Glutathione reductase-deficient erythrocytes as host cells of malarial parasites

The Evaluation of Metal Co-ordinating Bis-Thiosemicarbazones as Potential Anti-malarial Agents

Background:

The emergence of resistance to the artemisinins which are the current mainstays for antimalarial chemotheraphy has created an environment where the development of new drugs acting in a mechanistally discrete manner is a priority.

Objective:

The goal of this work was to synthesize ane evaluate bis-thiosemicarbazones as potential antimalarial agents. </P><P> Methods: Fifteen compounds were generated using two condensation protocols and evaluated in vitro against the NF54 (CQ sensitive) strain of Plasmodium falciparum. A preliminary assessment of the potential for human toxicity was conducted in vitro against the MRC5 human lung fibroblast line.

Results:

The activity of the bis-thiosemicarbazones was highly dependent on the nature of the arene at the core of the structure. The inclusion of a non-coordinating benzene core resulted in inactive compounds, while the inclusion of a pyridyl core resulted in compounds of moderate or potent antimalarial activity (4 compounds showing IC50 < 250 nM).

Conclusion:

Bis-thiosemicarbazones containing a central pyridyl core display potent antimalarial activity in vitro. Sequestration and activation of ferric iron appears to play a significant role in this activity. Ongoing studies are aimed at further development of this series as potential antimalarials.

A practical toxicity bioassay for vicine and convicine levels in faba bean (Vicia faba)

BACKGROUND

Faba bean (Vicia faba) vicine and convicine (V-C) aglycones (divicine and isouramil respectively) provoke an acute hemolytic anemia called favism in individuals with a glucose-6-phosphate dehydrogenase (G6PD) enzyme defect in their red blood cells. Geneticists/plant breeders are working with faba bean to decrease V-C levels to improve public acceptance of this high-protein pulse crop. Here, we present a fast and simple ex vivo in vitro bioassay for V-C toxicity testing of faba bean or faba bean food products.

RESULTS

We have shown that 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU)-treated (i.e., sensitized) normal red blood cells, like G6PD-defective blood, displayed (i) continuous glutathione (GSH) depletion with no regeneration as incubation time and the dose of aglycones increased, (ii) progressive accumulation of denatured hemoglobin products into high molecular weight (HMW) proteins with increased aglycone dose, (iii) both band 3 membrane proteins and hemichromes, in HMW protein aggregates. We have also demonstrated that sensitized red blood cells can effectively differentiate various levels of toxicity among faba bean varieties through the two hemolysis biomarkers: GSH depletion and HMW clumping.

CONCLUSION

BCNU-sensitized red blood cells provide an ideal model for favism blood, to assess and compare the toxicity of faba bean varieties and their food products. © 2018 Society of Chemical Industry.

1,4-naphthoquinones and other NADPH-dependent glutathione reductase-catalyzed redox cyclers as antimalarial agents

The homodimeric flavoenzyme glutathione reductase catalyzes NADPH-dependent glutathione disulfide reduction. This reaction is important for keeping the redox homeostasis in human cells and in the human pathogen Plasmodium falciparum. Different types of NADPH-dependent disulfide reductase inhibitors were designed in various chemical series to evaluate the impact of each inhibition mode on the propagation of the parasites. Against malaria parasites in cultures the most potent and specific effects were observed for redox-active agents acting as subversive substrates for both glutathione reductases of the Plasmodium-infected red blood cells. In their oxidized form, these redox-active compounds are reduced by NADPH-dependent flavoenzyme-catalyzed reactions in the cytosol of infected erythrocytes. In their reduced forms, these compounds can reduce molecular oxygen to reactive oxygen species, or reduce oxidants like methemoglobin, the major nutrient of the parasite, to indigestible hemoglobin. Furthermore, studies on a fluorinated suicide-substrate of the human glutathione reductase indicate that the glutathione reductase-catalyzed bioactivation of 3-benzylnaphthoquinones to the corresponding reduced 3-benzoyl metabolites is essential for the observed antimalarial activity. In conclusion, the antimalarial lead naphthoquinones are suggested to perturb the major redox equilibria of the targeted cells. These effects result in developmental arrest of the parasite and contribute to the removal of the parasitized erythrocytes by macrophages.

Low-molecular-mass antioxidants in parasites

SIGNIFICANCE

Parasitic infections continue to be a major problem for global human health. Vaccines are practically not available and chemotherapy is highly unsatisfactory. One approach toward a novel antiparasitic drug development is to unravel pathways that may be suited as future targets. Parasitic organisms show a remarkable diversity with respect to the nature and functions of their main low-molecular-mass antioxidants and many of them developed pathways that do not have a counterpart in their mammalian hosts.

RECENT ADVANCES

Work of the last years disclosed the individual antioxidants employed by parasites and their distinct pathways. Entamoeba, Trichomonas, and Giardia directly use cysteine as main low-molecular-mass thiol but have divergent cysteine metabolisms. Malarial parasites rely exclusively on cysteine uptake and generate glutathione (GSH) as main free thiol as do metazoan parasites. Trypanosomes and Leishmania have a unique trypanothione-based thiol metabolism but employ individual mechanisms for their cysteine supply. In addition, some trypanosomatids synthesize ovothiol A and/or ascorbate. Various essential parasite enzymes such as trypanothione synthetase and trypanothione reductase in Trypanosomatids and the Schistosoma thioredoxin GSH reductase are currently intensively explored as drug target molecules.

