The anomalous kinetics of coupled aspartate aminotransferase and malate dehydrogenase. Evidence for compartmentation of oxaloacetate.

The optimum cofactor requirements for triacylglycerol biosynthesis in rat adipose-tissue homogenates containing mitochondrial, microsomal and cytosolic fractions were investigated. In general the optimum concentrations of cofactors for triacylglycerol biosynthesis were found to differ from those for total fatty acid esterification. The results provided further evidence for the key role of phosphatidate phosphohydrolase in the regulation of triacylglycerol biosynthesis. Albumin was included in the incubation medium to permit the use of concentrations of added fatty acids that would swamp the effects of endogenous fatty acids. The addition of albumin had little effect on the incorporation of palmitic acid and stearic acid into lipids including triacylglycerols. By contrast, a critical concentration of albumin (about 60 muM) was required before incorporation of oleic acid or linoleic acid into triacylglycerols occurred. The system was used to study the incorporation of different 1-14C-labelled fatty acids from a mixture of unesterified fatty acids [palmitic acid 30%; stearic acid 10%; oleic acid 40%; linoleic acid 20% (molar percentages)] separately into the positions 1,2 and 3 of triacyl-sn-glycerols. In general the stereo-specific distribution of the labelled fatty acids incorporated into triacylglycerols paralleled the normal distribution of fatty acids within rat adipose-tissue triacylglycerols, suggesting that the specificities of the relevant acyltrasferases have the major role in determining the positional distribution of fatty acids within triacylglycerols.     

Palmitic acid incorporation into intramuscular acylglycerols depends on both total and unbound to albumin palmitic acid concentration.

Palmitic acid incorporation into the intramuscular acylglycerols in rat skeletal muscles of different fiber types was investigated at various total and unbound to albumin concentrations by means of the hind-limb perfusion technique. It was found that at simultaneously increasing total and unbound to albumin palmitic acid concentrations in the perfusion medium the incorporation of palmitic acid into acylglycerols increased. However, when the concentration of palmitic acid not bound to albumin was kept constant and the total palmitic acid concentration was increased, the incorporation also increased although markedly less than under former conditions. The increase was most apparent in the muscles composed of slow-twitch oxidative and fast-twitch oxidative-glycolytic fibers where fatty acid uptake is the greatest. These findings suggest that fatty acid incorporation into intramuscular acylglycerols depends not only on the unbound to albumin fatty acid concentration but also, to some extent, on the total fatty acid concentration.     

Essential fatty acid uptake and esterification in primary culture of rat hepatocytes

Primary cultures of adult rat hepatocytes were used to compare the uptake and esterification of essential polyunsaturated fatty acids (18:2, 20:3 and 20:4 of the n-6 series) with those of palmitic and oleic acids. The uptake of unesterified fatty acids was linearly related to the free fatty acid/albumin molar ratio for 14 h and did not depend on the unbound free fatty acid level. Whatever the initial free fatty acid/albumin molar ratio, it dropped to 0.5 +/- 0.1 mM after 14 h, thus showing that hepatocytes have a high capacity for clearing free fatty acids from the medium at high free fatty acid/albumin molar ratios. The free fatty acid uptake become saturable when the free fatty acid and albumin concentrations were raised and the free fatty acid/albumin ratio remained constant. This strongly suggests that albumin-hepatocyte interaction mediates free fatty acid uptake. This uptake was identical whatever the fatty acid tested and did not depend on the relative amounts of fatty acids when they were added simultaneously. Triacylglycerol accumulation and synthesis, monitored by labelled fatty acids, were related to the free fatty acid/albumin molar ratio and exhibited no specificity for the series of fatty acids tested. Triacylglycerols were enriched in all the fatty acids tested by up to 60%, and fatty acid incorporation into diacylglycerols and triacylglycerols reflected the free fatty acid composition of the medium. By contrast, neither the level nor the synthesis of phospholipids varied with free fatty acid/albumin, but the rate of phospholipid turnover depended on the fatty acids tested. Accumulation of these acids was smaller in phospholipids than in triacylglycerols. When linoleic and arachidonic acids were added together, phospholipids (especially phosphatidylethanolamine and phosphatidylinositol) were more enriched in arachidonic acid than triacylglycerols. This might be due to the specificity for fatty acid of the enzymes involved in phospholipid metabolism.     

