Plasmodium hemozoin formation mediated by histidine-rich proteins

On the mechanism of hemozoin production in malaria parasites: activated erythrocyte membranes promote beta-hematin synthesis

The ferriprotoporphyrin IX (FP) molecules released by intraerythrocytic malaria parasites during hemoglobin digestion are converted to beta-hematin and are stored in the parasites' food vacuoles. It has been demonstrated in cell-free medium that the incorporation of FP into beta-hematin under physiological conditions requires a catalyst from parasite lysates or pre-formed beta-hematin. In the present studies, lysates of Plasmodium falciparum-infected erythrocytes were suspended in 1 M NaOH and were washed with phosphate buffer, pH 7.6. When the cell extracts were incubated with hematin in 0.5 M sodium acetate buffer, pH 5, for 20 hr at 37 degrees C, a large quantity of beta-hematin was formed. To determine whether parasite components were necessary for the beta-hematin formation, normal erythrocyte ghosts were similarly treated with 1 M NaOH and then incubated with hematin. In repeated experiments it was found that, on the average, 70% of the hematin was converted to beta-hematin. Membranes treated with HCl or CH(3)COOH also promoted the formation of beta-hematin, while untreated membranes were ineffective. The possibility that metabolic activities in the food vacuoles of malaria parasites may activate membrane fragments, from hemoglobin vesicles, to promote beta-hematin formation is discussed in this paper.

Emerging targets for antimalarial drugs

The absence of an effective vaccine against malaria and the ability of the parasite to develop resistance to known antimalarial drugs makes it mandatory to unravel newer drug targets with a view to developing newer pharmacophores. While conventional targets such as the purine, pyrimidine and folate pathways are still being investigated in the light of newer knowledge, a new opportunity has emerged from an understanding of certain unique features of the parasite biology. These include the food vacuole, haemoglobin catabolism, haeme biosynthesis, apicoplasts and their metabolism as well as macromolecular transactions, import of host proteins, parasite induced alterations in the red cell surface and transport phenomena. This review seeks to emphasise the new and emerging targets, while giving a brief account of the targets that have already been exploited.

Histidine-rich protein 2 of the malaria parasite, Plasmodium falciparum, is involved in detoxification of the by-products of haemoglobin degradation

The histidine-rich protein 2 (PfHRP2) of Plasmodium falciparum has been implicated in the detoxification of ferriprotoporphyrin IX (FP) moieties that are produced as by-products of the digestion of haemoglobin. In this work, we have used a spectroscopic analysis to confirm that recombinant PfHRP2 binds FP. A monoclonal antibody that recognises both recombinant and authentic PfHRP2 was used in immunofluorescence microscopy studies. We found that PfHRP2 is mainly located in the erythrocyte cytosol of infected erythrocytes, however, dual labelling studies suggest that the location of a sub-population of the PfHRP2 molecules overlaps with that of the food vacuole-associated protein, P-glycoprotein homologue (Pgh-1). A semi-quantitative analysis of the level of PfHRP2 in infected erythrocytes suggests a concentration of a few micromolar in the food vacuole. Under conditions designed to mimic the parasite food vacuole, we found that 1.2 microM PfHRP2 is sufficient to catalyse the conversion of about 30% of a 100 microM sample of FP to beta-haematin within 24 h. Moreover, PfHRP2 is capable of promoting the H(2)O(2)-induced degradation of FP at pH 5.2. PfHRP2 also efficiently enhances the ability of FP to catalyse the H(2)O(2)-mediated oxidation of the model co-factor, ortho-phenylene diamine (OPD). These data suggest that PfHRP2 may promote the detoxification of FP and reactive oxygen species within the food vacuole. By contrast, PfHRP2 inhibits the destruction of FP by glutathione (GSH) at pH 7.4. This suggests that PfHRP2 is not a catalyst of FP degradation outside the food vacuole.

Mechanism of malarial haem detoxification inhibition by chloroquine

The haem detoxification pathway of the malaria parasite Plasmodium falciparum is a potential biochemical target for drug development. Free haem, released after haemoglobin degradation, is polymerized by the parasite to form haemozoin pigment. Plasmodium falciparum histidine-rich protein-2 (Pfhrp-2) has been implicated as the catalytic scaffold for detoxification of haem in the malaria parasite. Previously we have shown that a hexapeptide repeat sequence (Ala-His-His-Ala-Ala-Asp), which appears 33 times in Pfhrp-2, may be the major haem binding site in this protein. The haem binding studies carried out by ourselves indicate that up to 18 equivalents of haem could be bound by this protein with an observed K(d) of 0.94 microM. Absorbance spectroscopy provides evidence that chloroquine is capable of extracting haem bound to Pfhrp-2. This was supported by the K(d) value, of 37 nM, observed for the haem-chloroquine complex. The native PAGE studies reveal that the formation of the haem-Pfhrp-2 complex is disrupted by chloroquine. These results indicate that chloroquine may be acting by inhibiting haem detoxification/binding to Pfhrp-2. Moreover, the higher affinity of chloroquine for haem than Pfhrp-2 suggests a possible mechanism of action for chloroquine; it may remove the haem bound to Pfhrp-2 and form a complex that is toxic to the parasite.

Hemoglobin metabolism in the malaria parasite Plasmodium falciparum

Hemoglobin degradation in intraerythrocytic malaria parasites is a vast process that occurs in an acidic digestive vacuole. Proteases that participate in this catabolic pathway have been defined. Studies of protease biosynthesis have revealed unusual targeting and activation mechanisms. Oxygen radicals and heme are released during proteolysis and must be detoxified by dismutation and polymerization, respectively. The quinoline antimalarials appear to act by preventing sequestration of this toxic heme. Understanding the disposition of hemoglobin has allowed identification of essential processes and metabolic weakpoints that can be exploited to combat this scourge of mankind.

