Evidence for a functional glyoxylate cycle in the leishmaniae

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.

Recent advances in trypanosomatid research: genome organization, expression, metabolism, taxonomy and evolution

Unicellular flagellates of the family Trypanosomatidae are obligatory parasites of invertebrates, vertebrates and plants. Dixenous species are aetiological agents of a number of diseases in humans, domestic animals and plants. Their monoxenous relatives are restricted to insects. Because of the high biological diversity, adaptability to dramatically different environmental conditions, and omnipresence, these protists have major impact on all biotic communities that still needs to be fully elucidated. In addition, as these organisms represent a highly divergent evolutionary lineage, they are strikingly different from the common 'model system' eukaryotes, such as some mammals, plants or fungi. A number of excellent reviews, published over the past decade, were dedicated to specialized topics from the areas of trypanosomatid molecular and cell biology, biochemistry, host-parasite relationships or other aspects of these fascinating organisms. However, there is a need for a more comprehensive review that summarizing recent advances in the studies of trypanosomatids in the last 30 years, a task, which we tried to accomplish with the current paper.

Apparent isocitrate lyase activity in Leishmania amazonensis

Early reports have demonstrated the occurrence of glyoxylate cycle enzymes in several Leishmania species. However, these results have been underestimated because genes for the two key enzymes of the cycle, isocitrate lyase (ICL) and malate synthase (MS), are not annotated in Leishmania genomes. We have re-examined this issue in promastigotes of Leishmania amazonensis. Enzyme activities were assayed spectrophotometrically in cellular extracts and characterized partially. A 40 kDa band displaying ICL activity was visualized on zymograms of the extracts. By immunoblotting with mouse antibodies against ICL from Bacillus stearothermophilus, a band of approximately 40 kDa was identified, coincident with the relative molecular mass of the activity band revealed on zymograms. Indirect immunofluorescence of intact promastigotes showed that the recognized antigen is distributed as a punctuated pattern, mainly distributed beneath the subpellicular microtubules, over a diffused cytoplasmic stain. These results clearly demonstrate the existence of an apparent ICL activity in L. amazonensis promastigotes, which is associated to a 40 kDa polypeptide and distributed both diffused and as punctuate aggregates in the cytoplasm. The relevance of this activity is discussed.

Glucose deprivation induced upregulation of phosphoenolpyruvate carboxykinase modulates virulence in Leishmania donovani

Various physiological stimuli trigger the conversion of noninfective Leishmania donovani promastigotes to the infective form. Here, we present the first evidence of the effect of glucose starvation, on virulence and survival of these parasites. Glucose starvation resulted in a decrease in metabolically active parasites and their proliferation. However, this was reversed by supplementation of gluconeogenic amino acids. Glucose starvation induced metacyclogenesis and enhanced virulence through protein kinase A regulatory subunit (LdPKAR1) mediated autophagy. Glucose starvation driven oxidative stress upregulated the antioxidant machinery, culminating in increased infectivity and greater parasitic load in primary macrophages. Interestingly, phosphoenolpyruvate carboxykinase (LdPEPCK), a gluconeogenic enzyme, exhibited the highest activity under glucose starvation to regulate growth of L. donovani by alternatively utilising amino acids. Deletion of LdPEPCK (Δpepck) decreased virulent traits and parasitic load in primary macrophages but increased autophagosome formation in the mutant parasites. Furthermore, Δpepck parasites failed to activate the Pentose Phosphate Pathway shunt, abrogating NADPH/NADP⁺ homoeostasis, conferring increased susceptibility towards oxidants following glucose starvation. In conclusion, this study showed that L. donovani undertakes metabolic rearrangements via gluconeogenesis under glucose starvation for acquiring virulence and its survival in the hostile environment.

