependent changes in membrane fatty acid compositions of Brevibacterium ammoniagenes

Fatty acids newly synthesized by Brevibacterium ammoniagenes grown at different temperatures were analyzed. The assay temperature, not the growth temperature, was found to be the major factor affecting the unsaturated/saturated ratio of newly synthesized fatty acids in logarithmic-phase cells. However, in the stationary-phase cells the growth temperature also affected the product profile significantly; cells grown at 7 degrees C produced relatively more oleate and stearate and less palmitate and hexadecenoate when shifted up to 37 degrees C than did cells grown and assayed at 37 degrees C. The unsaturated/saturated ratio as well as average chain length of fatty acids also varied along with the progress of isothermal growth phase. These changes in fatty acid product profiles observed in vivo could be mimicked in vitro assays of the fatty acid synthetase by changing malonyl-CoA concentrations. Our results suggest that the malonyl-CoA concentration is a factor which, in addition to temperature, determines growth-phase-dependent and growth-temperature-dependent changes in the unsaturated/saturated ratios of fatty acids.     

The hydrogenation of phospholipid-bound unsaturated fatty acids by a homogeneous, water-soluble, palladium catalyst

The hydrogenation of unsaturated phospholipids by palladium di(sodium alizarine monosulphonate) activated for 5 min under H2 proceeded rapidly at 20 degrees C and 1 atm. H2. Multibilayer liposomes of dioleoyl- and dilinolenoylphosphatidylcholine were hydrogenated at similar rates while dilinoleoyl- and 1-palmitoyl-2-oleoylphosphatidylcholine were hydrogenated at slightly slower rates. The reduction of polyunsaturated fatty acids gave rise to a variety of natural and unnatural positional cis and trans isomers which were largely reduced further to saturated fatty acids as the hydrogenation continued. Dioleoylphosphatidylethanolamine was attacked by the catalyst more slowly at 20 degrees C than was the equivalent phosphatidylcholine molecular species. Experiments conducted using mixtures of phosphatidylethanolamine and phosphatidylcholine in varying proportions also suggested that phospholipids are slightly more susceptible to catalytic hydrogenation in the bilayer phase than in the hexagonalII phase. Understanding the sequence of hydrogenation reactions involving these one and two component lipid preparations is useful in interpreting the action of the palladium catalyst on living cells under the same mild conditions.