Heme lysis of the bloodstream forms of Trypanosoma brucei

Heme crystallization in a Chagas disease vector acts as a redox-protective mechanism to allow insect reproduction and parasite infection

Heme crystallization as hemozoin represents the dominant mechanism of heme disposal in blood feeding triatomine insect vectors of the Chagas disease. The absence of drugs or vaccine for the Chagas disease causative agent, the parasite Trypanosoma cruzi, makes the control of vector population the best available strategy to limit disease spread. Although heme and redox homeostasis regulation is critical for both triatomine insects and T. cruzi, the physiological relevance of hemozoin for these organisms remains unknown. Here, we demonstrate that selective blockage of heme crystallization in vivo by the antimalarial drug quinidine, caused systemic heme overload and redox imbalance in distinct insect tissues, assessed by spectrophotometry and fluorescence microscopy. Quinidine treatment activated compensatory defensive heme-scavenging mechanisms to cope with excessive heme, as revealed by biochemical hemolymph analyses, and fat body gene expression. Importantly, egg production, oviposition, and total T. cruzi parasite counts in R. prolixus were significantly reduced by quinidine treatment. These effects were reverted by oral supplementation with the major insect antioxidant urate. Altogether, these data underscore the importance of heme crystallization as the main redox regulator for triatomine vectors, indicating the dual role of hemozoin as a protective mechanism to allow insect fertility, and T. cruzi life-cycle. Thus, targeting heme crystallization in insect vectors represents an innovative way for Chagas disease control, by reducing simultaneously triatomine reproduction and T. cruzi transmission.

Chagas disease is a fatal illness caused by Trypanosoma cruzi parasites, which are transmitted by blood sucking triatomine insect vectors. Although blood is a natural food source for these insects, its digestion releases toxic products, which poses a dietary challenge for both triatomine insects and trypanosomes. To overcome this, triatomines eliminate these toxic blood products by a unique process of heme crystallization into hemozoin that take place in their digestive tract. Here we describe that this detoxification process represents the major mechanism for redox balance control, and is necessary to allow triatomine insect reproduction, and Trypanosoma cruzi infection. Disruption of heme crystallization in triatomine insects thus represents a new venue for Chagas disease control, by targeting at the same time insect reproduction and parasite transmission.

3-Hydroxykynurenine, a Tryptophan Metabolite Generated during the Infection, Is Active Against Trypanosoma cruzi

The antiparasitic activity of 3-hydroxykynurenine (3-HK), one of the major tryptophan catabolites of the kynurenine pathway, against both Trypanosoma cruzi evolutive forms that are important for human infection, trypomastigotes (Tps) and amastigotes (Am), possible targets in the parasite and the drug toxicity to mammalian cells have been investigated. 3-HK showed a potent activity against Am with IC₅₀ values in the micromolar concentration range, while the IC₅₀ values to cause Tps death was ∼6000-times higher, indicating that the replicative form present in the vertebrate hosts is much more susceptible to 3-HK than bloodstream Tps. In addition, 3-HK showed activity against Tps and Am, at concentrations that did not exhibit toxicity to mammalian cells. Ultrastructural analysis and flow cytometry studies indicated that Am and Tps mitochondrion and nuclei contain 3-HK targets. The potency and selectivity of 3-HK, which is generated during T. cruzi infection in human and mice, suggest that 3-HK may be a suitable candidate for drug research and development for Chagas disease.

Effects of ingested vertebrate-derived factors on insect immune responses

During the process of blood feeding insect vectors are exposed to an array of vertebrate-derived blood factors ranging from byproducts of blood meal digestion to naturally occurring products in the blood including growth hormones, cytokines and factors derived from blood-borne pathogens themselves. In this review, we examine the ability of these ingested vertebrate blood factors to alter the innate pathogen defenses of insect vectors. The ability of these factors to modify the immune responses of insect vectors offers new intriguing targets for blocking or reducing transmission of human disease-causing pathogens.

The effects of ingested mammalian blood factors on vector arthropod immunity and physiology

The blood feeding behavior of disease-transmitting arthropods creates a unique intersection between vertebrate and invertebrate physiology. Here, we review host blood-derived factors that persist through blood digestion to affect the lifespan, reproduction, and immune responses of some of the most common arthropod vectors of human disease.

On the mechanisms involved in biological heme crystallization

Blood-feeding organisms digest hemoglobin, releasing large quantities of heme inside their digestive tracts. Free heme is very toxic, and these organisms have evolved several mechanisms to protect against its deleterious effects. One of these adaptations is the crystallization of heme into the dark-brown pigment hemozoin (Hz). Here we review the process of Hz formation, focusing on organisms other than Plasmodium that have contributed to a better understanding of heme crystallization. Hemozoin has been found in several distinct classes of organisms including protozoa, helminths and insects and Hz formation is the predominant form of heme detoxification. The available evidence indicates that amphiphilic structures such as phospholipid membranes and lipid droplets accompanied by specific proteins play a major role in heme crystallization. Because this process is specific to a number of blood-feeding organisms and absent in their hosts, Hz formation is an attractive target for the development of novel drugs to control illnesses associated with these hematophagous organisms.

