Ouabain-sensitive Na+,K(+)-ATPase in the plasma membrane of Leishmania mexicana

P-type transport ATPases in Leishmania and Trypanosoma

P-type ATPases are critical to the maintenance and regulation of cellular ion homeostasis and membrane lipid asymmetry due to their ability to move ions and phospholipids against a concentration gradient by utilizing the energy of ATP hydrolysis. P-type ATPases are particularly relevant in human pathogenic trypanosomatids which are exposed to abrupt and dramatic changes in their external environment during their life cycles. This review describes the complete inventory of ion-motive, P-type ATPase genes in the human pathogenic Trypanosomatidae; eight Leishmania species (L. aethiopica, L. braziliensis, L. donovani, L. infantum, L. major, L. mexicana, L. panamensis, L. tropica), Trypanosoma cruzi and three Trypanosoma brucei subspecies (Trypanosoma brucei brucei TREU927, Trypanosoma brucei Lister strain 427, Trypanosoma brucei gambiense DAL972). The P-type ATPase complement in these trypanosomatids includes the P_1B (metal pumps), P_2A (SERCA, sarcoplasmic-endoplasmic reticulum calcium ATPases), P_2B (PMCA, plasma membrane calcium ATPases), P_2D (Na⁺ pumps), P_3A (H⁺ pumps), P₄ (aminophospholipid translocators), and P_5B (no assigned specificity) subfamilies. These subfamilies represent the P-type ATPase transport functions necessary for survival in the Trypanosomatidae as P-type ATPases for each of these seven subfamilies are found in all Leishmania and Trypanosoma species included in this analysis. These P-type ATPase subfamilies are correlated with current molecular and biochemical knowledge of their function in trypanosomatid growth, adaptation, infectivity, and survival.

Anti-parasitic effect of the diuretic and Na+-ATPAse inhibitor furosemide in cutaneous leishmaniasis

Leishmania amazonensis promastigotes are known to express furosemide (Lasix®)-sensitive P-type membrane Na+-ATPase. In the present study, furosemide activity was studied in intracellular amastigotes and infected BALB/c mice to investigate its efficacy in cutaneous leishmaniasis (CL). Intracellular parasites, but not macrophages, were found to be sensitive to killing by furosemide (IC50 = 87 µ m vs CC50 ≫ 1000 µ m, respectively). Although furosemide did not induce nitric oxide production or intracellular pH changes in infected macrophages, it led to a significant reactive oxygen species (ROS) burst. Freshly isolated tissue parasites expressed a high degree of Na+-ATPase activity that decreased with culture, indicative of a higher enzyme expression in amastigotes than in promastigotes. Both intraperitoneal and oral treatment of L. amazonensis-infected mice with furosemide dosages equivalent to that prescribed as a diuretic significantly reduced the parasite's growth compared with the situation in untreated mice. Combination with oral furosemide increased the efficacy and safety of intraperitoneal treatment with sodium stibogluconate (SSG). To summarize, furosemide control of intracellular leishmanial growth by means of parasite Na+-ATPase inhibition, and macrophage ROS activation may help explain its sole and SSG-combined therapeutic effect against murine CL.

Modulation of Na+/K+ ATPase Activity by Hydrogen Peroxide Generated through Heme in L. amazonensis

Leishmania amazonensis is a protozoan parasite that occurs in many areas of Brazil and causes skin lesions. Using this parasite, our group showed the activation of Na⁺/K⁺ ATPase through a signaling cascade that involves the presence of heme and protein kinase C (PKC) activity. Heme is an important biomolecule that has pro-oxidant activity and signaling capacity. Reactive oxygen species (ROS) can act as second messengers, which are required in various signaling cascades. Our goal in this work is to investigate the role of hydrogen peroxide (H₂O₂) generated in the presence of heme in the Na⁺/K⁺ ATPase activity of L. amazonensis. Our results show that increasing concentrations of heme stimulates the production of H₂O₂ in a dose-dependent manner until a concentration of 2.5 μM heme. To confirm that the effect of heme on the Na⁺/K⁺ ATPase is through the generation of H₂O₂, we measured enzyme activity using increasing concentrations of H₂O₂ and, as expected, the activity increased in a dose-dependent manner until a concentration of 0.1 μM H₂O₂. To investigate the role of PKC in this signaling pathway, we observed the production of H₂O₂ in the presence of its activator phorbol 12-myristate 13-acetate (PMA) and its inhibitor calphostin C. Both showed no effect on the generation of H₂O₂. Furthermore, we found that PKC activity is increased in the presence of H₂O₂, and that in the presence of calphostin C, H₂O₂ is unable to activate the Na⁺/K⁺ ATPase. 100 μM of Mito-TEMPO was capable of abolishing the stimulatory effect of heme on Na⁺/K⁺ ATPase activity, indicating that mitochondria might be the source of the hydrogen peroxide production induced by heme. The modulation of L. amazonensis Na⁺/K⁺ ATPase by H₂O₂ opens new possibilities for understanding the signaling pathways of this parasite.

