Inactivation of the 2-oxo acid dehydrogenase complexes upon generation of intrinsic radical species

The emerging importance of the α-keto acid dehydrogenase complexes in serving as intracellular and intercellular signaling platforms for the regulation of metabolism

The α-keto acid dehydrogenase complex (KDHc) class of mitochondrial enzymes is composed of four members: pyruvate dehydrogenase (PDHc), α-ketoglutarate dehydrogenase (KGDHc), branched-chain keto acid dehydrogenase (BCKDHc), and 2-oxoadipate dehydrogenase (OADHc). These enzyme complexes occupy critical metabolic intersections that connect monosaccharide, amino acid, and fatty acid metabolism to Krebs cycle flux and oxidative phosphorylation (OxPhos). This feature also imbues KDHc enzymes with the heightened capacity to serve as platforms for propagation of intracellular and intercellular signaling. KDHc enzymes serve as a source and sink for mitochondrial hydrogen peroxide (mtH2O2), a vital second messenger used to trigger oxidative eustress pathways. Notably, deactivation of KDHc enzymes through reversible oxidation by mtH2O2 and other electrophiles modulates the availability of several Krebs cycle intermediates and related metabolites which serve as powerful intracellular and intercellular messengers. The KDHc enzymes also play important roles in the modulation of mitochondrial metabolism and epigenetic programming in the nucleus through the provision of various acyl-CoAs, which are used to acylate proteinaceous lysine residues. Intriguingly, nucleosomal control by acylation is also achieved through PDHc and KGDHc localization to the nuclear lumen. In this review, I discuss emerging concepts in the signaling roles fulfilled by the KDHc complexes. I highlight their vital function in serving as mitochondrial redox sensors and how this function can be used by cells to regulate the availability of critical metabolites required in cell signaling. Coupled with this, I describe in detail how defects in KDHc function can cause disease states through the disruption of cell redox homeodynamics and the deregulation of metabolic signaling. Finally, I propose that the intracellular and intercellular signaling functions of the KDHc enzymes are controlled through the reversible redox modification of the vicinal lipoic acid thiols in the E2 subunit of the complexes.

Fatty acid oxidation drives mitochondrial hydrogen peroxide production by α-ketoglutarate dehydrogenase

In the present study, we examined the mitochondrial hydrogen peroxide (mH2O2) generating capacity of α-ketoglutarate dehydrogenase (KGDH) and compared it to components of the electron transport chain using liver mitochondria isolated from male and female C57BL6N mice. We show for the first time there are some sex dimorphisms in the production of mH2O2 by electron transport chain complexes I and III when mitochondria are fueled with different substrates. However, in our investigations into these sex effects, we made the unexpected and compelling discovery that 1) KGDH serves as a major mH2O2 supplier in male and female liver mitochondria and 2) KGDH can form mH2O2 when liver mitochondria are energized with fatty acids but only when malate is used to prime the Krebs cycle. Surprisingly, 2-keto-3-methylvaleric acid (KMV), a site-specific inhibitor for KGDH, nearly abolished mH2O2 generation in both male and female liver mitochondria oxidizing palmitoyl-carnitine. KMV inhibited mH2O2 production in liver mitochondria from male and female mice oxidizing myristoyl-, octanoyl-, or butyryl-carnitine as well. S1QEL 1.1 (S1) and S3QEL 2 (S3), compounds that inhibit reactive oxygen species generation by complexes I and III, respectively, without interfering with OxPhos and respiration, had a negligible effect on the rate of mH2O2 production when pyruvate or acyl-carnitines were used as fuels. However, inclusion of KMV in reaction mixtures containing S1 and/or S3 almost abolished mH2O2 generation. Together, our findings suggest KGDH is the main mH2O2 generator in liver mitochondria, even when fatty acids are used as fuel.

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.

Reactive nitrogen species inhibit branched chain alpha-ketoacid dehydrogenase complex and impact muscle cell metabolism

Branched chain α-ketoacid dehydrogenase complex (BCKDC) is the rate-limiting enzyme in branched chain amino acid (BCAA) catabolism, a metabolic pathway with great importance for human health. BCKDC belongs to the mitochondrial α-ketoacid dehydrogenase complex family, which also includes pyruvate dehydrogenase complex and oxoglutarate dehydrogenase complex. Here, we revealed that BCKDC can be substantially inhibited by reactive nitrogen species (RNS) via a mechanism similar to what we recently discovered with pyruvate dehydrogenase complex and oxoglutarate dehydrogenase complex-RNS can cause inactivating covalent modifications of the lipoic arm on its E2 subunit. In addition, we showed that such reaction between RNS and the lipoic arm of the E2 subunit can further promote inhibition of the E3 subunits of α-ketoacid dehydrogenase complexes. We examined the impacts of this RNS-mediated BCKDC inhibition in muscle cells, an important site of BCAA metabolism, and demonstrated that the nitric oxide production induced by cytokine stimulation leads to a strong inhibition of BCKDC activity and BCAA oxidation in myotubes and myoblasts. More broadly, nitric oxide production reduced the level of functional lipoic arms across the multiple α-ketoacid dehydrogenases and led to intracellular accumulation of their substrates (α-ketoacids), decrease of their products (acyl-CoAs), and a lower cellular energy charge. In sum, this work revealed a new mechanism for BCKDC regulation, demonstrated that RNS can generally inhibit all α-ketoacid dehydrogenases, which has broad physiological implications across multiple cell types, and elucidated the mechanistic connection between RNS-driven inhibitory modifications on the E2 and E3 subunits of α-ketoacid dehydrogenases.

Reactive nitrogen species inhibit branched chain alpha-ketoacid dehydrogenase complex and impact muscle cell metabolism

Branched chain α-ketoacid dehydrogenase complex (BCKDC) is the rate limiting enzyme in branched chain amino acid (BCAA) catabolism, a metabolic pathway with great importance for human health. BCKDC belongs to the mitochondrial α-ketoacid dehydrogenase complex family, which also includes pyruvate dehydrogenase complex (PDHC) and oxoglutarate dehydrogenase complex (OGDC). Here we revealed that BCKDC can be substantially inhibited by reactive nitrogen species (RNS) via a mechanism similar to what we recently discovered with PDHC and OGDC - modifying the lipoic arm on its E2 subunit. In addition, we showed that such reaction between RNS and the lipoic arm of the E2 subunit can further promote inhibition of the E3 subunits of α-ketoacid dehydrogenase complexes. We examined the impacts of this RNS-mediated BCKDC inhibition in muscle cells, an important site of BCAA metabolism, and demonstrated that the nitric oxide production induced by cytokine stimulation leads to a strong inhibition of BCKDC activity and BCAA oxidation in myotubes and myoblasts. More broadly, nitric oxide production reduced the level of functional lipoic arms across the multiple α-ketoacid dehydrogenases and led to intracellular accumulation of their substrates (α-ketoacids), reduction of their products (acyl-CoAs), and a lower cellular energy charge. This work revealed a new mechanism for BCKDC regulation, demonstrated its biological significance, and elucidated the mechanistic connection between RNS-driven inhibitory modifications on the E2 and E3 subunits of α-ketoacid dehydrogenases. Together with previous work, we revealed a general mechanism for RNS to inhibit all α-ketoacid dehydrogenases, which has numerous physiological implications across multiple cell types.

