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 Oleic acid was withdrawn from negative, neutral or cholesterol-containing monolayers by FABP with rates up to 10%/min.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.