Trypanosoma brucei brucei: effects of ferrous iron and heme on ecto-nucleoside triphosphate diphosphohydrolase activity

Trypanosoma brucei brucei is the causative agent of animal African trypanosomiasis, also called nagana. Procyclic vector form resides in the midgut of the tsetse fly, which feeds exclusively on blood. Hemoglobin digestion occurs in the midgut resulting in an intense release of free heme. In the present study we show that the magnesium-dependent ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) activity of procyclic T. brucei brucei is inhibited by ferrous iron and heme. The inhibition of E-NTPDase activity by ferrous iron, but not by heme, was prevented by pre-incubation of cells with catalase. However, antioxidants that permeate cells, such as PEG-catalase and N-acetyl-cysteine prevented the inhibition of E-NTPDase by heme. Ferrous iron was able to induce an increase in lipid peroxidation, while heme did not. Therefore, both ferrous iron and heme can inhibit E-NTPDase activity of T. brucei brucei by means of formation of reactive oxygen species, but apparently acting through distinct mechanisms.

Hemoglobin is a co-factor of human trypanosome lytic factor

Trypanosome lytic factor (TLF) is a high-density lipoprotein (HDL) subclass providing innate protection to humans against infection by the protozoan parasite Trypanosoma brucei brucei. Two primate-specific plasma proteins, haptoglobin-related protein (Hpr) and apolipoprotein L-1 (ApoL-1), have been proposed to kill T. b. brucei both singularly or when co-assembled into the same HDL. To better understand the mechanism of T. b. brucei killing by TLF, the protein composition of TLF was investigated using a gentle immunoaffinity purification technique that avoids the loss of weakly associated proteins. HDL particles recovered by immunoaffinity absorption, with either anti-Hpr or anti-ApoL-1, were identical in protein composition and specific activity for T. b. brucei killing. Here, we show that TLF-bound Hpr strongly binds Hb and that addition of Hb stimulates TLF killing of T. b. brucei by increasing the affinity of TLF for its receptor, and by inducing Fenton chemistry within the trypanosome lysosome. These findings suggest that TLF in uninfected humans may be inactive against T. b. brucei prior to initiation of infection. We propose that infection of humans by T. b. brucei causes hemolysis that triggers the activation of TLF by the formation of Hpr-Hb complexes, leading to enhanced binding, trypanolytic activity, and clearance of parasites.

Effects of combined parenteral vitamins C and E administration on the severity of anaemia, hepatic and renal damage in Trypanosoma brucei brucei infected rabbits

Rabbits infected with Trypanosoma brucei brucei (Basa isolate) were intraperitoneally administered with vitamins C and E at 100 mg/kg and 10 mg/kg body weight, respectively, from day 7 before infection to day 12 post-infection (p.i.). Another group of rabbits were similarly infected, but received no vitamin treatment. The uninfected (control) rabbits were either untreated or treated with vitamins like the infected group. Treatment of the infected animals did not affect the onset and level of parasitaemia. On day 12 p.i., the anaemia tended to be ameliorated, but insignificantly, by the treatment. The infection increased (p<0.05) serum urea and creatinine concentrations to similar levels in treated and untreated groups. However, the increase (p<0.05) in alanine and aspartate transaminases in the untreated infected animals was prevented in the treated infected ones. Therefore, it seemed that the treatment with antioxidant vitamins boosted their storage in hepatic cells, but not in erythrocytes and glomeruli, to annul any cellular injury due to infection. It is concluded that this may be an indirect evidence that the hepatic damage may be principally due to oxidative injury.

The biochemical basis of arsenical-diamidine crossresistance in African trypanosomes

Resistance to currently used drugs is a serious problem in most fields of antimicrobial chemotherapy. Crossresistance between two of the major classes of drug used in the treatment of African trypanosomiasis, the melaminophenyl arsenicals and diamidines is easily selected in the laboratory. Here, Mike Barrett and Alan Fairlamb outline the mechanism underlying this crossresistance, which appears to arise as a result of alterations in an unusual adenosine transporter involved in the uptake of these drugs.