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.

OXIDATIVE STRESS AND REPRODUCTIVE FUNCTION: Oxidative stress and in vitro ageing of the post-ovulatory oocyte: an update on recent advances in the field

In brief

Post-ovulatory ageing of oocytes leads to poor oocyte and embryo quality as well as abnormalities in offspring. This review provides an update on the contributions of oxidative stress to this process and discusses the current literature surrounding the use of antioxidant media to delay post-ovulatory oocyte ageing.

Abstract

Following ovulation, the metaphase II stage oocyte has a limited functional lifespan before succumbing to a process known as post-ovulatory oocyte ageing. This progressive demise occurs both in vivo and in vitro and is accompanied by a deterioration in oocyte quality, leading to a well-defined sequelae of reduced fertilisation rates, poor embryo quality, post-implantation errors, and abnormalities in the offspring. Although the physiological consequences of post-ovulatory oocyte ageing have largely been characterised, less is known regarding the molecular mechanisms that drive this process. This review presents an update on the established relationships between the biochemical changes exhibited by the ageing oocyte and the myriad of symptoms associated with the ageing phenotype. In doing so, we consider the molecular events that are potentially involved in orchestrating post-ovulatory ageing with a particular focus on the role of oxidative stress. We highlight the mounting evidence that oxidative stress acts as an initiator for a cascade of events that create the aged oocyte phenotype. Specifically, oxidative stress has the capacity to disrupt mitochondrial function and directly damage multiple intracellular components of the oocyte such as lipids, proteins, and DNA. Finally, this review addresses emerging strategies for delaying post-ovulatory oocyte ageing with emphasis placed on the promise afforded by the use of selected antioxidants to guide the development of media tailored for the preservation of oocyte integrity during in vitro fertilisation procedures.

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.

Reverse and Forward Electron Flow-Induced H2O2 Formation Is Decreased in α-Ketoglutarate Dehydrogenase (α-KGDH) Subunit (E2 or E3) Heterozygote Knock Out Animals

α-ketoglutarate dehydrogenase complex (KGDHc), or 2-oxoglutarate dehydrogenase complex (OGDHc) is a rate-limiting enzyme in the tricarboxylic acid cycle, that has been identified in neurodegenerative diseases such as in Alzheimer’s disease. The aim of the present study was to establish the role of the KGDHc and its subunits in the bioenergetics and reactive oxygen species (ROS) homeostasis of brain mitochondria. To study the bioenergetic profile of KGDHc, genetically modified mouse strains were used having a heterozygous knock out (KO) either in the dihydrolipoyl succinyltransferase (DLST^+/−) or in the dihydrolipoyl dehydrogenase (DLD^+/−) subunit. Mitochondrial oxygen consumption, hydrogen peroxide (H₂O₂) production, and expression of antioxidant enzymes were measured in isolated mouse brain mitochondria. Here, we demonstrate that the ADP-stimulated respiration of mitochondria was partially arrested in the transgenic animals when utilizing α-ketoglutarate (α-KG or 2-OG) as a fuel substrate. Succinate and α-glycerophosphate (α-GP), however, did not show this effect. The H₂O₂ production in mitochondria energized with α-KG was decreased after inhibiting the adenine nucleotide translocase and Complex I (CI) in the transgenic strains compared to the controls. Similarly, the reverse electron transfer (RET)-evoked H₂O₂ formation supported by succinate or α-GP were inhibited in mitochondria isolated from the transgenic animals. The decrease of RET-evoked ROS production by DLST^+/− or DLD^+/− KO-s puts the emphasis of the KGDHc in the pathomechanism of ischemia-reperfusion evoked oxidative stress. Supporting this notion, expression of the antioxidant enzyme glutathione peroxidase was also decreased in the KGDHc transgenic animals suggesting the attenuation of ROS-producing characteristics of KGDHc. These findings confirm the contribution of the KGDHc to the mitochondrial ROS production and in the pathomechanism of ischemia-reperfusion injury.

Oxidative Stress and Redox Signaling in the Pathophysiology of Liver Diseases

The increased production of derivatives of molecular oxygen and nitrogen in the form of reactive oxygen species (ROS) and reactive nitrogen species (RNS) lead to molecular damage called oxidative stress. Under normal physiological conditions, the ROS generation is tightly regulated in different cells and cellular compartments. Any disturbance in the balance between the cellular generation of ROS and antioxidant balance leads to oxidative stress. In this article, we discuss the sources of ROS (endogenous and exogenous) and antioxidant mechanisms. We also focus on the pathophysiological significance of oxidative stress in various cell types of the liver. Oxidative stress is implicated in the development and progression of various liver diseases. We narrate the master regulators of ROS-mediated signaling and their contribution to liver diseases. Nonalcoholic fatty liver diseases (NAFLD) are influenced by a "multiple parallel-hit model" in which oxidative stress plays a central role. We highlight the recent findings on the role of oxidative stress in the spectrum of NAFLD, including fibrosis and liver cancer. Finally, we provide a brief overview of oxidative stress biomarkers and their therapeutic applications in various liver-related disorders. Overall, the article sheds light on the significance of oxidative stress in the pathophysiology of the liver. © 2022 American Physiological Society. Compr Physiol 12:3167-3192, 2022.

Identifying Site-Specific Superoxide and Hydrogen Peroxide Production Rates From the Mitochondrial Electron Transport System Using a Computational Strategy

Mitochondrial reactive oxygen species (ROS) play important roles in cellular signaling; however, certain pathological conditions such as ischemia/reperfusion (I/R) injury disrupt ROS homeostasis and contribute to cell death. A major impediment to developing therapeutic measures against oxidative stress-induced cellular damage is the lack of a quantitative framework to identify the specific sources and regulatory mechanisms of mitochondrial ROS production. We developed a thermodynamically consistent, mass-and-charge balanced, kinetic model of mitochondrial ROS homeostasis focused on redox sites of electron transport chain complexes I, II, and III. The model was calibrated and corroborated using comprehensive data sets relevant to ROS homeostasis. The model predicts that complex I ROS production dominates other sources under conditions favoring a high membrane potential with elevated nicotinamide adenine dinucleotide (NADH) and ubiquinol (QH₂) levels. In general, complex I contributes to significant levels of ROS production under pathological conditions, while complexes II and III are responsible for basal levels of ROS production, especially when QH₂ levels are elevated. The model also reveals that hydrogen peroxide production by complex I underlies the non-linear relationship between ROS emission and O₂ at low O₂ concentrations. Lastly, the model highlights the need to quantify scavenging system activity under different conditions to establish a complete picture of mitochondrial ROS homeostasis. In summary, we describe the individual contributions of the electron transport system complex redox sites to total ROS emission in mitochondria respiring under various combinations of NADH- and Q-linked respiratory fuels under varying workloads.

