Figure 4. Effect of pH on the initial rate of l-tryptophan uptake by brush border (A) and basal (B) membrane vesicles.

Membrane vesicles were suspended in a medium containing 0.1 mm MgSO₄, 300 mmd-mannitol and 2 mm Tris-Hepes (pH 7.5). The uptake was initiated by adding 20 μl of the membrane suspension to 90 μl of an incubation medium composed of 2.45 μml-[³H]tryptophan, 0.1 mm MgSO₄, 300 mmd-mannitol and either 20 mm Mes-Tris (pH 5.5-7.0) or 20 mm Tris-Hepes (pH 7.0-8.5). •, Mes-Tris; ○, Tris-Hepes. Data represent the means ±s.d. of three separate experiments with triplicate assays from three different placentae.