Haemoproteus and Schistosoma synthesize heme polymers similar to Plasmodium hemozoin and beta-hematin

Many parasites digest hemoglobin as an amino acid source, but only a few produce heme polymer pigment instead of catabolizing heme via heme oxygenase. This work compares purified heme polymers produced by Haemoproteus columbae and Schistosoma mansoni to that of Plasmodium falciparum hemozoin and synthetic beta-hematin. Fourier-transform infrared spectroscopy identifies the signature peaks of the common iron-carboxylate bond characteristic in all four heme polymers. However, all pigments could be distinguished by quite different three-dimensional structure visualized by Field Emission Inlens Scanning Electron Microscopy. Both P. falciparum and H. columbae heme polymers had a symmetrical shape unlike the amorphous S. mansoni heme polymer and beta-hematin. All four heme pigments serve as templates for heme polymer extension, which was inhibitable by chloroquine and other quinoline antimalarials. The polymers showed different levels of resistance to hydrogen peroxide degradation. This work identifies another genus, Haemoproteus, capable of intracellular heme polymer formation. The different three-dimensional structures of each pigment implicate genus specific formation of heme polymer, variation of inhibition of polymer extension by the quinolines and degradation by hydrogen peroxide.

Possible modes of action of the artemisinin-type compounds

Artemisinin-type compounds are used for the treatment of uncomplicated and severe forms of malaria. They reduce parasitaemia more rapidly than any other antimalarial compound known, and are effective against multidrug-resistant parasites. However, uncertainties remain as to how they act on the parasite and cause toxicity. In this review, we summarize current ideas.

Plasmodium falciparum: histidine-rich protein II is expressed during gametocyte development

Both early gametocytes (I-II) and asexual trophozoite stages of Plasmodium falciparum digest hemoglobin and detoxify haem by polymerizing it into parasite pigment called hemozoin. The mechanism of polymerization is unclear but it has been proposed that histidine-rich protein II may facilitate transport of hemoglobin to the food vacuole and catalyze the polymerization in asexual stages. We describe the transcription of histidine-rich protein II in gametocytes by Northern blot analysis and the expression of the protein in these stages by immunoprecipitation and Western blotting. Localization of histidine-rich protein II within the gametocyte by immunofluorescence assay and immunoelectron microscopy clearly illustrated the presence of this molecule in the infected red cell cytosol in the early stages of gametocyte development and internalization in the later gametocyte as it matures. There is a strong correlation between the stage-specific trafficking of histidine-rich protein II in gametocytes and the susceptibility of early but not late gametocytes to the antimalarial drug chloroquine.

Hematin polymerization assay as a high-throughput screen for identification of new antimalarial pharmacophores

Hematin polymerization is a parasite-specific process that enables the detoxification of heme following its release in the lysosomal digestive vacuole during hemoglobin degradation, and represents both an essential and a unique pharmacological drug target. We have developed a high-throughput in vitro microassay of hematin polymerization based on the detection of (14)C-labeled hematin incorporated into polymeric hemozoin (malaria pigment). The assay uses 96-well filtration microplates and requires 12 h and a Wallac 1450 MicroBeta liquid scintillation counter. The robustness of the assay allowed the rapid screening and evaluation of more than 100, 000 compounds. Random screening was complemented by the development of a pharmacophore hypothesis using the "Catalyst" program and a large amount of data available on the inhibitory activity of a large library of 4-aminoquinolines. Using these methods, we identified "hit" compounds belonging to several chemical structural classes that had potential antimalarial activity. Follow-up evaluation of the antimalarial activity of these compounds in culture and in the Plasmodium berghei murine model further identified compounds with actual antimalarial activity. Of particular interest was a triarylcarbinol (Ro 06-9075) and a related benzophenone (Ro 22-8014) that showed oral activity in the murine model. These compounds are chemically accessible and could form the basis of a new antimalarial medicinal chemistry program.

Import of host delta-aminolevulinate dehydratase into the malarial parasite: identification of a new drug target

The parasite Plasmodium berghei imports the enzyme delta-aminolevulinate dehydratase (ALAD), and perhaps the subsequent enzymes of the pathway from the host red blood cell to sustain heme synthesis. Here we have studied the mechanism of this import. A 65-kDa protein on the P. berghei membrane specifically bound to mouse red blood cell ALAD, and a 93-amino-acid fragment (ALAD-DeltaNC) of the host erythrocyte ALAD was able to compete with the full-length enzyme for binding to the P. berghei membrane. ALAD-DeltaNC was taken up by the infected red blood cell when added to a culture of P. falciparum and this led to a substantial decrease in ALAD protein and enzyme activity and, subsequently, heme synthesis in the parasite, resulting in its death.

Haemozoin formation in the midgut of the blood-sucking insect Rhodnius prolixus

Malaria parasites digest haemoglobin and detoxify the free haem by its sequestration into an insoluble dark-brown pigment known as haemozoin (Hz). Until recently, this pigment could be found only in Plasmodium parasites. However, we have shown that Hz is also present in the midgut of the blood-sucking insect Rhodnius prolixus [Oliveira et al. (1999) Nature 400, 517-518]. Here we show that Hz synthesis in the midgut of this insect is promoted by a particulate fraction from intestine lumen. Haem aggregation activity is heat-labile and is inhibited in vitro by chloroquine (CLQ). Inhibition of Hz formation in vivo by feeding insects with CLQ leads to increased levels of haem in the haemolymph of the insect, which resulted in increased lipid peroxidation. Taken together, these results indicate that a factor capable of promoting Hz crystallisation is present in R. prolixus midgut and that this activity represents an important physiological defence of this insect against haem toxicity.