Trypanosoma brucei gamma-glutamylcysteine synthetase. Characterization of the kinetic mechanism and the role of Cys-319 in cystamine inactivation

The parasitic protozoan Trypanosoma brucei utilizes a conjugate of glutathione and spermidine, termed trypanothione, in place of glutathione to maintain cellular redox balance. The first committed step in the biosynthesis of glutathione and thereby trypanothione, is catalyzed by gamma-glutamylcysteine synthetase (gamma-GCS). We have determined the kinetic mechanism for T. brucei gamma-GCS. The kinetics are best described by a rapid equilibrium random ter-reactant mechanism, in which the model derived Kd values for the binding of L-Glu, L-alpha-aminobutyrate, and ATP to free enzyme are 2.6, 5.1, and 1.4 mM, respectively. However, significant dependences exist between the binding of some of the substrate pairs. The binding of either ATP or L-Glu to the enzyme increases the binding affinity of the other by 18-fold, whereas the binding of L-Glu or L-alpha-aminobutyrate decreases the binding affinity of the other by 6-fold. Similarly to the mammalian enzyme, cystamine is a time-dependent, irreversible inhibitor of T. brucei gamma-GCS. It has been suggested by several studies that cystamine labels an active site Cys residue essential for catalysis. Among the enzymes reported to be inactivated by cystamine, only one Cys residue is invariant (Cys-319 in T. brucei gamma-GCS). Mutation of Cys-319 to Ala in T. brucei gamma-GCS renders the enzyme insensitive to cystamine inactivation without significantly affecting the enzyme's catalytic efficiency, kinetic mechanism, or substrate affinities. These studies suggest that cystamine inactivates the enzyme by blocking substrate access to the active site and not by labeling an essential active site residue.

Is haemozoin a target for antimalarial drugs?

Malarial parasites utilize a unique pathway to degrade haemoglobin. Most of the haem which is released during haemoglobin degradation is incorporated into haemozoin. The mechanism of haemozoin synthesis and the structure of haemozoin are controversial. Neither chloroquine nor artemisinin appear to affect haemozoin synthesis in vivo. Artemisinin is activated by intraparasitic haem and iron into a free radical which then alkylates specific malarial proteins.

Characterization of Trypanosoma brucei gamma-glutamylcysteine synthetase, an essential enzyme in the biosynthesis of trypanothione (diglutathionylspermidine)

The parasitic protozoan Trypanosoma brucei maintains redox balance by synthesizing a conjugate of glutathione and spermidine termed trypanothione. The first committed step in the biosynthesis of glutathione, and thereby trypanothione, is catalyzed by the enzyme gamma-glutamylcysteine synthetase (gammaGCS). We have cloned and sequenced the 2037-base pair gene coding for the catalytic subunit of T. brucei gammaGCS. T. brucei gammaGCS appears to be encoded by a single copy gene. A transcript of about 2.3 kilobases was observed in procyclic trypomastigotes. The deduced amino acid sequence of 679 amino acids shares 45, 41, and 36% sequence identity with mammalian, Caenorhabditis elegans, and yeast gammaGCS, respectively. The T. brucei gammaGCS gene was expressed in E. coli; the purified 77.4-kDa enzyme catalyzes the ligation of L-Glu to L-Cys with a kcat of 10 s-1, confirming that the gene encodes the functional catalytic subunit of gammaGCS. The apparent Km values measured for the three natural substrates L-Glu, L-Cys, and ATP are 0.24, 0.69, and 0.07 mM, respectively. Unlike the mammalian enzyme, L-alpha-aminobutyrate (apparent Km = 10 mM) is a poor substitute for L-Cys in the T. brucei gammaGCS-catalyzed reaction. T. brucei gammaGCS is feedback-inhibited by glutathione (apparent KI = 1.1 mM), and it is inactivated by cystamine and buthionine sulfoximine. The kinetic properties of recombinant T. brucei gammaGCS suggest that the substrate binding pocket and the mechanism of enzyme regulation differ from the mammalian enzyme, providing evidence that T. brucei gammaGCS could be a selective chemotherapeutic target for the treatment of trypanosomiasis.

Effect of acute Trypanosoma vivax infection on cattle erythrocyte glutathione and susceptibility to in vitro peroxidation

During acute Trypanosoma vivax infection of calves, produced by intravenous inoculation, the mean packed cell volume and red blood cell counts of the infected animals decreased significantly (P < 0.05) between Days 6 and 13 post-infection (pi). The moderately severe normocytic anaemia started to develop during the first wave of parasitaemia which occurred from Day 2 pi and peaked between Days 4 and 5 pi. The mean erythrocyte glutathione (GSH) concentration of the infected calves decreased significantly (P < 0.05) from 58.4 +/- 11.4 mg 100ml-1 red blood cells (RBC) on Day 0 pi to 44.5 +/- 12.8 mg 100ml-1 RBC on Day 5 pi. As the GSH values recovered on Day 6 pi and increased thereafter, another slight decrease (P > 0.05) in GSH concentration occurred on Day 12 pi at the second peak of parasitaemia followed by a significant (P < 0.05) increase to 79.1 +/- 14.6 mg 100ml-1 RBC on Day 13 pi. In the uninfected calves, the mean GSH values ranged from 47.7 +/- 7.0 to 60.8 +/- 6.8 mg 100ml-1 RBC. When washed, erythrocytes of the infected and uninfected calves were separately challenged with hydrogen peroxide. They produced comparable amounts of thiobarbituric acid reactive substances as a measure of by-products of lipid peroxidation. This suggested that the ability of the erythrocytes to prevent peroxidative injury was not reduced, because GSH regeneration was probably enhanced and the antioxidant capacity of the erythrocytes was maintained.

