Antileishmanial Anthracene Endoperoxides: Efficacy In Vitro, Mechanisms and Structure-Activity Relationships.

Leishmaniasis is a vector-borne disease caused by protozoal Leishmania parasites. Previous studies have shown that endoperoxides (EP) can selectively kill Leishmania in host cells. Therefore, we studied in this work a set of new anthracene-derived EP (AcEP) together with their non-endoperoxidic analogs in model systems of Leishmania tarentolae promastigotes (LtP) and J774 macrophages for their antileishmanial activity and selectivity. The mechanism of effective compounds was explored by studying their reaction with iron (II) in chemical systems and in Leishmania. The correlation of structural parameters with activity demonstrated that in this compound set, active compounds had a LogPOW larger than 3.5 and a polar surface area smaller than 100 Å2. The most effective compounds (IC50 in LtP < 2 µM) with the highest selectivity (SI > 30) were pyridyl-/tert-butyl-substituted AcEP. Interestingly, also their analogs demonstrated activity and selectivity. In mechanistic studies, it was shown that EP were activated by iron in chemical systems and in LtP due to their EP group. However, the molecular structure beyond the EP group significantly contributed to their differential mitochondrial inhibition in Leishmania. The identified compound pairs are a good starting point for subsequent experiments in pathogenic Leishmania in vitro and in animal models.

The leishmanicidal activity of artemisinin is mediated by cleavage of the endoperoxide bridge and mitochondrial dysfunction.

Endoperoxides kill malaria parasites via cleavage of their endoperoxide bridge by haem or iron, leading to generation of cytotoxic oxygen-centred radicals. In view of the Leishmania parasites having a relatively compromised anti-oxidant defense and high iron content, this study aims to establish the underlying mechanism(s) accounting for the apoptotic-like death of Leishmania promastigotes by artemisinin, an endoperoxide. The formation of reactive oxygen species was confirmed by flow cytometry and was accompanied by inhibition of mitochondrial complexes I-III and II-III. However, this did not translate into a generation of mitochondrial superoxide or decrease in oxygen consumption, indicating minimal impairment of the electron transport chain. Artemisinin caused depolarization of the mitochondrial membrane along with a substantial depletion of adenosine triphosphatase (ATP), but it was not accompanied by enhancement of ATP hydrolysis. Collectively, the endoperoxide-mediated radical formation by artemisinin in Leishmania promastigotes was the key step for triggering its antileishmanial activity, leading secondarily to mitochondrial dysfunction indicating that endoperoxides represent a promising therapeutic strategy against Leishmania worthy of pharmacological consideration.

Techniques to study phagocytosis and uptake of Leishmania tarentolae by J774 macrophages.

Leishmaniasis is a vector borne parasitic disease affecting millions of people worldwide and is spreading into further areas because of global warming. The development of new active substances against these single-cell eukaryotic parasites is of great importance. Leishmania tarentolae promastigotes (LtP) are non-pathogenic for mammals and serve as model organisms for pathogenic Leishmania in basic research. However, it is important to refine methods to study the process of the infection of mammalian macrophages by LtP and pathogenic Leishmania. Important stages of the infection are phagocytosis by macrophages and multiplication of Leishmania amastigotes in the phagolysosome of macrophages. In this study, advanced methods using electron spin resonance (ESR) spectroscopy and genetically manipulated LtP were used to monitor the infection of adherent J774 macrophages with LtP. An ESR method was established to detect the formation of superoxide radicals directly in adherent J774 cells and to investigate the effect of LtP on this activity. J774 cells responded with a burst of superoxide radicals in the presence of phorbol myristate acetate as positive control. In contrast, challenging J774 cells with LtP resulted in a much lower burst of superoxide radicals. To facilitate LtP detection in the phagolysosome of J774 macrophages, LtP expressing enhanced green fluorescent protein (EGFP-LtP) were constructed. After different infection times with EGFP-LtP, the J774 cells were visualized by phase contrast microscopy and the cell number was determined. The intramacrophage Leishmania tarentolae amastigotes (LtA) expressing EGFP were detected by fluorescence microscopy and then counted with ImageJ. These experiments showed that LtP are taken up by J774 cells and form intraphagolysosomal amastigotes. LtA under our conditions multiplied intracellularly and were able to persist about 48 h in J774 cells. These experiments showed that ESR spectroscopy of attached macrophages and the use of the EGFP-LtP are suitable methods to study the initial phase of Leishmania infection in vitro.

