Superoxide dependence of the toxicity of short chain sugars

Cathodic Hydroxide Ions Induce Tetrose Formation during Glycolaldehyde Electroreduction to Alcohols: A Potential CO2-to-Carbohydrate Pathway

The electrochemical synthesis of organic compounds from CO2 can potentially alleviate climate change by hampering the atmospheric accumulation of greenhouse gases. The production of carbohydrates from CO2 reduction will have promising applications for the manufacturing of valuable, multi-carbon compounds that are traditionally produced from the petrochemical or agricultural industries. In this work, we analyzed the copper-catalyzed electrochemical reduction of glycolaldehyde, a commonly observed trace CO2RR product that has been previously proposed as an intermediate for alcohol formation. We determine that glycolaldehyde is not the main intermediate on polycrystalline copper-based electrocatalysts that selectively produce ethanol. In an unbuffered electrolyte, the cathodic hydroxide ions produced induce the coupling of glycolaldehyde to tetroses in the solution phase, yielding a maximum glycolaldehyde-to-sugar conversion of 47.2% under ambient conditions. Using in situ infrared spectroscopy coupled with density functional theory (DFT) calculations, we show that glycolaldehyde reduction to alcohols proceeds via adsorption of its enol tautomer, η2(C,C)─CHOH═CHOH. Our findings not only shed light on the C2 alcohol formation pathways during CO2RR, but also imply that a CO2 electrolyzer can potentially produce C4 carbohydrates via CO2 reduction to glycolaldehyde followed by C─C coupling in the solution phase, with only a high local pH needed to drive the tetrose formation step.

From plastic waste to bioprocesses: Using ethylene glycol from polyethylene terephthalate biodegradation to fuel Escherichia coli metabolism and produce value-added compounds

Polyethylene Terephthalate (PET) is a petroleum-based plastic polymer that, by design, can last decades, if not hundreds of years, when released into the environment through plastic waste leakage. In the pursuit of sustainable solutions to plastic waste recycling and repurposing, the enzymatic depolymerization of PET has emerged as a promising green alternative. However, the metabolic potential of the resulting PET breakdown molecules, such as the two-carbon (C2) molecule ethylene glycol (EG), remains largely untapped. Here, we review and discuss the current state of research regarding existing natural and synthetic microbial pathways that enable the assimilation of EG as a carbon and energy source for Escherichia coli. Leveraging the metabolic versatility of E. coli, we explore the viability of this widely used industrial strain in harnessing EG as feedstock for the synthesis of target value-added compounds via metabolic and protein engineering strategies. Consequently, we assess the potential of EG as a versatile alternative to conventional carbon sources like glucose, facilitating the closure of the loop between the highly available PET waste and the production of valuable biochemicals. This review explores the interplay between PET biodegradation and EG metabolism, as well as the key challenges and opportunities, while offering perspectives and suggestions for propelling advancements in microbial EG assimilation for circular economy applications.

Haemophilus influenzae strains possess variations in the global transcriptional profile in response to oxygen levels and this influences sensitivity to environmental stresses

An alcohol dehydrogenase, AdhC, is required for Haemophilus influenzae Rd KW20 growth with high oxygen. AdhC protects against both exogenous and metabolically generated, endogenous reactive aldehydes. However, adhC in the strain 86-028NP is a pseudogene. Unlike the Rd KW20 adhC mutant, 86-028NP does grow with high oxygen. This suggests the differences between Rd KW20 and 86-028NP include broader pathways, such as for the maintenance of redox and metabolism that avoids the toxicity related to oxygen. We hypothesized that these differences affect their resistance to relevant toxic chemicals, including reactive aldehydes. Across a range of oxygen concentrations, despite the growth profiles of Rd KW20 and 86-028NP being similar, there was a significant variation in their sensitivity to reactive aldehydes. 86-028NP is more sensitive to methylglyoxal, formaldehyde and glycolaldehyde under high oxygen than low oxygen as well as compared to Rd KW20. Also, as oxygen levels changed the whole genome gene expression profiles of Rd KW20 and 86-028NP revealed distinctions in their transcriptomes (the iron, FNR and ArcAB regulons). These were indicative of a difference in their intracellular redox properties and we show it is this that underpins their survival against reactive aldehydes.

