Cloning of the glucose 6-phosphate dehydrogenase gene from Plasmodium falciparum

Fused Enzyme Glucose-6-Phosphate Dehydrogenase::6-Phosphogluconolactonase (G6PD::6PGL) as a Potential Drug Target in Giardia lamblia, Trichomonas vaginalis, and Plasmodium falciparum

Several microaerophilic parasites such as Giardia lamblia, Trichomonas vaginalis, and Plasmodium falciparum are major disease-causing organisms and are responsible for spreading infections worldwide. Despite significant progress made in understanding the metabolism and molecular biology of microaerophilic parasites, chemotherapeutic treatment to control it has seen limited progress. A current proposed strategy for drug discovery against parasitic diseases is the identification of essential key enzymes of metabolic pathways associated with the parasite's survival. In these organisms, glucose-6-phosphate dehydrogenase::6-phosphogluconolactonase (G6PD:: 6PGL), the first enzyme of the pentose phosphate pathway (PPP), is essential for its metabolism. Since G6PD:: 6PGL provides substrates for nucleotides synthesis and NADPH as a source of reducing equivalents, it could be considered an anti-parasite drug target. This review analyzes the anaerobic energy metabolism of G. lamblia, T. vaginalis, and P. falciparum, with a focus on glucose metabolism through the pentose phosphate pathway and the significance of the fused G6PD:: 6PGL enzyme as a therapeutic target in the search for new drugs.

Molecular explorations of the Leishmania donovani 6-phosphogluconolactonase enzyme, a key player in the pentose phosphate pathway

The enzymes of the pentose phosphate pathway are vital to survival in kinetoplastids. The second step of the pentose phosphate pathway involves hydrolytic cleavage of 6-phosphogluconolactone to 6-phosphogluconic acid by 6- phosphogluconolactonase (6PGL). In the present study, Leishmania donovani 6PGL (Ld6PGL) was cloned and overexpressed in bacterial expression system. Comparative sequence analysis revealed the conserved sequence motifs, functionally and structurally important residues in 6PGL family. In silico amino acid substitution study and interacting partners of 6PGL were predicted. The Ld6PGL enzyme was found to be active in the assay and in the parasites. Specificity was confirmed by western blot analysis. The ∼30 kDa protein was found to be a dimer in MALDI, glutaraldehyde crosslinking and size exclusion chromatography studies. Kinetic analysis and structural stability studies of Ld6PGL were performed with denaturants and at varied temperature. Computational 3D Structural modelling of Ld6PGL elucidates that it has a similar α/β hydrolase fold structural topology as in other members of 6PGL family. The three loops are found in extended form when the structure is compared with the human 6PGL (Hs6PGL). Further, enzyme substrate binding mode and its mechanism were investigated using the molecular docking and molecular simulation studies. Interesting dynamics action of substrate 6-phosphogluconolactone was observed into active site during MD simulation. Interesting differences were observed between host and parasite enzyme which pointed towards its potential to be explored as an antileishmanial drug target. This study forms the basis for further analysis of the role of Ld6PGL in combating oxidative stress in Leishmania.

Boosting the Discovery of Small Molecule Inhibitors of Glucose-6-Phosphate Dehydrogenase for the Treatment of Cancer, Infectious Diseases, and Inflammation

We present an overview of small molecule glucose-6-phosphate dehydrogenase (G6PD) inhibitors that have potential for use in the treatment of cancer, infectious diseases, and inflammation. Both steroidal and nonsteroidal inhibitors have been identified with steroidal inhibitors lacking target selectivity. The main scaffolds encountered in nonsteroidal inhibitors are quinazolinones and benzothiazinones/benzothiazepinones. Three molecules show promise for development as antiparasitic (25 and 29) and anti-inflammatory (32) agents. Regarding modality of inhibition (MOI), steroidal inhibitors have been shown to be uncompetitive and reversible. Nonsteroidal small molecules have exhibited all types of MOI. Strategies to boost the discovery of small molecule G6PD inhibitors include exploration of structure-activity relationships (SARs) for established inhibitors, employment of high-throughput screening (HTS), and fragment-based drug discovery (FBDD) for the identification of new hits. We discuss the challenges and gaps associated with drug discovery efforts of G6PD inhibitors from in silico, in vitro, and in cellulo to in vivo studies.

