FIGURE 2.

Substrate specificity of Glt_Ph. A, l-[³H]aspartate transport (●) by Glt_Ph in the presence of an inwardly directed Na⁺ gradient at symmetrical pH 7.5. l-[³H]Glutamate uptake (○) in these conditions overlays that measured for aspartate in the absence of a Na⁺ gradient (▾). B, initial rate of uptake as a function of l-[³H]aspartate. Data fit to the Hill equation yield a K_m of 120 ± 30 nm, a V_max of 3.7 ± 0.3 nmol/mg protein/min, and a Hill coefficient of 1.0 ± 0.1. Same data are plotted as a Haynes plot in the inset. C, initial rates of uptake in the presence of 100 nm [³H]aspartate (normalized to no addition, n.a.) and 10 μm unlabeled competitor as indicated on the y axis. All amino acids were l-isomers except *D (d-aspartate). 1, TBOA; 2, cysteate; 3, serine O-sulfate; 4, kainate; 5, dihydrokainate; 6, (±)-threo-3-methylglutamate; 7, (2S,4R)-4-methylglutamate; 8, homocysteate.