Berberine chloride mediates its antileishmanial activity by inhibiting Leishmania mitochondria.

Berberine chloride, a plant-derived isoquinoline alkaloid, has been demonstrated to have leishmanicidal activity, which is mediated by generation of a redox imbalance and depolarization of the mitochondrial membrane, resulting in a caspase-independent apoptotic-like cell death. However, its impact on mitochondrial function remains to be delineated and is the focus of this study. In UR6 promastigotes, berberine chloride demonstrated a dose-dependent increase in generation of reactive oxygen species and mitochondrial superoxide, depolarization of the mitochondrial membrane potential, a dose-dependent inhibition of mitochondrial complexes I-III and II-III, along with a substantial depletion of ATP, collectively suggesting inhibition of parasite mitochondria. Accordingly, the oxidative stress induced by berberine chloride resulting in an apoptotic-like cell death in Leishmania can be exploited as a potent chemotherapeutic strategy, mitochondria being a prime contributor.

Interaction of ascaridole, carvacrol, and caryophyllene oxide from essential oil of Chenopodium ambrosioides L. with mitochondria in Leishmania and other eukaryotes.

The antileishmanial activity of the essential oil (EO) from Chenopodium ambrosioides L. has been demonstrated in vitro and in animal models, attributed to the major components of the EO. This study focused on the effects of the three major EO compounds carvacrol, caryophyllene oxide (Caryo), and the antileishmanial endoperoxide ascaridole (Asc) on mitochondrial functions in Leishmania tarentolae promastigotes (LtP). EO and Caryo were able to partially inhibit the leishmanial electron transport chain, whereas other components failed to demonstrate a direct immediate effect. Caryo demonstrated inhibition of complex III activity in LtP and in isolated complex III from other species. The formation of superoxide radicals was studied in Leishmania by electron spin resonance spectroscopy in the presence of iron chelators wherein selected compounds failed to trigger a significant immediate additional superoxide production in LtP. However, upon prolonged incubation of Leishmania with Asc and especially in the absence of iron chelators (allowing the activation of Asc), an increased superoxide radical production and significant impairment of mitochondrial coupling in Leishmania was observed. Prolonged incubation with all EO components resulted in thiol depletion. Taken together, the major components of EO mediate their leishmanicidal activity via different mitochondrial targets and time profiles. Further studies are required to elucidate possible synergistic effects of carvacrol and Asc and the influence of minor compounds.

Activation of Anthracene Endoperoxides in Leishmania and Impairment of Mitochondrial Functions.

Leishmaniasis is a vector-borne disease caused by protozoal Leishmania. Because of resistance development against current drugs, new antileishmanial compounds are urgently needed. Endoperoxides (EPs) are successfully used in malaria therapy, and experimental evidence of their potential against leishmaniasis exists. Anthracene endoperoxides (AcEPs) have so far been only technically used and not explored for their leishmanicidal potential. This study verified the in vitro efficiency and mechanism of AcEPs against both Leishmania promastigotes and axenic amastigotes (L. tarentolae and L. donovani) as well as their toxicity in J774 macrophages. Additionally, the kinetics and radical products of AcEPs' reaction with iron, the formation of radicals by AcEPs in Leishmania, as well as the resulting impairment of parasite mitochondrial functions were studied. Using electron paramagnetic resonance combined with spin trapping, photometry, and fluorescence-based oximetry, AcEPs were demonstrated to (i) show antileishmanial activity in vitro at IC50 values in a low micromolar range, (ii) exhibit host cell toxicity in J774 macrophages, (iii) react rapidly with iron (II) resulting in the formation of oxygen- and carbon-centered radicals, (iv) produce carbon-centered radicals which could secondarily trigger superoxide radical formation in Leishmania, and (v) impair mitochondrial functions in Leishmania during parasite killing. Overall, the data of different AcEPs demonstrate that their structures besides the peroxo bridge strongly influence their activity and mechanism of their antileishmanial action.

