A novel suicide inhibitor strategy for antiparasitic drug development

DL-alpha-Difluoromethylornithine (DFMO), a suicide inhibitor of eukaryotic ornithine decarboxylase (ODC), has therapeutic activities against African trypanosomiasis. The Ki value of DFMO for ODC of Trypanosoma brucei is somewhat higher than that for mouse ODC. The therapeutic efficacy of DFMO cannot therefore be attributed to a preferential inhibition of the parasite enzyme. The T. brucei gene encoding ODC was cloned and sequenced, and the derived amino acid sequence has 61.5% homology with that of mouse ODC, except that the C-terminal 36 amino acids of the mouse enzyme are missing from the parasite enzyme. The cloned T. brucei and mouse ODC genes were expressed in ODC-deficient Chinese hamster ovary cells (CHO) where the T. brucei enzyme was stable, but mouse ODC was unstable. Thus, the observed difference in intracellular stability is a property of the ODC protein itself, rather than of the cellular environment in which it is expressed. A chimeric ODC composed of the amino terminus of trypanosome ODC and the C-terminus of mouse ODC also was rapidly degraded in CHO cells, suggesting that peptide sequences in the mouse ODC carboxy-terminus determine its stability. The relatively slow turnover of the parasite enzyme constitutes the basis of selective antitrypanosomal action of DFMO. By this same token, many other proteins known to perform crucial functions in bacteria, fungi, protozoa, helminths, etc., also may have shorter half-lives in the mammalian hosts than in parasites. Suicide inhibitors of these proteins may have desirable characteristics as good chemotherapeutic agents. This new approach could provide an additional strategy for controlling infectious diseases.