The kinetics of oxidation of GSH by protein radicals

Tracing the serendipitous genesis of radiation resistance

Free-living organisms frequently encounter unfavorable abiotic environmental factors. Those who adapt and cope with sudden changes in the external environment survive. Desiccation is one of the most common and frequently encountered stresses in nature. On the contrary, ionizing radiations are limited to high local concentrations of naturally occurring radioactive materials and related anthropogenic activities. Yet, resistance to high doses of ionizing radiation is evident across the tree of life. The evolution of desiccation resistance has been linked to the evolution of ionizing radiation resistance, although, evidence to support the idea that the evolution of desiccation tolerance is a necessary precursor to ionizing radiation resistance is lacking. Moreover, the presence of radioresistance in hyperthermophiles suggests multiple paths lead to radiation resistance. In this minireview, we focus on the molecular aspects of damage dynamics and damage response pathways comprising protective and restorative functions with a definitive survival advantage, to explore the serendipitous genesis of ionizing radiation resistance.

Radiation-Chemical Oxygen Depletion Depends on Chemical Environment and Dose Rate: Implications for the FLASH Effect

PURPOSE

FLASH (dose rates >40 Gy/s) radiation therapy protects normal tissues from radiation damage, compared with conventional radiation therapy (∼Gy/m). Radiation-chemical oxygen depletion (ROD) occurs when oxygen reacts with radiation-induced free radicals, so a possible mechanism for FLASH involves radioprotection by the decreased oxygen as ROD occurs. High ROD rates would favor this mechanism, but prior studies have reported low ROD values (∼0.35 µM/Gy) in chemical environments such as water and protein/nutrient solutions. We proposed that intracellular ROD might be much larger, possibly promoted by its strongly reducing chemical environment.

METHODS AND MATERIALS

ROD was measured, using precision polarographic sensors, from ∼100 µM to zero in solutions containing intracellular reducing agents ± glycerol (1M), to simulate intracellular reducing and hydroxyl-radical-scavenging capacity. Cs irradiators and a research proton beamline allowed dose rates from 0.0085 to 100 Gy/s.

RESULTS

Reducing agents significantly altered ROD values. Most greatly increased ROD but some (eg, ascorbate) actually decreased ROD and additionally imposed an oxygen dependence of ROD at low oxygen concentrations. The highest values of ROD were found at low dose rates, but these montonically decreased with increasing dose rate.

CONCLUSIONS

ROD was greatly augmented by some intracellular reducing agents but others (eg, ascorbate) effectively reversed this effect. Ascorbate had its greatest effect at low oxygen concentrations. ROD decreased with increasing dose rate in most cases.

Endogenous Photosensitizers in Human Skin

Endogenous photosensitizers play a critical role in both beneficial and harmful light-induced transformations in biological systems. Understanding their mode of action is essential for advancing fields such as photomedicine, photoredox catalysis, environmental science, and the development of sun care products. This review offers a comprehensive analysis of endogenous photosensitizers in human skin, investigating the connections between their electronic excitation and the subsequent activation or damage of organic biomolecules. We gather the physicochemical and photochemical properties of key endogenous photosensitizers and examine the relationships between their chemical reactivity, location within the skin, and the primary biochemical events following solar radiation exposure, along with their influence on skin physiology and pathology. An important take-home message of this review is that photosensitization allows visible light and UV-A radiation to have large effects on skin. The analysis presented here unveils potential causes for the continuous increase in global skin cancer cases and emphasizes the limitations of current sun protection approaches.

