Methylglyoxal disrupts the functionality of rat liver mitochondria

Methylglyoxal (MG) is a reactive metabolite derived from different physiological pathways. Its production can be harmful to cells via glycation reactions of lipids, DNA, and proteins. But, the effects of MG on mitochondrial functioning and bioenergetic responses are still elusive. Then, the effects of MG on key parameters of mitochondrial functionality were examined here. Isolated rat liver mitochondria were exposed to 0.1-10 mM of MG to determine its toxicity in the mitochondrial viability, membrane potential (Δψm), swelling and the superoxide (O2•-) production. Besides, mitochondrial oxidative phosphorylation parameters were analyzed by high-resolution respiratory (HRR) assay. In this set of experiments, routine state, PM state (pyruvate/malate), oxidative phosphorylation (OXPHOS), LEAK respiration, electron transport system (ETS) and oxygen residual (ROX) states were evaluated. HRR showed that PM state, OXPHOS CI-Linked, LEAK respiration, ETS CI/CII-Linked and ETS CII-Linked/ROX were significantly inhibited by MG exposure. MG also inhibited the complex II activity, and decreased Δψm and the viability of mitochondria. Taken together, our data indicates that MG is an inductor of mitochondrial dysfunctions and impairs important steps of respiratory chain, effects that can alter bioenergetics responses.

Crystallographic investigation of the ubiquinone binding site of respiratory Complex II and its inhibitors

The quinone binding site (Q-site) of Mitochondrial Complex II (succinate-ubiquinone oxidoreductase) is the target for a number of inhibitors useful for elucidating the mechanism of the enzyme. Some of these have been developed as fungicides or pesticides, and species-specific Q-site inhibitors may be useful against human pathogens. We report structures of chicken Complex II with six different Q-site inhibitors bound, at resolutions 2.0-2.4 Å. These structures show the common interactions between the inhibitors and their binding site. In every case a carbonyl or hydroxyl oxygen of the inhibitor is H-bonded to Tyr58 in subunit SdhD and Trp173 in subunit SdhB. Two of the inhibitors H-bond Ser39 in subunit SdhC directly, while two others do so via a water molecule. There is a distinct cavity that accepts the 2-substituent of the carboxylate ring in flutolanil and related inhibitors. A hydrophobic "tail pocket" opens to receive a side-chain of intermediate-length inhibitors. Shorter inhibitors fit entirely within the main binding cleft, while the long hydrophobic side chains of ferulenol and atpenin A5 protrude out of the cleft into the bulk lipid region, as presumably does that of ubiquinone. Comparison of mitochondrial and Escherichia coli Complex II shows a rotation of the membrane-anchor subunits by 7° relative to the iron‑sulfur protein. This rotation alters the geometry of the Q-site and the H-bonding pattern of SdhB:His216 and SdhD:Asp57. This conformational difference, rather than any active-site mutation, may be responsible for the different inhibitor sensitivity of the bacterial enzyme.

The disappearance of IPO in myocardium of diabetes mellitus rats is associated with the increase of succinate dehydrogenase-flavin protein

BACKGROUND

The aim of the present study was to investigate whether the disappearance of ischemic post-processing (IPO) in the myocardium of diabetes mellitus (DM) is associated with the increase of succinate dehydrogenase-flavin protein (SDHA).

METHODS

A total of 50 Sprague Dawley rats, weighing 300-400 g, were divided into 5 groups according to the random number table method, each with 10 rats. After DM rats were fed a high-fat and -sugar diet for 4 weeks, they were injected with Streptozotocin to establish the diabetic rat model. Normal rats were fed the same regular diet for the same number of weeks. Next, the above rats were taken to establish a cardiopulmonary bypass (CPB) model. Intraperitoneal glucose tolerance test (IPGTT) and oral glucose tolerance test (OGTT) were used to detect whether the DM rat model was established successfully. Taking blood from the femoral artery to collect the blood-gas analysis indicators, and judged whether the CPB model is established. After perfusion was performed according to the experimental strategy, the area of myocardial infarction (MI), and serum creatine kinase isoenzyme (CK-MB) and cardiac troponin (CTnI) levels were measured. Finally, the relative mRNA and protein expression of SDHA was detected.

RESULTS

The OGTT and IPGTT suggested that the DM rat model was successfully established. The arterial blood gas analysis indicated that the CPB model was successfully established. As compared with the N group, the heart function of the IR group was significantly reduced, the levels of myocardial enzyme markers, the area of MI, as well as the relative mRNA and protein expression of SDHA, were all increased. As compared with the IR group, the CK-MB and CTnI levels in the IPO group, the MI area, relative mRNA and protein expression of SDHA decreased. As compared with the IPO group, the myocardial enzyme content in the DM + IPO group, the MI area and the relative mRNA and protein expression of SDHA increased. As compared with the DM + IPO group, in the DM + IPO + dme group, the myocardial enzyme content, area of MI and relative mRNA and protein expression were all decreased.

CONCLUSION

IPO can inhibit the expression of SDHA, reduce MIRI and exert a cardioprotective effect in the normal rats. However, the protective effect of IPO disappears in the diabetic rats. The inhibitor dme combined with IPO can increase the expression of SDHA and restore the protective effect of IPO in DM myocardia.

Inhibition of the mitochondrial respiratory components (Complex I and Complex III) as stimuli to induce oxidative damage in Oryza sativa L. under thiocyanate exposure

Repression of the electron transport in mitochondria can result in an increase of reactive oxygen species (ROS) in plant cells. This study was to clarify inhibition of the mitochondrial respiratory components (Complex I and Complex III) as stimuli to induce oxidative damage in Oryza sativa L. under exogenous SCN^- exposure with special emphasis on lipid peroxidation, protein modification, and DNA damage at the biochemical and molecular levels. Our results showed that enzymatic activity and gene expression of cytochrome c reductase (Complex III) in roots and shoots of rice seedlings were significantly repressed by SCN^- exposure, where significant inhibition of NADH dehydrogenase (Complex I) was only detected in shoots, suggesting that Complex III was the main target attacked by SCN^- ligand in rice roots, and both components were arrested in shoots. ROS analysis in tissues indicated that SCN^- exposure caused significant accumulation of H₂O₂ and O₂^-•, increased malondialdehyde (MDA) and carbonyl content in rice materials in a dose-dependent manner. Similarly, a remarkable elevation of electrolyte leakage was observed in rice tissue samples. The comet assay indicated a positive correlation between DNA damage and external SCN^- exposure. In conclusion, oxidative burst generated from the inhibitions of the electron transport in mitochondria in rice seedlings under SCN^- exposure can cause lipid peroxidation, protein modification and DNA damage, eventually decreasing fresh weight of rice seedlings.

Cysteine depletion targets leukemia stem cells through inhibition of electron transport complex II

We have previously demonstrated that oxidative phosphorylation is required for the survival of human leukemia stem cells (LSCs) from patients with acute myeloid leukemia (AML). More recently, we demonstrated that LSCs in patients with de novo AML rely on amino acid metabolism to drive oxidative phosphorylation. Notably, although overall levels of amino acids contribute to LSC energy metabolism, our current findings suggest that cysteine may be of particular importance for LSC survival. We demonstrate that exogenous cysteine is metabolized exclusively to glutathione. Upon cysteine depletion, glutathione synthesis is impaired, leading to reduced glutathionylation of succinate dehydrogenase A (SDHA), a key component of electron transport chain complex (ETC) II. Loss of SDHA glutathionylation impairs ETC II activity, thereby inhibiting oxidative phosphorylation, reducing production of ATP, and leading to LSC death. Given the role of cysteine in driving LSC energy production, we tested cysteine depletion as a potential therapeutic strategy. Using a novel cysteine-degrading enzyme, we demonstrate selective eradication of LSCs, with no detectable effect on normal hematopoietic stem/progenitor cells. Together, these findings indicate that LSCs are aberrantly reliant on cysteine to sustain energy metabolism, and that targeting this axis may represent a useful therapeutic strategy.

Acute exposure to a glyphosate-containing herbicide formulation inhibits Complex II and increases hydrogen peroxide in the model organism Caenorhabditis elegans

Glyphosate-based herbicides, such as Touchdown (TD) and Roundup, are among the most heavily-used herbicides in the world. While the active ingredient is generally considered non-toxic, the toxicity resulting from exposure to commercially-sold formulations is less clear. In many cases, cell cultures or various model organisms exposed to glyphosate formulations show toxicity and, in some cases, lethality. Using Caenorhabditis elegans, we assessed potential toxic mechanisms through which a highly-concentrated commercial formulation of TD promotes neurodegeneration. Following a 30-min treatment, we assayed mitochondrial electron transport chain function and reactive oxygen species (ROS) production. Initial oxygen consumption studies indicated general mitochondrial inhibition compared to controls (*p < 0.05). When Complex II activity was further assessed, inhibition was observed in all TD-treated groups (*p < 0.05). Complex IV activity, however, was not adversely affected by TD. This electron transport chain inhibition also resulted in reduced ATP levels (*p < 0.05). Furthermore, hydrogen peroxide levels, but not other ROS, were increased (*p < 0.05). Taken together, these data indicate that commercially-available formulations of TD may exert neurotoxicity through Complex II (succinate dehydrogenase) inhibition, decreased ATP levels, and increased hydrogen peroxide production.