Ferryl-oxo heme intermediate in the antimalarial mode of action of artemisinin

Fourier transform infrared (FTIR) and resonance Raman (RR) spectroscopies have been employed to investigate the reductive cleavage of the O-O bond of the endoperoxide moiety of the antimalarial drug artemisinin and its analog trioxane alcohol by hemin dimer. We have recorded FTIR spectra in the nu(O-O) and nu(as)(Fe-O-Fe) regions of artemisinin and of the hemin dimer that show the cleavage of the endoperoxide and that of the hemin dimer, respectively. We observed similar results in the trioxane alcohol/hemin dimer reaction. The RR spectrum of the artemisinin/hemin dimer reaction displays a vibrational mode at 850 cm(-1) that shifts to 818 cm(-1) when the experiment is repeated with (18)O-O(18) endoperoxide enriched trioxane alcohol. The frequency of this vibration and the magnitude of the (18)O-O(18) isotopic shift led us to assign the 850 cm(-1) mode to the Fe(IV) = O stretching vibration of a ferryl-xoxo heme intermediate that occurs in the artemisinin/hemin dimer and trioxane alcohol/hemin reactions. These results provide the first direct characterization of the antimalarial mode of action of artemisinin and its trioxane analog, and suggest that artemisinin appears to react with heme molecules that have been incorporated into hemozoin and subsequently the heme performs cytochrome P450-type chemistry.

Heme metabolism of Plasmodium is a major antimalarial target

The malarial parasite manifests unique features of heme metabolism. In the intraerythrocyte stage it utilizes the host hemoglobin to generate amino acids for its own protein synthesis, but polymerizes the acquired heme as a mechanism for detoxification. At the same time the parasite synthesizes heme de novo for metabolic use. The heme biosynthetic pathway of the parasite is similar to that of hepatocytes and erythrocytes. However, while the parasite makes its own delta-aminolevulinate (ALA) synthase that is immunochemically different from that of the host, it imports ALA dehydrase and perhaps the subsequent enzymes of the pathway from the host red cell. Many schizonticidal drugs such as chloroquine and artemisinin act by interfering with the heme metabolism of the parasite and there is scope to design new molecules based on the unique features of this metabolic machinery in the parasite.

The polyprotein lipid binding proteins of nematodes

The nematode polyprotein allergens/antigens (NPAs) are specific to nematodes, and are synthesised as tandemly repetitive polypeptides comprising 10 or more repeated units. The polyproteins are post-translationally cleaved at consensus sites to yield multiple copies of the approximately 15-kDa NPA units. These units can be highly diverse in their amino acid sequences, but absolutely conserved signature amino acid positions are identifiable. NPA units are helix-rich and possibly fold as four helix bundle proteins. The NPA units have relatively non-specific lipid binding activities, binding fatty acids and retinoids, with dissociation constants similar to those of lipid transport proteins of vertebrates. Fluorescence-based analysis has indicated that, like most lipid transport proteins, the ligand is taken into the binding site in its entirety, but the binding site environment is unusual. NPAs are synthesised in the gut of nematodes, and presumably act to distribute small lipids from the gut, via the pseudocoelomic fluid, to consuming tissues (muscles, gonads, etc.). In some species, one of the units has a histidine-rich extension peptide which binds haems and certain divalent metal ions. NPAs appear to be released by parasitic nematodes, and may thereby be involved in modification of the local inflammatory and immunological environment of the tissues they inhabit by delivering or sequestering pharmacologically active lipids - they are known to bind arachidonic acids and some of its metabolites, lysophospholipids, and retinoids. NPAs are the only known lipid binding protein made as polyproteins, and are exceptions to the rule that repetitive polyproteins are only produced by cells undergoing programmed cell death and producing specialist products.

Metalloporphyrins inhibit beta-hematin (hemozoin) formation

Metal-substituted protoporphyrin IXs (Cr(III)PPIX (1), Co(III)PPIX (2), Mn(III)PPIX (3), Cu(II)PPIX (4), Mg(II)PPIX (5), Zn(II)PPIX (6), and Sn(IV)PPIX (7)) act as inhibitors to beta-hematin (hemozoin) formation, a critical detoxification biopolymer of malarial parasites. The central metal ion plays a significant role in the efficacy of the metalloprotoporphyrins to inhibit beta-hematin formation. The efficacy of these compounds correlates well with the water exchange rate for the octahedral aqua complexes of the porphyrin's central metal ion. Under these in vitro reaction conditions, metalloporphyrins 5, 6 and 7 are as much as six times more efficacious than the free ligand protoporphyrin IX in preventing beta-hematin formation and four times as efficacious as chloroquine, while metalloporphyrins 3 and 4 are three to four times more effective at preventing beta-hematin formation than the free protoporphyrin IX base. In contrast, the relatively exchange inert metalloporphyrins 1 and 2 are only as efficacious as the free ligand and only two-thirds as effective as chloroquine. Aggregation studies of the heme:MPPIX using UV-Vis and fluorescence spectroscopies are indicative of the formation of pi-pi hetero-metalloporphyrin assemblies. Thus, hemozoin inhibition is likely prevented by the formation of heme:MPPIX complexes through pi-stacking interactions. The ramifications of such hetero-metalloporphyrin assemblies, in the context of the emerging structural picture of hemozoin, are discussed.