The expression of UCP3 directly correlates to UCP1 abundance in brown adipose tissue.

UCP1 and UCP3 are members of the uncoupling protein (UCP) subfamily and are localized in the inner mitochondrial membrane. Whereas UCP1's central role in non-shivering thermogenesis is acknowledged, the function and even tissue expression pattern of UCP3 are still under dispute. Because UCP3 properties regarding transport of protons are qualitatively identical to those of UCP1, its expression in brown adipose tissue (BAT) alongside UCP1 requires justification. In this work, we tested whether any correlation exists between the expression of UCP1 and UCP3 in BAT by quantification of protein amounts in mouse tissues at physiological conditions, in cold-acclimated and UCP1 knockout mice. Quantification using recombinant UCP3 revealed that the UCP3 amount in BAT (0.51ng/(µg total tissue protein)) was nearly one order of magnitude higher than that in muscles and heart. Cold-acclimated mice showed an approximate three-fold increase in UCP3 abundance in BAT in comparison to mice in thermoneutral conditions. Surprisingly, we found a significant decrease of UCP3 in BAT of UCP1 knockout mice, whereas the protein amount in skeletal and heart muscles remained constant. UCP3 abundance decreased even more in cold-acclimated UCP1 knockout mice. Protein quantification in UCP3 knockout mice revealed no compensatory increase in UCP1 or UCP2 expression. Our results do not support the participation of UCP3 in thermogenesis in the absence of UCP1 in BAT, but clearly demonstrate the correlation in abundance between both proteins. The latter is important for understanding UCP3's function in BAT.

The antileishmanial activity of xanthohumol is mediated by mitochondrial inhibition.

Xanthohumol (Xan) is a natural constituent of human nutrition. Little is known about its actions on leishmanial parasites and their mitochondria as putative target. Therefore, we determined the antileishmanial activity of Xan and resveratrol (Res, as alternative compound with antileishmanial activity) with respect to mitochondria in Leishmania amazonensis promastigotes/amastigotes (LaP/LaA) in comparison with their activity in peritoneal macrophages from mouse (PMM) and macrophage cell line J774A.1 (J774). Mechanistic studies were conducted in Leishmania tarentolae promastigotes (LtP) and mitochondrial fractions isolated from LtP. Xan and Res demonstrated antileishmanial activity in LaA [half inhibitory concentration (IC50): Xan 7 µ m, Res 14 µ m]; while they had less influence on the viability of PMM (IC50: Xan 70 µ m, Res >438 µ m). In contrast to Res, Xan strongly inhibited oxygen consumption in Leishmania (LtP) but not in J774 cells. This was based on the inhibition of the mitochondrial electron transfer complex II/III by Xan, which was less pronounced with Res. Neither Xan nor Res increased mitochondrial superoxide release in LtP, while both decreased the mitochondrial membrane potential in LtP. Bioenergetic studies showed that LtP mitochondria have no spare respiratory capacity in contrast to mitochondria in J774 cells and can therefore much less adapt to stress by mitochondrial inhibitors, such as Xan. These data show that Xan may have antileishmanial activity, which is mediated by mitochondrial inhibition.

Role of mitochondria in the leishmanicidal effects and toxicity of acyl phloroglucinol derivatives: nemorosone and guttiferone A.

