Presence of X-AG carrier in frog (Rana esculenta) red blood cells

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 (Km = 3 microM) and low capacity (Vmax 0.4 mumol 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 (Km 39 microM) but high capacity (Vmax 1.8 mumol L-Glu.Kg-1 dry cell mass.10 min-1).