Glyoxylate, a new marker metabolite of type 2 diabetes

Type 2 diabetes (T2D) is characterized by a variety of metabolic impairments that are closely linked to nonenzymatic glycation reactions of proteins and peptides resulting in advanced glycation end-products (AGEs). Reactive aldehydes derived from sugars play an important role in the generation of AGEs. Using metabolite profiling to characterize human plasma from diabetic versus nondiabetic subjects we observed in a recent study that the reactive aldehyde glyoxylate was increased before high levels of plasma glucose, typical for a diabetic condition, could be measured. Following this observation, we explored the relevance of increased glyoxylate in diabetic subjects and in diabetic C57BLKS/J-Lepr (db/db (-/-)) mice in the pathophysiology of diabetes. A retrospective study using samples of long-term blood donors revealed that glyoxylate levels unlike glucose levels became significantly elevated up to 3 years prior to diabetes diagnosis (difference to control P = 0.034). Elevated glyoxylate levels impact on newly identified mechanisms linking hyperglycemia and AGE production with diabetes-associated complications such as diabetic nephropathy. Glyoxylate in its metabolic network may serve as an early marker in diabetes diagnosis with predictive qualities for associated complications and as potential to guide the development of new antidiabetic therapies.

A global perspective of the genetic basis for carbonyl stress resistance

The accumulation of protein adducts caused by carbonyl stress (CS) is a hallmark of cellular aging and other diseases, yet the detailed cellular effects of this universal phenomena are poorly understood. An understanding of the global effects of CS will provide insight into disease mechanisms and can guide the development of therapeutics and lifestyle changes to ameliorate their effects. To identify cellular functions important for the response to carbonyl stress, multiple genome-wide genetic screens were performed using two known inducers of CS. We found that different cellular functions were required for resistance to stress induced by methylglyoxal (MG) and glyoxal (GLY). Specifically, we demonstrate the importance of macromolecule catabolism processes for resistance to MG, confirming and extending known mechanisms of MG toxicity, including modification of DNA, RNA, and proteins. Combining our results with related studies that examined the effects of ROS allowed a comprehensive view of the diverse range of cellular functions affected by both oxidative and carbonyl stress. To understand how these diverse cellular functions interact, we performed a quantitative epistasis analysis by creating multimutant strains from those individual genes required for glyoxal resistance. This analysis allowed us to define novel glyoxal-dependent genetic interactions. In summary, using multiple genome-wide approaches provides an effective approach to dissect the poorly understood effects of glyoxal in vivo. These data, observations, and comprehensive dataset provide 1) a comprehensive view of carbonyl stress, 2) a resource for future studies in other cell types, and 3) a demonstration of how inexpensive cell-based assays can identify complex gene-environment toxicities.

Aldehyde sources, metabolism, molecular toxicity mechanisms, and possible effects on human health

Aldehydes are organic compounds that are widespread in nature. They can be formed endogenously by lipid peroxidation (LPO), carbohydrate or metabolism ascorbate autoxidation, amine oxidases, cytochrome P-450s, or myeloperoxidase-catalyzed metabolic activation. This review compares the reactivity of many aldehydes towards biomolecules particularly macromolecules. Furthermore, it includes not only aldehydes of environmental or occupational concerns but also dietary aldehydes and aldehydes formed endogenously by intermediary metabolism. Drugs that are aldehydes or form reactive aldehyde metabolites that cause side-effect toxicity are also included. The effects of these aldehydes on biological function, their contribution to human diseases, and the role of nucleic acid and protein carbonylation/oxidation in mutagenicity and cytotoxicity mechanisms, respectively, as well as carbonyl signal transduction and gene expression, are reviewed. Aldehyde metabolic activation and detoxication by metabolizing enzymes are also reviewed, as well as the toxicological and anticancer therapeutic effects of metabolizing enzyme inhibitors. The human health risks from clinical and animal research studies are reviewed, including aldehydes as haptens in allergenic hypersensitivity diseases, respiratory allergies, and idiosyncratic drug toxicity; the potential carcinogenic risks of the carbonyl body burden; and the toxic effects of aldehydes in liver disease, embryo toxicity/teratogenicity, diabetes/hypertension, sclerosing peritonitis, cerebral ischemia/neurodegenerative diseases, and other aging-associated diseases.

