Insights into the inhibitory mechanisms of NADH on the αγ heterodimer of human NAD-dependent isocitrate dehydrogenase

Quantification of heavy metal exposure in a British population cohort links total mercury levels in plasma with skin tissue-specific changes in mitochondrial-related gene expression

Heavy metals in our direct environment have profound effects on human health and while some are essential for life, others can be toxic. In vivo studies often focus on clinical features caused by overexposure to, or by deprivation of a heavy metal. However, to understand the cellular impact of heavy metals on health, studies in healthy volunteers before symptom onset are needed. Here, we explored the impact of mercury, lead and selenium in over 800 British female twins on multi-tissue gene expression levels as an intermediate phenotype. Total mercury, lead and selenium concentrations were determined in plasma as a proxy for heavy metal exposure. We identified significant associations between total mercury levels measured in plasma, that fall within normal ranges, and expression of 873 genes within skin tissue, including PUSL1, SAMD10, ERCC1, MRPL17, NDUFB8, SELENOH, SEC31A, and KAT7P1. Functional analysis of genes associated with total mercury levels in plasma show a strong link to the mitochondrial oxidative phosphorylation pathway (p-value = 3.02 × 10-10). Analysis of mitochondrial-specific gene expression supported involvement of genes of oxidative phosphorylation complexes (MT-ND4L, and MT-ND5), which are encoded in mitochondrial DNA. These results suggest that mercury is likely detrimental to the energy metabolism of mitochondria. We also tested for associations between total mercury levels in plasma and gene expression in adipose and whole blood samples, but did not identify significant associations in these tissues, nor with lead or selenium in any tissue. Our results demonstrate that subtoxic mercury exposure leaves a clear molecular signature. It also underscores the necessity of conducting tissue-specific association studies to accurately capture the molecular impact of environmental exposures, as only relevant tissues will manifest a response to environmental exposures.

CYP3A5 promotes glioblastoma stemness and chemoresistance through fine-tuning NAD+/NADH ratio

BACKGROUND

Glioblastoma multiforme (GBM) exhibits a cellular hierarchy with a subpopulation of stem-like cells known as glioblastoma stem cells (GSCs) that drive tumor growth and contribute to treatment resistance. NAD(H) emerges as a crucial factor influencing GSC maintenance through its involvement in diverse biological processes, including mitochondrial fitness and DNA damage repair. However, how GSCs leverage metabolic adaptation to obtain survival advantage remains elusive.

METHODS

A multi-step process of machine learning algorithms was implemented to construct the glioma stemness-related score (GScore). Further in silico and patient tissue analyses validated the predictive ability of the GScore and identified a potential target, CYP3A5. Loss-of-function or gain-of-function genetic experiments were performed to assess the impact of CYP3A5 on the self-renewal and chemoresistance of GSCs both in vitro and in vivo. Mechanistic studies were conducted using nontargeted metabolomics, RNA-seq, seahorse, transmission electron microscopy, immunofluorescence, flow cytometry, ChIP‒qPCR, RT‒qPCR, western blotting, etc. The efficacy of pharmacological inhibitors of CYP3A5 was assessed in vivo.

RESULTS

Based on the proposed GScore, we identify a GSC target CYP3A5, which is highly expressed in GSCs and temozolomide (TMZ)-resistant GBM patients. This elevated expression of CYP3A5 is attributed to transcription factor STAT3 activated by EGFR signaling or TMZ treatment. Depletion of CYP3A5 impairs self-renewal and TMZ resistance of GSCs. Mechanistically, CYP3A5 maintains mitochondrial fitness to promote GSC metabolic adaption through the NAD⁺/NADH-SIRT1-PGC1α axis. Additionally, CYP3A5 enhances the activity of NAD-dependent enzyme PARP to augment DNA damage repair. Treatment with CYP3A5 inhibitor alone or together with TMZ effectively suppresses tumor growth in vivo.

