Mechanism for binding of fatty acids to hepatocyte plasma membranes

Purification and characterization of a protein capable of binding to fatty acids and bile salts in Giardia lamblia

A specific fatty acid binding protein was isolated from Giardia lamblia, using an affinity column with butyric acid acting as a ligand in place of stearic acid. This method has proved to be more efficient than the one previously described using stearic acid as ligand. The purified fraction showed 8 electrophoretic bands of proteins, with molecular weights ranging between 8 and 80 kDa. This pattern is a consequence of the aggregation of a protein with a molecular weight of 8,215 Da, corresponding to the lower molecular weight band, the only one capable of binding to fatty acids. The labeled oleic acid bound to these purified proteins was replaced by a 100-fold greater concentration of taurocholate, glycocholate, deoxycholate, palmitic acid, and arachidonic acid, having a greater displacement of the bile salts than the free fatty acids.

Measuring the adsorption of Fatty acids to phospholipid vesicles by multiple fluorescence probes

Fatty acids (FA) are important nutrients that the body uses to regulate the storage and use of energy resources. The predominant mechanism by which long-chain fatty acids enter cells is still debated widely as it is unclear whether long-chain fatty acids require protein transporters to catalyze their transmembrane movement. We use stopped-flow fluorescence (millisecond time resolution) with three fluorescent probes to monitor different aspects of FA binding to phospholipid vesicles. In addition to acrylodan-labeled fatty acid binding protein, a probe that detects unbound FA in equilibrium with the lipid bilayer, and cis-parinaric acid, which detects the insertion of the FA acyl chain into the membrane, we introduce fluorescein-labeled phosphatidylethanolamine as a new probe to measure the binding of FA anions to the outer membrane leaflet. We combined these three approaches with measurement of intravesicular pH to show very fast FA binding and translocation in the same experiment. We validated quantitative predictions of our flip-flop model by measuring the number of H(+) delivered across the membrane by a single dose of FA with the probe 6-methoxy-N-(3-sulfopropyl) quinolinium. These studies provide a framework and basis for evaluation of the potential roles of proteins in binding and transport of FA in biological membranes.

New insights into the roles of proteins and lipids in membrane transport of fatty acids

Recent calculations of the apparent permeability coefficients for long-chain fatty acids (LCFA) in phospholipid bilayers provide a new perspective on their transport in a membrane. LCFA have permeabilities that are many orders of magnitude higher than glucose, amino acids, and ions. Transport of LCFA through membranes must therefore be considered to be much different from these nutrients, and there is no a priori requirement for catalysis by a membrane protein. New evidence indicates that the plasma membrane proteins postulated as catalysts for transporting LCFA into the cell fall into three categories. Some act as enzymes, mainly for the activation of LCFA to the acyl CoA, which is required for subsequent intracellular metabolism of LCFA. Other proteins appear to participate in sequestering and trafficking of LCFA. Finally, some proteins have undefined mechanisms. The established mechanisms are consistent with biophysical properties of LCFA in membranes, including fast free diffusion by "flip-flop" in the phospholipid bilayer.

Fluorescence assays for measuring fatty acid binding and transport through membranes

The authors' laboratory has applied a series of different fluorescence assays for monitoring the binding and transport of fatty acids (FA) in model and biological membranes. The authors recently expanded their fluorescent assays for monitoring the adsorption of FA to membranes to a total of three probes that measure different aspects of FA binding: (1) an acrylodan-labeled FA-binding protein, which measures the partitioning of FA between membranes and the aqueous buffer; (2) the naturally occurring fluorescent cis-parinaric acid, which specifically measures the insertion of the FA acyl chain into the hydrophobic core of the phospholipid bilayer, and (3) a fluorescein-labeled phospholipid (N-fluorescein-5-thiocarbomoyl-1,2,dihexadecanoyl-sn-glycero-3-phosphoethanolamine), which specifically measures the arrival of the FA carboxyl at the outer leaflet of the membrane. None of these probes allow the transmembrane movement of FA to the inner leaflet to be measured. FA translocation (flip-flop) is typically measured directly, using a pH-sensitive fluorophore such as 8-hydroxypyrene-1.3.6-trisulfonic acid or 2',7'-bis-(2-carboxyethyl)-5-(and-6)- carboxyfluorescein. These probes detect the release of protons from unionized FA that have diffused through the membrane to the inner leaflet. Because adsorption of FA to the outer leaflet must occur before flip-flop, these probes measure the effects of the combined steps of adsorption and translocation. In this chapter, detailed methods are provided on how to monitor the transport of FA through protein-free model membranes, and some of the fluorescent artifacts that may arise with the use of these probes are addressed. Also, experiments designed to investigate such artifacts, and improve the reliability and interpretation of the data are described.