Structure-function relationships in aminoquinolines: effect of amino and chloro groups on quinoline-hematin complex formation, inhibition of beta-hematin formation, and antiplasmodial activity

Comparison of 19 aminoquinolines supports the hypothesis that chloroquine and related antimalarials act by complexing ferriprotoporphyrin IX (Fe(III)PPIX), inhibiting its conversion to beta-hematin (hemozoin) and hence its detoxification. The study suggests that a basic amino side chain is also essential for antiplasmodial activity. 2- And 4-aminoquinolines are unique in their strong affinity for Fe(III)PPIX, and attachment of side chains to the amino group has relatively little influence on the strength of complex formation. Association with Fe(III)PPIX is necessary, but not sufficient, for inhibiting beta-hematin formation. Presence of a 7-chloro group in the 4-aminoquinoline ring is a requirement for beta-hematin inhibitory activity, and this is also unaffected by side chains attached to the amino group. In turn, beta-hematin inhibitory activity is necessary, but not sufficient, for antiplasmodial activity as the presence of an aminoalkyl group attached to the 4-amino-7-chloroquinoline template is essential for strong activity. We thus propose that the 4-aminoquinoline nucleus of chloroquine and related antimalarials is responsible for complexing Fe(III)PPIX, the 7-chloro group is required for inhibition of beta-hematin formation, and the basic amino side chain is required for drug accumulation in the food vacuole of the parasite.

The production of nitric oxide and TNF-alpha in peritoneal macrophages is inhibited by Dichroa febrifuga Lour

Nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha) have been suggested to play an important role in endotoxin-mediated shock and inflammation. In this study, we investigated the effect of aqueous extract of Dichroa febrifuga Lour. (Saxifragaceae) roots, a traditional antimalarial drug, on the production of NO and TNF-alpha. The aqueous extract of D. febrifuga roots (AEDF) inhibited the secretion of NO and TNF-alpha in lipopolysaccharide (LPS) and/or interferon-gamma (IFN-gamma)-stimulated mouse peritoneal macrophages, without affecting cell viability. The protein level of inducible nitric oxide synthase (iNOS) in peritoneal macrophages was also decreased by AEDF. In addition, the serum level of NO was reduced by i.p. administration of AEDF. These results suggest that AEDF suppresses the endotoxin-induced inflammatory responses through inhibiting the production of NO and TNF-alpha, and could be used as an anti-inflammatory drug.

Heme binding and polymerization by Plasmodium falciparum histidine rich protein II: influence of pH on activity and conformation

The histidine rich protein II (HRPII) from Plasmodium falciparum has been implicated as a heme polymerase which detoxifies free heme by its polymerization to inactive hemozoin. Histidine-iron center coordination is the dominant mechanism of interaction between the amino acid and heme. The protein also contains aspartate allowing for ionic/coordination interactions between the carboxylate side chain and the heme metal center. The pH profile of heme binding and polymerization shows the possibility of these two types of binding sites being differentiated by pH. Circular dichroism studies of the protein show that pH and heme binding cause a change in conformation above pH 6 implying the involvement of His-His+ transitions. Heme binding at pHs above 6 perturbs HRPII conformation, causing an increase in helicity.

Characterization of the Main Radical and Products Resulting from a Reductive Activation of the Antimalarial Arteflene (Ro 42-1611)

The peroxide-containing antimalarial drug arteflene (Ro 42-1611) generates an alkyl radical after the reductive homolytic cleavage of the peroxide bond in the presence of a heme model Mn(II)(TPP). This alkyl radical has been trapped by TEMPO, and the different products of the reduction activation of arteflene have been characterized. These data suggest that, in these experimental conditions, arteflene is not a significant alkylating agent compared to artemisinin, a trioxane-containing antimalarial drug.

Artemisinin, an endoperoxide antimalarial, disrupts the hemoglobin catabolism and heme detoxification systems in malarial parasite

Endoperoxide antimalarials based on the ancient Chinese drug Qinghaosu (artemisinin) are currently our major hope in the fight against drug-resistant malaria. Rational drug design based on artemisinin and its analogues is slow as the mechanism of action of these antimalarials is not clear. Here we report that these drugs, at least in part, exert their effect by interfering with the plasmodial hemoglobin catabolic pathway and inhibition of heme polymerization. In an in vitro experiment we observed inhibition of digestive vacuole proteolytic activity of malarial parasite by artemisinin. These observations were further confirmed by ex vivo experiments showing accumulation of hemoglobin in the parasites treated with artemisinin, suggesting inhibition of hemoglobin degradation. We found artemisinin to be a potent inhibitor of heme polymerization activity mediated by Plasmodium yoelii lysates as well as Plasmodium falciparum histidine-rich protein II. Interaction of artemisinin with the purified malarial hemozoin in vitro resulted in the concentration-dependent breakdown of the malaria pigment. Our results presented here may explain the selective and rapid toxicity of these drugs on mature, hemozoin-containing, stages of malarial parasite. Since artemisinin and its analogues appear to have similar molecular targets as chloroquine despite having different structures, they can potentially bypass the quinoline resistance machinery of the malarial parasite, which causes sublethal accumulation of these drugs in resistant strains.

