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

In the studies described here rat liver microsomes containing labeled palmitic, stearic, oleic or linoleic acids were incubated with fatty acid binding protein (FABP) and the rate of removal of¹⁴C-labeled fatty acids from the membrane by the soluble protein was measured using a model system. More unsaturated than saturated fatty acids were removed from native liver microsomes incubated with similar amounts of FABP. Thein vitro peroxidation of microsomal membranes mediated by ascorbate-Fe⁺⁺, modified its fatty acid composition with a considerable decrease of the peroxidizability index. These changes in the microsomes facilitated the removal of oleic and linoeic acids by FABP, but the removal of palmitic and stearic acids was not modified. This effect is proposed to result from a perturbation of membrane structure following peroxidation with release of free fatty acids from susceptible domains.Abbreviations BSA bovine serum albumin - FABP fatty acid binding protein     

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

Fatty acid-binding proteins (FABP) are abundant cytosolic proteins whose level is responsive to nutritional, endocrine, and a variety of pathological states. Although FABPs have been investigatedin 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 accomodate one fatty acid each. One fatty acid binding site is identical to the cholesterol binding site. To test whether these observations obtainedin 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.Abbreviations FABP fatty acid-binding protein - L-FABP liver fatty acid-binding protein - I-FABP intestinal fatty acid-binding protein - H-FABP heart fatty acid-binding protein - A-FABP adipocyte fatty acid-binding protein - PMFABP plasma membrane fatty acid-binding protein - SCP-2 sterol carrier protein-2 - Dehydroergosterol (DHE) d-5,7,9(11),22-ergostatetraene-3b-ol - cis-parinaric acid-9Z, 11E, 13E, 15Z-octatetraenoic acid - trans parinaric acid, 9E, 11E, 13E, 14E-octatetraenoic acid - BSA bovine serum albumin - KRH Krebs-Ringer-Henseleit buffer     

Fatty acid oxidation capacity and fatty acid-binding protein content of different cell types isolated from rat heart

Summary Heart tissue contains appreciable amounts of fatty acid-binding protein (FABP). FABP is thought to play a crucial role in the transport of fatty acids from the cellular membrane to the intracellular site of oxidation and also, in case of endothelial cells, in the transfer of fatty acids from the vascular to the interstitial compartment through the endothelial cytoplasm. The present study was designed to delineate a possible quantitative relationship between the capacity of different cell types in the heart to oxidize fatty acids and the presence of FABP. Palmitate oxidation capacity, measured in homogenates of cells isolated from adult rat hearts, was 2 nmol/min per mg tissue protein in freshly isolated cardiomyocytes (CMC), but only 0.09 and 0.31 nmol/min per mg tissue protein in cultivated endothelial (CEC) and fibroblast-like cells (CFLC), respectively. Palmitate oxidation rates were closely related to the cytochrome C oxidase activity and, hence, to the mitochondrial density in the cells under investigation. In CMC the content of cytosolic H-FABP (H-FABP_c) was about 4.51 µg/mg tissue protein. However, in CEC and CFLC the FABP content was less than 0.01 and 0.004 µg/mg tissue protein, respectively, corresponding to at maximum 0.2% of the FABP content of CMC. These findings indicate a marked difference between CMC and non-myocytal cells in the heart regarding their capacity to oxidize fatty acids, and a marked disproportion between the fatty acid oxidation capacity and immunochemically determined FABP content in both CEC and CFLC. The functional implication of these observations remains to be elucidated.     

