Function and regulation of hepatic and intestinal fatty acid binding proteins

Expression of human liver fatty acid-binding protein in Escherichia coli and comparative analysis of its binding characteristics with muscle fatty acid-binding protein

Human liver fatty acid-binding protein (L-FABP) has been efficiently expressed in Escherichia coli. The cDNA encoding human liver FABP was under the control of T7 RNA polymerase promoter in the expression vector pET-3b. Expression required overnight induction with isopropyl beta-D-thiogalactopyranoside in the presence of the bacterial RNA polymerase inhibitor, rifampicin. The protein could be purified by (NH4)2SO4 fractionation, anion-exchange and gel filtration chromatography, and was recognized by anti-(human L-FABP) antiserum. The binding characteristics of delipidated recombinant human L-FABP and muscle FABP (M-FABP) for fatty acids of different chain length and saturation grade, and for various hydrophobic ligands, were determined by radiochemical analysis and also by fluorescence for L-FABP. The apparent binding affinity of the ligands was calculated by using displacement curves of oleic acid and dansylamino-undecanoic acid (DAUDA). L-FABP showed a preference for the binding of long-chain saturated and unsaturated fatty acids up to C24:1, whereas the M-FABP has a preference for unsaturated fatty acids, especially those with 18 C atoms. L-FABP also binds palmitoyl derivatives and many other hydrophobic ligands--however, generally with a lower affinity than fatty acids. M-FABP binds--besides with fatty acids--only with oestradiol and testosterone with high affinity. Fatty acids with fluorescent reporter groups are also more tightly bound by L-FABP. A direct assay and displacement study of oleic acid gave the same Kd value of DAUDA for L-FABP. Fluorescence enhancement and displacement studies indicate that the binding of fluorescent fatty acids is determined by both the fluorescent reporter group and the acyl carbon chain.

Experimental conditions affecting in vitro intestinal incorporation of palmitic acid: a methodological approach

In the rat, a large number of in vitro studies on intestinal fatty acid uptake have been carried out. However, the results obtained under different experimental conditions are often contradictory. The present work is a critical approach to the experimental aspects which may modify in vitro intestinal uptake of fatty acids. Different kinds of intestinal tissue samples (intact, everted or opened rings) were used. The histological changes and the uptake of palmitic acid were measured for each type of sample under different stirring rates, at different incubation times and with micellar solutions of varying composition. It is concluded that 1) opened rings have the highest absorptive capacity with the lowest dispersion; 2) incubation periods longer than 30 minutes do not result in additional palmitic acid uptake and may lead to severe tissue hypoxia as indicated by extensive vacuolization; 3) stirring rates over 1 cycle/sec do not result in increased PA uptake and cause extensive mucosal disruption, particularly in jejunal samples.

Effects of oleic and elaidic acids on in vitro intestinal uptake of cholesterol in the rat

The effects of oleic and elaidic acid upon the in vitro intestinal uptake of cholesterol were studied in 9 weeks-old male and female rats. Elaidic acid increases the uptake of cholesterol when compared to oleic acid in both males and females. It is suggested that elaidic acid may enhance the esterification of cholesterol within the enterocyte through being more available for esterification and/or through being preferentially incorporated into cholesterol esters.

Characterization and binding properties of human fetal lung fatty acid-binding proteins

When delipidated Mr > 10,000 cut-off human fetal lung cytosol was separated on gel filtration and ion-exchange chromatography on Auto-FPLC system, two fatty acid-binding proteins (FABPs) of pI 6.9 and pI 5.4 were purified to homogeneity. On Western blotting analysis with the anti-human fetal lung pI 6.9 FABP, these two proteins showed immunochemical cross reactivity with each other and with purified hepatic FABPs but not with cardiac or gut FABP. These two FABPs have identical molecular mass of 15.2 kDa, which is slightly higher than that of the hepatic proteins (14.2 kDa). Carbohydrate covalently linked to FABPs, that may substantially add to the molecular mass, was not detected in the purified protein preparations. Amino acid analysis revealed that both the proteins have same amino acid composition each containing one Trp residue that is lacking in hepatic FABP. Different isoforms of lung FABP exhibited different binding ability for their natural ligands. These proteins bind palmitoyl CoA with higher affinity than oleic acid. pI 6.9 FABP can more rapidly and efficiently transfer fatty acid than can pI 5.4 FABP from unilammelar liposomes. Thus these FABPs may play a critical role in fatty acid transport during human fetal lung development.

Two cytosolic protein families implicated in lipid-binding: main structural and functional features

1. According to the important biological role of fatty acids and phospholipids in cell membranes, two cytosolic proteins implicated in their binding and transport in brain were considered, namely: Fatty Acid-Binding Protein and basic 21 kDa protein. 2. They were reviewed as well as their related protein families. 3. Although the two protein groups do not present significant sequence homologies, they share several similar properties and might thus be implicated in common physiological functions.

