Cloning of a trypanosomatid gene coding for an ornithine decarboxylase that is metabolically unstable even though it lacks the C-terminal degradation domain

Trypanosoma cruzi Coexpressing Ornithine Decarboxylase and Green Fluorescence Proteins as a Tool to Study the Role of Polyamines in Chagas Disease Pathology

Polyamines are essential for Trypanosoma cruzi, the causative agent of Chagas disease. As T. cruzi behaves as a natural auxotrophic organism, it relies on host polyamines biosynthesis. In this paper we obtained a double-transfected T. cruzi parasite that expresses the green fluorescent protein (GFP) and a heterologous ornithine decarboxylase (ODC), used itself as a novel selectable marker. These autotrophic and fluorescent parasites were characterized; the ODC presented an apparent Km for ornithine of 0.51 ± 0.16 mM and an estimated V_max value of 476.2 nmoles/h/mg of protein. These expressing ODC parasites showed higher metacyclogenesis capacity than the auxotrophic counterpart, supporting the idea that polyamines are engaged in this process. This double-transfected T. cruzi parasite results in a powerful tool—easy to follow by its fluorescence—to study the role of polyamines in Chagas disease pathology and in related processes such as parasite survival, invasion, proliferation, metacyclogenesis, and tissue spreading.

Polyamine metabolism in Trypanosoma cruzi: studies on the expression and regulation of heterologous genes involved in polyamine biosynthesis

Biochemical studies have shown that Trypanosoma cruzi and Toxoplasma gondii are the only eukaryotic organisms so far described which are auxotrophic for polyamines. Both parasites are unable to carry out the de novo biosynthesis of putrescine, and therefore they need the addition of exogenous polyamines to the culture medium for their normal proliferation. Further investigations at the molecular level have demonstrated that the wild-type T. cruzi genome does not contain ornithine or arginine decarboxylase-like nucleic acid sequences, and that the corresponding genes have been presumably lost during evolution. Since T. cruzi behaves as a deletion mutant for ornithine decarboxylase (ODC) and arginine decarboxylase (ADC) genes, this parasite has been selected to study the regulation of the expression of heterologous genes involved in polyamine biosynthesis in other organisms. The resulting transgenic parasites have been useful tools to analyze the different stages of gene expression after transformation, as well as the mechanisms of drug resistance induction and the post-translational processing of enzyme precursors.

Trypanosoma cruzi as a model system to study the expression of exogenous genes coding for polyamine biosynthetic enzymes. Induction of DFMO resistance in transgenic parasites

Trypanosoma cruzi, the etiologic agent of Chagas' disease, is a polyamine auxotroph organism because its genome contains neither ornithine decarboxylase (ODC) nor arginine decarboxylase (ADC) genes, presumably lost during evolution. After transformation with a recombinant plasmid bearing the complete coding region of Crithidia fasciculata ODC gene, the transgenic parasites were able to synthesize putrescine and simultaneously became susceptible to alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ODC. We have studied the emergence of DFMO-resistant T. cruzi after one-step selection of ODC-transformed parasites cultivated in the presence of high levels of the drug (5 mM). Our results have indicated a duplication of the ODC gene copy number in the drug-resistant cell line. The ODC transcripts and the corresponding translation products showed very significant increases (about 7- and 25-fold, respectively) in DFMO-resistant parasites, while the ODC enzymatic activity was 5 times higher than in drug-sensitive T. cruzi. The unequal increases of ODC protein and enzymatic activity in DFMO-resistant protozoa strongly suggest that in addition to gene amplification and enhanced transcription and translation, the assembly of ODC polypeptide chains into dimeric active enzyme molecules might also contribute to regulate the development of DFMO resistance.

Drug resistance in Leishmania: similarities and differences to other organisms

The main line of defense available against parasitic protozoa is chemotherapy. Drug resistance has emerged however, as a primary obstacle to the successful treatment and control of parasitic diseases. Leishmania spp., the causative agents of leishmaniasis, have served as a useful model for studying mechanisms of drug resistance in vitro. Antimonials and amphotericin B are the first line drugs to treat Leishmania followed by pentamidine and a number of other drugs. Parasites resistant against all these classes of drugs have been selected under laboratory conditions. A multiplicity of resistance mechanisms has been detected, the most prevalent being gene amplification and transport mutations. With the tools now available, it should be possible to elucidate the mechanisms that govern drug resistance in field isolates and develop more effective chemotherapeutic agents.

