Rates of hydration of fatty acids bound to unilamellar vesicles of phosphatidylcholine or to albumin

Cyclophospholipids Enable a Protocellular Life Cycle

There is currently no plausible path for the emergence of a self-replicating protocell, because prevalent formulations of model protocells are built with fatty acid vesicles that cannot withstand the concentrations of Mg2+ needed for the function and replication of nucleic acids. Although prebiotic chelates increase the survivability of fatty acid vesicles, the resulting model protocells are incapable of growth and division. Here, we show that protocells made of mixtures of cyclophospholipids and fatty acids can grow and divide in the presence of Mg2+-citrate. Importantly, these protocells retain encapsulated nucleic acids during growth and division, can acquire nucleotides from their surroundings, and are compatible with the nonenzymatic extension of an RNA oligonucleotide, chemistry needed for the replication of a primitive genome. Our work shows that prebiotically plausible mixtures of lipids form protocells that are active under the conditions necessary for the emergence of Darwinian evolution.

Progress in synthesizing protocells

IMPACT STATEMENT

Advances in the understanding of the biophysics of membranes, the nonenzymatic and enzymatic polymerization of RNA, and in the design of complex chemical reaction networks have led to a new, integrated way of viewing the shared chemistry needed to sustain life. Although a protocell capable of Darwinian evolution has yet to be built, the seemingly disparate pieces are beginning to fit together. At the very least, better cellular mimics are on the horizon that will likely teach us much about the physicochemical underpinnings of cellular life.

Different Mechanisms of Free Fatty Acid Flip-Flop and Dissociation Revealed by Temperature and Molecular Species Dependence of Transport across Lipid Vesicles

The mechanism of free fatty acid (FFA) transport across membranes is a subject of intense investigation. We have demonstrated recently that flip-flop is the rate-limiting step for transport of oleic acid across phospholipid vesicles (Cupp, D., Kampf, J. P., and Kleinfeld, A. M. (2004) Biochemistry 43, 4473-4481). To better understand the nature of the flip-flop barrier, we measured the temperature dependence of a series of saturated and monounsaturated FFA. We determined the rate constants for flip-flop and dissociation for small (SUV), large (LUV), and giant (GUV) unilamellar vesicles composed of egg phosphatidylcholine. For all FFA and vesicle types, dissociation was faster than flip-flop, and for all FFA, flip-flop and dissociation were faster in SUV than in LUV or GUV. Rate constants for both flip-flop and dissociation decreased exponentially with increasing FFA size. However, only the flip-flop rate constants increased significantly with temperature; the barrier to flip-flop was virtually entirely due to an enthalpic activation free energy. The barrier to dissociation was primarily entropic. Analysis in terms of a simple free volume (V(f)) model revealed V(f) values for flip-flop that ranged between approximately 12 and 15 Angstroms(3), with larger values for SUV than for LUV or GUV. V(f) values increased with temperature, and this temperature dependence generated the enthalpic barrier to flip-flop. The barrier for dissociation and its size dependence primarily reflect the aqueous solubility of FFA. These are the first results to distinguish the energetics of flipflop and dissociation. This should lead to a better understanding of the mechanisms governing FFA transport across biological membranes.

Stereospecific recognition of a spirosuccinimide type aldose reductase inhibitor (AS-3201) by plasma proteins: A significant role of specific binding by serum albumin in the improved potency and stability