Hepatic disposition of acetaminophen and metabolites. Pharmacokinetic modeling, protein binding and subcellular distribution

Successful pharmacokinetic modeling often requires the ability of a simple model to describe a complex series of physiological processes. However, a simple model may be inappropriate. Physiologically-relevant modeling may offer a more appropriate description, but requires further support from in vitro/in vivo data. A well-stirred hepatic model with linear processes was proposed to describe in vivo disposition of acetaminophen and metabolites after a 100 mg/kg bolus of acetaminophen to vehicle- or phenobarbital-pretreated, renal-ligated rats. Model simulations underpredicted acetaminophen glucuronide (AG) concentrations at early time points in serum, and were inconsistent with AG biliary excretion-rate profiles. Intracellular binding of AG by ligandin was hypothesized, and a cytosolic compartment with reversible binding was incorporated into the model. In this second model, only AG bound in the cytosolic compartment was available for excretion into bile. Model 2 better described the AG biliary excretion rate-time profiles based on calculated Akaike's information criterion values. However, no apparent change was observed in the underprediction of AG serum concentrations. Parameter estimates derived from the two models also were different. The rate constants regulating AG formation and sinusoidal egress were increased significantly after phenobarbital pretreatment according to model 1, while the AG biliary excretion rate constant was decreased significantly. Parameter estimates based on model 2 suggested that phenobarbital pretreatment impaired the cytosolic binding of AG but increased significantly the AG biliary excretion rate constant. The physiologic relevance of model 2 was not supported by a subsequent investigation of the protein binding and subcellular distribution of acetaminophen and metabolites. Acetaminophen, AG and acetaminophen sulfate (AS) were not bound extensively in hepatic cytosol (mean +/- SD unbound fractions were 0.90 +/- 0.08, 0.97 +/- 0.08, and 0.88 +/- 0.06, respectively). Phenobarbital pretreatment did not alter significantly the unbound fractions of acetaminophen, AG or AS in hepatic cytosol. Acetaminophen was distributed to a greater extent in lysosomes than in the nuclear, mitochondrial, microsomal and cytosolic fractions. Distribution of AS predominated in cytosolic and lysosomal fractions. AG was detected only in cytosol. Phenobarbital pretreatment decreased the content of acetaminophen, AG and AS in all hepatic fractions. This study demonstrates the utility of pharmacokinetic modeling in exploring mechanistic hypotheses. However, these results underscore the importance of obtaining pivotal data from in vitro/in vivo studies to validate hypothesized mechanisms.

Expression of rat L-FABP in mouse fibroblasts: role in fat absorption

Fatty acid-binding proteins (FABP) are abundant cytosolic proteins whose levels is responsive to nutritional, endocrine, and a variety of pathological states. Although FABPs have been investigated in vitro for several decades, little is known of their physiological function. Liver L-FABP binds both fatty acids and cholesterol. Competitive binding analysis and molecular modeling studies of L-FABP indicate the presence of two ligand binding pockets that accommodate one fatty acid each. One fatty acid binding site is identical to the cholesterol binding site. To test whether these observations obtained in vitro were physiologically relevant, the cDNA encoding L-FABP was transfected into L-cells, a cell line with very low endogenous FABP and sterol carrier proteins. Uptake of both ligands did not differ between control cells and low expression clones. In contrast, both fatty acid uptake and cholesterol uptake were stimulated in the high expression cells. In high expression cells, uptake of fluorescent cis-parinaric acid was enhanced more than that of trans-parinaric acid. This is consistent with the preferential binding of cis-fatty acids to L-FABP but in contrast to the preferential binding of trans-parinaric acid to the L-cell plasma membrane fatty acid transporter (PMFABP). These data show that the level of cytosolic fatty acids in intact cells can regulate both the extent and specificity of fatty acid uptake. Last, sphingomyelinase treatment of L-cells released cholesterol from the plasma membrane to the cytoplasm and stimulated microsomal acyl-CoA: cholesteryl acyl transferase (ACAT). This process was accelerated in high expression cells. These observations show for the first time in intact cells that L-FABP, a protein most prevalent in liver and intestine where much fat absorption takes place, may have a role in fatty acid and cholesterol absorption.

Fatty acyl-CoA binding activity of the nuclear thyroid hormone receptor

Long-chain fatty acids and their acyl-CoA esters are potent inhibitors of nuclear thyroid hormone (T3) receptor in vitro. In the present study, we obtained evidence for acyl-CoA binding activity in the nuclear extract from rat liver. The activity sedimented at a position (3.5 S) identical with that of the T3 receptor, and the two activities sedimented together. Similarly, they coeluted on DEAE-Sephadex. After partial purification of the receptor, it was again inhibited strongly by acyl-CoAs. Heat stability and a partial trypsin digestion of the receptor both suggested that the action site of oleoyl-CoA overlapped the T3-binding domain of the receptor. In addition, thyroid hormone receptor beta 1, synthesized in vitro, bound oleoyl-CoA specifically and its T3-binding activity was inhibited. The dissociation constant for oleoyl-CoA binding to the partially purified receptor was 1.2 x 10(-7) M. This value as well as its molecular size distinguished the nuclear binding sites from the cytoplasmic fatty acid/acyl-CoA binding proteins. Oleoyl-CoA had no effect on the glucocorticoid receptor, another member of the nuclear hormone-receptor superfamily. From these results, we propose that thyroid hormone receptor is a specific acyl-CoA binding protein of the cell nucleus.

Interaction of fatty acids, acyl-CoA derivatives and retinoids with microsomal membranes: effect of cytosolic proteins

This paper reviews characteristics of microsomal membrane structure; long chain fatty acids, acyl CoA derivatives, retinoids and the microsomal formation of acyl CoA derivatives and retinyl esters. It is analyzed how the movement of these molecules at the intracellular level is affected by their respective binding proteins (Fatty acid binding protein, acyl CoA binding protein and cellular retinol binding protein). Studies with model systems using these hydrophobic ligands and the lipid-binding or transfer proteins are also described. This topic is of interest especially because in the esterification of retinol the three substrates and the three binding proteins may interact.