Sequence elements essential for the rapid turnover of Crithidia fasciculata ornithine decarboxylase

Ornithine decarboxylase (ODC) has a very fast turnover in mammalian cells, but is a stable enzyme in T. brucei and other trypanosmatid parasites like Leishmania donovani. However, Crithidia fasciculata, which is a phylogenetically closely related trypanosomatid to L. donovani, has an ODC with a rapid turnover. Interestingly, C. fasciculata ODC, but not L. donovani ODC, is rapidly degraded also in mammalian systems. In order to obtain information on what sequences are important for the rapid degradation of C. fasciculata ODC, we produced a variety of C. fasciculata/L. donovani ODC hybrid proteins and characterized their turnover using two different mammalian expression systems. The results obtained indicate that C. fasciculata ODC contains several sequence elements essential for the rapid turnover of the protein and that these regions are mainly located in the central part of the enzyme.

Ornithine decarboxylase and S-adenosylmethionine decarboxylase in trypanosomatids

The production of polyamines has been shown to be an effective target for a drug against the West African form of sleeping sickness caused by Trypanosoma brucei gambiense. T. brucei belongs to the group of protozoan parasites classed as trypanosomatids. Parasitic species of this group are the causative agents of various tropical diseases besides African sleeping sickness, e.g. Chagas' disease (Trypanosoma cruzi), cutaneous (Lesihmania spp.) and visceral (Leishmania donovani) leishmaniasis. The metabolism of polyamines in the parasites is a potential target for the development of new drugs for treatment of these diseases. The key steps in polyamine synthesis are catalysed by ODC (ornithine decarboxylase) and AdoMetDC (S-adenosylmethionine decarboxylase). In the present paper, some of the available information on ODC and AdoMetDC in trypanosomatids will be described and discussed.

An endosymbiont positively modulates ornithine decarboxylase in host trypanosomatids

Some trypanosomatids, such as Crithidia deanei, are endosymbiont-containing species. Aposymbiotic strains are obtained after antibiotic treatment, revealing interesting aspects of this symbiotic association. Ornithine decarboxylase (ODC) promotes polyamine biosynthesis and contributes to cell proliferation. Here, we show that ODC activity is higher in endosymbiont-bearing trypanosomatids than in aposymbiotic cells, but isolated endosymbionts did not display this enzyme activity. Intriguingly, expressed levels of ODC were similar in both strains, suggesting that ODC is positively modulated in endosymbiont-bearing cells. When the aposymbiotic strain was grown in conditioned medium, obtained after cultivation of the endosymbiont-bearing strain, cellular proliferation as well as ODC activity and localization were similar to that observed in the endosymbiont-containing trypanosomatids. Furthermore, dialyzed-heated medium and trypsin treatment reduced ODC activity of the aposymbiont strain. Taken together, these data indicate that the endosymbiont can enhance the protozoan ODC activity by providing factors of protein nature, which increase the host polyamine metabolism.

Characterization of a methionine adenosyltransferase over-expressing strain in the trypanosomatid Leishmania donovani

Methionine adenosyltransferase (MAT: EC 2.5.1.6) catalyzes the synthesis of S-adenosylmethionine (AdoMet) in two sequential steps, AdoMet formation and subsequent tripolyphosphate (PPPi) cleavage, induced by AdoMet. In pursuit of a better understanding of the biological function of the enzyme, the MAT gene was cloned into vector PX63NEO to induce episomal overexpression in leishmania parasites. Neomycin-selected clones originated a strain of such overexpressing parasites that accumulated more than 3-fold AdoMet than mock-transfected cells and showed over ten times the wild type MAT activity, concurring with a significant accumulation of the MAT protein during the early logarithmic phase and MAT transcripts throughout the growth cycle. The rate of AdoMet efflux, practically nil in the control promastigotes, was exceptionally high in the MAT-overexpressing parasites, whilst growth in this strain was comparable to development in control cells, i.e., it was not affected by deleterious hypermethylation. Moreover, the modified strain was 10-fold more resistant to sinefungin, a S-adenosylmethionine-like antibiotic, than control cells. The effects of overexpression on polyamine metabolism and transport were likewise studied.

