Lipoprotein levels in genetically selected mice with increased susceptibility to atherosclerosis

Male mice of two inbred strains, C57BR/cdJ and CBA/J, were maintained either on an atherogenic diet rich in cholesterol (5%) and saturated fat (30%) or on a control Purina Laboratory Chow diet. After 3 weeks on a 90% atherogenic/10% chow diet, the C57BR/cdJ mice showed a fivefold increase in plasma total cholesterol levels, while the CBA/J strain indicated only a twofold increase. On the atherogenic diet, both groups of animals showed marked increases in a lipoprotein of a broad density spectrum (1.006 greater than d less than 1.063) with prebeta and beta mobility on agarose gel electrophoresis, but the C57BR/cdJ strain showed a greater increase than the CBA/J strain. The very low density lipoprotein (VLDL) contained predominantly lipoprotein of prebeta mobility, while intermediate density lipoprotein (IDL) and low density lipoprotein (LDL) contained lipoprotein of beta-mobility. In both strains, the major lipid component in this lipoprotein was cholesteryl ester and both strains showed large amounts of an apoprotein (apo) with E mobility as defined for known apoproteins by sodium dodecyl sulfate polyacrylamide gel electrophoresis or by isoelectric focusing on polyacrylamide-urea disc gels. The phosphatidylcholine/free cholesterol ratio of the VLDL, IDL, and LDL fractions isolated from the C57BR/cdJ mice on the atherogenic diet (0.76 to 0.95) was noticeably lower than that (1.10 to 1.19) from the CBA/J mice maintained on the same diet. There was a marked decrease in the high density lipoprotein (HDL) fraction in both strains of mice on the atherogenic diet, but a greater reduction in the HDL level of the C57BR/cdJ strain with a 50% decrease in the phosphatidylcholine/free cholesterol ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and preliminary characterization of 2-monoacylglycerol acyltransferase from rat intestinal villus cells

We have purified the monoacylglycerol acyltransferase from rat small intestinal mucosa to homogeneity by a combination of hydrophobic absorption, guanidine dissociation, and gel filtration. The purified enzyme gives a single band of 37 000 daltons on sodium dodecyl sulphate--polyacrylamide gel electrophoresis. The enzyme has a specific activity of about 5900 nmol/mg per hour and represents 0.12% of total cell protein, corresponding to about a 600-fold purification. The enzyme does not acylate diacylglycerols to triacylglycerols, which is consistent with the separate physical existence of the mono- and di-acylglycerol acyltransferases. The enzyme acylates the 2-monoacylglycerols to yield an essentially racemic mixture of diacylglycerols. It does not acylate glycerol-3-phosphate.

Comparative studies of triacylglycerol structure of very low density lipoproteins and chylomicrons of normolipemic subjects and patients with type II hyperlipoproteinemia

The triacylglycerols of very low density lipoproteins (VLDL) and of chylomicrons were analyzed in the fasting and postabsorptive states from normolipemic subjects and patients with Frederickson's Type II hyperlipoproteinemia, who subsisted on free choice diets, standard diets excluding lard, or were given a breakfast enriched in lard. The VLDL and chylomicrons were obtained by conventional ultracentrifugation, and the triacylglycerols were isolated by thin-layer chromatography (TLC). Representative sn-1,2-, sn-2-3- and sn-1,3-diacylglycerols were generated by partial Grignard degradation of the triacylglycerols and a stereospecific hydrolysis by phospholipase C of the mixed sn-1,2(2,3)-diacyl phosphatidylcholines prepared as intermediates. Representative sn-2-acylglycerols were obtained by hydrolysis with pancreatic lipase. Positional distribution of the fatty acids was established by subtracting in turn the fatty acid composition of the sn-2-position from the fatty acid composition of the sn-1,2- and sn-2,3-diacylglycerols. The molecular association of the fatty acids in the diacylglycerol moieties was determined by gas-liquid chromatography with mass spectrometry (GC/MS) of the tertiary-butyldimethylsilyl (t-BDMS) ethers. The molecular association of the fatty acids in the triacylglycerols was determined by 1-random 2-random 3-random calculation following experimental validation of the distribution. The results confirm a marked asymmetry in the positional distribution of the fatty acids in all triacylglycerol samples, with the palmitic acid predominantly in the sn-1-position, the unsaturated acids about equally divided between the sn-2- and sn-3-positions, and the stearic acid divided about equally between the sn-1- and sn-3-positions. The overall structure of the VLDL and chylomicron triacylglycerols from patients and control subjects was characterized by a non-correlative distribution of fatty acids under all dietary conditions.