CONCLUSION

Together, this study suggests that GSCs activate STAT3 to upregulate CYP3A5 to fine-tune NAD⁺/NADH for the enhancement of mitochondrial functions and DNA damage repair, thereby fueling tumor stemness and conferring TMZ resistance, respectively. Thus, CYP3A5 represents a promising target for GBM treatment.

Alternative NADH dehydrogenase: A complex I backup, a drug target, and a tool for mitochondrial gene therapy

Alternative NADH dehydrogenase, also known as type II NADH dehydrogenase (NDH-2), catalyzes the same redox reaction as mitochondrial respiratory chain complex I. Specifically, it oxidizes reduced nicotinamide adenine dinucleotide (NADH) while simultaneously reducing ubiquinone to ubiquinol. However, unlike complex I, this enzyme is non-proton pumping, comprises of a single subunit, and is resistant to rotenone. Initially identified in bacteria, fungi and plants, NDH-2 was subsequently discovered in protists and certain animal taxa including sea squirts. The gene coding for NDH-2 is also present in the genomes of some annelids, tardigrades, and crustaceans. For over two decades, NDH-2 has been investigated as a potential substitute for defective complex I. In model organisms, NDH-2 has been shown to ameliorate a broad spectrum of conditions associated with complex I malfunction, including symptoms of Parkinson's disease. Recently, lifespan extension has been observed in animals expressing NDH-2 in a heterologous manner. A variety of mechanisms have been put forward by which NDH-2 may extend lifespan. Such mechanisms include the activation of pro-longevity pathways through modulation of the NAD+/NADH ratio, decreasing production of reactive oxygen species (ROS) in mitochondria, or then through moderate increases in ROS production followed by activation of defense pathways (mitohormesis). This review gives an overview of the latest research on NDH-2, including the structural peculiarities of NDH-2, its inhibitors, its role in the pathogenicity of mycobacteria and apicomplexan parasites, and its function in bacteria, fungi, and animals.

The role of SIRT3 in homeostasis and cellular health

Mitochondria are responsible for maintaining cellular energy levels, and play a major role in regulating homeostasis, which ensures physiological function from the molecular to whole animal. Sirtuin 3 (SIRT3) is the major protein deacetylase of mitochondria. SIRT3 serves as a nutrient sensor; under conditions of mild metabolic stress, SIRT3 activity is increased. Within the mitochondria, SIRT3 regulates every complex of the electron transport chain, the tricarboxylic acid (TCA) and urea cycles, as well as the mitochondria membrane potential, and other free radical scavengers. This article reviews the role of SIRT3 in regulating homeostasis, and thus physiological function. We discuss the role of SIRT3 in regulating reactive oxygen species (ROS), ATP, immunological function and mitochondria dynamics.

Two Different Isocitrate Dehydrogenases from Pseudomonas aeruginosa: Enzymology and Coenzyme-Evolutionary Implications

Pseudomonas aeruginosa PAO1, as an experimental model for Gram-negative bacteria, harbors two NADP+-dependent isocitrate dehydrogenases (NADP-IDHs) that were evolved from its ancient counterpart NAD-IDHs. For a better understanding of PaIDH1 and PaIDH2, we cloned the genes, overexpressed them in Escherichia coli and purified them to homogeneity. PaIDH1 displayed higher affinity to NADP+ and isocitrate, with lower Km values when compared to PaIDH2. Moreover, PaIDH1 possessed higher temperature tolerance (50 °C) and wider pH range tolerance (7.2-8.5) and could be phosphorylated. After treatment with the bifunctional PaIDH kinase/phosphatase (PaIDH K/P), PaIDH1 lost 80% of its enzymatic activity in one hour due to the phosphorylation of Ser115. Small-molecule compounds like glyoxylic acid and oxaloacetate can effectively inhibit the activity of PaIDHs. The mutant PaIDH1-D346I347A353K393 exhibited enhanced affinity for NAD+ while it lost activity towards NADP+, and the Km value (7770.67 μM) of the mutant PaIDH2-L589 I600 for NADP+ was higher than that observed for NAD+ (5824.33 μM), indicating a shift in coenzyme specificity from NADP+ to NAD+ for both PaIDHs. The experiments demonstrated that the mutation did not alter the oligomeric state of either protein. This study provides a foundation for the elucidation of the evolution and function of two NADP-IDHs in the pathogenic bacterium P. aeruginosa.