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.

Reactive Oxygen Species in intestinal stem cell metabolism, fate and function

Long dismissed as merely harmful respiratory by-products, Reactive Oxygen Species (ROS) have emerged as critical intracellular messengers during cell growth and differentiation. ROS's signaling roles are particularly prominent within the intestine, whose high regenerative capacity is maintained by Intestinal Stem Cells (ISCs). In this review, we outline roles for ROS in ISCs as revealed by studies using Drosophila and mouse model systems. We focus particularly on recent studies highlighting how ROS ties to metabolic adaptations, which ensure energy supply matches demand during ISC activation and differentiation. We describe how declines in these adaptive mechanisms, through aging or pathology, promote reciprocal changes in ISC metabolism and ROS signaling. These changes ultimately contribute to aberrant ISC function, a loss of tissue homeostasis, and a shortened lifespan.

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.

Oxidative Stress in Amyotrophic Lateral Sclerosis: Pathophysiology and Opportunities for Pharmacological Intervention

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease or Charcot disease, is a fatal neurodegenerative disease that affects motor neurons (MNs) and leads to death within 2–5 years of diagnosis, without any effective therapy available. Although the pathological mechanisms leading to ALS are still unknown, a wealth of evidence indicates that an excessive reactive oxygen species (ROS) production associated with an inefficient antioxidant defense represents an important pathological feature in ALS. Substantial evidence indicates that oxidative stress (OS) is implicated in the loss of MNs and in mitochondrial dysfunction, contributing decisively to neurodegeneration in ALS. Although the modulation of OS represents a promising approach to protect MNs from degeneration, the fact that several antioxidants with beneficial effects in animal models failed to show any therapeutic benefit in patients raises several questions that should be analyzed. Using specific queries for literature search on PubMed, we review here the role of OS-related mechanisms in ALS, including the involvement of altered mitochondrial function with repercussions in neurodegeneration. We also describe antioxidant compounds that have been mostly tested in preclinical and clinical trials of ALS, also describing their respective mechanisms of action. While the description of OS mechanism in the different mutations identified in ALS has as principal objective to clarify the contribution of OS in ALS, the description of positive and negative outcomes for each antioxidant is aimed at paving the way for novel opportunities for intervention. In conclusion, although antioxidant strategies represent a very promising approach to slow the progression of the disease, it is of utmost need to invest on the characterization of OS profiles representative of each subtype of patient, in order to develop personalized therapies, allowing to understand the characteristics of antioxidants that have beneficial effects on different subtypes of patients.

Crystal structure and interaction studies of human DHTKD1 provide insight into a mitochondrial megacomplex in lysine catabolism

DHTKD1 is a lesser-studied E1 enzyme among the family of 2-oxoacid de-hydrogenases. In complex with E2 (di-hydro-lipo-amide succinyltransferase, DLST) and E3 (dihydrolipo-amide de-hydrogenase, DLD) components, DHTKD1 is involved in lysine and tryptophan catabolism by catalysing the oxidative de-carboxyl-ation of 2-oxoadipate (2OA) in mitochondria. Here, the 1.9 Å resolution crystal structure of human DHTKD1 is solved in complex with the thi-amine diphosphate co-factor. The structure reveals how the DHTKD1 active site is modelled upon the well characterized homologue 2-oxoglutarate (2OG) de-hydrogenase but engineered specifically to accommodate its preference for the longer substrate of 2OA over 2OG. A 4.7 Å resolution reconstruction of the human DLST catalytic core is also generated by single-particle electron microscopy, revealing a 24-mer cubic scaffold for assembling DHTKD1 and DLD protomers into a megacomplex. It is further demonstrated that missense DHTKD1 variants causing the inborn error of 2-amino-adipic and 2-oxoadipic aciduria impact on the complex formation, either directly by disrupting the interaction with DLST, or indirectly through destabilizing the DHTKD1 protein. This study provides the starting framework for developing DHTKD1 modulators to probe the intricate mitochondrial energy metabolism.

Activation of Mitochondrial 2-Oxoglutarate Dehydrogenase by Cocarboxylase in Human Lung Adenocarcinoma Cells A549 Is p53/p21-Dependent and Impairs Cellular Redox State, Mimicking the Cisplatin Action

Genetic up-regulation of mitochondrial 2-oxoglutarate dehydrogenase is known to increase reactive oxygen species, being detrimental for cancer cells. Thiamine diphosphate (ThDP, cocarboxylase) is an essential activator of the enzyme and inhibits p53–DNA binding in cancer cells. We hypothesize that the pleiotropic regulator ThDP may be of importance for anticancer therapies. The hypothesis is tested in the present work on lung adenocarcinoma cells A549 possessing the p53–p21 pathway as fully functional or perturbed by p21 knockdown. Molecular mechanisms of ThDP action on cellular viability and their interplay with the cisplatin and p53–p21 pathways are characterized. Despite the well-known antioxidant properties of thiamine, A549 cells exhibit decreases in their reducing power and glutathione level after incubation with 5 mM ThDP, not observed in non-cancer epithelial cells Vero. Moreover, thiamine deficiency elevates glutathione in A549 cells. Viability of the thiamine deficient A549 cells is increased at a low (0.05 mM) ThDP. However, the increase is attenuated by 5 mM ThDP, p21 knockdown, specific inhibitor of the 2-oxoglutarate dehydrogenase complex (OGDHC), or cisplatin. Cellular levels of the catalytically competent ThDP·OGDHC holoenzyme are dysregulated by p21 knockdown and correlate negatively with the A549 viability. The inverse relationship between cellular glutathione and holo-OGDHC is corroborated by their comparison in the A549 and Vero cells. The similarity, non-additivity, and p21 dependence of the dual actions of ThDP and cisplatin on A549 cells manifest a common OGDHC-mediated mechanism of the viability decrease. High ThDP saturation of OGDHC compromises the redox state of A549 cells under the control of p53–p21 axes.

Redox-Driven Signaling: 2-Oxo Acid Dehydrogenase Complexes as Sensors and Transmitters of Metabolic Imbalance

SIGNIFICANCE

This article develops a holistic view on production of reactive oxygen species (ROS) by 2-oxo acid dehydrogenase complexes. Recent Advances: Catalytic and structural properties of the complexes and their components evolved to minimize damaging effects of side reactions, including ROS generation, simultaneously exploiting the reactions for homeostatic signaling.