Antioxidant defense mechanisms in parasitic protozoa

Many of the parasitic protozoa, such as Entamoeba histolytica, Giardia, Trypanosoma, Leishmania, and Plasmodium, are considered to be anaerobes because they can be grown in vitro only under conditions of reduced oxygen tension. However, these parasitic protozoa have been found to be aerotolerant or microaerophilic, and also to consume oxygen to a certain extent. Furthermore, these organisms are highly susceptible to exogenous reactive oxygen species, such as hydrogen peroxide. They must, therefore, detoxify both oxygen and free radical products of enzymatic reactions. However, they lack some or all of the usual antioxidant defense mechanisms present in aerobic or other aerotolerant cells, such as catalase, superoxide dismutase, reduced glutathione, and the glutathione-recycling enzymes glutathione peroxidase and glutathione reductase. Instead, they possess alternative mechanisms for detoxification similar to those known to exist in certain prokaryotes. Although the functional aspects of these alternative mechanisms are yet to be understood completely, they could provide new insights into the biochemical peculiarities of these enigmatic pathogens.

Killing of trypanosomes by the human haptoglobin-related protein

African trypanosomes cause disease in humans and animals. Trypanosoma brucei brucei affects cattle but not humans because of its sensitivity to a subclass of human high density lipoproteins (HDLs) called trypanosome lytic factor (TLF). TLF contains two apolipoproteins that are sufficient to cause lysis of T. b. brucei in vitro. These proteins were identified as the human haptoglobin-related protein and paraoxonase-arylesterase. An antibody to haptoglobin inhibited TLF activity. TLF was shown to exhibit peroxidase activity and to be inhibited by catalase. These results suggest that TLF kills trypanosomes by oxidative damage initiated by its peroxidase activity.

Effects of antimalarials and protease inhibitors on plasmodial hemozoin production

Malarial hemozoin may play an important role as a target for antimalarial drugs and in disease pathogenesis. A new assay for hemozoin was developed in which the hemozoin was separated from cells by filtration. Trophozoites have substantially more hemozoin than rings, but there are relatively small differences between chloroquine-sensitive and chloroquine-resistant strains. The effects of hemozoin content of chloroquine and artemisinin, two antimalarial drugs, and E64 and Pepstatin A, two protease inhibitors, were measured. At concentrations at which hypoxanthine incorporation was unaffected, the hemozoin content of rings was decreased by E64, but not by the other three compounds. Artemisinin and Pepstatin A also had little effect on the hemozoin content of trophozoites. Chloroquine and E64 inhibited trophozoite hemozoin formation, but inhibited hypoxanthine uptake to a similar or greater extent. When either rings or trophozoites were exposed to several higher concentrations of chloroquine, hemozoin content was diminished, but significantly less than hypoxanthine uptake. Various concentrations of E64, in contrast, inhibited hemozoin production by both rings and trophozoites significantly more than hypoxanthine incorporation, suggesting that hemozoin production may be directly affected by E64.

Developmental regulation of mitochondrial biogenesis in Trypanosoma brucei

The metabolism of Trypanosoma brucei undergoes a significant change as the parasite differentiates from the mammalian bloodstream form to the form found in the tse-tse fly vector. Because the mitochondria of bloodstream form cells lack cytochromes and several key citric acid cycle enzymes, the metabolism of these cells is mostly limited to glycolysis. The reducing equivalents generated by this process are passed to oxygen by a plant-like alternative oxidase. As cells differentiate to the insect form, they begin to oxidatively metabolize proline. The mitochondria of insect form cells contain functional, cytochrome-mediated electron transport chains and have complete complements of citric acid cycle enzymes. Although the characterization is far from complete, the nuclear and mitochondrial genes involved in the expression of these mitochondrial functions appear to be developmentally regulated at posttranscriptional and posttranslational levels. This review outlines some of the molecular processes that are associated with the developmental regulation of mitochondrial biogenesis and suggests some possible mechanisms of regulation.

Trypanocidal effects of catecholamines and indolealkylamines

Catecholamines, indolealkylamines and their analogues are oxidized at neutral or alkaline pH, producing hydrogen peroxide, quinones and free radicals. Several of these amines were tested for trypanocidal effects on Trypanosoma brucei, which possess a well-documented vulnerability to such oxidation products. Dopamine, 5-hydroxydopamine (5-OHDA), 6-hydroxydopamine (6-OHDA), 5-hydroxytryptamine (5-HT), 5,6-dihydroxytryptamine (5,6-DHT) and 5,7-dihydroxytryptamine (5,7-DHT) killed the parasites in vitro, using a fibroblast feeder layer cell culture system, in four to 48 hours at concentrations of 10(-5)-10(-7) M. The 5-OHDA, 6-OHDA, 5,6-DHT and 5,7-DHT were also effective in vivo when tested by intraperitoneal injection of infected mice.

