Inhibition of the alpha-ketoglutarate dehydrogenase-mediated reactive oxygen species generation by lipoic acid

Mitochondrial Alpha-Keto Acid Dehydrogenase Complexes: Recent Developments on Structure and Function in Health and Disease

The present work delves into the enigmatic world of mitochondrial alpha-keto acid dehydrogenase complexes discussing their metabolic significance, enzymatic operation, moonlighting activities, and pathological relevance with links to underlying structural features. This ubiquitous family of related but diverse multienzyme complexes is involved in carbohydrate metabolism (pyruvate dehydrogenase complex), the citric acid cycle (α-ketoglutarate dehydrogenase complex), and amino acid catabolism (branched-chain α-keto acid dehydrogenase complex, α-ketoadipate dehydrogenase complex); the complexes all function at strategic points and also participate in regulation in these metabolic pathways. These systems are among the largest multienzyme complexes with at times more than 100 protein chains and weights ranging up to ~10 million Daltons. Our chapter offers a wealth of up-to-date information on these multienzyme complexes for a comprehensive understanding of their significance in health and disease.

Defective function of α-ketoglutarate dehydrogenase exacerbates mitochondrial ATP deficits during complex I deficiency

The NDUFS4 knockout (KO) mouse phenotype resembles the human Complex I deficiency Leigh Syndrome. The irreversible succination of protein thiols by fumarate is increased in select regions of the NDUFS4 KO brain affected by neurodegeneration. We report that dihydrolipoyllysine-residue succinyltransferase (DLST), a component of the α-ketoglutarate dehydrogenase complex (KGDHC) of the tricarboxylic acid (TCA) cycle, is succinated in the affected regions of the NDUFS4 KO brain. Succination of DLST reduced KGDHC activity in the brainstem (BS) and olfactory bulb (OB) of KO mice. The defective production of KGDHC derived succinyl-CoA resulted in decreased mitochondrial substrate level phosphorylation (SLP), further aggravating the existing oxidative phosphorylation (OXPHOS) ATP deficit. Protein succinylation, an acylation modification that requires succinyl-CoA, was reduced in the KO mice. Modeling succination of a cysteine in the spatial vicinity of the DLST active site or introduction of succinomimetic mutations recapitulates these metabolic deficits. Our data demonstrate that the biochemical deficit extends beyond impaired Complex I assembly and OXPHOS deficiency, functionally impairing select components of the TCA cycle to drive metabolic perturbations in affected neurons.

(+)-Lipoic Acid Reduces Lipotoxicity and Regulates Mitochondrial Homeostasis and Energy Balance in an In Vitro Model of Liver Steatosis

Non-alcoholic fatty liver disease (NAFLD) is characterized by the accumulation of lipids within hepatocytes, which compromises liver functionality following mitochondrial dysfunction and increased production of reactive oxygen species (ROS). Lipoic acid is one of the prosthetic groups of the pyruvate dehydrogenase complex also known for its ability to confer protection from oxidative damage because of its antioxidant properties. In this study, we aimed to investigate the effects of lipoic acid on lipotoxicity and mitochondrial dynamics in an in vitro model of liver steatosis. HepG2 cells were treated with palmitic acid and oleic acid (1:2) to induce steatosis, without and with 1 and 5 µM lipoic acid. Following treatments, cell proliferation and lipid droplets accumulation were evaluated. Mitochondrial functions were assessed through the evaluation of membrane potential, MitoTracker Red staining, expression of genes of the mitochondrial quality control, and analysis of energy metabolism by HPLC and Seahorse. We showed that lipoic acid treatment restored membrane potential to values comparable to control cells, as well as protected cells from mitochondrial fragmentation following PA:OA treatment. Furthermore, our data showed that lipoic acid was able to determine an increase in the expression of mitochondrial fusion genes and a decrease in mitochondrial fission genes, as well as to restore the bioenergetics of cells after treatment with palmitic acid and oleic acid. In conclusion, our data suggest that lipoic acid reduces lipotoxicity and improves mitochondrial functions in an in vitro model of steatosis, thus providing a potentially valuable pharmacological tool for NAFLD treatment.

The Effects of Lipoic Acid on Yolk Nutrient Utilization, Energy Metabolism, and Redox Balance over Time in Artemia sp

Lipoic acid (LA) is a mitochondrial coenzyme that, depending on the concentration and exposure time, can behave as an antioxidant or pro-oxidant agent and has a proven ability to modulate metabolism by promoting lipid and glucose oxidation for energy production. To assess the effects of LA on energy metabolism and redox balance over time, Artemia sp. nauplii was used as an animal model. The administered concentrations of the antioxidant were 0.05, 0.1, 0.5, 1.0, 5.0, and 10.0 µM. Therefore, possible differences in protein, triglyceride, glucose, and lactate concentrations in the artemia samples and total ammoniacal nitrogen (TAN) in the culture water were evaluated. We also measured the effects of LA on in vivo activity of the electron transport system (ETS), antioxidant capacity, and production of reactive oxygen species (ROS) at 6, 12, 18, and 24 h post-hatching. There was a decrease in glucose concentration in the LA-treated animals, and a decrease in ammonia production was observed in the 0.5 µM LA treatment. ETS activity was positively regulated by the addition of LA, with the most significant effects at concentrations of 5.0 and 10.0 µM at 12 and 24 h. For ETS activity, treatments with LA presented the highest values at 24 h, a period when ROS production decreased significantly, for the treatment with 10.0 µM. LA showed positive regulation of energy metabolism together with a decrease in ROS and TAN excretion.

Structural and Biochemical Investigation of Selected Pathogenic Mutants of the Human Dihydrolipoamide Dehydrogenase

Clinically relevant disease-causing variants of the human dihydrolipoamide dehydrogenase (hLADH, hE3), a common component of the mitochondrial α-keto acid dehydrogenase complexes, were characterized using a multipronged approach to unravel the molecular pathomechanisms that underlie hLADH deficiency. The G101del and M326V substitutions both reduced the protein stability and triggered the disassembly of the functional/obligate hLADH homodimer and significant FAD losses, which altogether eventually manifested in a virtually undetectable catalytic activity in both cases. The I12T-hLADH variant proved also to be quite unstable, but managed to retain the dimeric enzyme form; the LADH activity, both in the forward and reverse catalytic directions and the affinity for the prosthetic group FAD were both significantly compromised. None of the above three variants lent themselves to an in-depth structural analysis via X-ray crystallography due to inherent protein instability. Crystal structures at 2.89 and 2.44 Å resolutions were determined for the I318T- and I358T-hLADH variants, respectively; structure analysis revealed minor conformational perturbations, which correlated well with the residual LADH activities, in both cases. For the dimer interface variants G426E-, I445M-, and R447G-hLADH, enzyme activities and FAD loss were determined and compared against the previously published structural data.

Lipoamide dehydrogenase (LADH) deficiency: medical perspectives of the structural and functional characterization of LADH and its pathogenic variants

(Dihydro)lipoamide dehydrogenase (LADH) deficiency is an autosomal recessive genetic metabolic disorder. It generally presents with an onset in the neonatal age and premature death. The clinical picture usually involves metabolic decompensation and lactic acidosis that lead to neurological, cardiological, and/or hepatological outcomes. Severity of the disease is due to the fact that LADH is a common E3 subunit to the pyruvate, alpha-ketoglutarate, alpha-ketoadipate, and branched-chain alpha-keto acid dehydrogenase complexes and is also part of the glycine cleavage system; hence, a loss in LADH activity adversely affects several central metabolic pathways simultaneously. The severe clinical manifestations, however, often do not parallel the LADH activity loss, which implies the existence of auxiliary pathological pathways; stimulated reactive oxygen species (ROS) production as well as dissociation from the relevant multienzyme complexes proved to be auxiliary exacerbating pathomechanisms for selected disease-causing LADH mutations. This review provides an overview on the therapeutic challenges of inherited metabolic diseases, structural and functional characteristics of the mitochondrial alpha-keto acid dehydrogenase complexes, molecular pathogenesis and structural basis of LADH deficiency, and relevant potential future medical perspectives.