Computational evidence for protein-mediated fatty acid transport across the sarcolemma

Long-chain fatty acids (FAs) are important substrates used by the heart to fulfil its energy requirements. Prior to mitochondrial oxidation, blood-borne FAs must pass through the cell membrane of the cardiac myocyte (sarcolemma). The mechanism underlying the sarcolemmal transport of FAs is incompletely understood. The aim of the present study was to estimate the trans-sarcolemmal FA uptake rate using a comprehensive computer model, in which the most relevant mechanisms proposed for cardiac FA uptake were incorporated. Our in silico findings show that diffusion of FA, present in its unbound form (uFA) in close proximity to the outer leaflet of the sarcolemma and serving as sole FA source, is insufficient to account for the physiological FA uptake rate. The inclusion of a hypothetical membrane-associated FA-TFPC (FA-transport-facilitating protein complex) in the model calculations substantially increased the FA uptake rate across the sarcolemma. The model requires that the biological properties of the FA-TFPC allow for increasing the rate of absorption of FA into the outer leaflet and the 'flip-flop' rate of FA from the outer to the inner leaflet of the sarcolemma. Experimental studies have identified various sarcolemma-associated proteins promoting cardiac FA uptake. It remains to be established whether these proteins possess the properties predicted by our model. Our findings also indicate that albumin receptors located on the outer leaflet of the sarcolemma facilitate the transfer of FA across the membrane to a significant extent. The outcomes of the computer simulations were verified with physiologically relevant FA uptake rates as assessed in the intact, beating heart in experimental studies.

Fatty acid transport and metabolism in HepG2 cells

The mechanism(s) of fatty acid uptake by liver cells is not fully understood. We applied new approaches to address long-standing controversies of fatty acid uptake and to distinguish diffusion and protein-based mechanisms. Using HepG2 cells containing an entrapped pH-sensing fluorescence dye, we showed that the addition of oleate (unbound or bound to cyclodextrin) to the external buffer caused a rapid (seconds) and dose-dependent decrease in intracellular pH (pH(in)), indicating diffusion of fatty acids across the plasma membrane. pH(in) returned to its initial value with a time course (in min) that paralleled the metabolism of radiolabeled oleate. Preincubation of cells with the inhibitors phloretin or triacsin C had no effect on the rapid pH(in) drop after the addition of oleate but greatly suppressed pH(in) recovery. Using radiolabeled oleate, we showed that its esterification was almost completely inhibited by phloretin or triacsin C, supporting the correlation between pH(in) recovery and metabolism. We then used a dual-fluorescence assay to study the interaction between HepG2 cells and cis-parinaric acid (PA), a naturally fluorescent but slowly metabolized fatty acid. The fluorescence of PA increased rapidly upon its addition to cells, indicating rapid binding to the plasma membrane; pH(in) decreased rapidly and simultaneously but did not recover within 5 min. Phloretin had no effect on the PA-mediated pH(in) drop or its slow recovery but decreased the absolute fluorescence of membrane-bound PA. Our results show that natural fatty acids rapidly bind to, and diffuse through, the plasma membrane without hindrance by metabolic inhibitors or by an inhibitor of putative membrane-bound fatty acid transporters.