AS-3201 [(3R)-2'-(4-bromo-2-fluorobenzyl)spiro[pyrrolidine-3,4'(1'H)-pyrrolo[1,2-a]pyrazine]-1',2,3',5(2'H)-tetrone] is a structurally novel and stereospecifically potent aldose reductase (AKR1B; EC 1.1.1.21) inhibitor, which contains a succinimide ring that undergoes ring-opening at physiological pH levels. To delineate intermolecular interactions governing its favorable pharmacokinetic profile, the interaction of AS-3201 (R-isomer) with plasma proteins, especially human serum albumin (HSA), was examined in comparison with that of the optical antipode (S-isomer). Fluorescence, kinetic, and high-performance frontal analyses showed that the R-isomer is more strongly bound than the S-isomer to sites I and II on HSA, and the R-isomer is particularly protected from hydrolysis, suggesting that the stable HSA-R-isomer complex contributes to its prolonged activity. The thermodynamic parameters for the specific binding indicated that in addition to hydrophobic interactions, hydrogen bonds contribute significantly to the R-isomer complex formation. (13)C NMR observations of the succinimide ring (5-(13)C enriched), which are sensitive to its ionization state, suggested the presence of a hydrogen bond between the R-isomer and HSA, and (19)F NMR of the pendent benzyl ring (2-(19)F) evaluated the equilibrium exchange dynamics between the specific sites. Furthermore, fatty acid binding or glycation (both are site II-oriented perturbations) inhibited the binding to one of the specific sites and reduced the stereospecificity of HSA toward the isomers, although the clinical influence of these perturbations on the R-isomer binding ratio seemed to be minor. Thus, the difference in the interaction mode at site II might be a major cause of the stereospecificity; this is discussed on the basis of putative binding modes. The present results, together with preliminary absorption and distribution profiles, provide valuable information on the stereospecific pharmacokinetic and pharmacodynamic properties of the R-isomer relevant for the therapeutic treatment of diabetic complications.

Changes in plasma membrane properties and phosphatidylcholine subspecies of insect Sf9 cells due to expression of scavenger receptor class B, type I, and CD36

In mammalian cells scavenger receptor class B, type I (SR-BI), mediates the selective uptake of high density lipoprotein (HDL) cholesteryl ester into hepatic and steroidogenic cells. In addition, SR-BI has a variety of effects on plasma membrane properties including stimulation of the bidirectional flux of free cholesterol (FC) between cells and HDL and changes in the organization of plasma membrane FC as indicated by increased susceptibility to exogenous cholesterol oxidase. Recent studies in SR-BI-deficient mice and in SR-BI-expressing Sf9 insect cells showed that SR-BI has significant effects on plasma membrane ultrastructure. The present study was designed to test the range of SR-BI effects in Sf9 insect cells that typically have very low cholesterol content and a different phospholipid profile compared with mammalian cells. The results showed that, as in mammalian cells, SR-BI expression increased HDL cholesteryl ester selective uptake, cellular cholesterol mass, FC efflux to HDL, and the sensitivity of membrane FC to cholesterol oxidase. These activities were diminished or absent upon expression of the related scavenger receptor CD36. Thus, SR-BI has fundamental effects on cholesterol flux and membrane properties that occur in cells of evolutionarily divergent origins. Profiling of phospholipid species by electrospray ionization mass spectrometry showed that scavenger receptor expression led to the accumulation of phosphatidylcholine species with longer mono- or polyunsaturated acyl chains. These changes would be expected to decrease phosphatidylcholine/cholesterol interactions and thereby enhance cholesterol desorption from the membrane. Scavenger receptor-mediated changes in membrane phosphatidylcholine may contribute to the increased flux of cholesterol and other lipids elicited by these receptors.

Fast flip-flop of cholesterol and fatty acids in membranes: implications for membrane transport proteins

PURPOSE OF REVIEW

The rates by which unesterified fatty acids and cholesterol move through and desorb from membranes have been difficult to measure, in part because of the simple structures of these lipids but also because methods have generally not clearly distinguished the two steps of membrane transport. Lack of definitive knowledge has given rise to speculation about the mechanism(s) of membrane 'transport' proteins for fatty acids and cholesterol.

RECENT FINDINGS

New biophysical and biochemical approaches have provided evidence that fatty acids and cholesterol exhibit rapid diffusion (flip-flop), as fast as milliseconds, across both protein-free phospholipid bilayers and cell membranes. In contrast, desorption of the cholesterol molecule from a membrane surface (hours) is much slower than that of common dietary fatty acids (milliseconds to seconds).

SUMMARY

Knowledge of these properties provides a framework for understanding transport and metabolism of cholesterol and fatty acids and how their putative membrane and intracellular transporters might function.