Kinetics of inhibition of glutathione-mediated degradation of ferriprotoporphyrin IX by antimalarial drugs

We have shown previously that chloroquine and amodiaquine inhibit the glutathione-dependent degradation of ferriprotoporphyrin IX (FP). We have also demonstrated that treatment of human erythrocytes infected with Plasmodium falciparum with chloroquine or amodiaquine results in a dose- and time-dependent accumulation of FP in the membrane fraction of these cells in correlation with parasite killing. High levels of membrane FP are known to perturb the barrier properties of cellular membranes, and could thereby irreversibly disturb the ion homeostasis of the parasite and cause parasite death. We here report on the effect of various 4-aminoquinolines, as well as pyronaridine, halofantrine and some bis-quinolines, on glutathione-mediated destruction of FP in aqueous solution, when FP was bound non-specifically to a protein, and when it was dissolved in human erythrocyte ghost membranes. We showed that all drugs were capable of inhibiting FP degradation in solution. The inhibitory efficacy of some drugs declined when FP was bound non-specifically to protein. Quinine and mefloquine were unable to inhibit the degradation of membrane-associated FP, in line with their inability to increase membrane-associated FP levels in malaria-infected cells following drug treatment. The discrepancy between chloroquine and amodiaquine on the one hand, and quinine and mefloquine on the other, is discussed in terms of the particular location of drugs and FP in the phospholipid membrane, and may suggest differences in the mechanistic details of the antimalarial action of these drugs.

Assay of beta-hematin formation by malaria parasite

Novel leads are urgently required for designing antimalarials due to the reduced efficacy of presently available drugs. The malaria parasite has a unique reaction of heme polymerization, which has attracted much attention in the recent past as a target for the design of antimalarial drugs. The process is hampered by non-availability of a proper assay method. Currently available methods are cumbersome and require advanced instrumentation or radioactive substrates. Here, we are describing an assay for hemozoin formation that is simple and reproducible. This assay has routinely been used by us for the identification of potential compounds with antimalarial activity.

Involvement of lipids in ferriprotoporphyrin IX polymerization in malaria

Approximately 70% of the initial ferriprotoporphyrin IX polymerizing activity in cell-free preparations of erythrocytes infected with Plasmodium berghei was recovered in a chloroform extract. No polymerizing activity remained in the residue. In studies to identify substances that promote FP polymerization, arachidonic, linoleic, oleic, and palmitoleic acids, 1-mono- and di-oleoylglycerol, and the detergents, SDS, Tween 80, and n-octyl-glucopyranoside, were active. Tri-oleoylglycerol, cholesterol, di-oleoylphosphatidylethanolamine, and stearic and palmitic acids were inactive. The model lipid, mono-oleoylglycerol (250 nmol), co-precipitated with FP from a 0.09 M acetate medium at pH 5 and promoted the polymerization of 215 nmol (61%) of the ferriprotoporphyrin IX in the precipitate during a 24-h incubation at 37 degrees C. Polymerization was maximal at pH 5, it was approximately linear for 2 h, and it continued at a decreasing rate for 24 h. The polymer contained exclusively ferriprotoporphyrin IX (97+/-1.3%, mean+/-S.E., n=4) and exhibited the solubility and the electronic absorption and infrared spectral characteristics of the sequestered ferriprotoporphyrin IX of hemozoin. Detergents presumably promote polymerization in an acid medium by helping to dissolve monomeric FP. We suggest that unsaturated lipids co-precipitate with FP in the parasite's acidic food vacuole and also dissolve sufficient monomeric FP to allow polymerization.

Hemozoin stability and dormant induction of heme oxygenase in hemozoin-fed human monocytes

Human monocytes avidly ingest malarial pigment, hemozoin. Phagocytosed hemozoin persists in the monocyte for a long time and modifies important monocyte functions. Stability of phagocytosed hemozoin may depend on modifications of the hemozoin heme moiety or reduced ability to express heme-inducible heme oxygenase. We show here that the spectral characteristics of alkali-solubilized hemozoin were identical to those of authentic heme, although hemozoin was solubilized by alkali much more slowly than authentic heme. Alkali-solubilized hemozoin was a substrate of microsomal rat heme oxygenase and bilirubin reductase, with bilirubin as the main final product. Hemozoin feeding to human monocytes did not induce heme oxygenase, but authentic heme and alkali-solubilized hemozoin supplemented to hemozoin-fed monocytes induced heme oxygenase and were degraded normally. Lysosomes isolated from hemozoin-fed monocytes released only traces of heme while lysosomes from erythrocyte-fed monocytes liberated considerable quantities of heme. Lack of heme release from hemozoin did not depend on proteolysis-resistant, heme-binding proteins, since lysosomal proteases fully degraded hemozoin-associated proteins but did not solubilize hemozoin. In conclusion, our data indicate that lack of induction of HO1 is due to the intrinsic structural characteristics of hemozoin and not to hemozoin-mediated impairment of the mechanism of HO1 induction.

Malaria parasite metabolism in sickle cells

Intraerythrocytic malaria parasites meet part of their growth requirements by ingesting and digesting haemoglobin (Hb). To see whether this process is affected by Hb types, the growth and metabolism of Plasmodium falciparum in normal AA, heterozygous AS and homozygous SS erythrocytes were compared in this study. Parasites that have been adapted to continuous growth in AA and AS erythrocytes in vitro were used, and the cultures were incubated in an environment that contained 3% oxygen, 4% carbon dioxide and 93% nitrogen. It was found that exposure of the cultures to this gas mixture caused 5-10% of the AS and up to 90% of the SS erythrocytes to sickle. Parasite growth was essentially normal in the 3 cell types, although multiplication was significantly lower in SS than in AS and AA erythrocytes. Parasite metabolism was evaluated through measurement of haemozoin production. The mean quantity of haemozoin produced by the parasites in AA was comparable to that produced in AS, but significantly higher than that produced in SS erythrocytes. This finding suggests that P. falciparum metabolism is impaired in SS but not in AS erythrocytes. The impairment may be related to polymerization of Hb S.

