Studies on fatty-acid-binding proteins. The purification of rat liver fatty-acid-binding protein and the role of cysteine-69 in fatty acid binding

Metalloproteomic approach to liver tissue of rats exposed to mercury

The aims of this study were to identify mercury-associated protein spots in the liver tissue of rats exposed to low concentrations of mercury and to elucidate the physiological and functional aspects of the proteins identified in the protein spots. Therefore, proteomic analysis of the liver tissue of Wistar rats exposed to mercury chloride (4.60 μg kg^-1 in Hg²⁺) was performed for thirty days (Hg-30 group) and sixty days (Hg-60 group). The proteomic profile of the liver tissue of the rats was obtained by two-dimensional electrophoresis (2D-PAGE), and the determinations of total mercury in the liver tissue, pellets and protein spots were performed by graphite furnace atomic absorption spectrometry (GFAAS). ImageMaster 2D Platinum 7.0 software was used to identify the differentially expressed mercury-associated protein spots, which were then characterized by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). The determinations by GFAAS indicated a total mercury bioaccumulation of 2812% in the Hg-30 group and 3298% in the Hg-60 group and 10 mercury-associated protein spots with a concentration range of 51 ± 1.0 to 412 ± 6.00 mg kg^-1 in the 2D PAGE gels from the liver tissue of the Hg-60 group. The LC-MS/MS analyses allowed the identification of 11 metal binding proteins in mercury-associated protein spots that presented fold change with upregulation >1.5, downregulation < -1.7 or that were expressed only in the Hg-60 group. Using the FASTA sequences of the proteins identified in the mercury-associated protein spots, bioinformatics analyses were performed to elucidate the physiological and functional aspects of the metal binding proteins, allowing us to infer that enzymes such as GSTM2 presented greater mercury concentrations and downregulation < -3; Acaa2 and Bhmt, which showed expression only in the Hg-60 group, among others, may act as potential mercury exposure biomarkers.

Metalloproteomic approach of mercury-binding proteins in liver and kidney tissues of Plagioscion squamosissimus (corvina) and Colossoma macropomum (tambaqui) from Amazon region: Possible identification of mercury contamination biomarkers

Fish is an important source of protein, vitamins, and minerals. However, this food is also a major source of human exposure to toxic contaminants such as mercury. Thus, this paper aimed to evaluate mercury-binding proteins for possible application as biomarkers of mercury contamination in hepatic and renal tissues of Plagioscion squamosissimus (carnivorous fish) and Colossoma macropomum (omnivorous fish) from the Amazon region using metalloproteomic approach. The proteome of hepatic and renal tissues of fish species was separated by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE), and the mercury concentrations in protein spots were determined by graphite furnace atomic absorption spectrometry (GFAAS). Finally, the protein spots associated to mercury were characterized by electrospray ionization mass spectrometry (ESI-MS/MS). The activity of antioxidant enzymes (SOD, CAT, GPx, and GST) and lipid peroxidation (LPO) were also determined. The results showed that the highest concentrations of mercury were found in the carnivorous species (P. squamosissimus) and that the accumulation pattern of this metal was higher in hepatic tissues than in renal tissues for both species. A tendency was observed for greater enzymatic activity in the hepatic and renal tissues of P. squamosissimus, the species with the highest concentration of mercury. Only GPx activity in the kidney and GST in the liver were lower for the P. squamosissimus species, and this finding can be explained by the interaction of mercury with these enzymes. The data obtained by ESI-MS/MS allowed for the characterization of the protein spots associated to mercury, revealing proteins involved in energy metabolism, biomolecules transport, protein synthesis and degradation, cell differentiation, gene regulation, and the antioxidant system. The results obtained in the present study can contribute to understanding the physiological processes underlying mercury toxicity and have provided new perspectives on possible candidates for mercury contamination biomarkers in fish.

Liver fatty acid-binding protein and obesity

While low levels of unesterified long chain fatty acids (LCFAs) are normal metabolic intermediates of dietary and endogenous fat, LCFAs are also potent regulators of key receptors/enzymes and at high levels become toxic detergents within the cell. Elevated levels of LCFAs are associated with diabetes, obesity and metabolic syndrome. Consequently, mammals evolved fatty acid-binding proteins (FABPs) that bind/sequester these potentially toxic free fatty acids in the cytosol and present them for rapid removal in oxidative (mitochondria, peroxisomes) or storage (endoplasmic reticulum, lipid droplets) organelles. Mammals have a large (15-member) family of FABPs with multiple members occurring within a single cell type. The first described FABP, liver-FABP (L-FABP or FABP1), is expressed in very high levels (2-5% of cytosolic protein) in liver as well as in intestine and kidney. Since L-FABP facilitates uptake and metabolism of LCFAs in vitro and in cultured cells, it was expected that abnormal function or loss of L-FABP would reduce hepatic LCFA uptake/oxidation and thereby increase LCFAs available for oxidation in muscle and/or storage in adipose. This prediction was confirmed in vitro with isolated liver slices and cultured primary hepatocytes from L-FABP gene-ablated mice. Despite unaltered food consumption when fed a control diet ad libitum, the L-FABP null mice exhibited age- and sex-dependent weight gain and increased fat tissue mass. The obese phenotype was exacerbated in L-FABP null mice pair fed a high-fat diet. Taken together with other findings, these data suggest that L-FABP could have an important role in preventing age- or diet-induced obesity.