Binding of a fluorescent lipid amphiphile to albumin and its transfer to lipid bilayer membranes

Kinetics and thermodynamics of the binding of a fluorescent lipid amphiphile, Rhodamine Green(TM)-tetradecylamide (RG-C(14:0)), to bovine serum albumin were characterized in an equilibrium titration and by stopped-flow fluorimetry. The binding equilibrium of RG-C(14:0) to albumin was then used to reduce its concentration in the aqueous phase to a value below its critical micelle concentration. Under these conditions, the only two species of RG-C(14:0) in the system were the monomer in aqueous solution in equilibrium with the protein-bound species. After previous determination of the kinetic and thermodynamic parameters for association of RG-C(14:0) with albumin, the kinetics of insertion of the amphiphile into and desorption off lipid bilayer membranes in different phases (solid, liquid-ordered, and liquid-disordered phases, presented as large unilamellar vesicles) were studied by stopped-flow fluorimetry at 30 degrees C. Insertion and desorption rate constants for association of the RG-C(14:0) monomer with the lipid bilayers were used to obtain lipid/water equilibrium partition coefficients for this fluorescent amphiphile. The direct measurement of these partition coefficients is shown to provide a new method for the indirect determination of the equilibrium partition coefficient of similar molecules between two defined lipid phases if they coexist in the same membrane.

How fatty acids bind to proteins: the inside story from protein structures

The interactions of fatty acids with proteins have been probed with a great variety of techniques and strategies. Many approaches have substituted covalently labeled fatty acids or structurally related molecules. Information from such studies ultimately requires validation by studies with natural fatty acids. However, even the best conventional approaches with natural fatty acids generally have revealed only limited aspects of fatty acid-protein interactions. In contrast, recent crystallographic and NMR studies of several proteins with bound fatty acids provide complete three-dimensional structures with molecular details of these interactions. This presentation reviews three examples of proteins that are indirectly or directly involved in cell signaling: a protein in the plasma compartment (human serum albumin); a protein family in the cytosolic compartment of mammalian cells (fatty-acid-binding proteins), and a nuclear protein (peroxisome proliferator-activated receptor): it also discusses the structures of these proteins and their binding pocket(s), compares their specific modes of interactions with fatty acids, and discusses established and potential roles of fatty acid-protein interactions in cell signaling.

Stopped-flow kinetic analysis of long-chain fatty acid dissociation from bovine serum albumin

The kinetics of the interaction of long-chain fatty acids (referred to as fatty acids) with albumin is critical to understanding the role of albumin in fatty acid transport. In this study we have determined the kinetics of fatty acid dissociation from BSA and the BSA-related fatty acid probe BSA-HCA (BSA labelled with 7-hydroxycoumarin-4-acetic acid) by stopped-flow methods. Fatty acid-albumin complexes of a range of natural fatty acid types and albumin molecules (donors) were mixed with three fatty acid-binding acceptor proteins. Dissociation of fatty acids from the donor was monitored by either the time course of donor fluorescence/absorbance or the time course of acceptor fluorescence. The results of these measurements indicate that fatty acid dissociation from BSA as well as BSA-HCA is well described by a single exponential function over the entire range of fatty acid/albumin molar ratios used in these measurements, from 0.5:1 to 6:1. The observed rate constants (k(obs)) for the dissociation of each fatty acid type reveal little or no dependence on the initial fatty acid/albumin ratio. However, dissociation rates were dependent upon the type of fatty acid. In the case of native BSA with an initial fatty acid/BSA molar ratio of 3:1, the order of k(obs) values was stearic acid (1.5 s(-1)) < oleic acid < palmitic acid congruent with linoleic acid<arachidonic acid (8 s(-1)) at 37 degrees C. The corresponding values for BSA-HCA were about half the values for BSA. The results of this study show that the rate of fatty acid dissociation from native BSA is more than 10-fold faster than reported previously and that the off-rate constants for the five primary fatty acid-binding sites differ by less than a factor of 2. We conclude that for reported rates of fatty acid transport across cell membranes, dissociation of fatty acids from the fatty acid-BSA complexes used in the transport studies should not be rate-limiting.