Glucose-6-phosphate dehydrogenase deficiency

Glucose 6-phosphate dehydrogenase (G6PD) deficiency is 1 of the commonest human enzymopathies, caused by inherited mutations of the X-linked gene G6PD. G6PD deficiency makes red cells highly vulnerable to oxidative damage, and therefore susceptible to hemolysis. Over 200 G6PD mutations are known: approximately one-half are polymorphic and therefore common in various populations. Some 500 million persons with any of these mutations are mostly asymptomatic throughout their lifetime; however, any of them may develop acute and sometimes very severe hemolytic anemia when triggered by ingestion of fava beans, by any of a number of drugs (for example, primaquine, rasburicase), or, more rarely, by infection. Approximately one-half of the G6PD mutations are instead sporadic: rare patients with these mutations present with chronic nonspherocytic hemolytic anemia. Almost all G6PD mutations are missense mutations, causing amino acid replacements that entail deficiency of G6PD enzyme activity: they compromise the stability of the protein, the catalytic activity is decreased, or a combination of both mechanisms occurs. Thus, genotype-phenotype correlations have been reasonably well clarified in many cases. G6PD deficiency correlates remarkably, in its geographic distribution, with past/present malaria endemicity: indeed, it is a unique example of an X-linked human polymorphism balanced through protection of heterozygotes from malaria mortality. Acute hemolytic anemia can be managed effectively provided it is promptly diagnosed. Reliable diagnostic procedures are available, with point-of-care tests becoming increasingly important where primaquine and its recently introduced analog tafenoquine are required for the elimination of malaria.

Plasmodium falciparum uses vitamin E to avoid oxidative stress

BACKGROUND

Plasmodium falciparum is sensitive to oxidative stress in vitro and in vivo, and many drugs such as artemisinin, chloroquine and cercosporin interfere in the parasite's redox system. To minimize the damage caused by reactive radicals, antioxidant enzymes and their substrates found in parasites and in erythrocytes must be functionally active. It was shown that P. falciparum synthesizes vitamin E and that usnic acid acts as an inhibitor of its biosynthesis. Vitamin E is a potent antioxidant that protects polyunsaturated fatty acids from lipid peroxidation, and this activity can be measured by detecting its oxidized product and by evaluating reactive oxygen species (ROS) levels.

RESULTS

Here, we demonstrated that ROS levels increased in P. falciparum when vitamin E biosynthesis was inhibited by usnic acid treatment and decreased to basal levels if exogenous vitamin E was added. Furthermore, we used metabolic labelling to demonstrate that vitamin E biosynthesized by the parasite acts as an antioxidant since we could detect its radiolabeled oxidized product. The treatment with chloroquine or cercosporin of the parasites increased the ratio between α-tocopherolquinone and α-tocopherol.

CONCLUSIONS

Our findings demonstrate that vitamin E produced endogenously by P. falciparum is active as an antioxidant, probably protecting the parasite from the radicals generated by drugs.

Glucose-6-phosphate metabolism in Plasmodium falciparum

Malaria is still one of the most threatening diseases worldwide. The high drug resistance rates of malarial parasites make its eradication difficult and furthermore necessitate the development of new antimalarial drugs. Plasmodium falciparum is responsible for severe malaria and therefore of special interest with regard to drug development. Plasmodium parasites are highly dependent on glucose and very sensitive to oxidative stress; two observations that drew interest to the pentose phosphate pathway (PPP) with its key enzyme glucose-6-phosphate dehydrogenase (G6PD). A central position of the PPP for malaria parasites is supported by the fact that human G6PD deficiency protects to a certain degree from malaria infections. Plasmodium parasites and the human host possess a complete PPP, both of which seem to be important for the parasites. Interestingly, there are major differences between parasite and human G6PD, making the enzyme of Plasmodium a promising target for antimalarial drug design. This review gives an overview of the current state of research on glucose-6-phosphate metabolism in P. falciparum and its impact on malaria infections. Moreover, the unique characteristics of the enzyme G6PD in P. falciparum are discussed, upon which its current status as promising target for drug development is based.

Glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase: a unique bifunctional enzyme from Plasmodium falciparum

The survival of malaria parasites in human RBCs (red blood cells) depends on the pentose phosphate pathway, both in Plasmodium falciparum and its human host. G6PD (glucose-6-phosphate dehydrogenase) deficiency, the most common human enzyme deficiency, leads to a lack of NADPH in erythrocytes, and protects from malaria. In P. falciparum, G6PD is combined with the second enzyme of the pentose phosphate pathway to create a unique bifunctional enzyme named GluPho (glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase). In the present paper, we report for the first time the cloning, heterologous overexpression, purification and kinetic characterization of both enzymatic activities of full-length PfGluPho (P. falciparum GluPho), and demonstrate striking structural and functional differences with the human enzymes. Detailed kinetic analyses indicate that PfGluPho functions on the basis of a rapid equilibrium random Bi Bi mechanism, where the binding of the second substrate depends on the first substrate. We furthermore show that PfGluPho is inhibited by S-glutathionylation. The availability of recombinant PfGluPho and the major differences to hG6PD (human G6PD) facilitate studies on PfGluPho as an excellent drug target candidate in the search for new antimalarial drugs.

Data mining of the transcriptome of Plasmodium falciparum: the pentose phosphate pathway and ancillary processes

The general paradigm that emerges from the analysis of the transcriptome of the malaria parasite Plasmodium falciparum is that the expression clusters of genes that code for enzymes engaged in the same cellular function is coordinated. Here the consistency of this perception is examined by analysing specific pathways that metabolically-linked. The pentose phosphate pathway (PPP) is a fundamental element of cell biochemistry since it is the major pathway for the recycling of NADP+ to NADPH and for the production of ribose-5-phosphate that is needed for the synthesis of nucleotides. The function of PPP depends on the synthesis of NADP+ and thiamine pyrophosphate, a co-enzyme of the PPP enzyme transketolase. In this essay, the transcription of gene coding for enzymes involved in the PPP, thiamine and NAD(P)+ syntheses are analysed. The genes coding for two essential enzymes in these pathways, transaldolase and NAD+ kinase could not be found in the genome of P. falciparum. It is found that the transcription of the genes of each pathway is not always coordinated and there is usually a gene whose transcription sets the latest time for the full deployment of the pathway's activity. The activity of PPP seems to involve only the oxidative arm of PPP that is geared for maximal NADP+ reduction and ribose-5-phosphate production during the early stages of parasite development. The synthesis of thiamine diphosphate is predicted to occur much later than the expression of transketolase. Later in the parasite cycle, the non-oxidative arm of PPP that can use fructose-6-phosphate and glyceraldehyde-3-phosphate supplied by glycolysis, becomes fully deployed allowing to maximize the production of ribose-5-phosphate. These discrepancies require direct biochemical investigations to test the activities of the various enzymes in the developing parasite. Notably, several transcripts of PPP enzyme-coding genes display biphasic pattern of transcription unlike most transcripts that peak only once during the parasite cycle. The physiological meaning of this pattern requires further investigation.

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.

Expression of Plasmodium falciparum G6PD-6PGL in laboratory parasites and in patient isolates in G6PD-deficient and normal Nigerian children

As the production of NADPH in the pentose phosphate pathway is the main antioxidant defence mechanism available to the Plasmodium falciparum, we have studied the expression of P. falciparum glucose 6-phosphate dehydrogenase-6-phosphogluconolactonase (PfG6PD-6PGL) in G6PD-deficient and normal erythrocyte host cells. Both erythrocytes infected in vitro with a laboratory isolate and erythrocytes from natural human infections were used. Total RNA was prepared from parasites collected from five G6PD-deficient and nine G6PD-normal children in Ibadan, Nigeria, selected after screening 189 rural schoolchildren and 68 clinical malaria patients, and was subjected to Northern blot analysis. The probe was a cDNA fragment of the G6PD domain of the PfG6PD-6PGL gene, with an internal control probe of P. falciparum 18S ribosomal RNA. Quantification was performed using a phosphoimager. Relative to internal control, the abundance of PfG6PD-6PGL mRNA (mean +/- standard deviation) was lower in parasites from G6PD-deficient children (0.29 +/- 0.27) than in G6PD-normal control subjects (0.74 +/- 0.26) (P = 0.014, Mann-Whitney U-test). Although confirmation in a larger study is required, our results suggest a lower relative abundance of PfG6PD-6PGL, and presumably antioxidant activity, in malaria parasites from G6PD-deficient hosts, thus extending the current knowledge of the mechanism of G6PD-deficiency related host protection.

Murine hexose-6-phosphate dehydrogenase: a bifunctional enzyme with broad substrate specificity and 6-phosphogluconolactonase activity

Murine hexose-6-phosphate dehydrogenase has been purified from liver microsomes by affinity chromatography on 2('),5(')-ADP-Sepharose. The purified enzyme has 6-phosphogluconolactonase activity and glucose-6-phosphate dehydrogenase activity and has a native molecular mass of 178 kDa and a subunit molecular mass of 89 kDa. Glucose 6-phosphate, galactose 6-phosphate, 2-deoxyglucose 6-phosphate, glucosamine 6-phosphate, and glucose 6-sulfate are substrates for murine hexose-6-phosphate dehydrogenase, with either NADP or deamino-NADP as coenzyme. This study confirms that hexose-6-phosphate dehydrogenase is a bifunctional enzyme which can catalyze the first two reactions of the pentose phosphate pathway.

