Analogs of natural lipids. II. Polymorphic behavior of the tris-Homo acyl derivatives of cyclopentane-1,2,3-triols

The polymorphic behavior of the three series of tris-homoacyl (C_14:0, C_16:0 and C_18:0) cyclopentane-1,2,3-triol analogs of the natural saturated triglycerides has been studied using differential thermal analysis, Fourier transform infrared spectroscopy, and X-ray diffraction. It was found that the triglyceride analogs derived from the 1,2,3/0 and 1,2/3 cyclopentanetriols exhibit different polymorphic behavior than that of the natural triglycerides. The analogs derived from 1,3/2 cyclopentanetriol, however, were found to parallel the polymorphic behavior of the natural triglycerides quite closely. This polymorphic behavior is discussed in terms of the different configurations which the chains assume in each of the triglyceride analogs.     

Synthesis of all-cis-1,3-diacylcyclopentane-1,2,3-triol-2-phosphate via acyl group migration in a cyclic diglyceride analog.

The acid-catalyzed isomerization of the diglyceride analog (1,2,3/0)-1,2-dipalmitoylcyclopentane-1,2,3-triol has been used to generate syn-syn-1,3-diacyl-cyclopentane-1,2,3-triol, a required intermediate in the synthesis of a symmetrical all-cis-1,2,3/0-2P cyclopentanoid phosphatidic acid analog. The all-cis cyclo-phosphatidic acid analog has therefore been obtained in the free acid form and as the diphenyl ester, dimethyl ester, and dipotassium salt derivatives. The compounds have been characterized by microanalysis and spectroscopic methods. The 1,2,3/0-2P analog is now available for comparative studies with the corresponding all-trans cyclophosphatidic acid (1,3/2-2P).     

Design,synthesis and structure–activity relationships of azole acids as novel,potent dual PPAR α/γ agonists

The design, synthesis and structure–activity relationships of a novel series of N-phenyl-substituted pyrrole, 1,2-pyrazole and 1,2,3-triazole acid analogs as PPAR ligands are outlined. The triazole acid analogs 3f and 4f were identified as potent dual PPARα/γ agonists both in binding and functional assays in vitro. The 3-oxybenzyl triazole acetic acid analog 3f showed excellent glucose and triglyceride lowering in diabetic db/db mice.     

Cyclopentanoid analogs of dipalmitoyl phosphatidic acid: effect of backbone geometry on thermotropic properties

Seven geometrical or positional isomers of dipalmitoyl cyclopentanophosphoric acid (DPCPA) have been synthesized and studied: 1,3/2-1P (I); 1,2/3-1P (II); 1,2/3-3P (III), 1,2,3/0-1P (IV); 1,2,3/0-2P (V); 1,3/2-2P (VI); 1,2/3-2P (VII). When dispersed in 0.1 M Tris-HCl at pH 7.4, I-VII gave thermal transitions (Tc) of 60.0 degrees, 59.0 degrees, 56.8 degrees, 55.3 degrees, 38.3 degrees, 36.8 degrees and 34.0 degrees C, respectively, as measured by differential scanning calorimetry (DSC). When the lipids were dispersed at pH 9.5 in 0.1 M borate, Tc of I-IV decreased, whereas Tc of V-VII increased. In contrast, at pH 1.5 in 0.1 M HCl/KCl, Tc of I-IV decreased slightly, but Tc of V-VII rose markedly. To determine the effect of head group geometry and substitution pattern on acyl chain motion, EPR spectra of 1-palmitoyl, 2-[16-doxylstearoyl]-glycero-3-phosphoric acid in bilayers of DPCPA isomers were acquired. Abrupt spectral changes occurred at temperatures closely correlating with transition temperatures observed by DSC. These results have led to the conclusions that: (i) isomers I-IV containing vicinal acyl chains form bilayers that exhibit structural transitions at temperatures higher than those at which transitions are exhibited by isomers V-VII which have a polar phosphate group interposed between the two chains; (ii) the effects of differences in backbone structure are transmitted down the entire length of the acyl chains; (iii) the orientation of the cyclopentane ring in the isomers I-IV is significantly different from that in isomers V-VII at pH values where the phosphate group is doubly negatively charged.     