An experimental model system for leishmaniasis. Effects of porphyrin-compounds and menadione on Leishmania parasites engulfed by cultured macrophages

In order to facilitate studies on the effects of chemotherapeutic agents on the host-parasite interactions in leishmaniasis, we have developed an experimental model for infecting human monocyte-derived- and mouse peritoneal macrophages in culture with recently-isolated Leishmania donovani promastigots (LDP). The chemotherapeutic agents studied were protoporphyrin, hematoporphyrin, menadione, and combinations of hematoporphyrin plus menadione. Since the Leishmania donovani amastigotes survived poorly in mouse macrophages and protoporphyrin was quite toxic to the latter, our investigations were focused on the effects of hematoporphyrin and menadione on amastigotes engulfed by human macrophages. Treatment of Leishmania donovani amastigotes-infested human macrophages with either 50 microM hematoporphyrin or 10 microM menadione did not influence significantly the survival of either Leishmania donovani amastigotes or the macrophages themselves. Larger individual doses of hematoporphyrin and menadione were toxic to both parasites and macrophages. The combination of 50 microM hematoporphyrin and 10 microM menadione, however, caused the destruction of the parasites without affecting the host macrophage. The enhanced deleterious effect from combined low doses of hematoporphyrin and menadione is discussed in terms of the production of reactive oxygen species, such as superoxide anion radical and hydrogen peroxide, originating from cellular redox cycling of menadione, and followed by decomposition of the formed hydrogen peroxide by hematoporphyrin to produce the more reactive hydroxyl radical.

Chemiluminescence enhancement by trypanocidal drugs and by inhibitors of antioxidant enzymes in Trypanosoma cruzi

The spontaneous emission of chemiluminescence by Trypanosoma cruzi epimastigotes was 133 +/- 5 counts s-1 (mg protein)-1. The measured intracellular steady state concentration of hydrogen peroxide in the same cells was 1.5 +/- 0.5 microM. These two values are about 12- and 15-times higher than the corresponding ones for isolated rat hepatocytes. The intracellular steady state concentrations of superoxide radical and hydrogen peroxide were apparently increased by inhibiting superoxide dismutase (with diethyldithiocarbamate or KCN addition) and by the addition of two different trypanocidal agents (beta-lapachone and nifurtimox) capable of intracellular redox cycling and in each case an increased chemiluminescence was observed. Depletion of intracellular reduced non-protein SH groups by 80% increased 3-fold the chemiluminescence of T. cruzi cells. It is apparent that both an increase in the intracellular steady state concentration of superoxide anion or hydrogen peroxide and a decrease in the level of reduced SH groups lead to an increase in the level of peroxy radicals which are the precursor species for light emission.

Hydrogen peroxide metabolism in Trypanosoma brucei

Contrary to previous reports in the literature, bloodstream forms of the haemoflagellate protozoan Trypanosoma brucei brucei are not deficient in their ability to metabolize hydrogen peroxide, although they either lack or only possess the normal enzymes for H2O2 detoxification, catalase (EC 1.11.1.6) and glutathione peroxidase (EC 1.11.1.9), at extremely low levels. The hydrogen peroxide which is consumed appears to be reduced by NADPH derived from glucose via the pentose phosphate pathway. This process requires the newly discovered cofactor trypanothione.

The inhibitory effects of ferriprotoporphyrin IX on platelet aggregation and release of serotonin

Ferriprotoporphyrin IX (FP) dissolved in 0.025N NaoH in concentrations of 0.01 - 0.04 microM/ul platelet suspension competitively inhibited platelet aggregation induced by a low concentration of collagen. 14C-serotonin release was also inhibited. Higher concentrations of collagen overcame the aggregation inhibition. A similar pattern of results was obtained with thrombin-, and arachidonic acid-induced aggregation and release. With ristocetin, there was little inhibition of aggregation although serotonin release was inhibited. ADP-induced aggregation was partially inhibited except at FP concentrations of 0.91 microM/mul. FP caused only platelet shape change and serotonin release of up to 8.1%. These changes were not associated with significant platelet lysis and could also not be attributed to pH or temperature changes. There was no inhibition of collagen-induced aggregation in PRP, but FP precipitates aggregated washed platelets and caused serotonin release. These results show that FP in solution inhibited platelet aggregation induced by the different agents studied. It did not interfere with platelet agglutination induced by ristocetin. The mechanism(s) of aggregation inhibition remains to be clarified.