Importance of polyamines in cell cycle kinetics as studied in a transgenic system

Polyamines are organic cations, which are considered essential for normal cell cycle progression. This view is based on results from numerous studies using a variety of enzyme inhibitors or polyamine analogues interfering with either the metabolism or the physiological functions of the polyamines. However, the presence of non-specific effects may be hard to rule out in such studies. In the present study, we have for the first time used a transgenic cell system to analyze the importance of polyamines in cell growth. We have earlier shown that expression of trypanosomal ODC in an ODC-deficient variant of CHO cells (C55.7) supported growth of these otherwise polyamine auxotrophic cells. However, one of the transgenic cell lines grew much slower than the others. As shown in the present study, the level of ODC activity was much lower in these cells, and that was reflected in a reduction of cellular polyamine levels. Analysis of cell cycle kinetics revealed that reduction of growth was correlated to prolongation of the G1, S, and G2+M phases in the cells. Providing exogenous putrescine to the cells resulted in a normalization of polyamine levels as well as cell cycle kinetics indicating a causal relationship.

Polyamine transport in parasites: a potential target for new antiparasitic drug development

The metabolism of the naturally occurring polyamines-putrescine, spermidine and spermine-is a highly integrated system involving biosynthesis, uptake, degradation and interconversion. Metabolic differences in polyamine metabolism have long been considered to be a potential target to arrest proliferative processes ranging from cancer to microbial and parasitic diseases. Despite the early success of polyamine inhibitors such as alpha-difluoromethylornithine (DFMO) in treating the latter stages of African sleeping sickness, in which the central nervous system is affected, they proved to be ineffective in checking other major diseases caused by parasitic protozoa, such as Chagas' disease, leishmaniasis or malaria. In the use and design of new polyamine-based inhibitors, account must be taken of the presence of up-regulated polyamine transporters in the plasma membrane of the infectious agent that are able to circumvent the effect of the drug by providing the parasite with polyamines from the host. This review contains information on the polyamine requirements and molecular, biochemical and genetic characterization of different transport mechanisms in the parasitic agents responsible for a number of the deadly diseases that afflict underdeveloped and developing countries.

Ornithine decarboxylase activity during the development of Anastrepha fraterculus (Diptera, Tephritidae)

Ornithine decarboxylase (ODC) (EC 4.1.1.17) is very important for polyamine biosynthesis, which is required for main biological events. In the present study, ODC activity was measured in samples of Anastrepha fraterculus's egg, larva, pupa body and abdomen, adult body, ovaries, and fat body of young females, and in ovaries of mature flies. The kinetic parameters (Km app and Vmax) for ODC activity were determined for pupa, larva, and young ovary. ODC activity showed fluctuations during A. fraterculus's life development. In its earlier stages, prior to emergence, the egg has high ODC-specific activity probably due to embryogenesis, which is characterized by a high rate of cell division. This enzyme activity is also significantly high in the ovary and fat body of young females possibly related to the increased oogenesis and vitellogenesis. The kinetic parameters (Km app and Vmax) had great variation. Our results using GTP showed that the great variation in kinetic parameters can be accounted for by post-translational modifications.

Biogenic amines and polyamines: similar biochemistry for different physiological missions and biomedical applications

Biogenic amines are organic polycations derived from aromatic or cationic amino acids. All of them have one or more positive charges and a hydrophobic skeleton. Nature has evolved these molecules to play different physiological roles in mammals, but maintains similar patterns for their metabolic and intracellular handling. As deduced from this review, many questions still remain to be solved around their biochemistry and molecular biology, blocking our aims to control the relevant pathologies in which they are involved (cancer and immunological, neurological, and gastrointestinal diseases). Advances in this knowledge are dispersed among groups working on different biomedical areas. In these pages, we put together the most relevant information to remark how fruitful it can be to learn from Nature and to take advantage of the biochemical similarities (key protein structures and their regulation data on metabolic interplays and binding properties) to generate new hypothesis and develop different biomedical strategies based on biochemistry and molecular biology of these compounds.