The fate of ferriprotorphyrin IX in malaria infected erythrocytes in conjunction with the mode of action of antimalarial drugs

The intraerythrocytic malaria parasite digests considerable amounts of its host cell cytosol, which consists mostly of hemoglobin. In order to avert the toxicity of ferriprotorphyrin IX (FP) thus produced, it is generally accepted that FP is polymerized to the non-toxic hemozoin. Investigating the fate of FP in cultured Plasmodium falciparum -infected human red blood cells, revealed a straight correlation between amounts of digested hemoglobin and hemozoin, but the latter contained less FP than produced. The efficacy of FP polymerization is stage-dependent, increasing with parasite maturation. Different strains display dissimilar efficacy in hemozoin production. Unpolymerized FP possibly exits the food vacuole and is degraded by glutathione, thus accounting for the low levels of free FP found in infected cells. 4-aminoquinoline antimalarials demonstrably form complexes with FP and inhibit hemozoin production in vitro. Chloroquine, amodiaquine, quinine and mefloquine were found to inhibit hemozoin production in intact infected cells, but only the first two drugs caused a dose-dependent accumulation of FP in the membrane fraction of infected cells that correlated well with parasite killing, due to the permeabilization of membranes to ions. This differential effect is explained by the ability of chloroquine and amodiaquine to inhibit the degradation of membrane-associated FP by glutathione and the incapacity of quinine and mefloquine to do so. This discrepancy implies that the antimalarial mode of action of chloroquine and amodiaquine is different in its mechanistic details from that of quinine and mefloquine and is compatible with the diametric sensitivity of most strains to chloroquine and mefloquine and the disparate interaction of these drugs with enhancers of their antimalarial action.

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.

A common mechanism for blockade of heme polymerization by antimalarial quinolines

The antimalarial quinolines are believed to work by blocking the polymerization of toxic heme released during hemoglobin proteolysis in intraerythrocytic Plasmodium falciparum. In the presence of free heme, chloroquine and quinidine associate with the heme polymer. We have proposed that this association of the quinoline-heme complex with polymer caps the growing heme polymer, preventing further sequestration of additional heme that then accumulates to levels that kill the parasite. In this work results of binding assays demonstrate that the association of quinoline-heme complex with heme polymer is specific, saturable, and high affinity and that diverse quinoline analogs can compete for binding. The relative quinoline binding affinity for heme polymer rather than free heme correlates with disruption of heme polymerization. Mefloquine, another important antimalarial quinoline, associated with polymer in a similar fashion, both in cultured parasites and in the test tube. In parasite culture, blocking heme release with protease inhibitor was antagonistic to mefloquine action, as it is to chloroquine action. These data suggest a common mechanism for quinoline antimalarial action dependent on drug interaction with both heme and heme polymer.

Inhibition of glutathione-dependent degradation of heme by chloroquine and amodiaquine as a possible basis for their antimalarial mode of action

We propose here a new and detailed model for the antimalarial action of chloroquine (CQ), based on the its ability to inhibit degradation of heme by glutathione. Heme, which is toxic to the malaria parasite, is formed when the intraerythrocytic malaria parasite ingests and digests inside its food vacuole its host cell cytosol, which consists mainly of hemoglobin. The parasite protects itself against the toxicity of heme by polymerizing some of it to insoluble hemozoin (HZ). We show here that in Plasmodium falciparum at the trophozoite stage only ca. 30% of the heme is converted into hemozoin. We suggest that nonpolymerized heme exits the food vacuole and is subsequently degraded by glutathione, as has been shown before for uninfected erythrocytes. Marginal amounts of free heme could be detected in the membrane fraction of infected cells but nowhere else. It is well established that CQ and amodiaquine (AQ) accumulate in the parasite's food vacuole and inhibit heme polymerization, thereby increasing its efflux out of the food vacuole. We found that these drugs competitively inhibit the degradation of heme by glutathione, thus allowing heme to accumulate in membranes. Incubation of intact infected cells with CQ and AQ results in a marked increase in membrane-associated heme in a dose- and time-dependent manner, and a relationship exists between membrane heme levels and the extent of parasite killing. Heme has been shown to disrupt the barrier properties of membranes and to upset ion homeostasis in CQ-treated malaria-infected cells. In agreement with the predictions of our model, increasing the cellular levels of glutathione leads to increased resistance to CQ, whereas decreasing them results in enhanced sensitivity to the drug. These results insinuate a novel mechanism of drug resistance.

An assessment of drug-haematin binding as a mechanism for inhibition of haematin polymerisation by quinoline antimalarials

Chloroquine is thought to exert its antimalarial activity by preventing the polymerisation of toxic haematin released during proteolysis of haemoglobin in the Plasmodium digestive vacuole. However, the molecular mechanisms by which this inhibition occurs and the universality of this mechanism for other quinoline antimalarials remain to be established. We demonstrate here a correlation for eight antimalarial quinolines between inhibition of haematin polymerisation in vitro and inhibition of P. falciparum growth in culture, confirming haematin polymerisation as the likely target of quinoline blood schizonticides. Furthermore, using isothermal titration microcalorimetry, a correlation was observed between the haematin binding constant of these compounds and their ability to inhibit haematin polymerisation, suggesting that these compounds mediate their activity through binding to haematin. It was also observed that the compounds bind primarily to the mu-oxo dimer form of haematin rather than the monomeric form. It is postulated that this binding inhibits haematin polymerisation by shifting the haematin dimerisation equilibrium to the mu-oxo dimer, thus reducing the availability of monomeric haematin for incorporation into haemozoin. These data reconcile the haematin polymerisation theory with the Fitch hypothesis, which states that chloroquine mediates its activity through binding to haematin.

