FACILITATING EFFECT OF ALBUMIN ON L-TRYPTOPHAN TRANSPORT INTO ISOLATED RAT HEPATOCYTES

The direct role of albumin in L-tryptophan transport into hepatocytes was studied by examining the effect of bovine serum albumin (BSA) on L -tryptophan uptake by isolated rat hepatocytes in comparison with that on the uptake of L-phenylalanine and L-leucine, which are known to be taken up into hepatocytes via the transport systems common to L-trypt ophan. When the uptake rates of these amino acids were determined in a n incubation medium containing BSA and at the same free-form substrate concentration, the rate of L-tryptophan uptake increased, while the r ates Of L-phenylalanine and L-leucine uptakes remained unchanged. L-Tr yptophan was well bound to BSA, while L-phenylalanine and L-leucine we re scarcely bound to the protein. BSA was clearly bound to the incubat ed cells. From the kinetic analysis of L-tryptophan uptake by hepatocy tes with and without BSA, BSA was found to enhance the affinity betwee n L-tryptophan and its transport site on the surface of liver cells. I n hepatocytes pretreated with BSA, only L-tryptophan uptake was stimul ated, in which the affinity between L-tryptophan and its transport sit e was enhanced. A significant amount of BSA was bound to the cells pre treated with the protein. When the effects of BSA co-addition and its pretreatment on the binding of L-tryptophan to plasma membranes prepar ed from rat livers were examined, the protein was found to promote the binding Of L-tryptophan to the membranes. The present results suggest that albumin can facilitate L-tryptophan transport into hepatocytes b y enhancing the affinity between the amino acid and its transport site present on the cell surface through its direct interaction with both the amino acid and the plasma membrane.