Nemorosone (Nem) and guttiferone A (GutA) are acyl phloroglucinol derivatives (APD) that are present in different natural products. For both compounds anti-cancer and anti-microbial properties have been reported. In particular, an anti-leishmanial activity of both compounds was demonstrated. The aim of this study was to explore the possible role of mitochondria in the anti-leishmanial activity of Nem and GutA in comparison with their action on mammalian mitochondria. Both APD inhibited the growth of promastigotes of Leishmania tarentolae (LtP) with half maximal inhibitory concentration (IC50) values of 0·67 ± 0·17 and 6·2 ± 2·6 µ m; while IC50 values for cytotoxicity against peritoneal macrophages from BALB/c mice were of 29·5 ± 3·7 and 9·2 ± 0·9 µ m, respectively. Nemorosone strongly inhibited LtP oxygen consumption, caused species-specific inhibition (P < 0·05) of succinate:ubiquinone oxidoreductase (complex II) from LtP-mitochondria and significantly increased (P < 0·05) the mitochondrial superoxide production. In contrast, GutA caused only a moderate reduction of respiration in LtP and triggered less superoxide radical production in LtP compared with Nem. In addition, GutA inhibited mitochondrial complex III in bovine heart submitochondrial particles, which is possibly involved in its mammalian toxicity. Both compounds demonstrated at low micromolar concentrations an effect on the mitochondrial membrane potential in LtP. The present study suggests that Nem caused its anti-leishmanial action due to specific inhibition of complexes II/III of mitochondrial respiratory chain of Leishmania parasites that could be responsible for increased production of reactive oxygen species that triggers parasite death.

Tocopheramine succinate and tocopheryl succinate: mechanism of mitochondrial inhibition and superoxide radical production.

Tocopherols (TOH) are lipophilic antioxidants which require the phenolic OH group for their redox activity. In contrast, non-redox active esters of α-TOH with succinate (α-TOS) were shown to possess proapoptotic activity in cancer cells. It was suggested that this activity is mediated via mitochondrial inhibition with subsequent O2(-) production triggering apoptosis and that the modification of the linker between the succinate and the lipophilic chroman may modulate this activity. However, the specific mechanism and the influence of the linker are not clear yet on the level of the mitochondrial respiratory chain. Therefore, this study systematically compared the effects of α-TOH acetate (α-TOA), α-TOS and α-tocopheramine succinate (α-TNS) in cells and submitochondrial particles (SMP). The results showed that not all cancer cell lines are highly sensitive to α-TOS and α-TNS. In HeLa cells α-TNS did more effectively reduce cell viability than α-TOS. The complex I activity of SMP was little affected by α-TNS and α-TOS while the complex II activity was much more inhibited (IC50=42±8µM α-TOS, 106±8µM α-TNS, respectively) than by α-TOA (IC50 >1000µM). Also the complex III activity was inhibited by α-TNS (IC50=137±6µM) and α-TOS (IC50=315±23µM). Oxygen consumption of NADH- or succinate-respiring SMP, involving the whole electron transfer machinery, was dose-dependently decreased by α-TOS and α-TNS, but only marginal effects were observed in the presence of α-TOA. In contrast to the similar inhibition pattern of α-TOS and α-TNS, only α-TOS triggered O2(-) formation in succinate- and NADH-respiring SMP. Inhibitor studies excluded complex I as O2(-) source and suggested an involvement of complex III in O2(-) production. In cancer cells only α-TOS was reproducibly able to increase O2(-) levels above the background level but neither α-TNS nor α-TOA. Furthermore, the stability of α-TNS in liver homogenates was significantly lower than that of α-TOS. In conclusion, this suggests that α-TNS although it has a structure similar to α-TOS is not acting via the same mechanism and that for α-TOS not only complex II but also complex III interactions are involved.

Radical Scavenging Is Not Involved in Thymoquinone-Induced Cell Protection in Neural Oxidative Stress Models.