CRITICAL ISSUES

Essentiality is a prerequisite but not a sufficient property of an enzyme to become a suited drug target. The availability of an appropriate in vivo screening system and many other factors are equally important.

FUTURE DIRECTIONS

The current organism-wide RNA-interference and proteome analyses are supposed to reveal many more interesting candidates for future drug development approaches directed against the parasite antioxidant defense systems.

Inherited glutathione reductase deficiency and Plasmodium falciparum malaria--a case study

In Plasmodium falciparum-infected red blood cells (RBCs), the flavoenzyme glutathione reductase (GR) regenerates reduced glutathione, which is essential for antioxidant defense. GR utilizes NADPH produced in the pentose phosphate shunt by glucose-6-phosphate dehydrogenase (G6PD). Thus, conditions affecting host G6PD or GR induce increased sensitivity to oxidants. Hereditary G6PD deficiency is frequent in malaria endemic areas and provides protection against severe malaria. Furthermore, GR deficiency resulting from insufficient saturation of the enzyme with its prosthetic group FAD is common. Based on these naturally occurring phenomena, GR of malaria parasites and their host cells represent attractive antimalarial drug targets. Recently we were given the opportunity to examine invasion, growth, and drug sensitivity of three P. falciparum strains (3D7, K1, and Palo Alto) in the RBCs from three homozygous individuals with total GR deficiency resulting from mutations in the apoprotein. Invasion or growth in the GR-deficient RBCs was not impaired for any of the parasite strains tested. Drug sensitivity to chloroquine, artemisinin, and methylene blue was comparable to parasites grown in GR-sufficient RBCs and sensitivity towards paraquat and sodium nitroprusside was only slightly enhanced. In contrast, membrane deposition of hemichromes as well as the opsonizing complement C3b fragments and phagocytosis were strongly increased in ring-infected RBCs of the GR-deficient individuals compared to ring-infected normal RBCs. Also, in one of the individuals, membrane-bound autologous IgGs were significantly enhanced. Thus, based on our in vitro data, GR deficiency and drug-induced GR inhibition may protect from malaria by inducing enhanced ring stage phagocytosis rather than by impairing parasite growth directly.

Eosin B as a novel antimalarial agent for drug-resistant Plasmodium falciparum

4',5'-Dibromo-2',7'-dinitrofluorescein, a red dye commonly referred to as eosin B, inhibits Toxoplasma gondii in both enzymatic and cell culture studies with a 50% inhibitory concentration (IC(50)) of 180 microM. As a non-active-site inhibitor of the bifunctional T. gondii dihydrofolate reductase-thymidylate synthase (DHFR-TS), eosin B offers a novel mechanism for inhibition of the parasitic folate biosynthesis pathway. In the present study, eosin B was further evaluated as a potential antiparasitic compound through in vitro and cell culture testing of its effects on Plasmodium falciparum. Our data revealed that eosin B is a highly selective, potent inhibitor of a variety of drug-resistant malarial strains, with an average IC(50) of 124 nM. Furthermore, there is no indication of cross-resistance with other clinically utilized compounds, suggesting that eosin B is acting via a novel mechanism. The antimalarial mode of action appears to be multifaceted and includes extensive damage to membranes, the alteration of intracellular organelles, and enzymatic inhibition not only of DHFR-TS but also of glutathione reductase and thioredoxin reductase. In addition, preliminary studies suggest that eosin B is also acting as a redox cycling compound. Overall, our data suggest that eosin B is an effective lead compound for the development of new, more effective antimalarial drugs.

The survival of Plasmodium Under oxidant stress

Oxidant stress is associated with the generation of reactive oxygen-derived species, which are considered as the ultimate agents responsible for the damage of a variety of cellular components. Transition metals such as iron ions serve as catalytic centers for the repeated conversion of superoxide radicals or ascorbate to the highly reactive and deleterious hydroxyl radicals and, indeed, increasing amounts of redox-active iron become available during plasmodial development within the parasitized erythrocytes. Thus, the survival of an intracellular parasite depends on the delicate balance of oxidant stress and defense mechanisms. This balance is continuously changing and the parasite must cope with increasing oxidant stress and the decline of protective capacity.

Dithiol Proteins as Guardians of the Intracellular Redox Milieu in Parasites: Old and New Drug Targets in Trypanosomes and Malaria-Causing Plasmodia

Parasitic diseases such as sleeping sickness, Chagas' heart disease, and malaria are major health problems in poverty-stricken areas. Antiparasitic drugs that are not only active but also affordable and readily available are urgently required. One approach to finding new drugs and rediscovering old ones is based on enzyme inhibitors that paralyze antioxidant systems in the pathogens. These antioxidant ensembles are essential to the parasites as they are attacked in the human host by strong oxidants such as peroxynitrite, hypochlorite, and H2O2. The pathogen-protecting system consists of some 20 thiol and dithiol proteins, which buffer the intraparasitic redox milieu at a potential of -250 mV. In trypanosomes and leishmania the network is centered around the unique dithiol trypanothione (N1,N8-bis(glutathionyl)spermidine). In contrast, malaria parasites have a more conservative dual antioxidative system based on glutathione and thioredoxin. Inhibitors of antioxidant enzymes such as trypanothione reductase are, indeed, parasiticidal but they can also delay or prevent resistance against a number of other antiparasitic drugs.