CRITICAL ISSUES

Side reactions of the complexes, characterized in vitro, are analyzed in view of protein interactions and conditions in vivo. Quantitative data support prevalence of the forward 2-oxo acid oxidation over the backward NADH oxidation in feeding physiologically significant ROS production by the complexes. Special focus on interactions between the active sites within 2-oxo acid dehydrogenase complexes highlights the central relevance of the complex-bound thiyl radicals in regulation of and signaling by complex-generated ROS. The thiyl radicals arise when dihydrolipoyl residues of the complexes regenerate FADH₂ from the flavin semiquinone coproduced with superoxide anion radical in 1e^- oxidation of FADH₂ by molecular oxygen.

FUTURE DIRECTIONS

Interaction of 2-oxo acid dehydrogenase complexes with thioredoxins (TRXs), peroxiredoxins, and glutaredoxins mediates scavenging of the thiyl radicals and ROS generated by the complexes, underlying signaling of disproportional availability of 2-oxo acids, CoA, and NAD⁺ in key metabolic branch points through thiol/disulfide exchange and medically important hypoxia-inducible factor, mammalian target of rapamycin (mTOR), poly (ADP-ribose) polymerase, and sirtuins. High reactivity of the coproduced ROS and thiyl radicals to iron/sulfur clusters and nitric oxide, peroxynitrite reductase activity of peroxiredoxins and transnitrosylating function of thioredoxin, implicate the side reactions of 2-oxo acid dehydrogenase complexes in nitric oxide-dependent signaling and damage.

Human 2-Oxoglutarate Dehydrogenase and 2-Oxoadipate Dehydrogenase Both Generate Superoxide/H2O2 in a Side Reaction and Each Could Contribute to Oxidative Stress in Mitochondria

According to recent findings, the human 2-oxoglutarate dehydrogenase complex (hOGDHc) could be an important source of the reactive oxygen species in the mitochondria and could contribute to mitochondrial abnormalities associated with multiple neurodegenerative diseases, including Alzheimer's disease, Huntington disease, and Parkinson's disease. The human 2-oxoadipate dehydrogenase (hE1a) is a novel protein, which is encoded by the DHTKD1 gene. Both missence and nonsense mutations were identified in the DHTKD1 that lead to alpha-aminoadipic and alpha-oxoadipic aciduria, a metabolic disorder with a wide variety of the neurological abnormalities, and Charcot-Marie-Tooth disease type 2Q, an inherited neurological disorder affecting the peripheral nervous system. Recently, the rare pathogenic mutations in DHTKD1 and an increased H₂O₂ production were linked to the genetic ethiology of Eosinophilic Esophagitis (EoE), a chronic allergic inflammatory esophageal disorder. In view of the importance of hOGDHc in the tricarboxylic acid cycle (TCA cycle) and hE1a on the L-lysine, L-hydroxylysine and L-tryptophan degradation pathway in mitochondria, and to enhance our current understanding of the mechanism of superoxide/H₂O₂ generation by hOGDHc, and by human 2-oxoadipate dehydrogenase complex (hOADHc), this review focuses on several novel and unanticipated recent findings in vitro that emerged from the Jordan group's research. Most significantly, the hE1o and hE1a now join the hE3 as being able to generate the superoxide/H₂O₂ in mitochondria.

Sources of Vascular Nitric Oxide and Reactive Oxygen Species and Their Regulation

Nitric oxide (NO) is a small free radical with critical signaling roles in physiology and pathophysiology. The generation of sufficient NO levels to regulate the resistance of the blood vessels and hence the maintenance of adequate blood flow is critical to the healthy performance of the vasculature. A novel paradigm indicates that classical NO synthesis by dedicated NO synthases is supplemented by nitrite reduction pathways under hypoxia. At the same time, reactive oxygen species (ROS), which include superoxide and hydrogen peroxide, are produced in the vascular system for signaling purposes, as effectors of the immune response, or as byproducts of cellular metabolism. NO and ROS can be generated by distinct enzymes or by the same enzyme through alternate reduction and oxidation processes. The latter oxidoreductase systems include NO synthases, molybdopterin enzymes, and hemoglobins, which can form superoxide by reduction of molecular oxygen or NO by reduction of inorganic nitrite. Enzymatic uncoupling, changes in oxygen tension, and the concentration of coenzymes and reductants can modulate the NO/ROS production from these oxidoreductases and determine the redox balance in health and disease. The dysregulation of the mechanisms involved in the generation of NO and ROS is an important cause of cardiovascular disease and target for therapy. In this review we will present the biology of NO and ROS in the cardiovascular system, with special emphasis on their routes of formation and regulation, as well as the therapeutic challenges and opportunities for the management of NO and ROS in cardiovascular disease.

Evidence for functional and regulatory cross-talk between the tricarboxylic acid cycle 2-oxoglutarate dehydrogenase complex and 2-oxoadipate dehydrogenase on the l-lysine, l-hydroxylysine and l-tryptophan degradation pathways from studies in vitro

Herein are reported findings in vitro suggesting both functional and regulatory cross-talk between the human 2-oxoglutarate dehydrogenase complex (hOGDHc), a key regulatory enzyme within the tricarboxylic acid cycle (TCA cycle), and a novel 2-oxoadipate dehydrogenase complex (hOADHc) from the final degradation pathway of l-lysine, l-hydroxylysine and l-tryptophan. The following could be concluded from our studies by using hOGDHc and hOADHc assembled from their individually expressed components in vitro: (i) Different substrate preferences (k_cat/K_m) were displayed by the two complexes even though they share the same dihydrolipoyl succinyltransferase (hE2o) and dihydrolipoyl dehydrogenase (hE3) components; (ii) Different binding modes were in evidence for the binary hE1o-hE2o and hE1a-hE2o subcomplexes according to fluorescence titrations using site-specifically labeled hE2o-derived proteins; (iii) Similarly to hE1o, the hE1a also forms the ThDP-enamine radical from 2-oxoadipate (electron paramagnetic resonance detection) in the oxidative half reaction; (iv) Both complexes produced superoxide/H₂O₂ from O₂ in the reductive half reaction suggesting that hE1o, and hE1a (within their complexes) could both be sources of reactive oxygen species generation in mitochondria from 2-oxoglutarate and 2-oxoadipate, respectively; (v) Based on our findings, we speculate that hE2o can serve as a trans-glutarylase, in addition to being a trans-succinylase, a role suggested by others; (vi) The glutaryl-CoA produced by hOADHc inhibits hE1o, as does succinyl-CoA, suggesting a regulatory cross-talk between the two complexes on the different metabolic pathways.