Thymoquinone (TQ), an active compound from Nigella sativa seeds, is often described as a pharmacologically relevant compound with antioxidative properties, while the synthesis of TQ in the plant via oxidations makes it inapplicable for scavenging radicals. Therefore, the present study was designed to reassess the radical scavenging properties of TQ and explore a potential mode of action. The effects of TQ were studied in models with mitochondrial impairment and oxidative stress induced by rotenone in N18TG2 neuroblastoma cells and rotenone/MPP+ in primary mesencephalic cells. Tyrosine hydroxylase staining revealed that TQ significantly protected dopaminergic neurons and preserved their morphology under oxidative stress conditions. Quantification of the formation of superoxide radicals via electron paramagnetic resonance showed an initial increase in the level of superoxide radicals in the cell by TQ. Measurements in both cell culture systems revealed that the mitochondrial membrane potential was tendentially lowered, while ATP production was mostly unaffected. Additionally, the total ROS levels were unaltered. In mesencephalic cell culture under oxidative stress conditions, caspase-3 activity was decreased when TQ was administered. On the contrary, TQ itself tremendously increased the caspase-3 activity in the neuroblastoma cell line. Evaluation of the glutathione level revealed an increased level of total glutathione in both cell culture systems. Therefore, the enhanced resistance against oxidative stress in primary cell culture might be a consequence of a lowered caspase-3 activity combined with an increased pool of reduced glutathione. The described anti-cancer ability of TQ might be a result of the pro-apoptotic condition in neuroblastoma cells. Our study provides evidence that TQ has no direct scavenging effect on superoxide radicals.

Lipophilicity as a determinant of thiazolidinedione action in vitro: findings from BLX-1002, a novel compound without affinity to PPARs.

The pharmacology of thiazolidinediones (TZDs) seems to be driven not only by activation of peroxisome proliferator-activated receptor-γ (PPARγ), but also by PPARγ-independent effects on mitochondrial function and cellular fuel handling. This study portrayed such actions of the novel hydrophilic TZD compound BLX-1002 and compared them to those of conventional TZDs. Mitochondrial function and fuel handling were examined in disrupted rat muscle mitochondria, intact rat liver mitochondria, and specimens of rat skeletal muscle. BLX-1002 was superior to most other TZDs as an inhibitor of respiratory complex 1 in disrupted mitochondria, but had less effect than any other TZD on oxygen consumption by intact mitochondria and on fuel metabolism by intact tissue. The latter finding was obviously related to the hydrophilic properties of BLX-1002, because high potentials of individual TZDs to shift muscle fuel metabolism from the aerobic into the anaerobic pathway were associated with high ClogP values indicative of high lipophilicity and low hydrophilicity (e.g., % increase in lactate release induced by 10 µmol/l of respective compound: BLX-1002, ClogP 0.39, +10 ± 8%, not significant; pioglitazone, ClogP 3.53, +68 ± 12%, P < 0.001; troglitazone, ClogP 5.58, +157 ± 14%, P < 0.001). The observed specific properties of BLX-1002 could result from relatively strong direct affinity to an unknown mitochondrial target, but limited access to this target. Results suggest 1) that impairment of mitochondrial function and increased anaerobic fuel metabolism are unlikely to account for PPARγ-independent glucose lowering by BLX-1002, and 2) that higher lipophilicity of an individual TZD is associated with stronger acceleration of anaerobic glycolysis.

Mitochondrial respiration and membrane potential are regulated by the allosteric ATP-inhibition of cytochrome c oxidase.

This paper describes the problems of measuring the allosteric ATP-inhibition of cytochrome c oxidase (CcO) in isolated mitochondria. Only by using the ATP-regenerating system phosphoenolpyruvate and pyruvate kinase full ATP-inhibition of CcO could be demonstrated by kinetic measurements. The mechanism was proposed to keep the mitochondrial membrane potential (DeltaPsi(m)) in living cells and tissues at low values (100-140 mV), when the matrix ATP/ADP ratios are high. In contrast, high DeltaPsi(m) values (180-220 mV) are generally measured in isolated mitochondria. By using a tetraphenyl phosphonium electrode we observed in isolated rat liver mitochondria with glutamate plus malate as substrates a reversible decrease of DeltaPsi(m) from 233 to 123 mV after addition of phosphoenolpyruvate and pyruvate kinase. The decrease of DeltaPsi(m) is explained by reversal of the gluconeogenetic enzymes pyruvate carboxylase and phosphoenolpyruvate carboxykinase yielding ATP and GTP, thus increasing the matrix ATP/ADP ratio. With rat heart mitochondria, which lack these enzymes, no decrease of DeltaPsi(m) was found. From the data we conclude that high matrix ATP/ADP ratios keep DeltaPsi(m) at low values by the allosteric ATP-inhibition of CcO, thus preventing the generation of reactive oxygen species which could generate degenerative diseases. It is proposed that respiration in living eukaryotic organisms is normally controlled by the DeltaPsi(m)-independent "allosteric ATP-inhibition of CcO." Only when the allosteric ATP-inhibition is switched off under stress, respiration is regulated by "respiratory control," based on DeltaPsi(m) according to the Mitchell Theory.