Oxidative stress in malaria parasite-infected erythrocytes: host–parasite interactions

Experimenta naturae, like the glucose-6-phosphate dehydrogenase deficiency, indicate that malaria parasites are highly susceptible to alterations in the redox equilibrium. This offers a great potential for the development of urgently required novel chemotherapeutic strategies. However, the relationship between the redox status of malarial parasites and that of their host is complex. In this review article we summarise the presently available knowledge on sources and detoxification pathways of reactive oxygen species in malaria parasite-infected red cells, on clinical aspects of redox metabolism and redox-related mechanisms of drug action as well as future prospects for drug development. As delineated below, alterations in redox status contribute to disease manifestation including sequestration, cerebral pathology, anaemia, respiratory distress, and placental malaria. Studying haemoglobinopathies, like thalassemias and sickle cell disease, and other red cell defects that provide protection against malaria allows insights into this fine balance of redox interactions. The host immune response to malaria involves phagocytosis as well as the production of nitric oxide and oxygen radicals that form part of the host defence system and also contribute to the pathology of the disease. Haemoglobin degradation by the malarial parasite produces the redox active by-products, free haem and H(2)O(2), conferring oxidative insult on the host cell. However, the parasite also supplies antioxidant moieties to the host and possesses an efficient enzymatic antioxidant defence system including glutathione- and thioredoxin-dependent proteins. Mechanistic and structural work on these enzymes might provide a basis for targeting the parasite. Indeed, a number of currently used drugs, especially the endoperoxide antimalarials, appear to act by increasing oxidant stress, and novel drugs such as peroxidic compounds and anthroquinones are being developed.

Glutathione--functions and metabolism in the malarial parasite Plasmodium falciparum

When present as a trophozoite in human erythrocytes, the malarial parasite Plasmodium falciparum exhibits an intense glutathione metabolism. Glutathione plays a role not only in antioxidative defense and in maintaining the reducing environment of the cytosol. Many of the known glutathione-dependent processes are directly related to the specific lifestyle of the parasite. Reduced glutathione (GSH) supports rapid cell growth by providing electrons for deoxyribonucleotide synthesis and it takes part in detoxifying heme, a product of hemoglobin digestion. Free radicals generated in the parasite can be scavenged in reaction sequences involving the thiyl radical GS* as well as the thiolate GS-. As a substrate of glutathione S-transferase, glutathione is conjugated to non-degradable compounds including antimalarial drugs. Furthermore, it is the coenzyme of the glyoxalase system which detoxifies methylglyoxal, a byproduct of the intense glycolysis taking place in the trophozoite. Proteins involved in GSH-dependent processes include glutathione reductase, glutaredoxins, glyoxalase I and II, glutathione S-transferases, and thioredoxins. These proteins, as well as the ATP-dependent enzymes of glutathione synthesis, are studied as factors in the pathophysiology of malaria but also as potential drug targets. Methylene blue, an inhibitor of the structurally known P. falciparum glutathione reductase, appears to be a promising antimalarial medication when given in combination with chloroquine.

Kinetic characterization of glutathione reductase from the malarial parasite Plasmodium falciparum. Comparison with the human enzyme

The homodimeric flavoenzyme glutathione reductase (GR) maintains high intracellular concentrations of the antioxidant glutathione (GSSG + NADPH + H(+) <--> 2 GSH + NADP(+)). Due to its central function in cellular redox metabolism, inhibition of GR from the malarial parasite Plasmodium falciparum represents an important approach to antimalarial drug development; therefore, the catalytic mechanism of GR from P. falciparum has been analyzed and compared with the human host enzyme. The reductive half-reaction is similar to the analogous reaction with GR from other species. The oxidative half-reaction is biphasic, reflecting formation and breakdown of a mixed disulfide between the interchange thiol and GSH. The equilibrium between the E(ox)-EH(2) and GSSG-GSH couples has been modeled showing that the Michaelis complex, mixed disulfide-GSH, is the predominant enzyme form as the oxidative half-reaction progresses; rate constants used in modeling allow calculation of an K(eq) from the Haldane relationship, 0.075, very similar to the K(eq) of the same reaction for the yeast enzyme (0.085) (Arscott, L. D., Veine, D. M., and Williams, C. H., Jr. (2000) Biochemistry 39, 4711-4721). Enzyme-monitored turnover indicates that E(FADH(-))(S-S). NADP(+) and E(FAD)(SH)(2).NADPH are dominant enzyme species in turnover. Since the individual forms of the enzyme differ in their susceptibility to inhibitors, the prevailing states of GR in the cell are of practical relevance.