The selective antifungal activity of Drosophila melanogaster metchnikowin reflects the species-dependent inhibition of succinate-coenzyme Q reductase

Insect-derived antifungal peptides have a significant economic potential, particularly for the engineering of pathogen-resistant crops. However, the nonspecific antifungal activity of such peptides could result in detrimental effects against beneficial fungi, whose interactions with plants promote growth or increase resistance against biotic and abiotic stress. The antifungal peptide metchnikowin (Mtk) from Drosophila melanogaster acts selectively against pathogenic Ascomycota, including Fusarium graminearum, without affecting Basidiomycota such as the beneficial symbiont Piriformospora indica. Here we investigated the mechanism responsible for the selective antifungal activity of Mtk by using the peptide to probe a yeast two-hybrid library of F. graminearum cDNAs. We found that Mtk specifically targets the iron-sulfur subunit (SdhB) of succinate-coenzyme Q reductase (SQR). A functional assay based on the succinate dehydrogenase (SDH) activity of mitochondrial complex II clearly demonstrated that Mtk inhibited the SDH activity of F. graminearum mitochondrial SQR by up to 52%, but that the equivalent enzyme in P. indica was unaffected. A phylogenetic analysis of the SdhB family revealed a significant divergence between the Ascomycota and Basidiomycota. SQR is one of the key targets of antifungal agents and we therefore propose Mtk as an environmentally sustainable and more selective alternative to chemical fungicides.

Structure-Based Discovery of Potential Fungicides as Succinate Ubiquinone Oxidoreductase Inhibitors

A series of diphenyl ether-containing pyrazole-carboxamide derivatives was designed and synthesized as new succinate ubiquinone oxidoreductase (SQR) inhibitors. This highly potent molecular scaffold was developed from a moderately activie hit 3, obtained from our previous pharmacophore-linked fragment virtual screening (PFVS) method. The results of greenhouse tests indicated that some analogues showed good SQR inhibitory activity, with promising fungicidal activity against Rhizoctonia solani and Sphaerotheca fuliginea at a dosage of 200 mg/L. Most surprisingly, compound 62 showed the highest SQR inhibitory activity with a K_i value of 0.081 μM, about 4-fold more potent than penthiopyrad (K_i = 0.307 μM). In addition, compounds 43 and 62 displayed excellent fungicidal activity even at a dosage as low as 6.25 mg/L, which was superior to thifluzamide. Moreover, compound 62 exhibited excellent protection effect against R. solani and provided about 81.2% protective control efficancy after 21 days with two sprayings. The present work indicated that these two compounds could be used as potential agricultural fungicides targeting SQR.

Discovery of Potent Succinate-Ubiquinone Oxidoreductase Inhibitors via Pharmacophore-linked Fragment Virtual Screening Approach

Succinate-ubiquinone oxidoreductase (SQR) is an attractive target for fungicide discovery. Herein, we report the discovery of novel SQR inhibitors using a pharmacophore-linked fragment virtual screening approach, a new drug design method developed in our laboratory. Among newly designed compounds, compound 9s was identified as the most potent inhibitor with a Ki value of 34 nM against porcine SQR, displaying approximately 10-fold higher potency than that of the commercial control penthiopyrad. Further inhibitory kinetics studies revealed that compound 9s is a noncompetitive inhibitor with respect to the substrate cytochrome c and DCIP. Interestingly, compounds 8a, 9h, 9j, and 9k exhibited good in vivo preventive effects against Rhizoctonia solani. The results obtained from molecular modeling showed that the orientation of the R(2) group had a significant effect on binding with the protein.

Inhibitory Activities of Alkyl Syringates and Related Compounds on Aflatoxin Production

Inhibitors of aflatoxin production of aflatoxigenic fungi are useful for preventing aflatoxin contamination in crops. As methyl syringate weakly inhibits aflatoxin production, aflatoxin production inhibitory activities of additional alkyl syringates with alkyl chains from ethyl to octyl were examined. Inhibitory activity toward aflatoxin production of Aspergillus flavus became stronger as the length of the alkyl chains on the esters became longer. Pentyl, hexyl, heptyl, and octyl syringates showed strong activity at 0.05 mM. Heptyl and octyl parabens, and octyl gallate also inhibited aflatoxin production as strongly as octyl syringate. Alkyl parabens and alkyl gallates inhibit the complex II activity of the mitochondrial respiration chain; thus, whether alkyl syringates inhibit complex II activity was examined. Inhibitory activities of alkyl syringates toward complex II also became stronger as the length of the alkyl chains increased. The complex II inhibitory activity of octyl syringate was comparable to that of octyl paraben and octyl gallate. These results suggest that alkyl syringates, alkyl parabens, and alkyl gallates, including commonly used food additives, are useful for aflatoxin control.

Folate Deficiency Triggered Apoptosis of Synoviocytes: Role of Overproduction of Reactive Oxygen Species Generated via NADPH Oxidase/Mitochondrial Complex II and Calcium Perturbation

Despite a plethora of literature has documented that osteoarthritis (OA) is veritably associated with oxidative stress-mediated chondrocyte death and matrix degradation, yet the possible involvement of synoviocyte abnormality as causative factor of OA has not been thoroughly investigated. For this reason, we conduct the current studies to insight into how synoviocytes could respond to an episode of folate-deprived (FD) condition. First, when HIG-82 synoviocytes were cultivated under FD condition, a time-dependent growth impediment was observed and the demise of these cells was demonstrated to be apoptotic in nature mediated through FD-evoked overproduction of reactive oxygen species (ROS) and drastically released of cytosolic calcium (Ca²⁺) concentrations. Next, we uncovered that FD-evoked ROS overproduction could only be strongly suppressed by either mitochondrial complex II inhibitors (TTFA and carboxin) or NADPH oxidase (NOX) inhibitors (AEBSF and apocynin), but not by mitochondrial complex I inhibitor (rotenone) and mitochondrial complex III inhibitor (antimycin A). Interestingly, this selective inhibition of FD-evoked ROS by mitochondrial complex II and NOX inhibitors was found to correlate excellently with the suppression of cytosolic Ca²⁺ release and reduced the magnitude of the apoptotic TUNEL-positive cells. Taken together, we present the first evidence here that FD-triggered ROS overproduction in synoviocytes is originated from mitochondrial complex II and NOX. Both elevated ROS in tandem with cytosolic Ca²⁺ overload serve as final arbitrators for apoptotic lethality of synoviocytes cultivated under FD condition. Thus, folate supplementation may be beneficial to patients with OA.

Complex I and complex III inhibition specifically increase cytosolic hydrogen peroxide levels without inducing oxidative stress in HEK293 cells

Inhibitor studies with isolated mitochondria demonstrated that complex I (CI) and III (CIII) of the electron transport chain (ETC) can act as relevant sources of mitochondrial reactive oxygen species (ROS). Here we studied ROS generation and oxidative stress induction during chronic (24h) inhibition of CI and CIII using rotenone (ROT) and antimycin A (AA), respectively, in intact HEK293 cells. Both inhibitors stimulated oxidation of the ROS sensor hydroethidine (HEt) and increased mitochondrial NAD(P)H levels without major effects on cell viability. Integrated analysis of cells stably expressing cytosolic- or mitochondria-targeted variants of the reporter molecules HyPer (H2O2-sensitive and pH-sensitive) and SypHer (H2O2-insensitive and pH-sensitive), revealed that CI- and CIII inhibition increased cytosolic but not mitochondrial H2O2 levels. Total and mitochondria-specific lipid peroxidation was not increased in the inhibited cells as reported by the C11-BODIPY(581/591) and MitoPerOx biosensors. Also expression of the superoxide-detoxifying enzymes CuZnSOD (cytosolic) and MnSOD (mitochondrial) was not affected. Oxyblot analysis revealed that protein carbonylation was not stimulated by CI and CIII inhibition. Our findings suggest that chronic inhibition of CI and CIII: (i) increases the levels of HEt-oxidizing ROS and (ii) specifically elevates cytosolic but not mitochondrial H2O2 levels, (iii) does not induce oxidative stress or substantial cell death. We conclude that the increased ROS levels are below the stress-inducing level and might play a role in redox signaling.

Mitochondrial respiratory capacity and coupling control decline with age in human skeletal muscle

Mitochondrial health is critical to physiological function, particularly in tissues with high ATP turnover, such as striated muscle. It has been postulated that derangements in skeletal muscle mitochondrial function contribute to impaired physical function in older adults. Here, we determined mitochondrial respiratory capacity and coupling control in skeletal muscle biopsies obtained from young and older adults. Twenty-four young (28 ± 7 yr) and thirty-one older (62 ± 8 yr) adults were studied. Mitochondrial respiration was determined in permeabilized myofibers from the vastus lateralis after the addition of substrates oligomycin and CCCP. Thereafter, mitochondrial coupling control was calculated. Maximal coupled respiration (respiration linked to ATP production) was lower in muscle from older vs. young subjects (P < 0.01), as was maximal uncoupled respiration (P = 0.06). Coupling control in response to the ATP synthase inhibitor oligomycin was lower in older adults (P < 0.05), as was the mitochondria flux control ratio, coupled respiration normalized to maximal uncoupled respiration (P < 0.05). Calculation of respiratory function revealed lower respiration linked to ATP production (P < 0.001) and greater reserve respiration (P < 0.01); i.e., respiratory capacity not used for phosphorylation in muscle from older adults. We conclude that skeletal muscle mitochondrial respiratory capacity and coupling control decline with age. Lower respiratory capacity and coupling efficiency result in a reduced capacity for ATP production in skeletal muscle of older adults.

