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1.
The phase behavior of 1-palmitoyl-2,3-distearoyl-sn-glycerol (PSS)/tristearoylglycerol (SSS) binary system was investigated in terms of polymorphism, crystallization and melting behavior, microstructure and solid fat content (SFC) using widely different constant cooling rates. Kinetic phase diagrams were experimentally determined from the DSC heating thermograms and analyzed using a thermodynamic model to account for non-ideality of mixing. The kinetic phase diagram presented a typical eutectic behavior with a eutectic point at the 0.5(PSS) mixture with a probable precipitation line from 0.5(PSS) to 1.0(PSS), regardless of the rate at which the sample was cooled. The eutectic temperature decreased only slightly with increasing cooling rate. PSS has a strong effect on the physical properties of the PSS-SSS mixtures. In fact, the overall phase behavior of the PSS-SSS binary system was determined, for a very large part, by the asymmetrical TAG. Moreover, PSS is a key driver of the high stability observed in crystal growth, polymorphism and phase development. Levels as low as 10% PSS, when cooled slowly, and 30% when cooled rapidly, were found to be sufficient to suppress the effect of thermal processing. 相似文献
2.
Mendoza LD Rodriguez JA Leclaire J Buono G Fotiadu F Carrière F Abousalham A 《Journal of lipid research》2012,53(1):185-194
In the present study, we propose a continuous assay for the screening of sn-2 lipases by using triacylglycerols (TAGs) from Aleurites fordii seed (tung oil) and a synthetic TAG containing the α-eleostearic acid at the sn-2 position and the oleic acid (OA) at the sn-1 and sn-3 positions [1,3-O-dioleoyl-2-O-α-eleostearoyl-sn-glycerol (sn-OEO)]. Each TAG was coated into a microplate well, and the lipase activity was measured by optical density increase at 272 nm due to transition of α-eleostearic acid from the adsorbed to the soluble state. The sn-1,3-regioselective lipases human pancreatic lipase (HPL), LIP2 lipase from Yarrowia lipolytica (YLLIP2), and a known sn-2 lipase, Candida antarctica lipase A (CALA) were used to validate this method. TLC analysis of lipolysis products showed that the lipases tested were able to hydrolyze the sn-OEO and the tung oil TAGs, but only CALA hydrolyzed the sn-2 position. The ratio of initial velocities on sn-OEO and tung oil TAGs was used to estimate the sn-2 preference of lipases. CALA was the enzyme with the highest ratio (0.22 ± 0.015), whereas HPL and YLLIP2 showed much lower ratios (0.072 ± 0.026 and 0.038 ± 0.016, respectively). This continuous sn-2 lipase assay is compatible with a high sample throughput and thus can be applied to the screening of sn-2 lipases. 相似文献
3.
Catalytic pyrolysis was used to generate gasoline-compatible fuel from peanut oil soap stock (PSS), a high free fatty acid feedstock, using a fixed-bed reactor at temperatures between 450 and 550°C with a zeolite catalyst (HZSM-5). PSS fed at 81 gh(-1) along with 100 mL min(-1) inert gas was passed across a 15 g catalyst bed (WHSV=5.4h(-1), gas phase residence time=34s). Results indicate that fuel properties of PSS including viscosity, heating value, and O:C ratio were improved significantly. For PSS processed at 500°C, viscosity was reduced from 59.6 to 0.9 mm(2)s(-1), heating value was increased from 35.8 to 39.3 MJL(-1), and the O:C ratio was reduced from 0.07 to 0.02. Aromatic gasoline components (e.g., BTEX), were formed in concentrations as high as 94% (v/v) in catalytically-cracked PSS with yields ranging from 22% to 35% (v/v of PSS feed). 相似文献
4.