Trypanosomes lacking trypanothione reductase are avirulent and show increased sensitivity to oxidative stress

In Kinetoplastida, trypanothione and trypanothione reductase (TRYR) provide an intracellular reducing environment, substituting for the glutathione-glutathione reductase system found in most other organisms. To investigate the physiological role of TRYR in Trypanosoma brucei, we generated cells containing just one trypanothione reductase gene, TRYR, which was under the control of a tetracycline-inducible promoter. This enabled us to regulate TRYR activity in the cells from less than 1% to 400% of wild-type levels by adjusting the concentration of added tetracycline. In normal growth medium (which contains reducing agents), trypanosomes containing less than 10% of wild-type enzyme activity were unable to grow, although the levels of reduced trypanothione and total thiols remained constant. In media lacking reducing agents, hypersensitivity towards hydrogen peroxide (EC50 = 3.5 microM) was observed compared with the wild type (EC50 = 223 microM). The depletion of TRYR had no effect on susceptibility to melarsen oxide. The infectivity and virulence of the parasites in mice was dependent upon tetracycline-regulated TRYR activity: if the trypanosomes were injected into mice in the absence of tetracycline, no infection was detectable; and when tetracycline was withdrawn from previously infected animals, the parasitaemia was suppressed.

Enzymes of parasite thiol metabolism as drug targets

The potential for chemotherapeutic exploitation of thiol metabolism in parasitic protozoa is reviewed here by Luise Krauth-Siegel and Graham Coombs. The review is based largely on discussions held at a meeting of the COST B9 Action entitled 'Chemotherapy of Protozoal Infections'*. The major questions posed were: which enzymes are the best to target; what further information is required to allow their use for rational drug development; and how can this be achieved most efficiently? Not surprisingly, only partial answers could be obtained in many cases, but the interactive discussion between the multidisciplinary group of participants provided thought-provoking ideas and will help direct future research.

Management of HIV-infected pregnant patients in malaria-endemic areas: therapeutic and safety considerations in concomitant use of antiretroviral and antimalarial agents

Chemotherapy in pregnancy is an intricate process requiring prudent use of pharmacologic agents. Malarial infection during pregnancy is often fatal, and prophylaxis against the causative parasite necessitates rational therapeutic intervention. Various agents have been used for prophylaxis against malaria during pregnancy, including chloroquine, mefloquine, proguanil, pyrimethamine, and pyrimethamine-sulfadoxine. Use of these agents has been based on a risk-benefit criterion, without appropriate toxicologic or teratologic evaluation. Some of the aforementioned prophylactic agents have been shown to alter glutathione levels and may exacerbate the oxidation-reduction imbalance attendant on HIV infection. HIV-infected patients traveling to or residing in malaria-endemic areas require protection from malarial infection to avoid placing themselves in double jeopardy. Zidovudine (AZT) is recommended for the prevention of vertical transmission of HIV-1 from mother to child. Other agents, such as lamivudine alone or in combination with AZT, nevirapine, or the HIV-1 protease inhibitors, are either being considered or are currently undergoing trials for use in preventing vertical transmission of HIV-1 or managing HIV infection in infants and children. Although the potential for antimalarial agents to cause congenital malformations is low when they are used alone, their ability to cause problems when combined with antiretroviral drugs needs to be evaluated. In developing countries that have high birth rates, a high endemicity of malaria, and alarming rates of new cases of HIV, prophylaxis against both diseases with combination agents during pregnancy is a challenge.

An overview of chemotherapeutic targets for antimalarial drug discovery

The need for new antimalarials comes from the widespread resistance to those in current use. New antimalarial targets are required to allow the discovery of chemically diverse, effective drugs. The search for such new targets and new drug chemotypes will likely be helped by the advent of functional genomics and structure-based drug design. After validation of the putative targets as those capable of providing effective and safe drugs, targets can be used as the basis for screening compounds in order to identify new leads, which, in turn, will qualify for lead optimization work. The combined use of combinatorial chemistry--to generate large numbers of structurally diverse compounds--and of high throughput screening systems--to speed up the testing of compounds--hopefully will help to optimize the process. Potential chemotherapeutic targets in the malaria parasite can be broadly classified into three categories: those involved in processes occurring in the digestive vacuole, enzymes involved in macromolecular and metabolite synthesis, and those responsible for membrane processes and signalling. The processes occurring in the digestive vacuole include haemoglobin digestion, redox processes and free radical formation, and reactions accompanying haem release followed by its polymerization into haemozoin. Many enzymes in macromolecular and metabolite synthesis are promising potential targets, some of which have been established in other microorganisms, although not yet validated for Plasmodium, with very few exceptions (such as dihydrofolate reductase). Proteins responsible for membrane processes, including trafficking and drug transport and signalling, are potentially important also to identify compounds to be used in combination with antimalarial drugs to combat resistance.

Inhibitors of glutathione reductase as potential antimalarial drugs. Kinetic cooperativity and effect of dimethyl sulphoxide on inhibition kinetics