Photoreactive fatty acid analogues that bind to the rat liver fatty-acid binding protein: 11-(5'-azido-salicylamido)-undecanoic acid derivatives

Photoreactive probes for the hydrophobic pocket of the liver fatty acid-binding protein, 11-(5'-azido-salicylamido)-undecanoic acid (5' ASU) and its acetyl ester (Ac5' ASU), were synthesized and their interaction with the protein was assessed. Fatty acid-binding proteins are closely related proteins which are abundantly expressed in tissues with active lipid metabolism. A simple model that assumes that the protein possesses a single kind of sites fitted the binding of radioiodinated 5' ASU to L-FABP satisfactorily. The apparent dissociation constant, 1.34 x 10(-7) M, evidenced a slightly higher affinity than that reported for C16-C20 fatty acids. Consistent with the binding curve, 5' ASU effectively competed with palmitic acid for the hydrophobic sites and the effect was nearly complete for concentrations of 1 microM; oleic acid, in turn, displaced the radiolabelled probe. Irradiation at 366 nm of 125I-5' ASU bound to L-FABP caused the covalent cross-linking of the reagent. The amount of radioactivity covalently bound reached a maximum after 2 min thus agreeing with the photo-activation kinetics of the unlabelled compound that evidenced a t1/2 of 31.1 sec. The yield with which probes bound to L-FABP became covalently linked to the protein, appraised after SDS-PAGE of irradiated samples, was estimated as 23 and 26 per cent for 5' ASU and Ac5' ASU respectively. In turn, irradiation of L-FABP incubated with 5' ASU or Ac5' ASU resulted in the irreversible loss of about one fourth its ability to bind palmitic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of ligand binding to acyl-CoA-binding protein

Ligand binding to recombinant bovine acyl-CoA-binding protein (rACBP) was examined using a Lipidex 1000 competition assay and an e.p.r. spectroscopy displacement assay. Of all putative ligands tested, rACBP exhibited a high binding affinity only for acyl-CoA esters. No alternative ligands could be found in rat liver fractions purified on an affinity of column on which ACBP was coupled to Sepharose 4B. E.p.r. data indicate that both the acyl chain and the CoA head group of acyl-CoA are involved in binding and that the 3'-phosphate group on the ribose moiety of acyl-CoA esters plays a crucial role in the binding of acyl-CoA to ACBP. E.p.r. competition binding studies show that the binding affinity of acyl-CoA esters for rACBP is strongly dependent on the length of the acyl chain with a clear preference for acyl-CoA esters with 14-22 carbon atoms in the acyl chain. No correlation between the number of double bonds in the acyl chain and the binding affinity was observed. The experimental results strongly indicate that ACBP specifically binds long-chain acyl-CoA esters with a very high affinity, results that indicate that ACBP is likely to be involved in the intracellular transport and pool formation of these compounds.

Characterization of cardiac fatty-acid-binding protein from human placenta. Comparison with placenta hepatic types

When a 105,000 x g supernatant of human placenta was incubated with [1-14C]oleate and subjected to Sephadex G-75 gel filtration and HPLC, two fatty-acid-binding protein (FABP) peaks were obtained. One of these, when further purified by carboxymethyl-cellulose, gave one 15.3-kDa FABP with pI5.3. The other, when chromatographed on DEAE cellulose, separated into two 14.2-kDa FABP with pI6.9 and 5.4. Purity of the proteins was checked by SDS/PAGE. Molecular mass, pI, immunochemical properties and amino acid compositions all indicated that 15.3-kDa FABP was of the cardiac type, whereas both 14.2-kDa FABP were of the hepatic type. Cardiac FABP did not cross-react with hepatic proteins. When tested for the acceptor/donor properties of these FABP, hepatic types were found to be better candidates than cardiac in uptaking fatty acids from liposomes. Cardiac FABP, on the other hand, acted in a more efficient way as a donor, indicating a distinct role of these proteins in human placenta, which furnishes a multiorgan system for the developing fetus.

Ligand-protein electrostatic interactions govern the specificity of retinol- and fatty acid-binding proteins

Cellular retinol-binding protein II (CRBP-II) and intestinal fatty acid-binding protein (I-FABP) are both expressed in small intestinal enterocytes and exhibit 31% sequence identity. I-FABP binds a single molecule of long-chain fatty acid and forms an ion-pair electrostatic interaction between the cationic side chain of arginine-106 and the anionic fatty acid carboxyl group. In contrast, CRBP-II binds all-trans-retinol or -retinal and contains a glutamine residue in the corresponding position, residue 109. We have characterized and compared the interactions of fatty acids and retinoids with I-FABP, CRBP-II, and two reciprocal mutant proteins. The mutants were designated CRBP-II(Q109R), where glutamine-109 was replaced by arginine, and I-FABP(R106Q), where arginine-106 was replaced by glutamine. As monitored by titration calorimetry and carbon-13 NMR spectroscopy, the fatty acid-binding properties of CRBP-II(Q109R) were found to be essentially identical to those of wild-type I-FABP. Both proteins bound 1 molecule of fatty acid with identical affinities (Kd = 0.2 microM). The enthalpic contribution to the total free energy of binding was large for both proteins: 66% and 87%, respectively. In addition, the carboxyl groups of fatty acids bound to both proteins were solvent-inaccessible. There was little or no change in the ionization state of the bound fatty acid over a wide pH range, as monitored by the chemical shift of the fatty acid carboxyl 13C resonance. Furthermore, the binding of fatty acid to both proteins was accompanied by a selective perturbation of the guanidino 13C resonance of a single arginine residue.(ABSTRACT TRUNCATED AT 250 WORDS)