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.

A unique insertion in Plasmodium berghei glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase: evolutionary and functional studies

Plasmodium berghei glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase (G6PD-6PGL) is a bifunctional enzyme with significant sequence similarity in both the 6PGL and G6PD domains to the Plasmodium falciparum enzyme. A recombinant form of the P. berghei enzyme was found to have both G6PD and 6PGL activities, and therefore catalyses the first two steps in the pentose phosphate pathway. Genes encoding very similar proteins are also found in three other malarial parasites, Plasmodium yoelii, Plasmodium chabaudi and Plasmodium knowlesi. All of these predicted enzymes contain unique parasite insertions in corresponding positions in the G6PD domain but the insertions differ in size and sequence. Such insertions are a common feature of malarial proteins but their origin and function is unknown. Excision of the insertion sequence in the P. berghei protein renders the G6PD domain inactive, although the 6PGL activity is unaffected. Replacing the insertion sequence in P. berghei with the insertion sequence from P. falciparum restores some of the G6PD activity and also enhances 6PGL activity. We conclude that although the insertions are evolving rapidly they have an essential role in the activity of the bifunctional enzyme.

NMR spectroscopic analysis of the first two steps of the pentose-phosphate pathway elucidates the role of 6-phosphogluconolactonase

The pentose-phosphate pathway provides reductive power and nucleotide precursors to the cell through oxidative and nonoxidative branches, respectively. 6-Phosphogluconolactonase is the second enzyme of the oxidative branch and catalyzes the hydrolysis of 6-phosphogluconolactones, the products of glucose 6-phosphate oxidation by glucose-6-phosphate dehydrogenase. The role of 6-phosphogluconolactonase was still questionable, because 6-phosphogluconolactones were believed to undergo rapid spontaneous hydrolysis. In this work, nuclear magnetic resonance spectroscopy was used to characterize the chemical scheme and kinetic features of the oxidative branch. We show that 6-phosphogluconolactones have in fact a nonnegligible lifetime and are highly electrophilic compounds. The delta form (1-5) of the lactone is the only product of glucose 6-phosphate oxidation. Subsequently, it leads to the gamma form (1-4) by intramolecular rearrangement. However, only the delta form undergoes spontaneous hydrolysis, the gamma form being a "dead end" of this branch. The delta form is the only substrate for 6-phosphogluconolactonase. Therefore, 6-phosphogluconolactonase activity accelerates hydrolysis of the delta form, thus preventing its conversion into the gamma form. Furthermore, 6-phosphogluconolactonase guards against the accumulation of delta-6-phosphogluconolactone, which may be toxic through its reaction with endogenous cellular nucleophiles. Finally, the difference between activity of human, Trypanosoma brucei, and Plasmodium falciparum 6-phosphogluconolactonases is reported and discussed.

Glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase. A novel bifunctional enzyme in malaria parasites

Plasmodium falciparum glucose 6-phosphate dehydrogenase (Pf Glc6PD), compared to other Glc6PDs has an additional 300 amino acids at the N-terminus. They are not related to Glc6PD but are similar to a family of proteins (devb) of unknown function, some of which are encoded next to Glc6PD in certain bacteria. The human devb homologue has recently been shown to have 6-phosphogluconolactonase (6PGL) activity. This suggests Pf Glc6PD may be a bifunctional enzyme, the evolution of which has involved the fusion of adjacent genes. Further functional analysis of Pf Glc6PD has been hampered because parts of the gene could not be cloned. We have isolated and sequenced the corresponding Plasmodium berghei gene and shown it encodes an enzyme (Pb Glc6PD) with the same structure as the P. falciparum enzyme. Pb Glc6PD is 950 amino acids long with significant sequence similarity in both the devb and Glc6PD domains with the P. falciparum enzyme. The P. berghei enzyme does not have an asparagine-rich segment between the N and C halves and it contains an insertion at the same point in the Glc6PD region as the P. falciparum enzyme but the insertion in the P. berghei is longer (110 versus 62 amino acids) and unrelated in sequence to the P. falciparum insertion. Though expression of this enzyme in bacteria produced largely insoluble protein, conditions were found where the full-length enzyme was produced in a soluble form which was purified via a histidine tag. We show that this enzyme has both Glc6PD and 6PGL activities. Thus the first two steps of the pentose phosphate pathway are catalysed by a single novel bifunctional enzyme in these parasites.