Correlation between glucose uptake and membrane potential in Leishmania parasites isolated from DCL patients with therapeutic failure: a proof of concept

Besides infection with drug-resistant parasites, therapeutic failure in leishmaniasis may be caused by altered drug pharmacokinetics, re-infection, and host immunologic compromise. Our aim has been to evaluate if relapses that occur in patients suffering from diffuse cutaneous leishmaniasis (DCL) associate with changes in the fitness of infecting organisms. Therefore, in isolates from patients suffering DCL, we correlated glucose uptake and plasma membrane potential and compared the results with those obtained from reference strains. The data demonstrate that Leishmania parasites causing DCL incorporate glucose at an efficient rate, albeit without significant changes in the plasma membrane potential as their corresponding reference strains. The isolate that did not change its accumulation rate of glucose compared to its reference strain expressed a less polarized membrane potential that was insensitive to mitochondrial inhibitors, suggesting a metabolic dysfunction that may result in glycolysis being the main source of ATP. The results constitute a proof of concept that indicates that parasites causing DCL adapted well to drug pressure and expressed an increased fitness. That is, that in Leishmania mexicana and Leishmania amazonensis, parasites isolated from DCL patients, a strong modification of the parasite physiology might occur. As consequences, the parasites adapted well to drug pressure, increased their fitness, and they had an efficient glucose uptake rate albeit not significant changes in membrane potential as their corresponding reference strains. Further validation of the concepts herein established and whether or not the third isolate corresponds with a drug-resistant phenotype need to be demonstrated at the genetic level.

A novel sucrose/H+ symport system and an intracellular sucrase in Leishmania donovani

The flagellated form of pathogenic parasitic protozoa Leishmania, resides in the alimentary tract of its sandfly vector, where sucrose serves as a major nutrient source. In this study we report the presence of a sucrose transport system in Leishmania donovani promastigotes. The kinetics of sucrose uptake in promastigotes are biphasic in nature with both high affinity K(m) (K(m) of ∼ 75 μM) and low affinity K(m) (K(m)∼ 1.38 mM) components. By contrast the virulent amastigotes take up sucrose via a low affinity process with a K(m) of 2.5mM. The transport of sucrose into promastigotes leads to rapid intracellular acidification, as indicated by changes in the fluorescence of the pH indicator 2',7'-bis-(2-carboxyethyl)-5-(6) Carboxyfluorescein (BCECF). In experiments with right side-out plasma membrane vesicles derived from L. donovani promastigotes, an artificial pH gradient was able to drive the active accumulation of sucrose. These data are consistent with the operation of a H(+)-sucrose symporter. The symporter was shown to be independent of Na(+) and to be insensitive to cytochalasin B, to the flavonoid phloretin and to the Na(+)/K(+) ATPase inhibitor ouabain. However, the protonophore carbonylcyanide P- (trifluromethoxy) phenylhydrazone (FCCP) and a number of thiol reagents caused significant inhibition of sucrose uptake. Evidence was also obtained for the presence of a stable intracellular pool of the sucrose splitting enzyme, sucrase, in promastigote stage parasites. The results are consistent with the hypothesis that L. donovani promastigotes take up sucrose via a novel H(+)-sucrose symport system and that, on entering the cell, the sucrose is hydrolysed to its component monosaccharides by an intracellular sucrase, thereby providing an energy source for the parasites.