Caenorhabditis elegans is a useful model for anthelmintic discovery

Parasitic nematodes infect one quarter of the world's population and impact all humans through widespread infection of crops and livestock. Resistance to current anthelmintics has prompted the search for new drugs. Traditional screens that rely on parasitic worms are costly and labour intensive and target-based approaches have failed to yield novel anthelmintics. Here, we present our screen of 67,012 compounds to identify those that kill the non-parasitic nematode Caenorhabditis elegans. We then rescreen our hits in two parasitic nematode species and two vertebrate models (HEK293 cells and zebrafish), and identify 30 structurally distinct anthelmintic lead molecules. Genetic screens of 19 million C. elegans mutants reveal those nematicides for which the generation of resistance is and is not likely. We identify the target of one lead with nematode specificity and nanomolar potency as complex II of the electron transport chain. This work establishes C. elegans as an effective and cost-efficient model system for anthelmintic discovery.

Mitochondrial targeting of α-tocopheryl succinate enhances its anti-mesothelioma efficacy

Malignant mesothelioma (MM) is a fatal neoplastic disease with no therapeutic option. Therefore, the search for novel therapies is of paramount importance.

METHODS

Since mitochondrial targeting of α-tocopheryl succinate (α-TOS) by its tagging with triphenylphosphonium enhances its cytotoxic effects to cancer cells, we tested its effect on MM cells and experimental mesotheliomas.

RESULTS

Mitochondrially targeted vitamin E succinate (MitoVES) was more efficient in killing MM cells than α-TOS with IC₅₀ lower by up to two orders of magnitude. Mitochondrial association of MitoVES in MM cells was documented using its fluorescently tagged analogue. MitoVES caused apoptosis in MM cells by mitochondrial destabilization, resulting in the loss of mitochondrial membrane potential, generation of reactive oxygen species, and destabilization of respiratory supercomplexes. The role of the mitochondrial complex II in the activity of MitoVES was confirmed by the finding that MM cells with suppressed succinate quinone reductase were resistant to MitoVES. MitoVES suppressed mesothelioma growth in nude mice with high efficacy.

DISCUSSION

MitoVES is more efficient in killing MM cells and suppressing experimental mesotheliomas compared with the non-targeted α-TOS, giving it a potential clinical benefit.

Effect of remifentanil on mitochondrial oxygen consumption of cultured human hepatocytes

During sepsis, liver dysfunction is common, and failure of mitochondria to effectively couple oxygen consumption with energy production has been described. In addition to sepsis, pharmacological agents used to treat septic patients may contribute to mitochondrial dysfunction. This study addressed the hypothesis that remifentanil interacts with hepatic mitochondrial oxygen consumption. The human hepatoma cell line HepG2 and their isolated mitochondria were exposed to remifentanil, with or without further exposure to tumor necrosis factor-α (TNF-α). Mitochondrial oxygen consumption was measured by high-resolution respirometry, Caspase-3 protein levels by Western blotting, and cytokine levels by ELISA. Inhibitory κBα (IκBα) phosphorylation, measurement of the cellular ATP content and mitochondrial membrane potential in intact cells were analysed using commercial ELISA kits. Maximal cellular respiration increased after one hour of incubation with remifentanil, and phosphorylation of IκBα occurred, denoting stimulation of nuclear factor κB (NF-κB). The effect on cellular respiration was not present at 2, 4, 8 or 16 hours of incubation. Remifentanil increased the isolated mitochondrial respiratory control ratio of complex-I-dependent respiration without interfering with maximal respiration. Preincubation with the opioid receptor antagonist naloxone prevented a remifentanil-induced increase in cellular respiration. Remifentanil at 10× higher concentrations than therapeutic reduced mitochondrial membrane potential and ATP content without uncoupling oxygen consumption and basal respiration levels. TNF-α exposure reduced respiration of complex-I, -II and -IV, an effect which was prevented by prior remifentanil incubation. Furthermore, prior remifentanil incubation prevented TNF-α-induced IL-6 release of HepG2 cells, and attenuated fragmentation of pro-caspase-3 into cleaved active caspase 3 (an early marker of apoptosis). Our data suggest that remifentanil increases cellular respiration of human hepatocytes and prevents TNF-α-induced mitochondrial dysfunction. The results were not explained by uncoupling of mitochondrial respiration.

DYT1 knock-in mice are not sensitized against mitochondrial complex-II inhibition

DYT1 is caused by a partly penetrant dominant mutation in TOR1A that leads to a glutamic acid deletion (ΔE) in torsinA. Identifying environmental factors that modulate disease pathogenesis and penetrance could help design therapeutic strategies for dystonia. Several cell-based studies suggest that expression of torsinA(ΔE) increases the susceptibility of neuronal cells to challenges to their oxidative/energy metabolism. Based on those reports, we hypothesized that mice expressing torsinA(ΔE) would be more susceptible than control littermates to the effects of oxidative stress and ATP deficits caused by disruption of the mitochondrial respiratory chain in neurons. To test this hypothesis, we administered 20 or 50 mg/kg/day of the irreversible complex-II inhibitor 3-nitropropionic acid (3-NP) intraperitoneally for 15 consecutive days to young heterozygote DYT1 knock-in (KI) mice and wild type littermates. Repeated phenotypic assessments were performed at baseline, during and after the injections. Animals were then sacrificed and their brains processed for protein analysis. The administration of 20 mg/kg 3-NP led to increased levels of torsinA in the striatum, the main target of 3-NP, but did not cause motor dysfunction in DYT1 KI or control mice. The administration of 50 mg/kg/day of 3-NP caused the death of ∼40% of wild type animals. Interestingly, DYT1 KI animals showed significantly reduced mortality. Surviving animals exhibited abnormal motor behavior during and right after the injection period, but recovered by 4 weeks postinjection independent of genotype. In contrast to the findings reported in cultured cells, these studies suggest the DYT1 mutation does not sensitize central neurons against the toxic effects of oxidative stress and energy deficits.

The pro-oxidant chromium(VI) inhibits mitochondrial complex I, complex II, and aconitase in the bronchial epithelium: EPR markers for Fe-S proteins

Hexavalent chromium (Cr(VI)) compounds (e.g., chromates) are strong oxidants that readily enter cells, where they are reduced to reactive Cr species that also facilitate reactive oxygen species generation. Recent studies demonstrated inhibition and oxidation of the thioredoxin system, with greater effects on mitochondrial thioredoxin (Trx2). This implies that Cr(VI)-induced oxidant stress may be especially directed at the mitochondria. Examination of other redox-sensitive mitochondrial functions showed that Cr(VI) treatments that cause Trx2 oxidation in human bronchial epithelial cells also result in pronounced and irreversible inhibition of aconitase, a TCA cycle enzyme that has an iron-sulfur (Fe-S) center that is labile with respect to certain oxidants. The activities of electron transport complexes I and II were also inhibited, whereas complex III was not. Electron paramagnetic resonance (EPR) studies of samples at liquid helium temperature (10K) showed a strong signal at g=1.94 that is consistent with the inhibition of electron flow through complex I and/or II. A signal at g=2.02 was also observed, which is consistent with oxidation of the Fe-S center of aconitase. The g=1.94 signal was particularly intense and remained after extracellular Cr(VI) was removed, whereas the g=2.02 signal declined in intensity after Cr(VI) was removed. A similar inhibition of these activities and analogous EPR findings were noted in bovine airways treated ex vivo with Cr(VI). Overall, the data support the hypothesis that Cr(VI) exposure has deleterious effects on a number of redox-sensitive core mitochondrial proteins. The g=1.94 signal could prove to be an important biomarker for oxidative damage resulting from Cr(VI) exposure. The EPR spectra simultaneously showed signals for Cr(V) and Cr(III), which verify Cr(VI) exposure and its intracellular reductive activation.

The mitochondrial complex II and ATP-sensitive potassium channel interaction: quantitation of the channel in heart mitochondria

The mitochondrial ATP-sensitive potassium channel (mK(ATP)) is important in cardioprotection, although the channel remains molecularly undefined. Several studies have demonstrated that mitochondrial complex II inhibitors activate the mK(ATP), suggesting a potential role for complex II in channel composition or regulation. However, these inhibitors activate mK(ATP) at concentrations which do not affect bulk complex II activity. Using the potent complex II inhibitor Atpenin A5, this relationship was investigated using tight-binding inhibitor theory, to demonstrate that only 0.4 % of total complex II molecules are necessary to activate the mK(ATP). These results estimate the mK(ATP) content at 15 channels per mitochondrion.

Multiple resistance of Botrytis cinerea from kiwifruit to SDHIs, QoIs and fungicides of other chemical groups

BACKGROUND

Botrytis cinerea Pers.: Fr. is a high-risk pathogen for fungicide resistance development that has caused resistance problems on many crops throughout the world. This study investigated the fungicide sensitivity profile of isolates from kiwifruits originating from three Greek locations with different fungicide use histories. Sensitivity was measured by in vitro fungitoxicity tests on artificial nutrient media.