Cognitive and Mood Effect of Alpha-Lipoic Acid Supplementation in a Nonclinical Elder Sample: An Open-Label Pilot Study

BACKGROUND

Memory disorders are common among elder people, and nonclinical cognitive decline is commonly experienced with age. Preclinical investigations have explored the possible role of alpha-lipoic acid (ALA), a known antioxidant compound abundant in vegetables and animal tissues, in reducing oxidative stress in the aging brain and preventing cognitive decline. However, clinical evidence is limited, and the few existing results are contrasting. In addition, while most of the existing trials have been focused on the effects of ALA administration in Alzheimer's disease (AD) or other types of dementia, studies evaluating its effects on nonclinical elder population are still missing.

METHODS

In the present open-label, pilot study, fifteen elder patients (mean age: 84.5 ± 5.77) received ALA at a daily dose of 600 mg/day for 12 weeks. General cognitive function, executive function, and mood symptom assessment were carried out at baseline and at the endpoint.

RESULTS

Overall, ALA administration was generally well-tolerated (only one dropout due to gastrointestinal side effects). However, no statistically significant effects either on cognitive function, executive function, or mood were found.

CONCLUSIONS

Despite several limitations, our study found no evidence of positive effects on cognition and mood after ALA administration in elder people without the diagnosis of AD or cognitive impairment. Further clinical trials are needed to better investigate ALA effectiveness on cognition and mood in elder subjects.

The α-Ketoglutarate Dehydrogenase Complex as a Hub of Plasticity in Neurodegeneration and Regeneration

Abnormal glucose metabolism is central to neurodegeneration, and considerable evidence suggests that abnormalities in key enzymes of the tricarboxylic acid (TCA) cycle underlie the metabolic deficits. Significant recent advances in the role of metabolism in cancer provide new insight that facilitates our understanding of the role of metabolism in neurodegeneration. Research indicates that the rate-limiting step of the TCA cycle, the α-ketoglutarate dehydrogenase complex (KGDHC) and its substrate alpha ketoglutarate (KG), serve as a signaling hub that regulates multiple cellular processes: (1) is the rate-limiting step of the TCA cycle, (2) is sensitive to reactive oxygen species (ROS) and produces ROS, (3) determines whether KG is used for energy or synthesis of compounds to support growth, (4) regulates the cellular responses to hypoxia, (5) controls the post-translational modification of hundreds of cell proteins in the mitochondria, cytosol, and nucleus through succinylation, (6) controls critical aspects of transcription, (7) modulates protein signaling within cells, and (8) modulates cellular calcium. The primary focus of this review is to understand how reductions in KGDHC are translated to pathologically important changes that underlie both neurodegeneration and cancer. An understanding of each role is necessary to develop new therapeutic strategies to treat neurodegenerative disease.

Anoxia-reoxygenation alters H2O2 efflux and sensitivity of redox centers to copper in heart mitochondria

Mitochondrial reactive oxygen species (ROS) have been implicated in organ damage caused by environmental stressors, prompting studies on the effect of oxygen deprivation and metal exposure on ROS metabolism. However, how anoxia and copper (Cu) jointly influence heart mitochondrial ROS metabolism is not understood. We used rainbow trout heart mitochondria to probe the effects of anoxia-reoxygenation and Cu on hydrogen peroxide (H₂O₂) emission during oxidation of palmitoylcarnitine (PC), succinate, or glutamate-malate. In addition, we examined the influence of anoxia-reoxygenation and Cu on site-specific H₂O₂ emission capacities and key antioxidant enzymes, glutathione peroxidase (GPx) and thioredoxin reductase (TrxR). Results showed that anoxia-reoxygenation suppressed H₂O₂ emission regardless of substrate type or duration of anoxia. Anoxia-reoxygenation reduced mitochondrial sensitivity to Cu during oxidation of succinate or glutamate-malate whereas high Cu concentration additively stimulated H₂O₂ emission in mitochondria oxidizing PC. Prolonged anoxia-reoxygenation stimulated H₂O₂ emission from sites O_F and I_F, inhibited emission from sites I_Q, II_F and III_Qo, and disparately altered the sensitivity of the sites to Cu. Interestingly, anoxia-reoxygenation increased GPx and TrxR activities, more prominently when reoxygenation followed a short duration of anoxia. Cu did not alter GPx but reduced TrxR activity in normoxic and anoxic-reoxygenated mitochondria. Overall, our study revealed potential mechanisms that may reduce oxidative damage associated with anoxia-reoxygenation and Cu exposure in heart mitochondria. The increased and decreased H₂O₂ emission from NADH/NAD⁺ and QH₂/Q isopotential sites, respectively, may represent a balance between H₂O₂ required for oxygen deprivation-induced signaling and prevention of ROS burst associated with anoxia-reoxygenation.

The moonlighting activities of dihydrolipoamide dehydrogenase: Biotechnological and biomedical applications

Dihydrolipoamide dehydrogenase (DLDH) is a homodimeric flavin-dependent enzyme that catalyzes the NAD⁺ -dependent oxidation of dihydrolipoamide. The enzyme is part of several multi-enzyme complexes such as the Pyruvate Dehydrogenase system that transforms pyruvate into acetyl-co-A. Concomitantly with its redox activity, DLDH produces Reactive Oxygen Species (ROS), which are involved in cellular apoptotic processes. DLDH possesses several moonlighting functions. One of these is the capacity to adhere to metal-oxides surfaces. This was first exemplified by the presence of an exocellular form of the enzyme on the cell-wall surface of Rhodococcus ruber. This capability was evolutionarily conserved and identified in the human, mitochondrial, DLDH. The enzyme was modified with Arg-Gly-Asp (RGD) groups, which enabled its interaction with integrin-rich cancer cells followed by "integrin-assisted-endocytosis." This allowed harnessing the enzyme for cancer therapy. Combining the TiO₂ -binding property with DLDH's ROS-production, enabled us to develop several medical applications including improving oesseointegration of TiO₂ -based implants and photodynamic treatment for melanoma. The TiO₂ -binding sites of both the bacterial and human DLDH's were identified on the proteins' molecules at regions that overlap with the binding site of E3-binding protein (E3BP). This protein is essential in forming the multiunit structure of PDC. Another moonlighting activity of DLDH, which is described in this Review, is its DNA-binding capacity that may affect DNA chelation and shredding leading to apoptotic processes in living cells. The typical ROS-generation by DLDH, which occurs in association with its enzymatic activity and its implications in cancer and apoptotic cell death are also discussed.

Elucidating the contribution of mitochondrial glutathione to ferroptosis in cardiomyocytes

Ferroptosis is a programmed iron-dependent cell death associated with peroxidation of lipids particularly, phospholipids. Several studies suggested a possible contribution of mitochondria to ferroptosis although the mechanisms underlying mitochondria-mediated ferroptotic pathways remain elusive. Reduced glutathione (GSH) is a central player in ferroptosis that is required for glutathione peroxidase 4 to eliminate oxidized phospholipids. Mitochondria do not produce GSH, and although the transport of GSH to mitochondria is not fully understood, two carrier proteins, the dicarboxylate carrier (DIC, SLC25A10) and the oxoglutarate carrier (OGC, SLC25A11) have been suggested to participate in GSH transport. Here, we elucidated the role of DIC and OGC as well as mitochondrial bioenergetics in ferroptosis in H9c2 cardioblasts. Results showed that mitochondria are highly sensitive to ferroptotic stimuli displaying fragmentation, and lipid peroxidation shortly after the onset of ferroptotic stimulus. Inhibition of electron transport chain complexes and oxidative phosphorylation worsened RSL3-induced ferroptosis. LC-MS/MS analysis revealed a dramatic increase in the levels of pro-ferroptotic oxygenated phosphatidylethanolamine species in mitochondria in response to RSL3 (ferroptosis inducer) and cardiac ischemia-reperfusion. Inhibition of DIC and OGC aggravated ferroptosis and increased mitochondrial ROS, membrane depolarization, and GSH depletion. Dihydrolipoic acid, an essential cofactor for several mitochondrial multienzyme complexes, attenuated ferroptosis and induced direct reduction of pro-ferroptotic peroxidized phospholipids to hydroxy-phospholipids in vitro. In conclusion, we suggest that ferroptotic stimuli diminishes mitochondrial bioenergetics and stimulates GSH depletion and glutathione peroxidase 4 inactivation leading to ferroptosis.