Extremely rapid turnover of S-adenosylmethionine decarboxylase inCrithidia fasciculata

The activity of S-adenosylmethionine decarboxylase (AdoMetDC) in Crithidia fasciculata was shown to be correlated to the growth of the parasite. An increase in activity was observed during exponential growth. Inhibition of protein synthesis induced an extremely rapid decay of AdoMetDC activity. The half-life of the enzyme was estimated to be about 3 min, which is the shortest half-life ever recorded for an eukaryotic AdoMetDC. The reduction in AdoMetDC activity was correlated with a decrease in AdoMetDC protein content, demonstrating a rapid turnover of the enzyme. No polyamine-mediated feedback regulation of AdoMetDC was observed in the parasite.

Regulation of ornithine decarboxylase by antizymes and antizyme inhibitor in zebrafish (Danio rerio)

Mammalian polyamine synthesis is regulated by a unique feedback mechanism. When cellular polyamine levels increase, antizyme, an ornithine decarboxylase (ODC) inhibitory protein, is induced by polyamine-dependent translational frameshifting. Antizyme not only inhibits ODC, a key enzyme in polyamine synthesis, it also targets the enzyme degradation by the 26S proteasome. Furthermore, it suppresses cellular uptake of polyamines. Previously, we isolated two zebrafish antizymes with different expressions and activities. This suggested that a common feedback mechanism of polyamine metabolism might operate in mammals and zebrafish (Danio rerio). In the present study, cDNAs of zebrafish ODC and antizyme inhibitor, another regulatory protein that inhibits antizyme action, were cloned. The presence of ODC and antizyme inhibitor mRNAs was confirmed by Northern blotting in embryos and adult fish, as well as in a zebrafish-derived cell line (BRF41). The activity of the ODC cDNA expression product was inhibited by short and long zebrafish antizymes, and recombinant zebrafish antizyme inhibitor reversed this inhibition. In the BRF41 cells, the ODC half-life was considerably longer than that of mammalian ODC but shorter than that of Schizosaccharomyces pombe. Spermidine elicited a rapid decay of ODC activity and ODC protein in a protein synthesis-dependent manner.

The Plasmodium falciparum bifunctional ornithine decarboxylase, S-adenosyl-L-methionine decarboxylase, enables a well balanced polyamine synthesis without domain-domain interaction

In the human malaria parasite Plasmodium falciparum (Pf), polyamines are synthesized by a bifunctional enzyme that possesses both ornithine decarboxylase (ODC) and S-adenosyl-l-methionine decarboxylase (AdoMetDC) activities. The mature enzyme consists of the heterotetrameric N-terminal AdoMetDC and the C-terminal dimeric ODC, which results in the formation of a heterotetrameric complex. For the native bifunctional protein a half-life longer than 2 h was determined, which is in contrast to the extreme short half-life of its mammalian monofunctional counterparts. The biological advantage of the plasmodial bifunctional ODC/AdoMetDC might be that the control of polyamine synthesis is achieved by only having to regulate the abundance and activity of one protein. An interesting feature in the regulation of the bifunctional protein is that putrescine inhibits PfODC activity approximately 10-fold more efficiently than the mammalian ODC activity, and in contrast to the mammalian AdoMetDC the activity of the PfAdoMetDC domain is not stimulated by the diamine. To analyze post-translational processing, polymerization, and domain-domain interactions, several mutant proteins were generated that have single mutations in either the PfODC or PfAdoMetDC domains. The exchange of amino acids essential for the activity of one domain had no effect on the enzyme activity of the other domain. Even prevention of the post-translational cleavage of the AdoMetDC domain or ODC dimerization and thus the interference with the folding of the protein hardly affected the activity of the partner domain. In addition, inhibition of the activity of the PfODC domain had no effect on the activity of the PfAdoMetDC domain and vice versa. These results demonstrate that no domain-domain interactions occur between the two enzymes of the bifunctional PfODC/AdoMetDC and that both enzymatic activities are operating as independent catalytic sites that do not affect each other.

Targeting polyamines of parasitic protozoa in chemotherapy

All parasitic protozoa contain polyamines and in recent years they, and their associated enzymes, have attracted attention as drug targets because they might reveal novel antiparasite therapies. How justified is this approach to drug discovery? In this review, Sylke Müller, Graham Coombs and Rolf Walter summarize the current status of research into drugs that exploit polyamine metabolism of trypanosomatid and malaria parasites, and propose priorities for research into such drugs. This review was inspired by an Expert Meeting entitled 'Polyamine Metabolism of Parasitic Protozoa as a Drug Target'.