1H NMR studies of lymph chylomicra and very low density lipoproteins from nonhuman primates

1H NMR spectroscopy at 200 MHz was used to study triglyceride crystalline leads to liquid transitions which occurred on heating between 10 and 50 degrees C in very low density lipoprotein and subfractionated chylomicron particles from nonhuman primates fed a saturated fat (butter fat) diet. Model system studies of pure triglycerides (triolein, tripalmitin and a 1:1 mixture) and emulsion particles consisting of these triglycerides with a surface of egg phosphatidylcholine showed that high resolution spectra were obtained only from liquid triglycerides. In lipoprotein spectra, changes in 1H NMR peak intensities and line widths accompanied the solid leads to liquid transition of the constituent triglycerides. Peak areas of fatty acyl resonances were proportional to the percentage of melted triglyceride determined by differential scanning calorimetry. NMR peak area measurements showed that the calorimetric transition involved the melting of relatively greater numbers of saturated fatty acyl chains than unsaturated chains; at temperatures well below the solid leads to liquid transition, the lipoproteins contained a significant fraction (approximately 33%) of liquid triglycerides which were relatively enriched in unsaturated fatty acyl chains. For model systems containing mixtures of solid and liquid triglycerides, the temperature dependence of line widths of fatty acyl resonances demonstrated that solid triglycerides decreased the mobility of the liquid triglycerides. A similar temperature dependence for the lipoprotein resonances suggested that solid and liquid species are co-mixed in individual lipoprotein particles within a purified subfraction. Temperature-dependent line width and intensity changes were observed for the phospholipid-choline methyl resonance in lipoprotein spectra and were apparently independent of the core transition.     

Effects of cis and trans unsaturation on the structure of phospholipid bilayers: a high-pressure infrared spectroscopic study

In order to compare the effects of cis and trans unsaturation on the structure and packing of phospholipid bilayers, infrared spectra of aqueous dispersions of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dielaidoyl-sn-glycero-3-phosphocholine (DEPC) were measured in a diamond anvil cell at 28 degrees C as a function of pressure up to 36 kbar. The infrared spectra indicate that DEPC and DOPC undergo pressure-induced liquid-crystalline to gel phase transitions at critical pressures of 0.7 and 5.2 kbar, respectively. Below their respective critical pressures, the infrared spectra of DOPC and DEPC are essentially indistinguishable, whereas above these pressures, there are very pronounced differences in the barotropic behavior of these two lipids. Specifically, at the 5.2-kbar transition in DOPC, there are significant changes in the frequencies, intensities, and widths of bands associated with the interfacial C = O groups, the olefinic CH = CH groups, and the terminal CH3 groups, whereas the corresponding bands of DEPC are, by contrast, relatively insensitive to the pressure-induced phase transition. The unusual band shape changes in DOPC are attributed to a unique packing arrangement of the oleoyl acyl chains required to accommodate the bent geometries of adjacent cis double bonds. Moreover, above 5 kbar in DEPC, well-defined correlation field splittings of the CH2 scissoring and rocking modes are observed, with magnitudes very similar to those observed at comparable pressures in saturated lipid systems. The absence of correlation field splittings of the corresponding bands of DOPC up to 36 kbar suggests that the bent oleoyl acyl chains are closely packed with all chains oriented parallel to each other.     

Interaction of cholesterol with conformationally restricted phospholipids in vesicles.