High-affinity fatty acid-binding activity in epidermis and cultured keratinocytes is attributable to high-molecular-weight and not low-molecular-weight fatty acid-binding proteins

Fatty acid-binding proteins (FABPs) are abundant low-molecular-weight cytosolic proteins in tissues involved in fatty acid (FA) metabolism. Because epidermis is also an active lipogenic tissue, we examined cytosols from murine and porcine epidermis and cultured human keratinocytes and fibroblasts for FABPs. High-affinity FA-binding activity was present in both epidermis and differentiated keratinocytes, whereas no high-affinity FA-binding activity was found in cultured human fibroblasts or undifferentiated keratinocytes. By column chromatography, a single binding peak was identified in the high (90-100 kDa)-molecular-weight range and no binding activity was evident in the low (14-15 kDa)-molecular-weight range, where conventional FABPs elute. Moreover, rabbit anti-rat heart FABP, anti-rat intestine FABP, and anti-rat liver FABP antisera did not identify proteins in the 14-15-kDa range in murine epidermal cytosol by Western immunoblots, whereas the anti-rat-heart antibody recognized a protein of approximately 32 kDa. Isoelectric focusing of differentiated keratinocyte cytosol demonstrated a single FA-binding peak having a pI of approximately 4.0. Analysis of this binding peak by SDS-PAGE revealed peptides of approximately 66 and 38 kDa. These findings suggest the possibility that the FA-binding protein in keratinocyte cytosol normally exists as a heterodimer. Western immunoblots of both differentiated keratinocyte cytosol and keratinocyte-conditional media stained with a rabbit anti-human serum albumin antibody identified a protein of approximately 67 kDa, but the electrofocused fraction did not react with this antibody. Thus, epidermis and differentiated keratinocytes possess high-affinity cytosolic FA-binding activity that cannot be ascribed either to conventional low-molecular-weight FABPs or to albumin.

Purification and characterization of a binding protein related to the Z class of cytosolic proteins in guinea-pig liver cytosol (guinea-pig Z protein)

The purification and characterization of a low-molecular-mass binding protein from female guinea-pig liver cytosol is reported. Its molecular mass (14.4 kDa), amino acid composition, abundance and biological properties identify it as belonging to the Z class of liver cytosolic proteins [Levi, A.J., Gatmaitan, Z. & Arias, I.M. (1969) J. Clin. Invest. 48, 2956-2167]. Among the most important members of this class of proteins are the fatty-acid-binding proteins (FABPs) and the sterol carrier protein2 (SCP2). The guinea-pig Z protein (G-ZP) has some similarities in its amino acid composition and NH2-terminal sequence with those of the rat liver FABP, but its isoelectric point is basic (pI 8.85), like that of SCP2. We also examined its binding affinities for a number of ligands bound by these two proteins. The results show that the purified G-ZP binds dehydroepiandrosterone sulfate, estrone sulfate, oleic acid and cholesterol, but shows no affinity for free steroids such as estrone and DHEA. Thus it can be said that G-ZP has some characteristics of FABPs and some of SCP2 but seems, however, to be different from both these proteins. The purified G-ZP inhibits microsomal DHEA sulfate sulfatase activity in a mixed noncompetitive way. This protein could be involved in the transport and/or metabolism of sulfated steroids.

Binding of bile acids, organic anions, and fatty acids by bovine intestinal Z protein

Z protein from bovine small intestinal mucosa was purified and its binding affinities for bile acids, organic anions, and fatty acids were compared with those of bovine hepatic Z protein. Purification of Z protein from intestinal and hepatic cytosol was performed by gel filtration, chromatofocusing, and hydroxyapatite chromatography. Both purified proteins had the same molecular weight (Mr 14,000) and eluted from a chromatofocused gel at about pH 10. Binding studies were performed by the competitive displacement of 8-anilinonaphthalene-1-sulfonic acid and by equilibrium dialysis. Binding affinities for bile acids, organic anions, and fatty acids were very similar between intestinal and hepatic Z proteins. Although the real physiologic role of Z protein remains to be further elucidated, these data indicate that intestinal Z protein participates in the mechanism of intracellular bile acid transfer in enterocytes.

Postnatal development of the brush cells in the common bile duct of the rat

The postnatal development of brush cells in the distal segment of the common bile duct of the rat was examined with respect to cell number and immunoreactivity for liver fatty acid-binding protein (L-FABP). The brush cells, distinguishable from the principal cells by scanning electron microscopy, first appeared in the common bile duct 4 weeks after birth. They showed a remarkable increase in number, with a sex difference in time, i.e., between 8 and 12 weeks in the male and between 10 and 14 weeks in the female. In both sexes, the frequency of brush cells reached approximately 30% of total epithelial cells by 16 weeks and remained constant until 40 weeks of age. Cells with positive immunoreactivity for L-FABP first appeared in small numbers at 8 weeks. Immuno-electron microscopy revealed that all immunoreactive cells were brush cells. They increased in number gradually from 16 to 40 weeks with no sex difference. At 40 weeks, the immunoreactive cells reached approximately 7.5% of total epithelial cells, corresponding to one-fourth of the number of brush cells. These results indicate that the occurrence of the brush cell population in the common bile duct is a late event in the postnatal development of the rat and that its functional maturation progresses with aging.