Structure of the dihydrolipoamide succinyltransferase (E2) component of the human alpha-ketoglutarate dehydrogenase complex (hKGDHc) revealed by cryo-EM and cross-linking mass spectrometry: Implications for the overall hKGDHc structure

BACKGROUND

The human mitochondrial alpha-ketoglutarate dehydrogenase complex (hKGDHc) converts KG to succinyl-CoA and NADH. Malfunction of and reactive oxygen species generation by the hKGDHc as well as its E1-E2 subcomplex are implicated in neurodegenerative disorders, ischemia-reperfusion injury, E3-deficiency and cancers.

METHODS

We performed cryo-EM, cross-linking mass spectrometry (CL-MS) and molecular modeling analyses to determine the structure of the E2 component of the hKGDHc (hE2k); hE2k transfers a succinyl group to CoA and forms the structural core of hKGDHc. We also assessed the overall structure of the hKGDHc by negative-stain EM and modeling.

RESULTS

We report the 2.9 Å resolution cryo-EM structure of the hE2k component. The cryo-EM map comprises density for hE2k residues 151-386 - the entire (inner) core catalytic domain plus a few additional residues -, while residues 1-150 are not observed due to the inherent flexibility of the N-terminal region. The structure of the latter segment was also determined by CL-MS and homology modeling. Negative-stain EM on in vitro assembled hKGDHc and previous data were used to build a putative overall structural model of the hKGDHc.

CONCLUSIONS

The E2 core of the hKGDHc is composed of 24 hE2k chains organized in octahedral (8 × 3 type) assembly. Each lipoyl domain is oriented towards the core domain of an adjacent chain in the hE2k homotrimer. hE1k and hE3 are most likely tethered at the edges and faces, respectively, of the cubic hE2k assembly.

GENERAL SIGNIFICANCE

The revealed structural information will support the future pharmacologically targeting of the hKGDHc.

Pleiotropic effects of alpha-ketoglutarate as a potential anti-ageing agent

An intermediate of tricarboxylic acid cycle alpha-ketoglutarate (AKG) is involved in pleiotropic metabolic and regulatory pathways in the cell, including energy production, biosynthesis of certain amino acids, collagen biosynthesis, epigenetic regulation of gene expression, regulation of redox homeostasis, and detoxification of hazardous substances. Recently, AKG supplement was found to extend lifespan and delay the onset of age-associated decline in experimental models such as nematodes, fruit flies, yeasts, and mice. This review summarizes current knowledge on metabolic and regulatory functions of AKG and its potential anti-ageing effects. Impact on epigenetic regulation of ageing via being an obligate substrate of DNA and histone demethylases, direct antioxidant properties, and function as mimetic of caloric restriction and hormesis-induced agent are among proposed mechanisms of AKG geroprotective action. Due to influence on mitochondrial respiration, AKG can stimulate production of reactive oxygen species (ROS) by mitochondria. According to hormesis hypothesis, moderate stimulation of ROS production could have rather beneficial biological effects, than detrimental ones, because of the induction of defensive mechanisms that improve resistance to stressors and age-related diseases and slow down functional senescence. Discrepancies found in different models and limitations of AKG as a geroprotective drug are discussed.

MicroRNA and ROS Crosstalk in Cardiac and Pulmonary Diseases

Reactive oxygen species (ROS) affect many cellular functions and the proper redox balance between ROS and antioxidants contributes substantially to the physiological welfare of the cell. During pathological conditions, an altered redox equilibrium leads to increased production of ROS that in turn may cause oxidative damage. MicroRNAs (miRNAs) regulate gene expression at the post-transcriptional level contributing to all major cellular processes, including oxidative stress and cell death. Several miRNAs are expressed in response to ROS to mediate oxidative stress. Conversely, oxidative stress may lead to the upregulation of miRNAs that control mechanisms to buffer the damage induced by ROS. This review focuses on the complex crosstalk between miRNAs and ROS in diseases of the cardiac (i.e., cardiac hypertrophy, heart failure, myocardial infarction, ischemia/reperfusion injury, diabetic cardiomyopathy) and pulmonary (i.e., idiopathic pulmonary fibrosis, acute lung injury/acute respiratory distress syndrome, asthma, chronic obstructive pulmonary disease, lung cancer) compartments. Of note, miR-34a, miR-144, miR-421, miR-129, miR-181c, miR-16, miR-31, miR-155, miR-21, and miR-1/206 were found to play a role during oxidative stress in both heart and lung pathologies. This review comprehensively summarizes current knowledge in the field.

Underlying molecular alterations in human dihydrolipoamide dehydrogenase deficiency revealed by structural analyses of disease-causing enzyme variants

Human dihydrolipoamide dehydrogenase (hLADH, hE3) deficiency (OMIM# 246900) is an often prematurely lethal genetic disease usually caused by inactive or partially inactive hE3 variants. Here we report the crystal structure of wild-type hE3 at an unprecedented high resolution of 1.75 Å and the structures of six disease-causing hE3 variants at resolutions ranging from 1.44 to 2.34 Å. P453L proved to be the most deleterious substitution in structure as aberrations extensively compromised the active site. The most prevalent G194C-hE3 variant primarily exhibited structural alterations close to the substitution site, whereas the nearby cofactor-binding residues were left unperturbed. The G426E substitution mainly interfered with the local charge distribution introducing dynamics to the substitution site in the dimer interface; G194C and G426E both led to minor structural changes. The R460G, R447G and I445M substitutions all perturbed a solvent accessible channel, the so-called H+/H2O channel, leading to the active site. Molecular pathomechanisms of enhanced reactive oxygen species (ROS) generation and impaired binding to multienzyme complexes were also addressed according to the structural data for the relevant mutations. In summary, we present here for the first time a comprehensive study that links three-dimensional structures of disease-causing hE3 variants to residual hLADH activities, altered capacities for ROS generation, compromised affinities for multienzyme complexes and eventually clinical symptoms. Our results may serve as useful starting points for future therapeutic intervention approaches.

An Updated View on the Molecular Pathomechanisms of Human Dihydrolipoamide Dehydrogenase Deficiency in Light of Novel Crystallographic Evidence

Dihydrolipoamide dehydrogenase (LADH, E3) deficiency is a rare (autosomal, recessive) genetic disorder generally presenting with an onset in the neonatal age and early death; the highest carrier rate has been found among Ashkenazi Jews. Acute clinical episodes usually involve severe metabolic decompensation and lactate acidosis that result in neurological, cardiological, and/or hepatological manifestations. Clinical severity is due to the fact that LADH is a common E3 subunit to the alpha-ketoglutarate, pyruvate, alpha-ketoadipate, and branched-chain alpha-keto acid dehydrogenase complexes, and is also a constituent in the glycine cleavage system, thus a loss in LADH function adversely affects multiple key metabolic routes. However, the severe clinical pictures frequently still do not parallel the LADH activity loss, which implies the involvement of auxiliary biochemical mechanisms; enhanced reactive oxygen species generation as well as affinity loss for multienzyme complexes proved to be key auxiliary exacerbating pathomechanisms. This review provides an overview and an up-to-date molecular insight into the pathomechanisms of this disease in light of the structural conclusions drawn from the first crystal structure of a disease-causing hE3 variant determined recently in our laboratory.

Crystal structures of the disease-causing D444V mutant and the relevant wild type human dihydrolipoamide dehydrogenase

We report the crystal structures of the human (dihydro)lipoamide dehydrogenase (hLADH, hE3) and its disease-causing homodimer interface mutant D444V-hE3 at 2.27 and 1.84 Å resolution, respectively. The wild type structure is a unique uncomplexed, unliganded hE3 structure with the true canonical sequence. Based on the structural information a novel molecular pathomechanism is proposed for the impaired catalytic activity and enhanced capacity for reactive oxygen species generation of the pathogenic mutant. The mechanistic model involves a previously much ignored solvent accessible channel leading to the active site that might be perturbed also by other disease-causing homodimer interface substitutions of this enzyme.