Metabolism, not autoxidation, plays a role in alpha-oxoaldehyde- and reducing sugar-induced erythrocyte GSH depletion: relevance for diabetes mellitus

Erythrocyte and lens reduced glutathione (GSH) levels are often lower in patients with diabetes whereas erythrocyte dicarbonyl levels are often higher. We hypothesise that high plasma carbohydrates may be metabolised by glycolytic and pentose phosphate pathways to form alpha-oxoaldehydes, which deplete cellular GSH. Our aims were: (1) to compare the effectiveness of various carbohydrates or metabolites at depleting erythrocyte GSH, (2) to determine if GSH loss is related to the autoxidation or metabolism of carbohydrates. It was found that erythrocyte GSH was depleted by 50% (ED-50) at t = 2.5 h when erythrocytes were incubated with the following: methylglyoxal (MG) 23 microM, glyoxal 75 microM, DL-glyceraldehyde 299 microM, deoxyribose 606 microM, xylitol 626 microM, and ribose 2 mM. The glycolytic inhibitors, sodium arsenate and KF prevented ribose, deoxyribose, xylitol and MG-induced GSH depletion in erythrocytes over 2 h. However, the antioxidant trolox and the ferric chelator detapac did not affect MG-induced GSH depletion. These data suggest that the carbohydrates or glyceraldehyde were metabolised to form carbonyls such as MG which depleted erythrocyte GSH as a result of catalysis by glyoxalase I. None of the carbohydrates were autoxidised to carbonyls over this time period. We speculate that as a result of GSH depletion, subsequent glycoxidative stress affects erythrocyte function and contributes to diabetic complications.

Hyperglycemia increases mitochondrial superoxide in retina and retinal cells

Oxidative stress is believed to play a significant role in the development of diabetic retinopathy. In this study, we have investigated the effects of elevated glucose concentration on the production of superoxide anion by retina and retinal cells, the cellular source of the superoxide, the effect of therapies that are known to inhibit diabetic retinopathy on the superoxide production, and the role of the superoxide in cell death in elevated glucose concentration. Superoxide release was measured from retinas collected from streptozotocin-diabetic rats (2 months) treated with or without aminoguanidine, aspirin, or vitamin E, and from transformed retinal Müller cells (rMC-1) and bovine retinal endothelial cells (BREC) incubated in normal (5 mM) and high (25 mM) glucose. Diabetes (retina) or incubation in elevated glucose concentration (rMC-1 and BREC cells) significantly increased superoxide production, primarily from mitochondria, because an inhibitor of mitochondrial electron transport chain complex II normalized superoxide production. Inhibition of reduced nicotinamine adenine dinucleotide phosphate (NADPH) oxidase or nitric oxide synthase had little or no effect on the glucose-induced increase in superoxide. Treatment of diabetic animals with aminoguanidine, aspirin, or vitamin E for 2 months significantly inhibited the diabetes-induced increase in production of superoxide in the retinas. Despite the increased production of superoxide, no increase in protein carbonyls was detected in retinal proteins from animals diabetic for 2-6 months or rMC-1 cells incubated in 25 mM glucose for 5 d unless the activities of calpain or the proteosome were inhibited. Addition of copper/zinc-containing superoxide dismutase to the media of rMC-1 and BREC cells inhibited the apoptotic death caused by elevated glucose. Diabetes-like glucose concentration increases superoxide production in retinal cells, and the superoxide contributes to impaired viability and increased cell death under those circumstances. Three therapies that inhibit the development of diabetic retinopathy all inhibit superoxide production, raising a possibility that these therapies inhibit retinopathy in part by inhibiting a hyperglycemia-induced increase in superoxide production.

