Recombinant hybrid toxin with dual enzymatic activities. Potential use in preparing highly effective immunotoxins

Bacterial toxins and ribosomal inhibitory proteins isolated from plants are used to prepare tumor-specific cytotoxic conjugates. The ability of these conjugates to kill tumor cells depends on binding, internalization, translocation to cytoplasm, and translation inhibition. Modulation of any one of these processes can improve cytotoxicity. Since bacterial and plant toxins act at a distinct step in translation, a combination of their activities could be more effective. Therefore, a chimeric protein was prepared by genetically fusing the coding region of the ricin A chain (RTA) and the fragment A of diphtheria toxin (DTA). The hybrid protein (RTA-DTA) expressed in bacteria retained the N-glycosidase activity of the RTA and ADP- ribosylation activity of the DTA. The hybrid toxin was more potent than the ricin A chain (11-fold) and the diphtheria toxin (50-fold) in inhibiting cell-free translation. Immunotoxin made with the hybrid toxin was about 100- and 1000-fold more effective than RTA or DTA conjugate, respectively, in inhibiting tumor cell growth in vitro. These results indicate that the hybrid toxin with dual activities could be useful in preparing potent immunotoxins with better anti-tumor cell activity.