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-tryptophan. When the uptake rates of these amino acids were determined in an incubation medium containing BSA and at the same free-form substrate concentration, the rate of l-tryptophan uptake increased, while the rates of l-phenylalanine and l-leucine uptakes remained unchanged. l-Tryptophan was well bound to BSA, while l-phenylalanine and l-leucine were scarcely bound to the protein. BSA was clearly bound to the incubated cells. From the kinetic analysis of l-tryptophan uptake by hepatocytes with and without BSA, BSA was found to enhance the affinity between l-tryptophan and its transport site on the surface of liver cells. In hepatocytes pretreated with BSA, only l-tryptophan uptake was stimulated, in which the affinity between l-tryptophan and its transport site was enhanced. A significant amount of BSA was bound to the cells pretreated with the protein. When the effects of BSA co-addition and its pretreatment on the binding of l-tryptophan to plasma membranes prepared 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 by 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.
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