Paromomycin: uptake and resistance in Leishmania donovani

Paromomycin is currently in phase IV clinical trials against leishmaniasis. In the present work we elucidate the effect and mechanism of uptake of paromomycin in Leishmania donovani. The in vitro sensitivities of both promastigotes and amastigotes were determined to this aminoglycoside. Association of paromomycin with L. donovani involved a rapid initial phase that was non-saturable up to 1mM of the drug. This initial phase was largely independent of temperature and not affected by metabolic inhibitors. Poly-lysine, a membrane impermeant polycation, caused profound inhibition of this association of the drug with the parasite indicating that it represented a binding of the cationic paromomycin to the negatively charged leishmanial glycocalyx. After 72h of exposure to the drug the mitochondrial membrane potential was significantly decreased, indicating that this organelle might be the ultimate target of the drug. Both cytoplasmic and mitochondrial protein synthesis were inhibited following paromomycin exposure. A line selected for resistance to the drug showed reduced paromomycin accumulation associated with a significant reduction in the initial binding to the cell surface. The drug induced reduction in membrane potential and inhibition of protein synthesis were less pronounced in the resistant strain in comparison to the wild-type.

Leishmania amazonensis: characterization of an ouabain-insensitive Na+-ATPase activity

We characterized ouabain-insensitive Na+-ATPase activity present in the plasma membrane of Leishmania amazonensis and investigated its possible role in the growth of the parasite. An increase in Na+ concentration in the presence of 1mM ouabain, increased the ATPase activity with a V(max) of 154.1+/-13.5nmol Pi x h(-1) x mg(-1) and a K0.5 of 28.9+/-7.7mM. Furosemide and sodium orthovanadate inhibited the Na+-stimulated ATPase activity with an IC(50) of 270microM and 0.10microM, respectively. Furosemide inhibited the growth of L. amazonensis after 48h incubation, with maximal effect after 96h. The IC50 for furosemide was 840. On the other hand, ouabain (1mM) did not change the growth of the parasite. Taken together, these results show that L. amazonensis expresses a P-type, ouabain-insensitive Na+-ATPase that could be involved with the growth of the parasite.

Entamoeba histolytica: ouabain-insensitive Na(+)-ATPase activity

Our aim was to determine the presence of sodium pumps in Entamoeba histolytica. It is shown through the measurement of ouabain-sensitive ATPase activity and immunoblotting that E. histolytica does not express (Na(+)+K(+))ATPase. On the other hand, we observed a Na(+)-ATPase with the following characteristics: (1) stimulated by Na(+) or K(+), but these effects are not addictive; (2) the apparent affinity is similar for Na(+) and K(+) (K(0.5) = 13.3 +/- 3.7 and 15.4 +/- 3.1mM, respectively), as well as the V(max) (24.9 +/- 1.5 or 27.5 +/- 1.6 nmol Pi mg(-1)min(-1), respectively); (3) insensitive up to 2mM ouabain; and (4) inhibited by furosemide with an IC(50) of 0.12 +/- 0.004 mM. Furthermore, this enzyme forms a Na(+)- or K(+)-stimulated, furosemide- and hydroxylamine-sensitive ATP-driven acylphosphate phosphorylated intermediate.

Leishmania amazonensis: PKC-like protein kinase modulates the (Na++K+)ATPase activity

The present study aimed to identify the presence of protein kinase C-like (PKC-like) in Leishmania amazonensis and to elucidate its possible role in the modulation of the (Na(+)+K(+))ATPase activity. Immunoblotting experiments using antibody against a consensus sequence (Ac 543-549) of rabbit protein kinase C (PKC) revealed the presence of a protein kinase of 80 kDa in L. amazonensis. Measurements of protein kinase activity showed the presence of both (Ca(2+)-dependent) and (Ca(2+)-independent) protein kinase activity in plasma membrane and cytosol. Phorbol ester (PMA) activation of the Ca(2+)-dependent protein kinase stimulated the (Na(+)+K(+))ATPase activity, while activation of the Ca(2+)-independent protein kinase was inhibitory. Both effects of protein kinase on the (Na(+)+K(+))ATPase of the plasma membrane were lower than that observed in intact cells. PMA induced the translocation of protein kinase from cytosol to plasma membrane, indicating that the maximal effect of protein kinase on the (Na(+)+K(+))ATPase activity depends on the synergistic action of protein kinases from both plasma membrane and cytosol. This is the first demonstration of a protein kinase activated by PMA in L. amazonensis and the first evidence for a possible role in the regulation of the (Na(+)+K(+))ATPase activity in this trypanosomatid. Modulation of the (Na(+)+K(+))ATPase by protein kinase in a trypanosomatid opens up new possibilities to understand the regulation of ion homeostasis in this parasite.

