Utilization of long chain fatty acids by rat liver: Studies of the role of fatty acid binding protein

Abstract

To investigate the possible role of fatty acid binding protein (FABP) in the utilization of long chain fatty acids by rat liver we studied uptake and metabolic disposition of albumin-bound 14C-oleate by isolated rat liver cell suspensions, both under control conditions and in the presence of flavaspidic acid, a known inhibitor of fatty acid binding to FABP. In incubations lasting up to 4 min, 0.02 mM flavaspidic acid effected a modest but consistent, significant and reversible inhibition of oleate oxidation (22%), esterification of phospholipids (17%) and triglycerides (15%), and uptake (18%), but had no effect on O2 consumption, ATP concentration, or oxidation of acetate or octanoate.In a second series of experiments, the mechanism of these effects was investigated in cell-free systems by studying mitochondrial and microsomal enzymes involved in the initial stages of fatty acid utilization. In the presence of physiologic concentrations of partially purified FABP, the activities of these enzymes were significantly enhanced in response to small changes in FABP concentrations, and 0.02 mM flavaspidic acid was not inhibitory.Finally, FABP partially purified from rat liver cytosol was found to contain 38.4 nmol of endogenous long chain fatty acid/mg protein, a more than 25-fold enrichment relative to the 105,000 g supernatant. Of these endogenous FABP-associated fatty acids, 26% were accounted for by 18:2 and 23% by 20:4, a substantial enrichment in these essential polyunsaturates compared with whole homogenate and serum.These experiments show that a low concentration of flavaspidic acid inhibits hepatocyte utilization of oleate. It is suggested that flavaspidic acid interferes with the entry of fatty acids into oxidation and esterification pathways by inhibiting formation of the fatty acid-FABP complex. Although its possible role as a fatty acid “carrier” remains uncertain, FABP influences the activity of enzyme reactions in which fatty acid and fatty acid acyl CoA are substrates, is associated with large quantities of endogenous unsaturated fatty acids, and may be important in the utilization of fatty acids formed within the cell, de novo or by ester hydrolysis. These and other studies support the concept that FABP plays a broad role in the cellular economy of long chain fatty acids.