Heart fatty acid uptake is decreased in heart fatty acid-binding protein gene-ablated mice

Cell culture systems have demonstrated a role for cytoplasmic fatty acid-binding proteins (FABP) in lipid metabolism, although a similar function in intact animals is unknown. We addressed this issue using heart fatty acid-binding protein (H-FABP) gene-ablated mice. H-FABP gene ablation reduced total heart fatty acid uptake 40 and 52% for [1-(14)C]16:0 and [1-(14)C]20:4n-6 compared with controls, respectively. Similarly, the amount of fatty acid found in the aqueous fraction was reduced 40 and 52% for [1-(14)C]16:0 and [1-(14)C]20:4n-6, respectively. Less [1-(14)C]16:0 entered the triacylglycerol pool, with significant redistribution of fatty acid between the triacylglycerol pool and the total phospholipid pool. Less [1-(14)C]20:4n-6 entered each lipid pool measured, but these changes did not alter the distribution of tracer among these pools. In gene-ablated mice, significantly more [1-(14)C]16:0 was targeted to choline and ethanolamine glycerophospholipids, whereas more [1-(14)C]20:4n-6 was targeted to the phosphatidylinositol (PtdIns) pool. H-FABP gene ablation significantly increased PtdIns mass 1.4-fold but reduced PtdIns 20:4n-6 mass 30%. Consistent with a reported effect of FABP on plasmalogen mass, ethanolamine plasmalogen mass was reduced 30% in gene-ablated mice. Further, 20:4n-6 mass was reduced in each of the three other major phospholipid classes, suggesting H-FABP has a role in maintaining steady-state 20:4n-6 mass in heart. In summary, H-FABP was important for heart fatty acid uptake and targeting of fatty acids to specific heart lipid pools as well as for maintenance of phospholipid pool mass and acyl chain composition.     

A comparison of heart and liver fatty acid-binding proteins: interactions with fatty acids and possible functional differences studied with fluorescent fatty acid analogues

Summary Fatty acid-binding proteins (FABP) are distinct but related gene products which are found in many mammalian cell types. They are generally present in high abundance, and are found in those tissues where free fatty acid (ffa) flux is high. The function(s) of FABP is unknown. Also not known is whether all FABP function similarly in their respective cell types, or whether different FABP have unique functions. The purpose of these studies was to assess whether different members of the FABP family exhibit different structural and functional properties. Two fluorescent analogues of ffa were used to compare the liver (L-FABP) and heart (H-FABP) binding proteins. The propionic acid derivative of diphenylhexatriene (PADPH) was used to examine the physical properties of the ffa binding site on L- and H-FABP, as well as the relative distribution of ffa between FABP and membranes. An anthroyloxy-derivative of palmitic acid, 2AP, was used to monitor the transfer kinetics of ffa from liver or heart FABP to acceptor membranes, using a resonance energy transfer assay. The results demonstrate that the ffa binding sites of both FABP are hydrophobic in nature, although the L-FABP site is more nonpolar than the H-FABP site. Equilibration of PADPH between L-FABP and phosphatidylcholine (PC) bilayers resulted in a molar partition preference of > 20: 1, L-FABP : PC. Similar studies with H-FABP resulted in a PADPH partition preference of only 3:1, H-FABP : PC. Finally, the transfer of 2AP from H-FABP to acceptor membranes was found to be 50-fold faster than transfer from L-FABP. These studies demonstrate that important structural and functional differences exist between different members of the FABP family, and therefore imply that the roles of different FABP may be unique.Abbreviations FABP Fatty Acid-Binding Protein - L-FABP Liver FABP - H-FABP Heart FABP - SUV Small Unilamellar Vesicle - PADPH 3-[p-(6-Phenyl)-1,3,5-Hexatrienyl]-phenylpropionic acid - 2AP 2-(9-Anthroyloxy)Palmitic acid - Q Quantum yield - _F Fluorescence lifetime     

Brain arachidonic acid incorporation is decreased in heart fatty acid binding protein gene-ablated mice