Modification of seed oil content and acyl composition in the brassicaceae by expression of a yeast sn-2 acyltransferase gene. 总被引:12,自引:1,他引:11
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J Zou V Katavic E M Giblin D L Barton S L MacKenzie W A Keller X Hu D C Taylor 《The Plant cell》1997,9(6):909-923
A putative yeast sn-2 acyltransferase gene (SLC1-1), reportedly a variant acyltransferase that suppresses a genetic defect in sphingolipid long-chain base biosynthesis, has been expressed in a yeast SLC deletion strain. The SLC1-1 gene product was shown in vitro to encode an sn-2 acyltransferase capable of acylating sn-1 oleoyl-lysophosphatidic acid, using a range of acyl-CoA thioesters, including 18:1-, 22:1-, and 24:0-CoAs. The SLC1-1 gene was introduced into Arabidopsis and a high erucic acid-containing Brassica napus cv Hero under the control of a constitutive (tandem cauliflower mosaic virus 35S) promoter. The resulting transgenic plants showed substantial increases of 8 to 48% in seed oil content (expressed on the basis of seed dry weight) and increases in both overall proportions and amounts of very-long-chain fatty acids in seed triacylglycerols (TAGs). Furthermore, the proportion of very-long-chain fatty acids found at the sn-2 position of TAGs was increased, and homogenates prepared from developing seeds of transformed plants exhibited elevated lysophosphatidic acid acyltransferase (EC 2.3.1.51) activity. Thus, the yeast sn-2 acyltransferase has been shown to encode a protein that can exhibit lysophosphatidic acid acyltransferase activity and that can be used to change total fatty acid content and composition as well as to alter the stereospecific acyl distribution of fatty acids in seed TAGs. 相似文献
5.
Microsomal Lyso-Phosphatidic Acid Acyltransferase from a Brassica oleracea Cultivar Incorporates Erucic Acid into the sn-2 Position of Seed Triacylglycerols 总被引:3,自引:0,他引:3
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Taylor DC Barton DL Giblin EM MacKenzie SL Van Den Berg C McVetty P 《Plant physiology》1995,109(2):409-420
Developing seeds from Brassica oleracea (L.) var botrytis cv Sesam were examined for the ability to biosynthesize and incorporate erucic acid into triacylglycerols (TAGs). Seed embryos at mid-development contained a high concentration of erucic acid in diacylglycerols and TAGs, and substantial levels were also detected in free fatty acids, acyl-coenzyme A (CoA), phosphatidic acid, and phosphatidylcholine. Homogenates and microsomal fractions from seeds at mid-development produced [14C]eicosenoyl- and [14C]erucoyl-CoAs from [14C]oleoyl-CoA in the presence of malonyl-CoA and reducing equivalents in vitro. These fatty acids were incorporated into TAGs via the Kennedy pathway. However, unlike most Brassicaceae, the B. oleracea was able to insert significant erucic acid into the sn-2 position of TAGs. It was shown that the lyso-phosphatidic acid acyltransferase (LPAT) incorporated erucic acid into the sn-2 position of lyso-phosphatidic acid. The erucoyl-CoA:LPAT activity during seed development and the sn-2 erucic acid content of the TAG fraction in mature seed were compared to those in B. napus, Tropaeolum majus, and Limnanthes douglasii. There was a correlation between the in vitro erucoyl-CoA:LPAT activity and the sn-2 erucic acid content in seed TAGs. To our knowledge, this is the first member of the Brassicaceae reported to have an LPAT able to use erucoyl-CoA. This observation has important implications for efforts being made to increase the erucic acid content in B. napus, to supply strategic industrial feedstocks. 相似文献
6.
The determination of the asymmetrical stereochemical distribution of fatty acids in triacylglycerols
The fatty acid distribution in triacylglycerols (TAGs) is a factor that contributes to the intrinsic properties of oils from different species variants. Many hypotheses have been proposed to explain the specific distribution of fatty acids in the different naturally occurring oils. Currently, the 1,3-random-2-random theory is more or less accepted, but it has been widely shown that most vegetable oils do not behave randomly in the sn-1 and sn-3 stereochemical positions. For this reason, complex methodologies have been developed to analyze the fatty acid composition of the three stereochemical positions in TAGs. In this article, we propose that by calculating the asymmetric alpha coefficient, the stereochemical asymmetry of fatty acids in TAG molecular species can be defined. This coefficient reflects the relative content of fatty acids at the sn-1 and sn-3 positions and may overcome the problems found mainly with complex sn-1, sn-2, and sn-3 stereochemical analysis of fatty acids in TAG. The alpha coefficient is calculated from the fatty acid, sn-2 fatty acid, and TAG composition of the oil. Indeed, through this coefficient, it has been possible to show that, despite having the same overall content, the stearic acid distribution in the sn-1 and sn-3 positions is not random in some oils. 相似文献
7.
