Localization of glycerol-3-phosphate oxidase in the mitochondrion and particulate NAD+-linked glycerol-3-phosphate dehydrogenase in the microbodies of the bloodstream form to Trypanosoma brucei

Trypanosomes as a magnifying glass for cell and molecular biology

The African trypanosome, Trypanosoma brucei, has developed into a flexible and robust experimental model for molecular and cellular parasitology, allowing us to better combat these and related parasites that cause worldwide suffering. Diminishing case numbers, due to efficient public health efforts, and recent development of new drug treatments have reduced the need for continued study of T. brucei in a disease context. However, we argue that this pathogen has been instrumental in revolutionary discoveries that have widely informed molecular and cellular biology and justifies continuing research as an experimental model. Ongoing work continues to contribute towards greater understanding of both diversified and conserved biological features. We discuss multiple examples where trypanosomes pushed the boundaries of cell biology and hope to inspire researchers to continue exploring these remarkable protists as tools for magnifying the inner workings of cells.

Mitochondrial adaptations throughout the Trypanosoma brucei life cycle

The unicellular parasite Trypanosoma brucei has a digenetic life cycle that alternates between a mammalian host and an insect vector. During programmed development, this extracellular parasite encounters strikingly different environments that determine its energy metabolism. Functioning as a bioenergetic, biosynthetic, and signaling center, the single mitochondrion of T. brucei is drastically remodeled to support the dynamic cellular demands of the parasite. This manuscript will provide an up-to-date overview of how the distinct T. brucei developmental stages differ in their mitochondrial metabolic and bioenergetic pathways, with a focus on the electron transport chain, proline oxidation, TCA cycle, acetate production, and ATP generation. Although mitochondrial metabolic rewiring has always been simply viewed as a consequence of the differentiation process, the possibility that certain mitochondrial activities reinforce parasite differentiation will be explored.

Depletion of cardiolipin induces major changes in energy metabolism in Trypanosoma brucei bloodstream forms

The mitochondrial inner membrane glycerophospholipid cardiolipin (CL) associates with mitochondrial proteins to regulate their activities and facilitate protein complex and supercomplex formation. Loss of CL leads to destabilized respiratory complexes and mitochondrial dysfunction. The role of CL in an organism lacking a conventional electron transport chain (ETC) has not been elucidated. Trypanosoma brucei bloodstream forms use an unconventional ETC composed of glycerol-3-phosphate dehydrogenase and alternative oxidase (AOX), while the mitochondrial membrane potential (ΔΨm) is generated by the hydrolytic action of the F_o F₁ -ATP synthase (aka F_o F₁ -ATPase). We now report that the inducible depletion of cardiolipin synthase (TbCls) is essential for survival of T brucei bloodstream forms. Loss of CL caused a rapid drop in ATP levels and a decline in the ΔΨm. Unbiased proteomic analyses revealed a reduction in the levels of many mitochondrial proteins, most notably of F_o F₁ -ATPase subunits and AOX, resulting in a strong decline of glycerol-3-phosphate-stimulated oxygen consumption. The changes in cellular respiration preceded the observed decrease in F_o F₁ -ATPase stability, suggesting that the AOX-mediated ETC is the first pathway responding to the decline in CL. Select proteins and pathways involved in glucose and amino acid metabolism were upregulated to counteract the CL depletion-induced drop in cellular ATP.

Alternative oxidase inhibitors: Mitochondrion-targeting as a strategy for new drugs against pathogenic parasites and fungi

The alternative oxidase (AOX) is a ubiquitous terminal oxidase of plants and many fungi, catalyzing the four-electron reduction of oxygen to water alongside the cytochrome-based electron transfer chain. Unlike the classical electron transfer chain, however, the activity of AOX does not generate adenosine triphosphate but has functions such as thermogenesis and stress response. As it lacks a mammalian counterpart, it has been investigated intensely in pathogenic fungi. However, it is in African trypanosomes, which lack cytochrome-based respiration in their infective stages, that trypanosome alternative oxidase (TAO) plays the central and essential role in their energy metabolism. TAO was validated as a drug target decades ago and among the first inhibitors to be identified was salicylhydroxamic acid (SHAM), which produced the expected trypanocidal effects, especially when potentiated by coadministration with glycerol to inhibit anaerobic energy metabolism as well. However, the efficacy of this combination was too low to be of practical clinical use. The antibiotic ascofuranone (AF) proved a much stronger TAO inhibitor and was able to cure Trypanosoma vivax infections in mice without glycerol and at much lower doses, providing an important proof of concept milestone. Systematic efforts to improve the SHAM and AF scaffolds, aided with the elucidation of the TAO crystal structure, provided detailed structure-activity relationship information and reinvigorated the drug discovery effort. Recently, the coupling of mitochondrion-targeting lipophilic cations to TAO inhibitors has dramatically improved drug targeting and trypanocidal activity while retaining target protein potency. These developments appear to have finally signposted the way to preclinical development of TAO inhibitors.

Workshop no. 1. Alternate metabolic pathways in protozoan energy metabolism

The purpose of this workshop was to collect together colleagues investigating the intermediary metabolism of protozoa, with a view to discussing those pathways involved in energy metabolism and the production of ATP and other high-energy compounds, together with the factors affecting energy balance. The aspects of energy metabolism chosen for discussion comprised the metabolic pathways ranging from the strictly anaerobic to highly oxidative; subcellular compartmentation of these pathways within the protozoa; the functional role of these pathways including a consideration of aero-tolerance; and the use of inhibitors as biochemical probes and potential chemotherapeuticagents. Hopefully this approach has produced a broad 'over-view' of important areas of protozoan energy metabolism which will enable both the specialist and non-specialist to appreciate the similarities and differences between the metabolic behaviour of a range of protozoa.

Probing the metabolic network in bloodstream-form Trypanosoma brucei using untargeted metabolomics with stable isotope labelled glucose

Metabolomics coupled with heavy-atom isotope-labelled glucose has been used to probe the metabolic pathways active in cultured bloodstream form trypomastigotes of Trypanosoma brucei, a parasite responsible for human African trypanosomiasis. Glucose enters many branches of metabolism beyond glycolysis, which has been widely held to be the sole route of glucose metabolism. Whilst pyruvate is the major end-product of glucose catabolism, its transamination product, alanine, is also produced in significant quantities. The oxidative branch of the pentose phosphate pathway is operative, although the non-oxidative branch is not. Ribose 5-phosphate generated through this pathway distributes widely into nucleotide synthesis and other branches of metabolism. Acetate, derived from glucose, is found associated with a range of acetylated amino acids and, to a lesser extent, fatty acids; while labelled glycerol is found in many glycerophospholipids. Glucose also enters inositol and several sugar nucleotides that serve as precursors to macromolecule biosynthesis. Although a Krebs cycle is not operative, malate, fumarate and succinate, primarily labelled in three carbons, were present, indicating an origin from phosphoenolpyruvate via oxaloacetate. Interestingly, the enzyme responsible for conversion of phosphoenolpyruvate to oxaloacetate, phosphoenolpyruvate carboxykinase, was shown to be essential to the bloodstream form trypanosomes, as demonstrated by the lethal phenotype induced by RNAi-mediated downregulation of its expression. In addition, glucose derivatives enter pyrimidine biosynthesis via oxaloacetate as a precursor to aspartate and orotate.