False susceptibility of enterococci to aminoglycosides with blood-enriched Mueller-Hinton agar for disk susceptibility testing

Disk diffusion susceptibility tests for enterococci are frequently modified by adding 5% sheep blood (SB) to Mueller-Hinton agar; the performance standards from the National Committee for Clinical Laboratory Standards sanction this addition. Susceptibility testing of aminoglycoside antibiotics is not recommended for enterococci; in actual practice, however, some laboratories do include aminoglycoside antibiotics routinely, and others may test upon request or in selected situations. In examining 50 clinical isolates of enterococci, SB-enriched Mueller-Hinton agar frequently gave enlarged zone sizes that falsely indicated susceptibility (72% for gentamicin and tobramycin), with the average increase in zone size being 6.3 and 7.6 mm, respectively. Comparison agar dilution MICs demonstrated uniform resistance, with or without added SB. The effect was shown to be caused by heme in concentrations as low as 0.03 micrograms/ml, which, when combined with aminoglycoside antibiotics, caused a synergistic growth inhibition of the enterococci, resulting in larger aminoglycoside antibiotic zones. We postulate that the heme effect is related to a catalytic cleavage of intracellular H2O2 and resultant lipid peroxidation. No other organism or antimicrobial agent tested demonstrated a similar effect, although other investigators have shown a similar phenomenon with the broad-spectrum cephalosporins. Because enterococci grow well and give accurate susceptibility results on Mueller-Hinton agar without SB supplementation and because of the spectrum of definable problems with a number of antimicrobial agents, we recommend that enterococci routinely be tested without SB.

The interaction of hemin with skeletal muscle actin

The ability of actin to interact with hemin was studied. It was found that the Soret absorption band of hemin changes in the presence of actin and that hemin is capable of quenching the fluorescence intensity of actin. These findings were indicative of hemin binding to actin. The binding constant for the high affinity site was calculated to be 5.3 X 10(6) M-1. The amounts of native G- and F-actin were estimated by their DNAase I inhibition activity. It was observed that the binding of hemin to G-actin is followed by a slow decrease in the ability of actin to inhibit DNAase I activity and to polymerize upon addition of salts. Binding of hemin to F-actin resulted in a gradual depolymerization of the filaments, to an inactivated form, as expressed by a reduction in the ability of hemin-bound F-actin to inhibit DNAase I activity in the absence as well as in the presence of guanidine-HCl. Electron microscopy studies further corroborated these findings by demonstrating that: (1) hemin-bound G-actin failed to show formation of polymers when salts were added; (2) a marked reduction in the amount of actin polymers was observed in the specimens examined 24 h after mixing with hemin. It is suggested that the elevated amounts of free hemin formed under pathological conditions, might be toxic to cells by interfering with actin polymerization cycles.

The in vitro trypanocidal activity of N-substituted p-benzoquinone imines: assessment of biochemical structure-activity relationships using the Hansch approach

It has previously been found that naphthoquinones can potentiate the rate of hydrogen peroxide production by mitochondrial preparations of Trypanosoma brucei brucei and that organisms treated with naphthoquinones are more susceptible to lysis, especially in the presence of compounds such as heme, which promote the homolytic cleavage of hydrogen peroxide. We have evaluated the lytic effect of various N-substituted p-benzoquinone imines both in vitro and in vivo and have attempted to correlate their structure with trypanocidal activity using the Hansch approach. While none of the compounds tested proved to be active in vivo, all caused the lysis of trypanosomes in vitro. The parameters that correlated best with trypanocidal activity were the conditional redox potential, the lipophilicity of the substituent attached to the nitrogen atom and the number of active hydrogens on the quinonoid ring. These findings suggest two possible modes of action, which may in fact be related. Conjugate nucleophilic addition and/or oxidative damage could be responsible for lysis of the parasites. These same compounds were previously found to be active against the ascitic sarcoma 180 in mice. The strong correlation between antineoplastic activity in vivo and trypanocidal activity in vitro suggests a similar mode of action in both cases. Further studies aimed at developing a quinonelike compound that will be active against trypanosomes in vivo are now in progress.

Cell-mediated killing of protozoa

Cell-mediated immunity represents an important host defence mechanism against protozoal infections. The effector cells directly involved are neutrophils, macrophages and, ultimately, activated macrophages. Within this simple scheme there are, however, considerable variations in activity. Effector cells from different animal species, and even from different strains of the same species, may be more or less effective in controlling a certain protozoal infection. Different protozoa differ in their susceptibility to cell-mediated killing according to genus, species, strain and morphological form. The most susceptible morphological form is that which occurs in the insect vector, and which has not yet adapted to protect itself from the vertebrate host. Epimastigotes of Trypanosoma and promastigotes of Leishmania are readily killed by phagocytic cells, while the corresponding trypomastigote and amastigote forms are considerably more resistant. Protozoa which live in macrophages, such as amastigotes of Leishmania, endozoites (tachyzoites) of Toxoplasma and amastigotes of reticulotropic strains of T. cruzi, have developed a remarkable resistance to the microbicidal activity of the host cell. Conversely, amastigotes of myotropic strains of T. cruzi, which live in muscle cells, have not developed this resistance to cell-mediated killing by macrophages. Readily accessible protozoa, such as T. brucei trypomastigotes and Plasmodium merozoites in the bloodstream, while they lack the marked resistance developed by reticulotropic protozoa, have a partial protection since they are attacked by phagocytic cells only when specific antibody is present. Granulocyte-mediated killing can be largely attributed to neutrophils. Eosinophils appear to play only a minor role and compete ineffectually when neutrophils are also present. The only group of protozoal species which may be significantly controlled by eosinophils are the stercorarian species of Trypanosoma. In vitro experiments show that antibody-coated trypomastigotes of T. cruzi can be killed by eosinophils, although there is little evidence that this occurs in vivo. Interestingly, this is the only species that has been reported to be susceptible to the major basic protein of eosinophils, a toxic component of the lysosomal granules which is very active against helminths. Neutrophils are not very active against endozoites of Toxoplasma gondii, Trypanosoma, trypomastigotes of salivarian Trypanosoma, free merozoites of Plasmodium, and promastigotes and amastigotes of Leishmania.(ABSTRACT TRUNCATED AT 400 WORDS)