The interaction of cholesterol with conformationally restricted analogs of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) in the liquid-crystalline phase has been studied in vesicles. These analogs contain one of three cyclopentane triols in place of the glycerol moiety found in natural phospholipids and make possible an analysis of whether a limitation of the conformational mobility in the glycerol backbone region affects the interaction with cholesterol. When cholesterol was incorporated into vesicles from cyclopentanoid phospholipids in which the acyl group vicinal to the head group is trans, the first-order rate constant for Cl- efflux is decreased similarly to that in vesicles from 'natural' DPPC or DPPG (about 50%). However, when the head group is in the unnatural 2 position, cholesterol has a much smaller effect on the rate of Cl- efflux (a decrease of about 20%). Cholesterol decreased the rate constants for valinomycin-mediated 86Rb+ efflux from vesicles of the cyclopentanoid PC analogs and of DPPC to a similar extent. The half-time values for spontaneous intervesicle cholesterol exchange were not markedly different using vesicles prepared with the natural glycerophospholipids and with the cyclopentano-phospholipids, suggesting that the geometrical orientation of the acyl chains or the head group has little influence on cholesterol desorption from the lipid/water interface.     

Infrared characterization of conformational differences in the lamellar phases of 1,3-dipalmitoyl-sn-glycero-2-phosphocholine

Fourier transform infrared spectroscopy was used to characterize the lamellar phases of 1,3-dipalmitoyl-sn-glycero-2-phosphocholine (1,3-DPPC), a positional isomer of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (1,2-DPPC). The molecule exists in three distinct phases over the temperature interval 0–70°C. In the low-temperature (L_c) phase, the spectra are indicative of acyl chains packed in an orthorhombic subcell, while the carbonyl groups and phosphate ester at the head group show evidence of only partial hydration. The transition from the low-temperature (L_c) phase to the intermediate-temperature (Lβ) phase at 25°C corresponds to a temperature-induced head-group hydration in which the hydration of the phosphate and carbonyl ester groups results in the reorganization of the hydrocarbon chain-packing subcell from orthorhombic to hexagonal. The transition from the intermediate (L_β) to the high-temperature (L_α) phase at 37°C is a gel-to-liquid-crystalline phase transition analogous to the 41.5°C transition of 1,2-DPPC. The spectra of the acyl-chain carbonyl groups show evidence of significant differences in molecular conformation at the carbonyl esters in the L_c phase. In the L_β and L_α phases, the carbonyl band contour becomes much more symmetric. However, two components are clearly present in the spectra indicating that the sn-1 and sn-3 carbonyls experience slightly different environments. The observed differences are likely due to a preferred conformation of the phosphocholine group relative to the glycerol backbone. Indications from the infrared spectra of differences in the structure of the C=O groups provide a possible explanation for the selection of the sn-1 chain of 1,3-DPPC by phospholipase A₂ on the basis of a preferred head group conformation.     

Quantitative determination of hydrocarbon chain conformational order in bilayers of saturated phosphatidylcholines of various chain lengths by Fourier transform infrared spectroscopy

The infrared spectra of aqueous dispersions of a homologous series of symmetric-chain, disaturated phosphatidylcholines, with fatty acyl chain lengths ranging from 12 to 19 carbons, have been measured at comparable reduced temperatures in their liquid-crystalline phases. The infrared spectra of these compounds contain bands that are dependent on the conformation of the fatty acyl chains. In particular, in the 1400-1300-cm-1 spectral region, there are bands due to CH2 wagging which are specific for the different types of gauche conformers. Thus, gauche-trans-gauché sequences (or kinks) give a band at 1367 cm-1, end-gauche conformers a band at 1341 cm-1, and double-gauche conformers a band at 1355 cm-1. The intensities of these bands were determined and normalized to the intensity of the conformation-insensitive band due to symmetric methyl bending at 1378 cm-1. The intensities of the different "gauche" bands yield a "per chain" intensity, which is directly related to the concentration of the different types of conformational defects. We find that, within experimental error, the concentration of end-gauche and double-gauche conformers is relatively low and practically invariant with chain length when a series of homologous phosphatidylcholines are compared at the same reduced temperature. In contrast, the concentration of gauche-trans-gauché sequences (kink defects) is much higher and increases as the chain length increases. For dipalmitoylphosphatidylcholine we find that there are about 1.2 kink, 0.5-0.6 end-gauche, and 0.4 double-gauche conformers per hydrocarbon chain.(ABSTRACT TRUNCATED AT 250 WORDS)     

Crystal structure of cutinase covalently inhibited by a triglyceride analogue.