In this work we have followed the distribution of carbon derived from glucose in bloodstream form trypanosomes, the causative agent of African trypanosomiasis, revealing it to enter a diverse range of metabolites. The work involved using ¹³C-labelled glucose and following the fate of the labelled carbon with an LC-MS based metabolomics platform. Beyond glycolysis and the oxidative branch of the pentose phosphate pathway the label entered lipid biosynthesis both through glycerol 3-phosphate and also acetate. Glucose derived carbon also entered nucleotide synthesis through ribose and pyrimidine synthesis through oxaloacetate-derived aspartate. Appreciable quantities of the carboxylic acids succinate and malate were identified, although labelling patterns indicate they are not TCA cycle derived. Amino sugars and sugar nucleotides were also labelled as was inositol used in protein modification but not in inositol phospholipid headgroup production. We confirm active and essential oxaloacetate production in bloodstream form trypanosomes and show that phosphoenolpyruvate carboxykinase is essential to these parasites using RNA interference. The amount of glucose entering these metabolites is minor compared to the quantity that enters pyruvate excreted from the cell, but the observation that enzymes contributing to the metabolism of glucose beyond glycolysis can be essential offers potential new targets for chemotherapy against trypanosomiasis.

Characterization of two mitochondrial flavin adenine dinucleotide-dependent glycerol-3-phosphate dehydrogenases in Trypanosoma brucei

Glycerol-3-phosphate dehydrogenases (G3PDHs) constitute a shuttle that serves for regeneration of NAD(+) reduced during glycolysis. This NAD-dependent enzyme is employed in glycolysis and produces glycerol-3-phosphate from dihydroxyacetone phosphate, while its flavin adenine dinucleotide (FAD)-dependent homologue catalyzes a reverse reaction coupled to the respiratory chain. Trypanosoma brucei possesses two FAD-dependent G3PDHs. While one of them (mitochondrial G3PDH [mtG3PDH]) has been attributed to the mitochondrion and seems to be directly involved in G3PDH shuttle reactions, the function of the other enzyme (putative G3PDH [putG3PDH]) remains unknown. In this work, we used RNA interference and protein overexpression and tagging to shed light on the relative contributions of both FAD-G3PDHs to overall cellular metabolism. Our results indicate that mtG3PDH is essential for the bloodstream stage of T. brucei, while in the procyclic stage the enzyme is dispensable. Expressed putG3PDH-V5 was localized to the mitochondrion, and the data obtained by digitonin permeabilization, Western blot analysis, and immunofluorescence indicate that putG3PDH is located within the mitochondrion.

Redox metabolism in mitochondria of trypanosomatids

SIGNIFICANCE

In the single mitochondrion of protozoan trypanosomatid parasites there are several sites for the generation and elimination of reactive oxygen species (ROS), a class of molecules that exhibit a dual role in cells, either as regulatory mediators or as cytotoxic effectors.

RECENT ADVANCES

Formation of ROS in trypanosomatid mitochondria can be induced by various drug compounds. Importantly, it can also be triggered by specific physiologic stimuli, indicating that this phenomenon may occur in living parasites as well. Elimination of ROS in these organelles is attributed to the activity of two iron-dependent superoxide dismutases (FeSODs) and up to three different peroxidases (a cytochrome c peroxidase and two thiol peroxidases).

CRITICAL ISSUES

Data regarding the formation of ROS in trypanosomatid mitochondria are limited and nonsystematic. Another critical issue refers to the exact contribution of mitochondrial FeSODs and peroxidases for ROS removal, given that their antioxidant activity is not essential when abrogated individually. This suggests some level of functional overlapping or that ROS produced in mitochondria under normal conditions can be removed noncatalytically. Also still unsolved is the mechanism by which mitochondrial thiol peroxidases are regenerated to their reduced (active) form.

FUTURE DIRECTIONS

The production of intramitochondrial ROS under physiologic conditions and their implication in parasite biology should be further clarified. The relative importance of enzymatic versus nonenzymatic mechanisms for ROS elimination in trypanosomatid mitochondria also requires investigation. Simultaneous depletion of several redundant antioxidant enzymes and determination of noncatalytic antioxidants are possible ways to achieve this.

Structure of the trypanosome cyanide-insensitive alternative oxidase

In addition to haem copper oxidases, all higher plants, some algae, yeasts, molds, metazoans, and pathogenic microorganisms such as Trypanosoma brucei contain an additional terminal oxidase, the cyanide-insensitive alternative oxidase (AOX). AOX is a diiron carboxylate protein that catalyzes the four-electron reduction of dioxygen to water by ubiquinol. In T. brucei, a parasite that causes human African sleeping sickness, AOX plays a critical role in the survival of the parasite in its bloodstream form. Because AOX is absent from mammals, this protein represents a unique and promising therapeutic target. Despite its bioenergetic and medical importance, however, structural features of any AOX are yet to be elucidated. Here we report crystal structures of the trypanosomal alternative oxidase in the absence and presence of ascofuranone derivatives. All structures reveal that the oxidase is a homodimer with the nonhaem diiron carboxylate active site buried within a four-helix bundle. Unusually, the active site is ligated solely by four glutamate residues in its oxidized inhibitor-free state; however, inhibitor binding induces the ligation of a histidine residue. A highly conserved Tyr220 is within 4 Å of the active site and is critical for catalytic activity. All structures also reveal that there are two hydrophobic cavities per monomer. Both inhibitors bind to one cavity within 4 Å and 5 Å of the active site and Tyr220, respectively. A second cavity interacts with the inhibitor-binding cavity at the diiron center. We suggest that both cavities bind ubiquinol and along with Tyr220 are required for the catalytic cycle for O2 reduction.

