Selective inhibition of long-chain fatty acid uptake in short-term cultured rat hepatocytes by an antibody to the rat liver plasma membrane fatty acid-binding protein

Uptake of long-chain fatty acids by short-term cultured hepatocytes was studied. Rat hepatocytes, which were cultured for 16 h on plastic dishes (3.6 X 10(6) cells/dish), were incubated with [3H]oleate in the presence of various concentrations of bovine serum albumin as a function of the concentration of unbound [3H]oleate in the medium. At 37 degrees C initial uptake velocity (V0) was saturable (Km = 9 X 10(-8) M; Vmax = 835 pmol/min per mg protein). V0 was temperature dependent with an optimum at 37 degrees C and markedly reduced at 4 degrees C and 70 degrees C. To evaluate the biologic significance of a previously isolated rat liver plasma membrane fatty acid-binding protein as putative carrier protein in the hepatocellular uptake of fatty acids, cultured hepatocytes were treated with a monospecific rabbit antibody (IgG-fraction) to this membrane protein or the IgG-fraction of the pre-immune serum as controls. Uptake kinetics of [3H]oleate in antibody pretreated short-term cultured hepatocytes revealed a depression of Vmax by 70%, while Km was only reduced by 16% compared to controls, indicating a predominant non-competitive type of inhibition. V0 of a variety of long-chain fatty acids (oleic acid, arachidonic acid, palmitic acid, stearic acid) was reduced by 56-69%, while V0 of [35S]sulfobromophthalein, [3H]cholic acid and [14C]taurocholic acid remained unaltered. These data support the concept that in the system of cultured hepatocytes, uptake of long-chain fatty acids is mediated by the rat liver plasma membrane fatty acid-binding protein.     

CTP:phosphorylcholine cytidylyltransferase in rat lung. The effect of free fatty acids on the translocation of activity between microsomes and cytosol

Phosphatidylglycerol and oleic acid had differential effects on cytidylyltransferase activity in cytosol and microsomes. The low-molecular-weight cytidylyltransferase in cytosol was stimulated more by phosphatidylglycerol than by oleic acid, whereas microsomal activity was stimulated more by oleic acid than by phosphatidylglycerol. Microsomal activity was stimulated by several unsaturated fatty acids but was not stimulated by saturated fatty acids. Bovine serum albumin decreased cytidylyltransferase activity in microsomes in the presence or absence of oleic acid but did not alter the activity measured in the presence of phosphatidylglycerol. The addition of oleic acid to albumin/microsome mixtures in amounts exceeding the binding capacity of albumin lead to complete recovery of the oleic acid stimulation. The addition of oleic acid to postmitochondrial supernatants resulted in a translocation of cytidylyltransferase activity from cytosol to microsome. The magnitude of the shift was severalfold greater with fetal preparations than adult. The free fatty acid content of microsomes increased coincident with the translocation. Bovine serum albumin, added to postmitochondrial supernatants, caused a release of cytidylyltransferase from microsomes to cytosol and a corresponding decrease in microsomal free fatty acid content. The amount of cytidylyltransferase activity in microsomes increased shortly after birth. The increase was accompanied by an increase in free fatty acid content of the microsomes. The increase in cytidylyltransferase activity and free fatty acids which occurred in vivo following birth was nearly identical to that obtained by adding oleic acid to postmitochondrial supernatants from fetal lung. We conclude that free fatty acids may affect the intracellular activity of cytidylyltransferase by promoting the translocation of inactive cytosolic forms to microsomes as well as by stimulating microsomal bound activity.     

Comparative metabolism of free and esterified fatty acids by the perfused rat heart and rat cardiac myocytes

Studies have been conducted on the uptake and metabolism of unesterified oleic acid and lipoprotein triacylglycerol by the perfused rat heart, and of oleic acid, free glycerol and lipoprotein triacylglycerol by rat cardiac myocytes. The perfused heart efficiently extracted and metabolized unesterified fatty acid and the fatty acid released during lipolysis of the recirculating triacylglycerol. The released glyceride glycerol, however, was largely accumulated in the perfusion media. Cardiac myocytes also extracted and rapidly metabolized unesterified fatty acid. As with the intact heart, free glycerol was poorly utilized by cardiac myocytes. Although the cells appeared to extract a small amount of available extracellular triacylglycerol presented as very low density lipoprotein, this was shown to be unmetabolized, suggesting adsorption rather than surface lipolysis and uptake of the released fatty acid. The data suggest that myocytes are unable to metabolize triacylglycerol fatty acids without prior lipolysis by extracellular (capillary endothelial) lipoprotein lipase.     