Deacylation of endogenously deuterated rat liver microsomal phospholipids by endogenous phospholipases

The relative deacylation of microsomal phospholipid molecular species was reexamined. Microsomal membranes were prepared from the livers of rats injected, over a period of 20 h, with perdeuterated ethanol. The phosphatidylcholine and phosphatidylethanolamine were isolated by thin-layer chromatography of the total lipid extracts and the distribution of deuterium among the molecular species of the diacylglycerol moieties was determined by reversed-phase high pressure liquid chromatography in combination with chemical ionization mass spectrometry. Deuterium was found to be incorporated into newly formed glycerol and newly synthesized palmitic and stearic acids (4-22% replacement) which were distributed throughout the molecular species in proportion to their relative rates of turnover (10-45% replacement). Within each unsaturation class, the palmitoyl species were labelled more extensively than the corresponding stearoyl species. Following in vitro incubation of the membranes with 10 mM Ca2+ at pH 8.5, there occurred a rapid degradation of the phosphatidylethanolamines (to 60% of control values after 90 min), while the phosphatidylcholines remained essentially unaffected. The various molecular species of the phosphatidylethanolamines were degraded linearly and in proportion to their masses. The deuterium content of the phosphatidylethanolamine and phosphatidylcholine remained constant throughout the incubation. It was concluded that under the present experimental conditions all molecular species of phosphatidylethanolamine, both old and newly synthesized, are equally accessible to the endogenous phospholipase.

Rapid isolation of a triacylglycerol synthetase complex from rat intestinal mucosa

A triacylglycerol synthetase complex made up of acyl-CoA synthetase, acyl-CoA:monoacylglycerol acyltransferase, and acyl-CoA:diacylglycerol acyltransferase has been solubilized by sodium taurocholate and isolated by chromatography on phenyl-Sepharose. For this purpose microsomes of the villus cells of rat intestinal mucosa were dissolved in 2% sodium taurocholate prepared in 1 M (NH4)2SO4 and 25 mM Tris-HCl (pH 8.5) (buffer A). After dialysis against buffer A, the sample was loaded on a phenyl-Sepharose column and the enzyme complex was eluted with 25 mM Tris-HCl (pH 8.5) (buffer B). The enzymatically active fractions were pooled and rechromatographed on a Bio-Gel A-0.5m column equilibrated with buffer B and 150 mM NaCl. The recovered triacylglycerol synthetase complex accounted for over 75% of the original enzyme activity and represented a 145-fold purification from villus cells and a 10-fold purification from microsomes. It exhibited maximal activity at pH 8.0-9.0. On the basis of Bio-Gel A-0.5m chromatography the three enzymic activities appeared as a single fraction in the molecular weight range of 350 000-375 000. The complex migrated as a single peak on high performance liquid chromatography on an ion-exchange column using a NaCl gradient. The ratio of the activities of the three enzymes remained constant during the purification. The purified enzyme complex lost about 50% of its diacylglycerol acyltransferase activity on storage for 2 weeks at -20 degrees C in presence of phenylmethylsulfonyl fluoride.