Phosphatidylserine (PS), the major anionic phospholipid in eukaryotic cell membranes, is synthesized by the integral membrane enzymes PS synthase 1 (PSS1) and 2 (PSS2). PSS2 is highly expressed in specific tissues, such as brain and testis, where docosahexaenoic acid (DHA, 22:6n-3) is also highly enriched. The purpose of this work was to characterize the hydrocarbon-chain preference of PSS2 to gain insight on the specialized role of PSS2 in PS accumulation in the DHA-abundant tissues. Flag-tagged PSS2 was expressed in HEK cells and immunopurified in a functionally active form. Purified PSS2 utilized both PE plasmalogen and diacyl PE as substrates. Nevertheless, the latter was six times better utilized, indicating the importance of an ester linkage at the sn-1 position. Although no sn-1 fatty acyl preference was noted, PSS2 exhibited significant preference toward DHA compared with 18:1 or 20:4 at the sn-2 position. Preferential production of DHA-containing PS (DHA-PS) was consistently observed with PSS2 purified from a variety of cell lines as well as with microsomes from mutant cells in which PS synthesis relies primarily on PSS2. These findings suggest that PSS2 may play a key role in PS accumulation in brain and testis through high activity toward DHA-containing substrates that are abundant in these tissues. 相似文献
8.
Yli-Jokipii K Kallio H Schwab U Mykkänen H Kurvinen JP Savolainen MJ Tahvonen R 《Journal of lipid research》2001,42(10):1618-1625
The effects of positional distribution of triacylglycerol (TAG) fatty acids to TAG structures in chylomicrons and VLDL, and to postprandial lipemia, were studied in 10 healthy premenopausal women using a 6-h oral fat load test and a randomized, double-blind cross-over design. Molecular level information of TAG regioisomerism was obtained with a tandem mass spectrometric method. The positional distribution of fatty acids in chylomicron TAGs was similar to the respective dietary fat; 79% of the analyzed regioisomers in palm oil and 84% of the analyzed regioisomers in transesterified oil were found in chylomicron TAGs 3 h after the oral fat loads. VLDL TAGs were equal after the two fat loads in all but one regioisomer. Similarities in the fatty acid compositions of chylomicron TAGs suggest that palmitic acid was absorbed equally from both test fats. The proportion of palmitoleic acid in the chylomicrons was increased. Fat with palmitic acid predominantly in the sn-1 and sn-3 positions caused a larger incremental area of total TAGs in plasma and reduced plasma insulin values at the beginning of the postprandial response (0-90 min) compared with fat with palmitic acid randomly distributed. The relationship between TAG molecular structures in dietary fats and in lipoproteins provides new means for understanding the effects of fatty acid positional distribution on human lipid metabolism. 相似文献
9.
M.V. Boodhoo 《Chemistry and physics of lipids》2009,160(1):11-32
The binary phase behavior of purified 1, 3-dipalmitoyl-2-stearoyl-sn-glycerol (PSP) and 1, 2-dipalmitoyl-3-stearoyl-sn-glycerol (PPS) was investigated at a very slow (0.1 °C/min) and a relatively fast (3.0 °C/min) cooling rate. Mixtures with molar fractions of 0.1 increments were studied in terms of melting and crystallization, polymorphism, solid fat content (SFC), hardness and microstructure. Only the α-form of a double chain length (DCL) structure was detected for all mixtures in both experiments. The kinetic phase diagram, constructed using heating DSC thermograms, displayed two distinct behaviors separated by a singularity at the 0.5PSP composition: a eutectic in the XPSP ≤ 0.5 and a monotectic in the XPSP ≤ 0.5 concentration region. The singularity was attributed to the formation of a 1:1 (mol:mol) molecular compound. Apart from the segment from 0.0PSP to the eutectic point, XE, the simulation of the liquidus line using a model based on the Hildebrand equation suggested that the molecular interactions are strong and tend to favor the formation of unlike pairs in the liquid state and that the miscibility is not significantly dependent on cooling rate. The kinetic effects are manifest in all measured properties, particularly dramatically in the XPSP ≤ XE concentration region. An analysis of induction time as measured by pulse nuclear magnetic resonance (pNMR) showed that PPS retards crystal growth, an effect which can explain the peculiarity of this concentration region. At both cooling rates, fit of the SFC (%) versus time curves to a modified form of the Avrami model revealed two common growth modes for all the mixtures. The polarized light microscope (PLM) of the PSP-PPS mixtures revealed networks made of spherulitic crystallites of size, growth direction and boundaries that are varied and sensitive to composition and cooling rate. The change in the microstructure and final SFC (%), particularly noticeable at compositions close to the eutectic, explain in part the differences seen in relative hardness. 相似文献
10.