Induction of the soxRS regulon of Escherichia coli by glycolaldehyde

The ability of short-chain sugars to cause oxidative stress has been examined using glycolaldehyde as the simplest sugar. Short-chain sugars autoxidize in air, producing superoxide and alpha,beta-dicarbonyls. In Escherichia coli the soxRS regulon mediates an oxidative stress response, which protects the cell against both superoxide-generating agents and nitric oxide. In superoxide dismutase-deficient E. coli mutants, glycolaldehyde induces fumarase C and nitroreductase A, which are regulated as members of the soxRS regulon. A mutational defect in soxRS eliminates that induction. This establishes that glycolaldehyde can cause induction of this defensive regulon. This effect of glycolaldehyde was oxygen-dependent, was not shown by glyoxal, and was not seen in the superoxide dismutase-replete parental strain, and it was abolished by a cell-permeable SOD mimetic. All of these suggest that superoxide radicals produced by the oxidation of glycolaldehyde played a key role in the induction.

Metabolic switches of T-cell activation and apoptosis

The signaling networks that mediate activation, proliferation, or programmed cell death of T lymphocytes are dependent on complex redox and metabolic pathways. T lymphocytes are primarily activated through the T-cell receptor and co-stimulatory molecules. Although activation results in lymphokine production, proliferation, and clonal expansion, it also increases susceptibility to apoptosis upon crosslinking of cell-surface death receptors or exposure to toxic metabolites. Activation signals are transmitted by receptor-associated protein tyrosine kinases and phosphatases through calcium mobilization to a secondary cascade of kinases, which in turn activate transcription factors initiating cell proliferation and cytokine production. Initiation and activity of cell death-mediating proteases are redox-sensitive and dependent on energy provided by ATP. Mitochondria play crucial roles in providing ATP for T-cell activation through the electron transport chain and oxidative phosphorylation. The mitochondrial transmembrane potential (DeltaPsi(m)) plays a decisive role not only by driving ATP synthesis, but also by controlling reactive oxygen species production and release of cell death-inducing factors. DeltaPsi(m) and reactive oxygen species levels are regulated by the supply of reducing equivalents, glutathione and thioredoxin, as well as NADPH generated in the pentose phosphate pathway. This article identifies redox and metabolic checkpoints controlling activation and survival of T lymphocytes.

The role of alpha,beta -dicarbonyl compounds in the toxicity of short chain sugars

The extent to which sugars serve as targets for superoxide was examined using glycolaldehyde as the simplest sugar and using superoxide dismutase (SOD)-replete and SOD-null strains growing under aerobic and anaerobic conditions. Glycolaldehyde was more toxic to the SOD-null strain than to its SOD-replete parent, and this differential effect was oxygen-dependent. The product, glyoxal, could be trapped in the medium by 1,2-diaminobenzene and assayed as quinoxaline. The SOD-null strain produced more glyoxal and eliminated it more slowly than the SOD-replete parent strain. Glyoxal was approximately 10 times more toxic than glycolaldehyde and was more toxic to the SOD-null strain than to the parental strain. 1,2-Diaminobenzene protected against the toxicity of glycolaldehyde. These Escherichia coli strains contained the glutathione-dependent glyoxalases I and II, as well as the glutathione-independent glyoxalase III. Of these enzymes, glyoxalase III was most abundant, and it was inactivated within the aerobic SOD-null strain and also in extracts when exposed to the flux of superoxide and hydrogen peroxide imposed by the xanthine oxidase reaction. Thus, it appears that short chain sugars are oxidized by superoxide yielding toxic dicarbonyls. Moreover, the defensive glyoxalase III is also inactivated by the oxidative stress imposed by the lack of SOD, thereby exacerbating the deleterious effect of sugar oxidation.