Serum Removal from Culture Induces Growth Arrest, Ploidy Alteration, Decrease in Infectivity and Differential Expression of Crucial Genes in Leishmania infantum Promastigotes

Leishmania infantum is one of the species responsible for visceral leishmaniasis. This species is distributed basically in the Mediterranean basin. A recent outbreak in humans has been reported in Spain. Axenic cultures are performed for most procedures with Leishmania spp. promastigotes. This model is stable and reproducible and mimics the conditions of the gut of the sand fly host, which is the natural environment of promastigote development. Culture media are undefined because they contain mammalian serum, which is a rich source of complex lipids and proteins. Serum deprivation slows down the growth kinetics and therefore, yield in biomass. In fact, we have confirmed that the growth rate decreases, as well as infectivity. Ploidy is also affected. Regarding the transcriptome, a high-throughput approach has revealed a low differential expression rate but important differentially regulated genes. The most remarkable profiles are: up-regulation of the GINS Psf3, the fatty acyl-CoA synthase (FAS1), the glyoxylase I (GLO1), the hydrophilic surface protein B (HASPB), the methylmalonyl-CoA epimerase (MMCE) and an amastin gene; and down-regulation of the gPEPCK and the arginase. Implications for metabolic adaptations, differentiation and infectivity are discussed herein.

Gluconeogenesis in Leishmania mexicana: contribution of glycerol kinase, phosphoenolpyruvate carboxykinase, and pyruvate phosphate dikinase

Gluconeogenesis is an active pathway in Leishmania amastigotes and is essential for their survival within the mammalian cells. However, our knowledge about this pathway in trypanosomatids is very limited. We investigated the role of glycerol kinase (GK), phosphoenolpyruvate carboxykinase (PEPCK), and pyruvate phosphate dikinase (PPDK) in gluconeogenesis by generating the respective Leishmania mexicana Δgk, Δpepck, and Δppdk null mutants. Our results demonstrated that indeed GK, PEPCK, and PPDK are key players in the gluconeogenesis pathway in Leishmania, although stage-specific differences in their contribution to this pathway were found. GK participates in the entry of glycerol in promastigotes and amastigotes; PEPCK participates in the entry of aspartate in promastigotes, and PPDK is involved in the entry of alanine in amastigotes. Furthermore, the majority of alanine enters into the pathway via decarboxylation of pyruvate in promastigotes, whereas pathway redundancy is suggested for the entry of aspartate in amastigotes. Interestingly, we also found that l-lactate, an abundant glucogenic precursor in mammals, was used by Leishmania amastigotes to synthesize mannogen, entering the pathway through PPDK. On the basis of these new results, we propose a revision in the current model of gluconeogenesis in Leishmania, emphasizing the differences between amastigotes and promastigotes. This work underlines the importance of studying the trypanosomatid intracellular life cycle stages to gain a better understanding of the pathologies caused in humans.

PLP-dependent enzymes as potential drug targets for protozoan diseases

The chemical properties of the B(6) vitamers are uniquely suited for wide use as cofactors in essential reactions, such as decarboxylations and transaminations. This review addresses current efforts to explore vitamin B(6) dependent enzymatic reactions as drug targets. Several current targets are described that are found amongst these enzymes. The focus is set on diseases caused by protozoan parasites. Comparison across a range of these organisms allows insight into the distribution of potential targets, many of which may be of interest in the development of broad range anti-protozoan drugs. This article is part of a Special Issue entitled: Pyridoxal Phosphate Enzymology.

Niche metabolism in parasitic protozoa

Complete or partial genome sequences have recently become available for several medically and evolutionarily important parasitic protozoa. Through the application of bioinformatics complete metabolic repertoires for these parasites can be predicted. For experimentally intractable parasites insight provided by metabolic maps generated in silico has been startling. At its more extreme end, such bioinformatics reckoning facilitated the discovery in some parasites of mitochondria remodelled beyond previous recognition, and the identification of a non-photosynthetic chloroplast relic in malarial parasites. However, for experimentally tractable parasites, mapping of the general metabolic terrain is only a first step in understanding how the parasite modulates its streamlined, yet still often puzzlingly complex, metabolism in order to complete life cycles within host, vector, or environment. This review provides a comparative overview and discussion of metabolic strategies used by several different parasitic protozoa in order to subvert and survive host defences, and illustrates how genomic data contribute to the elucidation of parasite metabolism.

