Mode of L-tryptophan uptake into rat hepatocytes via trypsin-sensitive high-affinity transport system

The effect of trypsin treatment on the mode of L-tryptophan uptake (L- and T-transport systems) into hepatocytes was investigated in comparison with that of L-leucine (L-type system) using isolated rat hepatocytes. L-Tryptophan uptake occurred via at least two saturable (one high-affinity and one low-affinity) and one non-saturable components in intact hepatocytes, while L-leucine was taken up via one saturable and one non-saturable component. Hepatocytes whose surface had been digested with trypsin lost only the high-affinity component of the two saturable L-tryptophan transport systems, while they retained the saturable component for L-leucine transport (Km = 3.3 mM). Km values for the trypsin-sensitive (high-affinity) and -insensitive (low-affinity) components of L-tryptophan uptake were 14 microM and 2.1 mM, respectively. However, inhibition studies with some substrates of the L-, T- and both transport systems as inhibitor demonstrated that L-tryptophan uptake via these two components could not be classified in terms of the T- and L-systems. The present results indicate the presence of a carrier with a very high affinity for L-tryptophan in rat hepatocytes.