Molecular insights into the catalysis and regulation of mammalian NAD-dependent isocitrate dehydrogenases

Eukaryotic NAD-dependent isocitrate dehydrogenases (NAD-IDHs) are mitochondria-localized enzymes which catalyze the oxidative decarboxylation of isocitrate to α-ketoglutarate using NAD as a cofactor. In mammals, NAD-IDHs (or IDH3) consist of three types of subunits (α, β, and γ), and exist as (α2βγ)2 heterooctamer. Mammalian NAD-IDHs are regulated allosterically and/or competitively by a diversity of metabolites including citrate, ADP, ATP, NADH, and NADPH, which are associated with cellular metabolite flux, energy demands, and redox status. Proper assembly of the component subunits is essential for the catalysis and regulation of the enzymes. Recently, crystal structures of human IDH3 have been solved in apo form and in complex with various ligands, revealing the molecular mechanisms for the assembly, catalysis, and regulation of the enzyme.

Effect of yhfS gene on Bt LLP29 antioxidant and UV ray resistance

BACKGROUND

Bacillus thuringiensis (Bt) is a widely used microbial insecticide. However, its persistence is limited because of ultraviolet (UV) rays or other environmental factors. The yhfS gene, which encodes acetyl-CoA acyltransferase, plays an important role in lipid transport and metabolism in many organisms. To explore whether it is related to the stress resistance of Bt LLP29, the yhfS gene knockout strain LLP29 Δ-yhfS and the complementary strain LLP29 R-yhfS were generated successfully by homologous recombination technology, and the related phenotypic changes were compared in this study.

RESULTS

Gene yhfS was found to be functional in response to UV radiation in Bt by comparing the survival rates of Bt LLP29 harboring yhfS or not under UV light. Enzyme activity assays of key enzymes showed the the Embden-Meyerhof-Parnas pathway was enhanced yet the tricarboxylic acid cycle as well as butanoate synthesis were repressed when the gene was deleted. At the same time, the amino acid content was decreased, but reduced nicotinamide adenine dinucleotide (NADH) and reactive oxygen species (ROS) content were increased. Most noteworthy, antioxidase (such as superoxide dismutase and peroxidase) activities and contents of some potent antioxidants (such as pyruvate, carotenoids and NADPH) were lower in LLP29 Δ-yhfS than in LLP29.

CONCLUSION

These tests revealed that the loss of the yhfS gene led to metabolic disorders and reduction of the antioxidant ability of Bt. Higher ROS level and lower anti-oxidative capacity might be responsible for the reduced UV resistance when the gene was deleted. These results not only greatly enrich understanding of the mechanism of Bt UV resistance, but also provide an important theoretical basis for Bt application. © 2023 Society of Chemical Industry.

Regulation and function of the mammalian tricarboxylic acid cycle

The tricarboxylic acid (TCA) cycle, otherwise known as the Krebs cycle, is a central metabolic pathway that performs the essential function of oxidizing nutrients to support cellular bioenergetics. More recently, it has become evident that TCA cycle behavior is dynamic, and products of the TCA cycle can be co-opted in cancer and other pathologic states. In this review, we revisit the TCA cycle, including its potential origins and the history of its discovery. We provide a detailed accounting of the requirements for sustained TCA cycle function and the critical regulatory nodes that can stimulate or constrain TCA cycle activity. We also discuss recent advances in our understanding of the flexibility of TCA cycle wiring and the increasingly appreciated heterogeneity in TCA cycle activity exhibited by mammalian cells. Deeper insight into how the TCA cycle can be differentially regulated and, consequently, configured in different contexts will shed light on how this pathway is primed to meet the requirements of distinct mammalian cell states.

