Concentration dependence of the initial (glucose-independent) uptake of isoleucine into isolated P falciparum trophozoites at 20°C. The uptake of [¹⁴C]isoleucine was measured at 15 seconds over an extracellular concentration range of 0.05 to 10 mM unlabeled isoleucine. Protein synthesis was inhibited by the addition of cycloheximide (40 μM) and anisomycin (150 μM). Symbols (•) show data averaged from 5 separate experiments, ± SEM, and the curve (solid line) was drawn using the equation υ = V_max[ile]/(K_m + [ile]) + k_d[ile], with K_m = 550 ± 160 μM, V_max = 2.1 ± 0.4 mmol/(10¹² cells · hour), and k_d = 0.3 ± 0.1 L/(10¹² cells · hour), fitted to the data by nonlinear regression. The 2 broken lines show the saturable component (V_max[ile]/(K_m + [ile]); dashed line) and the nonsaturable component (k_d[ile]; dotted line) of isoleucine uptake, separately. The inset shows the same data, magnified over the low end of the isoleucine concentration range, with the shaded bar indicating the normal isoleucine concentration range in human plasma (32-90 μM).¹⁹  Over this concentration range isoleucine uptake was predominantly via the saturable pathway.