Transport of 13C-oleate in adipocytes measured using multi imaging mass spectrometry

The mechanism of long chain free fatty acid (FFA) transport across cell membranes is under active investigation. Here we describe the use of multi imaging mass spectrometry (MIMS) to monitor intracellular concentrations of FFA and provide new insight into FFA transport in cultured adipocytes. Cells were incubated with 13C-oleate:BSA and either dried directly or dried after washing with a medium deprived of 13C-oleate:BSA. Cells were analyzed with MIMS using a scanning primary Cs+ ion beam and 12C-, 13C-, 12C14N-, 13C14N-) (or 12C 15N-) were imaged simultaneously. From these quantitative images the values of the 13C/ 12C ratios were determined in the intracellular lipid droplets, in the cytoplasm and outside the 3T3F442A adipocytes. The results indicate that after incubation with 13C-oleate:BSA the droplet 13C/ 12C ratio was 15 +/- 6%. This value is about 14-fold higher than the 13C/ 12C terrestrial ratio (1.12%). After washing the 13C-oleate:BSA, the droplet 13C/ 12C ratios decreased to 1.6 +/- 0.1%, about 40% greater than the natural abundance. Results for washed cells indicate that relatively little FFA was esterified. The unwashed cell results, together with the value of the lipid water partition coefficient, reveal that intracellular unbound FFA (FFAu) concentrations were on average about 4.5-fold greater than the extracellular FFAu concentrations. These results are consistent with the possibility that FFA may be pumped into adipocytes against their electro-chemical potential. This work demonstrates that MIMS can be used to image and quantitate stable isotope labeled fatty acid in intracellular lipid droplets.

Kinetics and mechanism of long-chain fatty acid transport into phosphatidylcholine vesicles from various donor systems

The kinetics of long-chain fatty acid (FA) transfer from three different donor systems to unilamellar egg phosphatidylcholine (EPC) vesicles containing the pH-sensitive fluorophore pyranine in the vesicle cavity were determined. The transfer of long-chain FA from three FA donors, FA vesicles, unilamellar EPC vesicles containing FA, and bovine serum albumin-FA complexes to pyranine-containing EPC vesicles is a true first-order process, indicating that the FA transfer proceeds through the aqueous phase and not through collisional contacts between the donor and acceptor. A collisional mechanism would be at least bimolecular, giving rise to second-order kinetics. Evidence from stopped-flow fluorescence spectroscopy using the pyranine assay (as developed by Kamp, F., and Hamilton, J. A. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11367-11370) shows that the transverse or flip-flop motion of long-chain FA (from 14 to 22 C atoms) is immeasurably fast in both small and large unilamellar EPC vesicles and characterized by half-times t(1/2) < 5 ms. The rate-limiting step of FA transfer from these different donor systems to pyranine-containing EPC vesicles is the dissociation or desorption of the FA molecule from the donor. The desorption of the FA molecule is chain-length-dependent, confirming published data (Zhang et al. (1996) Biochemistry 35, 16055-16060): the first-order rate constant k(1) decreases by a factor of about 10 with elongation of the FA chain by two CH(2) groups. Similar rates of desorption are observed for the transfer of oleic acid from the three donors to pyranine-containing EPC vesicles with rate constants k(1) ranging from 0.4 to 1.3 s(-1). We also show that osmotically stressed, pyranine-containing EPC vesicles can give rise to artifacts. In the presence of a chemical potential gradient across the lipid bilayer of these vesicles, fast kinetic processes are observed with stopped-flow fluorescence spectroscopy which are probably due to electrostatic and/or osmotic effects.ne

Liposomes as fatty acids carriers in isolated rat liver: effect on energy metabolism and on isolated mitochondria activity