RESULTS

Seventy-six single-spore isolates were tested for sensitivity to the SDHI fungicide boscalid, the QoI pyraclostrobin, the anilinopyrimidine cyprodinil, the hydroxyanilide fenhexamid, the phenylpyrrole fludioxonil, the dicarboxamide iprodione and the benzimidazole carbendazim. All isolates from Thessaloniki showed resistance to both boscalid and pyraclostrobin, while in the other two locations the fungal population was sensitive to these two fungicides. Sensitive isolates showed EC(50) values to boscalid and pyraclostrobin ranging from 0.9 to 5.2 and from 0.04 to 0.14 mg L(-1) respectively, while the resistant isolates showed EC(50) values higher than 50 mg L(-1) for boscalid and from 16 to > 50 mg L(-1) for pyraclostrobin. All QoI-resistant isolates carried the G143A mutation in cytb. Sensitivity determinations to the remaining fungicides revealed in total eight resistance phenotypes. No isolates were resistant to the fungicides fenhexamid and fludioxonil.

CONCLUSION

This is the first report of B. cinerea field isolates with resistance to both boscalid and pyraclostrobin, and it strongly suggests that there may be a major problem in controlling this important pathogen on kiwifruit.

Novel mitochondrial complex II isolated from Trypanosoma cruzi is composed of 12 peptides including a heterodimeric Ip subunit

Mitochondrial respiratory enzymes play a central role in energy production in aerobic organisms. They differentiated from the alpha-proteobacteria-derived ancestors by adding noncatalytic subunits. An exception is Complex II (succinate: ubiquinone reductase), which is composed of four alpha-proteobacteria-derived catalytic subunits (SDH1-SDH4). Complex II often plays a pivotal role in adaptation of parasites in host organisms and would be a potential target for new drugs. We purified Complex II from the parasitic protist Trypanosoma cruzi and obtained the unexpected result that it consists of six hydrophilic (SDH1, SDH2N, SDH2C, and SDH5-SDH7) and six hydrophobic (SDH3, SDH4, and SDH8-SDH11) nucleus-encoded subunits. Orthologous genes for each subunit were identified in Trypanosoma brucei and Leishmania major. Notably, the iron-sulfur subunit was heterodimeric; SDH2N and SDH2C contain the plant-type ferredoxin domain in the N-terminal half and the bacterial ferredoxin domain in the C-terminal half, respectively. Catalytic subunits (SDH1, SDH2N plus SDH2C, SDH3, and SDH4) contain all key residues for binding of dicarboxylates and quinones, but the enzyme showed the lower affinity for both substrates and inhibitors than mammalian enzymes. In addition, the enzyme binds protoheme IX, but SDH3 lacks a ligand histidine. These unusual features are unique in the Trypanosomatida and make their Complex II a target for new chemotherapeutic agents.

Differential sensitivity to cadmium of key mitochondrial enzymes in the eastern oyster, Crassostrea virginica Gmelin (Bivalvia: Ostreidae)

Combined effects of cadmium (Cd) and temperature on key mitochondrial enzymes [including Complexes I-IV of electron transport chain and Krebs cycle enzymes citrate synthase (CS), and NAD- and NADP-dependent isocitrate dehydrogenases (NAD-IDH and NADP-IDH)] were studied in a marine ectotherm, Crassostrea virginica in order to better understand the mechanisms of Cd-induced impairment of mitochondrial function. Matrix enzymes including CS and isocitrate dehydrogenases were the most sensitive to Cd making Krebs cycle a likely candidate to explain Cd-induced impairment of mitochondrial substrate oxidation. CS and NAD-IDH had IC(50) of 26 and 65 microM at the acclimation temperature (15 degrees C) and 65 (CS) and 1.5 (NAD-IDH) microM at elevated temperature (25 degrees C), respectively. Mitochondrial NADP-IDH was the most sensitive to Cd with IC(50) of 14 and 3.4 microM at 15 degrees and 25 degrees C, respectively. Electron transport chain (ETC) complexes were significantly less sensitive to the direct effects of Cd with IC(50) ranging from 260 to >>400 microM. Temperature increase led to a higher sensitivity of mitochondrial enzymes to the inhibitory effects of Cd as indicated by a decline in IC(50) with the exception of Complex III from gills and CS from gills and hepatopancreas. Cd exposure also resulted in a decrease in activation energy of mitochondrial enzymes suggesting that mitochondria from Cd-exposed oysters could exhibit reduced capacity to respond to temperature rise with an adequate increase in the substrate flux. These interactive effects of Cd and temperature on mitochondrial enzymes could negatively affect metabolic performance of oysters and possibly other ectotherms in polluted environments during temperature increase such as expected during the global climate change and/or tidal or seasonal warming in estuarine and coastal waters.

The nitrone disodium 2,4-sulphophenyl-N-tert-butylnitrone is without cytoprotective effect on sodium nitroprusside-induced cell death in N1E-115 neuroblastoma cells in vitro

Disodium 2,4-sulphophenyl-N-tert-butylnitrone (NXY-059) is a novel free radical-trapping compound that is neuroprotective in both rodent and primate models of acute ischaemic stroke. Neuroprotection in vitro by NXY-059 has not been reported previously, and we have now investigated whether such an effect can be detected using a simple cell culture model of neurotoxicity. Neuron-like cells of the neuroblastoma-derived clonal cell line N1E-115 were exposed to the free radical-generating agent sodium nitroprusside (SNP), which produced a concentration-dependent reduction in mitochondrial complex II activity 24 h later (EC(50) approximately 100 micromolar). Cell death induced by SNP (100 micromolar), assessed either by an increased proportion of apoptotic nuclear morphology or by mitochondrial complex II activity, was inhibited by a cocktail of known antioxidants (ascorbate, reduced glutathione, and dithiothreitol, all at 100 micromolar) but not by NXY-059 at a concentration known to be neuroprotective in vivo (300 micromolar). Disodium 2,4-sulphophenyl-N-tert-butylnitrone was also without effect on H(2)O(2)-mediated cytotoxicity. These results support recent data suggesting that in vivo NXY-059 probably acts at the neurovascular unit rather than at an intracellular site as a neuroprotective agent.

Target identification of drug induced mitochondrial toxicity using immunocapture based OXPHOS activity assays

Mitochondrial dysfunction has been shown to be a pharmacotoxicological response to a variety of currently-marketed drugs. In order to reduce attrition due to mitochondrial toxicity, high throughput-applicable screens are needed for early stage drug discovery. We describe, here, a set of immunocapture based assays to identify compounds that directly inhibit four of the oxidative phosphorylation (OXPHOS) complexes: I, II, IV, and V. Intra- and inter-assay variation were determined and specificity tested by using classical mitochondrial inhibitors. Twenty drugs, some with known mitochondrial toxicity and others with no known mitochondrial liability, were studied. Direct inhibition of one or more of the OXPHOS complexes was identified for many of the drugs. Novel information was obtained for several drugs including ones with previously unknown effects on oxidative phosphorylation. A major advantage of the immunocapture approach is that it can be used throughout drug screening from early compound evaluation to clinical trials.

The responses of HT22 cells to the blockade of mitochondrial complexes and potential protective effect of selenium supplementation

Mitochondria are the major reactive oxygen species (ROS) – generating sites in mammalian cells. Blockade of complexes in the electron transport chain (ETC) increases the leakage of single electrons to O₂ and therefore increases ROS levels. Complexes I and III have been reported to be the major ROS-generating sites in mitochondria. In this study, using mouse hippocampal HT22 cells as in vitro model, we monitored the change of intracellular ROS level in response to the blockade of ETC at different complex, and measured changes of gene expression of antioxidant enzymes and phase II enzymes, also evaluated potential protective effect of selenium (Se) supplementation to the cells under this oxidative stress. In summary, our results showed that complex I was the major ROS-generating site in HT22 cells. Complex I blockade upregulated the mRNA levels of glutamylcysteine synthetase heavy and light chains, glutathione-S-transferases omega1 and alpha 2, hemoxygenase 1, thioredoxin reductase 1, and selenoprotein H. Unexpectedly, the expression of the enzymes that directly scavenge ROS decreased, including superoxide dismutases 1 and 2, glutathione peroxidase 1, and catalase. Se supplementation increased glutathione levels and glutathione peroxidase activity, indicating a potential protective role in oxidative stress caused by ETC blockade.

Ferulenol specifically inhibits succinate ubiquinone reductase at the level of the ubiquinone cycle

The natural compound ferulenol, a sesquiterpene prenylated coumarin derivative, was purified from Ferula vesceritensis and its mitochondrial effects were studied. Ferulenol caused inhibition of oxidative phoshorylation. At low concentrations, ferulenol inhibited ATP synthesis by inhibition of the adenine nucleotide translocase without limitation of mitochondrial respiration. At higher concentrations, ferulenol inhibited oxygen consumption. Ferulenol caused specific inhibition of succinate ubiquinone reductase without altering succinate dehydrogenase activity of the complex II. This inhibition results from a limitation of electron transfers initiated by the reduction of ubiquinone to ubiquinol in the ubiquinone cycle. This original mechanism of action makes ferulenol a useful tool to study the physiological role and the mechanism of electron transfer in the complex II. In addition, these data provide an additional mechanism by which ferulenol may alter cell function and demonstrate that mitochondrial dysfunction is an important determinant in Ferula plant toxicity.