The CH2 proton NMR linewidths of sn-3 and sn-1 dipalmitoyl phosphatidylcholine respond differently to the addition of cholesterol to the lipid vesicles. This result points to a stereospecific phospholipid-cholesterol interaction in the "hydrogen belt" region. 相似文献
11.
The transformation between a gel and a fluid phase in dipalmitoyl-phosphatidylcholine (DPPC) bilayers has been simulated using a coarse grained (CG) model by cooling bilayer patches composed of up to 8000 lipids. The critical step in the transformation process is the nucleation of a gel cluster consisting of 20-80 lipids, spanning both monolayers. After the formation of the critical cluster, a fast growth regime is entered. Growth slows when multiple gel domains start interacting, forming a percolating network. Long-lived fluid domains remain trapped and can be metastable on a microsecond time scale. From the temperature dependence of the rate of cluster growth, the line tension of the fluid-gel interface was estimated to be 3+/-2 pN. The reverse process is observed when heating the gel phase. No evidence is found for a hexatic phase as an intermediate stage of melting. The hysteresis observed in the freezing and melting transformation is found to depend both on the system size and on the time scale of the simulation. Extrapolating to macroscopic length and time scales, the transition temperature for heating and cooling converges to 295+/-5 K, in semi-quantitative agreement with the experimental value for DPPC (315 K). The phase transformation is associated with a drop in lateral mobility of the lipids by two orders of magnitude, and an increase in the rotational correlation time of the same order of magnitude. The lipid headgroups, however, remain fluid. These observations are in agreement with experimental findings, and show that the nature of the ordered phase obtained with the CG model is indeed a gel rather than a crystalline phase. Simulations performed at different levels of hydration furthermore show that the gel phase is stabilized at low hydration. A simulation of a small DPPC vesicle reveals that curvature has the opposite effect. 相似文献
12.
J C Skou 《Biochimica et biophysica acta》1982,688(2):369-380
An increase in pH decreases the Na+ concentration (Na+ +K+ = 150 mM) necessary for half-maximum activation of the (Na+ +K+)-ATPase at non-saturating concentrations of ATP just as an increase in the concentration of ATP at a given pH. It also decreases the concentration of Na+ necessary for transformation from the K+-form to the Na+-form at equilibrium conditions (Na+ +K+ = 150 mM). An increase in pH increases the rate of the transformation from the K+-form to the Na+-form of the system and decreases the rate of the reverse reaction. The pH effect on the conformation suggests that the K+-form is a protonated form and the Na+-form a deprotonated one. The similarity between the effect of an increase in pH with non-saturating concentrations of ATP and that of an increase in ATP at a given pH suggests that ATP exerts its effect on the transformation from the K+ - to the Na+-form by a decrease in pK values of the system, i.e., by releasing protons, a Bohr effect. Enzyme modified by reaction with pyridoxal 5-phosphate terminated by NaBH4 behaves at a given pH as if it were non-modified enzyme but at a higher pH. The 'pH effect' is seen after modification by pyridoxal 5-phosphate in the presence of ATP, of Na+ without and with ATP, of K+ with ATP but not in the presence of K+ alone. The modification has also a 'pH effect' on the rate of the transformation from the K+ -form to the Na+ -form and on the reverse reaction. There are at least two different pyridoxal 5-phosphate-reactive groups (amino groups), one which can be protected by ATP and which is of importance for activity and another which is not protected by ATP and which is of importance for the pH effect on the conformation. The effect of a protonation-deprotonation of amino groups on the conformation is explained by an involvement of the amino groups in salt bridge formation in between and inside the polypeptide chains, a hemoglobin-like situation. The protonated K+ -form is then a tense T-structure with a high K+, low Na+ affinity and the deprotonated Na+ -form a relaxed, R-structure with high Na+, low K+ affinity. ATP facilitates deprotonation by decreasing pK values. Oligomycin has 'pH effect' on the K0.5 for Na+ under equilibrium and steady-state conditions, but oligomycin has no effect on the rate of the transformation from the K+ -form to the Na+ -form, but gives a pronounced decrease of the rate of the reverse reaction, indicating that oligomycin does not react with the K+ -form but with the Na+ -form of the system and prevents the protonation, the E1 to E2 transformation. 相似文献
13.