Significance of non-esterified fatty acids in iron uptake by intestinal brush-border membrane vesicles

Iron uptake from Fe/ascorbate by mouse brush-border membrane vesicles is not greatly inhibited by prior treatment with a variety of protein-modification reagents or heat. Non-esterified fatty acid levels in mouse proximal small intestine brush-border membrane vesicles show a close positive correlation with initial Fe uptake rates. Loading of rabbit duodenal brush-border membrane vesicles with oleic acid increases Fe uptake. Depletion of mouse brush-border membrane vesicle fatty acids by incubation with bovine serum albumin reduces Fe uptake. Iron uptake by vesicles from Fe/ascorbate is enhanced in an O2-free atmosphere. Iron uptake from Fe/ascorbate and Fe3+-nitrilotriacetate (Fe3+-NTA) were closely correlated. Incorporation of oleic acid into phosphatidylcholine/cholesterol (4:1) liposomes leads to greatly increased permeability to Yb3+, Tb3+, Fe2+/Fe3+ and Co2+. Ca2+ and Mg2+ are also transported by oleic acid-containing liposomes, but at much lower rates than transition and lanthanide metal ions. Fe3+ transport by various non-esterified fatty acids was highest with unsaturated acids. The maximal transport rate by saturated fatty acids was noted with chain length C14-16. It is suggested that Fe transport can be mediated by formation of Fe3+ (fatty acid)3 complexes.     

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.     

Insulin and leptin do not affect fatty acid uptake and metabolism in human placental choriocarcinoma (BeWo) cells

Placental transport of long chain polyunsaturated fatty acids is important for fetal growth and development. In order to examine the effects of leptin and insulin on fatty acid uptake by the placenta, placental choriocarcinoma (BeWo) cells were used. BeWo cells were incubated for 5h at 37 degrees C in the absence or presence of different concentrations of insulin (0.6, 60, and 100 ng) or leptin (10 ng) with 200 microM of various radiolabeled fatty acids (docosahexaenoic acid, arachidonic acid, eicosapentaenoic acid, and oleic acid, mixed with 1:1 bovine serum albumin (fat free). After incubation, the uptake and distribution of these fatty acids into different cellular lipid fractions were determined. The uptakes of oleic, eicosapentaenoic, arachidonic, and docosahexaenoic acids were 15.36+/-4.1, 19.95+/-3.6, 28.56+/-8.1, and 62.25+/-9.5 nmol/mg of protein, respectively, in BeWo cells. Incubation of these cells with insulin (0.6 or 60 ng/ml) or leptin (10 ng/ml) did not significantly alter uptake of any of these fatty acids (P>0.5). Insulin or leptin also did not affect beta oxidation of fatty acids in these cells. In contrast, leptin (10 ng/ml) and insulin (0.60 ng/ml)) stimulated the uptake of oleic acid (7.4+/-2.3 nmol/mg protein) in human adipose cells, SGBS cells by 1.28- and 2.48-fold (P<0.05), respectively. The distribution of fatty acids in different cellular lipid fractions was also not affected by these hormones. Our data indicate that unlike adipose tissue, fatty acid uptake and metabolism in placental trophoblasts is not regulated by insulin or leptin.     

Effects of hydrogen ion and fatty acid concentrations on the binding of aflatoxin B1 to human albumin

The influence of pH and long-chain fatty acids on the interaction between aflatoxin B1 and human albumin was investigated by fluorescence spectroscopy. Both the binding of aflatoxin B1 to albumin and the fluorescence of albumin-bound aflatoxin are pH-dependent over the pH range of 6-9.5. The data indicates that the carcinogen has a higher affinity for the basic(B) than for the neutral(N) conformation of human albumin. Palmitic, stearic and oleic acids up to a molar ratio of 2 over albumin, increases the binding strength of aflatoxin B1 by means of an allosteric mechanism. Furthermore, the pH-dependence of the aflatoxin-albumin interaction is affected by the presence of oleic acid by narrowing the pH range over which the dependence occurs. At molar ratios of oleic acid to albumin in excess of 4.25 at pH6, 3.1 at pH7.4 and 2.4 at pH9 cause a decrease in aflatoxin B1 fluorescence as a result of reduced binding to albumin.     

