RECOMBINANT HYBRID TOXIN WITH DUAL ENZYMATIC-ACTIVITIES - POTENTIAL USE IN PREPARING HIGHLY EFFECTIVE IMMUNOTOXINS

Bacterial toxins and ribosomal inhibitory proteins isolated from plant s are used to prepare tumor-specific cytotoxic conjugates. The ability of these conjugates to kill tumor cells depends on binding, internali zation, translocation to cytoplasm, and translation inhibition. Modula tion of any one of these processes can improve cytotoxicity. Since bac terial and plant toxins act at a distinct step in translation, a combi nation of their activities could be more effective. Therefore, a chime ric protein was prepared by genetically fusing the coding region of th e ricin A chain (RTA) and the fragment A of diphtheria toxin (DTA). Th e hybrid protein (RTA-DTA) expressed in bacteria retained the N-glycos idase activity of the RTA and ADP-ribosylation activity of the DTA. Th e hybrid toxin was more potent than the ricin A chain (11-fold) and th e diphtheria toxin (50-fold) in inhibiting cell-free translation. Immu notoxin made with the hybrid toxin was about 100- and 1000-fold more e ffective than RTA or DTA conjugate, respectively, in inhibiting tumor cell growth in vitro. These results indicate that the hybrid toxin wit h dual activities could be useful in preparing potent immunotoxins wit h better anti-tumor cell activity.