Marked acceleration of exogenous fatty acid incorporation into cellular triglycerides by fetuin.

Fetuin belongs to a group of fetal glycoproteins whose specific function is not known. In this study we investigated the effect of bovine fetuin on exogenous fatty acid incorporation into lipid classes by fetal rabbit aortic smooth muscle cells (SMC) and human fetal skin fibroblasts. When compared with albumin, the addition of fetuin to the culture medium caused a dramatic increase in labeled fatty acid incorporation (nanomoles/mg of protein) by SMC into triglycerides (albumin (control) 2.8 +/- 0.3 + fetuin 178.3 +/- 13.7). This effect was noted at a wide range of fetuin concentrations (0.2-5%) at oleate:fetuin molar ratios of 3.3-0.13, respectively. Similar effects were noted using human fetal skin fibroblasts with both labeled oleic and arachidonic acids (0.1 mM) as substrates (arachidonic acid incorporation into triglycerides, albumin (control) 76.9 +/- 16.2 + fetuin 684.6 +/- 64.1). Stimulation of fatty acid incorporation into di- and monoglycerides was also noted. Although the amount of unbound fatty acid in the presence of fetuin was greater than with albumin, experiments done under conditions that create identical unbound oleate levels (by varying fatty acid concentration) still showed increased fatty acid incorporation into triglycerides by SMC when exposed to fetuin. This marked effect of fetuin on triglyceride accumulation in cells was confirmed by lipid analysis, strong positive staining with oil red O, and transmission of electron microscopy. Furthermore, the potential physiological role of fetuin in terms of fatty acid and transport was attested by (a) the presence of significant amounts of free fatty acids associated with fetuin; and (b) by the stimulatory effect of fetuin, even when added to culture media containing other fatty acid carriers. These results show that (a) fetuin is far more efficient than albumin in incorporating fatty acids into cells; and (b) this might represent a novel function for fetuin during development.     

Influence of diet on the incorporation of radioactive fatty acids into the glycerolipids during development of the insectCeratitis capitata

1. Larvae ofCeratitis capitata were reared on three different lipid-containing diets. The levels of incorporation of palmitic and linoleic acids into triacylglycerols, phosphatidylethanolamine and phosphatidylcholine, from larvae and pharate adults, were determined.2. The modified diets cause an increase in the incorporation of linoleic and palmitic acids into the larvae triacylglycerols and pharate adult phospholipids.3. The addition of saturated fatty acids to the diet cause a higher incorporation of palmitic acid, than that of linoleic acid, into the larvae and pharate adult triacylglycerols.4. In pharate adults coming from larvae reared on modified diets, there is a higher incorporation of linoleic acid into phospholipids compared with those of larvae.5. These results indicate that dietary fatty acids have an influence on larvae triacylglycerol and pharate adults phospholipid biosynthesis.     

Effect of hippurate on tissue fatty acid metabolism

The authors studied 1-14C-palmitate metabolism in rat muscle, renal cortex and liver incubated with synthetic hippurate in vitro (1 mmol/l). a) Hippurate did not affect 1-14C-palmitate uptake and utilization in the muscle (hemidiaphragm). b) In the renal cortex it stimulated only the incorporation into total lipids and from the individual lipid fractions into mono- and diglycerides and free fatty acids (FFA). c) In the liver it stimulated the uptake, oxidation to 14CO2 and incorporation into total lipids and, out of the individual lipid fractions, into phospholipids, triacylglycerols and free fatty acids. d) Hippurate already had a significant effect in the concentration of 0.5 mmol/l, i.e. during the development of the disturbance and not just as a supplementary factor in advanced renal insufficiency. It is concluded that, by interfering with fatty acid metabolism, the hippurate present in the serum of patients with renal insufficiency plays an active role in the development of dyslipoproteinaemia in such patients.     

Biochemistry of development in insects. Stereospecific incorporation of fatty acids into triacylglycerols.