The yeast heat shock transcription factor changes conformation in response to superoxide and temperature

In vitro DNA-binding assays demonstrate that the heat shock transcription factor (HSF) from the yeast Saccharomyces cerevisiae can adopt an altered conformation when stressed. This conformation, reflected in a change in electrophoretic mobility, requires that two HSF trimers be bound to DNA. Single trimers do not show this change, which appears to represent an alteration in the cooperative interactions between trimers. HSF isolated from stressed cells displays a higher propensity to adopt this altered conformation. Purified HSF can be stimulated in vitro to undergo the conformational change by elevating the temperature or by exposing HSF to superoxide anion. Mutational analysis maps a region critical for this conformational change to the flexible loop between the minimal DNA-binding domain and the flexible linker that joins the DNA-binding domain to the trimerization domain. The significance of these findings is discussed in the context of the induction of the heat shock response by ischemic stroke, hypoxia, and recovery from anoxia, all known to stimulate the production of superoxide.

Genetic and metabolic control of the mitochondrial transmembrane potential and reactive oxygen intermediate production in HIV disease

Redox mechanims play important roles in replication of human immunodeficiency virus type 1 (HIV-1) and cellular susceptibility to apoptosis signals. Viral replication and accelerated turnover of CD4+ T cells occur throughout a prolonged asymptomatic phase in patients infected by HIV-1. Disease development is associated with steady loss of CD4+ T cells by apoptosis, increased rate of opportunistic infections and lymphoproliferative diseases, disruption of energy metabolism, and generalized wasting. Such pathological states are preceded by: (i) depletion of intracellular antioxidants, glutathione (GSH) and thioredoxin (TRX), (ii) increased reactive oxygen species (ROS) production, and (iii) changes in mitochondrial transmembrane potential (deltapsi(m)). Disruption of deltapsi(m) appears to be the point of no return in the effector phase of apoptosis. Viral proteins Tat, Nef, Vpr, protease, and gp120, have been implicated in initiation and/or intensification of oxidative stress and disruption of deltapsi(m). Redox-sensitive transcription factors, NF-kappaB, AP-1, and p53, support expression of viral genes and proinflammatory lymphokines. ROS regulate apoptosis signaling through Fas, tumor necrosis factor (TNF), and related cell death receptors, as well as the T-cell receptor. Oxidative stress in HIV-infected donors is accompanied by increased glucose utilization both on the cellular and organismal levels. Generation of GSH and TRX from their corresponding oxidized forms is dependent on NADPH provided through the pentose phosphate pathway of glucose metabolism. This article seeks to delineate the genetic and metabolic bases of HIV-induced oxidative stress. Such understanding should lead to development of effective antioxidant therapies in HIV disease.

Hydrogen peroxide in human urine: implications for antioxidant defense and redox regulation

The presence of hydrogen peroxide, at levels sometimes exceeding 100 microM, in human urine samples was established by three different assay methods: 2-oxoglutarate decarboxylation and the ferrous oxidation-xylenol orange (FOX) assay and an oxygen electrode. Detected levels of H(2)O(2) were decreased by addition of superoxide dismutase. We conclude that urine contains autooxidizable molecules that, upon exposure to 21% O(2), undergo rapid superoxide-dependent autooxidation reactions to generate H(2)O(2). The exposure of human tissues to hydrogen peroxide may be greater than is commonly supposed, which has implications in relation to the proposed role of this species in cell signaling.

Why superoxide imposes an aromatic amino acid auxotrophy on Escherichia coli. The transketolase connection

The lack of superoxide dismutase and the consequent elevation of [O2-] imposes, on Escherichia coli, auxotrophies for branched chain, sulfur-containing, and aromatic amino acids. The former two classes of auxotrophies have already been explained, whereas the third is explained herein. Thus O2- is shown to interfere with the production of erythrose-4-phosphate, which is essential for the first step of the aromatic biosynthetic pathway. It does so by oxidizing the 1, 2-dihydroxyethyl thiamine pyrophosphate intermediate of transketolase and inactivating this enzyme.