Exploring the Association between Glutathione Metabolism and Ferroptosis in Osteoblasts with Disuse Osteoporosis and the Key Genes Connecting them

Disused osteoporosis is a kind of osteoporosis, a common age-related disease. Neurological disorders are major risk factors for osteoporosis. Though there are many studies on disuse osteoporosis, the genetic mechanisms for the association between glutathione metabolism and ferroptosis in osteoblasts with disuse osteoporosis are still unclear. The purpose of this study is to explore the key genes and other related mechanism of ferroptosis and glutathione metabolism in osteoblast differentiation and disuse osteoporosis. By weighted gene coexpression network analysis (WGCNA), the process of osteoblast differentiation-related genes was studied in GSE30393. And the related functional pathways were found through the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. By combining GSE1367 and GSE100933 together, key genes which were separately bound up with glutathione metabolism and ferroptosis were located. The correlation of these key genes was analyzed by the Pearson correlation coefficient. GSTM1 targeted agonist glutathione (GSH) selected by connectivity map (CMap) analysis was used to interfere with the molding disused osteoporosis process in MC3T3-E1 cells. RT-PCR and intracellular reactive oxygen species (ROS) were performed. Two important pathways, glutathione metabolism and ferroptosis pathways, were found. GSTM1 and TFRC were thought as key genes in disuse osteoporosis osteoblasts with the two mechanisms. The two genes have a strong negative correlation. Our experiment results showed that the expression of TFRC was consistent with the negative correlation with the activation process of GSTM1. The strong relationship between the two genes was proved. Glutathione metabolism and ferroptosis are important in the normal differentiation of osteoblasts and the process of disuse osteoporosis. GSTM1 and TFRC were the key genes. The two genes interact with each other, which can be seen as a bridge between the two pathways. The two genes participate in the process of reducing ROS in disuse osteoporosis osteoblasts.

Chemical repair of damaged leucine and tryptophane by thiophenols at close to diffusion-controlled rates: Mechanisms and kinetics

Thiophenols are chemical species with multiple desirable biological properties, including their primary and secondary antioxidant capacity. In this work, the repairing antioxidant activity of eight different thiophenols has been investigated for damaged leucine and tryptophane. The investigation was carried out employing quantum mechanical and transition state methods to calculate the thermodynamic and kinetic data of the reactions involved, while simulating the biological conditions at physiological pH and aqueous and lipidic medium. The analysis of the atomic charges and the spin densities at each of the points on the potential energy surface was the tool that allowed the elucidation of the reaction mechanisms through which thiophenols repair the oxidative damage caused to the amino acids leucine and tryptophan. It was found that thiophenols can repair leucine via a hydrogen atom transfer mechanism in a manner which is similar to the one used by glutathione to repair the carbon-centered radicals of guanosine. In addition, thiophenols can also restore tryptophane, a nitrogen-centered radical, via proton-coupled electron transfer and single electron transfer mechanisms. Moreover, both processes occur at close to diffusion-controlled rates.

Oxidative Crosslinking of Peptides and Proteins: Mechanisms of Formation, Detection, Characterization and Quantification

Covalent crosslinks within or between proteins play a key role in determining the structure and function of proteins. Some of these are formed intentionally by either enzymatic or molecular reactions and are critical to normal physiological function. Others are generated as a consequence of exposure to oxidants (radicals, excited states or two-electron species) and other endogenous or external stimuli, or as a result of the actions of a number of enzymes (e.g., oxidases and peroxidases). Increasing evidence indicates that the accumulation of unwanted crosslinks, as is seen in ageing and multiple pathologies, has adverse effects on biological function. In this article, we review the spectrum of crosslinks, both reducible and non-reducible, currently known to be formed on proteins; the mechanisms of their formation; and experimental approaches to the detection, identification and characterization of these species.

Reactive Oxygen Species (ROS) Regulates Different Types of Cell Death by Acting as a Rheostat

Reactive oxygen species (ROS) are essential for cellular signaling and response to stress. The level of ROS and the type of ROS determine the ability of cells to undergo cell death. Furthermore, dysregulation of the antioxidant pathways is associated with many diseases. It has become apparent that cell death can occur through different mechanisms leading to the classifications of different types of cell death such as apoptosis, ferroptosis, and necroptosis. ROS play essential roles in all forms of cell death, but it is only now coming into focus that ROS control and determine the type of cell death that occurs in any given cell. Indeed, ROS may act as a rheostat allowing different cell death mechanisms to be engaged and crosstalk with different cell death types. In this review, we will describe the ROS regulatory pathways and how they control different types of cell death under normal and disease states. We will also propose how ROS could provide a mechanism of crosstalk between cell death mechanisms and act as a rheostat determining the type of cell death.