The uptake of oleic acid by rat small intestine: a comparison of methodologies

The interaction between long-chain and medium-chain lipids during intestinal absorption was examined using several model systems. A decrease in steady-state triolein (LCT) output in thoracic duct lymph after addition of trioctanoin (MCT) to the duodenal infusion confirmed previous studies in unanesthetized rats which demonstrated inhibition of steady-state LCT uptake from the small intestinal lumen by MCT. In slices of everted rat jejunum octanoic acid reduced incorporation into triglyceride and initial uptake of (14)C-labeled oleic acid from micellar solutions. Inhibition of uptake did not occur at 0 degrees C, when triglyceride synthesis was blocked. Incubation of slices at low pH (5.8) or in the presence of dimethyl sulfoxide also reduced uptake of oleic acid and its incorporation into triglyceride. However, when everted sacs of jejunum were similarly incubated, octanoate, dimethyl sulfoxide, or low pH caused no inhibition of oleic acid uptake or esterification. The results indicate that the significance of kinetic data describing intestinal fatty acid absorption which were obtained from experiments conducted in vitro is highly questionable, and that suitable models for in vivo uptake kinetics have yet to be developed. However, analysis of the in vitro kinetic data suggests that the intestinal mucosal membrane does not function as a simple lipid interface with respect to fatty acid absorption.     

Bactericidal effect of fatty acids on mycobacteria, with particular reference to the suggested mechanism of intracellular killing

The effect of fatty acids on Mycobacterium smegmatis was examined in vitro at pH 5.0 to 7.0 to determine the role of fatty acids in the intracellular killing of mycobacteria. Unsaturated fatty acids showed strong bactericidal activity in low concentrations (0.005 to 0.02 mM), whereas saturated fatty acids, except for lauric and myristic acids, were not very effective even at a concentration of 0.2 mM. Addition of a saturated fatty acid (palmitic or stearic acid) to an unsaturated fatty acid (oleic or linoleic acid) did not strongly interfere with the bactericidal effect of the unsaturated fatty acid at pH 5.0 and 6.0. Ca2+ (3.0 mM), Mg2+ (1.0 mM), and gamma-globulin (0.4%) showed weak reversal effects on the bactericidal activity of unsaturated fatty acids at pH 5.0 and 6.0. Serum albumin and serum showed strong reversal effects. The concentrations of each fatty acid in a mixture (molar ratio, 1:1:1:1) of oleic, linoleic, palmitic, and stearic acids required for the killing of M. smegmatis in the presence of 2% serum (bovine, rabbit, or human) were 0.05 to 0.10 mM at pH 5.0 and 6.0 and 0.05 to 0.20 mM at pH 7.0, depending on the serum used. The susceptibilities of M. kansasii, M. bovis strain BCG, and M. tuberculosis to the mixture of the four fatty acids in the presence of 2% bovine serum were similar to that of M. smegmatis, although M. fortuitum was more resistant.     

The ionization and distribution behavior of oleic acid in chylomicrons and chylomicron-like emulsion particles and the influence of serum albumin

A reproducible, fairly narrow-sized population of rat lymph chylomicrons, approximately 100 nm, was isolated by centrifugation and combined with low levels of [1-13C]oleic acid for NMR studies. The carboxyl chemical shift was monitored as a function of aqueous pH to characterize the ionization behavior of the fatty acid in these particles. The titration curves were very similar to those for oleic acid in equivalent-sized emulsion particles composed of egg phosphatidylcholine and triolein. A simple partition-ionization model was fitted to the data to derive values for apparent ionization constant, expressed as pKapp, of 7.4-7.5 and the "true" surface to core partition coefficient of approximately 7 for oleic acid in chylomicrons. The fatty acid in chylomicrons thus appeared to be largely associated with the surface regions of these particles. Addition of bovine serum albumin to the samples showed that near physiologic pH much of the fatty acid was bound to the albumin at fatty acid to albumin-binding stoichiometries as high as 5.1 and with mass ratios of greater than 2 in favor of the lipid or lipoprotein particles. Lowering the pH of the medium shifted the distribution of fatty acid away from albumin so that at pH 5 with the emulsion, virtually all the fatty acid was associated with the lipid. The behavior observed under physiologic conditions is consistent with the rapid clearance and redistribution of fatty acid generated in these particles by lipolytic processes. However, under conditions of severe acidosis, hyperlipidemia, and hypoalbuminemia a significant portion of fatty acids might be retained in triglyceride-rich lipoproteins and their remnants and affect subsequent metabolism.     