Effect of fatty acid-binding proteins on intermembrane fatty acid transport studies on different types and mutant proteins

Liposomes of different charge fixed to nitrocellulose filters were used to study the transfer of fatty acids to rat heart or liver mitochondria in the presence of fatty acid-binding protein (FABP) or albumin. [14C]Palmitate oxidation was used as a parameter. Different FABP types and heart FABP mutants were tested. The charge of the liposomes did not influence the solubilization and mitochondrial oxidation of palmitate without FABP and the amount of solubilized palmitate in the presence of FABP. Mitochondria did not show a preference for oxidation of FABP-bound palmitate over their tissue-specific FABP type. All FABP types increased palmitate oxidation by heart and liver mitochondria with neutral, positive and negative liposomes by 2.5-fold, 3.2-fold and twofold, respectively. Ileal lipid-binding protein and H-FABP mutants that do not bind fatty acid had no effect. Other H-FABP mutants had different effects, dependent on the site of mutation. The effect of albumin was similar to, but not dependent on, liposome charge. The ionic strength had only a slight effect. In conclusion, the transfer of palmitate from liposomal membranes to mitochondria was increased by all FABP types to a similar extent. The membrane charge had a large effect in contrast to the origin of the mitochondria.

Role of plasma membrane transporters in muscle metabolism

Muscle plays a major role in metabolism. Thus it is a major glucose-utilizing tissue in the absorptive state, and changes in muscle insulin-stimulated glucose uptake alter whole-body glucose disposal. In some conditions, muscle preferentially uses lipid substrates, such as fatty acids or ketone bodies. Furthermore, muscle is the main reservoir of amino acids and protein. The activity of many different plasma membrane transporters, such as glucose carriers and transporters of carnitine, creatine and amino acids, play a crucial role in muscle metabolism by catalysing the influx or the efflux of substrates across the cell surface. In some cases, the membrane transport process is subjected to intense regulatory control and may become a potential pharmacological target, as is the case with the glucose transporter GLUT4. The goal of this review is the molecular characterization of muscle membrane transporter proteins, as well as the analysis of their possible regulatory role.

The interaction of human serum albumin and model membranes

It is frequently observed that the interaction of human serum albumin (HSA) with different lipid membranes may affect molecular transport both in vivo and in vitro experiments. There was a lack of consensus however in the interpretation of results. Earlier studies on the serum albumin membrane association had different conclusions depending on the source of protein, the preparation and the composition of the membranes applied. In this work the change of heat capacity, a sensitive parameter of the interacting system, is compared for uni- and multilamellar liposomes (dimyristoyl-phosphatidylcoline/dimyristoyl-phosphatidylglycerol) at 0, 1x10(-3), 8x10(-3), 1.2x10(-2) and 3.3x10(-2) HSA-lipid ratios. The thermal properties of the sonicated and vortexed liposomes show remarkable differences. The presence of HSA in both types of liposomes also modified their thermal properties, providing clear evidence for protein-vesicle interaction, different in the uni- and multilamellar liposomes. In the case of unilamellar liposomes, two additional transitions were observed at lower temperature, independently of the HSA-lipid ratio, and the protein binding mode to smaller or larger sized liposomes was also distinguishable. The addition of HSA to the multilamellar liposomes resulted in an increase of the pretransition temperature only at the higher HSA-lipid ratio, but the main transition temperature was not affected.

Distinct roles for cellular retinoic acid-binding proteins I and II in regulating signaling by retinoic acid

The pleiotropic effects of retinoic acid (RA) in mammalian cells are mediated by two classes of proteins: the retinoic acid receptors (RAR) and cellular retinoic acid-binding proteins (CRABP-I and CRABP-II). Here we show that expression of CRABP-II, but not CRABP-I, markedly enhanced RAR-mediated transcriptional activation of a reporter gene in COS-7 cells. The equilibrium dissociation constants of complexes of CRABP-I or CRABP-II with RA were found to differ by 2-fold. It is thus unlikely that the distinct effects of the two proteins on transactivation stem from differential ligand-binding affinities. The mechanisms by which RA transfers from the CRABPs to RAR were thus investigated directly. The rate constant for movement of RA from CRABP-II, but not from CRABP-I, to RAR strongly depended on the concentration of the acceptor. The data suggest that transfer of RA from CRABP-I to RAR involves dissociation of the ligand from the binding protein, followed by association with the receptor. In contrast, movement of RA from CRABP-II to the receptor is facilitated by a mechanism that involves direct interactions between CRABP-II and RAR. These findings reveal a striking functional difference between CRABP-I and CRABP-II, and point at a novel mechanism by which the transcriptional activity of RA can be regulated by CRABP-II.

