Choline influx into P knowlesi-infected erythrocytes as a function of choline concentration in the presence of E4 (A), E6 (B), E7 (C), or E8 (D). Infected erythrocyte suspensions (98% parasitized, trophozoite stage) with 5.2 × 107cells/100 μL (0.7% final hematocrit) were preincubated for 5 minutes at 37°C in the presence of 750 μL special RPMI containing the drug (solid symbols) at 3 μmol/L (A and B) or 6 μmol/L (C and D) or no (open symbols) drug. Choline influx measurement was initiated by the rapid addition of 50 μL (3H)choline (specific activity, 0.2 Ci/mmol) at the indicated concentration. After 6 minutes of incubation at 37°C, the flux was stopped by adding 2.5 mL of ice-cold special RPMI, followed by centrifugation of a 1-mL aliquot through n-dibutyl phtalate at 4°C as described in the Materials and Methods. Nonspecific choline transport was determined in the presence of 1.2 mmol/L choline under the same conditions. The results are expressed by the double-reciprocal plot of the initial velocity of choline influx into infected erythrocytes (expressed as nanomoles per 1010 infected cells per minute) ± SEM. Each point is the mean of triplicate determinations in one typical experiment.