Dicoumarol impairs mitochondrial electron transport and pyrimidine biosynthesis in human myeloid leukemia HL-60 cells

Dicoumarol, a competitive inhibitor of NAD(P)H:quinone oxidoreductase 1 (NQO1), increases intracellular superoxide and affects cell growth of tumor cells. This work was set to establish a mechanistic link between dicoumarol, superoxide and cell cycle alterations in HL-60 cells. Using ES936, a mechanism-based irreversible inhibitor of NQO1, we demonstrate that NQO1 inhibition is not a major factor involved in superoxide boost. Mitochondrial Complexes II, III and IV were directly inhibited by dicoumarol. Succinate, which inhibits superoxide generation by reversed electron flow in Complex II, significantly decreased superoxide boost in dicoumarol-treated cells and in isolated mitochondria incubated with dicoumarol and decylubiquinol. Superoxide generation in cells was strongly potentiated by blocking the quinone site of Complex II with thenoyltrifluoroacetone, supporting the involvement of cytochrome b560 to drive electrons for increasing superoxide. Simultaneous inhibition of the mitochondrial chain upstream ubiquinone and displacement of succinate from the Complex II active site is proposed as a major mechanism to explain how dicoumarol increases superoxide in HL-60 cells. Dicoumarol-treated cells accumulated in S phase due to the impairment of pyrimidine biosynthesis at dihydroorotate dehydrogenase step because blockade was overcome by addition of exogenous uridine or orotate, but not by dihydroorotate. We demonstrate for the first time that dicoumarol inhibits mitochondrial electron transport, induces superoxide release by reversed electron flow in Complex II, and inhibits pyrimidines biosynthesis. These actions must be taken into account when considering dicoumarol effects on cells.

Mitochondrial Respiratory Dysfunction and Oxidative Stress after Chronic Malathion Exposure

Malathion is a pesticide used on a large scale and with high potential risk for human exposure. However, it is reasonable to hypothesize that while the malathion is metabolizing reactive oxygen species (ROS) can be generated and subsequently there is onset of an oxidative stress in central nervous system (CNS) structures: hippocampus, cortex, striatum and cerebellum of intoxicated rats due to mitochondrial respiratory chain disfunctions. The present study was therefore undertaken to evaluate malathion-induced lipid peroxidation (LPO), superoxide production from sub-mitochondrial particles and the activity of complexes II and IV of the mitochondrial respiratory chain. Malathion was administered in doses of 25, 50, 100 and 150 mg malathion/kg. After malathion administration LPO increased in hippocampus and striatum. This was accompanied by an increase in the formation of superoxide in submitochondrial particles in the hippocampus. Complex IV suffered significant inhibition of its activity. We could demonstrate in this study that malathion induces oxidative stress and it could be due to inactivation of mitochondrial respiratory complexes.

Crystallographic studies of the binding of ligands to the dicarboxylate site of Complex II, and the identity of the ligand in the "oxaloacetate-inhibited" state

Mitochondrial Complex II (succinate:ubiquinone oxidoreductase) is purified in a partially inactivated state, which can be activated by removal of tightly bound oxaloacetate (E.B. Kearney, et al., Biochem. Biophys. Res. Commun. 49 1115-1121). We crystallized Complex II in the presence of oxaloacetate or with the endogenous inhibitor bound. The structure showed a ligand essentially identical to the "malate-like intermediate" found in Shewanella Flavocytochrome c crystallized with fumarate (P. Taylor, et al., Nat. Struct. Biol. 6 1108-1112) Crystallization of Complex II in the presence of excess fumarate also gave the malate-like intermediate or a mixture of that and fumarate at the active site. In order to more conveniently monitor the occupation state of the dicarboxylate site, we are developing a library of UV/Vis spectral effects induced by binding different ligands to the site. Treatment with fumarate results in rapid development of the fumarate difference spectrum and then a very slow conversion into a species spectrally similar to the OAA-liganded complex. Complex II is known to be capable of oxidizing malate to the enol form of oxaloacetate (Y.O. Belikova, et al., Biochim. Biophys. Acta 936 1-9). The observations above suggest it may also be capable of interconverting fumarate and malate. It may be useful for understanding the mechanism and regulation of the enzyme to identify the malate-like intermediate and its pathway of formation from oxaloacetate or fumarate.

Hypoxic vasoconstriction of partial muscular intra-acinar pulmonary arteries in murine precision cut lung slices

Background

Acute alveolar hypoxia causes pulmonary vasoconstriction (HPV) which serves to match lung perfusion to ventilation. The underlying mechanisms are not fully resolved yet. The major vascular segment contributing to HPV, the intra-acinar artery, is mostly located in that part of the lung that cannot be selectively reached by the presently available techniques, e.g. hemodynamic studies of isolated perfused lungs, recordings from dissected proximal arterial segments or analysis of subpleural vessels. The aim of the present study was to establish a model which allows the investigation of HPV and its underlying mechanisms in small intra-acinar arteries.

Methods

Intra-acinar arteries of the mouse lung were studied in 200 μm thick precision-cut lung slices (PCLS). The organisation of the muscle coat of these vessels was characterized by α-smooth muscle actin immunohistochemistry. Basic features of intra-acinar HPV were characterized, and then the impact of reactive oxygen species (ROS) scavengers, inhibitors of the respiratory chain and Krebs cycle metabolites was analysed.

Results

Intra-acinar arteries are equipped with a discontinuous spiral of α-smooth muscle actin-immunoreactive cells. They exhibit a monophasic HPV (medium gassed with 1% O₂) that started to fade after 40 min and was lost after 80 min. This HPV, but not vasoconstriction induced by the thromboxane analogue U46619, was effectively blocked by nitro blue tetrazolium and diphenyleniodonium, indicating the involvement of ROS and flavoproteins. Inhibition of mitochondrial complexes II (3-nitropropionic acid, thenoyltrifluoroacetone) and III (antimycin A) specifically interfered with HPV, whereas blockade of complex IV (sodium azide) unspecifically inhibited both HPV and U46619-induced constriction. Succinate blocked HPV whereas fumarate had minor effects on vasoconstriction.

Conclusion

This study establishes the first model for investigation of basic characteristics of HPV directly in intra-acinar murine pulmonary vessels. The data are consistent with a critical involvement of ROS, flavoproteins, and of mitochondrial complexes II and III in intra-acinar HPV. In view of the lack of specificity of any of the classical inhibitors used in such types of experiments, validation awaits the use of appropriate knockout strains and siRNA interference, for which the present model represents a well-suited approach.

Differential inhibitory effects of methylmalonic acid on respiratory chain complex activities in rat tissues

Methylmalonic acidemia is an inherited metabolic disorder biochemically characterized by tissue accumulation of methylmalonic acid (MMA) and clinically by progressive neurological deterioration and kidney failure, whose pathophysiology is so far poorly established. Previous studies have shown that MMA inhibits complex II of the respiratory chain in rat cerebral cortex, although no inhibition of complexes I-V was found in bovine heart. Therefore, in the present study we investigated the in vitro effect of 2.5mM MMA on the activity of complexes I-III, II, II-III and IV in striatum, hippocampus, heart, liver and kidney homogenates from young rats. We observed that MMA caused a significant inhibition of complex II activity in striatum and hippocampus (15-20%) at low concentrations of succinate in the medium, but not in the peripheral tissues. We also verified that the inhibitory property of MMA only occurred after exposing brain homogenates for at least 10 min with the acid, suggesting that this inhibition was mediated by indirect mechanisms. Simultaneous preincubation with the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME) and catalase (CAT) plus superoxide dismutase (SOD) did not prevent MMA-induced inhibition of complex II, suggesting that common reactive oxygen (superoxide, hydrogen peroxide and hydroxyl radical) and nitric (nitric oxide) species were not involved in this effect. In addition, complex II-III (20-35%) was also inhibited by MMA in all tissues tested, and complex I-III only in the kidney (53%) and liver (38%). In contrast, complex IV activity was not changed by MMA in all tissues studied. These results indicate that MMA differentially affects the activity of the respiratory chain pending on the tissues studied, being striatum and hippocampus more vulnerable to its effect. In case our in vitro data are confirmed in vivo in tissues from methylmalonic acidemic patients, it is feasible that that the present findings may be related to the pathophysiology of the tissue damage characteristic of these patients.