Structures of a constitutively active mutant of human IDH3 reveal new insights into the mechanisms of allosteric activation and the catalytic reaction

Human NAD-dependent isocitrate dehydrogenase or IDH3 (HsIDH3) catalyzes the decarboxylation of isocitrate into α-ketoglutarate in the tricarboxylic acid cycle. It consists of three types of subunits (α, β, and γ) and exists and functions as the (αβαγ)₂ heterooctamer. HsIDH3 is regulated allosterically and/or competitively by numerous metabolites including CIT, ADP, ATP, and NADH. Our previous studies have revealed the molecular basis for the activity and regulation of the αβ and αγ heterodimers. However, the molecular mechanism for the allosteric activation of the HsIDH3 holoenzyme remains elusive. In this work, we report the crystal structures of the αβ and αγ heterodimers and the (αβαγ)₂ heterooctamer containing an α-Q139A mutation in the clasp domain, which renders all the heterodimers and the heterooctamer constitutively active in the absence of activators. Our structural analysis shows that the α-Q139A mutation alters the hydrogen-bonding network at the heterodimer-heterodimer interface in a manner similar to that in the activator-bound αγ heterodimer. This alteration not only stabilizes the active sites of both α_Q139Aβ and α_Q139Aγ heterodimers in active conformations but also induces conformational changes of the pseudo-allosteric site of the α_Q139Aβ heterodimer enabling it to bind activators. In addition, the α_Q139A^ICT+Ca+NADβ^NAD structure presents the first pseudo-Michaelis complex of HsIDH3, which allows us to identify the key residues involved in the binding of cofactor, substrate, and metal ion. Our structural and biochemical data together reveal new insights into the molecular mechanisms for allosteric regulation and the catalytic reaction of HsIDH3.

Over-Reduced State of Mitochondria as a Trigger of "β-Oxidation Shuttle" in Cancer Cells

A considerable amount of data have accumulated in the last decade on the pronounced mitochondrial fatty acid oxidation (mFAO) in many types of cancer cells. As a result, mFAO was found to coexist with abnormally activated fatty acid synthesis (FAS) and the mevalonate pathway. Recent studies have demonstrated that overactivated mitochondrial β-oxidation may aggravate the impaired mitochondrial redox state and vice versa. Furthermore, the impaired redox state of cancerous mitochondria can ensure the continuous operation of β-oxidation by disconnecting it from the Krebs cycle and connecting it to the citrate-malate shuttle. This could create a new metabolic state/pathway in cancer cells, which we have called the "β-oxidation-citrate-malate shuttle", or "β-oxidation shuttle" for short, which forces them to proliferate. The calculation of the phosphate/oxygen ratio indicates that it is inefficient as an energy source and must consume significantly more oxygen per mole of ATP produced when combined with acetyl-CoA consuming pathways, such as the FAS and mevalonate pathways. The "β-oxidation shuttle" is an unconventional mFAO, a separate metabolic pathway that has not yet been explored as a source of energy, as well as a source of cataplerosis, leading to biomass accumulation, accelerated oxygen consumption, and, ultimately, a source of proliferation. The role of the "β-oxidation shuttle" and its contribution to redox-altered cancer metabolism provides a new direction for the development of future anticancer strategies. This may represent the metabolic "secret" of cancer underlying hypoxia and genomic instability.