The crystal structure of Echinococcus granulosus fatty-acid-binding protein 1

We describe the 1.6 A crystal structure of the fatty-acid-binding protein EgFABP1 from the parasitic platyhelminth Echinococcus granulosus. E. granulosus causes hydatid disease, which is a major zoonosis. EgFABP1 has been implicated in the acquisition, storage, and transport of lipids, and may be important to the organism since it is incapable of synthesising most of its lipids de novo. Moreover, EgFABP1 is a promising candidate for a vaccine against hydatid disease. The crystal structure reveals that EgFABP1 has the expected 10-stranded beta-barrel fold typical of the family of intracellular lipid-binding proteins, and that it is structurally most similar to P2 myelin protein. We describe the comparison of the crystal structure of EgFABP1 with these proteins and with an older homology model for EgFABP1. The electron density reveals the presence of a bound ligand inside the cavity, which we have interpreted as palmitic acid. The carboxylate group of the fatty acid interacts with the protein's P2 motif, consisting of a conserved triad R em leader R-x-Y. The hydrophobic tail of the ligand assumes a fairly flat, U-shaped conformation and has relatively few interactions with the protein.We discuss some of the structural implications of the crystal structure of EgFABP1 for related platyhelminthic FABPs.

Binding of recombinant rat liver fatty acid-binding protein to small anionic phospholipid vesicles results in ligand release: a model for interfacial binding and fatty acid targeting

A number of intracellular proteins bind to negatively charged phospholipid membranes, and this interfacial binding results in a conformational change that modulates the activity of the protein. Using a fluorescent fatty acid analogue, 11-[5-(dimethylamino)naphthalenesulfonyl]undecanoic acid (DAUDA), it is possible to demonstrate the release of this ligand from recombinant rat liver FABP in the presence of phospholipid vesicles that contain a significant proportion of anionic phospholipids. The ligand release that is observed with anionic phospholipids is sensitive to the ionic strength of the assay conditions and the anionic charge density of the phospholipid at the interface, indicating that nonspecific electrostatic interactions play an important role in the process. The stoichiometric relationship between anionic phospholipid and liver FABP suggests that the liver FABP coats the surface of the phospholipid vesicle. The most likely explanation for ligand release is that interaction of FABP with an anionic membrane interface induces a rapid conformational change, resulting in a reduced affinity of DAUDA for the protein. The nature of this interaction involves both electrostatic and nonpolar interactions as maximal release of liver FABP from phospholipid vesicles with recovery of ligand binding cannot be achieved with high salt and requires the presence of a nonionic detergent. The precise interfacial mechanism that results in the rapid release of ligand from L-FABP remains to be determined, but studies with two mutants, F3W and F18W, suggest the possible involvement of the amino-terminal region of the protein in the process. The conformational change linked to interfacial binding of this protein could provide a mechanism for fatty acid targeting within the cell.

Liver fatty acid binding protein: species variation and the accommodation of different ligands

The crystal structure of rat liver fatty acid binding protein (LFABP) and an alignment of amino acid sequences of all known species have been used to demonstrate two groups or sub-classes. Based on estimates at neutral pH and the electrostatic field calculated using the crystal coordinates, some evidence of changes that occur in going from holo- to apo-forms has been obtained. LFABP belongs to a large family frequently referred to as the intracellular lipid binding proteins or iLBPs. LFABP, unlike other family members, has two fatty acid binding sites. The two cavity sites have been reviewed and arguments for interactions between the sites are presented. Based on the crystal structure of rat LFABP, differences between the A and B groups have been postulated. Last of all, hypothetical models have been built of complexes of LFABP and heme, and LFABP and oleoyl CoA. In both cases, the stoichiometry is one to one and the models show why this is likely.