Biochemistry and evolution of anaerobic energy metabolism in eukaryotes

Major insights into the phylogenetic distribution, biochemistry, and evolutionary significance of organelles involved in ATP synthesis (energy metabolism) in eukaryotes that thrive in anaerobic environments for all or part of their life cycles have accrued in recent years. All known eukaryotic groups possess an organelle of mitochondrial origin, mapping the origin of mitochondria to the eukaryotic common ancestor, and genome sequence data are rapidly accumulating for eukaryotes that possess anaerobic mitochondria, hydrogenosomes, or mitosomes. Here we review the available biochemical data on the enzymes and pathways that eukaryotes use in anaerobic energy metabolism and summarize the metabolic end products that they generate in their anaerobic habitats, focusing on the biochemical roles that their mitochondria play in anaerobic ATP synthesis. We present metabolic maps of compartmentalized energy metabolism for 16 well-studied species. There are currently no enzymes of core anaerobic energy metabolism that are specific to any of the six eukaryotic supergroup lineages; genes present in one supergroup are also found in at least one other supergroup. The gene distribution across lineages thus reflects the presence of anaerobic energy metabolism in the eukaryote common ancestor and differential loss during the specialization of some lineages to oxic niches, just as oxphos capabilities have been differentially lost in specialization to anoxic niches and the parasitic life-style. Some facultative anaerobes have retained both aerobic and anaerobic pathways. Diversified eukaryotic lineages have retained the same enzymes of anaerobic ATP synthesis, in line with geochemical data indicating low environmental oxygen levels while eukaryotes arose and diversified.

Proliferating bloodstream-form Trypanosoma brucei use a negligible part of consumed glucose for anabolic processes

Our quantitative knowledge of carbon fluxes in the long slender bloodstream form (BSF) Trypanosoma brucei is mainly based on non-proliferating parasites, isolated from laboratory animals and kept in buffers. In this paper we present a carbon balance for exponentially growing bloodstream form trypanosomes. The cells grew with a doubling time of 5.3h, contained 46 μ mol of carbon (10(8) cells)(-1) and had a glucose consumption flux of 160 nmol min(-1) (10(8) cells)(-1). The molar ratio of pyruvate excreted versus glucose consumed was 2.1. Furthermore, analysis of the (13)C label distribution in pyruvate in (13)C-glucose incubations of exponentially growing trypanosomes showed that glucose was the sole substrate for pyruvate production. We conclude that the glucose metabolised in glycolysis was hardly, if at all, used for biosynthetic processes. Carbon flux through glycolysis in exponentially growing trypanosomes was 10 times higher than the incorporation of carbon into biomass. This biosynthetic carbon is derived from other precursors present in the nutrient rich growth medium. Furthermore, we found that the glycolytic flux was unaltered when the culture went into stationary phase, suggesting that most of the ATP produced in glycolysis is used for processes other than growth.

Channel-forming activities in the glycosomal fraction from the bloodstream form of Trypanosoma brucei

BACKGROUND

Glycosomes are a specialized form of peroxisomes (microbodies) present in unicellular eukaryotes that belong to the Kinetoplastea order, such as Trypanosoma and Leishmania species, parasitic protists causing severe diseases of livestock and humans in subtropical and tropical countries. The organelles harbour most enzymes of the glycolytic pathway that is responsible for substrate-level ATP production in the cell. Glycolysis is essential for bloodstream-form Trypanosoma brucei and enzymes comprising this pathway have been validated as drug targets. Glycosomes are surrounded by a single membrane. How glycolytic metabolites are transported across the glycosomal membrane is unclear.

METHODS/PRINCIPAL FINDINGS

We hypothesized that glycosomal membrane, similarly to membranes of yeast and mammalian peroxisomes, contains channel-forming proteins involved in the selective transfer of metabolites. To verify this prediction, we isolated a glycosomal fraction from bloodstream-form T. brucei and reconstituted solubilized membrane proteins into planar lipid bilayers. The electrophysiological characteristics of the channels were studied using multiple channel recording and single channel analysis. Three main channel-forming activities were detected with current amplitudes 70-80 pA, 20-25 pA, and 8-11 pA, respectively (holding potential +10 mV and 3.0 M KCl as an electrolyte). All channels were in fully open state in a range of voltages ±150 mV and showed no sub-conductance transitions. The channel with current amplitude 20-25 pA is anion-selective (P(K+)/P(Cl-)∼0.31), while the other two types of channels are slightly selective for cations (P(K+)/P(Cl-) ratios ∼1.15 and ∼1.27 for the high- and low-conductance channels, respectively). The anion-selective channel showed an intrinsic current rectification that may suggest a functional asymmetry of the channel's pore.

CONCLUSIONS/SIGNIFICANCE

These results indicate that the membrane of glycosomes apparently contains several types of pore-forming channels connecting the glycosomal lumen and the cytosol.

Crystallization and preliminary crystallographic analysis of cyanide-insensitive alternative oxidase from Trypanosoma brucei brucei

Cyanide-insensitive alternative oxidase (AOX) is a mitochondrial membrane protein and a non-proton-pumping ubiquinol oxidase that catalyzes the four-electron reduction of dioxygen to water. In the African trypanosomes, trypanosome alternative oxidase (TAO) functions as a cytochrome-independent terminal oxidase that is essential for survival in the mammalian host; hence, the enzyme is considered to be a promising drug target for the treatment of trypanosomiasis. In the present study, recombinant TAO (rTAO) overexpressed in haem-deficient Escherichia coli was purified and crystallized at 293 K by the hanging-drop vapour-diffusion method using polyethylene glycol 400 as a precipitant. X-ray diffraction data were collected at 100 K and were processed to 2.9 A resolution with 93.1% completeness and an overall R(merge) of 9.5%. The TAO crystals belonged to the orthorhombic space group I222 or I2(1)2(1)2(1), with unit-cell parameters a = 63.11, b = 136.44, c = 223.06 A. Assuming the presence of two rTAO molecules in the asymmetric unit (2 x 38 kDa), the calculated Matthews coefficient (V(M)) was 3.2 A(3) Da(-1), which corresponds to a solvent content of 61.0%. This is the first report of a crystal of the membrane-bound diiron proteins, which include AOXs.

Differential expression of glycosomal and mitochondrial proteins in the two major life-cycle stages of Trypanosoma brucei

Label-free semi-quantitative differential three-dimensional liquid chromatography coupled to mass spectrometry (3D-LC-MS/MS) was used to compare the glycosomal and mitochondrial proteomes of the bloodstream- and insect-form of Trypanosoma brucei. The abundance of glycosomal marker proteins identified in the two life-cycle stages corresponded well with the relative importance of biochemical pathways present in the glycosomes of the two stages and the peptide spectral count ratios of selected enzymes were in good agreement with published data about their enzymatic specific activities. This approach proved extremely useful for the generation of large scale proteomics data for the comparison of different life-cycle stages. Several proteins involved in oxidative stress protection, sugar-nucleotide synthesis, purine salvage, nucleotide-monophosphate formation and purine-nucleotide cycle were identified as glycosomal proteins.

