Kinetics of choline uptake. (A) Time course of [³H]-choline uptake into free parasites of P falciparum. Uptake is measured at 22°C (○) and 0°C (•). Data points are means of duplicate observations from 2 individual experiments. (B) Concentration dependence of choline uptake into free parasites of P falciparum. Data points are means of duplicate observations from 2 individual experiments. Data were fitted to a function describing simple ligand binding at a single site by nonlinear regression analysis (Marquart method) using an iterative procedure to generate the best fit (χ²) of the curve to the data. Standard errors were calculated for each parameter using the matrix inversion method (Grafit user manual). (C) Hill plot of the data presented in panel B.