Brain bioenergetics in rats with acute hyperphenylalaninemia

Phenylketonuria (PKU) is a disorder of phenylalanine (Phe) metabolism caused by deficient phenylalanine hydroxylase (PAH) activity. The deficiency results in increased levels of Phe and its metabolites in fluids and tissues of patients. PKU patients present neurological signs and symptoms including hypomyelination and intellectual deficit. This study assessed brain bioenergetics at 1 h after acute Phe administration to induce hyperphenylalaninemia (HPA) in rats. Wistar rats were randomized in two groups: HPA animals received a single subcutaneous administration of Phe (5.2 μmol/g) plus p-Cl-Phe (PAH inhibitor) (0.9 μmol/g); control animals received a single injection of 0.9% NaCl. In cerebral cortex, HPA group showed lower mitochondrial mass, lower glycogen levels, as well as lower activities of complexes I-III and IV, ATP synthase and citrate synthase. Higher levels of free Pi and phospho-AMPK, and higher activities of LDH, α-ketoglutarate dehydrogenase and isocitrate dehydrogenase were also reported in cerebral cortex of HPA animals. In striatum, HPA animals had higher LDH (pyruvate to lactate) and isocitrate dehydrogenase activities, and lower activities of α-ketoglutarate dehydrogenase and complex IV, as well as lower phospho-AMPK immunocontent. In hippocampus, HPA rats had higher mRNA expression for MFN1 and higher activities of α-ketoglutarate dehydrogenase and isocitrate dehydrogenase, but decreased activities of pyruvate dehydrogenase and complexes I and IV. In conclusion, our data demonstrated impaired bioenergetics in cerebral cortex, striatum and hippocampus of HPA rats.

Inhibition of mitochondrial 2-oxoglutarate dehydrogenase impairs viability of cancer cells in a cell-specific metabolism-dependent manner

2-Oxoglutarate dehydrogenase (OGDH) of the tricarboxylic acid (TCA) cycle is often implied to be inactive in cancer, but this was not experimentally tested. We addressed the question through specific inhibition of OGDH by succinyl phosphonate (SP). SP action on different cancer cells was investigated using indicators of cellular viability and reactive oxygen species (ROS), metabolic profiling and transcriptomics. Relative sensitivity of various cancer cells to SP changed with increasing SP exposure and could differ in the ATP- and NAD(P)H-based assays. Glioblastoma responses to SP revealed metabolic sub-types increasing or decreasing cellular ATP/NAD(P)H ratio under OGDH inhibition. Cancer cell homeostasis was perturbed also when viability indicators were SP-resistant, e.g. in U87 and N2A cells. The transcriptomics database analysis showed that the SP-sensitive cells, such as A549 and T98G, exhibit the lowest expression of OGDH compared to other TCA cycle enzymes, associated with higher expression of affiliated pathways utilizing 2-oxoglutarate. Metabolic profiling confirmed the dependence of cellular SP reactivity on cell-specific expression of the pathways. Thus, oxidative decarboxylation of 2-oxoglutarate is significant for the interdependent homeostasis of NAD(P)H, ATP, ROS and key metabolites in various cancer cells. Assessment of cell-specific responses to OGDH inhibition is of diagnostic value for anticancer strategies.

Generation of superoxide and hydrogen peroxide by side reactions of mitochondrial 2-oxoacid dehydrogenase complexes in isolation and in cells

Mitochondrial 2-oxoacid dehydrogenase complexes oxidize 2-oxoglutarate, pyruvate, branched-chain 2-oxoacids and 2-oxoadipate to the corresponding acyl-CoAs and reduce NAD+ to NADH. The isolated enzyme complexes generate superoxide anion radical or hydrogen peroxide in defined reactions by leaking electrons to oxygen. Studies using isolated mitochondria in media mimicking cytosol suggest that the 2-oxoacid dehydrogenase complexes contribute little to the production of superoxide or hydrogen peroxide relative to other mitochondrial sites at physiological steady states. However, the contributions may increase under pathological conditions, in accordance with the high maximum capacities of superoxide or hydrogen peroxide-generating reactions of the complexes, established in isolated mitochondria. We assess available data on the use of modulations of enzyme activity to infer superoxide or hydrogen peroxide production from particular 2-oxoacid dehydrogenase complexes in cells, and limitations of such methods to discriminate specific superoxide or hydrogen peroxide sources in vivo.

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.

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.

Mitochondrial generation of superoxide and hydrogen peroxide as the source of mitochondrial redox signaling

This review examines the generation of reactive oxygen species by mammalian mitochondria, and the status of different sites of production in redox signaling and pathology. Eleven distinct mitochondrial sites associated with substrate oxidation and oxidative phosphorylation leak electrons to oxygen to produce superoxide or hydrogen peroxide: oxoacid dehydrogenase complexes that feed electrons to NAD⁺; respiratory complexes I and III, and dehydrogenases, including complex II, that use ubiquinone as acceptor. The topologies, capacities, and substrate dependences of each site have recently clarified. Complex III and mitochondrial glycerol 3-phosphate dehydrogenase generate superoxide to the external side of the mitochondrial inner membrane as well as the matrix, the other sites generate superoxide and/or hydrogen peroxide exclusively in the matrix. These different site-specific topologies are important for redox signaling. The net rate of superoxide or hydrogen peroxide generation depends on the substrates present and the antioxidant systems active in the matrix and cytosol. The rate at each site can now be measured in complex substrate mixtures. In skeletal muscle mitochondria in media mimicking muscle cytosol at rest, four sites dominate, two in complex I and one each in complexes II and III. Specific suppressors of two sites have been identified, the outer ubiquinone-binding site in complex III (site III_Qo) and the site in complex I active during reverse electron transport (site I_Q). These suppressors prevent superoxide/hydrogen peroxide production from a specific site without affecting oxidative phosphorylation, making them excellent tools to investigate the status of the sites in redox signaling, and to suppress the sites to prevent pathologies. They allow the cellular roles of mitochondrial superoxide/hydrogen peroxide production to be investigated without catastrophic confounding bioenergetic effects. They show that sites III_Qo and I_Q are active in cells and have important roles in redox signaling (e.g. hypoxic signaling and ER-stress) and in causing oxidative damage in a variety of biological contexts.

Manganese ions enhance mitochondrial H2O2 emission from Krebs cycle oxidoreductases by inducing permeability transition

Manganese-induced toxicity has been linked to mitochondrial dysfunction and an increased generation of reactive oxygen species (ROS). We could recently show in mechanistic studies that Mn²⁺ ions induce hydrogen peroxide (H₂O₂) production from the ubiquinone binding site of mitochondrial complex II (II_Q) and generally enhance H₂O₂ formation by accelerating the rate of superoxide dismutation. The present study with intact mitochondria reveals that manganese additionally enhances H₂O₂ emission by inducing mitochondrial permeability transition (mPT). In mitochondria fed by NADH-generating substrates, the combination of Mn²⁺ and different respiratory chain inhibitors led to a dynamically increasing H₂O₂emission which was sensitive to the mPT inhibitor cyclosporine A (CsA) as well as Ru-360, an inhibitor of the mitochondrial calcium uniporter (MCU). Under these conditions, flavin-containing enzymes of the mitochondrial matrix, e.g. the mitochondrial 2-oxoglutaratedehydrogenase (OGDH), were major sources of ROS. With succinate as substrate, Mn²⁺ stimulated ROS production mainly at complex II, whereby the applied succinate concentration had a marked effect on the tendency for mPT. Also Ca²⁺ increased the rate of H₂O₂ emission by mPT, while no direct effect on ROS-production of complex II was observed. The present study reveals a complex scenario through which manganese affects mitochondrial H₂O₂ emission: stimulating its production from distinct sites (e.g. site II_Q), accelerating superoxide dismutation and enhancing the emission via mPT which also leads to the loss of soluble components of the mitochondrial antioxidant systems and favors the ROS production from flavin-containing oxidoreductases of the Krebs cycle.

