Recognition of Fatty Acids by Hepatic Fatty Acid Binding Proteins

Are fatty acid-binding proteins involved in fatty acid transfer?

The possible function of fatty acid-binding protein (FABP) to act as a fatty acid carrier protein was investigated in model systems with regard to three aspects. (1) does FABP release fatty acids from membranes? (2) does it facilitate fatty acid transport in an aqueous environment? (3) are FABP-bound fatty acids released for use by mitochondria? FABPs could bind oleic acid from liposomes and mitochondrial membranes with a ratio of 1 mol per mol protein. Oleic acid was withdrawn from negative, neutral or cholesterol-containing monolayers by FABP with rates up to 10%/min. Only about 5% of FABP penetrated into the monolayer. Spontaneous transfer of oleic acid between mitochondria and vesicles or liposomes occurred so rapidly that an effect of FABP was not detectable. When the mitochondria were separated from the vesicles in an equilibrium dialysis cell, a stimulating effect of FABP on fatty acid transfer could be demonstrated. Injected FABP increased also transfer of oleic acid between two separate monolayers. FABP-bound fatty acid was well oxidized by rat liver mitochondria. The results indicate that the FABP-fatty acid complex may function as an intermediate in the transfer of fatty acids between membranes. No functional differences were detected between heart and liver FABPs in this respect.

Subcellular distribution of cardiac fatty acid-binding protein in bovine heart muscle and quantitation with an enzyme-linked immunosorbent assay

Several types of the 14-15 kDa fatty acid-binding proteins (FABPs) are known to occur in the cytosol of mammalian cells. With antibodies raised against the cardiac-type protein from bovine heart, immunoblots indicated a more widespread distribution of the cardiac FABP in subcellular fractions, such as mitochondria and nuclei. A detailed view was obtained when the post-embedding protein A-gold labeling method was applied to cross-sections of heart cells and isolated subcellular fractions. Cardiac FABP in myocytes was associated with myofibrils and localized within mitochondria and nuclei. After subfractionation of mitochondria, the binding protein was recovered with matrix proteins only. A non-competitive enzyme-linked immunosorbent assay (ELISA) of the direct type was developed specifically for bovine cardiac FABP. This assay was sensitive in the range of 0.05 to 1 ng, and concentrations of cardiac FABP per mg protein were found for cytosol, matrix and nuclei to be around 3.18, 0.18 and 0.03 micrograms, respectively. The newly found compartmentation of cardiac FABP in the heart cell must be considered when the true functions of the protein, yet to be defined, are studied.

Compartmentation of hepatic fatty-acid-binding protein in liver cells and its effect on microsomal phosphatidic acid biosynthesis

Fatty-acid-binding proteins are known to occur in the cytosol of mammalian cells and to bind fatty acids and their CoA-esters. Application of the postembedding protein A-gold labeling method with antibody against the hepatic type fatty-acid-binding protein (hFABP) to cross-sections of liver cells and a newly developed gel-chromatographic immunofluorescence assay established qualitatively (1) that hFABP in mitochondria was confined to outer mitochondrial membranes, (2) the presence of this protein in microsomes and (3) that nuclei were also filled with hFABP. Quantitative data elaborated with a non-competitive ELISA confirmed these results. A significant difference to the distribution of cardiac FABP in heart muscle cells, where this type of protein was found in cytosol, matrix and nuclei, was observed (Börchers et al. (1989) Biochim. Biophys. Acta, in the press). hFABP-containing rat liver microsomes were incubated with long-chain acyl-CoAs in the presence of hFABP (isolated from rat liver cytosol) in a study on the acylation of sn-glycerol-3-phosphate and lysophosphatidic acid. Both acyltransferases were stimulated by addition of hFABP to the incubation medium. The morphological, immunochemical as well as kinetic data infer a direct interaction of hFABP with microsomal membranes in liver cells.

