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 [H-3] palmitate ([H-3] PA) and 12-(N-methyl)-N-[(7-nitrobenz-2-oxa-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 [H-3] PA and 12- NBDS uptake rates increased linearly with an increase in BSA concentration ( p < 0.0001). In the presence of L-FABP antisera, [H-3] PA uptake showed a greater reduction in the presence of 100 mu M BSA than 5 mu M 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 [H-3] 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.