Cystamine and cysteamine prevent 3-NP-induced mitochondrial depolarization of Huntington's disease knock-in striatal cells

Abstract Cystamine significantly improved motor deficits and extended survival in mouse models of Huntington's disease (HD); however, the precise mechanism(s) by which cystamine and the related compound cysteamine are beneficial remain to be elucidated. Using clonal striatal cell lines from wild-type (STHdhQ7/HdhQ7) and mutant huntingtin knock-in (STHdhQ111/HdhQ111) mice, we have tested the hypothesis that cystamine and cysteamine could be beneficial by preventing the depolarization of mitochondria in cell cultures. Treatment with 3-nitroproprionic acid (3-NP), a mitochondrial complex II inhibitor, induces mitochondrial depolarization and cell death of mutant HD striatal cells but not of wild-type cells. The 3-NP-mediated decrease in the mitochondrial membrane potential was attenuated by 50 microm cystamine and completely inhibited by 250 microm cystamine. Similar results were obtained using cysteamine (50-500 microm). In addition, both cystamine and cysteamine significantly attenuated the 3-NP-induced cell death. Treatment of mutant HD striatal cells with 3-NP resulted in a robust decrease in the cellular and mitochondrial levels of glutathione (GSH) compared with cells exposed to the vehicle alone. Pre-treatment of the cells with cystamine and cysteamine completely prevented the 3-NP-mediated decrease in cellular and mitochondrial GSH levels. Incubation with L-buthionine (S,R) sulfoximine (BSO) 250 microm in combination with cystamine (250 microm) or cysteamine (250 microm) prior to being treated with 3-NP completely prevented the beneficial effects of cystamine and cysteamine on the 3-NP-mediated mitochondrial depolarization. These results demonstrate that cystamine and cysteamine prevent the 3-NP-induced mitochondrial depolarization of HD striatal cell cultures.

3-nitropropionic acid is a suicide inhibitor of mitochondrial respiration that, upon oxidation by complex II, forms a covalent adduct with a catalytic base arginine in the active site of the enzyme

We report three new structures of mitochondrial respiratory Complex II (succinate ubiquinone oxidoreductase, E.C. 1.3.5.1) at up to 2.1 A resolution, with various inhibitors. The structures define the conformation of the bound inhibitors and suggest the residues involved in substrate binding and catalysis at the dicarboxylate site. In particular they support the role of Arg(297) as a general base catalyst accepting a proton in the dehydrogenation of succinate. The dicarboxylate ligand in oxaloacetate-containing crystals appears to be the same as that reported for Shewanella flavocytochrome c treated with fumarate. The plant and fungal toxin 3-nitropropionic acid, an irreversible inactivator of succinate dehydrogenase, forms a covalent adduct with the side chain of Arg(297). The modification eliminates a trypsin cleavage site in the flavoprotein, and tandem mass spectroscopic analysis of the new fragment shows the mass of Arg(297) to be increased by 83 Da and to have the potential of losing 44 Da, consistent with decarboxylation, during fragmentation.

Inhibition of the electron transport chain and creatine kinase activity by ethylmalonic acid in human skeletal muscle

Ethylmalonic aciduria is a common finding in patients affected by short-chain acyl-CoA dehydrogenase (SCAD) deficiency and other diseases characterized by encephalopathy, muscular symptomatology, and lactic acidemia. Considering that the pathophysiological mechanisms of these disorders are practically unknown and that lactic acidosis suggest an impairment of energy production, the objective of the present work was to investigate the in vitro effect of ethylmalonic acid (EMA), at concentrations varying from 0.25 to 5.0 mM, on important parameters of energy metabolism in human skeletal muscle, such as the activities of the respiratory chain complexes and of creatine kinase, which are responsible for most of the ATP produced and transferred inside the cell. We verified that EMA significantly inhibited the activity of complex I-III at concentrations as low as 0.25 mM, complex II-III at 1 mM and higher concentrations, and complex II at the concentration of 5 mM. In contrast, complex IV was not inhibited by the acid. Finally, we observed that the activity of creatine kinase was significantly inhibited by EMA at the concentrations of 1 and 5 mM. These results suggest that EMA compromises energy metabolism in human skeletal muscle. In case the in vitro effects detected in the present study also occur in vivo, it is tempting to speculate that they may contribute, at least in part, to explain the hypotonia/myopathy, as well as the increased concentrations of lactic acid present in the patients affected by illnesses in which EMA accumulates.

Homocysteine-induced endothelin-1 release is dependent on hyperglycaemia and reactive oxygen species production in bovine aortic endothelial cells

BACKGROUND

Elevated plasma homocysteine (Hcy) is a risk factor for coronary disease. The objective of this study was to investigate whether Hcy either alone or in high glucose conditions induces endothelin-1 (ET-1) synthesis via the production of reactive oxygen species (ROS).

METHODS

Bovine aortic endothelial cells were grown in high (25 mmol/l) and low (5 mmol/l) glucose medium.

RESULTS

In high glucose, Hcy caused a time-dependent increase in ET-1 release, which was greatest with 50 micromol/l Hcy at 24 h (p < 0.01). This effect was not seen in low glucose conditions. In high glucose and 50 micromol/l Hcy, ET-1 mRNA levels were maximal after 1 h (p < 0.05). Tissue factor mRNA levels were raised at 4 h (p < 0.05) and functional activity was raised at 6 h (p < 0.01). Intracellular ROS production was increased by 50 micromol/l Hcy after 24 h (p < 0.05) but only in high glucose. To investigate the role of mitochondrial metabolism in ROS production, cells were incubated with thenoyltrifluoroacetone (inhibitor of complex II) or carbonyl cyanide m-chlorophenylhydrazone (uncoupler of oxidative phosphorylation). Both compounds abolished the Hcy-induced increase in ROS production and ET-1 release. There was an alteration in intracellular glutathione (GSH) levels with Hcy treatment with more oxidised GSH present.

CONCLUSION

The combined metabolic burden of Hcy and high glucose stimulates ET-1 synthesis in bovine aortic endothelial cells via a mechanism dependent on the production of mitochondrial ROS, but may not be generalisable to all types of endothelial cells.

Inhibition of mitochondrial complex II affects dopamine metabolism and decreases its uptake into striatal synaptosomes

The mitochondrial toxin, 3-nitropropionic acid (3-NP), is a specific inhibitor of succinate dehydrogenase, complex II in the mitochondrial respiratory chain. The aim of our study was to determine the relationship between inhibition of mitochondrial complex II and dopamine (DA) metabolism and its transport into rat striatal synaptosomes after exposure to 3-NP. The study was carried out using spectrophotometric, radiochemical and HPLC methods. Our data showed that inhibition of succinate dehydrogenase by intraperitoneal (i.p.) injection of 3-NP (cumulated dose 100 mg/kg in 4 days) significantly affected DA metabolism, leading to the accumulation of its metabolites, 3,4-dihydroxylphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the rat striatum. These experimental conditions had no effect on free radical dependent lipid peroxidation in the brain. In vitro experiments revealed that DA and DOPAC significantly decrease lipid peroxidation in the brain homogenate. Moreover, 3-NP significantly inhibited [3H]DA uptake into striatal synaptosomes by specific dopamine transporter (DAT). The scavengers of superoxide radical (O2-) Tempol and Trolox had no effect on DAT function, but the nitric oxide synthase (NOS) inhibitor N w-nitro-L-arginine (100 microM) prevented 3-NP-evoked DAT down-regulation. In summary, our results indicate that inhibition of mitochondrial complex II by 3-NP enhances DA degradation and decreases its uptake into synaptosomes. It is suggested that NO and energy failure are responsible for alteration of the dopaminergic system in the striatum.

Methamphetamine-induced inhibition of mitochondrial complex II: roles of glutamate and peroxynitrite

High-dose methamphetamine (METH) is associated with long-term deficits in dopaminergic systems. Although the mechanism(s) which contributes to these deficits is not known, glutamate and peroxynitrite are likely to play a role. These factors are hypothesized to inhibit mitochondrial function, increasing the free radical burden and decreasing neuronal energy supplies. Previous studies suggest a role for the mitochondrial electron transport chain (ETC) in mediating toxicity of METH. The purpose of the present studies was to determine whether METH administration selectively inhibits complex II of the ETC in rats. High-dose METH administration (10 mg/kg every 2 h x 4) rapidly (within 1 h) decreased complex II (succinate dehydrogenase) activity by approximately 20-30%. In addition, decreased activity of complex II-III, but not complex I-III, of the mitochondrial ETC was also observed 24 h after METH. This inhibition was not due to direct inhibition by METH or METH-induced hyperthermia and was specific to striatal brain regions. METH-induced decreases in complex II-III were prevented by MK-801 and the peroxynitrite scavenger 5,10,15,20-tetrakis (2,4,6-trimethyl-3,5-sulphonatophenyl) porphinato iron III. These findings provide the first evidence that METH administration, via glutamate receptor activation and peroxynitrite formation, selectively alters a specific site of the ETC.

Purification and characterization of succinate:menaquinone oxidoreductase from Corynebacterium glutamicum

Succinate:menaquinone oxidoreductase from Corynebacterium glutamicum, a high-G+C, Gram-positive bacterium, was purified to homogeneity. The enzyme contained two heme B molecules and three polypeptides with apparent molecular masses of 67, 29 and 23 kDa, which corresponded to SdhA (flavoprotein), SdhB (iron-sulfur protein), and SdhC (membrane anchor protein), respectively. In non-denaturating polyacrylamide gel electrophoresis, the enzyme migrated as a single band with an apparent molecular mass of 410 kDa, suggesting that it existed as a trimer. The succinate dehydrogenase activity assayed using 2,3-dimethoxy-5-methyl-6-decyl-1,4-benzoquinone and 2,6-dichloroindophenol as the electron acceptor was inhibited by 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO), and the Dixon plots were biphasic. In contrast, the succinate dehydrogenase activity assayed using phenazine methosulfate and 2,6-dichloroindophenol was inhibited by p-benzoquinone and not by HQNO. These findings suggested that the C. glutamicum succinate:menaquinone oxidoreductase had two quinone binding sites. In the phylogenetic tree of SdhA, Corynebacterium species do not belong to the high-G+C group, which includes Mycobacterium tuberculosis and Streptomyces coelicolor, but are rather close to the group of low-G+C, Gram-positive bacteria such as Bacillus subtilis. This situation may have arisen due to the horizontal gene transfer.