1. Previous experiments showed that fatty acids were incorporated into triacylglycerols by homogenates of Ceratitis capitata larvae far more efficiently than by pharate adult homogenates. This metabolic behaviour of both stages of development of the insect has been interpreted throughout the existence of a different acyltransferase activity. To obtain new data on the acyltransferase mechanism, a time-course of the stereospecific incorporation of labelled myristic, palmitic, oleic and linoleic acids into the sn-positions of triacylglycerols has been followed. 2. Studies on the stereospecific incorporation of labelled fatty acids confirmed previous results. Palmitic acid was mainly incorporated into sn-1 and sn-3 positions whereas position 2 exhibited a low incorporation. Myristic acid acylated sn-3 position at a higher rate than it acylated the other sn-positions. Oleic acid was more specifically distributed than palmitic acid and linoleic acid was more efficiently incorporated than the monounsaturated acid. All these data reflect substrate differences in the acyltransferase activity of larval homogenates. Pharate adult homogenates incorporated fatty acids very scarcely and mainly into positions (1 + 3). 3. Kinetics of incorporation of labelled fatty acids into the sn-positions points to a non-random distribution with respect to the major saturated and unsaturated fatty acids in triacylglycerols of larvae of Ceratitis capitata.     

In vitro and in vivo synthesis of long-chain fatty acids from (1-14C) acetate in the renal papillae of rats.

1. The relationship between the rate of [1-14C] acetate incorporation into the fatty acids of renal papillary lipids and the acetate concentration in the medium has been measured. 2. [1-14C] acetate was incorporated mainly into fatty acids of phospholipids and triacylglycerols. Only a few per cent of the radioactivity was found in the free fatty acid fraction. 3. The major part of the [1-14C] acetate was found to be incorporated by a chain elongation of prevalent fatty acids. The major component of the poly-unsaturated fatty acids in triacylglycerols and the major product of fatty acid synthesis from [1-14C] acetate in vitro was demonstrated by mass spectrometry to be docosa-7,10,13,16-tetraenoic acid. 4. The radioactivity of docosa-7,10,13,16-tetraenoic acid accounted for 40% of total radioactivity in triacylglycerol fatty acids (lipid droplet fraction) and 20% of total radioactivity in membrane phospholipid fatty acids.     

Selective retention of n-3 and n-6 fatty acids in human milk lipids in the face of increasing proportions of medium chain-length (C10-14) fatty acids

This paper reports the results of our analysis of the impact high levels of de novo fatty acids have on the proportions of essential and non-essential fatty acids in human milk lipids. The data for seven fatty acids (linoleic, alpha-linolenic, arachidonic (AA), docosahexaenoic (DHA), palmitic, stearic and oleic) were derived from several studies conducted in Nigeria. The proportion by weight of each of these fatty acids was plotted versus the proportion of C10-14 fatty acids. As the proportion of C10-14 fatty acids increased from 15 to 65%, there was not a proportional decrease in the percentages of all seven fatty acids, but, instead, preferential incorporation of the essential fatty acids, AA and DHA into the triacylglycerol component of the milk. At the same time, the proportions of stearic and oleic acid declined by 69% and 86%, respectively. However, the proportions of linoleic acid, palmitic acid, DHA, AA and alpha-linolenic acid, in milk lipids decreased by only 44%, 40%, 39%, 28% and 2.3%, respectively. These observations indicate that as the contribution of C10-14 fatty acids increases, essential fatty acids are preferentially incorporated into milk triacylglycerols at the expense of oleic acid and stearic acid.     

Synthesis and metabolism of arachidonyl- and eicosapentaenoyl-CoA in rat aorta

In studies on the metabolism of polyunsaturated fatty acids, acyl-CoA synthetase for 5,8,11,14-20:4 (arachidonic acid) and 5,8,11,14,17-20:5 (eicosapentaenoic acid) and the incorporation of these fatty acids into complex lipids and their conversion to CO2 were investigated in rat aorta. The activity of acyl-CoA synthetase was 35.9 for arachidonic acid and 63.0 for eicosapentaenoic acid (nmol/mg protein per 10 min) and the apparent Km values were 45 microM for arachidonic acid and 56 microM for eicosapentaenoic acid. Inhibition of eicosapentaenoyl-CoA synthesis by arachidonic acid was stronger than that of arachidonyl-CoA synthesis by eicosapentaenoic acid. Arachidonic acid and eicosapentaenoic acid were mostly incorporated into phospholipids. The incorporation of these fatty acids into cholesterol ester and their conversion to CO2 were less than those of palmitic acid, but their incorporation into triacyglycerol was greater. The incorporation of these fatty acids into phosphatidylserine + phosphatidylinositol and phosphatidylethanolamine was also greater than that of palmitic acid. The patterns of incorporation of arachidonic acid and eicosapentaenoic acid were similar. The physiological roles of these polyunsaturated fatty acids and the interference of eicosapentaenoic acid in arachidonic acid metabolism are discussed on the basis of these results.     