Trypanosome alternative oxidase: from molecule to function

Trypanosome alternative oxidase (TAO) is the cytochrome-independent terminal oxidase of the mitochondrial electron transport chain. TAO is a diiron protein that transfers electrons from ubiquinol to oxygen, reducing the oxygen to water. The mammalian bloodstream forms of Trypanosoma brucei depend solely on TAO for respiration. The inhibition of TAO by salicylhydroxamic acid (SHAM) or ascofuranone is trypanocidal. TAO is present at a reduced level in the procyclic form of T. brucei, where it is engaged in respiration and is also needed for developmental processes. Alternative oxidases similar to TAO have been found in a wide variety of organisms but not in mammals, thus rendering TAO an important chemotherapeutic target for African trypanosomiasis.

The mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase of Trypanosomatidae and the glycosomal redox balance of insect stages of Trypanosoma brucei and Leishmania spp

The genes for the mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase were identified in Trypanosoma brucei and Leishmania major genomes. We have expressed the L. major gene in Saccharomyces cerevisiae and confirmed the subcellular localization and activity of the produced enzyme. Using cultured T. brucei procyclic and Leishmania mexicana promastigote cells with a permeabilized plasma membrane and containing intact glycosomes, it was shown that dihydroxyacetone phosphate is converted into pyruvate, and stimulates oxygen consumption, indicating that all components of the glycerol 3-phosphate/dihydoxyacetone phosphate shuttle between glycosomes and mitochondrion are present in these insect stages of both organisms. A computer model has been prepared for the energy and carbohydrate metabolism of these cells. It was used in an elementary mode analysis to get insight into the metabolic role of the shuttle in these insect-stage parasites. Our analysis suggests that the shuttle fulfils important roles for these organisms, albeit different from its well-known function in the T. brucei bloodstream form. It allows (1) a high yield of further metabolizable glycolytic products by decreasing the need to produce a secreted end product of glycosomal metabolism, succinate; (2) the consumption of glycerol and glycerol 3-phosphate derived from lipids; and (3) to keep the redox balance of the glycosome finely tuned due to a highly flexible and redundant system.

In silico prediction of the glycosomal enzymes of Leishmania major and trypanosomes

In total, 37080 protein sequences of the three trypanosomatids Leishmania major, Trypanosoma brucei and Trypanosoma cruzi, were used to predict the trypanosomatid glycosomal proteome. All protein sequences were analyzed for the presence of either a C-terminal (PTS1) or an N-terminal (PTS2) peroxisomal targeting sequence. For L. major 191 potential PTS1-containing proteins and 68 potential PTS2-containing proteins with homologues in T. brucei and T. cruzi were identified. About 50% of them were hypothetical proteins to which no function was attributed. From those proteins with known function it appears that the predicted glycosomal proteome of L. major strongly resembles that of T. brucei and T. cruzi with respect to enzyme content. Glycosomes are not only involved in glycolysis, but are predicted to carry out also gluconeogenesis, reactions of the hexose-monophosphate pathway, purine salvage and pyrimidine biosynthesis, beta-oxidation of fatty acids, fatty acid elongation and the biosynthesis of ether lipids. In addition, they seem to catalyze several reactions of isoprenoid synthesis and are involved in oxidant stress protection.

Energy metabolism and its compartmentation in Trypanosoma brucei

African trypanosomes are parasitic protozoa of the order of Kinetoplastida, which cause sleeping sickness and nagana. Trypanosomes are not only of scientific interest because of their clinical importance, but also because these protozoa contain several very unusual biological features, such as their special energy metabolism. The energy metabolism of Trypanosoma brucei differs significantly from that of its host, not only because it comprises distinct enzymes and metabolic pathways, but also because some of the glycolytic enzymes are localized in organelles called glycosomes. Furthermore, the energy metabolism changes drastically during the complex life cycle of this parasite. This review will focus on the recent advances made in understanding the process of ATP production in T. brucei during its life cycle and the consequences of the special subcellular compartmentation.

The presence of four iron-containing superoxide dismutase isozymes in trypanosomatidae: characterization, subcellular localization, and phylogenetic origin in Trypanosoma brucei

Metalloenzymes such as the superoxide dismutases (SODs) form part of a defense mechanism that helps protect obligate and facultative aerobic organisms from oxygen toxicity and damage. Here, we report the presence in the trypanosomatid genomes of four SOD genes: soda, sodb1, sodb2, and a newly identified sodc. All four genes of Trypanosoma brucei have been cloned (Tbsods), sequenced, and overexpressed in Escherichia coli and shown to encode active dimeric FeSOD isozymes. Homology modeling of the structures of all four enzymes using available X-ray crystal structures of homologs showed that the four TbSOD structures were nearly identical. Subcellular localization using GFP-fusion proteins in procyclic insect trypomastigotes shows that TbSODB1 is mainly cytosolic, with a minor glycosomal component, TbSODB2 is mainly glycosomal with some activity in the cytosol, and TbSODA and TbSODC are both mitochondrial isozymes. Phylogenetic studies of all available trypanosomatid SODs and 106 dimeric FeSODs and closely related cambialistic dimeric SOD sequences suggest that the trypanosomatid SODs have all been acquired by more than one event of horizontal gene transfer, followed by events of gene duplication.

Chemotherapeutic efficacy of ascofuranone in Trypanosoma vivax-infected mice without glycerol

Ascofuranone, an antibiotic isolated from Ascochyta visiae, showed trypanocidal activity in Trypanosoma vivax-infected mice. A single dose of 50 mg/kg ascofuranone effectively cured the mice without the help of glycerol. Repeated administrations of this drug further enhanced its chemotherapeutic effect. After two, three, and four consecutive days treatment, the doses needed to cure the infection decreased to 25, 12, and 6 mg/kg, so that the total doses administered were 50, 36 and 24 mg/kg, respectively. Ascofuranone (50 mg/kg) also had a prophylactic effect against T. vivax infection within the first two days after administration. This prophylactic activity diminished to 80% by day 3 and completely disappeared four days after administration. Of particular interest in this study was that ascofuranone had trypanocidal activity in T. vivax-infected mice in the absence of glycerol, whereas co-administration of glycerol or repeated administrations of this drug are needed for Trypanosoma brucei brucei infection. Our present results strongly suggest that ascofuranone is also an effective tool in chemotherapy against African trypanosomiasis in domestic animals.