Virulence of Leishmania major in macrophages and mice requires the gluconeogenic enzyme fructose-1,6-bisphosphatase

Leishmania are protozoan parasites that replicate within mature phagolysosomes of mammalian macrophages. To define the biochemical composition of the phagosome and carbon source requirements of intracellular stages of L. major, we investigated the role and requirement for the gluconeogenic enzyme fructose-1,6-bisphosphatase (FBP). L. major FBP was constitutively expressed in both extracellular and intracellular stages and was primarily targeted to glycosomes, modified peroxisomes that also contain glycolytic enzymes. A L. major FBP-null mutant was unable to grow in the absence of hexose, and suspension in glycerol-containing medium resulted in rapid depletion of internal carbohydrate reserves. L. major Deltafbp promastigotes were internalized by macrophages and differentiated into amastigotes but were unable to replicate in the macrophage phagolysosome. Similarly, the mutant persisted in mice but failed to generate normal lesions. The data suggest that Leishmania amastigotes reside in a glucose-poor phagosome and depend heavily on nonglucose carbon sources. Feeding experiments with [(13)C]fatty acids showed that fatty acids are poor gluconeogenic substrates, indicating that amino acids are the major carbon source in vivo. The need for amino acids may have forced Leishmania spp. to adapt to life in the mature phagolysosome.

Substrate preferences and glucose uptake in glibenclamide-resistant Leishmania parasites

Several drug-resistant mammalian cell types exhibit increased glycolytic rates, preferential synthesis of ATP through oxidative phosphorylation, and altered glucose transport. Herein we analyzed the influence of parasite growth phase on energy substrate uptake and use in a Leishmania strain [NR(Gr)] selected for resistance against glibenclamide. Glibenclamide is an ABC-transporter blocker which modulates the function of glucose transporters in some mammalian cells. Our results demonstrate for the first time that compared to glibenclamide-sensitive Leishmania, exponential phase glibenclamide-resistant parasites exhibit decreased use of glucose as energy substrate, decreased glucose uptake and decreased glucose transporter expression. However, compared to glibenclamide-sensitive cells, stationary phase resistant parasites display an increased use of amino acids as energy substrate and an increased activity of the enzymes hexokinase, phosphoglucose isomerase, and especially NAD(+)-linked glutamate dehydrogenase. These results suggest that drug resistance in Leishmania involves a metabolic adaptation that promotes a stage dependent modulation of energy substrate uptake and use as a physiological response to the challenge imposed by drug pressure.

The crystal structure and active site location of isocitrate lyase from the fungus Aspergillus nidulans

BACKGROUND

Isocitrate lyase catalyses the first committed step of the carbon-conserving glyoxylate bypass, the Mg(2+)-dependent reversible cleavage of isocitrate into succinate and glyoxylate. This metabolic pathway is an inviting target for the control of a number of diseases, because the enzymes involved in this cycle have been identified in many pathogens including Mycobacterium leprae and Leishmania.

RESULTS

As part of a programme of rational drug design the structure of the tetrameric Aspergillus nidulans isocitrate lyase and its complex with glyoxylate and a divalent cation have been solved to 2.8 A resolution using X-ray diffraction. Each subunit comprises two domains, one of which adopts a folding pattern highly reminiscent of the triose phosphate isomerase (TIM) barrel. A 'knot' between subunits observed in the three-dimensional structure, involving residues towards the C terminus, implies that tetramer assembly involves considerable flexibility in this part of the protein.

CONCLUSIONS

Difference Fourier analysis together with the pattern of sequence conservation has led to the identification of both the glyoxylate and metal binding sites and implicates the C-terminal end of the TIM barrel as the active site, which is consistent with studies of other enzymes with this fold. Two disordered regions of the polypeptide chain lie close to the active site, one of which includes a critical cysteine residue suggesting that conformational rearrangements are essential for catalysis. Structural similarities between isocitrate lyase and both PEP mutase and enzymes belonging to the enolase superfamily suggest possible relationships in aspects of the mechanism.