Endotoxin causes functional endoplasmic reticulum failure, possibly mediated by mitochondria.

Inflammatory response has recently been shown to induce endoplasmic reticulum (ER) stress and the unfolded protein response (UPR), which either recovers proper ER function or activates apoptosis. Here we show that endotoxin (lipopolysaccharide = LPS) can lead to functional ER failure tentatively via a mitochondrion-dependent pathway in livers of rats. Histological examination did not reveal significant damage to liver in form of necroses. Electron microscopy displayed transparent rings appearing around morphologically unchanged mitochondria, which were identified as dilated ER. The spliced mRNA variant of X-box protein-1 (XBP1) and also the mRNA of 78 kDa glucose-regulated protein (GRP78) were up-regulated, both typical markers of ER stress. However, GRP78 was down-regulated at the protein level. A pro-apoptotic shift in the bax/bcl-XL mRNA ratio was not accompanied by translocation of apoptosis inducing factor (AIF) to the nucleus, suggesting that the cells entered a pre-apoptotic state, but apoptosis was not executed. Monooxygenase activity of p450, representing the detoxification system in ER, was decreased after administration of endotoxin. Biochemical analysis of proteins important for ER function revealed the impairment of protein folding, transport, and detoxification suggesting functional ER failure. We suggest that functional ER failure may be a reason for organ dysfunction upon excessive inflammatory response mediated by endotoxin.

Effect of estrogen on mitochondrial function and intracellular stress markers in rat liver and kidney following trauma-hemorrhagic shock and prolonged hypotension.

Trauma-hemorrhage (T-H) is known to impair tissue perfusion, leading to tissue hypoxia, and thus affecting mitochondria, the organelles with the highest oxygen demand. In a model of T-H and prolonged hypotension without fluid resuscitation, administration of a small volume of 17beta-estradiol (E2), but not vehicle, prolonged the survival of rats for 3 h, even in the absence of fluid resuscitation. The main finding of this study is that T-H followed by prolonged hypotension significantly affects mitochondrial function, endoplasmic reticulum (ER) stress markers and free iron levels, and that E2 ameliorated all these changes. All of these changes were observed in the liver but not in the kidney. The sensitivity of mitochondrial respiration to exogenous cytochrome c can reflect increased permeability of the outer mitochondrial membrane for cytochrome c. Increased levels of free iron are indicative of oxidative stress, but neither oxidative nor nitrosylative stress markers changed. The spliced isoform of XBP1 mRNA (an early marker of ER stress) and the expression of C/EBP homologous protein (CHOP) (a protein regulating ER stress-induced apoptosis) were elevated in T-H animals but remained unchanged if T-H rats received E2. Both the prevention of elevated sensitivity of mitochondrial respiration to cytochrome c and a decrease in ER stress by E2 maintain functional integrity of the liver and may help the organ during prolonged hypotension and following resuscitation. A decrease in free iron levels by E2 is more relevant for resuscitation, often accompanied by oxidative stress reaction. Thus, E2 appears to be a novel hormonal adjunct that prolongs permissive hypotension during lengthy transportation of the injured patient between the injury site and the hospital in both civilian and military injuries.

Modulation of the mitochondrial cytochrome bc1 complex activity by chromanols and related compounds.

