Toxicity of ricin, diphtheria toxin and alpha-Amanitin for Acanthamoeba castellanii (1983)

Selective cytotoxic agents, highly specific antibody coupled to potent toxin molecules, could, theoretically, be useful in the treatment of protozoan infections. To examine this possibility we began to synthesize immunotoxins for a model protozoan system, Acanthamoeba castellanii. We report here the selection of a suitable toxic moiety for this system. alpha-Amanitin was toxic for the amoeba, effecting a 50% decrease in in vivo protein synthesis at approximately 20 microM. However, the chemical modification of alpha-amanitin necessary for its covalent attachment to antibody molecules reduced A. castellanii toxicity to the extent that alpha-amanitin is unsuitable as a toxic moiety in the synthesis of A. castellanii immunotoxins. Ricin and diphtheria toxin were non-toxic for the amoeba. In addition, A. castellanii cell-free protein biosynthesis, unlike that of any other eukaryotic system examined to date, was resistant to inhibition by ricin A chain. However, diphtheria toxin A chain inhibited A. castellanii cell-free protein synthesis by 50% at 2.5 nM. The inhibition of diphtheria toxin was NAD+ dependent, suggesting that ADP-ribosylation of EF-2 could be the cause of the inhibition as it is in mammalian cell lines. The toxicity of diphtheria toxin A chain is sufficient for its use in the synthesis of immunotoxins for A. castellanii.