Membrane Binding Proteins are the Major Determinants for the Hepatocellular Transmembrane Flux of Long-Chain Fatty Acids Bound to Albumin

Purpose The hepatic transmembrane flux of long-chain fatty acids (LCFA) occurs through passive and fatty acid transport protein facilitated processes from blood. The extent that these transport processes can be related to the unbound and protein-bound fractions of LCFA in blood is not clear. Methods We used hepatocyte suspensions, hepatoma monolayers, and perfused rat livers to quantitate the transport of purified [ 3 H]palmitate ([ 3 H]PA) and 12-( N -methyl)- N -[(7-nitrobenz-2oxa-1,3-diazol-4yl-)amino]octadecanoicacid (12-NBDS) from solutions with a constant unbound LCFA concentration with varying bovine serum albumin (BSA) concentrations and in the presence and absence of antisera raised against cytosolic liver fatty acid binding protein (L-FABP). Results In the absence of L-FABP antisera, using an unbound ligand concentration that was adjusted to remain constant at each BSA concentration, hepatocyte [ 3 H]PA and 12-NBDS uptake rates increased linearly with an increase in BSA concentration ( p < 0.0001). In the presence of L-FABP antisera, [ 3 H]PA uptake showed a greater reduction in the presence of 100 μM BSA than 5 μM BSA. The calculated permeability surface area product (PS) confirmed that both unbound and bound fractions of LCFA contributed to the overall flux, but only the PS for the protein-bound fraction was reduced in the presence of L-FABP antisera. In situ rat liver perfusion studies showed that the only rate process for the disposition of [ 3 H]PA in the liver inhibited by L-FABP antisera was that for influx, as defined by PS, and that it reduced PS in the perfused liver by 42%. Conclusion These results suggest that, at physiological albumin concentrations, most of the LCFA uptake is mediated from that bound to albumin by a hepatocyte basolateral membrane transport protein, and uptake of unbound LCFA occurring by passive diffusion contributes a minor component.