We have developed inhibitors of glutathione reductase that improve on the inhibition of literature lead compounds by up to three orders of magnitude. Thus, analogues of Safranine O and menadione were found to be strong, reversible inhibitors of yeast glutathione reductase. Safranine O exhibited partial, uncompetitive inhibition with Ki and alpha values of 0.5 mM and 0.15, respectively. Thionine O was a partial (hyperbolic) uncompetitive inhibitor with Ki and alpha values of 0.4 microM and 0.15, respectively. LY83583 and 2-anilino-1,4-naphthoquinone also showed (hyperbolic) partial, uncompetitive inhibition with micromolar Ki values. For Nile Blue A a model for two-site binding with (parabolic) uncompetitive inhibition fitted the data with a Ki value of 11 microM and a kinetic cooperativity between the sites of 0.12, increased to 0.46 by preincubation of the enzyme and Nile Blue A in the presence of glutathione disulphide. Analysis of the effects of preincubation on the kinetics and cooperativity indicated the possibility of a slow conformational change in the homodimeric enzyme, the first such indication of kinetic cooperativity in the native enzyme to our knowledge. Further evidence of conformational changes for this enzyme came from studies of the effects of dimethyl sulphoxide which indicated that this co-solvent, which at low concentrations has no apparent effect on initial velocities under normal assay conditions, induced a slow conformational change in the enzyme. Thionine O, Nile Blue A and LY83583 were redox-cycling substrates producing superoxide ion, detectable by means of cytochrome c reduction, but leading to no loss of glutathione reductase activity, under aerobic or anaerobic conditions. The water-soluble Safranine analogues Methylene Blue, Methylene Green, Nile Blue A and Thionine O (5 mg/kg i.p. x 5) were effective antimalarial agents in vivo against P. berghei, but their effect was small and a higher dose (50 mg/kg i.p. x 1) was toxic in mice. Comparison was made with human glutathione reductase and its literature-reported interactions with several tricyclic inhibitors as studied by X-ray diffraction. It is possible that the conformational changes detected in the present study from alterations in detailed kinetic inhibition mechanisms may shed light on information transfer through the glutathione reductase molecule from the dimer interface ligand pocket to the active-site.

The malaria parasite supplies glutathione to its host cell--investigation of glutathione transport and metabolism in human erythrocytes infected with Plasmodium falciparum

Malaria-infected red blood cells are under a substantial oxidative stress. Glutathione metabolism may play an important role in antioxidant defense in these cells, as it does in other eukaryotes. In this work, we have determined the levels of reduced and oxidized glutathione (GSH and GSSG, respectively) and their distributions in the parasite, and in the host-cell compartments of human erythrocytes infected with the malaria parasite Plasmodium falciparum. In intact trophozoite-infected erythrocytes, [GSH] is low and [GSSG] is high, compared with the levels in normal erythrocytes. Normal erythrocytes and the parasite compartment display high GSH/GSSG ratios of 321.6 and 284.5, respectively, indicating adequate antioxidant defense. This ratio drops to 26.7 in the host-cell compartment, indicating a forceful oxidant challenge, the low ratios resulting from an increase in GSSG and a decline in GSH concentrations. On the other hand, the concentrations of GSH and GSSG in the parasite compartment remain physiological and comparable to their concentrations in normal red blood cells. This results from de novo glutathione synthesis and its recycling, assisted by the intensive activity of the hexose monophosphate shunt in the parasite. A large efflux of GSSG from infected cells has been observed, its rate being similar from free parasites and from intact infected cells. This result suggests that de novo synthesis by the parasite is the dominating process in infected cells. GSSG efflux from the intact infected cell is more than 60-fold higher than the rate observed in normal erythrocytes, and is mediated by permeability pathways that the parasite induces in the erythrocyte's membrane. The main route for GSSG efflux through the cytoplasmic membrane of the parasite seems to be due to a specific transport system and occurs against a concentration gradient. Gamma-glutamylcysteine [Glu(-Cys)] and GSH can penetrate through the pathways from the extracellular space into the host cytosol, but not into that of the parasite. This implies that the parasite membrane is impermeable to these peptides, and that the host cannot supply GSH to the parasite as suggested previously. Exogenous Glu(-Cys) is not converted into GSH in the host cell, arguing that GSH synthetase may not be functional. Compartment analysis of Mg2+ in infected erythrocytes revealed that the host compartment exhibits a low concentration of Mg2+ (0.5 mM) in comparison with the parasite compartment (4 mM) and the normal erythrocytes (1.5-3 mM). The drop in [Mg2+] results in cessation of Glu(-Cys) synthesis, and hence of GSH synthesis in the host-cell compartment. The decrease in [Mg2+] can affect other Mg2+-ATP-dependent functions, such as Na+ and Ca2+ active efflux. The present investigation confirms that the host-cell compartment is oxidatively distressed, whereas the parasite is efficiently equipped with anti-oxidant means that protect the parasite from the oxidative injury. The parasite has a huge capacity for de novo synthesis of GSH and for the reduction of GSSG. Part of the GSSG that is actively extruded from the parasite is reduced to GSH in the host cell whose own GSH synthesis is crippled.

Oxidative stress and antioxidative status of plasma and erythrocytes in patients with vivax malaria

OBJECTIVES

To investigate the oxidative stress and antioxidative status of plasma and erythrocytes in patients with vivax malaria and healthy persons.

DESIGN AND METHODS

Activities of antioxidative enzymes, rates of pathways of hexose monophosphate shunt and purine salvage, levels of lipid peroxidation, reduced glutathione, methemoglobin and sulfhemoglobin of erythrocytes were determined. Lipid peroxidation and levels of antioxidant substances were measured.

RESULTS

Antioxidants levels and antioxidative enzymes activities were lower and lipid peroxidation, purine salvage rate were higher in patients group than controls. Erythrocyte glucose-6 phosphate dehydrogenase (G-6-PD) activity was not different from that of healthy subjects.