Acyl selectivity in the transfer of molecular species of phosphatidylcholines from human erythrocytes

This report describes the molecular species composition of phosphatidylcholines (PC) transferred from human erythrocytes to acceptor vesicles composed of cholesterol and single PC species in the presence of PC-specific transfer protein from bovine liver. The compositions of the PC isolated from the vesicles were determined by capillary GLC as the diacylglycerol trimethylsilyl ethers. The cellular PC species appearing in the acceptor vesicles were enriched in unsaturated species and showed a low content of dipalmitoyl PC compared to untreated erythrocytes. This trend was independent of the composition of the PC used to construct the acceptor vesicles and it was possible to determine that the relative rates of efflux of the palmitoyl-containing phosphatidylcholines decreased in the order: palmitoyl-linoleoyl greater than palmitoyl-oleoyl greater than dipalmitoyl and in the stearoyl series, stearoyl-linoleoyl greater than stearoyl-oleoyl. No clear trend was distinguished for the influence of chain-length on the efflux, thus preventing an unambiguous assignment of the order of removal of all species from the cell membrane. Results derived for arachidonoyl-containing species were compromised by evidence for oxidation occurring during incubations at 37 degrees C. To confirm that acyl selectivity was also possible during transfer in the absence of the transfer protein, the efflux of 14C-labeled soya PC and [14C]dipalmitoyl PC from prelabeled erythrocytes was measured using plasma as the acceptor. As predicted by the chromatographic analyses, 14C-labeled soya PC effused up to 10-times faster than [14C]dipalmitoyl PC from the red cell membrane. Thus, the more rapid transfer of unsaturated PC cannot be explained entirely as a specificity of the transfer protein and is consistent with the hypothesis that intermolecular interactions involving PC molecules within the erythrocyte membrane, become weaker with increasing unsaturation. The results suggest a potential role of PC-specific transfer protein as a probe of the nature of PC interactions within biological membranes.

Milk and plasma lipid composition in a lactating patient with type I hyperlipoproteinemia

This report describes studies on the plasma and milk lipid composition of a patient with primary Type I hyperlipoproteinemia who had been followed through her second pregnancy. Post-partum she lactated, supplying milk for assay. It was abnormal in the low content of its total lipid and in the bizarre composition of its fatty acids. The proportion of long chain fatty acids was unusually low, and that of medium chain fatty acids unusually high. Furthermore, the fatty acids of the patient's milk differed greatly from those of her plasma triglycerides. This was in marked contrast to normal nursing mothers' milk, in which the fatty acid composition is comparable to that of plasma triglycerides. The patient's milk fatty acids were shorter in chain length and deficient in essential fatty acids. During the time of lactation, the patient remained hyperlipidemic and her post-heparin plasma had no lipolytic activity. These data and the differences between the plasma and milk fatty acids suggested that in the patient the circulating triglyceride fatty acids did not enter the mammary gland. Without preformed fatty acids entering it from plasma or adipose tissue, the lactating breast apparently synthesized fatty acids de novo. These newly synthesized fatty acids were of medium, rather than long chain length. This accounted for the abundance of medium chain length triglycerides in the patient's milk. The studies suggested that the deficit of lipoprotein lipase in Type I hyperlipoproteinemia extended to the mammary gland.

Determination of plasma total lipid profiles by capillary gas-liquid chromatography

We have obtained improved qualitative and quantitative information about plasma lipid composition by adopting capillary gas-liquid chromatography to the determination of plasma total lipid profiles. The new procedure takes advantage of the good recoveries of high molecular weight solutes provided by short capillary columns as well as of the absence of sample fractionation during on-column injection. The major improvements in the gas-liquid chromatographic determination of plasma lipid profiles are the greatly increased carbon number resolution of the solutes and the minimal sample size requirements on the capillary columns. In many instances there is a partial resolution of the saturated and unsaturated molecules within a carbon number, which provides clues to the fatty acid composition of the molecular species. In other respects the new plasma total lipid profiles are similar to those obtained on packed columns. The plasma lipids are dephosphorylated by phospholipase C and trimethylsilylated before analysis.