Induction of mitochondrial reactive oxygen species production by GSH mediated S-glutathionylation of 2-oxoglutarate dehydrogenase

2-Oxoglutarate dehydrogenase (Ogdh) is an important mitochondria redox sensor that can undergo S-glutathionylation following an increase in H₂O₂ levels. Although S-glutathionylation is required to protect Ogdh from irreversible oxidation while simultaneously modulating its activity it remains unknown if glutathione can also modulate reactive oxygen species (ROS) production by the complex. We report that reduced (GSH) and oxidized (GSSG) glutathione control O2∙-/H₂O₂ formation by Ogdh through protein S-glutathionylation reactions. GSSG (1 mM) induced a modest decrease in Ogdh activity which was associated with a significant decrease in O2∙-/H₂O₂ formation. GSH had the opposite effect, amplifying O2∙-/H₂O₂ formation by Ogdh. Incubation of purified Ogdh in 2.5 mM GSH led to significant increase in O2∙-/H₂O₂ formation which also lowered NADH production. Inclusion of enzymatically active glutaredoxin-2 (Grx2) in reaction mixtures reversed the GSH-mediated amplification of O2∙-/H₂O₂ formation. Similarly pre-incubation of permeabilized liver mitochondria from mouse depleted of GSH showed an approximately ~3.5-fold increase in Ogdh-mediated O2∙-/H₂O₂ production that was matched by a significant decrease in NADH formation which could be reversed by Grx2. Taken together, our results demonstrate GSH and GSSG modulate ROS production by Ogdh through S-glutathionylation of different subunits. This is also the first demonstration that GSH can work in the opposite direction in mitochondria-amplifying ROS formation instead of quenching it. We propose that this regulatory mechanism is required to modulate ROS emission from Ogdh in response to variations in glutathione redox buffering capacity.

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ROS formation by Ogdh is controlled by glutathione.

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GSH amplifies ROS production by Ogdh.

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Ogdh is S-glutathionylated by GSH.

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Grx2 deglutathionylates Ogdh.

Enzyme Complexes Important for the Glutamate-Glutamine Cycle

Transient multienzyme and/or multiprotein complexes (metabolons) direct substrates toward specific pathways and can significantly influence the metabolism of glutamate and glutamine in the brain. Glutamate is the primary excitatory neurotransmitter in brain. This neurotransmitter has essential roles in normal brain function including learning and memory. Metabolism of glutamate involves the coordinated activity of astrocytes and neurons and high affinity transporter proteins that are selectively distributed on these cells. This chapter describes known and possible metabolons that affect the metabolism of glutamate and related compounds in the brain, as well as some factors that can modulate the association and dissociation of such complexes, including protein modifications by acylation reactions (e.g., acetylation, palmitoylation, succinylation, SUMOylation, etc.) of specific residues. Development of strategies to modulate transient multienzyme and/or enzyme-protein interactions may represent a novel and promising therapeutic approach for treatment of diseases involving dysregulation of glutamate metabolism.

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.

Reactive oxygen species and energy machinery: an integrated dynamic model

The role of several important reactive oxygen species (ROS) on the Krebs cycle, the electron transport chain (ETC) and the two important shuttles has been modelled. Major part of the ROS is produced during oxygen reduction in the ETC, which has been kinetically simulated, and the changes in the final concentrations of several important metabolites were found. The simulation is based on chemical kinetics equation, and the associated set of differential equations was solved by the ordinary differential equation package in Octave. The validity of the model is checked by comparing the experimental results available in the literature with the simulations when a part of the ETC is blocked (80%) in the script. The present approach is versatile and flexible and has potential applications in various simulations. It is easy to study the change in concentrations of various metabolites when a particular enzyme or pathway is blocked (say by a drug). The Octave script is presented in the text.

Molecular mechanisms of the non-coenzyme action of thiamin in brain: biochemical, structural and pathway analysis

Thiamin (vitamin B1) is a pharmacological agent boosting central metabolism through the action of the coenzyme thiamin diphosphate (ThDP). However, positive effects, including improved cognition, of high thiamin doses in neurodegeneration may be observed without increased ThDP or ThDP-dependent enzymes in brain. Here, we determine protein partners and metabolic pathways where thiamin acts beyond its coenzyme role. Malate dehydrogenase, glutamate dehydrogenase and pyridoxal kinase were identified as abundant proteins binding to thiamin- or thiazolium-modified sorbents. Kinetic studies, supported by structural analysis, revealed allosteric regulation of these proteins by thiamin and/or its derivatives. Thiamin triphosphate and adenylated thiamin triphosphate activate glutamate dehydrogenase. Thiamin and ThDP regulate malate dehydrogenase isoforms and pyridoxal kinase. Thiamin regulation of enzymes related to malate-aspartate shuttle may impact on malate/citrate exchange, responsible for exporting acetyl residues from mitochondria. Indeed, bioinformatic analyses found an association between thiamin- and thiazolium-binding proteins and the term acetylation. Our interdisciplinary study shows that thiamin is not only a coenzyme for acetyl-CoA production, but also an allosteric regulator of acetyl-CoA metabolism including regulatory acetylation of proteins and acetylcholine biosynthesis. Moreover, thiamin action in neurodegeneration may also involve neurodegeneration-related 14-3-3, DJ-1 and β-amyloid precursor proteins identified among the thiamin- and/or thiazolium-binding proteins.

Pyruvate dehydrogenase complex and nicotinamide nucleotide transhydrogenase constitute an energy-consuming redox circuit

Cellular proteins rely on reversible redox reactions to establish and maintain biological structure and function. How redox catabolic (NAD+/NADH) and anabolic (NADP+/NADPH) processes integrate during metabolism to maintain cellular redox homoeostasis, however, is unknown. The present work identifies a continuously cycling mitochondrial membrane potential (ΔΨm)-dependent redox circuit between the pyruvate dehydrogenase complex (PDHC) and nicotinamide nucleotide transhydrogenase (NNT). PDHC is shown to produce H2O2 in relation to reducing pressure within the complex. The H2O2 produced, however, is effectively masked by a continuously cycling redox circuit that links, via glutathione/thioredoxin, to NNT, which catalyses the regeneration of NADPH from NADH at the expense of ΔΨm. The net effect is an automatic fine-tuning of NNT-mediated energy expenditure to metabolic balance at the level of PDHC. In mitochondria, genetic or pharmacological disruptions in the PDHC-NNT redox circuit negate counterbalance changes in energy expenditure. At the whole animal level, mice lacking functional NNT (C57BL/6J) are characterized by lower energy-expenditure rates, consistent with their well-known susceptibility to diet-induced obesity. These findings suggest the integration of redox sensing of metabolic balance with compensatory changes in energy expenditure provides a potential mechanism by which cellular redox homoeostasis is maintained and body weight is defended during periods of positive and negative energy balance.