Tocopherols (alpha-, beta-, gamma-, and delta-Toc) and tocopheryl quinones (alpha-, beta-, gamma-, and delta-TQ) were recently suggested to modulate mitochondrial electron transfer in mammals. Intriguingly, Tocs and stigmatellin, a potent inhibitor of the mitochondrial cytochrome (cyt) bc(1) complex, possess a common structural feature: the chroman core. Therefore, we studied the interference of Tocs as well as synthetic model compounds (low molecular weight TQ analogues and tetramethyl chromanones) at the mitochondrial cyt bc(1) complex. Enzymatic experiments revealed that besides the inhibitor stigmatellin, among natural vitamin E-related derivatives, gamma-TQ/delta-TQ and, among synthetic compounds, TMC2O (6-hydroxy-4,4,7,8-tetramethyl-chroman-2-one) were most effective in decreasing the cyt bc(1) activities. Stopped-flow photometric and low-temperature electron paramagnetic resonance spectroscopic experiments showed for TMC2O an inhibition of electron transfer to cyt c(1) and a modulation of the environment of the Rieske iron-sulfur protein (ISP). Docking experiments suggest a binding interaction of the 6-OH group and 1-O atom/2-C( horizontal lineO) group of TMC2O with Glu-271 (cyt b) and His-161 (ISP) in the cyt bc(1) complex, respectively. This binding pose is similar but not identical to the potent inhibitor stigmatellin. The data suggest that chroman-2-ones are possible templates for modulatory molecules for the cyt bc(1) target.

Activation of artemisinin and heme degradation in Leishmania tarentolae promastigotes: A possible link.

Endoperoxides (EPs) appear to be promising drug candidates against protozoal diseases, including malaria and leishmaniasis. Previous studies have shown that these drugs need an intracellular activation to exert their pharmacological potential. The efficiency of these drugs is linked to the extensive iron demand of these intracellular protozoal parasites. An essential step of the activation mechanism of these drugs is the formation of radicals in Leishmania. Iron is a known trigger for intracellular radical formation. However, the activation of EPs by low molecular iron or by heme iron may strongly depend on the structure of the EPs themselves. In this study, we focused on the activation of artemisinin (Art) in Leishmania tarentolae promastigotes (LtP) in comparison to reference compounds. Viability assays in different media in the presence of different iron sources (hemin/fetal calf serum) showed that IC50 values of Art in LtP were modulated by assay conditions, but overall were within the low micromolar range. Low temperature electron paramagnetic resonance (EPR) spectroscopy of LtP showed that Art shifted the redox state of the labile iron pool less than the EP ascaridole questioning its role as a major activator of Art in LtP. Based on the high reactivity of Art with hemin in previous biomimetic experiments, we focused on putative heme-metabolizing enzymes in Leishmania, which were so far not well described. Inhibitors of mammalian heme oxygenase (HO; tin and chromium mesoporphyrin) acted antagonistically to Art in LtP and boosted its IC50 value for several magnitudes. By inductively coupled plasma methods (ICP-OES, ICP-MS) we showed that these inhibitors do not block iron (heme) accumulation, but are taken up and act within LtP. These inhibitors blocked the conversion of hemin to bilirubin in LtP homogenates, suggesting that an HO-like enzyme activity in LtP exists. NADPH-dependent degradation of Art and hemin was highest in the small granule and microsomal fractions of LtP. Photometric measurements in the model Art/hemin demonstrated that hemin requires reduction to heme and that subsequently an Art/heme complex (λmax 474 nm) is formed. EPR spin-trapping in the system Art/hemin revealed that NADPH, ascorbate and cysteine are suitable reductants and finally activate Art to acyl-carbon centered radicals. These findings suggest that heme is a major activator of Art in LtP either via HO-like enzyme activities and/or chemical interaction of heme with Art.

Toxic effects of carvacrol, caryophyllene oxide, and ascaridole from essential oil of Chenopodium ambrosioides on mitochondria.