Metabolism of polyunsaturated fatty acids by rabbit reticulocytes

Rabbit reticulocytes obtained by repeated bleeding metabolize exogenous [1-14C]linoleic acid and [1-14C]arachidonic acid by three different pathways. 1. Incorporation into cellular lipids: 50% of the fatty acids metabolized are incorporated into phospholipids, mainly phosphatidylcholine (32.8%) but also into phosphatidylethanolamine (12%), whereas about 10% of the radioactivity was found in the neutral lipids (mono- di- and triacylglycerols, but not cholesterol esters). 2. Formation of lipoxygenase products: 30% of the fatty acids metabolized are converted via the lipoxygenase pathway mainly to hydroxy fatty acids. Their formation is strongly inhibited by lipoxygenase inhibitors such as 5,8,11,14-eicosatetraynoic acid or nordihydroguaiaretic acid. Inhibition of the lipoxygenase pathway results in an increase of the incorporation of the fatty acids into cellular lipids. 15-Hydroxy-5,8,11,13(Z,Z,Z,E)eicosatetraenoic acid and 13-hydroxy-9,11(Z,E)-octadecadienoic acid are incorporated by reticulocytes into cellular lipids and also are metabolized via beta-oxidation. The metabolism of arachidonic acid and linoleic acid is very similar except for a higher incorporation of linoleic acid into neutral lipids. 3. beta-Oxidation of the exogenous fatty acids: about 10% of the polyenoic fatty acids are metabolized via beta-oxidation to 14CO2. Addition of 5,8,11,14-eicosatetraynoic acid strongly increased the 14CO2 formation from the polyenoic fatty acids whereas antimycin A completely abolished beta-oxidation. Erythrocytes show very little incorporation of unsaturated fatty acids into phospholipids and neutral lipids. Without addition of calcium and ionophore A23187 lipoxygenase metabolites could not be detected.     

Composition and synthesis of fatty acids in atherosclerotic aortas of the pigeon

The composition, synthesis, and esterification of fatty acids were studied in aortas of White Carneau and Show Racer pigeons after perfusion of the aortas with a medium containing acetate-1-(14)C. For both breeds of pigeons the principal change in aortic fatty acids, in response to an atherogenic diet, was a marked increase in the percentage of oleic acid in the cholesteryl ester fraction. In atherosclerotic aortas incorporation of acetate-1-(14)C into the phospholipid and glyceride fractions increased 2-fold, while a much greater increase (up to 10-fold) was seen in incorporation into cholesteryl esters. In those birds receiving the atherogenic diet, palmitic acid accounted for approximately 50% of the fatty acid radioactivity, compared with approximately 25% from control aortas. Calculation of fatty acid synthesis showed the major newly synthesized fatty acids to be stearic acid in the phospholipid fraction; stearic, palmitic, and oleic acids in the glycerides; and oleic acid in the cholesteryl esters. The pattern of fatty acid synthesis was closely similar to the actual fatty acid composition of the aorta. In atherosclerotic aortas an increased synthesis of all fatty acids was seen, but the greatest increase was seen in the synthesis of oleic acid and its esterification to cholesterol.     

A comparison of the metabolic fate of Fatty acids of different chain lengths in developing oilseeds

To determine if medium and long chain fatty acids can be appropriately metabolized by species that normally produce 16 and 18 carbon fatty acids, homogenates of developing Cuphea wrightii, Carthamus tinctorius, and Crambe abyssinica seeds were incubated with radiolabeled lauric, palmitic, oleic, and erucic acids. In all three species, acyl-CoA synthetase showed broad substrate specificity in synthesis of acyl-coenzyme A (CoA) from any of the fatty acids presented. In Carthamus, two- to fivefold less of the foreign FAs, lauric, and erucic acid was incorporated into acyl-CoAs than palmitic and oleic acid. Lauric and erucic acid also supported less glycerolipid synthesis in Carthamus than palmitic and oleic acid, but the rate of acyl-CoA synthesis did not control rate of glycerolipid synthesis. In all species examined, medium and long chain fatty acids were incorporated predominantly into triacylglycerols and were almost excluded from phospholipid synthesis, whereas palmitic and oleic acid were found predominantly in polar lipids. However, the rate of esterification of unusual fatty acids to triacylglycerol is slow in species that do not normally synthesize these acyl substrates.     

