Figure 1.

Suramin is taken up rapidly and accumulates to high intracellular concentrations. Shown are scintillation counts from 1 × 10⁷ cells incubated in 200 nm [³H]suramin over 15 min and surface binding cold-chased. Bars, mean of three independent experiments for WT cells (wt), ISG75-overexpressing cells (ISG75O.E.), and ISG75- and ISG65-silenced cells (ISG75RNAi and ISG65RNAi), respectively, with the S.E. (error bars) and significance intervals from Student's unpaired t test (*, p < 0.05; ***, p < 0.001) indicated. The intracellular suramin concentration was calculated using a volume of 29 μm³ for T. brucei BSF (33). Inset, suramin uptake in the presence of 250 nm suramin monitored over 1 h (red line). Suramin is taken up in two phases, a rapid initial phase over 20 min with 19.0 pmol/min (linear regression shown as a black dashed line; r² = 0.97; p = 0.002, nonzero slope) and a slower phase (20–60 min) with 4.5 pmol/min (linear regression shown as a dark blue dashed line; r² = 0.99; p = 0.048, nonzero slope). Saturability of uptake was determined by incubation of cells with 250 nm [³H]suramin in the presence of 100 μm unlabeled suramin (p = 0.48, zero slope) (green line).