Heart fatty acid binding protein (H-FABP) is expressed in neurons, but its role in brain fatty acid incorporation and metabolism is poorly defined. We examined the effect of H-FABP gene ablation on brain incorporation of arachidonic ([1-(14)C]20:4n-6) or palmitic ([1-(14)C]16:0) acid in vivo. Analysis of brain mRNA confirmed gene ablation and demonstrated no compensatory changes in the levels of other FABP mRNA in the gene-ablated mice. In brains from H-FABP gene-ablated mice, the incorporation coefficient for [1-(14)C]20:4n-6 was reduced 24%, while that for [1-(14)C]16:0 was unaffected. Within the organic and aqueous fractions, significantly more [1-(14)C]20:4n-6 was distributed into the aqueous fraction, suggesting a disruption in the metabolic targeting of 20:4n-6 in these mice. There was less incorporation of [1-(14)C]20:4n-6 into total phospholipids and a marked reduction (51%) in the level of incorporation into the choline glycerophospholipids (ChoGpl). Because FABP can influence steady-state lipid mass, brain individual lipid masses were measured. The brain total phospholipid mass was reduced 17% by gene ablation, ascribed to a 27% and 32% reduction in the masses of ChoGpl and sphingomyelin, respectively. Plasmalogen subclass masses were also reduced, suggesting that H-FABP may augment brain plasmalogen synthesis. In gene-ablated mice, the phosphatidylinositol 20:4n-6 level was reduced 25%, while the proportion of total n-6 fatty acids was reduced in the major phospholipid classes. Thus, these results demonstrate for the first time that H-FABP expression influences brain 20:4n-6 uptake and trafficking as well as steady-state brain lipid levels.     

Histochemical localization of heart-type fatty-acid binding protein in human and murine tissues

Cellular fatty acid-binding proteins (FABP) are a highly conserved family of proteins consisting of several subtypes, among them the mammary-derived growth inhibitor (MDGI) which is quite homologous to or even identical with the heart-type FABP (H-FABP). The FABPs and MDGI have been suggested to be involved in intracellular fatty acid metabolism and trafficking. Recently, evidence for growth and differentiation regulating properties of MDGI and H-FABP was provided. Using four affinity-purified polyclonal antibodies against bovine and human antigen preparations, the cellular localization of MDGI/H-FABP in human and mouse tissues and organs was studied. The antibodies were weakly cross-reactive with adipose tissue extracts known to lack H-FABP, but failed to react by Western blot analysis with liver-type FABP (L-FABP) and intestinal-type FABP (I-FABP). MDGI/H-FABP protein was mainly detected in myocardium, skeletal and smooth muscle fibres, lipid and/or steroid synthesising cells (adrenals, Leydig cells, sebaceous glands, lactating mammary gland) and terminally differentiated epithelia of the respiratory, intestinal and urogenital tracts. The results provide evidence that expression of H-FABP is associated with an irreversibly postmitotic and terminally differentiated status of cells. Since all the antisera employed showed spatially identical and qualitatively equal immunostaining, it is suggested that human, bovine and mouse MDGI/H-FABP proteins share highly homologous epitopes.     

Molecular modeling and functional confirmation of a predicted fatty acid binding site of mitochondrial aspartate aminotransferase

Molecular interactions are necessary for proteins to perform their functions. The identification of a putative plasma membrane fatty acid transporter as mitochondrial aspartate aminotransferase (mAsp-AT) indicated that the protein must have a fatty acid binding site. Molecular modeling suggests that such a site exists in the form of a 500-ų hydrophobic cleft on the surface of the molecule and identifies specific amino acid residues that are likely to be important for binding. The modeling and comparison with the cytosolic isoform indicated that two residues (Arg201 and Ala219) were likely to be important to the structure and function of the binding site. These residues were mutated to determine if they were essential to that function. Expression constructs with wild-type or mutated cDNAs were produced for bacteria and eukaryotic cells. Proteins expressed in Escherichia coli were tested for oleate binding affinity, which was decreased in the mutant proteins. 3T3 fibroblasts were transfected with expression constructs for both normal and mutated forms. Plasma membrane expression was documented by indirect immunofluorescence before [³H]oleic acid uptake kinetics were assayed. The V_max for uptake was significantly increased by overexpression of the wild-type protein but changed little after transfection with mutated proteins, despite their presence on the plasma membrane. The hydrophobic cleft in mAsp-AT can serve as a fatty acid binding site. Specific residues are essential for normal fatty acid binding, without which fatty acid uptake is compromised. These results confirm the function of this protein as a fatty acid binding protein.     

