Identification of a new transport system (y+L) in human erythrocytes that recognizes lysine and leucine with high affinity

Abstract

1. The effect of neutral amino acids on the transport of L-lysine across the human erythrocyte membrane was studied. 2. All neutral amino acids tested (range 0.3-5 mM) inhibit the influx of L-[14C]lysine (1 microM). The inhibition pattern is biphasic, and tends to reach a maximum at approximately 50% of the original flux. The concentrations that give 25% inhibition are (mM): L-cysteine (2.7), L-alanine (1.3), L-serine (0.9), L-isoleucine (0.6), L-phenylalanine (0.35), L-methionine (< 0.3), L-leucine (< 0.3). L-lysine and L-arginine completely inhibit the rate at the highest concentration. 3. These results can be explained by assuming that L-lysine transport occurs through two independent transporters that differ in their affinity for neutral amino acids. A detailed kinetic analysis of the effect of L-leucine on L-lysine entry is consistent with this hypothesis. 4. Using a new experimental strategy, the substrate and inhibitor transport parameters for the two systems were determined. The half-saturation constants for lysine (+/- S.E.M.) are found to be: KmA, 0.014 +/- 0.002 mM and KmB, 0.112 +/- 0.017 mM. The maximum rates differ by a factor of 8.2 (VmaxB/VmaxA). The leucine inhibition constants are: KiA, 0.022 +/- 0.003 mM and KiB, 30.36 +/- 7.9 mM. If the sodium in the incubation medium is replaced by potassium, the apparent affinity for leucine (1/KiA) is reduced approximately 30-fold. 5. The maximum inhibition caused by leucine decreases as the lysine concentration is raised, showing that leucine acts upon the higher affinity system. 6. When added to the trans side, L-leucine, L-phenylalanine and L-isoleucine do not cause inhibition, but stimulate the flux by approximately 30%. This indicates that these analogues are also transported. 7. In conclusion, in the concentration range 1-100 microM, lysine crosses the red cell membrane through two distinct transport systems, one of which recognizes both neutral and cationic amino acids with high affinity.