Kinetic characterization of a carrier-mediated transport system for L-tryptophan in human blood platelets

We have characterized a membrane transport system on washed human blood platelets for tritiated L-tryptophan (L-TRP). This transport was extremely rapid, temperature dependent and markedly reduced by disruption of the platelet membranes. Kinetic studies within a large range of L-TRP concentrations have revealed the presence of a high affinity saturable transport system which follows simple Michaelis-Menten kinetics, with an apparent Km value of 10 microM and a Vmax of 200 pmol/min/10(8) cells. Platelet L-TRP accumulation was insensitive to changes in sodium concentrations and to the inclusion of ouabain in the incubation medium. Furthermore, uptake was unmodified by the presence of the metabolic inhibitor dinitrophenol, suggesting that it is mediated by facilitated diffusion. D-Tryptophan was a very poor inhibitor of L-TRP uptake. Transport was insensitive to serotonin and imipramine but was inhibited in a dose-dependent manner by L-tyrosine, L-phenylalanine and L-leucine, implying that it may be mediated by a system that is specific for aromatic and long chain amino acids. The results were compared to reports examining the L-TRP transport in other cell types.