Translocation of oleic acid across the erythrocyte membrane. Evidence for a fast process

To clarify divergent views concerning the mechanism of fatty acid translocation across biomembranes this issue was now investigated in human erythrocytes. Translocation rates of exogenously inserted radioactive oleic acid across the membrane of native cells were derived from the time-dependent increase of the fraction of radioactivity becoming non-extractable by albumin. No accumulation of non-extractable unesterified oleic acid occurred. The rate of transfer was markedly suppressed by SH-reagents and by ATP-depletion. The suppression, however, resulted from a mere decrease of incorporation of oleic acid into phospholipids and was not accompanied by an increase of non-extractable unesterified oleic acid. These findings were reconcilable with the concept of a slow, possibly carrier-mediated fatty acid transfer as well as a very fast presumably, diffusional process not resolvable by the albumin extraction procedure. This ambiguity was resolved by using resealed ghosts, which are unable to incorporate oleic acid into phospholipids. In such ghosts all of the oleic acid inserted into the membrane remains extractable by albumin even after prolonged incubation. On the other hand, ghosts containing albumin accumulated non-extractable oleic acid. The rate of accumulation was beyond the time resolution of the albumin extraction procedure at 4 degrees C. Oleic acid uptake into albumin-containing ghosts became kinetically resolvable when the fatty acid was added as a complex with albumin. Correspondingly, time-resolvable release of oleic acid, originally complexed to internal albumin, into an albumin-containing medium was demonstrated at 4 degrees C. Rate and extent of these redistributions of oleic acid were dependent on the concentrations of internal and external albumin. This indicates limitation by the dissociation of oleic acid from albumin and not its translocation across the membrane. Translocation of oleic acid, which is probably a simple diffusive flip-flop process, must therefore occur with a half-time of less than 15 s. These findings raise doubts on the physiological role of presently discussed concepts of a carrier-mediated translocation of fatty acids across plasma membranes.     

The distribution of oleic acid between chylomicron-like emulsions, phospholipid bilayers, and serum albumin. A model for fatty acid distribution between lipoproteins, membranes, and albumin

In the process of lipoprotein lipolysis, masses of fatty acid are generated at the surface of the lipoprotein. The newly generated fatty acid may at least partly redistribute from the site of lipolysis to phospholipid-rich membranes and to albumin. We have studied the distribution of [1-13C]oleic acid in model systems consisting of chylomicron-like triacylglycerol-rich emulsions, unilamellar phosphatidylcholine vesicles, and bovine serum albumin. By using high resolution 13C NMR spectroscopy it was possible to distinguish fatty acid in each compartment (emulsion, vesicle, albumin) and quantitate the fatty acid distribution under various conditions of lipid compartment concentration and aqueous pH. When emulsions and vesicles were present in equivalent mass amounts, fatty acid exhibited a profound preference for the lipid bilayers. The release of oleic acid to phospholipid bilayers was presumably also a function of its high molar stoichiometry (5:1) with the albumin present. More equitable distributions of fatty acid between vesicles and emulsions were seen when higher concentrations of emulsion were used. The distribution of fatty acid between compartments was in good agreement with predictions made using the apparent ionization constant, expressed as pKapp, of 7.5 and the surface to core (phospholipid to triacylglycerol) distribution coefficient of 7.0, measured for unionized oleic acid in chylomicron particles (Spooner, P. J. R., Bennett Clark, S., Gantz, D. L., Hamilton, J. A., and Small, D. M. (1988) J. Biol. Chem. 263, 1444-1455). These results indicate that the affinities of fatty acid for phospholipid bilayer and chylomicron-like emulsion surfaces are equivalent. Redistribution of lipolytically generated fatty acid from chylomicron surface to cell membrane may simply be driven by the predominant quantity of the cell membrane surfaces.     