Mitochondrial permeability transition in neuronal damage promoted by Ca2+ and respiratory chain complex II inhibition

Changes in mitochondrial integrity, reactive oxygen species release and Ca2+ handling are proposed to be involved in the pathogenesis of many neurological disorders including methylmalonic acidaemia and Huntington's disease, which exhibit partial mitochondrial respiratory inhibition. In this report, we studied the mechanisms by which the respiratory chain complex II inhibitors malonate, methylmalonate and 3-nitropropionate affect rat brain mitochondrial function and neuronal survival. All three compounds, at concentrations which inhibit respiration by 50%, induced mitochondrial inner membrane permeabilization when in the presence of micromolar Ca2+ concentrations. ADP, cyclosporin A and catalase prevented or delayed this effect, indicating it is mediated by reactive oxygen species and mitochondrial permeability transition (PT). PT induced by malonate was also present in mitochondria isolated from liver and kidney, but required more significant respiratory inhibition. In brain, PT promoted by complex II inhibition was stimulated by increasing Ca2+ cycling and absent when mitochondria were pre-loaded with Ca2+ or when Ca2+ uptake was prevented. In addition to isolated mitochondria, we determined the effect of methylmalonate on cultured PC12 cells and freshly prepared rat brain slices. Methylmalonate promoted cell death in striatal slices and PC12 cells, in a manner attenuated by cyclosporin A and bongkrekate, and unrelated to impairment of energy metabolism. We propose that under conditions in which mitochondrial complex II is partially inhibited in the CNS, neuronal cell death involves the induction of PT.

Shift in the localization of sites of hydrogen peroxide production in brain mitochondria by mitochondrial stress

We have determined the underlying sites of H(2)O(2) generation by isolated rat brain mitochondria and how these can shift depending on the presence of respiratory substrates, electron transport chain modulators and exposure to stressors. H(2)O(2) production was determined using the fluorogenic Amplex red and peroxidase system. H(2)O(2) production was higher when succinate was used as a respiratory substrate than with another FAD-dependent substrate, alpha-glycerophosphate, or with the NAD-dependent substrates, glutamate/malate. Depolarization by the uncoupler p-trifluoromethoxyphenylhydrazone decreased H(2)O(2) production stimulated by all respiratory substrates. H(2)O(2) production supported by succinate during reverse transfer of electrons was decreased by inhibitors of complex I (rotenone and diphenyleneiodonium) whereas in glutamate/malate-oxidizing mitochondria diphenyleneiodonium decreased while rotenone increased H(2)O(2) generation. The complex III inhibitors antimycin and myxothiazol decreased succinate-induced H(2)O(2) production but stimulated H(2)O(2) production in glutamate/malate-oxidizing mitochondria. Antimycin and myxothiazol also increased H(2)O(2) production in mitochondria using alpha-glycerophosphate as a respiratory substrate. In substrate/inhibitor experiments maximal stimulation of H(2)O(2) production by complex I was observed with the alpha-glycerophosphate/antimycin combination. In addition, three forms of in vitro mitochondrial stress were studied: Ca(2+) overload, cold storage for more than 24 h and cytochrome c depletion. In each case we observed (i) a decrease in succinate-supported H(2)O(2) production by complex I and an increase in succinate-supported H(2)O(2) production by complex III, (ii) increased glutamate/malate-induced H(2)O(2) generation by complex I and (iii) increased alpha-glycerophosphate-supported H(2)O(2) generation by complex III. Our results suggest that all three forms of mitochondrial stress resulted in similar shifts in the localization of sites of H(2)O(2) generation and that, in both normal and stressed states, the level and location of H(2)O(2) production depend on the predominant energetic substrate.

Inhibition of mitochondrial complex II alters striatal expression of genes involved in glutamatergic and dopaminergic signaling: possible implications for Huntington's disease

Huntington's disease (HD) is a genetic neurodegenerative disorder characterized by motor abnormalities and cognitive impairment. The irreversible succinate dehydrogenase (SD) inhibitor 3-nitropropionic acid (3NP) causes neurodegeration in the striatum resembling HD when administered to rodents or primates. Using corticostriatal brain slice preparations, we analyzed the pattern of gene expression following 3NP application utilizing cDNA microarrays. Acute 3NP treatment modulates the expression of several genes involved in dopaminergic and glutamatergic signaling in corticostriatal brain slices, and unbalances the downstream serine/threonine protein kinase and phosphatase network affecting the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32). Our data provide new information about the molecular events possibly underlying neurodegeneration induced by this mitochondrial toxin.

Neuronal vulnerability following inhibition of mitochondrial complex II: a possible ionic mechanism for Huntington's disease

An impaired complex II (succinate dehydrogenase, SD) striatal mitochondrial activity is one of the prominent metabolic alterations in Huntington's disease (HD), and intoxication with 3-nitropropionic acid (3-NP), an inhibitor of mitochondrial complex II, mimics the motor abnormalities and the pathology of HD. We found that striatal spiny neurons responded to this toxin with an irreversible membrane depolarization/inward current, while cholinergic interneurons showed a hyperpolarization/outward current. Both these currents were sensitive to intracellular concentration of ATP. The 3-NP-induced depolarization was associated with an increased release of endogenous GABA, while acetylcholine levels were reduced. Moreover, 3-NP induced a higher depolarization in presymptomatic R6/2 HD transgenic mice compared to wild-type (WT) mice, showing an increased susceptibility to SD inhibition. Conversely, the hyperpolarization did not significantly differ from the one recorded in WT mice. The diverse membrane changes induced by SD inhibition may contribute to the cell-type-specific neuronal death in HD.

Graded reoxygenation with chemical inhibition of oxidative phosphorylation improves posthypoxic recovery in murine hippocampal slices

Rapid and complete tissue reoxygenation is a prime goal of present stroke therapy. However, reoxygenation may trigger detrimental cascades that partially antagonize beneficial effects. It was our goal to investigate selective grading of reoxygenation with targeting of single mitochondrial complexes in murine hippocampal slices. Population spike amplitude (PSAP) and NADH were measured during hypoxic hypoxia (15 min) and recovery (45 min). With onset of reoxygenation, slices were treated for different times with amobarbital (1 mM), malonate (2 mM), or cyanide (1 mM), inhibitors of mitochondrial complex I, II, or IV, respectively. Other slices were treated with nicotinamide (1 mM). Posthypoxic recovery of PSAP increased from 32% +/- 43% of onset in control slices to 52% +/- 59% (P <.05) upon treatment with amobarbital for 1 min and to 62% +/- 37% (P <.05) upon treatment with malonate. With nicotinamide, posthypoxic recovery improved to 73% +/- 25% (P <.05). Oxidation of NADH was prolonged upon treatment with amobarbital, whereas no change in NADH oxidation was observed with malonate and nicotinamide. Thus, grading of reoxygenation with selective targeting of mitochondrial complex I or II but not of complex IV improves outcome upon reoxygenation in murine hippocampal slices.

Effect of oxygen on activation state of complex I and lack of oxaloacetate inhibition of complex II in Langendorff perfused rat heart

Two main entry points for electrons into the mitochondrial respiratory chain are NADH:ubiquinone oxidoreductase (complex I) and succinate:ubiquinone oxidoreductase (complex II). Metabolic regulation of these two respiratory complexes is not understood in detail. It has been suggested that the Krebs cycle metabolic intermediate oxaloacetate (OAA) inhibits complex II in vivo, whereas complex I undergoes a reversible active/de-active transition. In normoxic and anoxic hearts it has been shown that the proportion of complex I in the active and de-active states is different suggesting a possible mode of regulation of the enzyme by oxygen concentration. In the current studies rapid isolation of mitochondrial membranes in a state that preserves the activity of both complex I and complex II has been achieved using Langendorff perfused rat hearts. The findings indicate that the state of activation of complex I is controlled by the oxygen saturation in the perfusate. In addition, these studies show that complex II is fully active in the mitochondrion and not inhibited by OAA regardless of the oxygen concentration.

Striatal cells from mutant huntingtin knock-in mice are selectively vulnerable to mitochondrial complex II inhibitor-induced cell death through a non-apoptotic pathway

Extensive striatal neuronal loss occurs in Huntington's disease (HD), which is caused by an expanded polyglutamine tract in huntingtin (htt). Evidence suggests that mutant htt directly or indirectly compromises mitochondrial function, contributing to the neuronal loss. To determine the role of compromised mitochondrial function in the neuronal cell death in HD, immortalized striatal cells established from Hdh(Q7) (wild-type) and Hdh(Q111) (mutant) mouse knock-in embryos were treated with 3-nitropropionic acid (3-NP), a mitochondrial complex II toxin. 3-NP treatment caused significantly greater cell death in mutant striatal cells compared with wild-type cells. In contrast, the extent of cell death induced by rotenone, a complex I inhibitor, was similar in both cell lines. Although evidence of apoptosis was present in 3-NP-treated wild-type striatal cells, it was absent in 3-NP-treated mutant cells. 3-NP treatment caused a greater loss of mitochondrial membrane potential (deltapsim) in mutant striatal cells compared with wild-type cells. Cyclosporine A, an inhibitor of mitochondrial permeability transition pore (PTP), and ruthenium red, an inhibitor of the mitochondrial calcium uniporter, both rescued mutant striatal cells from 3-NP-induced cell death and prevented the loss of deltapsim. These data show that mutant htt specifically increases cell vulnerability to mitochondrial complex II inhibition and further switched the type of cell death induced by complex II inhibition from apoptosis to a non-apoptotic form, caused by mitochondrial membrane depolarization, probably initiated by mitochondrial calcium overload and subsequent PTP opening. These findings suggest that impaired mitochondrial complex II function in HD may contribute to non-apoptotic neuronal cell death.

