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Updated: Aug 26, 2026

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Evidence for distinct cellular internalization pathways of ricin and nigrin b
1Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid, Spain.
Abstract:
Nigrin b and ricin are type 2 (two chain) ribosome-inactivating proteins that exhibited nearly the same strong inhibitory activity on cell-free protein synthesis. Incubation of HeLa cells for 6 hr with ricin at 37 degrees C promoted protein synthesis inhibition with an IC50 of 0.2 ng/ml. Incubation of the cells for 6 hr at 18 degrees C abolished completely the inhibition. Incubation of HeLa cells with nigrin b for 6 hr at 37 degrees C was nearly 10(5) times less inhibitory than ricin. In contrast to the effects observed with ricin, incubation of HeLa cells with nigrin b at 18 degrees C slightly increased the inhibitory action on protein synthesis as compared with incubation at 37 degrees C. These results strongly support the hypothesis that the internalization of ricin and nigrin b could involve different receptors and therefore they could follow different intracellular pathways.
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