Administration of 5-methoxyindole-2-carboxylic acid that potentially targets mitochondrial dihydrolipoamide dehydrogenase confers cerebral preconditioning against ischemic stroke injury

The objective of this study was to investigate a possible role of mitochondrial dihydrolipoamide dehydrogenase (DLDH) as a chemical preconditioning target for neuroprotection against ischemic injury. We used 5-methoxyindole-2-carboxylic acid (MICA), a reportedly reversible DLDH inhibitor, as the preconditioning agent and administered MICA to rats mainly via dietary intake. Upon completion of 4 week's MICA treatment, rats underwent 1h transient ischemia and 24h reperfusion followed by tissue collection. Our results show that MICA protected the brain against ischemic stroke injury as the infarction volume of the brain from the MICA-treated group was significantly smaller than that from the control group. Data were then collected without or with stroke surgery following MICA feeding. It was found that in the absence of stroke following MICA feeding, DLDH activity was lower in the MICA treated group than in the control group, and this decreased activity could be partly due to DLDH protein sulfenation. Moreover, DLDH inhibition by MICA was also found to upregulate the expression of NAD(P)H-ubiquinone oxidoreductase 1(NQO1) via the Nrf2 signaling pathway. In the presence of stroke following MICA feeding, decreased DLDH activity and increased Nrf2 signaling were also observed along with increased NQO1 activity, decreased oxidative stress, decreased cell death, and increased mitochondrial ATP output. We also found that MICA had a delayed preconditioning effect four weeks post MICA treatment. Our study indicates that administration of MICA confers chemical preconditioning and neuroprotection against ischemic stroke injury.

Lipoic acid metabolism and mitochondrial redox regulation

Lipoic acid is an essential cofactor for mitochondrial metabolism and is synthesized de novo using intermediates from mitochondrial fatty-acid synthesis type II, S-adenosylmethionine and iron-sulfur clusters. This cofactor is required for catalysis by multiple mitochondrial 2-ketoacid dehydrogenase complexes, including pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, and branched-chain ketoacid dehydrogenase. Lipoic acid also plays a critical role in stabilizing and regulating these multienzyme complexes. Many of these dehydrogenases are regulated by reactive oxygen species, mediated through the disulfide bond of the prosthetic lipoyl moiety. Collectively, its functions explain why lipoic acid is required for cell growth, mitochondrial activity, and coordination of fuel metabolism.

The human Krebs cycle 2-oxoglutarate dehydrogenase complex creates an additional source of superoxide/hydrogen peroxide from 2-oxoadipate as alternative substrate

Recently, we reported that the human 2-oxoglutarate dehydrogenase (hE1o) component of the 2-oxoglutarate dehydrogenase complex (OGDHc) could produce the reactive oxygen species superoxide and hydrogen peroxide (detected by chemical means) from its substrate 2-oxoglutarate (OG), most likely concurrently with one-electron oxidation by dioxygen of the thiamin diphosphate (ThDP)-derived enamine intermediate to a C2α-centered radical (detected by Electron Paramagnetic Resonance) [Nemeria et al., 2014 [17]; Ambrus et al. 2015 [18]]. We here report that hE1o can also utilize the next higher homologue of OG, 2-oxoadipate (OA) as a substrate according to multiple criteria in our toolbox: (i) Both E1o-specific and overall complex activities (NADH production) were detected using OA as a substrate; (ii) Two post-decarboxylation intermediates were formed by hE1o from OA, the ThDP-enamine and the C2α-hydroxyalkyl-ThDP, with nearly identical rates for OG and OA; (iii) Both OG and OA could reductively acylate lipoyl domain created from dihydrolipoyl succinyltransferase (E2o); (iv) Both OG and OA gave α-ketol carboligaton products with glyoxylate, but with opposite chirality; a finding that could be of utility in chiral synthesis; (v) Dioxygen could oxidize the ThDP-derived enamine from both OG and OA, leading to ThDP-enamine radical and generation of superoxide and H₂O₂. While the observed oxidation-reduction with dioxygen is only a side reaction of the predominant physiological product glutaryl-CoA, the efficiency of superoxide/ H₂O₂ production was 7-times larger from OA than from OG, making the reaction of OGDHc with OA one of the important superoxide/ H₂O₂ producers among 2-oxo acid dehydrogenase complexes in mitochondria.

Human dihydrolipoamide dehydrogenase (E3) deficiency: Novel insights into the structural basis and molecular pathomechanism

This review summarizes our present view on the molecular pathogenesis of human (h) E3-deficiency caused by a variety of genetic alterations with a special emphasis on the moonlighting biochemical phenomena related to the affected (dihydro)lipoamide dehydrogenase (LADH, E3, gene: dld), in particular the generation of reactive oxygen species (ROS). E3-deficiency is a rare autosomal recessive genetic disorder frequently presenting with a neonatal onset and premature death; the highest carrier rate of a single pathogenic dld mutation (1:94-1:110) was found among Ashkenazi Jews. Patients usually die during acute episodes that generally involve severe metabolic decompensation and lactic acidosis leading to neurological, cardiological, and/or hepatological manifestations. The disease owes its severity to the fact that LADH is the common E3 subunit of the alpha-ketoglutarate (KGDHc), pyruvate (PDHc), and branched-chain α-keto acid dehydrogenase complexes and is also part of the glycine cleavage system, hence the malfunctioning of LADH simultaneously incapacitates several central metabolic pathways. Nevertheless, the clinical pictures are usually not unequivocally portrayed through the loss of LADH activities and imply auxiliary mechanisms that exacerbate the symptoms and outcomes of this disorder. Enhanced ROS generation by disease-causing hE3 variants as well as by the E1-E2 subcomplex of the hKGDHc likely contributes to selected pathogeneses of E3-deficiency, which could be targeted by specific drugs or antioxidants; lipoic acid was demonstrated to be a potent inhibitor of ROS generation by hE3 in vitro. Flavin supplementation might prove to be beneficial for those mutations triggering FAD loss in the hE3 component. Selected pathogenic hE3 variants lose their affinity for the E2 component of the hPDHc, a mechanism which warrants scrutiny also for other E3-haboring complexes.

Structural alterations induced by ten disease-causing mutations of human dihydrolipoamide dehydrogenase analyzed by hydrogen/deuterium-exchange mass spectrometry: Implications for the structural basis of E3 deficiency

Pathogenic amino acid substitutions of the common E3 component (hE3) of the human alpha-ketoglutarate dehydrogenase and the pyruvate dehydrogenase complexes lead to severe metabolic diseases (E3 deficiency), which usually manifest themselves in cardiological and/or neurological symptoms and often cause premature death. To date, 14 disease-causing amino acid substitutions of the hE3 component have been reported in the clinical literature. None of the pathogenic protein variants has lent itself to high-resolution structure elucidation by X-ray or NMR. Hence, the structural alterations of the hE3 protein caused by the disease-causing mutations and leading to dysfunction, including the enhanced generation of reactive oxygen species by selected disease-causing variants, could only be speculated. Here we report results of an examination of the effects on the protein structure of ten pathogenic mutations of hE3 using hydrogen/deuterium-exchange mass spectrometry (HDX-MS), a new and state-of-the-art approach of solution structure elucidation. On the basis of the results, putative structural and mechanistic conclusions were drawn regarding the molecular pathogenesis of each disease-causing hE3 mutation addressed in this study.

Physicochemical Profiling of α-Lipoic Acid and Related Compounds

Lipoic acid, the biomolecule of vital importance following glycolysis, shows diversity in its thiol/disulfide equilibria and also in its eight different protonation forms of the reduced molecule. In this paper, lipoic acid, lipoamide, and their dihydro derivatives were studied to quantify their solubility, acid-base, and lipophilicity properties at a submolecular level. The acid-base properties are characterized in terms of six macroscopic, 12 microscopic protonation constants, and three interactivity parameters. The species-specific basicities, the pH-dependent distribution of the microspecies, and lipophilicity parameters are interpreted by various intramolecular effects, and contribute to understanding the antioxidant, chelate-forming, and enzyme cofactor behavior of the molecules observed.