Isolation and characterization of two distinct forms of liver fatty acid binding protein from the rat

Liver fatty acid binding protein (L-FABP) appears to contain several different forms that may result from post-translational modification or bound ligand. To further assess this possibility, L-FABP was purified from rat liver homogenate and two putative isoforms separated using a sulfonyl column, a strong cation exchange resin. Fraction I eluted at 0.2 M NaCl, had a pI of 7.59, and following a final size exclusion step contained > 98% L-FABP. Fraction II eluted at 1.0 M NaCl, had a pI of 7.59, and following a final size exclusion step contained > 99% L-FABP. Both fractions contained approx. 0.15 moles of endogenous bound fatty acid per mole of protein, while L-FABP not subjected to the cation exchange step contained 0.75 moles of fatty acid per mole of protein. Fractions I and II had a greater proportion of saturated and monounsaturated fatty acids with a large reduction in polyunsaturated fatty acids compared to L-FABP not fractionated by cation exchange. Mass spectral analysis indicated the molecular mass of Fraction I was 14,315.02 +/- 0.35 Da and Fraction II was 14,315.86 +/- 0.34 Da. The peptide map for each fraction was determined by limited digestion of each fraction with either trypsin, Asp-N, or chymotrypsin to yield overlapping peptide fragments. Mass spectral analysis of these digests indicated the two proteins had identical amino acid fragments and that Cys69 was reduced and there were no Asn to Asp exchanges. Hence, these two forms of L-FABP were not isoforms and were not the result of differences in bound fatty acid. It is proposed that these two distinct forms of rat L-FABP were structural conformers based on two alternative folding pathways.

Changes in liver fatty acid-binding protein in rat enzyme-altered foci

The level of liver fatty acid-binding protein (L-FABP) was analyzed in enzyme-altered foci (EAF) positive for GST-P, or after classification of foci into different subclasses by haematoxylin and eosin staining. Rats were treated with either an initiating single dose of diethylnitrosamine (DEN) followed by no treatment, treatment with phenobarbital, PCB, nafenopin or repeated injections of DEN, or alternatively non-treated or treated with nafenopin alone. Changes in the level of L-FABP were detected in the majority of EAF and both L-FABP-positive and -negative foci were seen. However, in rats initiated with DEN, EAF were almost exclusively L-FABP-negative. The fraction of L-FABP-negative foci increased with increasing foci size, while the time of treatment or the dose of the promoter did not seem to have any effect. It was also found that treatment with DEN gave a higher fraction of L-FABP-negative foci as compared to treatment with phenobarbital or PCB, indicating a specific effect of DEN. These data together with previously published findings suggest that L-FABP expression in EAF is determined by the initiating carcinogenic regimen and that it might be possible to use the expression of L-FABP in tumours to differentiate initiating chemicals.

Combined effects of sphingomyelin and cholesterol on the hydrolysis of emulsion particle triolein by lipoprotein lipase

Sphingomyelin (SM) is one of the major lipids in lipoproteins. However, its function in lipoprotein metabolism is unknown. In an attempt to understand the role that this lipid plays in modulation of lipoprotein lipase (LPL)-mediated hydrolysis, triolein-based emulsion particles containing 15% (physiological concentration) and 30% of the phospholipid content as SM together with phosphatidyl choline were used as substrate for the enzyme. Using a continuous fluorescence displacement assay to measure triglyceride (triolein) hydrolysis, it is shown that LPL activity was not modified by physiological concentrations of SM. However, under these assay conditions the presence of 30% SM inhibited LPL hydrolysis. SM and cholesterol (a normal component of the lipoprotein surface monolayer) become closely associated in phospholipid monolayers and bilayers. Incorporation of cholesterol into emulsion particles containing only PC increased LPL activity, but this increase was reduced by the additional presence of a physiological concentration (15%) of SM. These model studies suggest that the ratio, cholesterol:SM, in the monolayer may regulate the hydrolytic activity of the LPL. The production of ceramide by sphingomyelinase pre-treatment of emulsion particles containing SM leads to a two- to three-fold increase in LPL activity. This effect was dependent on sphingomyelinase concentration and time of pre-incubation and was not seen with cholesterol containing substrates. The ability of apolipoprotein CII to enhance LPL-catalysed triolein hydrolysis was not affected by the presence of SM; however, the stimulatory effect of this apolipoprotein was attenuated by pre-treatment of emulsion particles with sphingomyelinase. In summary, physiological concentrations of SM can inhibit the hydrolysis of cholesterol-containing emulsion particles; while pre-treatment of SM containing emulsion particles with sphingomyelinase in the absence of cholesterol can increase LPL-mediated triglyceride hydrolysis.