Transport of fatty acids across membranes by the diffusion mechanism

In early research on fatty acid transport, passive diffusion seemed to provide an adequate explanation for movement of fatty acids through the membrane bilayer. This simple hypothesis was later challenged by the discovery of several proteins that appeared to be membrane-related fatty acid transporters. In addition, some biophysical studies suggested that fatty acids moved slowly through the simple model membranes (phospholipid bilayers), which would provide a rationale for protein-assisted transport. Furthermore, it was difficult to rationalize how fatty acids could diffuse passively across the bilayer as anions. Newer studies have shown that fatty acids are present in membranes in the un-ionized as well as the ionized form, and that the un-ionized form can cross a protein-free phospholipid bilayer quickly. This flip-flop mechanism has been validated in cells by intracellular pH measurements. The role of putative fatty acid transport proteins remains to be clarified.

Transport, metabolism and distribution space of octanoate in the perfused rat liver

The scope of the present work was to investigate the metabolism and the passage of octanoate from albumin into the phospholipid bilayer of the plasma membrane and from thence into the cell space. The experiments were done in the isolated perfused rat liver with infusions of albumin and octanoate at various concentrations. Once steady-state conditions were attained, trace amounts of [1-14C]-octanoate, [131 I]-albumin and [3H]-water were injected simultaneously and the effluent perfusate was fractionated. The normalized dilution curves were used for model analysis. The model which gives the best fit to the experimental results and which also produces the most consistent parameters is one that presupposes a rapid distribution of octanoate into the cell membrane and a slow transfer from the cell membrane into the cytosol. The concentration dependence of the distribution between the membrane and the extracellular space is parabolic, suggesting that octanoate changes the properties of the cell membrane when present at higher concentrations. The passage from the cell membrane into the cell space is relatively slow and limits metabolic transformation partly or totally, depending on the octanoate concentration in the plasma membrane. The rapid transfer of octanoate from the albumin space into the plasma membrane corroborates previous measurements of the dissociation of the albumin-octanoate complex.

Spontaneous transfer of monoacyl amphiphiles between lipid and protein surfaces

The kinetics of transfer of natural and fluorescent nonesterified fatty acids (NEFA) and lysolecithins (lysoPC) from phospholipid and protein surfaces were measured. The kinetics of transfer of 12-(1-pyrenyl)dodecanoic acid, from liquid crystalline and gel phase single unilamellar phospholipid vesicles, very low, low, and high density lipoproteins, human serum albumin, and rat liver fatty acid-binding protein, were first-order and characterized by similar rate constants. The halftimes (t1/2) of NEFA transfer from lipids and proteins were dependent on the acyl chain structure according to log t1/2 = -0.62n + 0.59m + 12.0, where n and m, respectively, are the numbers of carbon atoms and double bonds. The structure of the donor surface had a measurable but smaller effect on transfer rates. The kinetics of NEFA and lysoPC transfer are slow relative to the lipolytic processes that liberate them. Therefore, one would predict a transient accumulation of NEFA and lysoPC during lipolysis and an attendant modulation of many metabolic processes within living cells and within the plasma compartment of blood. These data will be useful in the refinement of current models of membrane and lipoprotein function and in the selection of fluorescent NEFA analogs for studying transport in living cells.

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.

Covalent complexes between serum albumin and 7-hydroxycoumarin-4-acetic acid: synthesis and applications in the spectrophotometric detection of long-chain fatty acids

Using a hydrophobic 8-aminooctanoic acid cross-linker, the pH-indicator dye 7-hydroxycoumarin-4-acetic acid (7-HCA) is covalently bound to bovine serum albumin (BSA) at the positions of reactive amino groups. A highly stable and water-soluble complex (BSA-HCA) with a 1:4 molar stoichiometry is synthesized. Appearance of a strong absorption band at gamma max = 372 nm is associated to ionization of the 7-HCA chromophore when it is transferred from water into a basic microenvironment on the BSA surface. This particular surface site is related to the region(s) for high-affinity binding of long-chain fatty acids (FA). BSA-HCA responds to binding of FA (14-20 carbons) with immediate spectral changes and a decrease in 372 nm absorption. BSA-HCA provides an indicator-protein having a range of practical applications for the quantitative determination of long-chain FA in biochemical studies. The lower detection limit in a spectrophotometric method is approximately 1 microM FA. BSA-HCA is usable both in various buffers and in the presence of detergents such as n-octylglucoside, Triton X-100 and CHAPS. A novel method for continuous assay of phospholipase A2 activity with BSA-HCA and a mixed phosphatidylcholine/CHAPS micellar substrate is reported.