Molecular characterization of the first two enzymes of the pentose-phosphate pathway of Trypanosoma brucei. Glucose-6-phosphate dehydrogenase and 6-phosphogluconolactonase

Trypanosomatids are parasitic protists that have part of their glycolytic pathway sequestered inside peroxisome-like organelles: the glycosomes. So far, at least one enzyme of the pentose-phosphate pathway has been found to be associated partially with glycosomes. Here, we describe how two genes from Trypanosoma brucei, coding for the first two enzymes of the pentose-phosphate pathway, i.e. glucose-6-phosphate dehydrogenase and 6-phosphogluconolactonase, were identified by in silico screening of trypanosome genome project data bases. These genes were cloned and sequenced. Analysis of the lactonase sequence revealed that it contained a C-terminal peroxisome targeting signal in agreement with its subcellular localization in the bloodstream form trypanosome (15% glycosomal and 85% cytosolic). However, the dehydrogenase sequence did not reveal any targeting signal, despite its localization inside glycosomes. The corresponding enzymes have been overexpressed in Escherichia coli and purified, and their biochemical characteristics have been determined.

A European perspective on Neospora caninum

Since the identification of Neospora caninum in 1984 as a parasite separate from Toxoplasma gondii by Bjerkas et al., and its subsequent characterization and classification in 1988 by Dubey and co-workers, this parasite has attracted increasing attention, primarily as an important causative agent of abortion in cattle and neuromuscular disease in dogs, but also as a complementary model system to T. gondii for investigating the basic biology of intracellular parasitism. During November 11-14, 1999, the COST 820 Annual meeting (Vaccines against coccidioses) took place in Interlaken, Switzerland. Almost half of the papers presented at that meeting were on N. caninum and neosporosis, reflecting the increasing awareness of the importance of this parasite on part of the scientific community in Europe. On the occasion of the meeting, participants in this COST Action involved in Neospora research in Europe were asked to participate in this invited review in order to document the growing interest in N. caninum and the disease it causes. Thus, this paper is a unique collection of contributions provided by several European experts in the field. It is comprised of 10 reviews or original papers on different aspects of Neospora research including epidemiology, immunology, application and development of serological tools, and molecular characterisation of the parasite currently carried out throughout Europe. In addition, two distinguished invited speakers from overseas (Milton McAllister and John Ellis) provided valuable contributions. This invited review demonstrates that the COST 820 Action has brought together scientists from all over Europe and other parts of the world, and has laid the basis for many fruitful collaborations. The studies described here will contribute in assessing the relevance of neosporosis as a potential risk factor not only for animals, but also for human health.

Human mutations in glucose 6-phosphate dehydrogenase reflect evolutionary history

Glucose 6-phosphate dehydrogenase (G6PD) is a cytosolic enzyme encoded by a housekeeping X-linked gene whose main function is to produce NADPH, a key electron donor in the defense against oxidizing agents and in reductive biosynthetic reactions. Inherited G6PD deficiency is associated with either episodic hemolytic anemia (triggered by fava beans or other agents) or life-long hemolytic anemia. We show here that an evolutionary analysis is a key to understanding the biology of a housekeeping gene. From the alignment of the amino acid (aa) sequence of 52 glucose 6-phosphate dehydrogenase (G6PD) species from 42 different organisms, we found a striking correlation between the aa replacements that cause G6PD deficiency in humans and the sequence conservation of G6PD: two-thirds of such replacements are in highly and moderately conserved (50-99%) aa; relatively few are in fully conserved aa (where they might be lethal) or in poorly conserved aa, where presumably they simply would not cause G6PD deficiency. This is consistent with the notion that all human mutants have residual enzyme activity and that null mutations are lethal at some stage of development. Comparing the distribution of mutations in a human housekeeping gene with evolutionary conservation is a useful tool for pinpointing amino acid residues important for the stability or the function of the corresponding protein. In view of the current explosive increase in full genome sequencing projects, this tool will become rapidly available for numerous other genes.