A fluorescence displacement assay for the measurement of arachidonoyl ethanolamide (anandamide) and oleoyl amide (octadecenoamide) hydrolysis

We describe a simple fluorescence displacement assay to measure hydrolysis of arachidonoyl ethanolamide and oleoyl amide, two important pharmacological compounds. Hydrolysis at the amide linkage of these ligands releases a fatty acid as one of the products. The displacement of a fluorescent fatty acid analogue from rat liver fatty acid-binding protein by the released fatty acid can thus be measured as a decrease in fluorescence. This process is time- and concentration-dependent and shows hyperbolic enzyme kinetics. Electrospray ionisation mass spectrometry was used to validate the assay.

Effect of lipid composition on lipoprotein lipase activity measured by a continuous fluorescence assay: effect of cholesterol supports an interfacial surface penetration model

The breakdown of normal substrates by lipases requires an interfacial binding step prior to hydrolysis. Interfacial binding and subsequent hydrolysis will be affected by the lipid components and hence physical properties of the substrate surface. In order to investigate in detail the effect of lipid structure on the activity of lipoprotein lipase (LPL), triolein-containing emulsion particles of defined composition have been used as substrates. In addition, lipase activity has been measured using a continuous fluorescence displacement assay that monitors the release of long-chain fatty acids as an alternative to normal radiochemical assays. Using this fluorescence assay, rates of hydrolysis of triolein were the same as when using a standard radiochemical assay under identical conditions. Activation by apolipoprotein CII was very similar by both methods; however, the extent of activation (2-3-fold) was less than has been reported previously using different assay conditions. In order to investigate the effect of cholesterol on LPL activity, emulsion particles were prepared in which the cholesterol/egg-phosphatidylcholine ratio was increased up to a 1:1 molar ratio. A pronounced stimulatory effect of cholesterol was observed under these assay conditions, with up to a 5-fold increase in rate compared with emulsion particles without cholesterol. Since high molar ratios of cholesterol are reported to exclude triacylglycerol from the phospholipid surface [Spooner and Small (1987) Biochemistry 26, 5820-5825], these results are not consistent with a mechanism involving LPL hydrolysis of surface triacylglycerol. Instead, they support an interfacial penetration model, allowing the enzyme's active site direct access to triacylglycerol in the lipoprotein core. Perturbation of the surface phospholipid monolayer of the emulsion particle as a result of hydrolysis by Naja naja phospholipase A2 resulted in a 10-fold activation of LPL, providing further support for an interfacial penetration model. The stimulatory effect of apolipoprotein CII was not modulated by modification of the interface with cholesterol.

Time-resolved fluorescence of intestinal and liver fatty acid binding proteins: role of fatty acyl CoA and fatty acid

The effect of fatty acyl CoA and fatty acid on the solution structure and dynamics of two intestinal enterocyte fatty acid binding proteins, intestinal (I-FABP) and liver (L-FABP), was examined by time-resolved fluorescence of FABP aromatic amino acid residues. I-FABP Trp displayed two rotational correlation times, 6.6 and 0.4 ns. reflecting motion of the protein as a whole and segmental mobility of Trp. Neither oleoyl CoA, oleic acid, nor CoASH altered overall I-FABP rotational correlation time. However, oleic acid and CoASH increased I-FABP Trp segmental mobility, while oleoyl CoA and CoASH decreased I-FABP Trp limiting anisotropy (order). The angle of I-FABP Trp "wobbling in a cone" was increased by ligands in the order oleoyl CoA > CoASH > oleic acid. L-FABP Trp segmental mobility. L-FABP overall rotational motion, in contrast to that of I-FABP, was significantly increased by ligands in the order oleoyl CoA > oleic acid > CoASH. cis-Parinaric acid and cis-parinaroyl CoA bound to L-FABP also reflected overall L-FABP motion but yielded longer rotational correlation times, 8.2 and 10.7 ns, than the respective apo-FABPs. Such effects were not observed with I-FABP. Finally, both cis-parinaric acid and cis-parinaroyl CoA were much less ordered in the I-FABP ligand binding site than with L-FABP. These observations suggest that the rotational dynamics of L-FABP and its conformation are more sensitive to ligands than I-FABP. Further, ligands such as fatty acids, fatty acyl CoAs, and/or CoASH differentially modulate the I-FABP and L-FABP dynamics, and the ligand binding sites of these proteins differ in their ability to order the ligands.

