THE BINDING-SPECIFICITY OF AMINO-ACID-TRANSPORT SYSTEM Y(+)L IN HUMANERYTHROCYTES IS ALTERED BY MONOVALENT CATIONS

System y(+)L is a broad-scope amino acid transporter which binds and t ranslocates cationic and neutral amino acids. Na+ replacement with Kdoes not affect lysine transport, but markedly decreases the affinity of the transporter for L-leucine and L-glutamine. This observation sug gests that the specificity of system y(+)L varies depending on the ion ic composition of the medium. Here we have studied the interaction of the carrier with various amino acids in the presence of Na+, K+, Li+ a nd guanidinium ion. In agreement with the prediction, the specificity of system y(+)L was altered by the monovalent cations. In the presence of Na+, L-leucine was the neutral amino acid that interacted more pow erfully. Elongation of the side chain (glycine-L-norleucine) strengthe ned binding. In contrast, bulkiness at the level of the beta carbon wa s detrimental. In K+, the carrier behaved as a cationic amino acid spe cific carrier, interacting weakly with neutral amino acids. Li+ was fo und to potentiate neutral amino acid binding and in general the appare nt affinities were higher than in Na+; elongation of the nonpolar side chain made a more important contribution to binding and the carrier w as more tolerant towards beta carbon substitution. Guanidinium stimula ted the interaction of the carrier with neutral amino acids, but the e ffect was restricted to certain analogues (e.g., L-leucine, L-glutamin e, L-methionine). Thus, in the presence of guanidinium, the carrier di scriminates sharply among different neutral amino acids. The results s uggest that the monovalent cations stabilize different carrier conform ations.