Glyceride synthesis by four kinds of microbial lipase

Apart from their usual mechanism of action, lipases from Aspergillus niger and Rhizopus delemar also catalyzed the synthesis of glycerides from oleic acid and glycerol. Lipases from Geotrichum candidum and Penicillium cyclopium were inactivated by oleic acid, but were stable in the presence of casein, albumin or buffer of appropriate pH. Lipases from Aspergillus niger and Rhizopus delemar synthesized glycerides from, not only fatty acid, but dibasic acids and aromatic acids, making ester bonds only at position 1 and 3 of glycerol. In contrast, lipases from Geotricum candidum and Penicillium cyclopium synthesized glycerides only from long chain fatty acids, and made ester bonds at all three available positions of the glycerol molecule.     

Effects of Free Fatty Acids on Synaptosomal Amino Acid Uptake Systems

Abstract: The Na⁺-dependent synaptosomal uptakes of proline, aspartic acid, glutamic acid, and γ-aminobutyric acid were strongly inhibited by monounsaturated fatty acids. With oleic acid, half-maximal inhibition was observed at about 15 μM. The Na⁺-independent uptakes of leucine, phenylalanine, histidine, and valine were less sensitive to inhibition by the unsaturated fatty acids. In contrast, the uptakes of all of these amino acids were unaffected by saturated fatty acids. The inhibition of proline uptake (and that of the other Na⁺-dependent amino acids) by oleic acid was overcome by the addition of serum albumin and the data presented further indicate that the previously reported stimulation of proline uptake by albumin could be related to its fatty acid binding properties.     

Displacement of sulphobromophthalein from albumin and fatty-acid-binding protein by oleic acid

The absorption of sulphobromophthalein changes upon addition of bovine serum albumin or fatty-acid-binding protein at pH 8.4. The sulphobromophthalein spectrum is changed most drastically after the addition of albumin than in the presence of fatty-acid-binding protein isolated from rat liver, suggesting as a first approximation that binding capacity of albumin is much higher than that of fatty-acid-binding protein. When both soluble proteins are saturated with oleic acid it is observed a decrease in the binding of sulphobromophthalein which suggests that the presence of fatty acids in those soluble proteins may affect the binding of other ligands.     

The interaction of albumin and fatty-acid-binding protein with membranes: oleic acid dissociation

Bovine serum albumin or fatty-acid-binding protein rapidly lose oleic acid when incubated in the presence of dimyristoyl lecithin liposomes. The phenomenon is dependent on vesicle concentration and no measurable quantities of protein are found associated with liposomes. Upon gel filtration on Sepharose CL-2B of incubated mixtures of microsomes containing [1-14C] oleic acid and albumin or fatty-acid-binding protein, association of fatty acid with the soluble proteins could be demonstrated. Both albumin and fatty-acid-binding protein stimulated the transfer of oleic acid from rat liver microsomes to egg lecithin liposomes. These results indicate that albumin is more effective in the binding of oleic acid than fatty-acid-binding protein, which allows a selective oleic acid dissociation during its interaction with membranes.     

Megalin is a receptor for albumin in astrocytes and is required for the synthesis of the neurotrophic factor oleic acid

We have previously shown that the uptake and transcytosis of albumin in astrocytes promote the synthesis of the neurotrophic factor oleic acid. Although the mechanism by which albumin induces oleic acid synthesis is well known, the mechanism of albumin uptake in astrocytes remains unknown. In this work, we found that astrocytes express megalin, an endocytic receptor for multiple ligands including albumin. In addition, when the activity of megalin is blocked by specific antibodies or by silencing megalin with specific siRNA, albumin binding and internalization is strongly reduced indicating that megalin is required for albumin binding and internalization in the astrocyte. Since the uptake of albumin in astrocytes aims at synthesizing the neurotrophic factor oleic acid, we tested the ability of megalin-silenced astrocytes to synthesize and release oleic acid in the presence of albumin. Our results showed that the amount of oleic acid found in the extracellular medium of megalin-silenced astrocytes was strongly reduced as compared with their controls. Together, the results of this work indicate that megalin is a receptor for albumin in astrocytes and is required for the synthesis of the neurotrophic factor oleic acid. Consequently, the possible involvement of albumin in the holoprosencephalic syndrome observed in megalin-deficient mice is suggested.     