Milk fat structure of a patient with type 1 hyperlipidemia

Structural analyses were performed on milk fat samples obtained 3-10 days postpartum from a lactating patient with primary Type 1 hyperlipidemia. The milk triacylglycerols contained 3-7% C10, 14-21% C12, 20-30% C14, 22-26% C16 and 20-30% C18 (largely oleic) acids. Gas liquid chromatographic (GLC) analyses of the X-1,3- and X-1,2-diacylglycerols on polar siloxane columns showed a markedly non-random association of acyl chains. Stereospecific analyses indicated that the short chain length fatty acids were confined essentially to the sn-3-position of the triacylglycerol molecule. Furthermore, these acids were largely absent from the phosphatidylcholines and the endogenous sn-1,2-diacylglycerols of the milk fat. It is concluded that the short chain fatty acids are incorporated into the milk triacylglycerols during the final stage of biosynthesis via the phosphatidic acid pathway, and that the overall fatty acid distribution is consistent with the 1-random 2-random 3-random hypothesis.

Studies of triacyglycerol structure of very low density lipoproteins of normolipemic subjects and patients with type III and type IV hyperlipoproteinemia

The triacylglycerols of very low density lipoproteins (VLDL-TG) were analyzed in samples from normal subjects and patients with Frederickson's Type III and Type IV hyperlipoproteinemia. VLDL were obtained by conventional ultracentrifugation, and the triacylglycerols were isolated by thin-layer chromatography (TLC). Representative sn-1,2(2,3)- and sn-1,3-diacylglycerols were generated by Grignard degradation of the triacylglycerols, and were resolved by TLC on borate-treated silica gel. The molecular association of the fatty acids in the diacylglycerol moieties was determined by gas-liquid chromatography with mass spectrometry (GC/MS) of the tertiary-butyldimethylsilyl ethers. The positional distribution of the fatty acids was established by the Brockerhoff stereospecific analysis. The results showed a marked asymmetry in the distribution of the fatty acids in all samples, with the saturated acids predominantly in the sn-1-position and the unsaturated fatty acids distributed about equally between the sn-2- and sn-3-positions. In all instances, the molecular species composition of the sn-1,2-, sn-2,3- and sn-1,3-diacylglycerols was found to be similar to that calculated for 1-random 2-random 3-random distribution of triacylglycerols. There were marked differences in the quantitative composition of the molecular species of the VLDL-TG between normal subjects and patients, but these discrepancies were attributed to differences in the fatty acid composition of the samples.

Molecular species of plant phosphatidylinositol with selective cytotoxicity towards tumor cells

The molecular species of highly purified phosphatidylinositol from soybeans were determined as an aid in the investigation of the mechanism of their reported selective cytotoxicity towards tumor cells. Unlike the animal phosphatidylinositol, which contains predominantly stearic acid in the sn-1 and arachidonic in the sn-2 position (18:0 20:4), the soybean phosphatidylinositol was found to contain mainly linoleic acid in the sn-2 position and palmitic (16:0 18:2), stearic (18:0 18:2) and linoleic (18:2 18:2) acids in the sn-1 position of its molecular species.

Resolution of alkenylacylglycerol moieties of natural glycerophospholipids by gas-liquid chromatography on polar capillary columns

A sensitive and practical procedure has been developed for the determination of the sn-1-alk-1-enyl-2-acylglycerol moieties of natural glycerophospholipids using capillary gas-liquid chromatography with flame ionization detection. The method gives excellent resolution and quantitative estimates for molecular species according to carbon number and degree of unsaturation. The diradylglycerols are released from the glycerophospholipids by hydrolysis with phospholipase C and the alk-1-enylacylglycerols are resolved by thin-layer chromatography. Either the trimethylsilyl or the tertiary-butyldimethylsilyl ethers are then resolved at 250 degrees C by gas-liquid chromatography on a glass capillary column wall-coated with a polar cyanopropylsiloxane polymer using hydrogen as the carrier gas. The method has been applied to the analysis of the alk-1-enylacyl- and diacyl-glycerol moieties of the phosphatidylethanolamines of rat heart and kidney.

