Figure 3. Dose-response curves of suramin and natural product extract 6990.

Nitrocellulose filter binding assays (closed symbols) and fluorescence polarization (FP) (open symbols) were used to measure the ability of either suramin (A; circles) or natural product extract 6990 (B; triangles) to inhibit binding of aptamer RNA to N. The competition assays involved incubating a fixed concentration of N with varying concentrations of compound and either fluorescent or radiolabeled RNA. For the filter binding assay the amount of radiolabeled (³²P-UTP) aptamer RNA retained on filters was measured by liquid scintillation counting and the percent RNA bound is plotted versus compound or natural product extract concentration. The fluorescence polarization experiments used fluorescently labeled aptamer RNA and the FP signal (mP) is plotted versus compound or natural product extract concentration. The IC₅₀ value calculated for suramin using a filter binding assay (A; closed circles) was 11.3 µM and using FP (A; open circles) was 16.3 µM. The IC50 value calculated for natural product extract 6990 using a filter binding assay (B; closed triangles) was .038 ng/µL and using FP (B; open triangles) was .015 ng/µL. Similar experiments were conducted to calculate IC₅₀ values for the other unique compounds and natural product extract (Table 2). The results are the average of at least two independent experiments.