Docosahexaenoic acid modulates the interactions of the interphotoreceptor retinoid-binding protein with 11-cis-retinal

Rapid transport of retinoids across the interphotoreceptor matrix is a critical part of the visual cycle, since it serves to replenish bleached rhodopsin with its chromophore 11-cis-retinal. The transport of retinoids in the interphotoreceptor matrix is believed to be mediated by the interphotoreceptor retinoid-binding protein (IRBP), a protein that, in addition to possessing two retinoid-binding sites, associates in vivo with long chain fatty acids. Here, the interrelationships between binding of the two types of ligands to IRBP were studied. The composition of fatty acids associated with IRBP in bovine retina was determined, and it was found that polyunsaturated fatty acids constitute a significant fraction of those. It was further found that docosahexaenoic acid, but not palmitic acid, induced a rapid and specific release of 11-cis-retinal from one of the protein's retinoid-binding sites. Based on these results and on the additional observation that a steep concentration gradient of docosahexaenoic acid exists between photoreceptor and pigment epithelium cells, a model for the mechanism by which IRBP may target 11-cis-retinal to photoreceptor cells is proposed.

Interactions of a very long chain fatty acid with model membranes and serum albumin. Implications for the pathogenesis of adrenoleukodystrophy

Adrenoleukodystrophy (ALD) is an inherited disorder of fatty acid metabolism marked by accumulation of very long chain saturated fatty acids (VLCFA), especially the 26-carbon acid, hexacosanoic acid (HA), in membranes and tissues. We have studied interactions of 13C-enriched HA with model membranes (phospholipid bilayer vesicles) and bovine serum albumin (BSA) by 13C NMR spectroscopy to compare properties of HA with those of typical dietary fatty acids. In phospholipid bilayers the carboxyl group of HA is localized in the aqueous interface, with an apparent pKa (7.4) similar to other fatty acids; the acyl chain must then penetrate very deeply into the membrane. Desorption of HA from vesicles (t1+2 = 3 h) is orders of magnitude slower than shorter chain fatty acids. In mixtures of vesicles and BSA, HA partitions much more favorably to phospholipid bilayers than typical fatty acids. BSA binds a maximum of only 1 mole of HA at one binding site. Calorimetric experiments show strong perturbations of acyl chains of phospholipids by HA. We predict that disruptive effects of VLCFA on cell membrane structure and function may explain the neurological manifestations of ALD patients. These effects will be further amplified by slow desorption of VLCFA from membranes and by the ineffective binding to serum albumin.

A direct role for serum albumin in the cellular uptake of long-chain fatty acids

The interaction of long-chain fatty acids with cells is important for their uptake and metabolism, as well as their involvement in signalling processes. The majority of long-chain fatty acids circulating in plasma exist as complexes with serum albumin. Thus an understanding of the involvement of serum albumin in these processes is vitally important. The effect of serum albumin on the uptake of long-chain fatty acids was studied in 3T3-L1 adipocytes. Serum albumin had a stimulatory effect on oleate uptake at all ratios of oleate: serum albumin tested. Furthermore, the rate of oleate uptake was saturable with increasing concentrations of serum albumin when the oleate: serum albumin ratio, and therefore the concentration of uncomplexed oleate, remained constant. This was not due to uptake being limited by dissociation of oleate from serum albumin, because oleate did not appear to be limiting. Furthermore, at very high ratios of oleate: serum albumin, when the concentration of uncomplexed oleate was predicted to be large relative to the amount of oleate taken up by cells, the rate of oleate uptake was still dependent on the albumin concentration. Serum albumin, covalently labelled with the photoreactive fatty acid 11-m-diazirinophenoxy[11-3H]undecanoate, bound to cells in a manner exhibiting both saturable (Kd 66.7 microM) and non-saturable processes. These results indicate that the stimulatory effect of serum albumin on the rate of oleate uptake is due to a direct interaction of serum albumin with the cells and point to an involvement of albumin binding sites in the cell surface in the cellular uptake of long-chain fatty acids.