The Origin of Dihydroorotate Dehydrogenase Genes of Kinetoplastids, with Special Reference to Their Biological Significance and Adaptation to Anaerobic, Parasitic Conditions

Trypanosoma cruzi dihydroorotate dehydrogenase (DHOD), the fourth enzyme of the de novo pyrimidine biosynthetic pathway, is localized in the cytosol and utilizes fumarate as electron acceptor (fumarate reductase activity), while the enzyme from other various eukaryotes is mitochondrial membrane-linked. Here we report that DHOD-knockout T. cruzi did not express the enzyme protein and could not survive even in the presence of pyrimidine nucleosides, substrates for the potentially active salvage pathway, suggesting a vital role of fumarate reductase activity in the regulation of cellular redox balance. Cloning and phylogenetic analysis of euglenozoan DHOD genes showed that the euglenoid Euglena gracilis had a mitochondrial DHOD and that biflagellated bodonids, a sister group of trypanosomatids within kinetoplastids, harbor the cytosolic DHOD. Further, Bodo saliens, a bodonid, had an ACT/DHOD gene fusion encoding aspartate carbamoyltransferase (ACT), the second enzyme of the de novo pyrimidine pathway, and DHOD. This is the first report of this novel gene structure. These results are consistent with suggestions that an ancient common ancestor of Euglenozoa had a mitochondrial DHOD whose descendant exists in E. gracilis and that a common ancestor of kinetoplastids (bodonids and trypanosomatids) subsequently acquired a cytosolic DHOD by horizontal gene transfer. The cytosolic DHOD gene thus acquired may have contributed to adaptation to anaerobiosis in the kinetoplastid lineage and further contributed to the subsequent establishment of parasitism in a trypanosomatid ancestor. Different molecular strategies for anaerobic adaptation in pyrimidine biosynthesis, used by kinetoplastids and by euglenoids, are discussed. Evolutionary implications of the ACT/DHOD gene fusion are also discussed.

Alternative oxidase present in procyclic Trypanosoma brucei may act to lower the mitochondrial production of superoxide

The mitochondrial electron transfer chain present in the procyclic form of the African trypanosome Trypanosoma brucei contains both cytochrome c oxidase and an alternative oxidase (TAO) as terminal oxidases that reduce oxygen to water. By contrast, the electron transfer chain of the primitive mitochondrion present in the bloodstream form of T. brucei contains only TAO as the terminal oxidase. TAO functions in the bloodstream forms to oxidize the ubiquinol produced by the glycerol-3-phosphate shuttle that results in the oxidation of the reduced nicotinamide adenine dinucleotide phosphate produced by glycolysis. The function, however, of TAO in the procyclic forms is unknown. In this study, we found that inhibition of TAO by the specific inhibitor salicylhydroxamic acid stimulates the formation of reactive oxygen species (ROS) in trypanosome mitochondria, resulting in mitochondrial alteration and increased oxidation of cellular proteins. Moreover, the activity and protein content of TAO in procyclic trypanosomes were increased when cells were incubated in the presence of hydrogen peroxide or antimycin A, the cytochrome bc1 complex inhibitor, which also results in increased ROS production. We suggest that one function of TAO in procyclic cells may be to prevent ROS production by removing excess reducing equivalents and transferring them to oxygen.

Trypanosoma brucei cytochrome c1 is imported into mitochondria along an unusual pathway

In most eukaryotic organisms, cytochrome c(1) is encoded in the nucleus, translated on cytosolic ribosomes, and directed to its final destination in the mitochondrial inner membrane by a bipartite, cleaved, amino-terminal presequence. However, in the kinetoplastids and euglenoids, the cytochrome c(1) protein has been shown to lack a cleaved presequence; a single methionine is removed from the amino terminus upon maturation, and the sequence upstream of the heme-binding site is generally shorter than that of the other eukaryotic homologs. We have used a newly developed mitochondrial protein import assay system from Trypanosoma brucei to demonstrate that the T. brucei cytochrome c(1) protein is imported along a non-conservative pathway similar to that described for the inner membrane carrier proteins of other organisms. This pathway requires external ATP and an external protein receptor but is not absolutely dependent on a membrane potential or on ATP hydrolysis in the mitochondrial matrix. We propose the cytochrome c(1) import in T. brucei is a two-step process first involving a membrane potential independent translocation across the outer mitochondrial membrane followed by heme attachment and a membrane potential-dependent insertion into the inner membrane.

Cell fractionation of parasitic protozoa: a review

Cell fractionation, a methodological strategy for obtaining purified organelle preparations, has been applied successfully to parasitic protozoa by a number of investigators. Here we present and discuss the work of several groups that have obtained highly purified subcellular fractions from trypanosomatids, Apicomplexa and trichomonads, and whose work have added substantially to our knowledge of the cell biology of these parasites.

Trypanosome alternative oxidase is regulated post-transcriptionally at the level of RNA stability

In the bloodstream form of African trypanosomes, trypanosome alternative oxidase (TAO), the non-cytochrome ubiquinol:oxidoreductase, is the only terminal oxidase of the mitochondrial electron transport system. TAO is developmentally regulated during mitochondrial biogenesis in this parasite. During in vitro differentiation of Trypanosoma brucei from the bloodstream to the procyclic form, the overall rate of oxygen consumption decreased about 80%. The mode of respiration changed over a 2- to 3-wk period from a cyanide-insensitive, SHAM-sensitive pathway to a predominantly cyanide-sensitive pathway. The TAO protein level gradually decreased to the level present in the procyclic forms during this 3-wk period. However, within the first week of differentiation, the TAO transcript level decreased about 90% and then in the following weeks it reached the level present in the established procyclic form, that is about 20% of that in bloodstream forms. Like other trypanosomatid genes TAO transcript synthesis remains unaltered in fully differentiated bloodstream and procyclic trypanosomes. The half-life of the TAO mRNA was about 3.2 h in the procyclic trypanosomes, whereas the TAO transcript level remained unaltered even after 4 h of incubation with actinomycin D in bloodstream forms. Inhibition of protein synthesis resulted in about a four-fold accumulation of the TAO transcript in the procyclic trypanosomes, comparable to the level present in the bloodstream forms. Thus, TAO is regulated at the level of mRNA stability and de novo protein synthesis is required for the reduction of the TAO mRNA pool in the procyclic form.

Glycerol kinase of Trypanosoma brucei. Cloning, molecular characterization and mutagenesis