Camptothecin-induced imbalance in intracellular cation homeostasis regulates programmed cell death in unicellular hemoflagellate Leishmania donovani

Leishmania, a unicellular trypanosomatid protozoan parasite, causes a wide range of human diseases ranging from the localized self-healing cutaneous lesions to fatal visceral leishmaniasis. However, it undergoes a process of programmed cell death during treatment with the topoisomerase I poison camptothecin (CPT). The present study shows that CPT-induced formation of reactive oxygen species increases the level of cytosolic calcium through the release of calcium ions from intracellular stores as well as by influx of extracellular calcium. Elevation of cytosolic calcium is responsible for depolarization of mitochondrial membrane potential (DeltaPsim), which is followed by a significant decrease in intracellular pH levels. CPT-induced oxidative stress also causes impairment of the Na+ - K+ -ATPase pump and subsequently decreases the intracellular K+ level in leishmanial cells. A decrease in both intracellular pH and K+ levels propagates the apoptotic process through activation of caspase 3-like proteases by rapid formation of cytochrome c-mediated apoptotic complex. In addition to caspase-like protease activation, a lower level of intracellular K+ also enhances the activation of apoptotic nucleases at the late stage of apoptosis. This suggests that the physiological level of pH and K+ are inhibitory for apoptotic DNA fragmentation and caspase-like protease activation in leishmanial cells. Moreover, unlike mammalian cells, the intracellular ATP level gradually decreases with an increase in the number of apoptotic cells after the loss of DeltaPsim. Taken together, the elucidation of biochemical events, which tightly regulate the process of growth arrest and death of Leishmania donovani promastigotes, allows us to define a more comprehensive view of cell death during treatment with CPT.

Osmoregulation in the parasitic protozoan Tritrichomonas foetus

Tritrichomonas foetus was shown to undergo a regulatory volume increase (RVI) when it was subjected to hyperosmotic challenge, but there was no regulatory volume decrease after hypoosmotic challenge, as determined by using both light-scattering methods and measurement of intracellular water space to monitor cell volume. An investigation of T. foetus intracellular amino acids revealed a pool size (65 mM) that was similar to that of Trichomonas vaginalis but was considerably smaller than those of Giardia intestinalis and Crithidia luciliae. Changes in amino acid concentrations in response to hyperosmotic challenge were found to account for only 18% of the T. foetus RVI. The T. foetus intracellular sodium and potassium concentrations were determined to be 35 and 119 mM, respectively. The intracellular K(+) concentration was found to increase considerably during exposure to hyperosmotic stress, and, assuming that there was a monovalent accompanying anion, this increase was estimated to account for 87% of the RVI. By using light scattering it was determined that the T. foetus RVI was enhanced by elevated external K(+) concentrations and was inhibited when K(+) and/or Cl(-) was absent from the medium. The results suggested that the well-documented Na(+)-K(+)-2Cl(-) cotransport system was responsible for the K(+) influx activated during the RVI. However, inhibitors of Na(+)-K(+)-2Cl(-) cotransport in other systems, such as quinine, ouabain, furosemide, and bumetanide, had no effect on the RVI or K(+) influx in T. foetus.

Identification of surface-membrane P-type ATPases resembling fungal K(+)- and Na(+)-ATPases, in Trypanosoma brucei, Trypanosoma cruzi and Leishmania donovani