In situ binding of fatty acids to the liver fatty acid binding protein: analysis using 3-[125I]iodo-4-azido-N-hexadecylsalicylamide

A photoactivatable radioiodinated fatty acid analogue, 3-[125I]iodo-4-azido-N-hexadecylsalicylamide (125I-AHS) has been synthesized and used to investigate the involvement of cellular lipid carriers in hepatic fatty acid utilization. Photoactivation of Hep G2 internalized 125I-AHS revealed that several cellular proteins were crosslinked with the radiolabeled fatty acid analogue. Three predominant proteins in the membrane fraction of the cell with molecular masses 17, 50 and 127 kDa were crosslinked with the lipid analogue, as determined using autoradiography after SDS-PAGE. Three other proteins in the soluble fraction of the cell, with molecular masses 14, 24 and 35 kDa, were also labeled in situ. In contrast to the other labeled proteins, the fatty acid analogue accumulated on the cytoplasmic 14 kDa protein in a time and temperature dependent fashion. The in situ-labeled 14 kDa protein was identified from primary rat hepatocytes as the liver fatty acid binding protein by partial purification and its ability to be immunoprecipitated with immunospecific L-FABP antiserum. Collectively the results indicate that fatty acids traverse the plasma membrane and are bound cytoplasmically by the liver fatty acid binding protein, as well as other proteins in the cell. This represents the first demonstration in intact hepatocytes that the liver fatty acid binding protein participates in the process of intracellular fatty acid trafficking, and supports a model in which cytoplasmic lipid carriers solubilize fatty acids as a step in their metabolic utilization.

Three-dimensional structure of fatty-acid-binding protein from bovine heart

The complex of fatty-acid-binding protein (FABP) from bovine heart (cFABP, pI4.9) with endogenous lipid was crystallized in the presence of ammonium sulfate as precipitant. The needle-shaped crystals belong to the monoclinic space group C2, with unit-cell constants a = 5.262(6) nm, b = 7.631(8) nm, c = 3.945(5) nm and beta = 94.47(9) degrees. A native data set to 0.35 nm resolution was collected using synchrotron radiation and film methods. An initial model for the three-dimensional structure of the protein was constructed based on the crystal structure of the related bovine P2 myelin protein [Jones, T.A., Bergfors, T., Sedzik, J. & Unge, T. (1988) EMBO J. 7, 1597-1604] to which the amino acid sequence of bovine cFABP was adapted. Energy minimizations were carried out under different conditions using both an all-atom and a united-atom force field. The optimized models were used to determine the crystal structure of cFABP by molecular-replacement techniques. The structure was refined by simulated annealing to R = 0.267. As the bound lipid is heterogeneous, it could not be located in the electron-density map and/or the attained resolution was not sufficient. Bovine cFABP is composed of ten antiparallel beta strands forming a beta barrel, and by two alpha helices. The structural features are similar to those of other members of the superfamily of hydrophobic molecule transporters.

Metabolism of ethanol by rat hepatocytes results in generation of a lipid chemotactic factor: studies using a cell-free system and role of oxygen-derived free radicals

We have reported that liver cells metabolizing ethanol release a lipid that has chemotactic activity for human polymorphonuclear leukocytes. The purpose of the present study was to investigate the role of ethanol and of oxygen-derived free radicals in generation of this activity. A cell-free, ethanol-metabolizing system consisting of rat liver cytosol was found to generate a polar lipid which possessed a chemotactic activity identical to the activity from intact cells. The importance of acetaldehyde in this process was established by the findings that (i) an inhibitor of the metabolism of ethanol to acetaldehyde, 4-methylpyrazole, blocked production of the chemotactic lipid; (ii) acetaldehyde could substitute for ethanol; and (iii) cytosol alone incubated with ethanol in the absence of mitochondrial acetaldehyde metabolism generated the activity. Acetaldehyde can serve as a substrate for cytosolic oxidases in reactions that generate radicals (e.g., superoxide anion) which have been shown to attack unsaturated lipids to yield lipid hydroperoxides. When scavengers of oxygen-derived free radicals (superoxide dismutase, catalase, and dimethylsulfoxide) were added to the ethanol-metabolizing system they prevented generation of the activity. The data are consistent with a mechanism of chemotactic factor generation by ethanol-metabolizing liver cytosol in which acetaldehyde causes formation of oxygen-derived free radicals that attack unsaturated cytosolic lipid to generate a polar chemotactic compound.

Interaction of fatty acids with recombinant rat intestinal and liver fatty acid-binding proteins

Intestinal enterocytes contain two homologous fatty acid-binding proteins, intestinal fatty acid-binding protein (I-FABP)2 and liver fatty acid-binding protein (L-FABP). Since the functional basis for this multiplicity is not known, the fatty acid-binding specificity of recombinant forms of both rat I-FABP and rat L-FABP was examined. A systematic comparative analysis of the 18 carbon chain length fatty acid binding parameters, using both radiolabeled (stearic, oleic, and linoleic) and fluorescent (trans-parinaric and cis-parinaric) fatty acids, was undertaken. Results obtained with a classical Lipidex-1000 binding assay, which requires separation of bound from free fatty acid, were confirmed with a fluorescent fatty acid-binding assay not requiring separation of bound and unbound ligand. Depending on the nature of the fatty acid ligand, I-FABP bound fatty acid had dissociation constants between 0.2 and 3.1 microM and a consistent 1:1 molar ratio. The dissociation constants for L-FABP bound fatty acids ranged between 0.9 and 2.6 microM and the protein bound up to 2 mol fatty acid per mole of protein. Both fatty acid-binding proteins exhibited relatively higher affinity for unsaturated fatty acids as compared to saturated fatty acids of the same chain length. cis-Parinaric acid or trans-parinaric acid (each containing four double bonds) bound to L-FABP and I-FABP were displaced in a competitive manner by non-fluorescent fatty acid. Hill plots of the binding of cis- and trans- parinaric acid to L-FABP showed that the binding affinities of the two sites were very similar and did not exhibit cooperativity. The lack of fluorescence self-quenching upon binding 2 mol of either trans- or cis-parinaric acid/mol L-FABP is consistent with the presence of two binding sites with dissimilar orientation in the L-FABP. Thus, the difference in binding capacity between I-FABP and L-FABP predicts a structurally different binding site or sites.