Trypanosoma brucei contains two tandemly arranged genes for glycerol kinase. The downstream gene was analysed in detail. It contains an ORF for a polypeptide of 512 amino acids. The polypeptide has a calculated molecular mass of 56 363 Da and a pI of 8.6. Comparison of the T. brucei glycerol kinase amino-acid sequence with the glycerol kinase sequences available in databases revealed positional identities of 39.0-50.4%. The T. brucei glycerol kinase gene was overexpressed in Escherichia coli cells and the recombinant protein obtained was purified and characterized biochemically. Its kinetic properties with regard to both the forward and reverse reaction were measured. The values corresponded to those determined previously for the natural glycerol kinase purified from the parasite, and confirmed that the apparent Km values of the trypanosome enzyme for its substrates are relatively high compared with those of other glycerol kinases. Alignment of the amino-acid sequences of T. brucei glycerol kinase and other eukaryotic and prokaryotic glycerol kinases, as well as inspection of the available three-dimensional structure of E. coli glycerol kinase showed that most residues of the magnesium-, glycerol- and ADP-binding sites are well conserved in T. brucei glycerol kinase. However, a number of remarkable substitutions was identified, which could be responsible for the low affinity for the substrates. Most striking is amino-acid Ala137 in T. brucei glycerol kinase; in all other organisms a serine is present at the corresponding position. We mutated Ala137 of T. brucei glycerol kinase into a serine and this mutant glycerol kinase was over-expressed and purified. The affinity of the mutant enzyme for its substrates glycerol and glycerol 3-phosphate appeared to be 3. 1-fold to 3.6-fold higher than in the wild-type enzyme. Part of the glycerol kinase gene comprising this residue 137 was amplified in eight different kinetoplastid species and sequenced. Interestingly, an alanine occurs not only in T. brucei, but also in other trypanosomatids which can convert glucose into equimolar amounts of glycerol and pyruvate: T. gambiense, T. equiperdum and T. evansi. In trypanosomatids with no or only a limited capacity to produce glycerol, a hydroxy group-containing residue is found as in all other organisms: T. vivax and T. congolense possess a serine while Phytomonas sp., Leishmania brasiliensis and L. mexicana have a threonine.

Cloning and analysis of the PTS-1 receptor in Trypanosoma brucei

Kinetoplastid organisms, such as the protozoan parasite Trypanosoma brucei, compartmentalise several important metabolic pathways in organelles called glycosomes. Glycosomes are related to peroxisomes of yeast and mammalian cells. A subset of glycosomal matrix proteins is routed to the organelles via the peroxisome-targeting signal type 1 (PTS-1). The PEX5 gene homologue has been cloned from T. brucei coding for a protein of the translocation machinery, the PTS-1 receptor. The gene codes for a polypeptide of 654 amino acids with a calculated molecular mass of 70 kDa. Like its homologue in other organisms T. brucei PTS-1 receptor protein (TbPEX5) is a member of the tetratricopeptide repeat (TPR) protein family and contains several copies of the pentapeptide W-X-X-X-F/Y. Northern and Western blot analysis showed that the protein is expressed at different stages of the life cycle of the parasite. The protein has been overproduced in Escherichia coli and purified using immobilized metal affinity chromatography. The purified protein specifically interacts in vitro with glycosomal phosphoglycerate kinase-C (PGK-C) of T. brucei, a PTS-1 containing protein. The equilibrium dissociation constant (Kd) of PGK-C for purified TbPEX5 is 40 nM. Using biochemical and cytochemical techniques a predominantly cytosolic localization was found for TbPEX5. This is consistent with the idea of receptor cycling between the glycosomes and the cytosol.

Compartmentation of phosphoglycerate kinase in Trypanosoma brucei plays a critical role in parasite energy metabolism

African trypanosomes compartmentalize glycolysis in a microbody, the glycosome. When growing in the mammalian bloodstream, trypanosomes contain only a rudimentary mitochondrion, and the first seven glycolytic enzymes, including phosphoglycerate kinase, are located in the glycosome. Procyclic trypanosomes, growing in the gut of tsetse flies, possess a fully developed mitochondrion that is active in oxidative phosphorylation. The first six glycolytic enzymes are still glycosomal, but phosphoglycerate kinase is now found in the cytosol. We demonstrate here that bloodstream trypanosomes are killed by expression of cytosolic phosphoglycerate kinase. The toxicity depends on both enzyme activity and cytosolic location. One possible explanation is that cytosolic phosphoglycerate kinase creates an ATP-generating shunt in the cytosol, thus preventing full ATP regeneration in the glycosome and ultimately inhibiting the first, ATP-consuming, steps of glycolysis.

Differences in Energy Metabolism Between Trypanosomatidae

Although various members of the family Trypanosomatidae generate energy in a similar way, fundamental differences also exist and are not always recognized. In this review, Louis Tielens and Jaap Van Hellemond discuss the known differences in carbohydrate metabolism among trypanosomatids, and especially compare Leishmania with trypanosomatids such as Trypanosoma brucei and Phytomonas spp. Special attention will be paid to differences in end-products of carbohydrate degradation, to differences in anaerobic capacities between the various trypanosomatids and to the components of their respiratory chains, including the presence or absence of a plant-like alternative oxidase. Furthermore, evidence will be discussed which indicates that the succinate produced by trypanosomatids is formed mainly via an oxidative pathway and not via reduction of fumarate, a process known to occur in parasitic helminths.

The presence of rotenone-sensitive NADH dehydrogenase in the long slender bloodstream and the procyclic forms of Trypanosoma brucei brucei

The mitochondrial electron-transport chain present in the procyclic and long slender bloodstream forms of Trypanosoma brucei brucei was investigated by means of several experimental approaches. The oxidation of proline, glycerol and glucose in procyclic cells was inhibited 80-90% by antimycin A or cyanide, 15-19% by salicylhydroxamic acid, and 30-35% by rotenone. Cytochrom-c-reductase activity, with proline or glycerol 3-phosphate as substrate, in a mitochondrial fraction isolated from these cells was inhibited by antimycin and rotenone, but not by malonate, while cytochrome-c-reductase activity with succinate as substrate was inhibited by antimycin A and malonate, but not by rotenone. In addition, the reduction of dichloroindophenol by NADH was inhibited by rotenone but not by malonate, which suggests that rotenone-sensitive NADH dehydrogenase (complex I) is present in these mitochondria. The presence of three subunits of NADH dehydrogenase was observed in immunoblots of mitochondrial proteins with specific antibodies raised against peptides corresponding to predicted antigenic regions of these proteins, which provides further evidence for the presence of NADH dehydrogenase. In long slender bloodstream forms, the oxidation of glucose or glycerol was inhibited 100% by salicyhydroxamic acid, unaffected by cyanide or antimycin A, and inhibited 40% or 75%, respectively, by rotenone, which suggests that NADH dehydrogenase is present in these cells. In a mitochondrial fraction isolated from the bloodstream forms, oxygen uptake with glycerol 3-phosphate as substrate was inhibited 65% by rotenone. Low levels of rotenone-sensitive NADH-dependent reduction of dichloroindophenol and the presence of subunits 7 and 8 of NADH dehydrogenase provided additional evidence for the presence of NADH dehydrogenase in bloodstream forms of T. brucei.