The binding of cholesterol and bile salts to recombinant rat liver fatty acid-binding protein

The physiological role of liver fatty acid-binding protein (L-FABP) has yet to be clarified. An important feature of this member of the family of intracellular lipid-binding proteins is the wide range of compounds that have been identified as potential physiological ligands. By using recombinant L-FABP, the binding of cholesterol, bile salts and their derivatives has been investigated under conditions that allow a direct comparison of the binding affinities of these ligands for fatty acids. The results demonstrate an inability of L-FABP to bind cholesterol, although the anionic derivative, cholesteryl sulphate, will bind under similar assay conditions. Of the bile salts examined, lithocholate and taurolithocholate sulphate showed the greatest binding to L-FABP. It is proposed that an important function of L-FABP is to bind certain physiological amphipathic anions, thus preventing the "free' concentrations of these compounds from exceeding their critical micelle concentration, which could result in cell damage.

Mutations of recombinant rat liver fatty acid-binding protein at residues 102 and 122 alter its structural integrity and affinity for physiological ligands

Rat liver fatty acid-binding protein (FABP) is able to accommodate a wide range of non-polar anions in addition to long-chain fatty acids. This property means that the liver protein is functionally different from other FABPs from intestine, muscle and adipose tissue that have a more restricted ligand specificity and stoichiometry. The availability of crystal structures for the latter proteins has highlighted the importance of two arginine residues that are involved in the binding of the fatty acid carboxylate. Only one of these arginine residues, arginine-122, is conserved in liver FABP, whereas the other arginine, at position 102, is replaced by a threonine. In order to gain further insight into the nature of ligand interactions with liver FABP these key residues (102 and 122) have been changed by site-directed mutagenesis. The results with an R122Q mutant highlight the critical role of this arginine in determining ligand affinity, while similar but less dramatic effects were observed with the T102Q mutant. The double mutant T102Q/R122Q was expressed but had lost the ability to bind fluorescent ligands. It is concluded that Arg-122 plays a role in accommodating the carboxylate group of at least one fatty acid. It is proposed that physiological ligands with more bulky headgroups, such as lysophospholipids, acyl-CoA and mono-olein, bind with the headgroups in a solvent-exposed location near the portal region of the protein. The portal region is suggested to be more flexible in the mutants (R122Q and to a lesser extent T102Q). The net result is that the ligand specificity of the R122Q mutant changes to that of a protein with enhanced affinity for acyl-CoA, lysophospholipids and mono-olein.

Induction of hepatic acyl-CoA-binding protein and liver fatty acid-binding protein by perfluorodecanoic acid in rats. Lack of correlation with hepatic long-chain acyl-CoA levels

Liver fatty acid-binding protein (L-FABP) and acyl-CoA-binding protein (ACBP) are involved in the intracellular trafficking and compartmentalization of fatty acids and fatty acyl-CoA esters, respectively, in the liver. Both proteins are induced in rat liver by the potent peroxisome proliferator perfluorodecanoic acid (PFDA). While it is believed that the peroxisome proliferator-activated receptor may mediate the responses to peroxisome proliferators by inducing responsive genes, the ligand(s) of this receptor remains unknown. We hypothesized that induction of L-FABP and ACBP in rat liver by PFDA is secondary to accumulation of long-chain acyl-CoA esters. However, neither dose-response nor time-course effects of PFDA on hepatic long-chain acyl-CoA, L-FABP, or ACBP concentrations confirmed this hypothesis. In a dose-response study, PFDA increased hepatic long-chain acyl-CoA concentrations (7 days after treatment) over the dose range of 20-50 mg/kg, whereas it increased ACBP and L-FABP over the wider dose range of 20-65 mg/kg. In the time-course study, PFDA treatment (50 mg/kg) elevated long-chain acyl-CoA esters in the liver beginning on day 3 post-treatment, yet hepatic L-FABP concentrations were increased earlier beginning on day 2 and ACBP was not induced until day 7. To determine if this dissociation of increases in hepatic long-chain acyl-CoA concentrations from increases in hepatic L-FABP and ACBP concentrations could be demonstrated under other conditions, control rats fasted for 24-48 hr were used. Fasting increased hepatic long-chain acyl-CoA levels to a greater extent than PFDA treatment, yet neither L-FABP nor ACBP was induced. We conclude that elevated concentrations of hepatic long-chain acyl-CoAs in PFDA-treated rats are not a major contributor to the induction of L-FABP or ACBP by peroxisome proliferators. A more plausible mechanism is that PFDA induces L-FABP and ACBP by activating the peroxisome proliferator receptor directly rather than indirectly through long-chain acyl-CoA esters.