Cutinase from Fusarium solani is a lipolytic enzyme that hydrolyses triglycerides efficiently. All the inhibited forms of lipolytic enzymes described so far are based on the use of small organophosphate and organophosphonate inhibitors, which bear little resemblance to a natural triglyceride substrate. In this article we describe the crystal structure of cutinase covalently inhibited by (R)-1,2-dibutyl-carbamoylglycero-3-O-p-nitrophenylbutyl-phos phonate, a triglyceride analogue mimicking the first tetrahedral intermediate along the reaction pathway. The structure, which has been solved at 2.3 A, reveals that in both the protein molecules of the asymmetric unit the inhibitor is almost completely embedded in the active site crevice. The overall shape of the inhibitor is that of a fork: the two dibutyl-carbamoyl chains point towards the surface of the protein, whereas the butyl chain bound to the phosphorous atom is roughly perpendicular to the sn-1 and sn-2 chains. The sn-3 chain is accommodated in a rather small pocket at the bottom of the active site crevice, thus providing a structural explanation for the preference of cutinase for short acyl chain substrates.     

Conversion of chlorinated propanes by Methylosinus trichosporium OB3b expressing soluble methane monooxygenase

Chlorinated propanes are important pollutants that may show persistent behaviour in the environment. The biotransformation of 1-chloropropane, 1,2-dichloropropane, 1,3-dichloropropane and 1,2,3-trichloropropane was studied using resting cell suspensions of Methylosinus trichosporium OB3b expressing soluble methane monooxygenase. The transformation followed first-order kinetics. The rate constants were in the order 1-chloropropane > 1,3-dichloropropane > 1,2-dichloropropane > 1,2,3-trichloropropane, and varied from 0.07 to 1.03 ml min⁻¹ mg of cells⁻¹ for 1,2,3-trichloropropane and 1-chloropropane respectively. Turnover-dependent inactivation occurred for all of the chloropropanes tested. The inactivation constants were lower for 1-chloropropane and 1,2-dichloropropane than for 1,2,3-trichloropropane and 1,3-dichloropropane. Not all the chloride was released during cometabolic transformation of the chlorinated propanes and production of monochlorinated- and dichlorinated propanols was found by gas chromatography. The reaction pathway of 1,2,3-trichloropropane conversion was studied by mass spectrometric analysis of products formed in ²H₂O, which indicated that 1,2,3-trichloropropane was initially oxidized to 2,3-dichloropropionaldehyde and 1,3-dichloroacetone, depending on whether oxygen insertion occurred on the C-3 or C-2 carbon of 1,2,3,-trichloropropane, followed by reduction to the corresponding propanols. The results show that chloropropanes are susceptible to cometabolic oxidation by methanotrophs, but that the transformation kinetics is worse than with cometabolic conversion of trichloroethylene. Received: 27 November 1997 / Received revision: 27 February 1998 / Accepted: 27 February 1998     

Circular dichroism as a reliable tool for anomeric assignment of glycosyl-2-phenyl-2H-1,2,3-triazole C-nucleoside analogs. A rule for prediction of their anomeric configuration

The circular dichroism (CD) of a series of acyclic C-nucleoside analogs; 4-(pentahydroxypentyl-1-yl)-2-phenyl-2H-1,2,3-triazoles [1-5] and 4-(D-glycero-D-gulo)-2-phenyl-2H-1,2,3-triazole 6, are reported. A correlation between the sign of the Cotton effect at the maximal UV absorption and the absolute configuration of the carbon atom alpha- to the triazole base moiety is reported. The CD of anomeric 4-(alpha,beta-D-arabinofuranosyl)- and 4-(alpha,beta-D-arabinopyranosyl)-2-phenyl-2H-1,2,3-triazole C-nucleosides are reported. The assignment of the anomeric configuration of C-glycosyl-2-phenyl-2H-1,2,3-triazoles from their CD spectra was found to be a simple method that relies on comparison of the sign of the Cotton effect at the maximal UV absorption and the absolute configuration of the anomeric carbon atom. A correlation between the anomeric configuration and the sign of the Cotton effect at the maximal UV absorption is deduced and generalized as a rule for prediction of the anomeric configuration of C-glycosyl-2-phenyl-2H-1,2,3-triazoles. Nuclear Overhauser effect and 13C NMR spectra supported the CD assignment rule.     