Mitochondrial ROS Metabolism: 10 Years Later

The role of mitochondria in oxidative stress is well recognized, but many questions are still to be answered. This article is intended to update our comprehensive review in 2005 by highlighting the progress in understanding of mitochondrial reactive oxygen species (ROS) metabolism over the past 10 years. We review the recently identified or re-appraised sources of ROS generation in mitochondria, such as p66(shc) protein, succinate dehydrogenase, and recently discovered properties of the mitochondrial antioxidant system. We also reflect upon some controversies, disputes, and misconceptions that confound the field.

Hyperglycemic Stress and Carbon Stress in Diabetic Glucotoxicity

Diabetes and its complications are caused by chronic glucotoxicity driven by persistent hyperglycemia. In this article, we review the mechanisms of diabetic glucotoxicity by focusing mainly on hyperglycemic stress and carbon stress. Mechanisms of hyperglycemic stress include reductive stress or pseudohypoxic stress caused by redox imbalance between NADH and NAD(+) driven by activation of both the polyol pathway and poly ADP ribose polymerase; the hexosamine pathway; the advanced glycation end products pathway; the protein kinase C activation pathway; and the enediol formation pathway. Mechanisms of carbon stress include excess production of acetyl-CoA that can over-acetylate a proteome and excess production of fumarate that can over-succinate a proteome; both of which can increase glucotoxicity in diabetes. For hyperglycemia stress, we also discuss the possible role of mitochondrial complex I in diabetes as this complex, in charge of NAD(+) regeneration, can make more reactive oxygen species (ROS) in the presence of excess NADH. For carbon stress, we also discuss the role of sirtuins in diabetes as they are deacetylases that can reverse protein acetylation thereby attenuating diabetic glucotoxicity and improving glucose metabolism. It is our belief that targeting some of the stress pathways discussed in this article may provide new therapeutic strategies for treatment of diabetes and its complications.

Formation of reactive oxygen species by human and bacterial pyruvate and 2-oxoglutarate dehydrogenase multienzyme complexes reconstituted from recombinant components

Individual recombinant components of pyruvate and 2-oxoglutarate dehydrogenase multienzyme complexes (PDHc, OGDHc) of human and Escherichia coli (E. coli) origin were expressed and purified from E. coli with optimized protocols. The four multienzyme complexes were each reconstituted under optimal conditions at different stoichiometric ratios. Binding stoichiometries for the highest catalytic efficiency were determined from the rate of NADH generation by the complexes at physiological pH. Since some of these complexes were shown to possess 'moonlighting' activities under pathological conditions often accompanied by acidosis, activities were also determined at pH 6.3. As reactive oxygen species (ROS) generation by the E3 component of hOGDHc is a pathologically relevant feature, superoxide generation by the complexes with optimal stoichiometry was measured by the acetylated cytochrome c reduction method in both the forward and the reverse catalytic directions. Various known affectors of physiological activity and ROS production, including Ca(2+), ADP, lipoylation status or pH, were investigated. The human complexes were also reconstituted with the most prevalent human pathological mutant of the E3 component, G194C and characterized; isolated human E3 with the G194C substitution was previously reported to have an enhanced ROS generating capacity. It is demonstrated that: i. PDHc, similarly to OGDHc, is able to generate ROS and this feature is displayed by both the E. coli and human complexes, ii. Reconstituted hPDHc generates ROS at a significantly higher rate as compared to hOGDHc in both the forward and the reverse reactions when ROS generation is calculated for unit mass of their common E3 component, iii. The E1 component or E1-E2 subcomplex generates significant amount of ROS only in hOGDHc; iv. Incorporation of the G194C variant of hE3, the result of a disease-causing mutation, into reconstituted hOGDHc and hPDHc indeed leads to a decreased activity of both complexes and higher ROS generation by only hOGDHc and only in its reverse reaction.

The Interplay Between Respiratory Supercomplexes and ROS in Aging

SIGNIFICANCE

The molecular mechanism of aging is still vigorously debated, although a general consensus exists that mitochondria are significantly involved in this process. However, the previously postulated role of mitochondrial-derived reactive oxygen species (ROS) as the damaging agents inducing functional loss in aging has fallen out of favor in the recent past. In this review, we critically examine the role of ROS in aging in the light of recent advances on the relationship between mitochondrial structure and function.

RECENT ADVANCES

The functional mitochondrial respiratory chain is now recognized as a reflection of the dynamic association of respiratory complexes in the form of supercomplexes (SCs). Besides providing kinetic advantage (channeling), SCs control ROS generation by the respiratory chain, thus providing a means to regulate ROS levels in the cell. Depending on their concentration, these ROS are either physiological signals essential for the life of the cell or toxic species that damage cell structure and functions.

CRITICAL ISSUES

We propose that under physiological conditions the dynamic nature of SCs reversibly controls the generation of ROS as signals involved in mitochondrial-nuclear communication. During aging, there is a progressive loss of control of ROS generation so that their production is irreversibly enhanced, inducing a vicious circle in which signaling is altered and structural damage takes place.

FUTURE DIRECTIONS

A better understanding on the forces affecting SC association would allow the manipulation of ROS generation, directing these species to their physiological signaling role.

Dysfunctional mitochondrial bioenergetics and the pathogenesis of hepatic disorders

The liver is involved in a variety of critical biological functions including the homeostasis of glucose, fatty acids, amino acids, and the synthesis of proteins that are secreted in the blood. It is also at the forefront in the detoxification of noxious metabolites that would otherwise upset the functioning of the body. As such, this vital component of the mammalian system is exposed to a notable quantity of toxicants on a regular basis. It therefore comes as no surprise that there are over a hundred disparate hepatic disorders, encompassing such afflictions as fatty liver disease, hepatitis, and liver cancer. Most if not all of liver functions are dependent on energy, an ingredient that is primarily generated by the mitochondrion, the power house of all cells. This organelle is indispensable in providing adenosine triphosphate (ATP), a key effector of most biological processes. Dysfunctional mitochondria lead to a shortage in ATP, the leakage of deleterious reactive oxygen species (ROS), and the excessive storage of fats. Here we examine how incapacitated mitochondrial bioenergetics triggers the pathogenesis of various hepatic diseases. Exposure of liver cells to detrimental environmental hazards such as oxidative stress, metal toxicity, and various xenobiotics results in the inactivation of crucial mitochondrial enzymes and decreased ATP levels. The contribution of the latter to hepatic disorders and potential therapeutic cues to remedy these conditions are elaborated.

Structural alterations by five disease-causing mutations in the low-pH conformation of human dihydrolipoamide dehydrogenase (hLADH) analyzed by molecular dynamics - Implications in functional loss and modulation of reactive oxygen species generation by pathogenic hLADH forms

Human dihydrolipoamide dehydrogenase (hLADH) is a flavoenzyme component (E3) of the human alpha-ketoglutarate dehydrogenase complex (α-KGDHc) and few other dehydrogenase complexes. Pathogenic mutations of hLADH cause severe metabolic diseases (atypical forms of E3 deficiency) that often escalate to cardiological or neurological presentations and even premature death; the pathologies are generally accompanied by lactic acidosis. hLADH presents a distinct conformation under acidosis (pH 5.5–6.8) with lower physiological activity and the capacity of generating reactive oxygen species (ROS). It has been shown by our laboratory that selected pathogenic mutations, besides lowering the physiological activity of hLADH, significantly stimulate ROS generation by hLADH, especially at lower pH, which might play a role in the pathogenesis of E3-deficiency in respective cases. Previously, we generated by molecular dynamics (MD) simulation the low-pH hLADH structure and analyzed the structural changes induced in this structure by eight of the pathogenic mutations of hLADH. In the absence of high resolution mutant structures these pieces of information are crucial for the mechanistic investigation of the molecular pathogeneses of the hLADH protein. In the present work we analyzed by molecular dynamics simulation the structural changes induced in the low-pH conformation of hLADH by five pathogenic mutations of hLADH; the structures of these disease-causing mutants of hLADH have never been examined before.

