The Binding Specificity of Amino Acid Transport System y+L in Human Erythrocytes is Altered by Monovalent Cations |
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Authors: | S Angelo C Irarrázabal R Devés |
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Institution: | (1) Department of Physiology and Biophysics, Faculty of Medicine, University of Chile, Santiago, Chile, CL |
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Abstract: | System y+L is a broad-scope amino acid transporter which binds and translocates cationic and neutral amino acids. Na+ replacement with K+ does not affect lysine transport, but markedly decreases the affinity of the transporter for l-leucine and l-glutamine. This observation suggests that the specificity of system y+L varies depending on the ionic composition of the medium. Here we have studied the interaction of the carrier with various
amino acids in the presence of Na+, K+, Li+ and 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 powerfully. Elongation of the side chain (glycine - l-norleucine) strengthened binding. In contrast, bulkiness at the level of the β carbon was detrimental. In K+, the carrier behaved as a cationic amino acid specific carrier, interacting weakly with neutral amino acids. Li+ was found to potentiate neutral amino acid binding and in general the apparent affinities were higher than in Na+; elongation of the nonpolar side chain made a more important contribution to binding and the carrier was more tolerant towards
β carbon substitution. Guanidinium stimulated the interaction of the carrier with neutral amino acids, but the effect was
restricted to certain analogues (e.g., l-leucine, l-glutamine, l-methionine). Thus, in the presence of guanidinium, the carrier discriminates sharply among different neutral amino acids.
The results suggest that the monovalent cations stabilize different carrier conformations.
Received: 22 January 1996/Revised: 26 April 1996 |
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Keywords: | : Lysine — Transport — Amino acids — Carrier — Specificity — System y+L |
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