Interactions between mitochondrial reactive oxygen species and cellular glucose metabolism

Mitochondrial reactive oxygen species (ROS) production and detoxification are tightly balanced. Shifting this balance enables ROS to activate intracellular signaling and/or induce cellular damage and cell death. Increased mitochondrial ROS production is observed in a number of pathological conditions characterized by mitochondrial dysfunction. One important hallmark of these diseases is enhanced glycolytic activity and low or impaired oxidative phosphorylation. This suggests that ROS is involved in glycolysis (dys)regulation and vice versa. Here we focus on the bidirectional link between ROS and the regulation of glucose metabolism. To this end, we provide a basic introduction into mitochondrial energy metabolism, ROS generation and redox homeostasis. Next, we discuss the interactions between cellular glucose metabolism and ROS. ROS-stimulated cellular glucose uptake can stimulate both ROS production and scavenging. When glucose-stimulated ROS production, leading to further glucose uptake, is not adequately counterbalanced by (glucose-stimulated) ROS scavenging systems, a toxic cycle is triggered, ultimately leading to cell death. Here we inventoried the various cellular regulatory mechanisms and negative feedback loops that prevent this cycle from occurring. It is concluded that more insight in these processes is required to understand why they are (un)able to prevent excessive ROS production during various pathological conditions in humans.

Alpha-ketoglutarate dehydrogenase complex-dependent succinylation of proteins in neurons and neuronal cell lines

Reversible post-translation modifications of proteins are common in all cells and appear to regulate many processes. Nevertheless, the enzyme(s) responsible for the alterations and the significance of the modification are largely unknown. Succinylation of proteins occurs and causes large changes in the structure of proteins; however, the source of the succinyl groups, the targets, and the consequences of these modifications on other proteins remain unknown. These studies focused on succinylation of mitochondrial proteins. The results demonstrate that the α-ketoglutarate dehydrogenase complex (KGDHC) can serve as a trans-succinylase that mediates succinylation in an α-ketoglutarate-dependent manner. Inhibition of KGDHC reduced succinylation of both cytosolic and mitochondrial proteins in cultured neurons and in a neuronal cell line. Purified KGDHC can succinylate multiple proteins including other enzymes of the tricarboxylic acid cycle leading to modification of their activity. Inhibition of KGDHC also modifies acetylation by modifying the pyruvate dehydrogenase complex. The much greater effectiveness of KGDHC than succinyl-CoA suggests that the catalysis owing to the E2k succinyltransferase is important. Succinylation appears to be a major signaling system and it can be mediated by KGDHC. Reversible post-translation modifications of proteins are common and may regulate many processes. Succinylation of proteins occurs and causes large changes in the structure of proteins. However, the source of the succinyl groups, the targets, and the consequences of these modifications on other proteins remains unknown. The results demonstrate that the mitochondrial α-ketoglutarate dehydrogenase complex (KGDHC) can succinylate multiple mitochondrial proteins and alter their function. Succinylation appears to be a major signaling system and it can be mediated by KGDHC.

Accumulation of electrophilic aldehydes during postovulatory aging of mouse oocytes causes reduced fertility, oxidative stress, and apoptosis

With increasing periods of time following ovulation, the metaphase II (MII)-stage oocyte experiences overproduction of reactive oxygen species and elevated levels of lipid peroxidation that are implicitly linked with functional deficiencies acquired during postovulatory oocyte aging. We have demonstrated that the electrophilic aldehydes 4-hydroxynonenal (4HNE), malondialdehyde, and acrolein are by-products of nonenzymatic lipid peroxidation in the murine MII-stage oocyte, adducting to multiple proteins within the cell. The covalent modification of oocyte proteins by these aldehydes increased with extended periods of time postovulation; the mitochondrial protein succinate dehydrogenase (SDHA) was identified as a primary target for 4HNE adduction. Time- and dose-dependent studies revealed that exposure to elevated levels of electrophilic aldehydes causes mitochondrial reactive oxygen species production, lipid peroxidation, loss of mitochondrial membrane potential, and eventual apoptosis within the MII oocyte, presumably as a consequence of electron transport chain collapse following SDHA adduction. Additionally, we have determined that short-term exposure to low doses of 4HNE dramatically impairs the oocyte's ability to participate in fertilization and support embryonic development; however, this loss of functionality can be prevented by supplementation with the antioxidant penicillamine. In conclusion, this study has revealed that the accumulation of electrophilic aldehydes is linked to postovulatory oocyte aging, causing reduced fertility, oxidative stress, and apoptosis of this highly specialized cell. These data highlight the importance of timely fertilization of the mammalian oocyte postovulation and emphasize the potential advantages associated with antioxidant supplementation of oocyte culture medium in circumstances where reinsemination of oocytes may be desirable (i.e., rescue intracytoplasmic sperm injection), or where in vitro fertilization may be delayed.

Mitochondrial production of reactive oxygen species

Numerous biochemical studies are aimed at elucidating the sources and mechanisms of formation of reactive oxygen species (ROS) because they are involved in cellular, organ-, and tissue-specific physiology. Mitochondria along with other cellular organelles of eukaryotes contribute significantly to ROS formation and utilization. This review is a critical account of the mitochondrial ROS production and methods for their registration. The physiological and pathophysiological significance of the mitochondrially produced ROS are discussed.

Riboflavin responsive mitochondrial myopathy is a new phenotype of dihydrolipoamide dehydrogenase deficiency. The chaperon-like effect of vitamin B2

Dihydrolipoamide dehydrogenase (DLD, E3) is a flavoprotein common to pyruvate, α-ketoglutarate and branched-chain α-keto acid dehydrogenases. We found two novel DLD mutations (p.I40Lfs*4; p.G461E) in a 19 year-old patient with lactic acidosis and a complex amino- and organic aciduria consistent with DLD deficiency, manifesting progressive exertional fatigue. Muscle biopsy showed mitochondrial proliferation and lack of DLD cross-reacting material. Riboflavin supplementation determined the complete resolution of exercise intolerance with the partial restoration of the DLD protein and disappearance of mitochondrial proliferation in the muscle. Morphological and functional studies support the riboflavin chaperon-like role in stabilizing DLD protein with rescue of its expression in the muscle.

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.