Purification, localisation and characterisation of glucose-6-phosphate dehydrogenase of Trypanosoma brucei

Cell-fractionation and digitonin titration of procyclic trypomastigotes of Trypanosoma brucei, revealed that almost half of the total NADP+ -dependent glucose-6-phosphate dehydrogenase (G6PDH) activity, the first enzyme of the pentose phosphate pathway (PPP), is associated with glycosomes. The specific activity of G6PDH in the purified organelles was increased 4-fold relative to a total cell extract and showed latency. Moreover, in the absence of detergents this activity was totally resistant to the action of trypsin. The cytosolic counterpart was neither latent, nor was it resistant to trypsin. Both cytosolic and glycosomal G6PDH activities behaved identically on phenyl-, CM-, heparin-, and Affigel-blue-Sepharose columns. Both isoenzymes had a subunit Mr of 62 000 and an isoelectric point of 6.85, while kinetic studies carried out on the partially purified G6PDH from both cell compartments did not reveal any differences. The purified enzyme had an apparent Km of 138 and 5.3 microM for glucose 6-phosphate (G6P), and for NADP+, respectively, and had a specific activity of 14 micromol. (min mg of protein)(-1). We conclude that while in procyclic stages of T. brucei G6PDH activity is present in two different cell compartments, i.e. the cytosol and the glycosomes, these two activities most likely represent one and the same isoenzyme.

Amino acid conservation and clinical severity of human glucose-6-phosphate dehydrogenase mutations

More than a hundred naturally occurring mutations of human glucose-6-phosphate dehydrogenase (G6PD) have been identified at the amino acid level. The abundance of distinct mutation sites and their clinical manifestations make this enzyme ideal for structure-function analysis studies. We present here a sequence and structure combined analysis by which the severity of clinical symptoms resulting from point mutations of this enzyme is correlated with quantified degrees of amino acid conservation within 23 G6PD sequences from different organisms. Our analysis verifies, on a quantitative basis, a widely held notion that clinically severer mutations of G6PD usually occur at conserved amino acids. However, marked exceptions to this general trend exist which are most notably revealed by a number of mutations associated with chronic nonspherocytic hemolytic anemia (class I variants). When mapped onto a homology-derived structural model of human G6PD, these class I mutational sites of low amino acid conservation appear to localize in two spatially distinct clusters, both of which are populated with mutations consisting mainly of clinically severer variants (i.e. class I and class II). These results of computer-assisted analyses contribute to a further understanding of the structure-function relationships of human G6PD deficiency.

Early Phagocytosis of Glucose-6-Phosphate Dehydrogenase (G6PD)-Deficient Erythrocytes Parasitized by Plasmodium falciparum May Explain Malaria Protection in G6PD Deficiency

In population-based studies it has been established that inherited deficiency of erythrocyte (E) glucose-6-phosphate dehydrogenase (G6PD) confers protection against severe Plasmodium falciparum (P falciparum) malaria. Impaired growth of parasites in G6PD-deficient E in vitro has been reported in some studies, but not in others. In a systematic analysis, we have found that with five different strains ofP falciparum (FCR-3, KI, C10, HB3B, and T9/96), there was no significant difference in either invasion or maturation when the parasites were grown in either normal or G6PD-deficient (Mediterranean variant) E. With all of these strains and at different maturation stages, we were unable to detect any difference in the amount of P falciparum–specific G6PD mRNA in normal versus deficient parasitized E. The rate of 14C-CO2 production from D-[1-14C] glucose (which closely reflects intracellular activity of G6PD) contributed by the parasite was very similar in intact normal and deficient E. By contrast, in studies of phagocytosis of parasitized E by human adherent monocytes, we found that when the parasites were at the ring stage (ring-stage parasitized E [RPE]), deficient RPE were phagocytosed 2.3 times more intensely than normal RPE (P = .001), whereas there was no difference when the parasites were at the more mature trophozoite stage (trophozoite-stage parasitized E [TPE]). Phagocytic removal markers (autologous IgG and complement C3 fragments) were significantly higher in deficient RPE than in normal RPE, while they were very similar in normal and deficient TPE. The level of reduced glutathione was remarkably lower in deficient RPE compared with normal RPE. We conclude that impaired antioxidant defense in deficient RPE may be responsible for membrane damage followed by phagocytosis. Because RPE, unlike TPE, are nontoxic to phagocytes, the increased removal by phagocytosis of RPE would reduce maturation to TPE and to schizonts and may be a highly efficient mechanism of malaria resistance in deficient subjects.