Resolution and quantitation of diacylglycerol moieties of natural glycerophospholipids by reversed-phase liquid chromatography with direct liquid inlet mass spectrometry

The sn-1,2-diacylglycerol moieties of natural phosphatidylcholines as the tertiary-butyldimethylsilyl ethers were resolved on the basis of their carbon number and degree of unsaturation by high pressure liquid chromatography on reversed-phase C18 columns. Using acetonitrile and propionitrile as eluting solvents and reagent gases, the yields of both quasi molecular and fragment ions were found to vary with the degree of unsaturation and positional distribution of the fatty acids in the diacylglycerol molecules, and appropriate calibration factors were necessary for accurate quantitation. In the absence of pure structural isomer and mixed acid standards, we have determined preliminary calibration factors for total and specific ion current responses, by comparing the peak area ratios obtained by liquid chromatography – mass spectrometry with the weight and mole proportions of molecular species known to be present in the samples from detailed analyses by capillary gas–liquid chromatography on polar liquid phases. It was found that the total chemical ionization current response agreed closely with the weight composition of the molecular species. The relative yields of the (MH – 132)+ ions varied over one- to three-fold, while those of the (MH – RCOOH)+ ions varied over a three- to four-fold range of intensities. After suitable calibration of the relative ion response, it was possible to determine the identity and quantity of all common molecular species in the test diacylglycerophospholipids. Although the derived factors include both chromatographic and mass spectrometrie effects and are obtained with a gradient of reagent gases, they appear to be generally applicable.

An examination of the stereochemical course of acylation of 2-monoacylglycerols by rat intestinal villus cells using [2H3]palmitic acid

The stereochemical course of acylation of 2-monoacylglycerols by rat intestinal mucosa was investigated using isolated villus cells with 2-lauroylglycerol and [2H3]palmitic acid as substrates. The newly synthesized X-1,2-diacylglycerols and triacylglycerols were recognized on the basis of the content, positional distribution and molecular association of the fatty acids by gas-liquid chromatography/mass spectrometry in combination with sterospecific analysis. It was found that the free X-1,2-[2H3]palmitoyllauroylglycerols contained 74% sn-1,2- and 26% sn-2,3-enantiomers, which were utilized for triacylglycerol formation in the same proportion. Some of the 2-monolauroylglycerol became hydrolyzed and the released lauric acid was utilized along with [2H3]palmitic acid for diacylglycerol and triacylglycerol formation via both the phosphatidic acid and the 2-monoacylglycerol pathway. The contribution of the phosphatidic acid pathway (12-16%) was determined by estimating the relative proportion of the newly synthesized triacylglycerols containing [2H3]palmitic acid in the sn-2 position. After correcting for the contribution of the phosphatidic acid pathway, the sn-1,2-/sn-2,3-enantiomer ratio in the X-1,2-[2H3]palmitoyllauroylglycerol was estimated to be 71/29. These results are consistent with those previously obtained with glycerol-labelled 2-monoacylglycerols and intact tissue or homogenates of rat intestinal mucosa. It is suggested that the 2-mono-acylglycerol transacylase, if a single enzyme, possesses a low stereospecificity, or that two enzymes exist of unequal concentration and/or activity.