CONCLUSIONS

Oxidative mechanisms were observed to be dominant compared with antioxidative mechanisms in patients with vivax malaria. Therefore, oxidative stress may be produced and maintained by the host defense mechanisms against malarial infection.

Oxidative stress in malaria; implications for prevention and therapy

Malaria affects world-wide more than 200 million people, of which 1-2 million die every year. New drugs and treatment strategies are needed to face the rapidly increasing problems of drug resistance. During a malaria infection, both host and parasite are under oxidative stress. Increased production levels of reactive oxygen species (ROS, e.g superoxide anion and the hydroxyl radical) are produced by activated neutrophils in the host and during degradation of haemoglobin in the parasite. The effects of ROS in malaria can be both beneficial and pathological, depending on the amount and place of production. Enhanced ROS production after the administration of pro-oxidants, which is directed against the intra-erythrocytic parasite, inhibits the infection both in vitro and in vivo. However, ROS are also involved in pathological changes in host tissue like damage of the vascular endothelial lining during a malaria infection (cerebral malaria). Pro-oxidants support the host defense against the parasite when working in or near the infected cell but potentially cause vascular damage when working on or near the vascular lining. Examples of pro-oxidants are found among xenobiotics and food components. Important new drugs belonging to the class of pro-oxidants are artemisinin and its derivatives. Anti-oxidants potentially counteract these agents. Treatment with anti-oxidants or chelators of metals to prevent their catalytic function in the generation of ROS may prevent vascular pathology. In addition, the iron chelator desferrioxamine, exhibits an antiparasitic activity, because iron is also essential for the proliferation of the parasite. Cytokines play an important role in ROS-related pathology of malaria, though their mechanism of action is not completely elucidated. This field might bring up new treatment concepts and drugs. Drugs which prevent host pathology, such as the cerebral complications might be life saving.

Lactose absorption in patients with glucose 6-phosphate dehydrogenase deficiency with and without favism

BACKGROUND

It has recently been suggested that primary lactase deficiency might have been selected for by malaria, as has been previously shown to occur for thalasaemia and glucose 6-phosphate dehydrogenase (G6PD) deficiency.

AIMS

To test this hypothesis, the prevalence of primary lactase deficiency in G6PD deficient subjects and in controls from the area of Sassari (Northern Sardinia) was determined, which in the past was characterised by an intermediate malarial endemicity.

SUBJECTS

70 adult subjects with G6PD deficiency, 34 of whom had a past history of favism, and 50 age matched control subjects.

METHODS

The capacity to absorb lactose was assessed by measuring breath hydrogen production after oral administration of lactose (50 g) by a gas chromatographic method.

RESULTS

Twenty per cent of G6PD deficient subjects with a positive history of favism and 22% of G6PD deficient subjects without a positive history of favism were lactose absorbers compared with 14% lactose absorbers in the control group. The differences were not statistically significant.

CONCLUSIONS

These data show that the prevalence of primary lactase deficiency in the area of Sassari is relatively high, but comparable to that seen in the adult population from another area of southern Italy (Naples) where malaria was less endemic.

Molecular cloning and characterization of a putative glutathione reductase gene, the PfGR2 gene, from Plasmodium falciparum

Recently, glutathione reductase (GR) has emerged as a promising target for antiparasitic drugs. The central role of GR in cellular antioxidant defence, the particular susceptibility of intracellular parasites like Plasmodium falciparum to oxidative stress, and successful inhibitor studies substantiate this approach. However, more information is required on the structural and functional characteristics of GR from malarial parasites and differences from the enzyme of host erythrocytes. We have identified a putative P. falciparum GR gene coding for a polypeptide (PfGR2) of 500 amino acids that exhibits 40-45% sequence identity with GR enzymes from other species. 18 out of 19 residues contributing to glutathione binding are identical in the putative PfGR2 and human GR. According to Southern blot analysis, the PfGR2 gene is present as a single-copy gene. It is expressed during the intraerythrocytic life cycle. Stage-specific Northern blot analysis demonstrates that the PfGR2 gene is only weakly transcribed in ring, early trophozoite, and segmenter stages; major transcription occurs in the late trophozoite/early schizont stage. This is consistent with the high glutathione reductase activity found in early schizonts. Other data also suggest that PfGR2 corresponds to the enzyme isolated from parasitized erythrocytes. These criteria include the subunit molecular mass (56.2 kDa), the N-terminal sequence (VYDLIVIGGGSGGMA), the presence of specific sequence motifs at ligand-binding sites, and, as demonstrated by Western blotting, the occurrence of a unique chain segment in the core of the central domain. In view of these data, the function(s) of PfGR2 as well as PfGR1, the product of another GR-like gene of P. falciparum (Müller et al., 1995) should be carefully assessed.

Molecular characterization of the glutathione peroxidase gene of the human malaria parasite Plasmodium falciparum

In this paper we report the isolation and the characterization of a gene encoding the antioxidant enzyme glutathione peroxidase from the human malaria parasite Plasmodium falciparum. This gene contains two introns of 208 and 168 bp and is present in a single copy on chromosome 13. The open reading frame encodes a protein with a predicted length of 205 amino acids, which possesses a potential cleavage site between residues 21 and 22 after a hydrophobic region with the characteristics of a signal sequence. Therefore, the mature protein is predicted to be 184 residues long with a molecular mass of 21404 Da. In comparison with other known glutathione peroxidases many amino acid residues implicated in catalysis are conserved in the malarial enzyme. Phylogenetic analysis indicates that the deduced protein sequence is more closely related to plant glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase. A 1.5-kb transcript was identified in asynchronous erythrocytic stages.