Membrane permeation and intracellular trafficking of long chain fatty acids: insights from Escherichia coli and 3T3-L1 adipocytes

Long chain fatty acids are important substrates for energy production and lipid synthesis in prokaryotes and eukaryotes. Their cellular uptake represents an important first step leading to metabolism. This step is induced in Escherichia coli by growth in medium containing long chain fatty acids and in murine 3T3-L1 cells during differentiation to adipocytes. Consequently, these have been used extensively as model systems to study the cellular uptake of long chain fatty acids. Here, we present an overview of our current understanding of long chain fatty acid uptake in these cells. It consists of several distinct steps, mediated by a combination of biochemical and physico-chemical processes, and is driven by conversion of long chain fatty acids to acyl-CoA by acyl-CoA synthetase. An understanding of long chain fatty acid uptake may provide valuable insights into the roles of fatty acids in the regulation of cell signalling cascades, in the regulation of a variety of metabolic and transport processes, and in a variety of mammalian pathogenic conditions such as obesity and diabetes.

Is there facilitated uptake of fatty acids by the liver? Interpretation and analysis of experimental data

Uptake of hydrophobic organic anions that are extensively bound to serum proteins has been a controversial issue for over 30 years. It is known that steady-state uptake is lower in the presence of binding proteins, but it is much higher than predicted on the basis of protein-ligand binding equilibrium. Several theories have been postulated to account for this observation. Recent work has shown how binding proteins are capable of enhancing the uptake rate of long-chain fatty acids by decreasing the diffusional resistance of the unstirred fluid layer. The enhanced transport via codiffusion is especially important for tightly bound ligands like long-chain fatty acids. Whether this model accounts for all experimental data or whether hepatocytes facilitate the uptake of protein-bound ligands, by for example mediating the protein-ligand dissociation rate, is not clear. We review the published reports to gain an understanding into the potential mechanism for the extraction of long-chain fatty acids. Understanding the uptake mechanism of these important metabolic substrates is vitally important in determining their overall utilization in a variety of clinical disorders as diverse as gallstones, obesity, and atherosclerosis.

Kinetics of bilirubin transfer between serum albumin and membrane vesicles. Insight into the mechanism of organic anion delivery to the hepatocyte plasma membrane

Unconjugated bilirubin is transported in the plasma bound primarily to serum albumin, from which it is taken up and metabolized by the liver. To better characterize the mechanism of bilirubin delivery to the hepatocyte, stopped-flow techniques were utilized to study the kinetics of bilirubin transfer between serum albumin and both model phospholipid and native hepatocyte plasma membrane vesicles. The transfer process was best described by a single exponential function, with rate constants of 0.93 +/- 0.04, 0.61 +/- 0.03, and 0.10 +/- 0.01 s-1 (+/- S.D.) at 25 degrees C for human, rat, and bovine serum albumins, respectively. The observed variations in rate with respect to donor and acceptor concentrations provide strong evidence for the diffusional transfer of free bilirubin. Thermodynamic analysis suggests that the binding site on bovine serum albumin demonstrates higher specificity for the bilirubin molecule than that on human or rat serum albumin, which exhibit similar binding characteristics. Kinetic analysis of bilirubin transfer from rat serum albumin to isolated rat basolateral liver plasma membranes indicates that the delivery of albumin-bound bilirubin to the hepatocyte surface occurs via aqueous diffusion, rather than a collisional process, thereby mitigating against the presence of an "albumin receptor."