A “Weird” Mitochondrial Fatty Acid Oxidation as a Metabolic “Secret” of Cancer

Cancer metabolism is an extensively studied field since the discovery of the Warburg effect about 100 years ago and continues to be increasingly intriguing and enigmatic so far. It has become clear that glycolysis is not the only abnormally activated metabolic pathway in the cancer cells, but the same is true for the fatty acid synthesis (FAS) and mevalonate pathway. In the last decade, a lot of data have been accumulated on the pronounced mitochondrial fatty acid oxidation (mFAO) in many types of cancer cells. In this article, we discuss how mFAO can escape normal regulation under certain conditions and be overactivated. Such abnormal activation of mitochondrial β-oxidation can also be combined with mutations in certain enzymes of the Krebs cycle that are common in cancer. If overactivated β-oxidation is combined with other common cancer conditions, such as dysfunctions in the electron transport complexes, and/or hypoxia, this may alter the redox state of the mitochondrial matrix. We propose the idea that the altered mitochondrial redox state and/or inhibited Krebs cycle at certain segments may link mitochondrial β-oxidation to the citrate-malate shuttle instead to the Krebs cycle. We call this abnormal metabolic condition “β-oxidation shuttle”. It is unconventional mFAO, a separate metabolic pathway, unexplored so far as a source of energy, as well as a source of cataplerosis, leading to biomass accumulation, accelerated oxygen consumption, and ultimately a source of proliferation. It is inefficient as an energy source and must consume significantly more oxygen per mole of ATP produced when combined with acetyl-CoA consuming pathways, such as the FAS and mevalonate pathway.

Mechanisms underlying neonate-specific metabolic effects of volatile anesthetics

Volatile anesthetics (VAs) are widely used in medicine, but the mechanisms underlying their effects remain ill-defined. Though routine anesthesia is safe in healthy individuals, instances of sensitivity are well documented, and there has been significant concern regarding the impact of VAs on neonatal brain development. Evidence indicates that VAs have multiple targets, with anesthetic and non-anesthetic effects mediated by neuroreceptors, ion channels, and the mitochondrial electron transport chain. Here, we characterize an unexpected metabolic effect of VAs in neonatal mice. Neonatal blood β-hydroxybutarate (β-HB) is rapidly depleted by VAs at concentrations well below those necessary for anesthesia. β-HB in adults, including animals in dietary ketosis, is unaffected. Depletion of β-HB is mediated by citrate accumulation, malonyl-CoA production by acetyl-CoA carboxylase, and inhibition of fatty acid oxidation. Adults show similar significant changes to citrate and malonyl-CoA, but are insensitive to malonyl-CoA, displaying reduced metabolic flexibility compared to younger animals.

Chemical Proteomics Approach for Profiling the NAD Interactome

Nicotinamide adenine dinucleotide (NAD⁺) is a multifunctional molecule. Beyond redox metabolism, NAD⁺ has an equally important function as a substrate for post-translational modification enzymes, the largest family being the poly-ADP-ribose polymerases (PARPs, 17 family members in humans). The recent surprising discoveries of noncanonical NAD (NAD⁺/NADH)-binding proteins suggests that the NAD interactome is likely larger than previously thought; yet, broadly useful chemical tools for profiling and discovering NAD-binding proteins do not exist. Here, we describe the design, synthesis, and validation of clickable, photoaffinity labeling (PAL) probes, 2- and 6-ad-BAD, for interrogating the NAD interactome. We found that 2-ad-BAD efficiently labels PARPs in a UV-dependent manner. Chemical proteomics experiments with 2- and 6-ad-BAD identified known and unknown NAD⁺/NADH-binding proteins. Together, our study shows the utility of 2- and 6-ad-BAD as clickable PAL NAD probes.

