A new Na+-independent transport system for dipolar amino acids apparently corresponding to systems persisting after erythrocyte maturation in some mammalian genotypes.

On the basis of its absence in transport-deficient phenotypes, Young and his associates have discovered in the erythrocytes of sheep and horses a type of Na+-independent transport system that resembles Na+-dependent ASC somewhat in its amino-acid selectivity. These authors have stressed this similarity and accordingly suggested that System ASC might have lost its Na+-dependence in the course of the maturation of the erythrocyte, thus to generate a system of the new type, except in the phenotypes showing the transport-deficient erythrocytes.'*2 We have argued, however, that the cosubstrate Na+ plays a decisive role in generating and determining the recognition site of System ASC for the amino acid; hence that system could hardly retain its amino-acid selectivity in the absence of Na+.' We find it more likely that amino-acid binding not only would lose its pattern of selectivity among the amino acids, but would, in the absence of Na+, disappear entirely. Whichever of these alternative possibilities pertains to the life history of this transport system, the mutually accepted provisional designation, System asc, serves to recognize the similarities to System ASC, and by a recently accepted convention: to take note by its lower case letters of its Na+independence. In the course of studying the heterogeneity of the Na'-independent uptake of amino acids by the nucleated erythrocyte of the pigeon, we can summarize as follows: (1) These features assist in discriminating the new Na+-independent System asc from the classical System L: (a) The time-courses of the Na+-independent uptake of serine, threonine, cysteine, and alanine are very different (see Fig. 1, in Vadagama and Christensen') from the time-courses observed for the uptake of leucine, tryptophan, and the analogue 3-endoaminobicyclo [3.2.l]octane-3-carboxylic acid (BCO). (b) During the 10-minute uptake of threonine, System asc dominates, and leucine and BCO as substrates of System L cause only partial inhibition, whereas threonine, cysteine, serine, and alanine show an apparent tendency to saturate threonine inhibition completely (see Fig. 4 in Vadgama and Christensen'). These properties allowed us to discriminate for several amino acids their components of transport dominated by the new System asc, or by theclassical System L (FIG. 1). (c) System asc was readily inhibited by N-ethylmaleimide, whereas System L is relatively insensitive (FIG. 2).