The binding of lysophospholipids to rat liver fatty acid-binding protein and albumin

The binding of lysophospholipids to rat liver fatty acid-binding protein (FABP) and to BSA and human serum albumin was investigated by using competitive displacement fluorescence assays by monitoring the displacement of the fluorescent fatty acid probe 11-(dansylamino)undecanoic acid (DAUDA). In addition, direct binding assays using changes in tryptophan fluorescence were possible with albumin. Liver FABP was able to bind a range of lysophospholipids, oleoyl-lysophosphatidic acid (lysoPA), oleoyl-lysophosphatidylcholine (lysoPC), oleoyl-lysophosphatidylethanolamine (lysoPE) and oleoyl-lysophosphatidylglycerol, with similar affinity and a Kd of about 1 microM. Liver FABP was also able to bind lysophospholipids generated by the action of phospholipase A2 or phospholipase A1 (triacylglycerol lipase) on phospholipid vesicles. A possible physiological role for liver FABP in lysophospholipid metabolism within the cell is discussed. Albumin was shown to bind lysoPA with higher affinity than either lysoPC or lysoPE, and the initial minimal DAUDA displacement by lysoPA indicated that lysoPA was binding to the primary high-affinity fatty acid-binding sites on albumin and that, like oleic acid, about 3 mol of ligand/mol was bound to these sites. Kd values in the microM range were indicated for lysoPC and lysoPE, whereas, by comparison with oleic acid, the Kd for lysoPA was significantly lower and high-affinity binding in the nM range was indicated. Overall, the data suggest that, because of structural similarity, lysoPA binds to albumin in a similar manner to long-chain fatty acids.

Characterization of binding and structural properties of rat liver fatty-acid-binding protein using tryptophan mutants

Rat liver fatty-acid-binding protein (FABP) does not contain tryptophan. Three mutant proteins have been produced in which a single tryptophan residue has been inserted by site-directed mutagenesis at positions 3 (F3W), 18 (F18W) and 69 (C69W). These tryptophans have been strategically located in order to provide fluorescent reporter groups to study the binding and structural characteristics of rat liver FABP. Two fluorescent fatty acid analogues, DAUDA (11-[(5-dimethylaminonaphthalene-1- sulphonyl)amino]undecanoic acid) and 3-[p-(6-phenyl)-hexa-1,3,5-trienyl]phenylpropionic acid, showed no significant difference in binding affinities for the different mutant proteins, although maximum fluorescence values were decreased for F3W and increased for C69W. These findings were confirmed by studies of DAUDA displacement by oleate. Protein-denaturation studies in the presence of urea indicated subtle differences for the three mutants which could be explained by multiple unfolding pathways. Fatty acid binding increased tryptophan fluorescence emission in the case of the F18W protein, but had no effect on the F3W and C69W proteins. Fluorescence quenching studies with 2-bromopalmitate showed that a fatty acid carboxylate is close to the tryptophan in the F18W protein. Energy-transfer studies showed that the fluorescent moiety of DAUDA is equidistant from the three mutated amino acids and is bound within the beta-clam solvent cavity of liver FABP. This interpretation of the fluorescence quenching and energy-transfer data supports the difference in ligand orientation between intestinal and liver FABP observed in previous studies.

Effect on ligand binding of arginine mutations in recombinant rat liver fatty acid-binding protein

Rat liver fatty acid-binding protein is able to accommodate a wide range of non-polar anions in addition to long-chain fatty acids. The two arginine residues of rat liver fatty acid-binding protein, Arg122 and Arg126, have been mutated and the effect of mutation on ligand binding investigated. No significant decrease in affinity for the fluorescent fatty acid analogue, 11-(5-dimethylaminonaphthalenesulphonyl amino)undecanoic acid, or oleate was observed. However, the apparent affinity for oleoyl-CoA was slightly increased with the mutations Ala122 and Gln122 such that oleoyl-CoA rather than oleate became the preferred ligand for these mutants. Small changes in protein stability were observed with the Arg122 mutations. The lack of notable ionic involvement of the conserved internal residue Arg122 in ligand binding is consistent with the hypothesis that the mode of ligand binding in liver fatty acid-binding protein is markedly different from that of other members of this lipid-binding protein family.

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.

Involvement of arginine in the binding of heme and fatty acids to fatty acid-binding protein from bovine liver

Fatty acid-binding protein from bovine liver but not from bovine heart binds hematin in a saturable manner with high affinity. This property is not confined to a particular isoform as both, pI 6.0- and pI 7.0 L-FABP, bind hematin similarly. In competition experiments hematin and oleic acid could replace each other demonstrating that they share at least parts of the same binding site. Common structural features, i.e. the presence of carboxylic groups and of hydrophobic carbon chains led to the hypothesis that both ligands interact similarly with L-FABP. This was supported by the decrease of binding affinity for either ligand upon modification with phenylglyoxal. Modification in the presence of fatty acid revealed the protection of one of the two arginines of L-FABP. By peptide mapping and Edman degradation Arg122 was identified as the counterpart of the fatty acids carboxylic group.