Genomic DNA fragments encoding nine, novel, P-type ATPases in trypanosomatid organisms were amplified in PCR, using degenerate oligonucleotide primers that recognize the ATP-binding and -phosphorylation sites present in all P-type ATPases. Subsequent phylogenetic analysis, based on the presence of conserved motifs in predicted peptide sequences for six Trypanosoma brucei, T. cruzi or Leishmania donovani PCR fragments, identified calcium-, proton- and phospholipid-translocating ATPases. DNA fragments that predict proteins homologous to the fungal, type-IID, P-type, ATPase pumps that transport Na(+) or K(+) ions were also present in T. brucei (TBCA1; 1022 nucleotides representing 340 amino acids), T. cruzi (TCNA1; 1022 nucleotides representing 340 amino acids) and L. donovani (LDCA1; 1031 nucleotides representing 343 amino acids). Southern blots showed that the Na(+)-ATPases were each present as a single-copy gene. The LDCA1 fragment was used to clone the complete LDCA1 gene from an L. donovani genomic-DNA library. The LDCA1 gene encodes a protein, of 1047 amino acids, with a predicted molecular mass of 115,501 Da. The results of analyses based on northern blots and the rapid amplification of cDNA ends (RACE) indicated that LDCA1 was expressed in promastigotes and amastigotes from axenic cultures and in animal-derived amastigotes. TBCA1 was expressed, as a 5.0-kb transcript, in procyclic culture stages and bloodstream trypomastigotes, with the 5.0-kb message up-regulated six-fold in the trypomastigote stage. Western blots probed with an antibody to the partial TBCA1 peptide identified a 150-kDa protein that was detected, by immunofluorescence, on the surface membrane of procyclic T. brucei.

Resistance to pentamidine in Leishmania mexicana involves exclusion of the drug from the mitochondrion

The uptake of [(3)H]pentamidine into wild-type and drug-resistant strains of Leishmania mexicana was compared. Uptake was carrier mediated. Pentamidine-resistant parasites showed cross-resistance to other toxic diamidine derivatives. A substantial decrease in accumulation of the drug accompanied the resistance phenotype, although the apparent affinity for pentamidine by its carrier was not altered when initial uptake velocity was measured. The apparent V(max), however, was reduced. An efflux of pentamidine could be measured in both wild-type and resistant cells. Only a relatively small proportion of the total accumulated pentamidine was available for efflux in wild-type cells, while in resistant cells the majority of loaded pentamidine was available for release. Pharmacological reagents which diminish the mitochondrial membrane potential reduced pentamidine uptake in wild-type parasites, and the mitochondrial membrane potential was shown to be reduced in resistant cells. A fluorescent analogue of pentamidine, 4',6'-diamidino-2-phenylindole, accumulated in the kinetoplast of wild-type but not resistant parasites. These data together indicate that diamidine drugs accumulate in the Leishmania mitochondrion and that the development of the resistance phenotype is accompanied by lack of mitochondrial accumulation of the drug and its exclusion from the parasites.

Choline transport in Leishmania major promastigotes and its inhibition by choline and phosphocholine analogs

Phosphatidylcholine is the most abundant phospholipid in the membranes of the human parasite Leishmania. The metabolic pathways leading to its biosynthesis are likely to play a critical role in parasite development and survival and may offer a good target for antileishmanial chemotherapy. Phosphatidylcholine synthesis via the CDP-choline pathway requires transport of the choline precursor from the host. Here, we report the first characterization of choline transport in this parasite, which is carrier-mediated and exhibits Michaelis-Menten kinetics with an apparent K(m) value of 2.5 microM for choline. This process is Na(+)-independent and requires an intact proton gradient to be fully functional. Choline transport into Leishmania is highly specific for choline and is inhibited by the choline carrier inhibitor hemicholinium-3, the channel blocker quinacrine, the antimalarial aminoquinolines quinine and quinidine, the antileishmanial phosphocholine analogs, miltefosine and edelfosine, and by choline analogs, most of which have antimalarial activities. Most importantly, choline analogs kill the promastigote form of the parasite in vitro in the low micromolar range. These results set the stage for the use of choline analogs in antileishmanial chemotherapy and shed new lights on the mechanism of action of the leishmanicidal phosphocholine analogs.

Secretory pathway of trypanosomatid parasites

The Trypanosomatidae comprise a large group of parasitic protozoa, some of which cause important diseases in humans. These include Trypanosoma brucei (the causative agent of African sleeping sickness and nagana in cattle), Trypanosoma cruzi (the causative agent of Chagas' disease in Central and South America), and Leishmania spp. (the causative agent of visceral and [muco]cutaneous leishmaniasis throughout the tropics and subtropics). The cell surfaces of these parasites are covered in complex protein- or carbohydrate-rich coats that are required for parasite survival and infectivity in their respective insect vectors and mammalian hosts. These molecules are assembled in the secretory pathway. Recent advances in the genetic manipulation of these parasites as well as progress with the parasite genome projects has greatly advanced our understanding of processes that underlie secretory transport in trypanosomatids. This article provides an overview of the organization of the trypanosomatid secretory pathway and connections that exist with endocytic organelles and multiple lytic and storage vacuoles. A number of the molecular components that are required for vesicular transport have been identified, as have some of the sorting signals that direct proteins to the cell surface or organelles in the endosome-vacuole system. Finally, the subcellular organization of the major glycosylation pathways in these parasites is reviewed. Studies on these highly divergent eukaryotes provide important insights into the molecular processes underlying secretory transport that arose very early in eukaryotic evolution. They also reveal unusual or novel aspects of secretory transport and protein glycosylation that may be exploited in developing new antiparasite drugs.