Oxygen-dependent microbicidal systems of phagocytes and host defense against intracellular protozoa

The role of oxygen-dependent microbicidal systems of leukocytes in the host defense against the major nonerythrocytic intracellular protozoa which infect man--Toxoplasma gondii, Trypanosoma cruzi, and the Leishmania species--is reviewed. The hydrogen peroxide-halide-peroxidase microbicidal system is uniformly cidal to these organisms in vitro. Peroxidase-independent oxygen product(s) toxicity is more variable. Studies to data indicate that phagocytes which contain granule peroxidase and which have the capacity to generate a vigorous respiratory burst; eg, neutrophils and monocytes, possess substantial activity against these protozoa. The absence of granule peroxidase together with the markedly attenuated respiratory burst of resident macrophages leaves these cells with a severe microbicidal defect. These protozoa can enter resident macrophages in the absence of antibody and survive and replicate within the intracellular environment. Enhancement of the antiparasite activity of resident macrophages can be accomplished either by activation of these cells by exposure to sensitized T-cell products, or by the introduction of exogenous peroxidase into the vacuole. Other factors influencing the ability of protozoa to survive intracellularly include the capacity of these organisms to avoid effective triggering of the macrophage respiratory burst and the levels of endogenous scavengers of oxygen products within the parasite.

Lysis of Plasmodium falciparum by ferriprotoporphyrin IX and a chloroquine-ferriprotoporphyrin IX complex

Ferriprotoporphyrin IX (FP) and a chloroquine-FP complex lysed isolated Plasmodium falciparum parasites as judged by decreases in the turbidity of parasite suspensions and by ultrastructural changes. Exposure of parasite suspensions to 50 microM FP or to a complex formed from 50 microM FP and 20 MicroM chloroquine reduced the number of identifiable parasites and caused swelling and loss of internal detail in those that were identifiable. The amount of lysis was dose-dependent over the range of 10 to 50 microM FP. Formation of a chloroquine-FP complex reduced, but did not eliminate, the toxicity of FP. Since there is evidence indicating that a chloroquine-FP complex forms when chloroquine-susceptible parasites are exposed to chloroquine, we suggest that accumulation of this complex may account for the chemotherapeutic effect of chloroquine against P. falciparum.

Hemin lyses malaria parasites

Malaria parasites isolated from mouse erythrocytes are lysed by ferriprotoporphyrin IX chloride (hemin) or by a chloroquine-hemin complex in amounts that could be produced by release of less than 0.1 percent of the heme in erythrocytic hemoglobin. This effect of hemin may explain the protection against malaria provided by thalassemia and other conditions causing intracellular denaturation of hemoglobin. The toxicity of the chloroquine-hemin complex may explain the selective antimalarial action of chloroquine.

Trypanosoma brucei brucei: a systematic screening for alternatives to the salicylhydroxamic acid-glycerol combination

Salicylhydroxamic acid (SHAM) and glycerol, when administered together, cause destruction of bloodstream forms of Trypanosoma brucei brucei, both in vitro and in vivo, but the dose required is exceedingly high. In an attempt to improve the efficacy of this drug combination, we examined the ability of various polyols and hydroxamic acids to substitute for glycerol and SHAM, respectively. No satisfactory substitute for glycerol was found. The inhibition of the trypanosomal alpha-glycerophosphate oxidase system (GPO) by SHAM (Ki 21 microM) was uncompetitive. Only primary and secondary aromatic hydroxamates were inhibitory. Among a series of 19 benzhydroxamates, no correlation existed between their acidity or their affinity for iron and their inhibition of the GPO in a cell free preparation. The Ki's of most of the primary hydroxamates ranged from 10 to 24 microM, with the more lipophilic derivatives being slightly more active. The Ki's of secondary hydroxamates were more variable, the best having Ki's of about 10 microM. Several other classes of iron chelators were also evaluated. Tropolones were active with 3-bromo-4,5-benzotropolone being as active as SHAM. 3,4-Dihydroxybenzaldehyde (Ki 15 microM) also inhibited the GPO. On the other hand, diphenylamine and 8-hydroxyquinoline, known inhibitors of the GPO, were 30 to 50 times less active. The results suggest that a lipophilic aromatic iron-chelating agent may be useful as a substitute for SHAM in combination therapy.