•

5 disease-causing mutants of hLADH were subjected to MD to reveal structural changes.

•

MD simulations were carried out both in vacuum and in water supplemented with ions.

•

Functional regions significantly affected by mutation were identified.

•

Implicated residues are to be targeted in mechanistic studies of hLADH dysfunction.

Enhanced hydrogen peroxide generation accompanies the beneficial bioenergetic effects of methylene blue in isolated brain mitochondria

The redox dye methylene blue (MB) is proven to have beneficial effects in various models of neurodegenerative diseases. Here we investigated the effects of MB (100 nM, 300 nM, and 1 μM) on key bioenergetic parameters and on H2O2 production/elimination in isolated guinea pig brain mitochondria under normal as well as respiration-impaired conditions. As measured by high-resolution Oxygraph the rate of resting oxygen consumption was increased, but the ADP-stimulated respiration was unaffected by MB with any of the substrates (glutamate malate, succinate, or α-glycerophosphate) used for supporting mitochondrial respiration. In mitochondria treated with inhibitors of complex I or complex III MB moderately but significantly increased the rate of ATP production, restored ΔΨm, and increased the rate of Ca(2+) uptake. The effects of MB are consistent with transferring electrons from upstream components of the electron transport chain to cytochrome c, which is energetically favorable when the flow of electrons in the respiratory chain is compromised. On the other hand, MB significantly increased the production of H2O2 measured by Amplex UltraRed fluorimetry under all conditions, in resting, ATP-synthesizing, and respiration-impaired mitochondria, with each substrate combination supporting respiration. Furthermore, it also decreased the elimination of H2O2. Generation of H2O2 without superoxide formation, observed in the presence of MB, is interpreted as a result of reduction of molecular oxygen to H2O2 by the reduced MB. The elevated generation and impaired elimination of H2O2 should be considered for the overall oxidative state of mitochondria treated with MB.

S-glutathionylation reactions in mitochondrial function and disease

Mitochondria are highly efficient energy-transforming organelles that convert energy stored in nutrients into ATP. The production of ATP by mitochondria is dependent on oxidation of nutrients and coupling of exergonic electron transfer reactions to the genesis of transmembrane electrochemical potential of protons. Electrons can also prematurely “spin-off” from prosthetic groups in Krebs cycle enzymes and respiratory complexes and univalently reduce di-oxygen to generate reactive oxygen species (ROS) superoxide (O₂•⁻) and hydrogen peroxide (H₂O₂), important signaling molecules that can be toxic at high concentrations. Production of ATP and ROS are intimately linked by the respiratory chain and the genesis of one or the other inherently depends on the metabolic state of mitochondria. Various control mechanisms converge on mitochondria to adjust ATP and ROS output in response to changing cellular demands. One control mechanism that has gained a high amount of attention recently is S-glutathionylation, a redox sensitive covalent modification that involves formation of a disulfide bridge between glutathione and an available protein cysteine thiol. A number of S-glutathionylation targets have been identified in mitochondria. It has also been established that S-glutathionylation reactions in mitochondria are mediated by the thiol oxidoreductase glutaredoxin-2 (Grx2). In the following review, emerging knowledge on S-glutathionylation reactions and its importance in modulating mitochondrial ATP and ROS production will be discussed. Major focus will be placed on Complex I of the respiratory chain since (1) it is a target for reversible S-glutathionylation by Grx2 and (2) deregulation of Complex I S-glutathionylation is associated with development of various disease states particularly heart disease. Other mitochondrial enzymes and how their S-glutathionylation profile is affected in different disease states will also be discussed.

Human 2-oxoglutarate dehydrogenase complex E1 component forms a thiamin-derived radical by aerobic oxidation of the enamine intermediate

Herein are reported unique properties of the human 2-oxoglutarate dehydrogenase multienzyme complex (OGDHc), a rate-limiting enzyme in the Krebs (citric acid) cycle. (a) Functionally competent 2-oxoglutarate dehydrogenase (E1o-h) and dihydrolipoyl succinyltransferase components have been expressed according to kinetic and spectroscopic evidence. (b) A stable free radical, consistent with the C2-(C2α-hydroxy)-γ-carboxypropylidene thiamin diphosphate (ThDP) cation radical was detected by electron spin resonance upon reaction of the E1o-h with 2-oxoglutarate (OG) by itself or when assembled from individual components into OGDHc. (c) An unusual stability of the E1o-h-bound C2-(2α-hydroxy)-γ-carboxypropylidene thiamin diphosphate (the "ThDP-enamine"/C2α-carbanion, the first postdecarboxylation intermediate) was observed, probably stabilized by the 5-carboxyl group of OG, not reported before. (d) The reaction of OG with the E1o-h gave rise to superoxide anion and hydrogen peroxide (reactive oxygen species (ROS)). (e) The relatively stable enzyme-bound enamine is the likely substrate for oxidation by O2, leading to the superoxide anion radical (in d) and the radical (in b). (f) The specific activity assessed for ROS formation compared with the NADH (overall complex) activity, as well as the fraction of radical intermediate occupying active centers of E1o-h are consistent with each other and indicate that radical/ROS formation is an "off-pathway" side reaction comprising less than 1% of the "on-pathway" reactivity. However, the nearly ubiquitous presence of OGDHc in human tissues, including the brain, makes these findings of considerable importance in human metabolism and perhaps disease.

Protein redox modification as a cellular defense mechanism against tissue ischemic injury

Protein oxidative or redox modifications induced by reactive oxygen species (ROS) or reactive nitrogen species (RNS) not only can impair protein function, but also can regulate and expand protein function under a variety of stressful conditions. Protein oxidative modifications can generally be classified into two categories: irreversible oxidation and reversible oxidation. While irreversible oxidation usually leads to protein aggregation and degradation, reversible oxidation that usually occurs on protein cysteine residues can often serve as an “on and off” switch that regulates protein function and redox signaling pathways upon stress challenges. In the context of ischemic tolerance, including preconditioning and postconditioning, increasing evidence has indicated that reversible cysteine redox modifications such as S-sulfonation, S-nitrosylation, S-glutathionylation, and disulfide bond formation can serve as a cellular defense mechanism against tissue ischemic injury. In this review, I highlight evidence of cysteine redox modifications as protective measures in ischemic injury, demonstrating that protein redox modifications can serve as a therapeutic target for attenuating tissue ischemic injury. Prospectively, more oxidatively modified proteins will need to be identified that can play protective roles in tissue ischemic injury, in particular, when the oxidative modifications of such identified proteins can be enhanced by pharmacological agents or drugs that are available or to be developed.

A strategically designed small molecule attacks alpha-ketoglutarate dehydrogenase in tumor cells through a redox process

Background

Targeting cancer cell metabolism is recognized as a promising arena for development of cancer chemotherapeutics. Moreover, redox metabolism is also systematically altered in tumor cells. Indeed, there is growing reason to believe that tumor-specific alteration of redox control of metabolism will be central to understanding and attacking malignancy. We report here that lipoate analog CPI-613 attacks a gate-keeping, lipoate-using metabolic enzyme, alpha-ketoglutarate dehydrogenase (KGDH), by a redox mechanism selectively in tumors cells.

Results

CPI-613 inhibited KGDH function strongly and rapidly, selectively in tumor cells. Moreover, CPI-613 induced a correspondingly rapid, powerful redox signal in tumor cell mitochondria. This signal was associated with redox modification of KGDH (including extensive enzyme glutathionylation and redox blockage of enzyme lipoate sulfhydryls), correlating with KGDH inactivation. The source of this tumor-specific mitochondrial redox modulatory signal was not electron transport complexes (I or III), but was largely or entirely the E3 (dihydrolipoamide dehydrogenase) component of dehydrogenases, including KGDH. Finally, we demonstrated that KGDH activity was redox regulated (in tumor cells), as expected if a tumor-specific redox process (auto)regulates KGDH.