Influence of Triglyceride Molecular Species on Autoxidation

The autoxidative stability of triglyceride molecular species was analyzed by determining the residual molecular species after incubating synthesized triglycerides (TG_s), interesterified TG_s and soybean oil TG_s, respectively. The autoxidative stability of each molecular species of TG depends on the degree of unsaturation of TG_s and the length of the saturated acyl chain present in glycerides. Thirteen types of soybean oil TG molecular species were isolated by high performance liquid chromatography. Among them, the TG groups most resistant to autoxidation were 2-oleoyl-1(3)-stearoyl-3(1)-palmitin, 1,3-dipalmitoyl-2-olein, 2-oleoyl-1(3)-oleoyl-3(1)-stearin and 2-oleoyl-1(3)-oleoyl-3(1)-palmitin.     

Cyclopentanoid analogs of phosphatidylcholine: susceptibility to phospholipase A2

Six isomers of dipalmitoylcyclopentanetriol phosphocholine (cyclopentano-lecithin) were tested as potential substrates for phospholipase A2. Since each of these analogs possesses a configuration that mimics a narrow range of conformations of a glycerophospholipid molecule, the analogs were used to assess the enzyme's conformational requirements. Studies showed that all of the analogs containing the phosphocholine at the C-1 (or C-3) position could be hydrolyzed, while only one of the three analogs that contains the polar head group at the C-2 position was susceptible. Kinetic studies, however, revealed that only the all-trans-(1,3/2-1P)-cyclopentano-lecithin gave initial rates of hydrolysis that were measurable by pH-stat. Acyl group specificity of the enzyme towards the all-trans isomer was determined with an analog was acyl groups were distinguishable. The synthesis of this mixed-acid-cyclopentano-PC is described herein. When this analog was enzymatically assayed, results unequivocally showed the enzyme to be specific for C-2 acyl hydrolysis. This specificity, and data showing that the all-trans analog is stereospecifically hydrolyzed, indicate that it is acted on in an analogous manner to dipalmitoylphosphatidylcholine. These studies indicate that although the configuration of the analog is not necessarily a prerequisite for hydrolysis, there does appear to be an optimal spatial orientation for enzymatic activity. The analogy between the susceptibilities of all-trans-(1,3/2-1P)-cyclopentano-lecithin and glycero-lecithin suggests that the conformation of the glycero-lecithin during phospholipase A2-mediated hydrolysis may be best simulated by the all-trans orientation of C-O bonds in the artificial substrate.     

Pseudononapeptide bombesin antagonists containing C-terminal Trp or Tpi.

Seven new antagonists of bombesin (Bn)/gastrin-releasing peptide (GRP) containing C-terminal Trp or Tpi (2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indol-3-carboxylic acid) in a reduced peptide bond were synthesized by solid phase methods and evaluated biologically. The reduced bond in four [Leu13 psi(CH2NH)Trp14]Bn(6-14) analogs was formed by reductive alkylation at the dipeptide stage. In the case of three [Leu13 psi(CH2N)Tpi14]Bn(6-14) analogs, the Trp dipeptide with reduced bond was reacted with formaldehyde to form the corresponding Tpi derivative. These Tpi-containing analogs have a new reduced bond which is structurally more constrained. Leu13 psi(CH2N)Tpi14 analogs inhibit [125I][Tyr4]bombesin binding to Swiss 3T3 cells with IC50 values of 2-4 nM, compared to 5-10 nM for Leu13 psi(CH2NH)Trp14 analogs. Leu13 psi(CH2N)Tpi14 analogs are also more potent than Leu13 psi(CH2NH)Trp14 analogs in growth inhibition studies using Swiss 3T3 cells. The two best bombesin antagonists of this series, [D-Trp6,Leu13 psi(CH2N)Tpi14]Bn(6-14) (RC-3415) and [Tpi6,Leu13 psi(CH2N)Tpi14]Bn(6-14) (RC-3440), inhibited GRP-stimulated growth of Swiss 3T3 cells with IC50 values less than 1 nM. RC-3440 was also active in vivo, suppressing GRP(14-27)-stimulated serum gastrin secretion in rats. Bombesin/GRP antagonists, such as RC-3440, containing the new reduced bond (CH2N) reported herein are very potent.     