The effects of fatty acids (FA)-carrier, egg-lecithin liposomes (LIPO) as alternative to BSA, on ATP, glycogen and glucose contents in isolated perfused liver of fed rats were non-invasively studied using 31P/13C nuclear magnetic resonance (NMR). Oxidative phosphorylation was studied in isolated mitochondria from the same liver consecutively to the NMR experiments. ATP content decreased slowly and ATP turnover was similar during the perfusion with saline solution (KHB) or LIPO. However, LIPO induced an enhancement of respiratory control ratio in isolated mitochondria. Tissue glycogen and glucose content decreased when FA (linoleate or linolenate) were perfused with defatted BSA (3%) or LIPO (600 mg/l) whereas glucose excretion level was unchanged and lactate excretion tended to increase, reflecting changes in the cytosolic redox state and/or an enhancement of glycolysis. Addition of FA (0.5 or 1.5 mM) to LIPO caused a dramatic fall in liver ATP, a mitochondrial uncoupling and an impairment of the phosphorylation activity. Perfusion with FA (1.5 mM) carried by BSA significantly increased the ATP degradation without change of mitochondrial function. Owing to the higher affinity of BSA than LIPO for FA, these latter could be more easily released from complex LIPO-FA, increasing their uncoupling effect. Hence, the FA concentrations have to be largely decreased from the above currently used concentrations to avoid this effect. It will then be possible to minimize the effector action of FA and to study their more specific metabolic function as fuel. It was concluded that LIPO were appropriate carriers to study the different metabolic effects of FA.

Transport of long-chain native fatty acids across human erythrocyte ghost membranes

Evidence from a number of laboratories suggests that membrane proteins may meditate the transport of physiologic fatty acids (FA) across cell membranes. However, actual transport of unbound free fatty acids (unbound FFA) from the aqueous phase on one side of a cell membrane to the aqueous phase on the other side has not been measured previously. In this study, we have used the fluorescent probe of unbound FFA, ADIFAB, to monitor the time course of FA movement from the outer to the inner aqueous compartments, and from the lipid membrane to the outer aqueous compartment of red cell ghosts. These two measurements, together with measurements of the lipid/aqueous partition coefficients, allowed the determination of the rate constants for binding (kon), flip-flop (kff), and dissociation (koff) for the transport of long-chain natural FA across red cell ghosts. Measurements done using palmitate, oleate, and linoleate at temperatures between 20 and 37 degreesC revealed that the overall transport times ranged from about 0.5 to more than 10 s, depending upon FA type and temperature. Analysis of these time courses yielded kff values between 0.3 and 3.0 s-1, and these values were consistent with those obtained using ghosts containing pyranine to detect intracellular acidification by the translocating FA. The measured koff values ranged from about 0.3 to 5 s-1, while the rate of binding, for the ghost concentrations used in this study (>50 microM phospholipid), exceed both kff and koff. Thus, long-chain FA transport across red cell ghost membranes is rate-limited by a combination of flip-flop and dissociation rates. Binding of FA to ghost membranes was well described by simple, nonsaturable, aqueous/membrane partition, and that partition appears to be governed by the aqueous solubility of the FA. Transport rates did not reveal any evidence of saturation and were not affected by a variety of protein-specific reagents. These FA binding and transport characteristics are similar to those observed previously for lipid vesicles, although the rate constants are generally about 2-3 fold larger for ghosts as compared to the lipid vesicles. We suggest, therefore, that FA transport across red cell ghosts is reasonably well described by transport across the lipid phase of the membrane.

Molecular mechanism of cellular uptake and intracellular translocation of fatty acids

The molecular mechanism of the transport of long-chain fatty acids across cellular membranes and the necessity and precise functioning of specific proteins in this process are still unclear. Various alternative mechanisms have been proposed. Studies with artificial phospholipid bilayers support the concept that fatty acids may enter and traverse the plasma membrane without the involvement of proteins. On the other hand, a number of membrane-associated fatty acid-binding proteins (FABPs) have been described which putatively function as acceptors for fatty acids released from albumin or from lipoproteins. Albumin binding proteins located at the outer cell surface could play an additional role in the delivery of fatty acids. The subsequent transmembrane translocation of fatty acids could take place by a membrane protein acting as a translocase, or by simple diffusion of fatty acids through either the phospholipid bilayer or a pore or channel formed by one or more membrane fatty acid transporters. At the inner side of the plasma membrane, the fatty acid is bound to a cytoplasmic FABP, which serves to buffer the intracellular aqueous fatty acid concentration. The direction of fatty acid migration through the plasma membrane most likely is governed by the transmembrane gradient of fatty acid concentration, assisted to some extent and in selected tissues by co-transport of sodium ions. The intracellular transport of fatty acids from the plasma membrane to the sites of metabolic conversion (oxidation, esterification) or subcellular target (signal transduction) is greatly facilitated by cytoplasmic FABPs. In conclusion, cellular uptake and intracellular translocation of long-chain fatty acids is a multi-step process that is facilitated by various membrane-associated and soluble proteins. The mechanism of cellular uptake of fatty acids probably involves both a passive and carrier-mediated transmembrane translocation.