Expression of a functionally active cardiac fatty acid-binding protein in the yeast,Saccharomyces cerevisiae

Summary The unicellular eukaryotic microorganism, Saccharomyces cerevisiae, transformed with a plasmid containing a cDNA fragment encoding bovine heart fatty acid-binding protein (H-FABP) under the control of the inducible yeast GAL10 promoter, expressed FABP during growth on galactose. The maximum level of immunoreactive FABP, identical in size to native protein as judged from SDS-polyacrylamide gel electrophoresis, was reached after approximately 16 hours of induction. Analysis of particulate and soluble subcellular fractions showed that FABP was exclusively associated with the cytosol. FABP expressed in yeast cells was functional as was demonstrated by its capacity to bind ¹⁴C-oleic acid in an in vitro assay. Growth of the transformants on galactose as the carbon source was significantly retarded at 37°C. Whereas the fatty acid pattern of total lipids was not altered in transformed cells, desaturation of exogenously added ¹⁴C-palmitic acid was significantly reduced both at 30 and 37°C. The lowest percentage of radioactively labeled unsaturated fatty acids was found in the phospholipid fraction.     

Fatty acid-binding protein expression in the liver: its regulation and relationship to the zonation of fatty acid metabolism

Summary Liver fatty acid-binding protein (L-FABP) is expressed in a declining gradient between the portal and central zones of the liver acinus. This paper discusses the results of experimental studies which address the questions: (a) What factors regulate L-FABP expression in liver and produce its acinar gradient? (b) What is the relationship between the acinar gradient of L-FABP and acinar gradients in the transport and metabolism of long-chain fatty acids? Both high-fat diets and clofibrate-treatment increase L-FABP proportionally at both extremes of the liver acinus and the small intestine, with preservation of the L-FABP gradient in both tissues. Female rats differ from males, however, in showing a greater hepatic abundance of L-FABP which is expressed almost equally throughout the acinus. Dietary studies show that L-FABP is induced with increased fatty acid flux derived from dietary fat but not from de novo hepatic fatty acid synthesis. Studies of the synthesis and utilization of fatty acids by hepatocytes isolated from the periportal and pericentral zones of the liver acinus suggest that the acinar gradient of L-FABP is not associated with differences in the instrinsic capacity of zone 1 and zone 3 hepatocytes to utilize or synthesize fatty acids. In addition, studies of the acinar uptake pattern of a fluorescent fatty acid derivative by isolated perfused livers indicate that the acinar distribution of L-FABP does not determine the pattern of fatty acid uptake in the intact acinus. Rather, the acinar gradient of L-FABP is most likely to represent a response to physiological conditions existing in the intact acinus which may include gradients in the flux of fatty acids, fatty acid metabolites and hormones.Abbreviations ALT Alanine Aminotransferase - FABP Fatty Acid Binding Protein - I-FABP Intestinal-type Fatty Acid Binding Protein - L-FABP Liver-type Fatty Acid Binding Protein - 12-NBD-stearate 12-(N-methyl)-N-(7-nitrobenzo-2-oxa-1, 3,-diazol-4-yl)amino)-octadecanoic acid     

Partial purification of molecular weight 12 000 fatty acid binding proteins from rat brain and their effect on synaptosomal Na+-dependent amino acid uptake