Two types of fatty acid-binding protein in human kidney. Isolation, characterization and localization

Two types of fatty acid-binding protein (FABP) were isolated from human kidney by gel filtration and ion-exchange chromatography. Northern-blot analysis showed the presence of two FABP transcripts in total kidney RNA, hybridizing with cDNA of human liver and muscle FABP respectively. Characterisation based on molecular mass, isoelectric point, fluorescence with dansylaminoundecanoic acid and immunological cross-reactivity showed that one, type B, was fairly similar to human heart FABP. The other, type A, showed, like human liver FABP, a high fluorescence enhancement and a wavelength shift with dansylaminoundecanoic acid as well as the binding of a variety of ligands. Antibodies raised against FABP type A and against liver FABP markedly cross-reacted in e.l.i.s.a., in Western blotting and in indirect immunoperoxidase staining on kidney and liver sections. Differences in amino acid composition and isoelectric points, however, indicate that type A is a new kidney-specific FABP type. The FABP type A is more abundant in kidney than the B type and is predominantly localized in the cortex, especially in the cells of the proximal tubules. The FABP type B is mainly present in the cells of the distal tubules. In conclusion, this study shows the presence of two types of FABP in the kidney. One type seems to be related to heart FABP, while the other type resembles, but is not identical with, liver FABP. Both types have a characteristic cellular distribution along the nephron.

Isolation and characterization of fatty acid binding protein in the liver of the nurse shark, Ginglymostoma cirratum

1. A 14.5 kDa fatty acid binding protein was isolated from the liver of the nurse shark, Ginglymostoma cirratum. 2. Purified shark liver FABP (pI = 5.4) bound oleic acid at a single site with an affinity similar to that of mammalian FABP. 3. The apparent size, pI and amino acid composition of shark liver FABP indicate a close structural relationship between this protein and mammalian heart FABP.

A methylsulfonyl metabolite of a polychlorinated biphenyl can serve as a ligand for liver fatty acid binding protein in rat intestinal mucosa

When a 100,000 x g supernatant from rat intestinal mucosa was incubated with 4,4'-bis([3H]methylsulfonyl)-2,2',5,5'-tetrachlorobiphenyl, [(CT3SO2)2TCB] a (CT3SO2)2TCB-protein complex was formed. The (CT3SO2)2TCB-protein complex was isolated and purified using gel filtration and ion-exchange chromatography. The protein portion of this complex was characterized to be liver fatty acid binding protein (L-FABP) by SDS-polyacrylamide gel electrophoresis and immunoblot analysis. No cross reactivity was observed in the immunoblot analysis between the purified protein and anti-heart or anti-intestinal fatty acid binding protein. (CT3SO2)2TCB was extractable from L-FABP and therefore not covalently bound to L-FABP.

Identification of two lipid binding proteins from liver of Gallus domesticus

1. Two low molecular weight (approximately 14,000 Da) proteins exhibiting lipid binding activity were purified from liver cytosol and identified as non-specific lipid binding protein (ns-LTP) and fatty acid binding protein (L-FABP). 2. Ligand binding assays indicated that ns-LTP exhibited greater binding activity for cholesterol and little binding of fatty acids. Conversely, L-FABP had higher relative binding activity for fatty acids but did not bind cholesterol. 3. Amino acid composition and pI data supported the identification of the chicken liver lipid binding proteins as L-FABP and ns-LTP. 4. Polyclonal antisera was prepared against each of the liver lipid binding proteins and monospecificity verified using Western blot analysis.

Isolation, characterization, and developmental expression of pig intestinal fatty acid-binding proteins

The goal of this study was to characterize and quantify intestinal fatty acid-binding proteins of the pig. Small intestinal mucosa from 13-19 kg pigs was homogenized and centrifuged to obtain cytosol. Isolation of fatty acid-binding proteins from delipidated cytosol was achieved using molecular sieve, oleic acid affinity, and ion exchange chromatography. Fatty acid-binding protein isolation was monitored using a fatty-acid binding assay in conjunction with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. Antisera to rat liver-fatty acid-binding protein cross reacted with an isolated intestinal fatty acid-binding protein of Mr = 13,000, whereas antisera to rat intestine-fatty acid-binding protein was not cross reactive with isolated pig intestinal proteins. These experiments identify a pig intestinal fatty acid-binding protein that exhibits strong immunochemical similarity to rat liver-fatty acid-binding protein. Cytosol prepared from intestinal mucosa of pigs at -4, 2, 4, 7, 15, 22, 28, and 35 d of age was assayed for fatty acid-binding protein activity. Preweaning fatty acid-binding protein activity in cytosol was maximal at 7 days of age when expressed as total jejunal fatty acid binding per kilogram bodyweight, intestinal or mucosal weight or milligram total protein. After weaning (21 d), fatty acid-binding protein activities declined to 28 days, but increased again by 35 days. Total soluble fatty acid-binding protein activity in pig intestine is regulated during postnatal development and this may account in part for the altered intestinal absorption of lipids observed in young pigs at weaning.