Leishmania donovani: in vitro culture and [1H] NMR characterization of amastigote-like forms

When Leishmania donovani promastigote forms, were cultured in TC-199 medium at 28 degrees C and subsequently incubated at 38 degrees C, they turned into aflagellate (amastigote-like) forms. A return of the incubation-culture temperature to 28 degrees C these amastigote-like forms to revert to promastigotes. The amastigotes obtained by heat-shock, were viable and retained antigenic capacity being recognized by the sera of naturally infected patients. These forms, remained also capable of multiplying inside the J-774A.1 macrophages. When the amastigote-like forms are kept in culture at 38 degrees C retained their rounded appearance and their biological characteristics for more than 3 months subculturing every 6 days. These amastigote-like forms, when used for subcultures at 28 degrees C, transformed into promastigotes capable of multiplying as flagellate forms. The amastigote-like forms obtained in vitro can be used in biochemical studies related to chemotherapy and immunology studies, as part of an effort to combat this parasite. The end-products of of glycolysis were studied in both the amastigote-like and promastigote forms of L. donovani, by proton magnetic resonance analysis of the culture media. Alanine, succinate, and acetate, were predominant, and to a lesser extent pyruvate, glycine and D-lactate. Our results suggest that both forms of Leishmania use different biochemical strategies to obtain their energy.

Energy metabolism in Leishmania

Alanine plays a key role in the response of promastigotes to osmotic stress and to hypoxia. It is rapidly released in response to hypo-osmolality, is consumed from its large intracellular pool under iso-osmotic conditions even in the presence of glucose, and is synthesized under hyperosmotic conditions even in the absence of glucose. Its rate of oxidation, in the presence or absence of any of ten other amino acids tested, is strongly inhibited by hyperosmolality. Glucose oxidation is also inhibited by hyperosmolality, but to a lesser extent than that of alanine, and is inhibited by alanine, glutamate, and aspartate. Hyperosmolality also inhibits the incorporation of label from [2-14C]acetate into the putative storage carbohydrate, mannan, which occurs via the glyoxylate bypass and the as yet unexplored "mannoneogenic" pathway. The rates of glycolysis and of oxidation of several amino acids decrease with increasing culture age, but the capacity to oxidize fatty acids increases, and in cells from 3-day stationary phase cultures hyperosmolality enhances rather than inhibits alanine oxidation.

Experimental visceral leishmaniasis: role of trans-aconitic acid in combined chemotherapy

We previously reported the effectiveness of trans-aconitic acid (TAA) as an antileishmanial compound. Inhibitory effects of TAA along with other antileishmanial compounds on transformation and in vitro multiplication in macrophage cultures of Leishmania donovani have been assessed. The efficacy of TAA in combined chemotherapy of experimental visceral leishmaniasis has also been evaluated along with those of commonly used antileishmanial compounds such as sodium stibogluconate, pentamidine, and allopurinol. TAA (2 mM) inhibited transformation of L. donovani amastigotes to promastigotes by 95.2%, whereas in combination with pentamidine (5 micrograms/ml), allopurinol (10 micrograms/ml), and sodium stibogluconate (50 micrograms of Sb per ml), it inhibited transformation by about 100, 99, and 98.5%, respectively. Sodium stibogluconate (20 micrograms of Sb per ml), pentamidine (2 micrograms/ml), and allopurinol (5 micrograms/ml) suppressed the amastigote burden in peritoneal macrophage cultures from BALB/c mice by 32.6, 56.1, and 46.3%, respectively. When these three drugs were used along with TAA (5 mM), the parasite loads were reduced by 100, 100, and 88.1%, respectively. TAA (5 mM) alone suppressed the amastigote burden by 59.5%. In experimental visceral leishmaniasis in hamsters (1-month model), TAA at a dose of 200 mg/kg of body weight per day suppressed the spleen parasite load by 73.5%, and TAA in combination with sodium stibogluconate (50 mg of Sb per kg per day), pentamidine (8 mg/kg/day), and allopurinol (15 mg/kg/day) inhibited the spleen parasite load by 98, 98.9, and 97%, respectively. Individually, these three drugs inhibited the parasite load by 35, 20, and 22%, respectively. TAA (400 mg/kg/day) inhibited the spleen parasite load by 99.8%, but an inhibitory effect of approximately 100% was noted when TAA was supplemented with an antileishmanial drug. TAA was administered in experimental animals through oral, intraperitoneal, and intramuscular routes; the intramuscular route was most effective.