Conclusions

Our data demonstrate that lipoate analog CPI-613 attacks redox control of KGDH activity in tumor cells, perhaps by modulation of an existing lipoate-sensitive allosteric process normally governing tumor cell KGDH activity. Together with its previously reported, mechanistically distinct (non-redox) effects on the other major, lipoate-using mitochondrial metabolic enzyme, pyruvate dehydrogenase, CPI-613’s KGDH effects indicate that this agent simultaneously attacks multiple central, essential components of tumor cell metabolic regulation.

Positive oxidative stress in aging and aging-related disease tolerance

It is now well established that reactive oxygen species (ROS), reactive nitrogen species (RNS), and a basal level of oxidative stress are essential for cell survival. It is also well known that while severe oxidative stress often leads to widespread oxidative damage and cell death, a moderate level of oxidative stress, induced by a variety of stressors, can yield great beneficial effects on adaptive cellular responses to pathological challenges in aging and aging-associated disease tolerance such as ischemia tolerance. Here in this review, I term this moderate level of oxidative stress as positive oxidative stress, which usually involves imprinting molecular signatures on lipids and proteins via formation of lipid peroxidation by-products and protein oxidation adducts. As ROS/RNS are short-lived molecules, these molecular signatures can thus execute the ultimate function of ROS/RNS. Representative examples of lipid peroxidation products and protein oxidation adducts are presented to illustrate the role of positive oxidative stress in a variety of pathological settings, demonstrating that positive oxidative stress could be a valuable prophylactic and/or therapeutic approach targeting aging and aging-associated diseases.

Molecular dynamics study of the structural basis of dysfunction and the modulation of reactive oxygen species generation by pathogenic mutants of human dihydrolipoamide dehydrogenase

Human dihydrolipoamide dehydrogenase (LADH, E3) is a component in the pyruvate-, alpha-ketoglutarate- and branched-chain ketoacid dehydrogenase complexes and in the glycine cleavage system. The pathogenic mutations of LADH cause severe metabolic disturbances, called E3 deficiency that often involve cardiological and neurological symptoms and premature death. Our laboratory has recently shown that some of the known pathogenic mutations augment the reactive oxygen species (ROS) generation capacity of LADH, which may contribute to the clinical presentations. A recent report concluded that elevated oxidative stress generated by the above mutants turns the lipoic acid cofactor on the E2 subunits dysfunctional. In the present contribution we generated by molecular dynamics (MD) simulation the conformation of LADH that is proposed to be compatible with ROS generation. We propose here for the first time the structural changes, which are likely to turn the physiological LADH conformation to its ROS-generating conformation. We also created nine of the pathogenic mutants of the ROS-generating conformation and again used MD simulation to detect structural changes that the mutations induced in this LADH conformation. We propose the structural changes that may lead to the modulation in ROS generation of LADH by the pathogenic mutations.

An update on the role of mitochondrial α-ketoglutarate dehydrogenase in oxidative stress

The activity of mitochondrial alpha-ketoglutarate dehydrogenase complex (KGDHC) is severely reduced in human pathologies where oxidative stress is traditionally thought to play an important role, such as familial and sporadic forms of Alzheimer's disease and other age-related neurodegenerative diseases. This minireview is focused on substantial data that were accumulated over the last 2 decades to support the concept that KGDHC can be a primary mitochondrial target of oxidative stress and at the same time a key contributor to it by producing reactive oxygen species. This article is part of a Special Issue entitled 'Mitochondrial function and dysfunction in neurodegeneration'.

Oxidative stress underlying axonal degeneration in adrenoleukodystrophy: a paradigm for multifactorial neurodegenerative diseases?

X-linked adrenoleukodystrophy (X-ALD) is an inherited neurodegenerative disorder expressed as four disease variants characterized by adrenal insufficiency and graded damage in the nervous system. X-ALD is caused by a loss of function of the peroxisomal ABCD1 fatty-acid transporter, resulting in the accumulation of very long chain fatty acids (VLCFA) in the organs and plasma, which have potentially toxic effects in CNS and adrenal glands. We have recently shown that treatment with a combination of antioxidants containing α-tocopherol, N-acetyl-cysteine and α-lipoic acid reversed oxidative damage and energetic failure, together with the axonal degeneration and locomotor impairment displayed by Abcd1 null mice, the animal model of X-ALD. This is the first direct demonstration that oxidative stress, which is a hallmark not only of X-ALD, but also of other neurodegenerative processes, such as Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD), contributes to axonal damage. The purpose of this review is, first, to discuss the molecular and cellular underpinnings of VLCFA-induced oxidative stress, and how it interacts with energy metabolism and/or inflammation to generate a complex syndrome wherein multiple factors are contributing. Particular attention will be paid to the dysregulation of redox homeostasis by the interplay between peroxisomes and mitochondria. Second, we will extend this analysis to the aforementioned neurodegenerative diseases with the aim of defining differences as well as the existence of a core pathogenic mechanism that would justify the exchange of therapeutic opportunities among these pathologies.

Mutations in the dimer interface of dihydrolipoamide dehydrogenase promote site-specific oxidative damages in yeast and human cells

Dihydrolipoamide dehydrogenase (DLD) is a multifunctional protein well characterized as the E3 component of the pyruvate dehydrogenase and α-ketoglutarate dehydrogenase complexes. Previously, conditions predicted to destabilize the DLD dimer revealed that DLD could also function as a diaphorase and serine protease. However, the relevance of these cryptic activities remained undefined. We analyzed human DLD mutations linked to strikingly different clinical phenotypes, including E340K, D444V, R447G, and R460G in the dimer interface domain that are responsible for severe multisystem disorders of infancy and G194C in the NAD(+)-binding domain that is typically associated with milder presentations. In vitro, all of these mutations decreased to various degrees dihydrolipoamide dehydrogenase activity, whereas dimer interface mutations also enhanced proteolytic and/or diaphorase activity. Human DLD proteins carrying each individual mutation complemented fully the respiratory-deficient phenotype of yeast cells lacking endogenous DLD even when residual dihydrolipoamide dehydrogenase activity was as low as 21% of controls. However, under elevated oxidative stress, expression of DLD proteins with dimer interface mutations greatly accelerated the loss of respiratory function, resulting from enhanced oxidative damage to the lipoic acid cofactor of pyruvate dehydrogenase and α-ketoglutarate dehydrogenase and other mitochondrial targets. This effect was not observed with the G194C mutation or a mutation that disrupts the proteolytic active site of DLD. As in yeast, lipoic acid cofactor was damaged in human D444V-homozygous fibroblasts after exposure to oxidative stress. We conclude that the cryptic activities of DLD promote oxidative damage to neighboring molecules and thus contribute to the clinical severity of DLD mutations.

Oxidative damage compromises energy metabolism in the axonal degeneration mouse model of X-adrenoleukodystrophy

AIMS

Chronic metabolic impairment and oxidative stress are associated with the pathogenesis of axonal dysfunction in a growing number of neurodegenerative conditions. To investigate the intertwining of both noxious factors, we have chosen the mouse model of adrenoleukodystrophy (X-ALD), which exhibits axonal degeneration in spinal cords and motor disability. The disease is caused by loss of function of the ABCD1 transporter, involved in the import and degradation of very long-chain fatty acids (VLCFA) in peroxisomes. Oxidative stress due to VLCFA excess appears early in the neurodegenerative cascade.

RESULTS

In this study, we demonstrate by redox proteomics that oxidative damage to proteins specifically affects five key enzymes of glycolysis and TCA (Tricarboxylic acid) cycle in spinal cords of Abcd1(-) mice and pyruvate kinase in human X-ALD fibroblasts. We also show that NADH and ATP levels are significantly diminished in these samples, together with decrease of pyruvate kinase activities and GSH levels, and increase of NADPH.

INNOVATION

Treating Abcd1(-) mice with the antioxidants N-acetylcysteine and α-lipoic acid (LA) prevents protein oxidation; preserves NADH, NADPH, ATP, and GSH levels; and normalizes pyruvate kinase activity, which implies that oxidative stress provoked by VLCFA results in bioenergetic dysfunction, at a presymptomatic stage.

