Sodium-dependent cysteine transport in human red blood cells

Abstract

ADEQUATE membrane transport of L-cysteine is necessary for red blood cell (RBC) survival, as this amino acid is essential for glutathione (GSH) synthesis and thus cell protection against oxidative damage. Thus, in sheep a genetically controlled L-cysteine transport defect leads to a reduced intracellular GSH concentration and shortened cell lifespan^1–4. Human RBCs do not have the same amino acid transport system as sheep red blood cells (SRBC)⁵, the principal component being a high capacity, medium affinity system with a specificity broadly directed towards large neutral amino acids, the L-system of Christensen⁶. L-cysteine and its analogue L-α-amino-n-buty-rate are carried by the L-system in human RBCs, but with a low affinity⁷. Most experiments on amino acid transport in RBCs, particularly involving the L-system, have revealed substrate affinities with apparent K_m values in the millimolar range, much higher than the physiological plasma levels of most amino acids. We have now investigated L-cysteine uptake in human RBCs over the concentration range 1–50 µm, as human plasma levels are around 20 µM^8,9. As the early work of Yunis and Arimura¹⁰ has shown that glycine and L-alanine transport are partially Na-dependent, we have also investigated the effects of removing external Na on L-cysteine transport. We report here that human RBCs transport L-cysteine by a previously unidentified high affinity, low capacity, Na-dependent uptake mechanism. This system has a uniquely high affinity for its substrate and is the first Na-dependent transport mechanism to be kinetically characterised in mammalian erythrocytes. At physiological substrate concentrations it accounts for approximately half of the L-cysteine uptake into the cell.