Chenopodium ambrosioides have been used for centuries in the Americas as a popular remedy for parasitic diseases. The essential oil of this plant possesses anthelmintic activity and is still used in some regions to treat parasitosis and leishmaniasis. However, the Chenopodium oil caused also some fatalities, leading to its commercial disuse. In this work, we studied the mechanism of toxicity of the essential oil and its major pure ingredients (carvacrol, caryophyllene oxide, and ascaridole, which was synthesized from alpha-terpinene) with respect to mammalian cells and mitochondria. We observed that all products, but especially caryophyllene oxide, inhibited the mitochondrial electron transport chain. This effect for carvacrol and caryophyllene oxide was mediated via direct complex I inhibition. Without Fe2+, ascaridole was less toxic to mammalian mitochondria than other major ingredients. However, evidence on the formation of carbon-centered radicals in the presence of Fe2+ was obtained by ESR spin-trapping. Furthermore, it was shown that Fe2+ potentiated the toxicity of ascaridole on oxidative phosphorylation of rat liver mitochondria. The increase of the alpha-tocopherol quinone/alpha-tocopherol ratio under these conditions indicated the initiation of lipid peroxidation by Fe2+-mediated ascaridole cleavage. Further ESR spin-trapping experiments demonstrated that in addition to Fe2+, reduced hemin, but not mitochondrial cytochrome c can activate ascaridole, explaining why ascaridole in peritoneal macrophages from BALB/c mice exhibited a higher toxicity than in isolated mitochondria.

Mechanism of ascaridole activation in Leishmania.

Endoperoxides (EP) are an emerging class of drugs which have potential in antiparasitic therapy, but also in other fields. For malaria therapy the EP artemisinin (Art) and its derivatives are successfully used. We have shown in the past that the EP ascaridole (Asc) is useful for the treatment of cutaneous leishmaniasis in a mouse model. Biomimetic experiments suggested that these drugs need activation in the respective target pathogens to exert their function. In spite of this idea, direct activation of EP to radicals inside cells has never been demonstrated. Therefore, this study was initiated to explore the activation of Asc in biomimetic systems and inside Leishmania in comparison to Art. Using electron paramagnetic resonance spectroscopy (EPR) in combination with spin-trapping we identified the secondary alkyl radical intermediates arising from reduction by Fe2+ in cell-free systems. Combined GC/NMR analysis confirmed the loss of isopropyl residues from Asc during this process as intermediates. This activation of Asc was stimulated by low molecular Fe2+ complexes or alternatively by hemin in conjunction with thiol reductants, such as cysteine (Cys). In Leishmania tarentolae promastigotes (LtP) as model for pathogenic forms of Leishmania carbon-centered radicals were identified in the presence of Asc by EPR spin-trapping. Both Asc and Art inhibited the viability in LtP with IC50 values in the low micromolar range while IC50 values for J774 macrophages were considerably higher. A similar structure without EP bridge (1,4-cineole) resulted in no detectable radicals and possessed much less cytotoxicity in LtP and no selectivity for LtP compared to J774 cells. The Asc-derived radical formation in LtP was inhibited by the iron chelator deferoxamine (DFO), and stimulated by Cys (a suitable reductant for hemin). The IC50 values for LtP viability in the presence of Asc or Art were increased significantly by the spin trap DMPO, while Cys and DFO increased only IC50 values for Art. In a heme association assay Asc demonstrated a lower binding affinity to heme than Art. ICP-OES measurements revealed that in LtP the total iron concentrations were twice as high as values in J774 macrophages. Since low molecular iron was important in Asc activation we studied the influence of Asc on the labile iron pool (LIP) in LtP. Low temperature EPR experiments demonstrated that Asc shifts the redox balance of iron in the LIP to its oxidized state. These data demonstrate that univalent cleavage of Asc/Art in LtP is an essential part of their pharmacological mechanism. The structure of the EP determines whether activation by low molecular iron or heme is favored and the availability of these intracellular activators modulates their cytotoxicity. These findings may be helpful for synthesis of new Asc derivatives and understanding the action of EP in other cell types.

Skeletal muscles, heart, and lung are the main sources of oxygen radicals in old rats.

The aim of this study was to compare rat tissues with respect to their reactive oxygen and nitrogen species (RONS) generating activities as a function of age. We quantified the RONS generation in vivo in young (6 months) and in old (30 months) male Sprague-Dawley rats using the recently developed spin trap 1-hydroxy-3-carboxy-pyrrolidine, applied intravenously. This spin trap reacts with superoxide radical and peroxynitrite yielding a stable spin adduct which is detectable by means of electron paramagnetic resonance (EPR) spectroscopy in frozen tissues. In old rats RONS generation was significantly increased compared to their young counterparts in the following order: blood