FIGURE 3.

Correlated internalization and cytotoxicity measurements indicate that a precise number of rGel molecules must be internalized by a single cell before cytotoxicity is observed. A, time and concentration dependence of rGel internalization by HT-1080 cells using the described quantitative internalization flow cytometry assay. B, concentration- and exposure-matched cytotoxicity was measured using the WST-1 assay. C, data from A and B were combined and plotted to show the dependence of cytotoxicity on the number of internalized gelonin molecules, resulting in the determination of the TN₅₀ near 5 × 10⁶.