High-affinity, Na+-dependent synaptosomal amino acid uptake systems are strongly stimulated by proteins which are known to bind fatty acids, including the Mr 12 000 fatty acid binding protein (FABP) from liver. To explore the possibility that such a function might be served by fatty acid binding proteins intrinsic to brain, we examined the 105000g supernatant of brain for fatty acid binding. Observed binding was accounted for mainly by components excluded by Sephadex G-50, and to a small degree by the Mr 12 000 protein fraction (brain FABP fraction). The partially purified brain FABP fraction contained a protein immunologically identical with liver FABP as well as a FABP electrophoretically distinct from liver FABP. Brain FABP fraction markedly stimulated synaptosomal Na+-dependent, but not Na+-independent, amino acid uptake, and also completely reversed the inhibition of synaptosomal Na+-dependent amino acid uptake induced by oleic acid. Palmitic, stearic, and oleic acids were endogenously associated with the brain FABP fraction. These data are consistent with the hypothesis that Mr 12 000 soluble FABPs intrinsic to brain may act as regulators of synaptosomal Na+-dependent amino acid uptake by sequestering free fatty acids which inhibit this process.     

Binding of cytochrome P450 monooxygenase and lipoxygenase pathway products by heart fatty acid-binding protein

Arachidonic acid metabolism by lipoxygenases and cytochrome P450 monooxygenases produces regioisomeric hydroperoxyeicosatetraenoic acids (HPETEs), hydroxyeicosatetraenoic acids (HETEs), epoxyeicosatrienoic acids (EETs), and dihydroxyeicosatrienoic acids (DHETs), which serve as components of cell signaling cascades. Intracellular fatty acid-binding proteins (FABPs) may differentially bind these nonprostanoid oxygenated fatty acids, thus modulating their metabolism and activities. Vascular cells, which express heart FABP (H-FABP), utilize oxygenated fatty acids for regulation of vascular tone. Therefore, the relative affinities of H-FABP for several isomeric series of these compounds were measured by fluorescent displacement of 1-anilinonaphthalene-8-sulfonic acid (ANS). In general, H-FABP rank order affinities (arachidonic acid > EETs > HETEs > DHETs) paralleled reversed-phase high-performance liquid chromatography retention times, indicating that the differences in H-FABP affinity were determined largely by polarity. H-FABP displayed a similar rank order of affinity for compounds derived from linoleic acid. H-FABP affinity for 20-HETE [apparent dissociation constant (K(d)') of 0.44 microM] was much greater than expected from its polarity, indicating unique binding interactions for this HETE. H-FABP affinity for 5,6-EET and 11,12-EET (K(d)' of approximately 0.4 microM) was approximately 20-fold greater than for DHETs (K(d)' of approximately 8 microM). The homologous proteins, liver FABP and intestinal FABP, also displayed selective affinity for EET versus DHET. Thus, FABP binding of EETs may facilitate their intracellular retention whereas the lack of FABP affinity for DHETs may partially explain their release from cells. The affinity of H-FABP for EETs suggests that this family of intracellular proteins may modulate the metabolism, activities, and targeting of these potent eicosanoid biomediators.     

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

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

Intracellular transport of lipids

Summary Translocation of lipids inside mammalian cells is considered to be facilitated by a number of low-molecular weight lipid binding proteins. An overview of these proteins is given, with particular reference to the heart. Three distinct phospholipid transfer proteins specifically stimulate the net transfer of individual phospholipid classes between membrane structures. In rat cardiac muscle their content is 15–140 pmol/g ww. Fatty acid-binding proteins (FABP) are abundantly present in tissues actively involved in the uptake or utilization of long-chain fatty acids, such as intestine, liver and heart. The four distinct FABP types now identified show a complex tissue distribution with some tissues containing more than one type. Heart (H-) FABP comprises about 5% of the cytosolic protein mass; its content in rat heart is 100 nmol/g ww. Immunochemical evidence has been obtained for the presence of H-FABP in several other tissues, including red skeletal muscle, mammary gland and kidney. Beside long-chain fatty acids FABP binds with similar affinity also fatty acyl-CoA and acyl-L-carnitines. In heart the latter compound may be the primary ligand, since normoxic acyl-L-carnitine levels are several fold higher than those of fatty acids. In addition, H-FABP was found to modulate cardiac energy production by controlling the transfer of acyl-L-carnitine to the mitochondrial -oxidative system. H-FABP may also protect the heart against the toxic effects of high intracellular levels of fatty acid intermediates that arise during ischemia.     