Fourier transform infrared (FTIR) spectroscopy was used to study the thermotropic behaviour of fully hydrated 1-palmitoyl-2-[10-(pyren-1-yl)-decanoyl]-sn-glycero-3-phosphatidyl choline (PPDPC) in the temperature range of 3-30 degrees C. Several changes in the spectral features of PPDPC were observed. Major alterations analogous to the gel-to-liquid crystalline phase transition of saturated phosphatidylcholines were evident at approximately 16 degrees C in both the wavenumbers and the halfbandwidths of five different vibrational modes of PPDPC, viz. asymmetric and symmetric CH2 stretching, C = O stretching, and CH2 bending. Also the pyrene ring deformation mode changed at this temperature. Using Fourier self-deconvolution technique we resolved the carbonyl stretching mode into two bands at approx. 1741 and 1726 cm-1. These bands are due to conformational differences in the ester linkages of the two acyl chains, and are further assigned on the basis of literature data to the sn-1 and sn-2 carbonyl groups, respectively. The ratio of the relative intensities of these two bands is shown to depend on the phase state of the phospholipid. 相似文献
14.
Effect of hydrostatic pressure on the bilayer phase behavior of symmetric and asymmetric phospholipids with the same total chain length 总被引:1,自引:0,他引:1
The bilayer phase transitions of palmitoylstearoyl-phosphatidylcholine (PSPC), diheptadecanoyl-PC (C17PC) and stearoylpalmitoyl-PC (SPPC) which have the same total carbon numbers in the two acyl chains were observed by differential scanning calorimetry and high-pressure optical method. As the temperature increased, these bilayers exhibited four phases of the subgel (Lc), lamellar gel (L beta'), ripple gel (P beta') and liquid crystal (L alpha), in turn. The Lc phase was observed only in the first heating scan after cold storage. The temperatures of the phase transitions were almost linearly elevated by applying pressure. The temperature-pressure phase diagrams and the thermodynamic quantities associated with the phase transitions were compared among the lipid bilayers. For all the bilayers studied, the pressure-induced interdigitated gel (L beta I) phase appeared above the critical interdigitation pressure (CIP) between the L beta' and P beta' phases. The CIPs for the PSPC, C17PC and SPPC bilayers were found to be 50.6, 79.1 and 93.0 MPa, respectively. Contribution of two acyl chains to thermodynamic properties for the phase transitions of asymmetric PSPC and SPPC bilayers was not even. The sn-2 acyl chain lengths of asymmetric PCs governed primarily the bilayer properties. The fluorescence spectra of Prodan in lipid bilayers showed the emission maxima characteristic of bilayer phases, which were dependent on the location of Prodan in the bilayers. Second derivative of fluorescent spectrum exhibited the original emission spectrum of Prodan to be composed of the distribution of Prodan into multiple locations in the lipid bilayer. The F'497/F'430 value, a ratio of second derivative of fluorescence intensity at 497 nm to that at 430 nm, is decisive evidence whether bilayer interdigitation will occur. With respect to the L beta'/L beta I phase transition in the SPPC bilayer, the emission maximum of Prodan exhibited the narrow-range red-shift from 441 to 449 nm, indicating that the L beta I phase in the SPPC bilayer has a less polar "pocket" formed by a space between uneven terminal methyl ends of the sn-1 and sn-2 chains, in which the Prodan molecule remains stably. 相似文献
15.