Early Phagocytosis of Glucose-6-Phosphate Dehydrogenase (G6PD)-Deficient Erythrocytes Parasitized by Plasmodium falciparum May Explain Malaria Protection in G6PD Deficiency

In population-based studies it has been established that inherited deficiency of erythrocyte (E) glucose-6-phosphate dehydrogenase (G6PD) confers protection against severe Plasmodium falciparum (P falciparum) malaria. Impaired growth of parasites in G6PD-deficient E in vitro has been reported in some studies, but not in others. In a systematic analysis, we have found that with five different strains ofP falciparum (FCR-3, KI, C10, HB3B, and T9/96), there was no significant difference in either invasion or maturation when the parasites were grown in either normal or G6PD-deficient (Mediterranean variant) E. With all of these strains and at different maturation stages, we were unable to detect any difference in the amount of P falciparum–specific G6PD mRNA in normal versus deficient parasitized E. The rate of 14C-CO2 production from D-[1-14C] glucose (which closely reflects intracellular activity of G6PD) contributed by the parasite was very similar in intact normal and deficient E. By contrast, in studies of phagocytosis of parasitized E by human adherent monocytes, we found that when the parasites were at the ring stage (ring-stage parasitized E [RPE]), deficient RPE were phagocytosed 2.3 times more intensely than normal RPE (P = .001), whereas there was no difference when the parasites were at the more mature trophozoite stage (trophozoite-stage parasitized E [TPE]). Phagocytic removal markers (autologous IgG and complement C3 fragments) were significantly higher in deficient RPE than in normal RPE, while they were very similar in normal and deficient TPE. The level of reduced glutathione was remarkably lower in deficient RPE compared with normal RPE. We conclude that impaired antioxidant defense in deficient RPE may be responsible for membrane damage followed by phagocytosis. Because RPE, unlike TPE, are nontoxic to phagocytes, the increased removal by phagocytosis of RPE would reduce maturation to TPE and to schizonts and may be a highly efficient mechanism of malaria resistance in deficient subjects.

A chloroquine resistance locus in the rodent malaria parasite Plasmodium chabaudi

We have located a possible chloroquine resistance locus in the genome of the rodent malaria parasite Plasmodium chabaudi. Two genetically distinct clones of the parasite were grown in vivo and allowed to undergo genetic crossing. The clones differed from each other in their susceptibility to chloroquine; AS(3CQ) had been selected for a low level of resistance to the drug whereas AJ is chloroquine-sensitive. Independent recombinant progeny (20) were cloned from the products of two crosses, phenotyped for their susceptibility to chloroquine, and genotyped for their inheritance of 46 chromosome-specific markers. No association was found between chloroquine susceptibility and the inheritance of pcmdr1, the P. chabaudi homologue of the pfmdr1 multi-drug resistance gene of P. falciparum. Also, there was no association between chloroquine susceptibility and the inheritance of a marker linked to a putative chloroquine resistance locus in a P. falciparum cross. However, 16 of the progeny clones showed co-segregation of four linked markers on chromosome 11 with their resistance phenotype. This result suggests that a locus for chloroquine resistance exists on this chromosome in P. chabaudi.

Malaria's Eve: evidence of a recent population bottleneck throughout the world populations of Plasmodium falciparum

We have analyzed DNA sequences from world-wide geographic strains of Plasmodium falciparum and found a complete absence of synonymous DNA polymorphism at 10 gene loci. We hypothesize that all extant world populations of the parasite have recently derived (within several thousand years) from a single ancestral strain. The upper limit of the 95% confidence interval for the time when this most recent common ancestor lived is between 24,500 and 57,500 years ago (depending on different estimates of the nucleotide substitution rate); the actual time is likely to be much more recent. The recent origin of the P. falciparum populations could have resulted from either a demographic sweep (P. falciparum has only recently spread throughout the world from a small geographically confined population) or a selective sweep (one strain favored by natural selection has recently replaced all others). The selective sweep hypothesis requires that populations of P. falciparum be effectively clonal, despite the obligate sexual stage of the parasite life cycle. A demographic sweep that started several thousand years ago is consistent with worldwide climatic changes ensuing the last glaciation, increased anthropophilia of the mosquito vectors, and the spread of agriculture. P. falciparum may have rapidly spread from its African tropical origins to the tropical and subtropical regions of the world only within the last 6,000 years. The recent origin of the world-wide P. falciparum populations may account for its virulence, as the most malignant of human malarial parasites.