Quantitation of natural triacylglycerols by reversed-phase liquid chromatography with direct liquid inlet mass spectrometry

Using acetonitrile and propionitrile as eluting solvents and reagent gases the yields of both quasi-molecular and fragment ions were found to vary with the molecular weight, degree of unsaturation and positional distribution of the fatty acids in the triacylglycerol molecule, and appropriate calibration factors were necessary for accurate quantitation. In the absence of pure structural isomers and mixed acid standards, preliminary calibration factors have been determined for total ion and specific ion current responses by comparing the peak area ratios obtained by liquid chromatography-mass spectrometry with the proportions of the molecular species known to be present in randomized oils and in natural oils of known chemical composition. Although the derived factors include both chromatographic and mass spectrometric effects and are obtained with a gradient of reagent gases, they appear to be generally applicable. It was shown that positional isomers affected the yield of the (MH-RCOOH)+ ions over a 1-3-fold range of intensities, while the nature of the fatty acid affected it over a range of 1.25-fold. After suitable calibration of the relative ion responses it was possible to determine the identities and amounts of the individual molecular species in natural fats and oils.

Effect of intravenous infusion of Intralipid, cholesterol, and plant sterols on hepatic cholesterogenesis

Male Wistar rats were infused intravenously with various amounts (2-20 mL/24 h) of Intralipid or Intralipid containing cholesterol or plant sterols (5-100 mg/24h), and hepatic cholesterogenesis was monitored by measuring the incorporation of [1-14C]acetate into the nonsaponifiable sterols of liver slices. It was observed that the infusion of Intralipid alone resulted in a hypercholesterolemia that varied with the amount of Intralipid administered and that it was accompanied by up to a threefold increase in hepatic cholesterogenesis. Inclusion of cholesterol in the Intralipid at 5 mg/mL prevented the increase in hepatic cholesterol biosynthesis, while an inhibition of up to 95% of control synthesis was achieved when a total of 33 mg of cholesterol in 20 mL Intralipid was infused over a 24-h period. It is concluded that the feedback regulation of cholesterol biosynthesis is operative even when the entry of cholesterol bypasses the intestine and the lipoprotein synthesis taking place there and that Intralipid is a suitable medium for the intravenous introduction of a large mass of metabolically active cholesterol molecules. Similar infusions of mixed plant sterols failed to prevent the activation or inhibition of cholesterol biosynthesis, but the administration of much larger doses of plant sterols (100 mg) brought about a partial inhibition of hepatic cholesterogenesis. It is concluded that the presence of an alkyl group in the side chain prevents the plant sterols from an effective interaction with the critical sites of the feedback regulatory system of cholesterogenesis. The effects of the larger doses of plant sterols were attributed to the displacement of increasing amounts of free cholesterol from the vascular tissues, which resulted in an effective elevation of plasma cholesterol levels. The infusion of either cholesterol or plant sterols over the 24-h period did not appear to have a consistent effect upon the composition or secretion of biliary bile acids.

Simultaneous quantitation of Krebs cycle and related acids by mass fragmentography

Methods are described for simultaneous quantitation of Krebs cycle and related acids by gas chromatography--mass spectrometry using deuterium-labelled acids and n-butyl-d9-esters of the organic acids as internal standards. Using sulphuric acid as esterification catalyst, only lactic, succinic, fumaric, malic, maleic and citric acids were found to be stable to hydrogen exchange and could be used as reference standards in the deuterated form. In contrast, pyruvic, oxalacetic, alpha-ketoglutaric and malonic acids were found to exchange their deuterium readily and could not be employed for this purpose. All the acids could be quantitated using n-butyl-d9-esters of reference organic acids as internal standards, following a separate preparation of the n-butyl derivatives of the unknown acids. The method is suitable for routine analysis of organic acids at the picogram level in perchloric acid extracts of tissues.