Leishmania plasma membrane Mg2+-ATPase is a H+/K+-antiporter involved in glucose symport. Studies with sealed ghosts and vesicles of opposite polarity

Experiments from other laboratories conducted with Leishmania donovani promastigote cells had earlier indicated that the plasma membrane Mg2+-ATPase of the parasite is an extrusion pump for H+. Taking advantage of the pellicular microtubular structure of the plasma membrane of the organism, we report procedures for obtaining sealed ghost and sealed everted vesicle of defined polarity. Rapid influx of H+ into everted vesicles was found to be dependent on the simultaneous presence of ATP (1 mm) and Mg2+ (1 mm). Excellent correspondence between rate of H+ entry and the enzyme activity clearly demonstrated the Mg2+-ATPase to be a true H+ pump. H+ entry into everted vesicle was strongly inhibited by SCH28080 (IC50 = approximately 40 microm) and by omeprazole (IC50 = approximately 50 microm), both of which are characteristic inhibitors of mammalian gastric H+,K+-ATPase. H+ influx was completely insensitive to ouabain (250 microm), the typical inhibitor of Na+,K+-ATPase. Mg2+-ATPase activity could be partially stimulated with K+ (20 mm) that was inhibitable (>85%) with SCH28080 (50 microm). ATP-dependent rapid efflux of 86Rb+ from preloaded vesicles was completely inhibited by preincubation with omeprazole (150 microm) and by 5,5'-dithiobis-(2-nitrobenzoic acid) (1 mm), an inhibitor of the enzyme. Assuming Rb+ to be a true surrogate for K+, an ATP-dependent, electroneutral stoichiometric exchange of H+ and K+(1:1) was established. Rapid and 10-fold active accumulation of [U-(14)C]2-deoxyglucose in sealed ghosts could be observed when an artificial pH gradient (interior alkaline) was imposed. Rapid efflux of [U-(14)C]d-glucose from preloaded everted vesicles could also be initiated by activating the enzyme, with ATP. Taken together, the plasma membrane Mg2+-ATPase has been identified as an electroneutral H+/K+ antiporter with some properties reminiscent of the gastric H+,K+-ATPase. This enzyme is possibly involved in active accumulation of glucose via a H+-glucose symport system and in K+ accumulation.

Putrescine and spermidine transport in Leishmania

The transport of putrescine and spermidine into Leishmnania donovani promastigotes and Leishmania mexicana promastigotes and amastigotes has been characterised. Polyamine transport was shown to be saturable and temperature-sensitive for both developmental stages of Leishmania. Transport was pH-dependent with pH optima of 7.4 and 5.5 for promastigotes and amastigotes, respectively. The uptake process was independent of extracellular Na+, but inhibited by protonophores and H+-ATPase inhibitors. Kinetic analyses of polyamine transport showed that Km and Vmax differed between promastigotes of the two species and between promastigotes and amastigotes of L. mexicana. Inhibition data suggest that putrescine and spermidine use different transporters. The aromatic diamidine pentamidine, the drug of choice for treatment of antimonial-resistant cases of leishmaniasis, inhibited both putrescine and spermidine transport non-competitively.

Intracellular pH in mammalian stages of Trypanosoma cruzi is K+-dependent and regulated by H+-ATPases