The specific volumes of seven 1,2-diacyl-sn-glycero-3-phosphocholines with symmetric, unbranched acyl chains containing one, four, or six cis double bonds per chain, or with a saturated sn-1 chain and one, four, or six cis double bonds in the sn-2 chain were determined by the neutral buoyancy method. Experiments were conducted in the liquid crystalline lamellar phase over the temperature range from 5 to 35 degrees C. It is demonstrated that the molecular volume of phosphatidylcholines can be well approximated as the sum of a constant volume of the polar lipid head region and the temperature-dependent volumes of hydrocarbon chain CH2, CH, and terminal CH3 groups. A linear dependence of chain segment volumes on temperature was observed. A self-consistent set of partially temperature-dependent volumes is obtained that allows prediction of phosphatidylcholine molecular volumes within very tight error margins. 相似文献
16.
Differential scanning calorimetric study of a homologous series of fully hydrated saturated mixed-chain C(X):C(X + 6) phosphatidylcholines 总被引:4,自引:0,他引:4
The successive high-resolution differential scanning calorimetric (DSC) thermograms for aqueous dispersions of a homologous series of mixed-chain phosphatidylcholines, C(X):C(X + 6)PC, have been recorded and analyzed. In this series of saturated mixed-chain phosphatidylcholines, the total number of carbon atoms in the sn-1 acyl chain increases from 11 to 20, and the sn-2 acyl chain is always 6 methylene units longer than the sn-1 acyl chain. In the initial heating DSC thermograms, two prominent endothermic transitions are detected for all the samples prepared from the various C(X):C(X + 6)PCs except C(12):C(18)PC. In contrast, a single exothermic transition is observed on cooling for all the samples except C(13):C(19)PC. The temperature difference between the two endothermic transitions increases linearly as the acyl chain length of C(X):C(X + 6)PC becomes progressively longer. Interestingly, the main phase transition occurs before the subtransition for C(11):C(17)PC dispersions. Our DSC data further demonstrate that the thermodynamic parameters (Tm, delta H, and delta S) associated with the main phase transition for fully hydrated C(13):C(19)PC and other identical MW phosphatidylcholines are inversely related to the corresponding values of the chain-length inequivalence (delta C/CL) for these lipids. This linear relationship can be employed to map the Tm values for aqueous dispersions prepared from a large number of mixed-chain phosphatidylcholines whose values of delta C/CL are within the range of 0.1-0.4. 相似文献
17.
Mixed-chain phosphatidylcholine bilayers: structure and properties 总被引:10,自引:0,他引:10
Calorimetric and X-ray diffraction data are reported for two series of saturated mixed-chain phosphatidylcholines (PCs), 18:0/n:0-PC and n:0/18:0-PC, where the sn-1 and sn-2 fatty acyl chains on the glycerol backbone are systematically varied by two methylene groups from 18:0 to 10:0 (n = 18, 16, 14, 12, or 10). Fully hydrated PCs were annealed at -4 degrees C and their multilamellar dispersions characterized by differential scanning calorimetry and X-ray diffraction. All mixed-chain PCs form low-temperature "crystalline" bilayer phases following low-temperature incubation, except 18:0/10:0-PC. The subtransition temperature (Ts) shifts toward the main (chain melting) transition temperature (Tm) as the sn-1 or sn-2 fatty acyl chain is reduced in length; for the shorter chain PCs (18:0/12:0-PC, 12:0/18:0-PC, and 10:0/18:0-PC), Ts is 1-2 degrees C greater than Tm, and the subtransition enthalpy (delta Hs) is much greater than for the longer acyl chain PCs. Tm decreases with acyl chain length for both series of PCs except 18:0/10:0-PC, while for the positional isomers, n:0/18:0-PC and 18:0/n:0-PC, Tm is higher for the isomer with the longer acyl chain in the sn-2 position of the glycerol backbone. The conversion from the crystalline bilayer Lc phase to the liquid-crystalline L alpha phase with melted hydrocarbon chains occurs through a series of phase changes which are chain length dependent. For example, 18:0/18:0-PC undergoes the phase changes Lc----L beta'----P beta'----L alpha, while the shorter chain PC, 10:0/18:0-PC, is directly transformed from the Lc phase to the L alpha phase. However, normalized enthalpy and entropy data suggest that the overall thermodynamic change, Lc----L alpha, is essentially chain length independent. On cooling, the conversion to the Lc phases occurs via bilayer gel phases, L beta', for the longer chain PCs or through triple-chain interdigitated bilayer gel phases, L beta, for the shorter chain PC 18:0/12:0-PC and possibly 10:0/18:0-PC. Molecular models indicate that the bilayer gel phases for the more asymmetric PC series, 18:0/n:0-PC, must undergo progressive interdigitation with chain length reduction to maintain maximum chain-chain interaction. The L beta phase of 18:0/10:0-PC is the most stable structure for this PC below Tm. The formation and stability of the triple-chain structures can be rationalized from molecular models. 相似文献
18.
