Concentration dependence of the inhibition by leucine and isoleucine of the initial (glucose-independent) uptake of [¹⁴C]isoleucine in isolated P falciparum trophozoites, at 20°C. The uptake of [¹⁴C]isoleucine was measured over 15 seconds. The extracellular concentration of leucine (○) or isoleucine (•) ranged from 0.1 to 10 mM. Protein synthesis was inhibited by the addition of cycloheximide (40 μM) and anisomycin (150 μM). Both curves were drawn using the following equation: distribution ratio at 15 seconds = AB/(B + [amino acid]) + C fitted to the averaged data by nonlinear least-squares regression. For the inhibition of [¹⁴C]isoleucine uptake by leucine: A = 0.89 ± 0.03 (15 seconds)⁻¹ B = 1.2 ± 0.1 mM, and C = 0.012 ± 0.002 (15 seconds)⁻¹ (n = 4), respectively. For the inhibition of [¹⁴C]isoleucine uptake by unlabeled isoleucine: A = 0.76 ± 0.03 (15 seconds)⁻¹ B = 0.190 ± 0.001 mM, and C = 0.08 ± 0.01 (15 seconds)⁻¹ (n = 5), respectively. The leucine data are averaged from 4 separate experiments performed on different days and are shown ± SEM. The isoleucine data are derived from those of Figure 3.