CONCLUSION

Our results provide mechanistic insight into the beneficial effects of antioxidants and enhance the rationale for translation into clinical trials for X-adrenoleukodystrophy.

Evidence for two sites of superoxide production by mitochondrial NADH-ubiquinone oxidoreductase (complex I)

Complex I (NADH-ubiquinone oxidoreductase) can form superoxide during forward electron flow (NADH-oxidizing) or, at sufficiently high protonmotive force, during reverse electron transport from the ubiquinone (Q) pool (NAD(+)-reducing). We designed an assay system to allow titration of the redox state of the superoxide-generating site during reverse electron transport in rat skeletal muscle mitochondria: a protonmotive force generated by ATP hydrolysis, succinate:malonate to alter electron supply and modulate the redox state of the Q pool, and inhibition of complex III to prevent QH(2) oxidation via the Q cycle. Stepwise oxidation of the QH(2)/Q pool by increasing malonate concentration slowed the rates of both reverse electron transport and rotenone-sensitive superoxide production by complex I. However, the superoxide production rate was not uniquely related to the resultant potential of the NADH/NAD(+) redox couple. Thus, there is a superoxide producer during reverse electron transport at complex I that responds to Q pool redox state and is not in equilibrium with the NAD reduction state. In contrast, superoxide production during forward electron transport in the presence of rotenone was uniquely related to NAD redox state. These results support a two-site model of complex I superoxide production; one site in equilibrium with the NAD pool, presumably the flavin of the FMN moiety (site I(F)) and the other dependent not only on NAD redox state, but also on protonmotive force and the reduction state of the Q pool, presumably a semiquinone in the Q-binding site (site I(Q)).

In situ kinetic trapping reveals a fingerprint of reversible protein thiol oxidation in the mitochondrial matrix

Reactive oxygen species (ROS) are released at the mitochondrial inner membrane by the electron transport chain (ETC). Increasing evidence suggests that mitochondrial H2O2 acts as a signaling molecule and participates in the (feedback) regulation of mitochondrial activity and turnover. It seems likely that key mitochondrial components contain redox-sensitive thiols that help to adapt protein function to changes in electron flow. However, the identity of most redox-regulated mitochondrial proteins remains to be defined. Thioredoxin 2 (Trx2) is the major protein-thiol-reducing oxidoreductase in the mitochondrial matrix. We used in situ mechanism-based kinetic trapping to identify disulfide-exchange interactions of Trx2 within functional mitochondria of intact cells. Mass spectrometry successfully identified known and suspected Trx2 target proteins and, in addition, revealed a set of new candidate target proteins. Our results suggest that the mitochondrial protein biosynthesis machinery is a major target of ETC-derived ROS. In particular, we identified mitochondrial methionyl-tRNA synthetase (mtMetRS) as one of the most prominent Trx2 target proteins. We show that an increase in ETC-derived oxidants leads to an increase in mtMetRS oxidation in intact cells. In conclusion, we find that in situ kinetic trapping provides starting points for future functional studies of intramitochondrial redox regulation.

Stimulation of reactive oxygen species generation by disease-causing mutations of lipoamide dehydrogenase

We investigated pathogenic mutations relevant in dihydrolipoamide dehydrogenase (LADH; gene: Dld) deficiency, a severe human disease, to elucidate how they alter reactive oxygen species (ROS) generation and associated biophysical characteristics of LADH. Twelve known disease-causing mutants of human LADH have been expressed and purified to homogeneity from E. coli. Detailed biophysical and biochemical characterization of the mutants has been performed applying circular dichroism (CD) spectroscopy, nano-spray mass spectrometry (MS), calibrated gel filtration and flavin adenine dinucleotide-content analysis. Functional analyses revealed that four of the pathogenic mutations significantly stimulated the ROS-generating activity of LADH and also increased its sensitivity to an acidic shift in pH. LADH activity was reduced by variable extents in the mutants exhibiting excessive ROS generation. It is remarkable that in the P453L mutant, enzyme activity was nearly completely lost with a ROS-forming activity becoming dominant, whereas the G194C mutation, common among Ashkenazi Jews, resulted in no alteration in LADH activity but a gain in the ROS-generating activity. There have been neither major conformational alterations nor monomerization of the functional homodimer of LADH associated with the higher ROS-generating capacity as measured by CD spectroscopy and size-exclusion chromatography combined with nano-spray MS, respectively. The excessive ROS generation of selected LADH mutants could be an important factor in the pathology and clinical presentation of human LADH deficiency and raises the possibility of an antioxidant therapy in the treatment of this condition.

Mitochondrial ROS generation and its regulation: mechanisms involved in H(2)O(2) signaling

Mitochondria are the main source of reactive oxygen species in the cell. These reactive oxygen species have long been known as being involved in oxidative stress. This is a review of the mechanisms involved in reactive oxygen species generation by the respiratory chain and some of the dehydrogenases and the control by thermodynamic and kinetic constraints. Mitochondrial ROS produced at the level of the bc1 complex as well at the level of complex I are discussed. It was recognized more than a decade ago that they can also function as signaling molecules. This signaling role will be developed both in terms of mechanism and in terms of mitochondrial ROS signaling. The notion that hydrogen peroxide acts not only as a damaging oxidant but also as a signaling molecule was proposed more than a decade ago. Hydrogen peroxide signaling can be either direct (oxidation of its target) or indirect (involving peroxiredoxins, for example). The consequences of ROS signaling on crucial biologic processes such as cell proliferation and differentiation are discussed.

Metabolic networks to combat oxidative stress in Pseudomonas fluorescens

Oxidative stress is an unavoidable peril that aerobic organisms have to confront. Thus, it is not surprising that intricate strategies are deployed in an effort to fend the dangers associated with living in an O(2) environment. In the classical models of anti-oxidative defense mechanisms, a variety of stratagems including the reactive oxygen species (ROS) scavenging systems, the NADPH-generating enzymes and the DNA repair machineries are highlighted. However, it is becoming increasingly clear that metabolism may be intimately involved in anti-oxidative defence. Recent data show that metabolic reprogramming plays a pivotal role in the survival of organisms exposed to oxidative stress. Here, we describe how Pseudomonas fluorescens, the metabolically-versatile soil microbe, manipulates its metabolic networks in an effort to counter oxidative stress. An intricate link between metabolism and anti-oxidative defense is presented. P. fluorescens reconfigures its metabolic processes in an effort to satisfy its need for NADPH during oxidative insult. Seemingly, disparate metabolic modules appear to partner together to concomitantly fine-tune the levels of the anti-oxidant NADPH and the pro-oxidant NADH. Central to this shift in the metabolic production of the pyridine nucleotides is the increase in NAD kinase with the concomitant decrease in NADP phosphatase. The tricarboxylic acid cycle is tweaked in an effort to limit the formation of NADH. This metabolic redox-balancing act appears to afford a potent tool against oxidative challenge and may be a more widespread ROS-combating tactic than hitherto recognized.

α-Ketoglutarate dehydrogenase: a mitochondrial redox sensor

α-Ketoglutarate dehydrogenase (KGDH), a key regulatory enzyme within the Krebs cycle, is sensitive to mitochondrial redox status. Treatment of mitochondria with H₂O₂ results in reversible inhibition of KGDH due to glutathionylation of the cofactor, lipoic acid. Upon consumption of H₂O₂, glutathione is removed by glutaredoxin restoring KGDH activity. Glutathionylation appears to be enzymatically catalysed or require a unique microenvironment. This may represent an antioxidant response, diminishing the flow of electrons to the respiratory chain and protecting sulphydryl residues from oxidative damage. KGDH is, however, also susceptible to oxidative damage. 4-Hydroxy-2-nonenal (HNE), a lipid peroxidation product, reacts with lipoic acid resulting in enzyme inactivation. Evidence indicates that HNE modified lipoic acid is cleaved from KGDH, potentially the first step of a repair process. KGDH is therefore a likely redox sensor, reversibly altering metabolism to reduce oxidative damage and, under severe oxidative stress, acting as a sentinel of mitochondrial viability.