Amino acid sequence and some ligand binding properties of fatty acid-binding protein from bovine brain

Summary A fatty acid-binding protein (FABP) from the cytosol of bovine brain was purified by Sephadex G-75 filtration and electrofocusing. The purified protein migrated as a single protein band in 15% polyacrylamide gel electrophoresis with an apparent molecular mass of 14.7 kDa. To ascertain that the purified protein was a FABP, it was submitted to fatty acid-binding tests. Oleic and palmitic acids bound to brain FABP but this was not the case for palmitoyl CoA. By Scatchard analysis the ligand binding values were: Kd = 0.28 µM, Bmax (mol/mol) = 0.6 for oleic acid and Kd = 0.8 µM, Bmax (mol/mol) = 2.1 for palmitic acid. The complete amino acid sequence of the brain FABP was determined and a microheterogeneity was observed. Sequence comparison with other FABPs of known sequence and the observed microheterogeneity demonstrated the presence in brain of several homologous FABPs closely related to heart FABP.This paper corresponds to a communication at the first international workshop on fatty acid binding proteins (Maastricht, the Netherlands, September 4–5, 1989).     

Fatty acid double bond orientation alters interaction with L-cell fibroblasts

Relatively little is known of fatty acid specificity in cellular fatty acid uptake. In this study L-cells, a fibroblastic cell line with very low levels of endogenous cytosolic fatty acid binding protein, were used to examine the role of cis and trans unsaturation on fatty acid uptake. The fluorescent fatty acids, trans-parinaric acid and cis-parinaric acid, were used as analogs of straight-chain saturated, and kinked-chain unsaturated fatty acids, respectively, in order to evaluate the fatty acid specificity of the uptake system. Parinaric acid is poorly metabolizable; greater than 97% was unesterified while ³H-oleic acid was almost totally metabolized after 30 min uptake. Cis- and trans-parinaric acid uptake was saturable and dependent on the concentration of fatty acid. However, the initial rate and maximal amount of trans-parinaric acid taken up by the L-cells was greater than for cis-parinaric acid under the same conditions. The affinity of L-cell uptake for trans-parinaric acid (K_m = 0.12 uM) was 35-fold higher than that for cis-parinaric acid (K_m = 4.17 uM) . Based on competition studies with oleic and stearic acids, it was concluded that the cis- and trans-parinaric acid were taken up by the same L-cell fatty acid uptake system. The results suggest that the L-cell fatty acid uptake system has selectivity for straight chain rather than kinked chain unsaturated fatty acids.Abbreviations Cis-parinaric acid 9Z, 11E, 13E, 15Z-octatetraenoic acid - trans-parinaric acid 9E, I IE, 13E, 15E-octatetraenoic acid - EGTA ethylene glycol-bis(beta-amlno-ethyl ether) N,N,N,N-tetratacetic acid - BSA bovine serum albumin - PBS phosphate buffered saline     

Fatty acid binding proteins from heart

Heart contains a fatty acid binding protein (FABP) concentration comparable to liver, when it is determined with a fatty acid-binding assay. The low concentration detected with anti-liver FABP antibodies is related to the different chemical forms and physiochemical properties of liver and heart FABP. The ratio of fatty acid bound per purified protein molecule is one or lower. Rat heart mitochondria oxidize FABP-bound fatty acids. The FABP content of rat heart is dependent on sex and diurnal cycle, but is not influenced by starvation or clofibrate feeding. It is also not different in the newborn rat. FABP was obtained from human heart in a yield of 11%. It shows similar binding characteristics to palmitic, oleic and arachidonic acid. The functional significance of the specific heart FABP is discussed in relation to myocardial fatty acid metabolism in normal and pathological conditions.