Studies on the efflux of heme from biological membranes

It is unknown how heme is distributed intracellularly from its site of synthesis in the mitochondria to other organelles. In previous work (Biochemistry 23, 3715, 1984) the transfer of heme from lipid bilayers to soluble proteins had been found to be independent of the recipient proteins' affinity for heme. Here, we investigated whether proteins are involved in the transfer of heme from biological membranes into aqueous media. We followed the release of 14C-labeled heme, from mitochondria preloaded with the heme, to BSA and found that only about 28%, of the heme was extracted on the first wash. After the third wash 35-50% of the heme that had been partitioned into the membranes was extracted. Fourth and fifth washes with BSA or a cytosolic heme-binding protein (HBP, also known as liver fatty acid binding protein) removed only insignificant amounts of 14C-labeled heme. Similarly, a large portion of the preloaded 14C-labeled heme could not be extracted from a variety of isolated membranes (inner and outer mitochondrial membranes, plasma membranes of liver cells, kidney cortex cells and erythrocyte membranes). By contrast, essentially all [14C]palmitate preloaded in biological membranes and all 14C-labeled heme preloaded in synthetic membranes was released to albumin (Biochemistry 23, 3715, 1984). These observations suggest that, in general, heme associates with membrane components which can be distinguished into two compartments. One compartment releases its heme spontaneously, while another compartment binds heme so tightly that a specific process has to be evoked for its release.

Interaction of rat liver microsomes containing saturated or unsaturated fatty acids with fatty acid binding protein: peroxidation effect

In the studies described here rat liver microsomes containing labeled palmitic, stearic, oleic or linoleic acids were incubated with fatty acid binding protein (FABP) and the rate of removal of 14C-labeled fatty acids from the membrane by the soluble protein was measured using a model system. More unsaturated than saturated fatty acids were removed from native liver mircrosomes incubated with similar amounts of FABP. The in vitro peroxidation of microsomal membranes mediated by ascorbate-Fe++, modified its fatty acid composition with a considerable decrease of the peroxidizability index. These changes in the microsomes facilitated the removal of oleic and linoeic acids by FABP, but the removal of palmitic and stearic acids was not modified. This effect is proposed to result from a perturbation of membrane structure following peroxidation with release of free fatty acids from susceptible domains.

Carrier-protein-mediated enhancement of fatty-acid binding and internalization in human T-lymphocytes

Albumin and alpha-fetoprotein (AFP) are members of a multigene family which also includes vitamin-D-binding protein. Previous work in our laboratory has provided experimental support for the suggestion that the entry of unsaturated fatty acids into growing, normal and neoplastic cells may be regulated by AFP. In the actual study we have examined the role of human serum albumin (HSA) as a carrier protein, when compared to AFP, on the uptake (binding and internalization) of fatty acids by resting and PHA-activated human lymphocytes. Radioiodinated human HSA and tritiated oleic and arachidonic acids were used under different experimental conditions to follow the binding of the protein and fatty acids (FA) to cells. Time-course uptake at 4 degrees C of HSA and of oleic and arachidonic acids bound to HSA (FA/HSA molar ratio = 1) by either resting or activated T-lymphocytes exhibited a steady state of binding. The amount of FA bound was much greater than the corresponding amount of HSA, suggesting that T-lymphocytes bear distinct binding sites for albumin and fatty acids. A saturable process of FA binding was observed at constant unbound FA concentration in the incubation medium when the HSA-to-FA molar ratio was fixed at 1 and the concentrations of both HSA and FA were increased simultaneously. This saturable component of binding reflects an intrinsic regulatory effect of HSA, probably operating throughout the interaction of the protein with specific cell receptors. At varying unbound FA concentrations, binding curves showed two distinct components: a non-linear portion which could indicate the presence of a saturable process operating at low concentrations of unbound, free FA, followed by a second part which increased linearly with the concentration of unbound FA. The amount of FA bound at 4 degrees C and bound and internalized at 37 degrees C by both types of cell was considerably higher in the presence than in the absence of carrier proteins. On the contrary, carrier proteins were without effect on the distribution pattern of internalized oleic or arachidonic acid. Taken together, these observations suggest that: (i) the binding and entry of FA into cells is enhanced by the two carrier-proteins at low concentrations of free, unbound fatty acids in the vicinity of the cell surface, and (ii) fatty-acid uptake seems regulated by a dual-receptor mechanism involving HSA and/or AFP receptors as well as plasma-membrane FA-binding proteins.

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.