Human erythrocyte ammonium transport is mediated by functional interaction of ammonium (RhAG) and anion (AE1) transporters

Abstract

The ammonia/ammonium (NH3/NH 4 + ) influx into red blood cells (RBCs) is mediated by surface glycoprotein RhAG that forms a structural complex with anion exchanger 1 (AE1, band 3). Owing to the activity of this complex, RBCs exposed to the isosmotic ammonium buffer swell and finally lyse. Isoosmotic NH 4 + -containing media alters the pH gradient in RBCs (intracellular alkalosis in response to NH3/NH 4 + influx) and triggers the AE1 activity resulting in redundant chloride and water influx and finally in cell swelling. Here we demonstrate that the ammonia/ammonium transport in human RBCs depends on the pH (pH optimum 7.4 ± 0.1), temperature (Q10 2.6 ± 0.3), HCO 3 − concentration (EC50 4.7 ± 0.3 mM), and AE1 function. The data confirm functional interactions between AE1 and RhAG. The initial velocity of cell swelling increased almost 50-fold in the isosmotic ammonium buffer containing 25 mM HCO 3 − (37°C) in comparison to the reaction in the same buffer without HCO 3 − . This indicates that the reaction is facilitated mostly by the carrier proteins, not just owing to the simple diffusion of NH3 across the erythrocyte membrane. We demonstrate that pHi reaches its maximum value much faster than the volume increase does. These data suggest that there is no direct correlation between pHi changes and the influx of NH3/NH 4 + . Taken together, our data show that the RhAG and AE1 complex activity enables erythrocytes to be ammonia/ammonium storage sites in order to maintain the physiological blood ammonia/ammonium equilibrium.