Intermembrane cholesterol transfer: role of sterol carrier proteins and phosphatidylserine

The effect of phosphatidylserine and sterol carrier proteins on cholesterol exchange was determined using an assay not requiring separation of donor and acceptor membrane vesicles. Sterol carrier protein-2 (SCP2, also called nonspecific lipid transfer protein), but not fatty acid binding protein (FABP, also called sterol carrier protein), enhanced the initial rate of sterol exchange between neutral zwitterionic phosphatidylcholine small unilamellar vesicles (SUV) 2.3-fold. Phosphatidylserine at 10 mol% increased the initial rate of spontaneous and of SCP2-mediated (but not FABP-mediated) sterol exchange by 22% and 44-fold, respectively. The SCP2 potentiation of sterol transfer was dependent on SCP2 concentration and on phosphatidylserine concentration. The SCP2-mediated sterol transfer was inhibited by a variety of cations including KCl, divalent metal ions, and neomycin. The data suggest that SCP2 increase in activity for sterol transfer may be partly ascribed to charge on the phospholipid.

Characteristics of fatty acid-binding proteins and their relation to mammary-derived growth inhibitor

Based on sequence relationships the cytoplasmic fatty acid-binding proteins (FABPs) of mammalian origin are divided into at least three distinct types, namely the hepatic-, intestinal- and cardiac-type. Highly conserved sequences of FABPs within the same type correlate with immunological crossreactivities. Isoforms of hepatic-type FABP are found in several mammalian species and for bovine liver FABP specific shifts in isoelectric points upon lipidation with fatty acids are observed. Isoforms of intestinal-type FABP are not known and the occurrence of cardiac-type isoforms so far is confined to bovine heart tissue. A bovine mammary-derived growth inhibitor (MDGI) is 95% homologous to the cardiac-type FABP from bovine heart. Dissociation constants of FABP/fatty acid complexes are in the range of 1 microM and 1:1 stoichiometries are usually found, but the neutral isoform of hepatic FABP from bovine liver binds 2 fatty acids. On subcellular levels hepatic- and cardiac-type FABPs are differently distributed. Though mainly cytosolic in either case, immunoelectron microscopy as well as a gelchromatographic immunofluorescence assay demonstrate the association of hepatic FABP in liver cells with microsomal and outer mitochondrial membranes and with nuclei, whereas in heart cells cardiac FABP is confined to mitochondrial matrix and nuclei. In mammary epithelial cells MDGI is associated with neither mitochondria nor endoplasmic reticulum, and is expressed in a strictly developmental-dependent spatial and temporal pattern. The specific role proposed for MDGI is to arrest growth of mammary epithelial cells when they become committed to differentiation in the mammary gland.

Cellular fatty acid-binding proteins: current concepts and future directions

At least three different proteins are implicated in the cellular transport of fatty acid moieties: a plasmalemmal membrane and a cytoplasmic fatty acid-binding protein (FABPPM and FABPC, respectively) and cytoplasmic acyl-CoA binding protein (ACBP). Their putative main physiological significance is the assurance that long-chain fatty acids and derivatives, either in transit through membranes or present in intracellular compartments, are largely complexed to proteins. FABPC distinguishes from the other proteins in that distinct types of FABPC are found in remarkable abundance in the cytoplasmic compartment of a variety of tissues. Although their mechanism of action is not yet fully elucidated, current knowledge suggests that the function of this set of proteins reaches beyond simply aiding cytoplasmic solubilization of hydrophobic ligands, but that they can be assigned several regulatory roles in cellular lipid homeostasis.

The chemical modification of cysteine-69 of rat liver fatty acid-binding protein (FABP): a fluorescence approach to FABP structure and function

Hepatic-FABP was labelled at cysteine-69 with the fluorescent environmentally sensitive reporter group AEDANS. The labelled protein had an emission maximum at 465 nm indicating that cysteine-69 was buried in a non-polar environment. The modified protein was still able to bind ligands such as oleic acid, oleoyl CoA and haem. The affinity of AEDANS-FABP for haem was unaltered as compared with the native protein indicating that cysteine-69 must be remote from the ligand binding site. The binding of oleic acid did not significantly perturb the fluorescence emission spectrum of the fluorescent reporter group suggesting that there are not large conformational changes in the region of cysteine-69 on fatty acid binding. The binding of stoichiometric amounts of oleoyl CoA was accompanied by a small fluorescence enhancement which suggests that fatty acyl CoAs may interact with other regions of the FABP molecule not involved in fatty acid binding.

