Interaction between parallel transport systems examined with tryptophan and related amino acids.

Most of the neutral amino acids are transported across the plasma membrane by two or more parallel transport systems. In this study, we have limited transport interactions to System A and L by studying amino acids not transported by System ASC. Results are presented to emphasize that such amino acids as leucine, phenylalanine and tryptophan do enter the Ehrlich cell to substantial degrees by System A. We show how the phenomenon of competitive stimulation presents a strong argument that these systems operate between the same two compartments, extracellular and cellular. To discover what is needed to bring two amino acids into the transport relation shown by tryptophan and methionine, we arbitrarily set up two classes of amino acids: (1) those whose steady-state gradients will be increased when System L is deleted; (2) those whose gradients will instead be decreased. On blockading System L with 2-aminorbornane-2-carboxylic acid applied in symmetry to the two sides of the plasma membrane, we show as predicted that the gradient maintained for methionine is strongly increased, that for trypotphan decreased. The initial rate of uptake of all the amino acids mentioned is highly sensitive to the nature of the cellular amino acid pool. A high influx by exchange, relative to net influx, appears to characterize the amino acids that respond to competitive stimulation. Albumin-bound tryptophan appears not directly accessible to the tested carriers.