High rates of [1-14C]acetate incorporation into the lipid of isolated spinach chloroplasts.

Spinach chloroplasts, isolated by techniques yielding preparations with high O2- evolving activity, showed rates of light-dependent acetate incorporation into lipids 3-4 fold higher than any previously reported. Incorporation rates as high as 500 nmol of acetate/h per mg of chlorophyll were measured in buffered sorbitol solutions containing only NaHCO3 and [1-14C]acetate, and as high as 800 nmol/h per mg of chlorophyll when 0.13 mM-Triton X-100 was also included in the reaction media. The fatty acids synthesized were predominantly oleic (70-80% of the total fatty acid radioactivity) and palmitic (20-25%) with only minor amounts (1-5%) of linoleic acid. Linolenic acid synthesis was not detected in the system in vitro. Free fatty acids accounted for 70-90% of the radioactivity incorporated and the remainder was shared fairly evenly between 1,2-diacylglycerols and polar lipids. Oleic acid constituted 80-90% of the free fatty acids synthesized, but the diacylglycerols and polar lipids contained slightly more palmitic acid than oleic acid. Triton X-100 stimulated the synthesis of diacylglycerols 3-6 fold, but stimulated free fatty acid synthesis only 1-1.5-fold. Added glycerol 1-phosphate stimulated both the synthesis of diacylglycerols and palmitic acid relative to oleic acid, but did not increase acetate incorporation into total chloroplast lipids. CoA and ATP, when added separately, stimulated acetate incorporation into chloroplast lipids to variable extents and had no effect on the types of lipid synthesized, but when added together resulted in 34% of the incorporated acetate appearing in long-chain acyl-CoA. Pyruvate was a much less effective precursor of chloroplast fatty acids than was acetate.     

Myristic acid, unlike palmitic acid, is rapidly metabolized in cultured rat hepatocytes

This study was designed to examine and compare the metabolism of myristic and palmitic acids in cultured rat hepatocytes. [1-(14)C]-Labeled fatty acids were solubilized with albumin at 0.1 mmol/L in culture medium. Incubation with 24-hr cultured hepatocytes was carried out for 12 hr. Myristic acid was more rapidly (P < 0.05) taken up by the cells than was palmitic acid (86.9 +/- 0.9% and 68.3 +/- 5.7%, respectively, of the initial radioactivity was cleared from the medium after 4 hr incubation). Incorporation into cellular lipids, however, was similar after the same time (33.4 +/- 2.8% and 34.9 +/- 9.3%, respectively, of initial radioactivity). In the early phase of the incubation (30 min), myristic acid was more rapidly incorporated into cellular triglycerides than was palmitic acid (7.4 +/- 0.9% and 3.6 +/- 1.9%, respectively, of initial radioactivity). However, after 12 hr incubation, the radioactivity of cellular triglycerides, cellular phospholipids, and secreted triglycerides was significantly higher with palmitic acid as precursor. Myristic acid oxidation was significantly higher than that of palmitic acid (14.9 +/- 2.2% and 2.3 +/- 0.6%, respectively, of the initial radioactivity was incorporated into the beta-oxidation products after 4 hr). Myristic acid was also more strongly elongated to radiolabeled palmitic acid (12.2 +/- 0.8% of initial radioactivity after 12 hr) than palmitic acid was to stearic acid (5.1 +/- 1.3% of initial radioactivity after 12 hr). The combination of elongation and beta-oxidation results in the rapid disappearance of C14:0 in hepatocytes whereas C16:0 is esterified to form glycerolipids. This study provides evidence that myristic acid is more rapidly metabolized in cultured hepatocytes than is palmitic acid.     