Increase in glutathione peroxidase activity in malaria parasite after selenium supplementation

Glutathione peroxidase (GPx), a key enzyme involved in the detoxification of many peroxides, has been investigated in two malaria parasite species: P. yoelii in vivo (murine malaria) and P. falciparum in vitro (human malaria). We demonstrate the presence of an endogenous GPx activity in these two Plasmodia species. Enzymatic assays and the use of specific substrates and inhibitors allowed us to determine that the activity is selenium dependent. As this activity was shown to be lower in P. falciparum than in P. yoelii, and selenium levels were found to be low in culture medium and culture red blood cells, we hypothesized that a severe selenium deficiency could be responsible for this difference. After selenium supplementation, with either sodium selenite or selenocystine, we observed an increase in growth of P. falciparum only in with sodium selenite, whereas higher GPx activities were noted in parasites grown in media supplemented with both. An increase in GPx activities was also observed in parasites that had undergone an experimental oxidative stress with TBOOH. As the erythrocyte is unable to synthesize new proteins, these results provide further evidence for the existence of an endogenous parasitic selenium-dependent glutathione peroxidase.

Hexose-monophosphate shunt activity in intact Plasmodium falciparum-infected erythrocytes and in free parasites

The hexose monophosphate shunt (HMS) produces NADPH for reductive antioxidant protection and for metabolic regulation, as well as ribose-5-phosphate needed for the synthesis of nucleic acids. Since malaria-infected red blood cells (RBC) are under endogenous oxidant stress, it was interesting to determine HMS activity in intact infected cells, as well as in free parasites. HMS activity was determined by measuring the evolution of 14CO2 from D-[1-14C]glucose in normal RBC, in intact Plasmodium falciparum-infected RBC (IRBC) and in free parasites. The HMS activity of IRBC was found to be 78 times higher than that of normal RBC. This activity increased with parasite maturation from the ring stage toward the trophozoite stage, and declined at the schizont stage. The HMS activity of the parasite contributes 82% of the total observed in the intact IRBC, and that of the host cell is increased some 24-fold. The increased reducing capacity of IRBC and free parasites were also evidenced by the larger ability for reductive accumulation of methylene blue. Since the endogenous oxidative stress is produced by the parasite digestion of the host cell's hemoglobin, inhibition of this process with protease inhibitors, by alkalinization of the parasite's food vacuole, or by the application of antimalarial drugs, resulted in 20-44% inhibition of IRBC HMS activity. A similar inhibition was observed in the presence of scavengers of oxidative radicals, uric and benzoic acids. These inhibitors had only a minor effect on the HMS activity of free parasites. D-[1-14C]glucose and D-[6-14C]glucose contributed equally to newly synthesized nucleic acids, suggesting that ribose-5-phosphate needed for this synthesis is contributed by the non-oxidative activity of HMS. These results imply that a major portion of parasite HMS activity and the activated HMS of the host cell are devoted to counteract the endogenously generated oxidative stress.

Plasmodium falciparum induced perturbations of the erythrocyte antioxidant system

Erythrocyte antioxidants catalase, superoxide dismutase, reduced glutathione and glutathione peroxidase were studied in cells harbouring different growth stages of Plasmodium falciparum. Catalase and superoxide dismutase showed significant decrease during parasite maturation indicating hampered metabolism of hydrogen peroxide and superoxide anions. Glutathione peroxidase also exhibited a downward trend during the growth of P. falciparum, while there was a moderate accumulation of reduced glutathione. These findings suggest decreased utilization of the reduction potential in detoxification of reactive oxygen species. The fall in all three antioxidant enzymes studied was highly significant (P less than 0.001) in erythrocytes with mature stages of the parasite (trophozoites, schizonts). The increased vulnerability of erythrocytes to damage, which parallels the growth phases of the parasite emphasizes the need for early treatment of P. falciparum malaria to minimise red cell destruction and the resulting anaemia.

