Transport systems for neutral amino acids in the pigeon erythrocyte.

Uptake and inhibition studies show that the uptake of neutral amino acids by the pigeon red blood cell is divided among four systems: a Naf-independent, nonexchanging system operative for amino acids with large hydrocarbon side chains; a Na+-dependent, exchanging system (ASCP) for alanine, serine, cysteine, proline, and probably also threonine and similar amino acids, very similar to systems seen in the Ehrlich cell (ASC) and the rabbit reticulocyte; a system effective for glycine and sarcosine already studied by Vidaver; and a system reactive with p-alanine and taurine but not with cr-amino acids. The transport of valine was shown to be divided between the principal Na+-independent systems; that of glycine, between the glycine system and the ASCP system. These relations were supported by differences in the competitive interactions among the amino acids for transport, by the activity of various amino acids in accelerating the exodus of labeled or endogenous amino acids from the cell by countertransport, which is limited to the ASCP system, and by interactions between amino acids and Na+ and H+. Uptake by the two broad spectrum systems was minimally sensitive to metabolic inhibitors, except for a special sensitivity to anionic sulfhydryl reagents, without, however, an interruption of countertransport. Energy to permit uphill transport of various amino acids may reach the exchanging system by way of the system for glycine. The character of the dependence of the velocity, the K, and the T/‘,,, for amino acid uptake on the Na+ concentration was different among the three Na+-dependent systems.