Mechanism of biosynthesis of unsaturated fatty acids in Pseudomonas sp. strain E-3, a psychrotrophic bacterium.

Biosynthesis of palmitic, palmitoleic, and cis-vaccenic acids in Pseudomonas sp. strain E-3 was investigated with in vitro and in vivo systems. [1-14C]palmitic acid was aerobically converted to palmitoleate and cis-vaccenate, and the radioactivities on their carboxyl carbons were 100 and 43%, respectively, of the total radioactivity in the fatty acids. Palmitoyl coenzyme A desaturase activity was found in the membrane fraction. [1-14C]stearic acid was converted to octadecenoate and C16 fatty acids. The octadecenoate contained oleate and cis-vaccenate, but only oleate was produced in the presence of cerulenin. [1-14C]lauric acid was aerobically converted to palmitate, palmitoleate, and cis-vaccenate. Under anaerobic conditions, palmitate (62%), palmitoleate (4%), and cis-vaccenate (34%) were produced from [1-14C]acetic acid, while they amounted to 48, 39, and 14%, respectively, under aerobic conditions. In these incorporation experiments, 3 to 19% of the added radioactivity was detected in released 14CO2, indicating that part of the added fatty acids were oxidatively decomposed. Partially purified fatty acid synthetase produced saturated and unsaturated fatty acids with chain lengths of C10 to C18. These results indicated that both aerobic and anaerobic mechanisms for the synthesis of unsaturated fatty acid are operating in this bacterium.     

Stereospecific distribution of plamitic acid in the triacylglycerols of rat adipocytes. Effects of varying the composition of the substrate fatty acid in vitro

The effects of inclusion of different fatty acids in the medium on the rate of esterification of palmitic acid and its stereospecific distribution among the three positions of the triacyl-sn-glycerols by preparations of rat adipocytes in vitro have been determined. Myristic acid, stearic acid, oleic acid and linoleic acid were used as diluents and the concentration of the combined unesterified fatty acids in the medium was held constant; only the proportion of palmitic acid was varied. The amount of palmitic acid esterified was always linearly related to its relative concentration in the medium and was not significantly affected by the nature of the diluent fatty acid chosen. Constant relative proportions were recovered in triacylglycerols and in intermediates in each instance. The amount of palmitic acid esterified to each of the positions of the triacyl-sn-glycerols was linearly dependent on the relative proportion in the medium but the nature of the relationship was markedly influenced by which fatty acid was present. When stearic acid was present, simple relationships were found over the whole range tested. When either myristic acid, oleic acid or linoleic acid was present, abrupt changes in the manner of esterification of palmitic acid were observed in position sn-1 when the relative concentrations of palmitic acid and the diluent reached critical values, which differed with each fatty acid. In position sn-2 when oleic acid or linoleic acid was present, a similar change was observed, and in position sn-3 it was obtained with myristic acid as diluent. The results are discussed in terms of changes in the relative affinities of the acyltransferases for palmitic acid. Palmitic acid was esterified into various molecular species in proportions that indicated acylation with non-correlative specificity at higher relative concentrations but not at lower.     

Long-term effects of cis and trans monounsaturated (18:1) and saturated (16:0) fatty acids on the synthesis and secretion of apolipoprotein A-I- and apolipoprotein B-containing lipoproteins in HepG2 cells

