Presence of an X ⁻_AG carrier in frog (Rana esculenta) red blood cells

Abstract

Evidence is presented that the high levels of internal l-glutamic and l-aspartic acid in frog Rana esculenta red blood cells are due to the existence of a specific carrier for acidic amino acids of high affinity K _m = 3 μm and low capacity (V_max) 0.4 μmol l-Glu · Kg⁻¹ dry cell mass · 10 min⁻¹. It is Na+ dependent and the incorporation of l-glutamic acid can be inhibited by l and d-aspartate and l-cysteic acid, while d-glutamic does not inhibit. Moreover, this glutamic uptake shows a bell-shaped dependence on the external pH. All these properties show that this carrier belongs to the system X ⁻_AG family. Besides the incorporation through this system, l-glutamic acid is also taken up through the ASC system, although, under physiological conditions, this transport is far less important, since it has relatively low affinity K _m 39 μm but high capacity (V _max) 1.8 μmol l-Glu · Kg⁻¹ dry cell mass · 10 min⁻¹.