Structure and allosteric regulation of human NAD-dependent isocitrate dehydrogenase

Human NAD-dependent isocitrate dehydrogenase or HsIDH3 catalyzes the decarboxylation of isocitrate into α-ketoglutarate in the TCA cycle. HsIDH3 exists and functions as a heterooctamer composed of the αβ and αγ heterodimers, and is regulated allosterically and/or competitively by numerous metabolites including CIT, ADP, ATP, and NADH. In this work, we report the crystal structure of HsIDH3 containing a β mutant in apo form. In the HsIDH3 structure, the αβ and αγ heterodimers form the α₂βγ heterotetramer via their clasp domains, and two α₂βγ heterotetramers form the (α₂βγ)₂ heterooctamer through insertion of the N-terminus of the γ subunit of one heterotetramer into the back cleft of the β subunit of the other heterotetramer. The functional roles of the key residues at the allosteric site, the pseudo allosteric site, the heterodimer and heterodimer-heterodimer interfaces, and the N-terminal of the γ subunit are validated by mutagenesis and kinetic studies. Our structural and biochemical data together demonstrate that the allosteric site plays an important role but the pseudo allosteric site plays no role in the allosteric activation of the enzyme; the activation signal from the allosteric site is transmitted to the active sites of both αβ and αγ heterodimers via the clasp domains; and the N-terminal of the γ subunit plays a critical role in the formation of the heterooctamer to ensure the optimal activity of the enzyme. These findings reveal the molecular mechanism of the assembly and allosteric regulation of HsIDH3.

CoRNeA: A Pipeline to Decrypt the Inter-Protein Interfaces from Amino Acid Sequence Information

Decrypting the interface residues of the protein complexes provides insight into the functions of the proteins and, hence, the overall cellular machinery. Computational methods have been devised in the past to predict the interface residues using amino acid sequence information, but all these methods have been majorly applied to predict for prokaryotic protein complexes. Since the composition and rate of evolution of the primary sequence is different between prokaryotes and eukaryotes, it is important to develop a method specifically for eukaryotic complexes. Here, we report a new hybrid pipeline for predicting the protein-protein interaction interfaces in a pairwise manner from the amino acid sequence information of the interacting proteins. It is based on the framework of Co-evolution, machine learning (Random Forest), and Network Analysis named CoRNeA trained specifically on eukaryotic protein complexes. We use Co-evolution, physicochemical properties, and contact potential as major group of features to train the Random Forest classifier. We also incorporate the intra-contact information of the individual proteins to eliminate false positives from the predictions keeping in mind that the amino acid sequence of a protein also holds information for its own folding and not only the interface propensities. Our prediction on example datasets shows that CoRNeA not only enhances the prediction of true interface residues but also reduces false positive rates significantly.

Antidotal effects of methylene blue against cyanide neurological toxicity: in vivo and in vitro studies

The aim of the present study was to determine whether methylene blue (MB) could directly oppose the neurological toxicity of a lethal cyanide (CN) intoxication. KCN, infused at the rate of 0.375 mg/kg/min intravenously, produced 100% lethality within 15 min in unanaesthetized rats (n = 12). MB at 10 (n = 5) or 20 mg/kg (n = 5), administered 3 min into CN infusion, allowed all animals to survive with no sequelae. No apnea and gasping were observed at 20 mg/kg MB (P < 0.001). The onset of coma was also significantly delayed and recovery from coma was shortened in a dose-dependent manner (median of 359 and 737 seconds, respectively, at 20 and 10 mg/kg). At 4 mg/kg MB (n = 5), all animals presented faster onset of coma and apnea and a longer period of recovery than at the highest doses (median 1344 seconds, P < 0.001). MB reversed NaCN-induced resting membrane potential depolarization and action potential depression in primary cultures of human fetal neurons intoxicated with CN. MB restored calcium homeostasis in the CN-intoxicated human SH-SY5Y neuroblastoma cell line. We conclude that MB mitigates the neuronal toxicity of CN in a dose-dependent manner, preventing the lethal depression of respiratory medullary neurons and fatal outcome.