Sensitivity of trypanosomatid protozoa to DFMO and metabolic turnover of ornithine decarboxylase

alpha-Difluoromethylornithine (DFMO), the specific and irreversible inhibitor of ornithine decarboxylase (ODC), was able to induce the arrest of proliferation in Leishmania mexicana and ODC-transformed Trypanosoma cruzi cultures grown in a semi-defined medium essentially free of polyamines. Conversely, Crithidia fasciculata and Phytomonas 274 were not affected by the inhibitor. The drug-resistance of Crithidia and Phytomonas was neither caused by an impairment of DFMO uptake nor by a decrease of the enzyme affinity for the inhibitor. We were also able to rule out the possibility of ODC overexpression in the drug-tolerant parasites. The measurements of ODC metabolic turnover indicated that the enzymes from Crithidia and Phytomonas have a short half-life of 20-40 min, while ODC from Leishmania and transgenic Trypanosoma cruzi are rather stable with a half-life longer than 6 hours. Analyses of polyamine internal pools under different growth conditions have shown that DFMO was able to markedly decrease the levels of putrescine and spermidine in all parasites, but the depletion of spermidine was higher in trypanosomatids containing an ODC with slow turnover. Our results suggest that in these parasites cultivated in the presence of the drug, spermidine might decrease below critical levels needed to maintain trypanothione concentrations or other conditions essential for normal proliferation.

In the human malaria parasite Plasmodium falciparum, polyamines are synthesized by a bifunctional ornithine decarboxylase, S-adenosylmethionine decarboxylase

The polyamines putrescine, spermidine, and spermine are crucial for cell differentiation and proliferation. Interference with polyamine biosynthesis by inhibition of the rate-limiting enzymes ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (AdoMetDC) has been discussed as a potential chemotherapy of cancer and parasitic infections. Usually both enzymes are individually transcribed and highly regulated as monofunctional proteins. We have isolated a cDNA from the malaria parasite Plasmodium falciparum that encodes both proteins on a single open reading frame, with the AdoMetDC domain in the N-terminal region connected to a C-terminal ODC domain by a hinge region. The predicted molecular mass of the entire transcript is 166 kDa. The ODC/AdoMetDC coding region was subcloned into the expression vector pASK IBA3 and transformed into the AdoMetDC- and ODC-deficient Escherichia coli cell line EWH331. The resulting recombinant protein exhibited both AdoMetDC and ODC activity and co-eluted after gel filtration on Superdex S-200 at approximately 333 kDa, which is in good agreement with the molecular mass of approximately 326 kDa determined for the native protein from isolated P. falciparum. SDS-polyacrylamide gel electrophoresis analysis of the recombinant ODC/AdoMetDC revealed a heterotetrameric structure of the active enzyme indicating processing of the AdoMetDC domain. The data presented describe the occurrence of a unique bifunctional ODC/AdoMetDC in P. falciparum, an organization which is possibly exploitable for the design of new antimalarial drugs.

In vitro study of proteolytic degradation of rat histidine decarboxylase

Mammalian ornithine decarboxylase (ODC) is a very unstable protein which is degraded in an ATP-dependent manner by proteasome 26S, after making contact with the regulatory protein antizyme. PEST regions are sequences described as signals for protein degradation. The C-terminal PEST region of mammalian ODC is essential for its degradation by proteasome 26S. Mammalian histidine decarboxylase (HDC) is also a short-lived protein. The full primary sequence of mammalian HDC contains PEST-regions at both the N- and C-termini. Rat ODC and different truncated and full versions of rat HDC were expressed in vitro. In vitro degradation of rat ODC and rat 1-512 HDC were compared. Like ODC, rat 1-512 HDC is degraded mainly by an ATP-dependent mechanism. However, antizyme has no effect on the degradation of 1-512 HDC. The use of the inhibitors MG-132 and lactacystine significantly inhibited the degradation of 1-512 HDC, suggesting that a ubiquitin-dependent, proteasome 26S proteolytic pathway is involved. Results obtained with the different modifications of rat HDC containing all three PEST regions (full version, 1-656 HDC), only the N-terminal PEST region (1-512 HDC), or no PEST region (69-512 HDC), indicate that the N-terminal (1-69) fragment, but not the C-terminal fragment, determines that the HDC protein is a proteasome substrate in vitro.