Utilization of exogenous free fatty acids for the production of very low density lipoprotein triglyceride by livers of carbohydrate-fed rats

High carbohydrate diets enhance the hepatic output of very low density lipoprotein triglycerides. The fatty acids of these triglycerides could come from exogenous sources (i.e., diet or adipose tissue) or from de novo fatty acid synthesis in the liver. The role of exogenous free fatty acids was evaluated in rats fed Purina Chow or diets containing 10% fructose for up to 14 wk. In carbohydrate-fed rats, serum triglycerides were twice normal, and VLDL accounted for about 60% of the increases. Pre-beta-lipoprotein was increased and alpha- and beta-lipoprotein were decreased. Phospholipid and cholesterol levels were unchanged. Livers were perfused with glucose and free fatty acids. Perfusate free fatty acids rose from 180 to 1800 micro eq/liter as the infused acids increased from 0 to 992 micro eq/3 hr; simultaneously, net free fatty acid uptake rose from < 1 to 18 micro eq/g/hr and triglyceride output by the liver doubled. However, rates of secretion of triglyceride became constant, and triglyceride accumulated in liver at uptakes of free fatty acids > 13 micro eq/g/hr. More lauric and myristic acid appeared in the perfusate than was infused, suggesting the hepatic discharge of free fatty acids. Livers of fructose-fed rats secreted twice as much oleate-(14)C-labeled triglyceride as controls at all levels of free fatty acid uptake. The ratios of the specific activities of perfusate triglyceride to free oleate-(14)C were unaffected by diet and were about 0.6 and 1.0 at low and high triglyceride secretion rates, respectively. Thus, carbohydrate feeding did not result in altered uptakes of free fatty acids or preferential secretion of triglycerides containing endogenously synthesized fatty acid. Instead, the increased secretion of triglyceride was accomplished by enhanced formation of VLDL triglyceride from exogenous free fatty acids.     

Synthesis of Novel Acyclic Nucleoside Analogues with Anti-Retroviral Activity

A series of novel acyclic thymine nucleoside analogues were prepared by the Mitsunobu reaction from appropriately protected chiral triols. The enantiomeric triols were obtained from substituted γ-lactone acids, prepared by asymmetric oxidation of 3-substituted-1,2-cyclopentanediones. The cytotoxic activity of new analogues was evaluated on MCF-7 human breast cancer and HeLa cells, and antiviral activities on human immunodeficiency virus type 1 and hepatitis C virus models. The synthesized compounds revealed specific anti-retroviral activity and no cytotoxic side effects.     

Determination of enantiomeric purity of glycerides with a chiral PMR shift reagent.

Tris(3-heptafluorobutyryl-d-camphorato)europium(III), Eu(hfbc)₃ was used to determine the optical purities of enantiomeric mixtures of tri-, di- and monoglycerides with various fatty acid chain lengths by proton magnetic resonance (PMR). Synthesized model enantiomers were used to assign PMR signals. Enantiomeric signal separation becomes more difficult if the chain length difference between the fatty acids in the 1- and 3-positions of glycerol becomes smaller. The sign of the enantiomeric shift difference (ΔΔδ) of the terminal acyl CH₃ group of 1-acyl-2,3-distearoyl-sn-glycerol vs its enantiometer remains the same in the series acyl is hexanoyl, butyryl, propionyl, but is reversed for acetyl.The absolute configuration of the main triglyceride of the seed oil of Euonymus alatus was determined to be 3-acetyl-1,2-distearoyl-sn-glycerol and that of a monobutyryl triglyceride fraction from hydrogenated bovine butterfat was confirmed to be mainly 1,2-diacyl-3-butyryl-sn-glycerol. The enantiotopic behaviour of the glycerol CH₂ groups in (nearly) symmetric di- and triglycerides is discussed.     

