Zero-trans uptake of L-tryptophan in the human erythrocyte

Some results from ongoing studies of the transport of some neutral amino acids across the human erythrocyte membrane (L-tryptophan and L-leucine) at 25 degrees C are presented. The unidirectional flux into the cell at initial rate as a function of the substrate concentration can be described by a 3-parameter model (i.e. a carrier in parallel with an apparent leak component). From the estimated kinetic parameters an apparent membrane permeability of the carrier (substrate concentrations S1 smaller than Kzt 12) has been calculated. The permeability of L-tryptophan (1.03 . 10(-8) cm/sec) was significantly lower than the permeability of L-leucine (2.36 . 10(-7) cm/sec). The contribution of the leak permeability to the total permeability at low concentrations was about 10% for L-tryptophan, but only 1% for L-leucine. From inhibition studies it is indicated that L-tryptophan passing the membrane via theapparent leak component is transported by the L-leucine carrier, the affinity being low. The saturable component of L-tryptophan transport does not correspond to any known amino acid transport system, and it is thus strongly suggested that a new L-tryptophan specific transport system operating at physyological concentrations has been described.