Plasmodium falciparum glutathione metabolism and growth are independent of glutathione system of host erythrocyte

Plasmodium falciparum parasites grew normally in glutathione (GSH)-depleted normal and G6PD-deficient (Mediterranean variant) erythrocytes (RBC). Growth inhibition was observed only at less than approximately 6-12% residual GSH. Parasites studied separately with the Sendai virus technique synthesized GSH de novo and regenerated reduced GSH 10-20 times faster than non-parasitized RBC. Electron spin resonance measurement of Tempol reduction indicated that the ability to reduce free radicals was restricted to the parasite. The marked efflux of oxidized GSH was mainly derived from the parasite. In conclusion, parasites are endowed with powerful and host-independent mechanisms which de novo synthesize or regenerate GSH and allow undisturbed parasite development in GSH-depleted RBC.

Molecular cloning of CTP:phosphocholine cytidylyltransferase from Plasmodium falciparum

CTP:phosphocholine cytidylyltransferase (CCT) is the rate-limiting and regulatory enzyme in the synthesis of phosphatidylcholine, the major membrane phospholipid, in Plasmodium. The structural gene encoding CCT was isolated from the human malaria parasite Plasmodium falciparum. This was achieved using the PCR to amplify genomic DNA with degenerate primers constructed on the basis of conserved regions identified within yeast and rat liver CCT molecules, and using the PCR product to screen a genomic library. The P. falciparum CCT gene encodes a protein of 370 amino acids (42. 6 kDa) and displays 41-43% similarity (28-29% identity) to CCT molecules of the other organisms cloned to date. The central domain of CCT, proposed as the catalytic domain of the CTP-transfer reaction, shows 68-72% similarity and 48-55% identity among P. falciparum, human, rat and yeast enzymes. This gene is present in a single copy, as determined by Southern-blotting of genomic DNA, and located on chromosome 13 of P. falciparum. Large transcripts were detected by Northern analysis and indicate that this gene is expressed in the asexual intraerythrocytic stages. The coding region of the P. falciparum CCT gene was inserted into an Escherichia coli expression vector to confirm the function of the CCT product. The recombinant CCT expressed in E. coli is catalytically active, as evidenced by the conversion of phosphocholine to CDP-choline.

Genomic structure and sequence of the Fugu rubripes glucose-6-phosphate dehydrogenase gene (G6PD)

To investigate the organization of the compact genome of the puffer fish Fugu rubripes at the level of an individual housekeeping gene, we have cloned and sequenced the glucose-6-phosphate dehydrogenase (G6PD) gene from this fish and compared it to the corresponding human gene. The intron/exon structure of the two genes is identical throughout the protein coding regions. The puffer fish gene is four times smaller than the human gene--the difference is accounted for by the fact that the puffer fish gene has smaller introns. Intron 2 is the largest intron in both species. We have constructed a molecular phylogeny for 10 G6PD protein sequences. The sequences fall in the expected arrangement based on established phylogenetic relationships, with the Plasmodium falciparum sequence the most widely diverged.

The three-dimensional structure of glucose 6-phosphate dehydrogenase from Leuconostoc mesenteroides refined at 2.0 A resolution

BACKGROUND

Glucose 6-phosphate dehydrogenase (G6PD) is the first enzyme of the pentose phosphate pathway. Normally the pathway is synthetic and NADP-dependent, but the Gram-positive bacterium Leuconostoc mesenteroides, which does not have a complete glycolytic pathway, also uses the oxidative enzymes of the pentose phosphate pathway for catabolic reactions, and selects either NAD or NADP depending on the demands for catabolic or anabolic metabolism.

RESULTS

The structure of G6PD has been determined and refined to 2.0 A resolution. The enzyme is a dimer, each subunit consisting of two domains. The smaller domain is a classic dinucleotide-binding fold, while the larger one is a new beta+ alpha fold, not previously seen, with a predominantly antiparallel nine-stranded beta-sheet. There are significant structural differences in the coenzyme-binding domains of the two subunits, caused by Pro 149 which is cis in one subunit and trans in the other.

CONCLUSIONS

The structure has allowed us to propose the location of the active site and the coenzyme-binding site, and suggests the role of many of the residues conserved between species. We propose that the conserved Arg46 would interact with both the adenine ring and the 2'-phosphate of NADP. Gln47, which is not conserved, may contribute to the change from NADP to dual coenzyme specificity. His178, in a nine-residue peptide conserved for all known sequences, binds a phosphate in the active site pocket. His240 is the most likely candidate for the base to oxidize the 1-hydroxyl group of the glucose 6-phosphate substrate.