A comparison and analysis of several ways to promote haematin (haem) polymerisation and an assessment of its initiation in vitro

We compared several methods for producing haematin polymerisation at physiological temperatures (i.e., 37 degrees) and found that a trophozoite lysate-mediated reaction was inappropriate for measuring compound inhibition of haematin polymerisation. Using this method, we obtained significantly higher IC50 values (concentration inhibiting haematin polymerisation by 50%) for certain compounds than when other methods were used, including a food vacuole lysate-mediated reaction. This difference was probably due to the binding of these compounds to cytosolic parasite proteins, as proteinase K treatment of the trophozoite lysate reversed this effect. The initiation of haematin polymerisation was also investigated using several assays. It was found that haematin polymerisation occurred spontaneously, in the absence of preformed haemozoin, over a period of several days, but that the process was more rapid when an acetonitrile extract of malarial trophozoites was added. This extract contained no detectable protein, and its activity could be replicated using an extract from uninfected erythrocytes and by using lipids. We therefore postulate that no protein or parasite-specific material is absolutely required for the initiation of haematin polymerisation. The formation of beta-haematin de novo using the acetonitrile extract is more pH-dependent than the generation of newly synthesised beta-haematin from preformed haemozoin and cannot proceed much above pH = 6. We postulate that the initiation of haematin polymerisation is more sensitive to the equilibrium of haematin between its monomeric and mu-oxo dimer form and requires a higher concentration of monomer than for the elongation phase of polymerisation.

Antagonism of Immunostimulatory CpG-Oligodeoxynucleotides by Quinacrine, Chloroquine, and Structurally Related Compounds

Phosphorothioate oligodeoxynucleotides containing CpG (CpG-ODN) activate immune responses. We report that quinacrine, chloroquine, and structurally related compounds completely inhibit the antiapoptotic effect of CpG-ODN on WEHI 231 murine B lymphoma cells and inhibit CpG-ODN-induced secretion of IL-6 by WEHI 231. They also inhibit IL-6 synthesis and thymidine uptake by human unfractionated PBMC induced by CpG-ODN. The compounds did not inhibit LPS-induced responses. Half-maximal inhibition required 10 nM quinacrine or 100 nM chloroquine. Inhibition was noncompetitive with respect to CpG-ODN. Quinine, quinidine, and primaquine were much less powerful. Quinacrine was effective even when added after the CpG-ODN. Near-toxic concentrations of ammonia plus bafilomycin A1 (used to inhibit vesicular acidification) did not reduce the efficacy of the quinacrine, but the effects of both quinacrine and chloroquine were enhanced by inhibition of the multidrug resistance efflux pump by verapamil. Agents that bind to DNA, including propidium iodide, Hoechst dye 33258, and coralyne chloride did not inhibit CpG-ODN effect, nor did 4-bromophenacyl bromide, an inhibitor of phospholipase A2. Examination of the structure-activity relationship of seventy 4-aminoquinoline and 9-aminoacridine analogues reveals that increased activity was conferred by bulky hydrophobic substituents on positions 2 and 6 of the quinoline nucleus. No correlation was found between published antimalarial activity and ability to block CpG-ODN-induced effects. These results are discussed in the light of the ability of quinacrine and chloroquine to induce remission of rheumatoid arthritis and lupus erythematosus.

Tough tendons. Mussel byssus has collagen with silk-like domains

The primary structure of the alpha-chain of preCol-D (molecular mass = 80 kDa), a tanned collagenous protein predominating in the distal portion of the byssal threads of the mussel Mytilus edulis, was deduced from cDNA to encode an unprecedented natural block copolymer with three major domain types: a central collagen domain flanked by fibroin-like domains and followed by histidine-rich termini. The fibroin-like domains have sequence motifs that strongly resemble the crystalline polyalanine-rich and amorphous glycine-rich regions of spider dragline silk fibroins. The terminal regions resemble the histidine-rich domains of a variety of metal-binding proteins. The silk domains may toughen the collagen by increasing its strength and extensibility. PreCol-D expression is limited to the mussel foot, which contains a longitudinal gradient of preCol-D mRNA. This gradient increases linearly in the proximal to distal direction and reaches a maximum just before the distal depression of the foot.

Synthetic peptides corresponding to a repetitive sequence of malarial histidine rich protein bind haem and inhibit haemozoin formation in vitro

Synthetic peptides containing a repetitive hexapeptide sequence (Ala-His-His-Ala-Ala-Asp) of malarial histidine-rich protein II were evaluated for binding with haem in vitro. The pattern of haem binding suggested that each repeat unit of this sequence provides one binding site for haem. Chloroquine inhibited the haem-peptide complex formation with preferential formation of a haem chloroquine complex. In vitro studies on haem polymerisation showed that none of the peptides could initiate haemozoin formation. However, they could inhibit haemozoin formation promoted by a malarial parasite extract, possibly by competitively binding free haem. These results indicate this hexapeptide sequence represents the haem binding site of the malarial histidine-rich protein and possibly the site of nucleation for haem polymerisation.

Extensible collagen in mussel byssus: a natural block copolymer

To adhere to solid surfaces, marine mussels produce byssal threads, each of which is a stiff tether at one end and a shock absorber with 160 percent extensibility at the other end. The elastic extensibility of proximal byssus is extraordinary given its construction of collagen and the limited extension (less than 10 percent) of most collagenous materials. From the complementary DNA, we deduced that the primary structure of a collagenous protein (preCol-P) predominating in the extensible proximal portion of the threads encodes an unprecedented natural block copolymer with three major domain types: a central collagen domain, flanking elastic domains, and histidine-rich terminal domains. The elastic domains have sequence motifs that strongly resemble those of elastin and the amorphous glycine-rich regions of spider silk fibroins. Byssal thread extensibility may be imparted by the elastic domains of preCol-P.