Molecular mechanism of the dual regulatory roles of ATP on the αγ heterodimer of human NAD-dependent isocitrate dehydrogenase

Human NAD-dependent isocitrate dehydrogenase (NAD-IDH) is responsible for the catalytic conversion of isocitrate into α-ketoglutarate in the Krebs cycle. This enzyme exists as the α₂βγ heterotetramer composed of the αβ and αγ heterodimers. Our previous biochemical data showed that the αγ heterodimer and the holoenzyme can be activated by low concentrations of ATP but inhibited by high concentrations of ATP; however, the molecular mechanism was unknown. Here, we report the crystal structures of the αγ heterodimer with ATP binding only to the allosteric site (α^Mgγ^Mg+CIT+ATP) and to both the allosteric site and the active site (α^Mg+ATPγ^Mg+CIT+ATP). Structural data show that ATP at low concentrations can mimic ADP to bind to the allosteric site, which stabilizes CIT binding and leads the enzyme to adopt an active conformation, revealing why the enzyme can be activated by low concentrations of ATP. On the other hand, at high concentrations ATP is competitive with NAD for binding to the catalytic site. In addition, our biochemical data show that high concentrations of ATP promote the formation of metal ion-ATP chelates. This reduces the concentration of free metal ion available for the catalytic reaction, and thus further inhibits the enzymatic activity. The combination of these two effects accounts for the inhibition of the enzyme at high concentrations of ATP. Taken together, our structural and biochemical data reveal the molecular mechanism for the dual regulatory roles of ATP on the αγ heterodimer of human NAD-IDH.

Molecular basis for the function of the αβ heterodimer of human NAD-dependent isocitrate dehydrogenase

Mammalian mitochondrial NAD-dependent isocitrate dehydrogenase (NAD-IDH) catalyzes the decarboxylation of isocitrate into α-ketoglutarate in the tricarboxylic acid cycle. It exists as the α₂βγ heterotetramer composed of the αβ and αγ heterodimers. Different from the αγ heterodimer that can be allosterically activated by CIT and ADP, the αβ heterodimer cannot be allosterically regulated by the activators; however, the molecular mechanism is unclear. We report here the crystal structures of the αβ heterodimer of human NAD-IDH with the α subunit in apo form and in Ca²⁺-bound, NAD-bound, and NADH-bound forms. Structural analyses and comparisons reveal that the αβ heterodimer has a similar yet more compact overall structure compared with the αγ heterodimer and contains a pseudo-allosteric site that is structurally different from the allosteric site. In particular, the β3-α3 and β12-α8 loops of the β subunit at the pseudo-allosteric site adopt significantly different conformations from those of the γ subunit at the allosteric site and hence impede the binding of the activators, explaining why the αβ heterodimer cannot be allosterically regulated by the activators. The structural data also show that NADH can compete with NAD to bind to the active site and inhibits the activity of the αβ heterodimer. These findings together with the biochemical data reveal the molecular basis for the function of the αβ heterodimer of human NAD-IDH.

Antidotal Effects of the Phenothiazine Chromophore Methylene Blue Following Cyanide Intoxication

Our study was aimed at (1) determining the efficacy of the dye methylene blue (MB), following a rapidly lethal cyanide (CN) intoxication in un-sedated rats; (2) clarifying some of the mechanisms responsible for the antidotal properties produced by this potent cyclic redox dye. Sixty-nine awake rats acutely intoxicated by CN (IP, KCN 7 mg/kg) received saline, MB (20 mg/kg) or hydroxocobalamin (HyCo, 150 mg/kg) when in deep coma. Survival in this model was very low, reaching 9% at 60 min without any treatment. Methylene blue significantly increased survival (59%, p < .001) at 60 min, versus 37% with HyCo (p < .01). In addition, 8 urethane-anesthetized rats were exposed to a sublethal CN intoxication (KCN, 0.75 mg/kg/min IV for 4 min); they received MB (20 mg/kg, IV) or saline, 5 min after the end of CN exposure. All MB-treated rats displayed a significant reduction in hyperlactacidemia, a restoration of pyruvate/lactate ratio-a marker of NAD/NADH ratio-and an increase in CO2 production, a marker of the activity of the TCA cycle. These changes were also associated with a 2-fold increase in the pool of CN in red cells. Based on series of in vitro experiments, looking at the effects of MB on NADH, as well as the redox effects of MB on hemoglobin and cytochrome c, we hypothesize that the antidotal properties of MB can in large part be accounted for by its ability to readily restore NAD/NADH ratio and to cyclically re-oxidize then reduce the iron in hemoglobin and the electron chain complexes. All of these effects can account for the rapid antidotal properties of this dye following CN poisoning.

