Inhibition of Translation in Eukaryotic Systems by Harringtonine

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Abstract
The Cephalotaxus alkaloids harringtonine, homoharringtonine and isoharringtonine inhibit protein synthesis in eukaryotic cells. The alkaloids do not inhibit, in model systems, any of the steps of the initiation process but block poly(U)-directed polyphenylalanine synthesis as well as peptide bond formation in the fragment reaction assay, the sparsomycin-induced binding of (C)U-A-C-C-A-[3H]Leu-Ac, and the enzymic and the non-enzymic binding of Phe-tRNA to ribosomes. These results suggest that the Cephalotaxus alkaloids inhibit the elongation phase of translation by preventing substrate binding to the acceptor site on the 60-S ribosome subunit and therefore block aminoacyl-tRNA binding and peptide bond formation. However, the Cephalotaxus alkaloids do not inhibit polypeptide synthesis and peptidyl-[3H]puromycin formation in polysomes. Furthermore, these alkaloids strongly inhibit [14C]trichodermin binding to free ribosomes but hardly affect the interaction of the antibiotic with yeast polysomes. These results clearly suggest that the Cephalotaxus alkaloids cannot interact with polysomes and therefore only inhibit the initial cycles of elongation. This explains the polysome run off that has been observed by some workers in the presence of harringtonine.

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