Incorporation of label from acetate and laurate into the mannan of Leishmania donovani via the glyoxylate cycle

Leishmania donovani promastigotes in late-stationary phase incorporated label from [2-14C]acetate and [1-14C]laurate into the mannose residues of mannan, thus confirming the presence of a functional glyoxylate bypass in these parasitic protozoa. Isolated, washed calls also incorporated label from [2-14C]acetate and [1-14C]laurate into mannan during a 1-hr incubation in buffer. Glucose had no effect on label incorporation into mannan, but glutamate caused over a four-fold increase in incorporation from [2-14C]acetate and a 2.4-fold increase from [1-14C]laurate. Staurosporine, a protein kinase inhibitor that inhibits glutamate and alanine oxidation, did not inhibit label incorporation from [2-14C]acetate into mannan. Hyperosmolality caused about a 33% inhibition of label incorporation into mannan. These results show the glyoxylate cycle and/or the subsequent biosynthetic pathway from fructose-6-phosphate to mannan are subject to regulation.

Intermediary metabolism of Leishmania

In the course of their existence, parasites develop several metabolic pathways that differ significantly from those of their hosts. Despite the fairly close evolutionary kinship between Leishmania donovani and Trypanosoma brucei, the forms that live in the insect vectors have evolved different strategies for the disposition of available food resources. In this brief review, Joseph Blum will focus on the data available from studies on Leishmania spp and will largely ignore the information available from Trypanosoma spp.

Utilization of a carbohydrate reserve comprised primarily of mannose by Leishmania donovani

Promastigotes from late log phase and 3-day stationary phase cultures of Leishmania donovani were collected, washed in buffer, and the cell pellet was treated with boiling KOH. A putative carbohydrate storage material was then precipitated and washed in ethanol/LiBr. This material did not liberate glucose when treated with amyloglucosidase, indicating that it was not glycogen. Acid hydrolysis released a hexose which was identified as mannose by several criteria. Considerably more of this mannan-like carbohydrate is present in cells from 3-day stationary phase than from late log phase cultures, consistent with the ability of 3-day stationary phase cells to survive in non-nutrient buffer and maintain oxygen consumption for longer than log phase cells. The amount of this mannan-like compound decreased by over 50% during a 3-h incubation in buffer of cells from 3-day stationary phase cultures. The presence of glucose during the incubation prevented the utilization of this carbohydrate, consistent with the possibility that it serves as an energy reserve.

A carbon-13 nuclear magnetic resonance analysis of the products of glucose metabolism in Leishmania pifanoi amastigotes and promastigotes

The major products of glucose metabolism were determined for amastigotes and promastigotes of Leishmania (mexicana) pifanoi under aerobic and anaerobic conditions using carbon-13 nuclear magnetic resonance. Under aerobic conditions, the major products for both forms were carbon dioxide, succinate, malate, acetate and alanine. Succinate was the dominant metabolite of promastigotes, whereas acetate and alanine were most abundant with amastigotes. Under anaerobic conditions, promastigotes produced glycerol as the dominant metabolite, along with lesser amounts of succinate, acetate and alanine; acetate and alanine remained major metabolites in amastigotes, with an increase in the relative amount of succinate and the production of some glycerol. Promastigotes generated carbon dioxide at a 5-fold greater rate than amastigotes under aerobic conditions, but this rate was reduced by more than 95% in the absence of oxygen. Amastigotes were relatively less affected by lack of oxygen and produced carbon dioxide at a rate comparable to promastigotes under anaerobic conditions. The presence of carbohydrates with a possible role in storage was detected in both promastigotes and amastigotes.