Degradation of ornithine decarboxylase by the 26S proteasome

Ornithine decarboxylase (ODC) is a key enzyme in polyamine biosynthesis. Turnover of ODC is extremely rapid and highly regulated, and is accelerated when polyamine levels increase. Polyamine-stimulated ODC degradation is mediated by association with antizyme (AZ), an ODC inhibitory protein induced by polyamines. ODC, in association with AZ, is degraded by the 26S proteasome in an ATP-dependent, but ubiquitin-independent, manner. The 26S proteasome irreversibly inactivates ODC prior to its degradation. The inactivation, possibly due to unfolding, is coupled to sequestration of ODC within the 26S proteasome. This process requires AZ and ATP, but not proteolytic activity of the 26S proteasome. The carboxyl-terminal region of ODC presumably exposed by interaction with AZ plays a critical role for being trapped by the 26S proteasome. Thus, the degradation pathway of ODC proceeds as a sequence of multiple distinct processes, including recognition, sequestration, unfolding, translocation, and ultimate degradation mediated by the 26S proteasome.

Trypanosoma cruzi epimastigotes lack ornithine decarboxylase but can express a foreign gene encoding this enzyme

Trypanosoma cruzi, a pathogenic protozoan causing Chagas disease, lacks ornithine decarboxylase (ODC), the enzyme catalyzing the first step of polyamine biosynthetic pathway in eukaryotic cells. Our results indicate that the auxotrophy for diamines of T. cruzi epimastigotes is due to the absence of an active ODC gene in these parasites and not to the inability for the expression of this gene. The introduction of an exogenous complete coding region from Crithidia fasciculata ODC gene inserted in an expression vector specific for trypanosomatids induces the normal expression of the foreign genetic information allowing the transformed T. cruzi to overcome the exogenous polyamine requirement for growth. The enzyme expressed in the transformed parasites has shown a considerably extended metabolic stability. The loss of ODC activity in T. cruzi might be related to the parasite adaptation to the intracellular stages of its life cycle.

The pest regions containing C-termini of mammalian ornithine decarboxylase and histidine decarboxylase play different roles in protein degradation

Proteasome 26S must recognize the PEST region-containing C-terminus of mammalian ornithine decarboxylase (ODC) monomer to proceed with degradation. We have detected PEST regions in both termini of mammalian histidine decarboxylase (HDC). In the present report, a chimaeric ODC/HDC was used to elucidate whether the PEST region-containing C-termini of ODC and HDC are exchangeable. Wild-type rat ODC had an expected antizyme and ATP-dependent degradation. This was not the case for both the chimaera and a C-terminus truncated rat ODC. Results suggest that the PEST region-containing C-terminus of rat HDC should have another role different to confering polypeptide availability to the proteasome.

Structure/function relationship studies on the T/S residues 173-177 of rat ODC

A well-conserved T/S cluster was detected among vertebrate ornithine decarboxylase by computer analysis (E. Viguera, O. Trelles, J.L. Urdiales, J.M. Matés, F. Sánchez-Jiménez, Trends Biochem. Sci. 19 (1994) 318-319). In the present report we studied the role of these residues (173, 176 and 177 in rat ornithine decarboxylase (ODC)) in enzymic activity and stability by in vitro expression, kinetic characterization and in vitro degradation of site-directed mutants. These T/S residues are substituted by a D/E-enriched fragment in other lower eukaryotic ODCs. The substitution of the T/S-enriched fragment (TLKTS) of rat ODC by the negative charged fragment of T. brucei ODC (KVEDC) did not affect protein stability, but increased Km values of the mutant enzyme. The substitution of the T/S residues by alanine also has a similar effect on rat ODC kinetic values. However, results indicate that polarity of the fragment must be an important factor for protein conformation, since the latter mutant, having no T/S or D/E residue in the fragment (ALKAA), showed reduced stability in vitro.

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.