Metallothionein-IIA promoter induction alters rat intestinal fatty acid binding protein expression, fatty acid uptake, and lipid metabolism in transfected L-cells

Mouse L-cell fibroblasts, transfected with the cDNA encoding for rat intestinal fatty acid-binding protein (I-FABP) under the control of the human metallothionein-IIA promoter, were tested for their protein inducibility by the heavy metals cadmium (Cd2+) and zinc (Zn2+). I-FABP levels were quantitated by Western immunoblotting. Expression of I-FABP in all transfected cell lines tested was induced several-fold by optimized levels of Cd2+ and Zn2+. Induction conditions had no effect on cell growth rates or cell densities for any of the cell lines. Induction of high I-FABP-expressing cells (H141) decreased the initial rate and extent of uptake of cis-parinaric acid, a nonmetabolizable fatty acid, and of [3H]oleic acid, an esterifiable fatty acid. These effects of induction were specific for I-FABP-expressing cells since they were not observed in control cells or cells expressing a high level of liver (L-) FABP. Induction of H141 cells also significantly altered the esterification and distribution of exogenous [3H]oleic acid, especially among triglycerides and phosphatidylcholine, but less so among other glycero-phospholipids, cholesteryl esters, and phosphatidylethanolamine. Induction of H141 cells normalized [3H]oleic acid esterification into cholesteryl esters, phosphatidylcholine, total neutral lipids, and total phospholipids such that they no longer differed from control levels. In contrast, induction did not normalize [3H]oleic acid esterification into triacylglycerols and phosphatidylethanolamine to control levels in H141 cells; both remained significantly increased over control cells. Therefore, promoter induction levels of Cd2+ and Zn2+ enhanced I-FABP expression in H141 cells, thereby modulating both fatty acid uptake and intracellular esterification into neutral and phospholipids.

Dissociation of long and very long chain fatty acids from phospholipid bilayers

Dissociation of fatty acids (FA) from and transbilayer movement (flip-flop) in small unilamellar phosphatidylcholine vesicles (SUV) were monitored by measuring the pH inside the vesicle with an entrapped water-soluble fluorophore, pyranin. With a pH gradient imposed upon SUV preloaded with FA, the rate of flip-flop of saturated very long chain FA (C20:0, C:22:0, and C24:0) was shown to be fast (t1/2 < 1 s); previously, we showed by stopped flow measurements that flip-flop of long chain (14-18 carbons) FA is very fast [t1/2 < 10 ms; Kamp, F., et al. (1995) Biochemistry 34, 11928-11937]. The rates of dissociation of FA from SUV were evaluated by incorporating FA into donor vesicles and measuring transfer to acceptor vesicles. The transfer was followed by changes in internal pH of either donor or acceptor vesicles with stopped flow (C14:0, C16:0, C17:0, C18:0, C18:1, and C18:2) or on-line (C20:0, C22:0, and C24:0) fluorescence. All FA showed a single-exponential transfer process that was slower than the lower limits established for the rate of flip-flop, with t1/2 of dissociation ranging from 20 ms for C14:0 to 1900 s for C24:0. The pseudo-unimolecular rate constant (koff) for dissociation of C14:0 to C26:0 showed a 10-fold decrease for each addition of two CH2 groups to the acyl chain and a delta (delta G) of -740 cal/CH2. The dissociation rate constants for oleic acid (18:1) and linoleic acid (18:2) were 5 and 10 times faster, respectively, than that of C18:0. The rates of dissociation for typical dietary FA are sufficiently rapid that complex mechanisms (e.g. protein-mediated) may not be required for their desorption from biological membranes. The very slow dissociation rates for C24:0 and C26:0 may accentuate their pathological effects in diseases in which they accumulate in tissues.