Positive selection systems for discovery of novel polyester biosynthesis genes based on fatty acid detoxification

The photosynthetic bacterium Rhodobacter capsulatus can grow with short- to long-chain fatty acids as the sole carbon source (R. G. Kranz, K. K. Gabbert, T. A. Locke, and M. T. Madigan, Appl. Environ. Microbiol. 63:3003-3009, 1997). Concomitant with growth on fatty acids is the production to high levels of the polyester storage compounds called polyhydroxyalkanoates (PHAs). Here, we describe colony screening and selection systems to analyze the production of PHAs in R. capsulatus. A screen with Nile red dissolved in acetone distinguishes between PHA producers and nonproducers. Unlike the wild type, an R. capsulatus PhaC- strain with the gene encoding PHA synthase deleted is unable to grow on solid media containing high concentrations of certain fatty acids. It is proposed that this deficiency is due to the inability of the PhaC- strain to detoxify the surrounding medium by consumption of fatty acids and their incorporation into PHAs. This fatty acid toxicity phenotype is used in selection for the cloning and characterization of heterologous phaC genes.

Probing the chloroquine resistance locus of Plasmodium falciparum with a novel class of multidentate metal(III) coordination complexes

The malaria organism Plasmodium falciparum detoxifies heme released during degradation of host erythrocyte hemoglobin by sequestering it within the parasite digestive vacuole as a polymer called hemozoin. Antimalarial agents such as chloroquine appear to work by interrupting the heme polymerization process, but their efficacy has been impaired by the emergence of drug-resistant organisms. We report here the identification of a new class of antimalarial compounds, hexadentate ethylenediamine-N, N'-bis[propyl(2-hydroxy-(R)-benzylimino)]metal(III) complexes [(R)-ENBPI-M(III)] and a corresponding ((R)-benzylamino)] analog [(R)-ENBPA-M(III)], a group of lipophilic monocationic leads amenable to metallopharmaceutical development. Racemic mixtures of Al(III), Fe(III), or Ga(III) but not In(III) (R)-ENBPI metallo-complexes killed intraerythrocytic malaria parasites in a stage-specific manner, the R = 4,6-dimethoxy-substituted ENBPI Fe(III) complex being most potent (IC50 approximately 1 microM). Inhibiting both chloroquine-sensitive and -resistant parasites, potency of these imino complexes correlated in a free metal-independent manner with their ability to inhibit heme polymerization in vitro. In contrast, the reduced (amino) 3-MeO-ENBPA Ga(III) complex (MR045) was found to be selectively toxic to chloroquine-resistant parasites in a verapamil-insensitive manner. In 21 independent recombinant progeny of a genetic cross, susceptibility to this agent mapped in perfect linkage with the chloroquine resistance phenotype suggesting that a locus for 3-MeO-ENBPA Ga(III) susceptibility was located on the same 36-kilobase segment of chromosome 7 as the chloroquine resistance determinant. These compounds may be useful as novel probes of chloroquine resistance mechanisms and for antimalarial drug development.

Expression and characterisation of plasmepsin I from Plasmodium falciparum

Two aspartic proteinases, plasmepsins I and II, are present in the digestive vacuole of the human malarial parasite Plasmodium falciparum and are believed to be essential for parasite degradation of haemoglobin. Here we report the expression and kinetic characterisation of functional recombinant plasmepsin I. In order to generate active plasmepsin I from its precursor, an autocatalytic cleavage site was introduced into the propart of the zymogen by mutation of Lys110P to Val (P indicates a propart residue). Appropriate refolding of the mutated zymogen then permitted pH-dependent autocatalytic processing of the zymogen to the active mature proteinase. A purification scheme was devised that removed aggregated and misfolded protein to yield pure, fully processable, proplasmepsin I. Kinetic constants for two synthetic peptide substrates and four inhibitors were determined for both recombinant plasmepsin I and recombinant plasmepsin II. Plasmepsin I had 5-10-fold lower k(cat)/Km values than plasmepsin II for the peptide substrates, while the aspartic proteinase inhibitors, selected for their ability to inhibit P. falciparum growth, were found to have up to 80-fold lower inhibition constants for plasmepsin I compared to plasmepsin II. The most active plasmepsin I inhibitors were antagonistic to the antimalarial action of chloroquine on cultured parasites. Northern blot analysis of RNA, isolated from specific stages of the erythrocytic cycle of P. falciparum, showed that the proplasmepsin I gene is expressed in the ring stages whereas the proplasmepsin II gene is not transcribed until the later trophozoite stage of parasite growth. The differences in kinetic properties and temporal expression of the two plasmepsins suggest they are not functionally redundant but play distinct roles in the parasite.

Characterization of the products of the heme detoxification pathway in malarial late trophozoites by X-ray diffraction

In a process inhibited by the quinoline antimalarial drugs, Plasmodia detoxify heme released during the degradation of hemoglobin by aggregating it into malarial pigment, an insoluble crystalline heme coordination polymer. Synchrotron x-ray powder diffraction patterns for intact desiccated malarial trophozoites and synthetic beta-hematin have been measured; both materials correspond to a single crystalline triclinic lattice with unit cell parameters a = 12.2176(4), b = 14.7184(5), c = 8.0456(3) A; alpha = 90.200(2), beta = 96.806(3), gamma = 97.818(3) degrees and Z = 2. These results unambiguously demonstrate that hemozoin crystallites are identical to synthetic beta-hematin.