title: Membrane binding proteins are the major determinants for the hepatocellular transmembrane flux of long-chain fatty acids bound to albumin.
abstract: OBJECTIVE
  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 muM BSA than 5 muM 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%.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Faculty of Pharmacy, University of Manitoba, Winnipeg, Manitoba, Canada.
authors.forename: G
authors.initials: G
authors.lastname: Rajaraman
authors.forename: M S
authors.initials: MS
authors.lastname: Roberts
authors.forename: D
authors.initials: D
authors.lastname: Hung
authors.forename: G Q
authors.initials: GQ
authors.lastname: Wang
authors.forename: F J
authors.initials: FJ
authors.lastname: Burczynski
mesh_terms: D000418:Albumins / Q000378:metabolism*; D000818:Animals; D006528:Carcinoma, Hepatocellular / Q000378:metabolism; D045744:Cell Line, Tumor; D050556:Fatty Acid-Binding Proteins / Q000502:physiology; D005227:Fatty Acids / Q000378:metabolism*; D005260:Female; D022781:Hepatocytes / Q000378:metabolism*; D008099:Liver / Q000378:metabolism; D008297:Male; D026901:Membrane Transport Proteins / Q000502:physiology*; D019308:Palmitic Acid / Q000378:metabolism; D011485:Protein Binding; D051381:Rats; D017207:Rats, Sprague-Dawley; D017208:Rats, Wistar
chemical_list: D000418:Albumins; D050556:Fatty Acid-Binding Proteins; D005227:Fatty Acids; D026901:Membrane Transport Proteins; D019308:Palmitic Acid