Plasma membrane fatty acid-binding protein (FABPpm) of the sheep placenta

Fatty acid-binding protein (FABPpm) has been identified and characterised from sheep placental membranes. Binding of [14C]oleate to placental membranes was found to be time- and temperature-dependent. Addition of a 20-fold excess unlabelled oleic, palmitic, or linoleic acid reduced the binding of [14C]oleate to the membranes to around 50% of total binding, whereas D-alpha-tocopherol at similar concentrations did not affect [14C]oleate binding. This indicates that the binding sites are specific to fatty acids. Specific binding of [14C]oleate was reduced by heat denaturation or trypsin digestion of the membranes, suggesting that the fatty acid-binding sites are protein in nature. FABPpm was then solubilised from sheep placental membranes, and subsequently purified to electrophoretic homogeneity using an oleate-agarose affinity column. The purified FABPpm had an apparent molecular mass of 40 kDa, as determined by SDS-PAGE and by gel permeation chromatography. The [14C]oleate-binding activity of the purified protein was also confirmed by PAGE followed by autoradioblotting. The specific binding for oleate was around 1.5 nmol per mg of membrane protein. Our data indicate the presence of FABPpm in sheep placental membranes.

Movement of fatty acids, fatty acid analogues, and bile acids across phospholipid bilayers

How lipophilic acids move across membranes, either model or biological, is the subject of controversy. We describe experiments which better define the mechanism and rates in protein-free phospholipid bilayers. The transbilayer movement of lipophilic acids [fatty acids (FA), covalently-labeled FA, bile acids, and retinoic acid] was monitored by entrapping pyranin, a water-soluble, pH-sensitive fluorescent molecule to measure pH inside unilamellar vesicles [Kamp, F., & Hamilton, J.A. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11367-11370]. Equations for the pseudo-unimolecular rate constants for transbilayer movement of un-ionized (kappa FAH) and ionized (kappa FA-) acids are derived. All FA studied (octanoic, lauric, myristic, palmitic, stearic, oleic, elaidic, linoleic, linolelaidic, and arachidonic) and retinoic acid exhibited rapid transbilayer movement (t 1/2 < 1 s) via the un-ionized form across small unilamellar egg phosphatidylcholine (PC) vesicles. FA produced by phospholipase A2 in the outer leaflet of PC vesicles equilibrated rapidly to the inner leaflet. Ionized FA showed enhanced transbilayer movement (kappa FA- = 0.029 s-1) in the presence of equimolar valinomycin. The three FA analogues [12-(9-anthroyloxy)stearic acid, 5-doxylstearic acid, and 1-pyrenenonanoic acid] moved across PC bilayers via the un-ionized form; except for the anthroyloxy FA (kappa FAH = 4.8 x 10(-3) s-1), the rates were too fast to measure (t 1/2 < 1 s). The rate for cholic acid (CA) transbilayer movement was slow (kappa CAH = 0.056 s-1) compared to that of the more hydrophobic bile acids, deoxy- and chenodeoxycholic acid (t 1/2 < 1 s). The taurine conjugates of the three bile acids did not cross the bilayer (t 1/2 > 1 h). A further application of the pyranin method was to measure the partitioning of FA and bile acids among water, albumin, and PC vesicles. Our results show that the ability of lipophilic acids to permeate a PC bilayer rapidly is dependent on the presence of the un-ionized acid in the membrane interface. Considering the fast unfacilitated movement of FA across protein-free phospholipid bilayers, it is unlikely that there is a universal need for a transport protein to enhance movement of FA across membrane bilayers. Physiological implications of proton movement accompanying fast movement of un-ionized lipophilic acids (and the consequent generation of a pH gradient) are discussed.