Cloning and characterization of the NAD-linked glycerol-3-phosphate dehydrogenases of Trypanosoma brucei brucei and Leishmania mexicana mexicana and expression of the trypanosome enzyme in Escherichia coli

A polyclonal antiserum raised against the purified glycosomal glycerol-3-phosphate dehydrogenase of Trypanosoma brucei brucei has been used to identify the corresponding cDNA clone in a T.b. brucei expression library. This cDNA was subsequently used to obtain genomic clones containing glycerol-3-phosphate dehydrogenase genes. Two tandemly arranged genes were detected in these clones. Characterization of one of the genes showed that it codes for a polypeptide of 353 amino acids, with a molecular mass of 37,651 Da and a calculated net charge of +8. Using the T.b. brucei gene as a probe, a corresponding glycerol-3-phosphate dehydrogenase gene was also identified in a genomic library of Leishmania mexicana mexicana. The L.m. mexicana gene codes for a polypeptide of 365 amino acids, with a molecular mass of 39,140 Da and a calculated net charge of +8. The amino-acid sequences of both polypeptides are 63% identical and carry a type-1 peroxisomal targeting signal (PTS1) SKM and -SKL at their respective C-termini. Moreover, the L.m. mexicana polypeptide also carries a short N-terminal extension reminiscent of a mitochondrial transit sequence. Subcellular localisation analysis showed that in L.m. mexicana the glycerol-3-phosphate dehydrogenase activity co-fractionated both with mitochondria and with glycosomes. This is not the case in T. brucei, where the enzyme is predominantly glycosomal. The two trypanosomatid sequences resemble their prokaryotic homologues (32-36%) more than their eukaryotic counterparts (25-31%) and carry typical prokaryotic signatures. The possible reason for this prokaryotic nature of a trypanosomatid glycerol-3-phosphate dehydrogenase is discussed.

Protein trafficking in kinetoplastid protozoa

The kinetoplastid protozoa infect hosts ranging from invertebrates to plants and mammals, causing diseases of medical and economic importance. They are the earliest-branching organisms in eucaryotic evolution to have either mitochondria or peroxisome-like microbodies. Investigation of their protein trafficking enables us to identify characteristics that have been conserved throughout eucaryotic evolution and also reveals how far variations, or alternative mechanisms, are possible. Protein trafficking in kinetoplastids is in many respects similar to that in higher eucaryotes, including mammals and yeasts. Differences in signal sequence specificities exist, however, for all subcellular locations so far examined in detail--microbodies, mitochondria, and endoplasmic reticulum--with signals being more degenerate, or shorter, than those of their higher eucaryotic counterparts. Some components of the normal array of trafficking mechanisms may be missing in most (if not all) kinetoplastids: examples are clathrin-coated vesicles, recycling receptors, and mannose 6-phosphate-mediated lysosomal targeting. Other aspects and structures are unique to the kinetoplastids or are as yet unexplained. Some of these peculiarities may eventually prove to be weak points that can be used as targets for chemotherapy; others may turn out to be much more widespread than currently suspected.

Identification and partial purification of a stage-specific 33 kDa mitochondrial protein as the alternative oxidase of the Trypanosoma brucei brucei bloodstream trypomastigotes

The glycerophosphate oxidase (GPO), the unique terminal oxidase of bloodstream trypanosome (TAO), appears to be functionally similar to the alternative oxidases of some plants and higher fungi. Immunoblotting of mitochondrial proteins of bloodstream trypomastigotes of Trypanosoma brucei with monoclonal or polyclonal antibodies to Sauromatum guttatum (voodoo lily) and Symplocarpus foetidus (skunk cabbage) alternative oxidases respectively revealed two proteins of about 33 kDa (p33) and 68 kDa (p68). These proteins are not present in procyclic trypomastigotes. Electrophoresis under rigorous denaturing conditions indicated p68 to be the dimer of p33. Indirect immunofluorescent studies of bloodstream and procyclic trypomastigotes with monoclonal antibody to plant alternative oxidase also showed the localization of 33 kDa protein in the mitochondria of the bloodstream trypomastigotes. The functional TAO activity could be solubilized efficiently from the mitochondrial membrane of the bloodstream trypomastigotes by 1% NP-40 or 10 mM lauryl maltoside. When fractionated by Superose 12 gel filtration chromatography, p33 was co-purified with the TAO enzymatic activity. The apparent molecular size of the active enzyme complex was found to be 160 kDa. Gradual disappearance of the 33 kDa protein and the TAO enzymatic activity were well correlated during in vitro differentiation of the bloodstream to procyclic trypomastigotes. This study implies that the net biosynthesis of p33, an essential subunit of TAO, is decreased during differentiation from bloodstream to procyclic trypomastigotes.

Developmental regulation of mitochondrial biogenesis in Trypanosoma brucei

The metabolism of Trypanosoma brucei undergoes a significant change as the parasite differentiates from the mammalian bloodstream form to the form found in the tse-tse fly vector. Because the mitochondria of bloodstream form cells lack cytochromes and several key citric acid cycle enzymes, the metabolism of these cells is mostly limited to glycolysis. The reducing equivalents generated by this process are passed to oxygen by a plant-like alternative oxidase. As cells differentiate to the insect form, they begin to oxidatively metabolize proline. The mitochondria of insect form cells contain functional, cytochrome-mediated electron transport chains and have complete complements of citric acid cycle enzymes. Although the characterization is far from complete, the nuclear and mitochondrial genes involved in the expression of these mitochondrial functions appear to be developmentally regulated at posttranscriptional and posttranslational levels. This review outlines some of the molecular processes that are associated with the developmental regulation of mitochondrial biogenesis and suggests some possible mechanisms of regulation.

Oxidation of NADH by a rotenone and antimycin-sensitive pathway in the mitochondrion of procyclic Trypanosoma brucei brucei

The pathway of NADH oxidation in the procyclic Trypanosoma brucei brucei was investigated in a crude mitochondrial membrane fraction and in whole cells permeabilized with digitonin. NADH:cytochrome c reductase activity was 75% inhibited by concentrations of antimycin that inhibited 95% succinate:cytochrome c reductase activity suggesting that the major pathway for NADH oxidation in the mitochondria involved the cytochrome bc1 complex of the electron transfer chain. Both NADH:cytochrome c and NADH:ubiquinone reductase activities were inhibited 80-90% by rotenone indicating the presence of a complex I-like NADH dehydrogenase in the mitochondrion of trypanosomes. In whole cells permeabilized with low concentrations of digitonin, the oxidation of malate, proline and glucose (in the presence of salicylhydroxamic acid, the inhibitor of the alternate oxidase) was inhibited 30-50% by rotenone. The presence of an alternative pathway for NADH oxidation involving fumarate reductase was indicated by the observation that malonate, the specific inhibitor of succinate dehydrogenase, inhibited 30-35% the rate of oxygen uptake with malate and glucose as substrates in the digitonin-permeabilized cells. We conclude that in the mitochondrion of the procyclic form of T. brucei, NADH is preferentially oxidized by a rotenone-sensitive NADH:ubiquinone oxidoreductase; however, NADH can also be oxidized to some extent by the enzyme fumarate reductase present in the mitochondrion of T. brucei.

