Concentration dependence of the second (glucose-dependent) component of the uptake of isoleucine into isolated P falciparum trophozoites at 20°C. The uptake of [¹⁴C]isoleucine (0.04 μM) was measured at 3 minutes over an extracellular concentration range of 0.5 to 5 μM unlabeled isoleucine. Protein synthesis was inhibited by the addition of cycloheximide (40 μM) and anisomycin (150 μM). The equation υ = V_max[ile]/(K_m + [ile]) + k_d[ile] was used to resolve the uptake data into a linear component (corresponding to the initial, glucose-independent transport of isoleucine into the parasite, which, with a K_m of 550 μM, shows an effectively linear dependence on isoleucine concentration over the concentration range used here) and a saturable component (ie, the second, glucose-dependent component of isoleucine uptake). The linear (glucose-independent) component (k_d[ile]) was subtracted from the data to give the concentration dependence of the second (glucose-dependent) component of isoleucine uptake. Symbols (•) show data averaged from 4 separate experiments, ± SEM, and the curve was fitted using the Michaelis-Menten equation. The apparent K_m and V_max values for the glucose-dependent uptake of isoleucine are 0.93 ± 0.26 μM and 1.73 ± 0.25 μmol/(10¹² cells · hour), respectively.