Inhibition of glutathione synthesis as a chemotherapeutic strategy for trypanosomiasis

With the expectation that trypanosomal glutathione (GSH) plays a major protective role against the endogenous oxidant stress that results form high intracellular levels of H2O2, we sought to deplete Trypanosoma brucei brucei of their GSH through inhibition of its biosynthesis. Administration of buthionine sulfoximine (BSO), a reversible inhibitor of gamma-glutamylcysteine synthetase, to parasitemic mice resulted in a progressive decrease in trypanosome GSH content, such that parasites isolated after 5 h or BSO treatment contained 50% of normal values. When BSO administration was continued for 18 h (intraperitoneal injection of 4 mmol/kg every 1.5 h), parasitemias temporarily cleared. When inhibitory plasma levels of BSO were maintained for about 27 h, two out of six infected mice were cured and the rest had significantly prolonged survival. These findings demonstrate the potential value of GSH depletion for the treatment of trypanosomiasis.

Deficient metabolic utilization of hydrogen peroxide in Trypanosoma cruzi

The glutathione peroxidase-glutathione reductase system, an alternative pathway for metabolic utilization of H2O2 [Chance, Sies & Boveris (1979) Physiol. Rev. 59, 527-605], was investigated in Trypanosoma cruzi, an organism lacking catalase and deficient in peroxidase [Boveris & Stoppani (1977) Experientia 33, 1306-1308]. The presence of glutathione (4.9 +/- 0.7 nmol of reduced glutathione/10(8) cells) and NADPH-dependent glutathione reductase (5.3 +/- 0.4 munit/10(8) cells) was demonstrated in the cytosolic fraction of the parasite, but with H2O2 as substrate glutathione peroxidase activity could not be demonstrated in the same extracts. With t-butyl hydroperoxide or cumene hydroperoxide as substrate, a very low NADPH-dependent glutathione peroxidase activity was detected (equivalent to 0.3-0.5 munit of peroxidase/10(8) cells, or about 10% of glutathione reductase activity). Blank reactions of the glutathione peroxidase assay (non-enzymic oxidation of glutathione by hydroperoxides and enzymic oxidation of NADPH) hampered accurate measurement of peroxidase activity. The presence of superoxide dismutase and ascorbate peroxidase activity in, as well as the absence of catalase from, epimastigote extracts was confirmed. Ascorbate peroxidase activity was cyanide-sensitive and heat-labile, but no activity could be demonstrated with diaminobenzidine, pyrogallol or guaiacol as electron donor. The summarized results support the view that T. cruzi epimastigotes lack an adequate enzyme defence against H2O2 and H2O2-related free radicals.

Ultrastructural alterations and peroxide formation induced by naphthoquinones in different stages of Trypanosoma cruzi

Addition of beta-lapachone, an o-naphthoquinone with bactericidal, cytotoxic, and trypanocidal activities, to Trypanosoma cruzi epimastigote and amastigote stages induced the release of O2- and H2O2 from the whole cells into the suspending medium. In the presence of reduced nicotinamide adenine dinucleotide as reductant beta-lapachone was also able to stimulate O2- and H2O2 production by homogenates of these stages. Electron micrographs showed that in beta-lapachone-treated amastigotes and trypomastigotes, the chromatin is arranged in patches, clearly differing from the normal pattern of chromatin distribution. Alterations of the nuclear, mitochondrial, and cytoplasmic membranes, as well as swelling of the mitochondria were also observed.

An approach to the development of new drugs for African trypanosomiasis

The inability of the bloodstream form of Trypanosoma brucei brucei to decompose hydrogen peroxide forms the basis of our attempt to develop new pharmacological agents to kill these organisms. Approximately 1-3% of the oxygen consumed by these parasites appears in the form of hydrogen peroxide. Our previous observation that free radical initiators such as heme and hematoporphyrin D proved to be trypanocidal in vitro and in vivo, respectively, prompted this investigation into the mechanism of action of this class of compounds to enhance their therapeutic efficacy. The locus of H2O2 production within the trypanosome was examined using cell-free homogenates. Experiments described herein suggest that H2O2 is formed by the alpha-glycerol phosphate dehydrogenase in an adventitious manner, and that no enzymatic means of disposing of this potentially toxic compound are present with the organisms. Naphthoquinones were found to substantially increase the rate of both oxygen consumption and H2O2 production by trypanosomal mitochondrial preparations. Presumably, the naphthoquinones are acting as coenzyme Q analogues. The addition of sublytic concentrations of both naphthoquinones and heme leads to a synergistic lysis of the organisms in vitro. Another approach to increasing the susceptibility of T. b. brucei to free radical damage involved reduction of the intracellular concentration of glutathione. This was accomplished through the use of trypanocidal arsenicals. Melarsenoxide and heme acted synergistically in vitro, an effect which was further enhanced via addition of a naphthoquinone. Moreover, hematoporphyrin D and tryparsamide were shown to have a synergistic effect in T. b. brucei-infected mice.