Effect of hyper-osmotic stress on alanine content of Leishmania major promastigotes

Earlier studies showed that Leishmania major promastigotes are sensitive to osmotic conditions. A reduction in osmolality caused the cells to shorten and to rapidly release most of their large internal pool of alanine. In this study some effects of hyper-osmotic stress were examined. An increase in osmolality of the culture medium from 308 to 625 mOsm/kg caused only a small decrease in growth rate. When cells grown in the usual culture medium (308 mOsm/kg) were washed, resuspended in iso-osmotic buffer, and subjected to acute hyper-osmotic stress by addition of mannitol, the alanine content increased even in the absence of exogenous substrate. Promastigotes, depleted of alanine by a 5-min exposure to hypo-osmotic conditions, also synthesized alanine when resuspended in iso-osmotic buffer. Washed cells resuspended in iso-osmotic buffer consume their internal pool of alanine under aerobic conditions. Rates of consumption decreased on addition of mannitol, becoming zero at about 440 mOsm/kg. At higher osmolalities, alanine synthesis occurred. To estimate whether proteolysis could account for alanine synthesis in the absence of exogenous substrate, cells that had been grown with [1-14C]leucine were washed and resuspended under hypo-, iso-, and hyper-osmotic conditions and the amounts of 14CO2 and 14C-labelled peptides released in 1 h were measured. Little proteolysis occurred under these conditions, but the possibility that proteolysis was the source of the alanine increase, observed in response to hyper-osmotic stress, cannot be ruled out.

Carbon dioxide abolishes the reverse Pasteur effect in Leishmania major promastigotes

The products released by Leishmania major promastigotes incubated with [1-13C]glucose as sole exogenous carbon source were identified using nuclear magnetic resonance (NMR). Under aerobic (95% O2/5% CO2) conditions, acetate, succinate, and small amounts of pyruvate, D-lactate, and glycerol were released in addition to CO2. Under anaerobic (95% N2/5% CO2) conditions, the relative amounts of products formed changed and alanine was also released. The changes in the rates of glucose consumption and product formation during the aerobic to anaerobic transition were measured. Under hypoxic conditions (O2 less than 0.2%), glucose consumption was decreased by about 50%. Under completely anaerobic conditions (100% N2), glucose consumption almost ceased (a total reverse Pasteur effect). The inclusion of 5% CO2 in the gas phase restored hypoxic and anaerobic glucose consumption to the aerobic rate, and increased production of succinate, pyruvate, and D-lactate. Thus, CO2 and very low concentrations of O2 have strong regulatory effects on L. major glucose metabolism. A quantitative carbon balance showed that the NMR-identified products accounted for only about 25% of the glucose carbons consumed under aerobic conditions. CO2, measured as the release of 14CO2 from [U-14C]glucose, accounted for an additional 25% of the glucose consumed. About 11% of the glucose carbon was incorporated into trichloroacetic acid-insoluble products, mostly lipid. Large amounts of label from [U-14C]glucose were incorporated into the intracellular pools of alanine, glutamate, glutamine, and aspartate, indicating that CO2 from unlabeled amino acids contributed to the carbon balance. Under anaerobic conditions, all the glucose carbons consumed could be accounted for solely by the NMR-identified products.

Purification and characterization of a metabolite-regulated pyruvate kinase from Leishmania major promastigotes

The pyruvate kinase (ATP:pyruvate 2-O-phosphotransferase, EC 2.7.1.40) of Leishmania major promastigotes is a multimer of 59 kDa subunits having an Mr 181000. It is activated by its substrate phosphoenolpyruvate (PEP) in a positively cooperative manner, and heterotropically by fructose 1,6-bisphosphate (FBP). Kinetics with regard to the phosphate acceptor adenosine 5'-diphosphate (ADP), MgCl2, and KCl are hyperbolic and unaffected by FBP. The enzyme is strongly inhibited by the reaction product ATP, as well as GTP and ITP, and to a lesser degree by citrate. Of seven amino acids reported to inhibit the pyruvate kinases of other organisms, none have any effect on the L. major pyruvate kinase in vitro. The enzyme shows its maximum activity at pH 7.0 in the absence of FBP, and at pH 7.6 in its presence. Contrary to previous suggestions, the enzyme appears to be well-suited for a regulatory role in the metabolism of an aerobic organism capable of net glucose synthesis.

