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Alterations in ornithine decarboxylase characteristics account for tolerance of Trypanosoma brucei rhodesiense to D,L-alpha-difluoromethylornithine

Ornithine decarboxylase (ODC), the target enzyme of D,L-alpha-difluoromethylornithine (DFMO), was investigated in four DFMO-tolerant Trypanosoma brucei rhodesiense isolates from East Africa and two DFMO-susceptible T. b. gambiense isolates from West Africa. Neither drug uptake nor inhibition of ODC activity by DFMO in cellular extracts differed in the two trypanosome subspecies. However, the specific ODC activity of the cellular extracts was three times as high in T. b. rhodesiense isolates as in T. b. gambiense isolates. Furthermore, a significant difference in the turnover rate of ODC was observed. The time required to induce a 50% reduction of T. b. rhodesiense ODC activity under cycloheximide pressure (tentative half-life) was about 4.3 h, whereas that required for T. b. gambiense ODC was longer than 18 h. We concluded that the higher specific ODC activity and faster enzyme turnover contributed to a substantial degree to the DFMO tolerance observed in the East African T. b. rhodesiense isolates.