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System y+L: the broad scope and cation modulated amino acid transporter
Author(s) -
Deves R,
Angelo S,
Rojas AM
Publication year - 1998
Publication title -
experimental physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.925
H-Index - 101
eISSN - 1469-445X
pISSN - 0958-0670
DOI - 10.1113/expphysiol.1998.sp004105
Subject(s) - amino acid , chemistry , stereochemistry , leucine , alanine , cationic polymerization , serine , xenopus , transporter , amino acid transporter , lysine , biochemistry , biophysics , enzyme , biology , organic chemistry , gene
The properties are discussed of system y+L, a broad scope amino acid transporter which was first identified in human erythrocytes. System y+L exhibits two distinctive properties: (a) it can bind and translocate cationic and neutral amino acids, and (b) its specificity varies depending on the ionic composition of the medium. In Na+ medium, the half‐saturation constant for L‐lysine influx was 9.5 +/− 0.67 microM and the inhibition constant (Ki) for L‐leucine was 10.7 +/− 0.72 microM. L‐Leucine is the neutral amino acid that binds more powerfully, whereas smaller analogues, such as L‐alanine and L‐serine interact less strongly (the corresponding inhibition constants were Ki,Ala, 0.62 +/− 0.11 mM; Ki,Ser, 0.49 +/− 0.08 mM). In the presence of K+, the carrier functions as a cationic amino acid specific carrier, but Li+ is able to substitute for Na+ facilitating neutral amino acid binding. The effect of the inorganic cations is restricted to the recognition of neutral amino acids; translocation occurs at similar rates in the presence of Na+, K+ and Li+. The only structural feature that appears to impair translocation is bulkiness and substrates with half‐saturation constants differing by more than 100‐fold translocate at the same rate. This suggests that translocation is largely independent of the forces of interaction between the substrate and the carrier site. System y+L activity has been observed in Xenopus laevis oocytes injected with the cRNA for the heavy chain of the 4F2 human surface antigen. 4F2hc is an integral membrane protein with a single putative membrane‐spanning domain and it remains to be clarified whether it is part of the transporter or an activator protein.