Mitochondrial calcium, oxidative stress and apoptosis in a neurodegenerative disease model induced by 3-nitropropionic acid

Intracellular calcium homeostasis is important for cell survival. However, increase in mitochondrial calcium (Ca2+m) induces opening of permeability transition pore (PTP), mitochondrial dysfunction and apoptosis. Since alterations of intracellular Ca2+ and reactive oxygen species (ROS) generation are involved in cell death, they might be involved in neurodegenerative processes such as Huntington's disease (HD). HD is characterized by the inhibition of complex II of respiratory chain and increase in ROS production. In this report, we studied the correlation between the inhibitor of the complex II, 3-nitropropionic acid (3NP), Ca2+ metabolism, apoptosis and behavioural alterations. We showed that 3NP (1 mm) is able to release Ca2+m, as neither Thapsigargin (TAP, 2 microm) nor free-calcium medium affected its effect. PTP inhibitors and antioxidants inhibited this process, suggesting an increase in ROS generation and PTP opening. In addition, 3NP (0.1 mm) also induces apoptotic cell death. Behavioural changes in animals treated with 3NP (20 mg/kg/day for 4 days) were also attenuated by pre- and co-treatment with vitamin E (VE, 20 mg/kg/day). Taken together, our results show that complex II inhibition could involve Ca2+m release, oxidative stress and cell death that may precede motor alterations in neurodegenerative processes such as HD.

Hypoxic increase in nitric oxidegeneration of rat sensory neurons requires activation of mitochondrial complex II and voltage-gated calcium channels

Recently, we have demonstrated that sensory neurons of rat lumbar dorsal root ganglia (DRG) respond to hypoxia with an activation of endothelial nitric oxide (NO) synthase (eNOS) resulting in enhanced NO production associated with mitochondria which contributes to resistance against hypoxia. Extracellular calcium is essential to this effect. In the present study on rat DRG slices, we set out to determine what types of calcium channels operate under hypoxia, and which upstream events contribute to their activation, thereby focusing upon mitochondrial complex II. Both the metallic ions Cd2+ and Ni2+, known to inhibit voltage-gated calcium channels and T-type channels, respectively, and verapamil and nifedipine, typical blocker of L-type calcium channels completely prevented the hypoxic neuronal NO generation. Inhibition of complex II by thenoyltrifluoroacetone at the ubiquinon binding site or by 3-nitropropionic acid at the substrate binding site largely diminished hypoxic-induced NO production while having an opposite effect under normoxia. An additional blockade of voltage-gated calcium channels entirely abolished the hypoxic response. The complex II inhibitor malonate inhibited both normoxic and hypoxic NO generation. These data show that complex II activity is required for increased hypoxic NO production. Since succinate dehydrogenase activity of complex II decreased at hypoxia, as measured by histochemistry and densitometry, we propose a hypoxia-induced functional switch of complex II from succinate dehydrogenase to fumarate reductase, which subsequently leads to activation of voltage-gated calcium channels resulting in increased NO production by eNOS.

Molecular Mechanism of Cell Death Induced by the Antioxidant tert-Butylhydroxyanisole in Human Monocytic Leukemia U937 Cells

A phenolic antioxidant 3-tert-butyl-4-hydroxyanisole (BHA) is a widely used food additive. BHA had cytotoxicity in human monocytic leukemia U937 cells. BHA at 0.75 mM caused nuclear condensation and fragmentation, structural damage in mitochondria, decrease in mitochondrial transmembrane potential, and internucleosomal DNA cleavage. It induced the activities of caspase-3 and/or -7, -6, -8 and -9, especially high when DEVD-MCA was the substrate (caspase-3 and/or -7). DEVDase activity increased in time- and dose-dependent manner and high activity was observed in lysates of cells treated for 3 h at 0.75 mM. Addition of GSH (reduced glutathione) during the treatment of cells with BHA inhibited the induction of DEVDase activity, and the intracellular GSH level decreased as the concentration of BHA was raised. Intracellular ATP levels decreased in time- and dose-dependent manner when the cells were treated with BHA in the presence or absence of glucose. Enzyme activities involved in the respiratory chain were assayed with the mitochondrial fraction prepared from U937 cells. BHA distinctly inhibited NADH-ubiquinone oxidoreductase (complex I) and cytochrome c oxidase (complex IV) at low concentrations. Succinate-ubiquinone oxidoreductase (complex II) was also inhibited, but to somewhat less extent. Without mitochondrial enzymes, BHA stimulated the ubiquinol-dependent reduction of cytochrome c (complex III), but it might have some detrimental effects on the mitochondrial enzyme reaction of complex III. The inhibition of mitochondrial oxidative phosphorylation might corroborate the mechanistic evidence for apoptosis of leukemia cells by BHA. Cell death induced by BHA is primarily ascribable to apoptosis.

Apoptotic death in Leishmania donovani promastigotes in response to respiratory chain inhibition: complex II inhibition results in increased pentamidine cytotoxicity

The biochemical changes consequent to respiratory chain inhibition and their relationship to cell death in Leishmania spp. remain elusive. Inhibitors of respiratory chain complexes I, II, and III were able to induce apoptotic death of the bloodstream form of Leishmania donovani. Complex I inhibition resulted in mitochondrial hyperpolarization that was preceded by increased superoxide production. Limitation of electron transport by thenoyltrifluoroacetone and antimycin A, inhibitors of complexes II and III, respectively, resulted in dissipation of mitochondrial membrane potential that was sensitive to cyclosporin A, a blocker of mitochondrial permeability transition pore. Further studies conducted with thenoyltrifluoroacetone showed maximal generation of hydrogen peroxide with a moderate elevation of superoxide levels. Complex III inhibition provoked superoxide generation only. Interference with complex II but not complexes I and III increased intracellular Ca(2+). A tight link between Ca(2+) and reactive oxygen species was demonstrated by antioxidant-induced diminution of the Ca(2+) increase. However, chelation of extracellular Ca(2+) could not abrogate the early increase of reactive oxygen species, providing evidence that Ca(2+) elevation was downstream to reactive oxygen species generation. Ca(2+) influx occurred through nonselective cation and L-type channels and Na(+)/Ca(2+) exchanger-like pathways. Antioxidants such as glutathione and Ca(2+) channel blockers reduced apoptotic death. This study provides a new possibility that concurrent inhibition of respiratory chain complex II with pentamidine administration increases cytotoxicity of the drug. This increased cytotoxicity was connected to a 4-fold elevation in intracellular Ca(2+) that was pooled only from intracellular sources. Therefore, inhibition of complexes I, II, and III leads to apoptosis and complex II inhibition in parallel with pentamidine administration-enhanced drug efficacy.

Adenosinergic protection of dopaminergic and GABAergic neurons against mitochondrial inhibition through receptors located in the substantia nigra and striatum, respectively

Mitochondrial dysfunction may contribute to dopaminergic (DAergic) cell death in Parkinson's disease and GABAergic cell death in Huntington's disease. In the present work, we tested whether blocking A1 receptors could enhance the damage to DAergic and GABAergic neurons caused by mitochondrial inhibition, and whether blocking A2a receptors could protect against damage in this model. Animals received an intraperitoneal injection of 8-cyclopentyl-1,3-dipropylxanthine (CPX) (A1 antagonist) or 3,7-dimethyl-1-propargylxanthine (DMPX) (A2a antagonist) 30 min before intrastriatal infusion of malonate (mitochondrial complex II inhibitor). Damage was assessed 1 week later by measuring striatal dopamine, tyrosine hydroxylase (TH), and GABA content. In mice and rats, malonate-induced depletion of striatal dopamine, TH, or GABA was potentiated by pretreatment with 1 mg/kg CPX and attenuated by pretreatment with 5 mg/kg DMPX. To determine the location of the A1 and A2a receptors mediating these effects, CPX or DMPX was infused directly into the striatum or substantia nigra of rats 30 min before intrastriatal infusion of malonate. When infused into the striatum, CPX (20 ng) potentiated, whereas DMPX (50 ng) prevented malonate-induced GABA loss, but up to 100 ng of CPX or 500 ng of DMPX did not alter malonate-induced striatal dopamine loss. Intranigral infusion of CPX (100 ng) or DMPX (500 ng), however, did exacerbate and protect, respectively, against malonate-induced striatal dopamine loss. Thus, A1 receptor blockade enhances and A2a receptor blockade protects against damage to DAergic and GABAergic neurons caused by mitochondrial inhibition. Interestingly, these effects are mediated by A1 and A2a receptors located in the substantia nigra for DAergic neurons and in the striatum for GABAergic neurons.