Hydrogen sulfide intoxication induced brain injury and methylene blue

Hydrogen sulfide (H₂S) remains a chemical hazard in the gas and farming industry. It is easy to manufacture from common chemicals and thus represents a potential threat for the civilian population. It is also employed as a method of suicide, for which incidence has recently increased in the US. H₂S is a mitochondrial poison and exerts its toxicity through mechanisms that are thought to result from its high affinity to various metallo-proteins (such as - but not exclusively- the mitochondrial cytochrome c oxidase) and interactions with cysteine residues of proteins. Ion channels with critical implications for the cardiac and the brain functions appear to be affected very early during and following H₂S exposure, an effect which is rapidly reversible during a light intoxication. However, during severe H₂S intoxication, a coma, associated with a reduction in cardiac contractility, develops within minutes or even seconds leading to death by complete electro-mechanical dissociation of the heart. If the level of intoxication is milder, a rapid and spontaneous recovery of the coma occurs as soon as the exposure stops. The risk, although probably very small, of developing long-term debilitating motor or cognitive deficits is present. One of the major challenges impeding our effort to offer an effective treatment against H₂S intoxication after exposure is that the pool of free/soluble H₂S almost immediately disappears from the body preventing agents trapping free H₂S (cobalt or ferric compounds) to play their protective role. This paper (1) presents and discusses the neurological symptoms and lesions observed in various animals models and in humans following an acute exposure to sub-lethal or lethal levels of H₂S, (2) reviews the potential interest of methylene blue (MB), a potent cyclic redox dye - currently used for the treatment of methemoglobinemia - which has potential rescuing effects on the mitochondrial activity, as an antidote against sulfide intoxication.

Methylene Blue Administration During and After Life-Threatening Intoxication by Hydrogen Sulfide: Efficacy Studies in Adult Sheep and Mechanisms of Action

Exposure to toxic levels of hydrogen sulfide (H2S) produces an acute cardiac depression that can be rapidly fatal. We sought to characterize the time course of the cardiac effects produced by the toxicity of H2S in sheep, a human sized mammal, and to describe the in vivo and in vitro antidotal properties of methylene blue (MB), which has shown efficacy in sulfide intoxicated rats. Infusing NaHS (720 mg) in anesthetized adult sheep produced a rapid dilation of the left ventricular with a decrease in contractility, which was lethal within about 10 min by pulseless electrical activity. MB (7 mg/kg), administered during sulfide exposure, maintained cardiac contractility and allowed all of the treated animals to recover. At a dose of 350 mg NaHS, we were able to produce an intoxication, which led to a persistent decrease in ventricular function for at least 1 h in nontreated animals. Administration of MB, 3 or 30 min after the end of exposure, whereas all free H2S had already vanished, restored cardiac contractility and the pyruvate/lactate (P/L) ratio. We found that MB exerts its antidotal effects through at least 4 different mechanisms: (1) a direct oxidation of free sulfide; (2) an increase in the pool of "trapped" H2S in red cells; (3) a restoration of the mitochondrial substrate-level phosphorylation; and (4) a rescue of the mitochondrial electron chain. In conclusion, H2S intoxication produces acute and long persisting alteration in cardiac function in large mammals even after all free H2S has vanished. MB exerts its antidotal effects against life-threatening sulfide intoxication via multifarious properties, some of them unrelated to any direct interaction with free H2S.