Coordinate induction of acyl-CoA binding protein, fatty acid binding protein and peroxisomal beta-oxidation by peroxisome proliferators

Acyl-CoA binding protein (ACBP) and fatty acid binding protein (FABP) are important intracellular lipid binding proteins. The purpose of the present experiments was to test the hypothesis that peroxisome proliferators induce ACBP in rat hepatocytes as has been shown previously for FABP. The effects of two structurally dissimilar peroxisome proliferators perfluorodecanoic acid (PFDA) and clofibric acid (CPIB) were examined in primary rat hepatocyte cultures in a chemically defined media. Both compounds alter lipid metabolism in primary rat hepatocytes in a similar fashion, although PFDA is more potent than CPIB at inducing peroxisomal beta-oxidation. In addition, PFDA and CPIB compete with long-chain fatty acids for binding to FABP but do not compete with long-chain acyl-CoA esters for binding to ACBP. The concentration of ACBP and FABP was increased in peroxisome proliferator-treated hepatocytes relative to vehicle controls within 48 h of treatment. Evidence is given to support increases in ACBP and FABP mRNA being the cause of the increased protein levels by peroxisome proliferators. In addition, the peroxisome proliferators PFDA, perfluorooctanoic acid and ciprofibrate induced hepatic ACBP following in vivo administration to rats indicating that this phenomena is not exclusive to in vitro systems. Therefore, ACBP appears to be a member of the peroxisome proliferator loci, a group of lipid metabolizing proteins, including FABP, which are regulated by peroxisome proliferators such as fibric acids and perfluorinated fatty acids.

The binding of the fluorescent ATP analogue 2'(3')-trinitrophenyladenosine-5'-triphosphate to rat liver fatty acid-binding protein

The less polar fluorescent analogue of ATP, 2'(3')-trinitrophenyl-5'-triphosphate bound to rat liver fatty acid-binding protein with high affinity (Kd 6.3 x 10(-6) M) and 1:1 molar stoichiometry. This probe bound to the fatty acid binding site of the protein and was displaced by oleic acid and oleoyl CoA. High concentrations of ATP did not cause significant displacement of the fluorescent ATP analogue. Since the anionic part of this molecule is the triphosphate group it is difficult to envisage this group being accommodated at an anion binding site within the non-polar core of this protein as is the case with other fatty acid binding proteins. Therefore it is anticipated that the ligand must bind to liver fatty acid-binding protein with this triphosphate group surface exposed. Caution must be exercised when using the more hydrophobic fluorescent analogue of ATP to investigate the ATP binding properties of proteins.

Interaction of LY171883 and other peroxisome proliferators with fatty-acid-binding protein isolated from rat liver

Fatty-acid-binding protein (FABP) is a 14 kDa protein found in hepatic cytosol which binds and transports fatty acids and other hydrophobic ligands throughout the cell. The purpose of this investigation was to determine whether LY171883, a leukotriene D4 antagonist, and other peroxisome proliferators bind to FABP and displace an endogenous fatty acid. [3H]Oleic acid was used to monitor the elution of FABP during chromatographic purification. [14C]LY171883 had a similar elution profile when substituted in the purification, indicating a common interaction with FABP. LY171883 and its structural analogue, LY189585, as well as the hypolipidaemic peroxisome proliferators clofibric acid, ciprofibrate, bezafibrate and WY14,643, displaced [3H]oleic acid binding to FABP. Analogues of LY171883 that do not induce peroxisome proliferation only weakly displaced oleate binding. [3H]Ly171883 bound directly to FABP with a Kd of 10.8 microM, compared with a Kd of 0.96 microM for [3H]oleate. LY171883 binding was inhibited by LY189585, clofibric acid, ciprofibrate and bezafibrate. These findings demonstrate that peroxisome proliferators, presumably due to their structural similarity to fatty acids, are able to bind to FABP and displace an endogenous ligand from its binding site. Interaction of peroxisome proliferators with FABP may be involved in perturbations of fatty acid metabolism caused by these agents as well as in the development of the pleiotropic response of peroxisome proliferation.

Synthesis of a gene for rat liver fatty-acid-binding protein and its expression in Escherichia coli

A gene coding for rat liver fatty-acid-binding protein (FABP) has been constructed by the single-step ligation of ten synthetic oligonucleotides. The gene has been cloned into the bacterial expression vector pKK223-3. Induction of protein synthesis from this gene results in over expression of an FABP that is indistinguishable in its structure and binding properties with that isolated from rat liver.