Regulation of intracellular pH (pHi) was investigated in Trypanosoma cruzi amastigotes and trypomastigotes using 2',7'-bis-(carboxyethyl)-5(and-6)-carboxyfluorescein (BCECF). pHi was determined to be 7.33 +/- 0.08 and 7.35 +/- 0.07 in amastigotes and trypomastigotes, respectively, and there were no significant differences in the regulation of pH, between the two stages. Steady-state pHi, recovery of pHi from acidification, and H+-efflux were all decreased markedly by the H+-ATPase inhibitors N,N'-dicyclohexylcarbodi-imide (DCCD), diethylstilbestrol (DES) and N-ethylmaleimide (NEM) supporting a significant role for a plasma membrane H+-ATPase in the regulation of pHi. pHi was maintained at neutrality over a range of external pH (pHe) from 5-8 in parasites suspended in a buffer containing Na+ and K+ (standard buffer) but was acidified at low pHe in the absence of these cations (choline buffer). The pHi of trypomastigotes decreased significantly when they transformed into amastigotes. The rate of recovery of pHi by acidified parasites was similar in Na+-free buffer and standard buffer but was slower in the absence of K+ (K+-free or choline buffer) and parasites suspended in choline buffer were acidic by 0.25 pH units as compared with controls. Ba2+ and Cs+ decreased the pHi of parasites suspended in standard but not choline buffer suggesting the presence of an inward directed K+ channel. The pHi of amastigotes and trypomastigotes suspended in Cl(-)-free buffer was decreased by 0.13 and 0.2 pH units, respectively, supporting the presence of a chloride conductive channel. No evidence of pH regulation via a Na+/H+ or Cl-/HCO3- exchanger was found. These results are consistent with the presence of a plasma membrane H+-ATPase that regulates pHi and is supported by K+ and Cl- channels.

Comparative phosphorylation of calmodulin from trypanosomatids and bovine brain by calmodulin-binding protein kinases

Calmodulin (CaM), a major intracellular Ca2+ receptor protein, has been identified and partially characterized in several trypanosomatids. The amino acid sequences of CaM from Trypanosoma cruzi and Trypanosoma brucei are known, while that from Leishmania mexicana is not. CaM from T. cruzi contains 18 amino acid substitutions, as compared with CaM from bovine brain. In addition, CaM from bovine brain contains two tyrosine residues (Tyr-99 and Tyr-138), while CaM from T. cruzi only contains Tyr-138. In the present work we show that a monoclonal antibody developed against the carboxyl-terminal region of bovine brain CaM fails to recognize CaM from both T. cruzi and L. mexicana. CaM from both parasites and from bovine brain were phosphorylated in vitro by a preparation of CaM-binding protein kinases enriched in the epidermal growth factor (EGF) receptor. Phosphoamino acids analysis demonstrated EGF-dependent phosphorylation of tyrosine residues in bovine brain CaM, while only trace amounts of tyrosine phosphorylation were detected in CaM from both trypanosomatids. These results demonstrate that the EGF receptor tyrosine kinase targets Tyr-99, but not Tyr-138, as the single major phosphorylatable residue of CaM. On the other hand, and in contrast to bovine brain CaM, there is a significant phosphorylation of serine residues in CaM from trypanosomatids which is activated by the EGF receptor via a protein-serine/threonine kinase cascade.

Proton-ATPase activities involved in the uptake of an S-adenosylmethionine analogue

Characteristics of the transport of sinefungin (SF) were studied in Leishmania donovani promastigotes grown in vitro in a semi-defined medium. The uptake is time and pH dependent, temperature sensitive, saturable and independent of the growth phase. Metabolic inhibitors decrease the influx, indicating that sinefungin uptake is an energy requiring process. The presence of Na+ is unnecessary for activity. The uptake is sensitive to valinomycin and nigericin and to the H+-ATPases inhibitors such as N'N'-dicyclohexylcarbodiimide, bafilomycin A and oligomycin. Sulfhydryl group(s) are involved in carrier activity. Use of SF analogues shows, stereospecificity of the transporter, recognition of the 6'-amino group and to a lesser degree of the 9'-amino group of the lateral chain, whereas the 9'-carboxyl group of the lateral chain is not implicated in the recognition. Adenosine and ornithine do not interfere with the uptake. No significant amount of SF is tightly bound to macromolecules. In a SF-resistant clone, though the uptake of SF is reduced (the apparent Vmax is 276 pmoles mg protein(-1) 30 min(-1) compared with 2061 pmoles mg protein(-1) 30 min(-1) for the wild-type clone), the apparent affinity for SF is similar to that of wild-type cells (Km 0.7 and 0.6 microM respectively). This lower uptake activity is not the reflection of an increased efflux of the drug. In these resistant cells, the susceptibility of SF uptake to variation of the external pH, as well as to azide, NaF, and valinomycin are decreased, that to nigericin is lost.