Critical role of ring structure in the differential uptake of cholesterol and plant sterols by membrane preparations in vitro

To determine the role of the ring structure in the differential absorption of sterols, we have used rat jejunal brush border vesicles and erythrocytes to examine the uptake of cholesterol, campesterol, and sitosterol following successive chemical degradations of rings A and B. The cell and membrane preparations were incubated with the sterols and sterol analogues (about 30 micromolar each) dissolved in 7 mM sodium taurocholate and 0.6 mM egg phospholipid. The uptake of the analogues was analyzed by high performance liquid chromatography and capillary gas--liquid chromatography. In both membrane preparations, the uptake of the 7-dehydroanalogues of cholesterol, campesterol, and sitosterol was linear with time. 7-Dehydrocholesterol was absorbed 4-5 times faster than 7-dehydrositosterol by both preparations. The uptake of the campesterol analogue was intermediate between that of the analogues of cholesterol and sitosterol at all time points. Following conversion of the 7-dehydrosterols to their calciferol derivatives, the 27-carbon sterols were absorbed only 1.9 and 1.4 times faster than those of the 29-carbon sterols by the erythrocyte and brush border membranes, respectively. A similar degree of selectivity was expressed in the erythrocytes during the uptake of a steroid series possessing keto-4-ene ring system. Complete oxidation of the calciferol derivatives to the des-AB-8-ones resulted in a total loss of discrimination among the various side-chain homologues during absorption from micellar solutions. It is concluded that the selective absorption of animal and plant sterols depends upon the presence of a ring system having the bulk of the cholestane nucleus, although not necessarily a rigid or planar one containing a hydroxyl group.

Adsorption of rat plasma phosphorylcholine-binding protein to synthetic lipid emulsions in the presence of divalent cations

Lipoproteinlike lipid emulsions were incubated with rat serum, plasma, or plasma-derived serum. Two previous unidentified components which bound to these emulsions only in the presence of divalent cations were identified as the rat plasma phosphorylcholine-binding proteins.

Studies on the substrate specificity of purified human milk bile salt-activated lipase

The fatty acid specificity of the bile salt-activated lipase purified from human milk was studied using C12 to C54 (total acyl carbon) saturated and the C54 unsaturated triacylglycerols. Kinetic studies indicated that the short chain triacylglycerols were hydrolyzed more readily than the long chain triacylglycerols, and that the long chain unsaturated triacylglycerols were attacked more readily than the long chain saturated triacylglycerols. This fatty acid specificity was also apparent intramolecularly, both short chain and unsaturated fatty acids being released at higher rates than the saturated long chain acids. The enzyme possessed neither positional specificity nor stereospecificity as indicated by the nearly simultaneous appearance of the sn-1,2-, sn-2,3-, and sn-1,3-dioleoylglycerols from trioleoylglycerol. The hydrolyses of these three ester bonds were approximately at their anticipated chemical reactivities. Synthetic rac-1-monooleoylglycerols were hydrolyzed about 2 times faster than the sn-2-monooleoylglycerols. It is concluded that the bile salt-activated lipase may possess a special potential for a rapid release of short chain and polyunsaturated fatty acids from dietary triacylglycerols in the intestinal lumen of infants.

Analysis of triacylglycerols by reversed-phase high pressure liquid chromatography with direct liquid inlet mass spectrometry

Natural triacylglycerols were resolved by high pressure liquid chromatography on Supelcosil LC-18 columns using a gradient of 30-90% propionitrile in acetonitrile as eluting solvent. The effluent was admitted to a quadrupole mass spectrometer via a direct liquid inlet interface. The mass spectra of the solutes were recorded in the chemical ionization mode. The triacylglycerol elution profile was obtained from the total ion current. Individual molecular species of the triacylglycerols were identified from the (MH)+ and the (MH--RCOOH)+ ions. The reversed-phase system allowed the separation of triacylglycerols on the basis of both carbon number and double-bond number, as well as of certain critical pairs of triplets of triacylglycerols with the same partition number. The method is applicable to the determination of triacylglycerol composition of both plant and animal fats. The minimum amount of sample which is required is about 50 ng in the scanning mode. However, 100 times more material must be injected in the liquid chromatograph because only 1/100th of the effluent is being injected into the mass spectrometer.