The interfacial structure of phospholipid bilayers: differential scanning calorimetry and Fourier transform infrared spectroscopic studies of 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine and its dialkyl and acyl-alkyl analogs.

The thermotropic phase behavior of aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) and its 1,2-dialkyl, 1-acyl 2-alkyl and 1-alkyl 2-acyl analogs was examined by differential scanning calorimetry, and the organization of these molecules in those hydrated bilayers was studied by Fourier transform infrared spectroscopy. The calorimetric data indicate that substitution of either or both of the acyl chains of DPPC with the corresponding ether-linked hydrocarbon chain results in relatively small increases in the temperature (< 4 degrees C) and enthalpy (< 1 kcal/mol) of the lipid chain-melting phase transition. The spectroscopic data reveal that replacement of one or both of the ester-linked hydrocarbon chains of DPPC with its ether-linked analog causes structural changes in the bilayer assembly, which result in an increase in the polarity of the local environments of the phosphate headgroups and of the ester carbonyl groups at the bilayer polar/apolar interface. The latter observation is unexpected, given that ester linkages are considered to be intrinsically more polar that ether linkages. This finding cannot be satisfactorily rationalized unless the conformation of the glycerol backbones of the analogs containing ether-linked hydrocarbon chains differs significantly from that of diacyl glycerolipids such as DPPC. A comparison of the alpha-methylene scissoring bands and the methylene wagging band progressions of these lipids with the corresponding absorption bands of specifically chain-perdeuterated analogs of DPPC also supports the conclusion that replacement of the ester-linked hydrocarbon chains of DPPC with the corresponding ether-linked analog induces conformational changes in the lipid glycerol backbone. The suggestion that the conformation of glycerol backbones in the alkyl-acyl and dialkyl derivatives of DPPC differs from that of the naturally occurring 1,2-diacyl glycerolipid suggests that mono- and di-alkyl glycerolipids may not be good models of their diacyl analogs. These results, and previously published evidence that DPPC analogs with ether-linked hydrocarbon chains spontaneously form chain-interdigitated gel phases at low temperatures, clearly indicate that the properties of lipid bilayers can be substantially altered by small changes in the chemical structures of their polar/polar interfaces, and highlight the critical role of the interfacial region as a determinant of the structure and organization of lipid assemblies.     

Intracellular lipase activities in heart and skeletal muscle homogenates. The absence of trierucin cleavage by the heart: a possible biochemical basis for erucic acid lipidosis.

Rat heart and skeletal muscle homogenates were compared for their intracellular lipolytic activity towards a series of saturated and unsaturated triglycerides from trilaurin (C12:0) to trierucin (C22:1). It is shown that for all triglycerides esterified with fatty acids from C12 to C18, lipolytic activity in heart homogenates was higher than in skeletal muscle homogenates. For these triglycerides there was no relationship between the fatty acid chain length and the lipolytic activity. In both homogenates cleavage of unsaturated triglycerides was higher than cleavage of the homologous saturated triglyceride. Lipolysis of tri-delta-11-eicosenoin (C20:1) was similar in both homogenates but much lower than lypolysis of other triglycerides. Although cleavage of trierucin (C22:1) was very low in skeletal muscle homogenates, it was undetectable in heart homogenates, even when enzyme concentration was increased. A mixture of triglycerides did not show preferential hydrolysis of any simple triglyceride. Trierucin was the only triglyceride that did not complete for lipolytic activity and only with heart homogenates, which shows that that lipase(s) do not cleave trierucin. The absence of lipolytic activity towards trierucin in heart homogenates could explain the selective accumulation of erucic acid-rich triglycerides in hearts of animals fed a diet with a high erucic acid content.