A kinetic analysis of L-tryptophan transport in human red blood cells.

1. The initial rate of L-tryptophan transport as a function of the cellular substrate concentration in human red blood cells was studied in zero-trans (= net) efflux and equilibrium-exchange efflux experiments at 25 degrees C, pH 7.4. 2. Efflux curves were resolved into two Michaelis-Menten components. Further evidence was found that L-tryptophan transport at physiological concentrations of substrate is mediated predominantly by a recently identified transport system, designated the T-system. 3. The results from a kinetic analysis according to the theory of Lieb and Stein ((1974) Biochim. Biophys. Acta 373, 178-196) were consistent with the T-system being a carrier-mediated type of transport.