Oligomycin occludes Na+ in an E1-form of the Na,K-ATPase. The rate constants for the release of Na+ from the E1-form and for the transition to the E2-form are about 0.5 s-1. The effect of oligomycin is not seen using other cations which also have a Na+-like effect on the enzyme conformation. The inhibitory effect of oligomycin on the ADP-ATP dependent Na:Na exchange but not on the accompanying ADP-ATP exchange can be explained from a decrease in the rate of release of Na+ from an E1 approximately phosphoform with Na+ occluded, E'1 approximately P (Na3), i.e. with Na+ in the membrane phase, to an E"1 approximately PNa3 form with Na+ not occluded. E"1 approximately PNa3 is at a step before formation of E2-P, and disappears at a high rate when ADP reacts with E"1 approximately P (Na3). 相似文献
19.
Rodriguez JA Mendoza LD Pezzotti F Vanthuyne N Leclaire J Verger R Buono G Carriere F Fotiadu F 《Analytical biochemistry》2008,375(2):196-208
In the present study, we propose a general and accessible method for the resolution of enantiomeric 1,2-sn- and 2,3-sn-diacylglycerols based on derivatization by isocyanates, which can be easily used routinely by biochemists to evaluate the stereopreferences of lipases in a time course of triacylglycerol (TAG) hydrolysis. Diacylglycerol (DAG) enantiomers were transformed into carbamates using achiral and commercially available reagents. Excellent separation and resolution factors were obtained for diacylglycerols present in lipolysis reaction mixtures. This analytical method was then applied to investigate the stereoselectivity of three model lipases (porcine pancreatic lipase, PPL; lipase from Rhizomucor miehei, MML; and recombinant dog gastric lipase, rDGL) in the time course of hydrolysis of prochiral triolein as a substrate. From the measurements of the diglyceride enantiomeric excess it was confirmed that PPL was not stereospecific (position sn-1 vs sn-3 of triolein), whereas MML and rDGL preferentially hydrolyzed the ester bond at position sn-1 and sn-3, respectively. The enantiomeric excess of DAGs was not constant with time, decreasing with the course of hydrolysis. This was due to the fact that DAGs can be products of the stereospecific hydrolysis of TAGs and substrates for stereospecific hydrolysis into monoacylglycerols. 相似文献
20.
近年来霜冻对我国果树产业的影响越来越大, 建立科学的模拟霜冻程序对于加强果树霜冻基础研究十分必要。基于对大田霜冻天气的实际观测, 分析了自然霜冻在降温速度、低温极限、升温速度以及霜后光照条件方面的特点, 建立了用于实验室环境下的霜冻处理程序。结果表明, 霜冻发生时气温的变化主要包括降温、低温维持和升温3个阶段。降温和升温阶段气温的变化近似线性; 霜后一般伴随较强的光照。经研究确定模拟霜冻条件为: 黑暗环境下, 气温在30分钟内从室温(20°C)降到5°C, 在5°C维持30分钟, 之后以0.8°C·h -1的速度降到-2°C, 在-2°C维持2小时, 再以4.7°C·h -1的速度回升到5°C结束霜冻处理。霜冻处理后的恢复条件为气温16°C及光强800 μmol·m -2·s -1。 相似文献