A continuous fluorescence-displacement assay for triacylglycerol lipase and phospholipase C that also allows the measurement of acylglycerols

A new continuous fluorescence-displacement assay for enzymes that release long-chain fatty acids [Wilton (1990) Biochem. J. 266, 435-439] is described in detail for pig pancreatic triacylglycerol lipase. The assay involves the displacement of the highly fluorescent fatty acid probe 11-(dansylamino)undecanoic acid from rat liver fatty acid-binding protein by long-chain fatty acids released as a result of enzyme activity. The assay is surprisingly effective for triacylglycerol lipase, allowing the expression of full activity with low concentrations of substrates in the absence of detergents. The initial rate of decrease in fluorescence is linearly related to enzyme concentration, and activity can be detected in the assay down to concentrations of 10 pg of pure enzyme/ml. The assays demonstrated the quantitative conversion of limiting amounts of substrate into the monacylglycerol. This observation allowed the assay to be used to measure substrates such as triacylglycerols and particularly 1,2-diacylglycerols at concentrations down to about 0.1 microM. Because phospholipase C releases 1,2-diacylglycerols, the coupling of this enzyme to excess lipase allowed the measurement of pure phospholipase C from Bacillus cereus at concentrations down to about 2 ng/ml, and the initial rate of fall in fluorescence in the assay was linearly related to enzyme activity.

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.

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.

Two-dimensional electrophoresis of the fatty acid binding protein from human heart: evidence for a thiol group which can form an intermolecular disulfide bond

A 100,000 g supernatant from human heart muscle, containing cytosolic proteins with some contaminating plasma proteins, was analyzed for fatty acid binding protein (FABP) by two-dimensional electrophoresis (2-DE) using isoelectric focusing under nondenaturing conditions in the first dimension. FABP purified from human heart muscle was found to comigrate with a major spot in 2-DE gels of the supernatant. This spot was comparable with those of the myoglobins in staining intensity. When purified FABP was charged with [3H]palmitate and subjected to nondenaturing 2-DE, radioactivity always comigrated with this protein. Under denaturing and reducing conditions in the second dimension, FABP was found to have a pI of 5.3 and an apparent molecular weight of 15,000. Isoforms of FABP, reported here for the first time to occur in human heart muscle, were observed as minor spots focusing at pH 5.1 and 5.7. When electrophoresis in the second dimension was carried out under denaturing but nonreducing conditions, an additional protein appeared at pH 5.3 with an apparent molecular weight of about 30,000. This protein was identified as a dimer of FABP and evidence for the involvement of an intermolecular disulfide bond in this dimerization is presented.

The fatty acid analogue 11-(dansylamino)undecanoic acid is a fluorescent probe for the bilirubin-binding sites of albumin and not for the high-affinity fatty acid-binding sites

1. The fluorescent fatty acid probe 11-(dansylamino)undecanoic acid (DAUDA) binds with high affinity to bovine and human serum albumin (BSA and HSA) at three sites. 2. The Kd of the primary binding site could not be determined; however, the two secondary sites appeared to be equivalent, with an apparent Kd of 8 x 10(-7) M for both BSA and HSA. 3. The spectral characteristics of DAUDA when bound to the primary site of the two albumins were different, with HSA producing a greater fluorescence enhancement and emission maximum at a shorter wavelength (480 nm) than for BSA (495 nm). 4. Displacement studies indicated that the DAUDA-binding sites were not equivalent to the primary long-chain fatty acid-binding sites on albumin, but corresponded to the bilirubin sites. Fatty acyl-CoAs also bind to the bilirubin sites, as do medium-chain fatty acids. 5. The solubility, stability and spectral properties of DAUDA make it an excellent probe for investigating the bilirubin-binding sites of albumin, particularly HSA.

A continuous fluorescence displacement assay for the measurement of phospholipase A2 and other lipases that release long-chain fatty acids

1. A new continuous fluorescence assay for phospholipase A2 is described which involves the displacement of the highly fluorescent fatty-acid probe 11-(dansylamino)undecanoic acid from rat liver fatty-acid-binding protein by long-chain fatty acids released as a result of phospholipase A2-catalysed hydrolysis of phospholipids. The initial rate of decrease in fluorescence is linearly related to enzyme activity. 2. The assay will detect enzyme activity down to about 10 pmol/min per ml and gives a linear response up to about 10 nmol/min per ml. 3. The assay will work with all phospholipids that have been tested including phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol and phosphatidylglycerol. Substrates carrying a net negative charge showed the highest rates of hydrolysis. 4. The assay will work, in principle, with an enzyme catalysing the release of long-chain fatty acids from a fatty-acylated substrate. This has been confirmed with pancreatic lipase and cholesterol esterase.