Characterization of FadL-specific fatty acid binding in Escherichia coli

The product of the fadL gene (FadL) is a central component of the long-chain fatty acid transport system of Escherichia coli. When fatty acid activation is blocked by a mutation in the structural gene for acyl CoA synthetase (fadD) transport is inhibited allowing a FadL-specific fatty acid binding activity to be measured. This binding activity was 4- to 6-fold greater in the fadL+ fadD strain LS6928 when compared to the delta fadLfadD strain LS6929. With long-chain fatty acids, this binding activity was saturable and it was estimated that there were approx. 35,000 FadL-specific oleic acid binding sites per cell in the fadL+ strain LS6928. The FadL-specific fatty acid binding affinity was highest for oleic acid (18:1) and palmitic acid (16:0) giving apparent KD values of 2.3.10(-7) M and 8.8.10(-7) M, respectively. FadL-specific binding affinity of myristic acid (14:0) was nearly an order of magnitude less and no FadL-specific binding of decanoic acid (10:0) could be measured. Two lines of evidence suggest that FadL-fatty acid binding occurs by a hydrophobic interaction: (1) There was a preference for the long-chain substrates oleic acid and palmitic acid; and (2) oleic acid binding activity was not significantly changed over the pH range 5.0 to 8.0. The FadL-specific binding of oleic acid in the fadL+ strain LS6928 could be blocked by preincubation with antisera raised against purified FadL providing a clear correlation between the activity and identity of FadL. The binding activity associated with FadL was measured in vesicles of the outer membrane following passage over the hydrophobic resin Lipidex 1000. The KD of oleic acid binding attributable to FadL in outer membranes vesicles (6.0.10(-7) M) was in close agreement with that determined in whole cells. Overall, these studies demonstrated that FadL binds long-chain fatty acids with a relatively high affinity prior to their transport across the outer membrane.

Functions of fatty acid binding proteins

Cytosolic fatty acid binding proteins (FABP) belong to a gene family of which eight members have been conclusively identified. These 14-15 kDa proteins are abundantly expressed in a highly tissue-specific manner. Although the functions of the cytosolic FABP are not clearly established, they appear to enhance the transfer of long-chain fatty acids between artificial and native lipid membranes, and also to have a stimulatory effect on a number of enzymes of fatty acid metabolism in vitro. These findings, as well as the tissue expression, ligand binding properties, ontogeny and regulation of these proteins provide a considerable body of indirect evidence supporting a broad role for the FABP in the intracellular transport and metabolism of long-chain fatty acids. The available data also support the existence of structure- and tissue-specific specialization of function among different members of the FABP gene family. Moreover, FABP may also have a possible role in the modulation of cell growth and proliferation, possibly by virtue of their affinity for ligands such as prostaglandins, leukotrienes and fatty acids, which are known to influence cell growth activity. FABP structurally unrelated to the cytosolic gene family have also been identified in the plasma membranes of several tissues (FABPpm). These proteins have not been fully characterized to date, but strong evidence suggest that they function in the transport of long-chain fatty acids across the plasma membrane.

Possible role of rat fatty acid-binding proteins in the intestine as carriers of phenol and phthalate derivatives

Fatty acid-binding proteins of hepatic and intestinal type and gastrotropin-like protein (GTLP) were purified from rat intestinal cytosol by Sephadex G-75 gel filtration and DEAE-cellulose, CM-cellulose, and hydroxylapatite chromatographies. In addition to fatty acids, butylated hydroxytoluene (BHT), phthalate dibutyl, and di(2-ethylhexyl) esters (DBP and DEHP) were identified by gas liquid chromatography and mass spectrometry as endogenous ligands from the extract of either fatty acid-binding protein superfamily. These protein families in the intestine may have an important role as carriers in the initial step of arresting these exogenous pollutants.

Immunohistochemical demonstration of liver fatty acid-binding protein in human hepatocellular malignancies

Twenty-three hepatoblastomas of childhood, sixty-two adult hepatocellular carcinomas, and two hepatic sarcomas were examined immunohistochemically with the use of a polyclonal antibody against rat liver fatty acid-binding protein (L-FABP), which cross-reacts to human L-FABP. All the hepatoblastomas and half of the hepatic cell carcinomas contained L-FABP immunoreactive tumour cells, whereas two hepatic sarcomas were negative. The overall frequency of immunostained tumour cells was 43.5 per cent in hepatoblastomas and 18.6 per cent in hepatocellular carcinomas, respectively. Histologically well-differentiated areas contained more numerous immunopositive cells than undifferentiated or immature ones. These results indicate that L-FABP immunoreactivity is a new candidate for a tumour cell marker in hepatic cell malignancies, although its biological role has not been elucidated.

Plasma membrane fatty acid-binding protein and mitochondrial glutamic-oxaloacetic transaminase of rat liver are related

The hepatic plasma membrane fatty acid-binding protein (h-FABPPM) and the mitochondrial isoenzyme of glutamic-oxaloacetic transaminase (mGOT) of rat liver have similar amino acid compositions and identical amino acid sequences for residues 3-24. Both proteins migrate with an apparent molecular mass of 43 kDa on SDS/polyacrylamide gel electrophoresis, have a similar pattern of basic charge isomers on isoelectric focusing, are eluted similarly from four different high-performance liquid chromatographic columns, have absorption maxima at 435 nm under acid conditions and 354 nm at pH 8.3, and bind oleate with a Ka approximately 1.2-1.4 x 10(7) M-1. Sinusoidally enriched liver plasma membranes and purified h-FABPPM have GOT enzymatic activity; the relative specific activities (units/mg) of the membranes and purified protein suggest that h-FABPPM constitutes 1-2% of plasma membrane protein in the rat hepatocyte. Monospecific rabbit antiserum against h-FABPPM reacts on Western blotting with mGOT, and vice versa. Antisera against both proteins produce plasma membrane immunofluorescence in rat hepatocytes and selectively inhibit the hepatocellular uptake of [3H]oleate but not that of [35S]sulfobromophthalein or [14C]taurocholate. The inhibition of oleate uptake produced by anti-h-FABPPM can be eliminated by preincubation of the antiserum with mGOT; similarly, the plasma membrane immunofluorescence produced by either antiserum can be eliminated by preincubation with the other antigen. These data suggest that h-FABPPM and mGOT are closely related.