Effect of oleic acid on mitochondrial oxidative phosphorylation in rat brain slices

We tested the effect of oleic acid on oxidative phosphorylation and free fatty acid composition in rat brain slices simultaneously to investigate the relationship between the change in respiratory control ratio and the uptake of oleic acid in the brain mitochondria. The uncoupling of mitochondria was observed when the ratio of oleic acid to stearic acid in the free fatty acid fraction was nearly doubled, but was not recovered even by the addition of fatty acid-free bovine serum albumin. The data suggest that the intactness of oxidative phosphorylation of brain mitochondria is maintained by the precise control of the free fatty acid composition in the mitochondrial membranes.     

Arachidonic Acid Inhibits Choline Uptake and Depletes Acetylcholine Content in Rat Cerebral Cortical Synaptosomes

The effects of arachidonic acid on [3H]choline uptake, on [3H]acetylcholine accumulation, and on endogenous acetylcholine content and release in rat cerebral cortical synaptosomes were investigated. Arachidonic acid (10-150 microM) produced a dose-dependent inhibition of high-affinity [3H]choline uptake. Low-affinity [3H]choline uptake was also inhibited by arachidonic acid. Fatty acids inhibited high-affinity [3H]choline uptake with the following order of potency: arachidonic greater than palmitoleic greater than oleic greater than lauric; stearic acid (up to 150 microM) had no effect. Inhibition of [3H]choline uptake by arachidonic acid was reversed by bovine serum albumin. In the presence of arachidonic acid, there was an increased accumulation of choline in the medium, but this did not account for the inhibition of [3H]choline uptake produced by the fatty acid. Arachidonic acid inhibited the synthesis of [3H]acetylcholine from [3H]choline, and this inhibition was equal in magnitude to the inhibition of high-affinity [3H]choline uptake produced by the fatty acid. A K+-stimulated increase in [3H]acetylcholine synthesis was inhibited completely by arachidonic acid. Arachidonic acid also depleted endogenous acetylcholine stores. Concentrations of arachidonic acid and hemicholinium-3 that produced equivalent inhibition of [3H]choline uptake also produced equivalent depletion of acetylcholine content. In the presence of eserine, arachidonic acid had no effect on acetylcholine release. The results suggest that arachidonic acid may deplete acetylcholine content by inhibiting high-affinity choline uptake and subsequent acetylcholine synthesis. This raises the possibility that arachidonic acid may play a role in the impairment of cholinergic transmission seen in cerebral ischemia and other conditions in which large amounts of the free fatty acid are released in brain.     

Effect of phosphatidylcholine on fatty acid and cholesterol absorption from mixed micellar solutions.

Using the experimental model of the everted sac prepared from rat jejuna, kinetic studies on [14C]oleic acid uptake from bile salt micelles were conducted in the presence and absence of phosphatidylcholine. The concentration of oleic acid was varied between 0.625 and 5 mM. At every level of fatty acid concentration studied the addition of 2 mM phosphatidylcholine produced a significant inhibition of fatty acid uptake. It was further noted that the intact phospholipid molecule was required for this effect as lysophosphatidylcholine produced little, if any, inhibition of [14C]oleic acid uptake. The effect of varying the concentration of phosphatidylcholine on fatty acid uptake was also studied. The degree of inhibition was noted to be correlated grossly with media concentrations of this phospholipid although the decrease of fatty acid uptake was not strictly proportional to concentration of this material in the medium. Studies were also performed analyzing in vitro absorption of [14C]oleic acid and [3H]cholesterol simultaneously from mixed micelles composed of sodium taurocholate, oleic acid, monoolein and cholesterol. Control medium contained no phospholipid while experimental medium contained either diester or diether phosphatidylcholine, 2 mM. Both types of phosphatidylcholine caused significant inhibition of fatty acid and cholesterol uptake. In vivo absorption studies were also performed using the isolated jejunal segment technique. A mixed micellar solution containing [3H]cholesterol and [14C]oleic acid was used as the test dose. Phospholipid in the test dose for controls was supplied as lysophosphatidylcholine and for experimentals it was in the form of diether phosphatidylcholine. Significantly less radioactively labeled cholesterol and fatty acid was absorbed by experimentals as compared to controls over a 10-min period. It is concluded that the intact molecule of phosphatidylcholine inhibits intestinal uptake of cholesterol and fatty acid from mixed micellar solutions under both in vitro and in vivo conditions.