Immunochemical quantitation of fatty-acid-binding proteins. I. Tissue and intracellular distribution, postnatal development and influence of physiological conditions on rat heart and liver FABP

Antisera against rat heart and liver fatty acid-binding protein (FABP) were applied in Western blotting analysis and ELISA to assess their tissue and intracellular distribution, and the influence of development, physiological conditions and several agents on the FABP content of tissue cytosols. The data obtained are compared with the oleic acid-binding capacity. Heart FABP is found in high concentrations in heart, skeletal muscles, diaphragm and lung, and in lower concentrations in kidney, brain and spleen, whereas liver FABP is limited to liver and intestine. In heart and liver, FABP is only present in the cytosol. The FABP content of both heart and liver shows a progressive increase during the first weeks of postnatal development, in contrast to their constant oleic acid-binding capacity. The reciprocally declining alpha-fetoprotein content of both tissues may partially account for the complementary fraction of the fatty acid-binding capacity. The FABP content and the fatty acid-binding capacity of adult heart and liver were in good accordance under various physiological conditions. Addition of clofibrate to the diet induces an increase of liver FABP content, whereas feeding of cholesterol, cholestyramine, mevinolin or cholate caused a marked decrease. The significance of the combined determination of fatty acid-binding capacity and FABP content (by immunochemical quantitation and blotting analysis) is indicated.

The binding affinity of fatty acid-binding proteins from human, pig and rat liver for different fluorescent fatty acids and other ligands

1. Two forms of fatty acid-binding proteins (FABPs) were isolated from human, pig and rat liver cytosols by gelfiltration and anion-exchange chromatography. 2. Both forms did not show physicochemical or chemical differences. They had an Mr of about 14.5 kDa for all species. pI Values were 5.8 for both forms of human and pig liver FABP and 6.4 for both forms of rat liver FABP. In contrast to heart FABPs no tryptophan was present in liver FABPs. 3. Liver FABPs show a much higher enhancement of fluorescence at binding of 11-dansylaminoundecanoic acid, 16-anthroyloxy-palmitic acid and 1-pyrene-dodecanoic acid than heart FABPs and additionally a blue shift in excitation and emission wavelengths with the first fatty acid. 4. The bulky side-chain did not affect fatty acid binding since binding constants of liver FABPs were comparable for these fluorescent fatty acids and oleic acid (0.3-0.7 microM). 5. A 1:1 binding stoichiometry was obtained for oleic acid binding with heart and liver FABPs. 6. Liver FABPs have a high binding affinity for C16-C22 saturated and unsaturated fatty acids, palmitoyl-CoA, bromo-substituted fatty acids, POCA, tetradecylglycidic acid and flavaspidic acid. 7. Fatty acid binding could be reduced to less than 50% by arginine modification with 2,3-butadione or by enzymatic degradation of FABPs with trypsin or pronase.

Interactions of fatty acids with neutral fatty-acid-binding protein from bovine liver

Hepatic-type fatty-acid-binding protein (hFABP) from the cytosol of bovine liver is a 14.4-kDa neutral protein with a blocked N-terminus and a disulfide system located on the surface of the protein. It binds two molecules of fatty acid in one binding site, apparent dissociation constants of the oleic acid/hFABP complex are 0.24 microM and 2.15 microM. Computer analysis of circular dichroic spectra predicts that hFABP contains about 12% alpha-helix, 45% beta-structure, 15% beta-turn and 27% unordered structure. Ellipticities indicative of secondary structure are not affected by fatty acid binding. Cationic amino acid residues of hFABP (1 His, 15 Lys, 2 Arg) were screened for ionic fatty acid/protein interactions. His was excluded, as 1H-NMR analysis of His-C2 and His-C4 protons indicated that binding of oleic acid shifts the pK of His from 6.9 to 7.1 only in hFABP with the disulfide system in the oxidized state; acylation of His with diethylpyrocarbonate does not affect the binding of the fatty acid. Acetylation of Lys reduces binding marginally, whereas modification of Arg with phenylglyoxal lowers the binding activity by 65%. From 1H-NMR investigations, conformational changes within the protein, due to a sort of disaggregation of hFABP upon fatty acid binding, were derived. Most of the proton resonances sharpen up with ligand binding, and some of the methyl resonances shift positions, possibly because they are directly involved in the fatty acid/protein interaction.