Identification of high affinity membrane-bound fatty acid-binding proteins using a photoreactive fatty acid

A photoaffinity labeling method was developed to identify and characterize high affinity fatty acid-binding proteins in membranes. The specific labeling of these sites requires the use of low concentrations (nanomolar) of the photoreactive fatty acid 11-m-diazirinophenoxy-[11-3H]undecanoate. It was delivered as a bovine serum albumin (BSA) complex which serves as a reservoir for fatty acid and thus allows precise control of unbound fatty acid concentrations. The fadL protein of E. coli, which is required for fatty acid permeation of its outer membrane, was labeled by the photoreactive fatty acid neither specifically nor saturably when the probe was added in the absence of BSA; however when a nanomolar concentration of the uncomplexed probe was maintained in the presence of BSA, the labeling of the fadL protein was highly specific and saturable. This photoaffinity labeling method was also used to characterize a 22 kDa, high affinity fatty acid-binding protein which we have recently identified in the plasma membrane of 3T3-L1 adipocytes. This protein bound the probe with a Kd of 216 nM. The approach described is easily capable of identifying membrane-bound fatty acid-binding proteins and can distinguish between those of high and low affinities for fatty acids. It represents a general method for the identification and characterization of fatty acid-binding proteins.

Characteristics and specificity of the inhibition of liver glucose-6-phosphatase by arachidonic acid. Lesser inhibitability of the enzyme of diabetic rats

The effect of arachidonic acid (delta 4Ach) on liver glucose-6-phosphatase (Glc6Pase) has been studied in vitro using untreated and detergent-treated microsomes prepared from fed and 48-h-fasted normal rats and from streptozotocin-induced diabetic rats. Glc6Pase of both untreated and detergent-treated microsomes (60 micrograms protein/ml) is inhibited by delta 4Ach in a dose-dependent manner between 10-100 microM. The inhibition is very rapid and does not depend on preincubation of microsomes in the presence of delta 4Ach. It does depend on the concentration of microsomal membranes and on the concentration of glucose 6-phosphate: it is more pronounced at low Glc6P concentrations than at high. As a consequence, the enzyme displays sigmoidal kinetics in the presence of delta 4Ach. Hill coefficients (equal to 1 in the control experiments) of about 1.4 were determined in the presence of 50 microM delta 4Ach, indicating a clear positive cooperative dependency of the Glc6Pase upon its substrate in the presence of delta 4Ach. The delta 4Ach inhibition is fully reversible in the presence of bovine serum albumin. The inhibition does not depend on the metabolism of delta 4Ach through the prostaglandin synthase (cyclooxygenase) or arachidonate 12-lipoxygenase pathways since it is not affected by indomethacin and nordihydroguaiaretic acid. Several other unsaturated fatty acids are able to inhibit the enzyme within the same concentration range. In contrast, saturated fatty acids, the arachidonic acid methyl ester and numerous other lipid compounds containing esterified unsaturated fatty acids do not inhibit Glc6Pase within the same concentration range. The enzyme of fed rats was inhibited in the same manner as the enzyme of 48-h-fasted rats. However, Glc6Pase of untreated microsomes from diabetic rats was less inhibitable by delta 4Ach than the Glc6Pase of normal rats. This difference does not persist after solubilization of the membrane lipids by detergent treatment.

Transfer of long-chain fluorescent fatty acids between small and large unilamellar vesicles

Transfer of 12-(9-anthroyloxy)stearic acid (12AS) was measured between small unilamellar vesicles (SUV) and between large unilamellar vesicles (LUV), over a temperature range of 5-50 degrees C. The results of this study clearly establish the biexponential nature of the time dependence of the transfer in a variety of vesicle types and confirm our previous results using egg phosphatidylcholine (EPC) SUV at 25 degrees C (Storch & Kleinfeld, 1986). In our previous study we developed a kinetic model of the transfer process and concluded that the observed time dependence of the transfer of long-chain 12-(9-anthroyloxy) fatty acids (AOFA) was due to transbilayer flip-flop that was much slower than the rate at which the fatty acids (FA) move from the vesicle and into the surrounding aqueous phase (the off step). In the present study, experimental and theoretical advances have allowed us to examine, in detail, predictions of the kinetic model that critically depend upon the slow rate of flip-flop. The current results verify these predictions and demonstrate that slow AOFA flip-flop is rate limiting in at least three different vesicle systems and at all temperatures studied. Moreover, both flip-flop and the off rate constants were almost an order of magnitude smaller in EPC-LUV than in EPC-SUV. Flip-flop was found to be asymmetric (the rate constant for transfer from the inner to outer hemileaflet of the bilayer is approximately twice that from the outer to inner hemileaflet) in SUV but virtually symmetric in LUV. The temperature dependence of transfer was used to determine the thermodynamic activation potentials for the flip-flop and off rate constants.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of oleate binding to liver plasma membranes and its uptake by isolated hepatocytes