Cloning and sequencing of mammalian glutathione reductase cDNA

The molecular cloning of a partial cDNA to mouse glutathione reductase mRNA and of a full-length cDNA to the mRNA of the human enzyme is described. An initial cDNA clone designated lambda GRM-B11 was isolated by plaque-screening of an induced mouse cDNA expression library in the lambda gt11 vector with a rabbit antibody probe to human glutathione reductase. 125Iodine-labelled whole anti-rabbit immunoglobulin was used as second antibody. EcoRI digestion of the lambda GRM-B11 clone released a 720-bp fragment which was identified as a partial mouse glutathione reductase cDNA by the following techniques. (a) Escherichia coli Y1089 lysogenized with lambda GRM-B11 could be induced to synthesize a recombinant polypeptide whose antigenicity to anti-(glutathione reductase) serum was established by SDS/polyacrylamide gel electrophoresis and subsequent immunoblotting. (b) The GRM-B11 sequence, recloned in the Bluescript vector to give the plasmid pGRM-B11, was found to code for a polypeptide consisting of 242 amino acid residues exhibiting 82% identities with the known amino acid sequence of the human glutathione reductase from position 77 to 318. The insert of the pGRM-B11 plasmid was used as a bona fide nucleic acid probe to screen mouse and human cDNA libraries prepared in the lambda gt11 or in the lambda gt10 vector. The first full-length cDNA clone (lambda GRH-Mev10) was identified in a human cDNA library based on RNA of human placental cells. Its insert was composed of three EcoRI fragments of 720, 613 and 336 bp. The three fragments were recloned in the Bluescript vector and sequenced. The largest fragment (pGRH-B) is colinear with the mouse sequence cloned in the pGRM-B11 plasmid. The fragment of intermediate size (pGRH-CT) comprises the 3' end of the mRNA and the poly(A) tail while the short fragment (pGRH-NT) corresponds to the 5' region of the mRNA. The amino acid sequence deduced from the nucleotide sequences of the three subclones is identical with the known sequence of the mature glutathione reductase from human erythrocytes in all 478 positions.

Antimalarial action of flavin analogues seems not be due to inhibition of glutathione reductase of host erythrocytes

A series of 10-(4'-chlorophenyl)-3-substituted flavins (1a-f) were examined with respect to their antimalarial properties. They were tested against Plasmodium falciparum in vitro and Plasmodium vinckei vinckei in vivo. The proposition that they might act through glutathione reductase (GR) (EC 1.6.4.2) inhibition has been studied. Inhibition of P. falciparum in vitro by these compounds shows only slight variation between analogues; in contrast, inhibition of human erythrocyte GR by members of the same series is highly variable, indicating that this is probably not their primary mode of antimalarial action. Results of the P. vinckei vinckei screen showed that 10-(4'-chlorophenyl)-3-methyl,3-ethyl and 3-propyl substituted flavins are active in vivo over the dose range screened (10-70 mg/kg).

Flavin analogs with antimalarial activity as glutathione reductase inhibitors

10-(4'-Chlorophenyl)-3-methylflavin has antimalarial activity in vitro and in vivo (Cowden et al., J Med Chem 31: 799, 1988). This flavin analog and two of its derivatives were found to inhibit the antioxidant flavoenzyme glutathione reductase from human erythrocytes in its isolated form as well as in hemolysates. The mixed-type inhibition was completely reversible, the Ki-values being of the order of 1 microM. Surprisingly, the drugs were not competitive with FAD, but with GSSG, one of the enzyme's substrates. Malaria parasite glutathione reductase, extracted from Plasmodium falciparum, could also be inhibited by the compounds. Studies on the effects of the substances on P. falciparum in vitro, which were demonstrated morphologically and by growth inhibition, confirmed previous observations with 10-(4'-chlorophenyl)-3-methylflavin and showed similar parasiticidal characteristics for the two new derivatives. The activities of five other erythrocytic enzymes tested were not impaired by the drugs, nor was the nucleotide metabolism of erythrocytes and/or parasites significantly changed. Permeation into red blood cells was demonstrated for one compound by 19F-NMR-spectroscopy. Inhibition of glutathione reductase might contribute to, or account for, the antimalarial activity of this group of flavin analogs.

Antimalarial properties of ebselen

The seleno-organic compound ebselen showed anti-malarial activity in vitro against the murine Plasmodium berghei and the human P. falciparum. In P. berghei, the uptake and incorporation of [3H]-methionine and [3H]-adenosine was inhibited and the infectivity of plasmodia was reduced. Ebselen affects the development of asexual stages of chloroquine-resistant and -sensitive P. falciparum strains. Its IC50 for P. falciparum was about 14 mumol/l and that for P. berghei, about 10 mumol/l. The growth of P. falciparum was blocked by ebselen at all stages, including the invasion of erythrocytes by merozoites. In a human hepatoma cell line and in mouse peritoneal macrophages, no cytostatic or cytotoxic effects were found, indicating selective inhibition of plasmodia by ebselen. Its in vitro inhibitory effect is discussed in relation to its possible reactivity with thiol groups and its lack of an anti-malarial effect in infected mice.

Glutathione reductase inhibitors as potential antimalarial drugs. Effects of nitrosoureas on Plasmodium falciparum in vitro

Malarial parasites are believed to be more susceptible to oxidative stress than their hosts. BCNU(1,3-bis(2-chloroethyl)-1-nitrosourea) and HeCNU(1-(2-chloroethyl)-3-(2-hydroxythyl)-1-nitrosourea), inhibitors of the antioxidant enzyme glutathione reductase, were found to prevent the growth of Plasmodium falciparum in all intraerythrocytic stages. When exposing infected red blood cells to 38 microM BCNU or 62 microM HeCNU for one life cycle of synchronously growing parasites, the parasitemia decreased by 90%. During the formation of new ring forms, the parasites are even more susceptible to these drugs. The treatment with BCNU or HeCNU produced a rapid depletion of GSH in the parasites and their host cells; in addition, protection against lipid peroxidation was impaired in these cells. Possible mechanisms for the antimalarial action of the inhibitors are discussed. Our results suggest that erythrocyte glutathione reductase, an enzyme of known structure, might be considered as a target for the design of antimalarial drugs.