Differential entry of ricin into malignant and normal rat hepatocytes.

We have compared the mechanisms of ricin binding to and entry into Zajdela hepatoma cells (ZHC) and normal rat hepatocytes (HyC). Lactose but not mannan was found to inhibit ricin binding to and toxicity on ZHC and HyC. This finding suggests that ricin binding, entry, and toxicity are expressed only through the galactose binding sites on ZHC and HyC. Nevertheless, the characteristics of ricin binding and its entry pathway appeared to be different in several respects in ZHC and HyC. Scatchard analysis of equilibrium data determined over a wide range of 125I-labeled ricin concentrations yielded a curvilinear plot for ZHC, while a straight line was obtained for HyC. These results indicate that only ZHC possess high-affinity receptors for ricin. Analysis of ricin toxicity on ZHC and HyC, in the presence of ammonium chloride or after K+-depletion in both cell types, suggests that the ricin bound to galactose receptors entered through neutral vesicles in ZHC, and through both neutral and acidic vesicles in HyC. The qualitative and quantitative differences found between the process of receptor-mediated endocytosis of ricin in ZHC and HyC might explain the differential sensitivity of the two cell types toward the toxin.