Regulation of aerobic fermentation in Leishmania donovani promastigotes by NADP+-dependent malic enzyme

NADP+-dependent malic enzyme (decarboxylating) was extracted from Leishmania donovani promastigotes with Triton X-100. The enzyme was specific for NADP+ and did not decarboxylate oxaloacetate (OA). The substrate activity relationship was hyperbolic for both L-malate and NADP+, and Km values were calculated as 0.18 and 0.12 mM, respectively. The enzyme exhibited a broad pH optimum of 7.5-8.0. Pyruvate, NADPH and OA inhibited the reaction in a competitive manner with apparent Ki values of 0.2, 0.04 and 0.04 mM, respectively, while oxalate inhibition was of the mixed type. The kinetic results obtained indicate that malic enzyme is involved in the regulation of carbon flow towards aerobic fermentation, complete oxidation of dicarboxylic acids or biosynthetic purposes.

Sugar synthesis in Leptospira. II. Presence of glyoxylate cycle enzymes

The presence and some properties of the key enzymes of the glyoxylate cycle, isocitrate lyase (threo-Ds-isocitrate glyoxylate-lyase, EC 4.1.3.1) and malate synthase (L-malate glyoxylate-lyase (CoA-acetylating) EC 4.1.3.2), were investigated in Leptospira biflexa. Isocitrate lyase activity was found for the first time in the organism. The enzyme was induced by ethanol but not by acetate. The optimum pH was 6.8. The activity was inhibited by phosphoenolpyruvate, a specific inhibitor of isocitrate lyase. The optimum pH of malate synthase of L. biflexa was about 8.5. The Km value for glyoxylate was 3.0 X 10(-3) M and the activity was inhibited by glycolate, the inhibitor. The results strongly suggested the presence of a glyoxylate cycle in Leptospira. The possibility that the glyoxylate cycle plays an essential role in the synthesis of sugars, amino acids and other cellular components as an anaplerotic pathway of the tricarboxylic acid cycle in Leptospira was discussed.

The free amino acid pool in Leishmania tropica promastigotes

The principal constituents of the free amino acid pool in Leishmania tropica promastigotes include alanine, glycine, serine, threonine, alpha-aminobutyric acid and ethanolamine. Changes in levels of the pool constituents during starvation, nutrient supplementation and treatment with a metabolic inhibitor (sodium fluoride) were followed. Several enzymes capable of converting amino acids to Krebs cycle intermediates were identified. It is suggested that during prolonged starvation which is accompanied by enhanced proteolysis, these enzymes enable the cells to utilize endogenous amino acids as carbon and energy sources. This is compatible with previous reports that Leishmania promastigotes in culture could grow on amino acids as growth substrates when carbohydrates are totally lacking.

Respiration of Leishmania mexicana amastigotes and promastigotes

Promastigotes of Leishmania mexicana mexicana recently derived from amastigotes by transformation in vitro respired at a rate (17 nmol O2/min per 10(8) parasites) 4-5 times higher than that of amastigotes, but when the difference in cell protein content between the two preparations was taken into account the rates were not significantly different (32 nmol O2/min per mg protein). The respiration of both amastigotes and promastigotes was sensitive to cyanide, azide, antimycin A, 2-n-heptyl-4-hydroxyquinoline-N-oxide and high concentrations of amytal, but insensitive to rotenone and salicyl-hydroxamic acid, indicating that the two developmental forms possess a similar cytochrome-containing respiratory chain. D-Glucose and non-esterified fatty acids stimulated promastigote respiration and amastigote transformation to promastigotes in vitro; possibly these substances are important exogenous energy substrates for both forms of the parasites. Amino acids (incuding L-proline) and proteins did not appear to be used as energy substrates. The respiration rate of promastigotes was found to rise significantly upon continued sub-culture in vitro; at the same time cell size and protein content increased.