Effect of fatty acid‐binding proteins on intermembrane fatty acid transport

Abstract

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. [ 14 C]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.Keywords: fatty acid oxidation; heart mitochondria; liver mitochondria; fixed liposomes; membrane charge.Fatty acid binding proteins (FABPs) have been isolated from cytosols of tissues of vertebrates and invertebrates as 13± 15 kDa proteins. Nine FABP types have been named up to now after the first tissue from which they were isolated. They show a characteristic tissue and cellular distribution [1]. In spite of the large amount of knowledge on their molecular structure, their binding properties and their genes, the functions of these different FABP types are still unclear. They are postulated to be involved in fatty acid uptake and targeting, in modulation of fatty acid concentration and, in this way, in regulation of metabolism, signal transduction, gene transcription and detoxification [1,2]. Theoretical considerations ascribed a role to FABPs in fatty acid transport in hepatocytes [3] and cardiomyocytes [4]. In model systems of liposomes, monolayers, mitochondria and microsomes, fatty acid transfer was demonstrated from FABP to membranes, from membranes to FABP and from membranes via FABP to membranes by use of radiolabeled or fluorescent fatty acids, or NMR ([1,2,5±8] and references therein). FABP facilitated the diffusion of oleate in a model cytosol system [9]. Photoactivatable radiolabeled fatty acids were taken up and labeled FABP in hepatocytes and adipocytes [10±11]. FABP-bound fatty acids were oxidized by mitochondria [8,12±14] and peroxisomes [14] or used for acyl-CoA synthesis by microsomes [15]. The technique of fluorescence recovery after photobleaching was used to measure the intracellular transport of a fluorescent fatty acid (12-N-methyl-(7-nitrobenzo-2-oxa-1,3-diazol)aminostearate; NBDstearate) [16,17]. Inhibition of ...

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