Characterization of translational inhibitors from Phytolacca americana. Amino-terminal sequence determination and antibody-inhibitor conjugates.

Two translational inhibitors (pokeweed antiviral protein and pokeweed antiviral protein II) isolated from the leaves of the pokeweed plant, Phytolacca americana, were characterized as to their behavior during reverse-phase HPLC and their amino-terminal sequences. Alignment of the sequences demonstrated that a substantial degree of homology was present (10 of 29 identical residues). Pokeweed antiviral protein was shown by reverse-phase chromatography to be composed of at least two components, pokeweed antiviral proteina and pokeweed antiviral proteinb, which comigrated on sodium dodecyl sulfate polyacrylamide gel electrophoresis, shared identical N-terminal amino-acid sequences through residue 31, and had similar specific activities in a cell-free translation inhibition assay. Pokeweed antiviral protein II was covalently coupled to a monoclonal antibody that recognizes the transferrin receptor (anti-transferrin receptor). The disulfide-linked conjugate inhibited protein synthesis in the human breast tumor cell line MCF-7, whereas anti-transferrin receptor, pokeweed antiviral protein II, or an immunotoxin composed of an irrelevant antiserum and pokeweed antiviral protein II, were nontoxic. The inhibitory dose 50% of anti-transferrin receptor-pokeweed antiviral protein II for MCF-7 cells was 0.7 nM, whereas the corresponding ricin A chain conjugate (anti-transferrin receptor-ricin A chain) was more potent with a inhibitory dose 50% of 0.1 nM. Pokeweed antiviral protein II can be added to the growing list of translation inhibitors that are effective as components of immunotoxins in vitro. Additional studies will be needed to determine whether pokeweed antiviral protein II immunotoxins provide advantageous properties for in vivo applications.