To clarify mechanisms of hepatic free fatty acid uptake, [3H]oleate uptake by isolated rat hepatocytes was studied, using solutions of 150 microM bovine serum albumin at oleate:albumin molar ratios of 0.033-6.7:1. Oleate partitioning between liver plasma membranes and albumin was also studied, and used to ascertain the membrane binding function for oleate. The experimental uptake curve was complex, but could be resolved by computer fitting into a sum of two components, one a saturable and the second a linear function of the unbound oleate concentration. The saturable component comprises > 90% of total oleate uptake when the oleate:albumin molar ratio is < 2.5, but < 50% when this ratio is > 5. Membrane binding also consisted of a sum of a saturable and a linear component. By comparison of the computer-fitted uptake and binding functions, separate rate constants for the transfer into the cell of the saturably and non-saturably bound oleate were estimated to be 0.7 s-1 and 0.05 s-1, respectively. The former is compatible with a specific, protein-mediated process. It is 15-times greater than the corresponding rate constant for transfer of non-saturably bound oleate into the cell, which in turn is similar to reported rates of non-specific 'flip-flop' of fatty acids across lipid bilayers. The observed kinetics are not consistent with models in which uptake occurs principally from the albumin-bound pool of oleate, or solely from the oleate which has partitioned passively into the lipid bilayer of the plasma membrane.

Uptake of a fluorescent-labeled fatty acid by spiroplasma floricola cells

12-(1-pyrene)dodecanoic fatty acid (P12) uptake by Spiroplasma floricola BNR-1 cells was characterized with regard to its kinetics, specificity, metabolism and susceptibility to protein and lipid inhibitors. The uptake process depended on temperature and pH, and exhibited biphasic saturation kinetics with a very low (2.7 microM) and a high (37 microM) apparent Km value. Lauric, myristic, palmitic, stearic and oleic fatty acids did not compete with P12 for transport. The fluorescence of P12 was exclusively recovered in the neutral lipid fraction, suggesting that this fatty acid is not further utilized for phospholipid biosynthesis. Valinomycin, carbonylcyanide m-chlorophenyldrazone (CCCP), dicyclohexylcarbodiimide (DCCD), and pronase strongly reduced P12 uptake by cells, but not by membrane vesicles, affecting the high affinity (low Km) component of the uptake system. Uptake of P12 by cells, as well as by membrane vesicles, was very sensitive to glutaraldehyde, chlorpromazine, phospholipase A21 and ascorbate with FeCl3, which affected the low affinity (high Km) component of a transport system. Digitonin stimulated P12 uptake. We suggest that the incorporation of P12 into spiroplasma cell membrane is a two-step process: a high specificity energy-dependent and protease-sensitive binding to the outer surface of membrane, and a low specificity and energy-independent diffusion and partition into the membrane lipid environment.

Interaction of amphiphilic substrates (acyl-CoAs) and their metabolites (free fatty acids) with microsomes from mouse sciatic nerves

We have measured the partition of stearoyl-CoA and oleoyl-CoA between an aqueous phase and the microsomes from mouse sciatic nerves. A method of microultracentrifugation was used which allowed us to study separately the aqueous phase and the biological membranes. We observed that the partition is dependent upon the amount of acyl-CoAs and membrane proteins but seems to be independent of time. A theoretical analysis of these data allowed interpretation of the binding and release in terms of acyl-CoA surface density in the vesicles. We have also analyzed the fate of the membrane-bound acyl-CoAs. We show that, whereas the apparent partition does not seem to vary, the hydrolysis of the membrane-bound acyl-CoAs followed by the release of free fatty acids from the membrane leads to a modification of the partition of acyl-CoAs between the membrane and the aqueous phase. We propose that there is a constant partition of the aliphatic chains (acyl-CoAs + free fatty acids).