Mitochondrial reactive oxygen species (ROS) and ROS-induced ROS release

Byproducts of normal mitochondrial metabolism and homeostasis include the buildup of potentially damaging levels of reactive oxygen species (ROS), Ca(2+), etc., which must be normalized. Evidence suggests that brief mitochondrial permeability transition pore (mPTP) openings play an important physiological role maintaining healthy mitochondria homeostasis. Adaptive and maladaptive responses to redox stress may involve mitochondrial channels such as mPTP and inner membrane anion channel (IMAC). Their activation causes intra- and intermitochondrial redox-environment changes leading to ROS release. This regenerative cycle of mitochondrial ROS formation and release was named ROS-induced ROS release (RIRR). Brief, reversible mPTP opening-associated ROS release apparently constitutes an adaptive housekeeping function by the timely release from mitochondria of accumulated potentially toxic levels of ROS (and Ca(2+)). At higher ROS levels, longer mPTP openings may release a ROS burst leading to destruction of mitochondria, and if propagated from mitochondrion to mitochondrion, of the cell itself. The destructive function of RIRR may serve a physiological role by removal of unwanted cells or damaged mitochondria, or cause the pathological elimination of vital and essential mitochondria and cells. The adaptive release of sufficient ROS into the vicinity of mitochondria may also activate local pools of redox-sensitive enzymes involved in protective signaling pathways that limit ischemic damage to mitochondria and cells in that area. Maladaptive mPTP- or IMAC-related RIRR may also be playing a role in aging. Because the mechanism of mitochondrial RIRR highlights the central role of mitochondria-formed ROS, we discuss all of the known ROS-producing sites (shown in vitro) and their relevance to the mitochondrial ROS production in vivo.

Krebs cycle intermediates regulate DNA and histone methylation: epigenetic impact on the aging process

Many aging theories have proposed that mitochondria and energy metabolism have a major role in the aging process. There are recent studies indicating that Krebs cycle intermediates can shape the epigenetic landscape of chromatin by regulating DNA and histone methylation. A growing evidence indicates that epigenetics plays an important role in the regulation of healthspan but also is involved in the aging process. 2-Oxoglutarate (α-ketoglutarate) is a key metabolite in the Krebs cycle but it is also an obligatory substrate for 2-oxoglutarate-dependent dioxygenases (2-OGDO). The 2-OGDO enzyme family includes the major enzymes of DNA and histone demethylation, i.e. Ten-Eleven Translocation (TETs) and Jumonji C domain containing (JmjC) demethylases. In addition, 2-OGDO members can regulate collagen synthesis and hypoxic responses in a non-epigenetical manner. Interestingly, succinate and fumarate, also Krebs cycle intermediates, are potent inhibitors of 2-OGDO enzymes, i.e. the balance of Krebs cycle reactions can affect the level of DNA and histone methylation and thus control gene expression. We will review the epigenetic mechanisms through which Krebs cycle intermediates control the DNA and histone methylation. We propose that age-related disturbances in the Krebs cycle function induce stochastic epigenetic changes in chromatin structures which in turn promote the aging process.

The 2-oxoacid dehydrogenase complexes in mitochondria can produce superoxide/hydrogen peroxide at much higher rates than complex I

Several flavin-dependent enzymes of the mitochondrial matrix utilize NAD(+) or NADH at about the same operating redox potential as the NADH/NAD(+) pool and comprise the NADH/NAD(+) isopotential enzyme group. Complex I (specifically the flavin, site IF) is often regarded as the major source of matrix superoxide/H2O2 production at this redox potential. However, the 2-oxoglutarate dehydrogenase (OGDH), branched-chain 2-oxoacid dehydrogenase (BCKDH), and pyruvate dehydrogenase (PDH) complexes are also capable of considerable superoxide/H2O2 production. To differentiate the superoxide/H2O2-producing capacities of these different mitochondrial sites in situ, we compared the observed rates of H2O2 production over a range of different NAD(P)H reduction levels in isolated skeletal muscle mitochondria under conditions that favored superoxide/H2O2 production from complex I, the OGDH complex, the BCKDH complex, or the PDH complex. The rates from all four complexes increased at higher NAD(P)H/NAD(P)(+) ratios, although the 2-oxoacid dehydrogenase complexes produced superoxide/H2O2 at high rates only when oxidizing their specific 2-oxoacid substrates and not in the reverse reaction from NADH. At optimal conditions for each system, superoxide/H2O2 was produced by the OGDH complex at about twice the rate from the PDH complex, four times the rate from the BCKDH complex, and eight times the rate from site IF of complex I. Depending on the substrates present, the dominant sites of superoxide/H2O2 production at the level of NADH may be the OGDH and PDH complexes, but these activities may often be misattributed to complex I.

The role of mitochondrial function and cellular bioenergetics in ageing and disease

Mitochondria constitute an important topic of biomedical enquiry (one paper in every 154 indexed in PubMed since 1998 is retrieved by the keyword 'mitochondria') because of widespread recognition of their importance in cell physiology and pathology. Mitochondrial dysfunction is widely implicated in ageing and in the diseases of ageing, through dysfunction in adenosine triphosphate (ATP) synthesis, Ca(2+) homeostasis, central metabolic pathways or radical production. Nonetheless, the mechanisms and regulation of superoxide and hydrogen peroxide formation by mitochondria remain poorly described. Measurement of the capacities of different sites of superoxide and hydrogen peroxide production in isolated skeletal muscle mitochondria show that the maximum capacities of sites in complexes I, II and III and in several associated redox enzymes greatly exceed the native rates observed in the absence of respiratory chain inhibitors. In vitro, the native rates and the relative importance of different sites both depend on the substrate being oxidized, with sites IQ, IIF, GPDH, IF and IIIQo each being important with particular substrates. The techniques involved in measuring rates from each site should become applicable to cell cultures and in vivo in the future.

Mitochondrial glutathione depletion reveals a novel role for the pyruvate dehydrogenase complex as a key H2O2-emitting source under conditions of nutrient overload

Once regarded as a "by-product" of aerobic metabolism, the production of superoxide/H2O2 is now understood to be a highly specialized and extensively regulated process responsible for exerting control over a vast number of thiol-containing proteins, collectively referred to as the redox-sensitive proteome. Although disruptions within this process, secondary to elevated peroxide exposure, have been linked to disease, the sources and mechanisms regulating increased peroxide burden remain poorly defined and as such are difficult to target using pharmacotherapy. Here we identify the pyruvate dehydrogenase complex (PDC) as a key source of H2O2 within skeletal muscle mitochondria under conditions of depressed glutathione redox buffering integrity. Treatment of permeabilized myofibers with varying concentrations of the glutathione-depleting agent 1-chloro-2,4-dinitrobenzene led to a dose-dependent increase in pyruvate-supported JH2O2 emission (the flux of H2O2 diffusing out of the mitochondrial matrix into the surrounding assay medium), with emission rates eventually rising to exceed those of all substrate combinations tested. This striking sensitivity to glutathione depletion was observed in permeabilized fibers prepared from multiple species and was specific to PDC. Physiological oxidation of the cellular glutathione pool after high-fat feeding in rodents was found to elevate PDC JH2O2 emission, as well as increasing the sensitivity of the complex to GSH depletion. These findings reveal PDC as a potential major site of H2O2 production that is extremely sensitive to mitochondrial glutathione redox status.