The objective of this study was to compare the long-term effects of oleic (cis 18:1), elaidic (trans 18:1), and palmitic (16:0) acids on hepatic lipoprotein production, using HepG2 cells as an experimental model. The net accumulation in the medium of apolipoprotein A-I (apoA-I) was not significantly altered by fatty acids, whereas that of apoB was increased with oleic and elaidic acids. Oleic acid, and to a lesser extent elaidic and palmitic acids, increased the mass of triglycerides in the medium and the incorporation of [(3)H]glycerol into secreted triglycerides. The incorporation of [(14)C]acetate into cellular and secreted total cholesterol was stimulated by 96% and 83%, respectively, with elaidic acid but was not significantly modified by oleic or palmitic acid. Relative to oleic acid, the secretion of (14)C-labeled phospholipids and triglycerides was decreased 28% to 31% with elaidic and palmitic acids whereas that of free cholesterol and cholesteryl esters was enhanced 93% and 73%, respectively, with elaidic acid but remained unchanged with palmitic acid. Compared with oleic acid, elaidic acid stimulated the secretion of very low density lipoprotein cholesterol (VLDL-Chol), low density lipoprotein cholesterol (LDL-Chol), and high density lipoprotein cholesterol (HDL-Chol) by 43%, 70%, and 34%, respectively, whereas palmitic acid decreased VLDL-Chol but had no significant effect on LDL-Chol and HDL-Chol. The ratios of total cholesterol to HDL-Chol were 3.17, 3.60, and 3.25 with oleic, elaidic, and palmitic acids, respectively; the corresponding ratios of LDL-Chol to HDL-Chol were 0.87, 1.10, and 0.93, respectively. Compared with oleic and palmitic acids, the LDL and HDL particles secreted in the presence of elaidic acid contained higher levels of free cholesterol and cholesteryl esters and a lower content of phospholipids. The phospholipid-to-total cholesterol ratios of HDL were 1.05, 0.40, and 0.76 with oleic, elaidic, and palmitic acids, respectively.Our results indicate that in comparison with cis monounsaturated and saturated fatty acids, trans fatty acids have more adverse effects on the concentration and composition of lipoproteins secreted by HepG2 cells.     

The influence of insulin on glucose and fatty acid metabolism in the isolated perfused rat hind quarter.

Glucose and fatty acid metabolism of resting skeletal muscle were studied by perfusion of the isolated rat hind leg with a hemoglobin-free medium. Tissue integrity was demonstrated by normal ATP, ADP and creatine phosphate levels, by a sufficient oxygen supply, and by a normal appearance of perfused muscle specimens under the electron microscope. The rates of glucose and fatty acid uptake, and of lactate, alanine, glycerol and fatty acid release were constant over a perfusion period of 60 min. Insulin (1 unit/l) caused a more than threefold increase in glucose uptake, a stimulation of lactate production, and a 20% increase in the muscular glycogen levels. Fatty acids and alanine release were significantly diminished by insulin, but glycerol release did not change. The uptake of oleate by the rat hind leg was dependent on the medium concentration in a range of 0.7-1.9mM oleate, and was stimulated by insulin. Glucose uptake was not influenced by oleate, whether sodium was present or not. When the leg was perfused with [1-14C]oleate, 75% of the incorporated fatty acids were found in muscle lipids, 10% were oxidized to CO2, and 5% were recovered in bone lipids. The absolute amount of oleate oxidation was not altered by insulin. In all experiments with and without glucose in the medium, 70-80% of the 14C label incorporated into muscle lipids was found in the triglyceride fraction. In the presence of glucose, insulin significantly increased the incorporation of [1-14C]oleate into muscle triglycerides, whereas no insulin effect, either on fatty acid uptake or on triglyceride formation, could be observed when glucose was omitted from the perfusate. The present results indicate that a "glucose-fatty acid cycle" as found in rat heart muscle does not operate in resting peripheral skeletal muscle tissue. They also demonstrate that the stimulating effect of insulin on muscular fatty acid uptake and triglyceride synthesis is dependent on glucose supply. This finding can be intrepreted as a stimulation of fatty acid esterification by sn-glycerol 3-phosphate derived from an increased glucose turnover, which is in turn due to insulin.     

Functional role of lipid metabolism in activated T-lymphocytes

Exogenous long-chain fatty acids are readily taken up by unstimulated lymphocytes derived from the thymus of calves or rabbits and esterified to complex lipids, primarily phospholipids and triacylglycerols. Compared to saturated fatty acids, unsaturated fatty acids are incorporated preferentially. Furthermore, unsaturated fatty acids are transferred from triacylglycerols to phospholipids. The transfer cannot be observed with palmitic acid. With regard to individual phospholipid species, oleic acid and linoleic acid are found primarily in phosphatidylcholine. Arachidonic acid, however, is transferred to phosphatidylethanolamine and phosphatidylinositol as well. This suggests an arachidonic-specific transfer between individual phospholipids. Stimulation of the cells with the mitogen concanavalin A results in an enhanced incorporation of the fatty acids and an enhanced transfer from triacylglycerols to phospholipids. Triacylglycerols may thus be regarded as a labile intracellular storage pool that may be activated upon mitogenic stimulation.