Possible localisation of dolichol-dependent mannosyltransferase of Trypanosoma brucei to the rough endoplasmic reticulum

The glycosylphosphatidylinositol membrane anchor of variant surface glycoprotein of the African trypanosome Trypanosoma brucei contains several mannosyl residues for which dolichol phosphoryl mannose is supposed to be the precursor; this itself is probably synthesised by a dolichol-dependent mannosyltransferase. We have characterised and localised a mannosyltransferase activity of T. brucei which transfers mannose from GDP-[14C]mannose to exogenously added dolichyl phosphate. The enzyme was saturable for both its substrates and had a Km of 7.8 microM and 3.3 microM, respectively, for dolichyl phosphate and GDP-mannose. Mannosyltransferase was labile at 37 degrees C in the presence of Triton X-100, but its activity remained constant for at least 60 min at temperatures between 10-15 degrees C. The enzyme was inhibited by amphomycin and this inhibition was potentiated by the presence of 10 mM CaCl2. After subcellular fractionation of cell homogenates by differential centrifugation, mannosyltransferase was recovered mainly in the microsomal fraction and its distribution was very similar to that of RNA, a marker for the rough endoplasmic reticulum. After isopycnic centrifugation in a linear sucrose gradient the distribution of mannosyltransferase also resembled that of RNA. Both constituents exhibited a shift towards lower densities after pre-treatment of microsomal membranes with inorganic pyrophosphate, while other membrane markers such as acid phosphatase and nucleoside diphosphatase did not. It is concluded that the formation of dolichol phosphoryl mannose from GDP-mannose and dolichyl phosphate in T. brucei occurs mainly in the rough endoplasmic reticulum.

The glycosomes of the Kinetoplastida

Glycosomes are the microbodies of the organisms belonging to the order of the Kinetoplastida, comprising trypanosomes and leishmanias, both pathogens to man. The organelles sequester a number of glycolytic enzymes that are normally located in the cytosol in other eukaryotic organisms, and share some enzymes with peroxisomes and glyoxysomes of other protists, plants and animals. Proteins enter the glycosome by a mechanism of post-translational translocation which involves in some, but not all, cases a C-terminal oligopeptide sequence.

Energization-dependent Ca2+ accumulation in Trypanosoma brucei bloodstream and procyclic trypomastigotes mitochondria

The permeabilization of Trypanosoma brucei procyclic and bloodstream trypomastigotes with digitonin permitted the quantitative estimation of a mitochondrial membrane potential of the order of 130-140 mV, in both forms, using safranine O. Dependence on substrate oxidation and response of the procyclic mitochondrial membrane potential to phosphate, FCCP, valinomycin, and Ca2+ indicate that these mitochondria behave similarly to vertebrate mitochondria regarding the properties of their electrochemical proton gradient. In contrast, in bloodstream mitochondria, development of a membrane potential was independent of substrate oxidation and dependent on hydrolysis of ATP by the mitochondrial oligomycin-sensitive ATPase, as demonstrated by collapse of the membrane potential by oligomycin and its insensitivity to the respiratory chain-inhibitor antimycin A. Mitochondria of T. brucei bloodstream forms were also able to take up Ca2+ by an electrophoretic mechanism. This is the first report of the presence of a Ca2+ transport mechanism in an eukaryotic cell devoid of complete tricarboxylic acid cycle and respiratory chain, the activities of which are known to be regulated by changes in intramitochondrial calcium concentration in other cells.

The mitochondrion in bloodstream forms of Trypanosoma brucei is energized by the electrogenic pumping of protons catalysed by the F1F0-ATPase

Bloodstream forms of Trypanosoma brucei were found to maintain a significant membrane potential across their mitochondrial inner membrane (delta psi m) in addition to a plasma membrane potential (delta psi p). Significantly, the delta psi m was selectively abolished by low concentrations of specific inhibitors of the F1F0-ATPase, such as oligomycin, whereas inhibition of mitochondrial respiration with salicylhydroxamic acid was without effect. Thus, the mitochondrial membrane potential is generated and maintained exclusively by the electrogenic translocation of H+, catalysed by the mitochondrial F1F0-ATPase at the expense of ATP rather than by the mitochondrial electron-transport chain present in T. brucei. Consequently, bloodstream forms of T. brucei cannot engage in oxidative phosphorylation. The mitochondrial membrane potential generated by the mitochondrial F1F0-ATPase in intact trypanosomes was calculated after solving the two-compartment problem for the uptake of the lipophilic cation, methyltriphenylphosphonium (MePh3P+) and was shown to have a value of approximately 150 mV. When the value for the delta psi m is combined with that for the mitochondrial pH gradient (Nolan and Voorheis, 1990), the mitochondrial proton-motive force was calculated to be greater than 190 mV. It seems likely that this mitochondrial proton-motive force serves a role in the directional transport of ions and metabolites across the promitochondrial inner membrane during the bloodstream stage of the life cycle, as well as promoting the import of nuclear-encoded protein into the promitochondrion during the transformation of bloodstream forms into the next stage of the life cycle of T. brucei.

Differential energetic metabolism during Trypanosoma cruzi differentiation. II. Hexokinase, phosphofructokinase and pyruvate kinase

The activities of hexokinase (ATP:hexose-6-phosphate transferase, E.C. 2.7.1.1), phosphofructokinase (ATP:fructose-6-phosphate 1-phosphotransferase, E.C.2.7.1.11) and pyruvate kinase (ATP:pyruvate transferase, E.C. 2.7.1.40), and their kinetic behaviour in two morphological forms of Trypanosoma cruzi (epimastigotes and metacyclic trypomastigotes) have been studied. The kinetic responses of the three enzymes to their respective substrates were normalized to hyperbolic forms on a velocity versus substrate concentration plots. Hexokinase and phosphofructokinase showed a higher activity in epimastigotes than in metacyclics, whereas pyruvate kinase had similar activity in both forms of the parasite. The specific activity of hexokinase from epimastigotes was 102.00 mUnits/mg of protein and the apparent Km value for glucose was 35.4 microM. Metacyclic forms showed a specific activity of 55.25 mUnits/mg and a Km value of 46.3 microM. The kinetic parameters (specific activity and Km for fructose 6-phosphate) of phosphofructokinase for epimastigotes were 42.60 mUnits/mg and 0.31 mM and for metacyclics 13.97 mUnits/mg and 0.16 mM, respectively. On the contrary, pyruvate kinase in both forms of T. cruzi did not show significant differences in its kinetic parameters. The specific activity in epimastigotes was 37.00 mUnits/mg and the Km for phosphoenolpyruvate was 0.47 mM, whereas in metacyclics these values were 42.94 mUnits/mg and 0.46 mM, respectively. The results presented in this work, clearly demonstrate a quantitative change in the glycolytic pathway of both culture forms of T. cruzi.