Differential transfer of C and E apolipoproteins from very-low-density lipoprotein to lysophosphatidylcholine-phosphatidylcholine micelles

Rat plasma VLDL was incubated with lysoPC/PC micelles consisting of 45-90 mol% lysoPC at micelle/VLDL phospholipid ratios of 0.33-6. Following incubation, the VLDL and micellar particles were reisolated by ultracentrifugation and the lipid and apolipoprotein composition determined by high-temperature gas chromatography and polyacrylamide gel electrophoresis, respectively. Lysophosphatidylcholine was found to equilibrate between the very-low-density lipoproteins and micellar fraction without transfer of phosphatidylcholine or sphingomyelin between these fractions. This produced reisolated VLDL and micellar particles with nearly identical lysoPC/PC ratios. Only apolipoprotein E transferred to reisolated micellar fractions with less than 35 mol% lysoPC. The C apolipoproteins were also transferred to the micellar fraction when the reisolated micelles contained more than 35 mol% lysoPC. It is concluded that apolipoproteins E and C can bind to HDL-size micellar particles of appropriate composition. The differential transfer of apolipoproteins E and C indicates that fundamental differences exist between these apolipoproteins in their interaction with lipid interfaces.

Plasma phosphatidylcholine/free cholesterol ratio as an indicator for atherosclerosis

Plasma lipid profiles were determined in two inbred strains of mice, C57BR/cdJ and CBA/J, fed either a normal chow or an atherogenic diet for a 15-week period, starting at 10 weeks of age. On the chow diet, the C57BR/cdJ had significantly higher mean free cholesterol, esterified cholesterol, and total lipid values, and a significantly lower mean phosphatidylcholine/free cholesterol ratio than the CBA/J mice. On the atherogenic diet, the C57BR/cdJ had significantly higher mean levels for all lipids classes, except triacylglycerols, than the CBA/J mice. The mean plasma free cholesterol and esterified cholesterol levels of the C57BR/cdJ were four times greater than those of the CBA/J strain on the atherogenic diet. The mean plasma phosphatidylcholine/free cholesterol ratio of the C57BR/cdJ mice on the high cholesterol diet was 0.87 compared to 1.91 for CBA/J mice. These plasma lipid changes were associated with a marked development of atheromatous deposits in the wall of the aortic sinus of the C57BR/cdJ compared to the CBA/J animals. The phosphatidylcholine/free cholesterol ratios of the liver lipids of both strains decreased from 2.5-2.7 on the chow diet to 1.0-1.1 on the high cholesterol diet. It is suggested that a plasma phosphatidylcholine/free cholesterol ratio less than 1 represents a supersaturation of the vascular system and the vessel wall with cholesterol, which leads to a destabilization of the plasma membranes of the endothelial and smooth muscle cells, and an infiltration of the vessel wall by the plasma lipids.

Further evidence for enhanced phospholipid synthesis by rat jejunal villus cells during fat absorption

The effect of fat absorption on the phospholipid turnover of rat intestinal mucosa was determined in animals receiving single fatty meals by stomach tube or multiple meals in the form of corn-oil-soaked laboratory chow diet. The specific activity and relative specific activity of the total phospholipids and of individual phospholipid classes were measured in the isolated jejunal villus cells of fasting and fat-fed animals following an injection of radioactive inorganic phosphate 0.5-31 h prior to sacrifice, which was scheduled to coincide with the peak of fat absorption (2.5-3 h after the last meal). It was shown that the relative specific activity of the fat-absorbing cells increased by about 33% when the samples were taken 0.5 h after intravenous injection of radioactive phosphate. Samples taken 11 and 31 h after the introduction of the radioactive phosphate showed about 16% decrease in the relative specific activity of the phospholipids of the fat-absorbing cells when compared with the fasting controls. These changes in the relative specific activity of the total phospholipids included all phospholipid classes and corresponded to the recently described expansion of the cellular phospholipid pool owing partly to increased de novo synthesis of the membrane phospholipids. The present results are consistent with the known biochemical and physiological changes taking place in the mucosal cells during fat absorption and transport and find support in various less direct biochemical and morphometric measurements.