Modelling the repair of carbon-centred protein radicals by the antioxidants glutathione and Trolox

GSH can repair carbon-centred protein radicals with rate constants in the diffusion limit, but Trolox repairs are much slower.

Deinococcus radiodurans' SRA-HNH domain containing protein Shp (Dr1533) is involved in faithful genome inheritance maintenance following DNA damage

BACKGROUND

Deinococcus radiodurans R1 (DR) survives conditions of extreme desiccation, irradiation and exposure to genotoxic chemicals, due to efficient DNA breaks repair, also through Mn²⁺ protection of DNA repair enzymes.

METHODS

Possible annotated domains of the DR1533 locus protein (Shp) were searched by bioinformatic analysis. The gene was cloned and expressed as fusion protein. Band-shift assays of Shp or the SRA and HNH domains were performed on oligonucleotides, genomic DNA from E. coli and DR. shp knock-out mutant was generated by homologous recombination with a kanamycin resistance cassette.

RESULTS

DR1533 contains an N-terminal SRA domain and a C-terminal HNH motif (SRA-HNH Protein, Shp). Through its SRA domain, Shp binds double-strand oligonucleotides containing 5mC and 5hmC, but also unmethylated and mismatched cytosines in presence of Mn²⁺. Shp also binds to Escherichia coli dcm⁺ genomic DNA, and to cytosine unmethylated DR and E. coli dcm^- genomic DNAs, but only in presence of Mn²⁺. Under these binding conditions, Shp displays DNAse activity through its HNH domain. Shp KO enhanced >100 fold the number of spontaneous mutants, whilst the treatment with DNA double strand break inducing agents enhanced up to 3-log the number of survivors.

CONCLUSIONS

The SRA-HNH containing protein Shp binds to and cuts 5mC DNA, and unmethylated DNA in a Mn²⁺ dependent manner, and might be involved in faithful genome inheritance maintenance following DNA damage.

GENERAL SIGNIFICANCE

Our results provide evidence for a potential role of DR Shp protein for genome integrity maintenance, following DNA double strand breaks induced by genotoxic agents.

Antioxidants in the Fight Against Atherosclerosis: Is This a Dead End?

PURPOSE OF REVIEW

The purpose of this review is to focus on the outcome of recent antioxidant interventions using synthetic and naturally occurring molecules established as adjuvant strategies to lipid-lowering or anti-inflammatory therapies designed to reduce the risk of cardiovascular disease.

RECENT FINDINGS

To date, accumulated evidence regarding oxidation as a pro-atherogenic factor indicates that redox biochemical events involved in atherogenesis are indeed a very attractive target for the management of cardiovascular disease in the clinic. Nevertheless, although evidence indicates that redox reactions are important in the initiation and progression of atherosclerosis, oxidation with a pro-atherogenic context does not eliminate the fact that oxidation participates in many cases as an essential messenger of important cellular signaling pathways. Therefore, disease management and therapeutic goals require not only high-precision and high-sensitivity methods to detect in plasma very low amounts of reducing and oxidizing molecules but also a much better understanding of the normal processes and metabolic pathways influenced and/or controlled by oxidative stress. As several methodologies have been specifically described for the quantification of the total antioxidant capacity and the oxidation state of diverse biological systems, a successful way to carefully study how redox reactions influence atherosclerosis can be achieved. Since there is still a lack of standardization with many of these methods, clinical trials studying antioxidant capacity have been difficult to compare and therefore difficult to use in order to reach a conclusion. We believe a comprehensive analysis of new knowledge and its relationship with the presence of plasma antioxidants and their reducing capacity will undoubtedly open new ways to understand and develop new therapeutic pathways in the fight not only against atherosclerosis but also against other degenerative diseases.

Reaction rates of glutathione and ascorbate with alkyl radicals are too slow for protection against protein peroxidation in vivo

Reaction kinetics of amino acid and peptide alkyl radicals with GSH and ascorbate, the two most abundant endogenous antioxidants, were investigated by pulse radiolysis. Rate constants in the order of 10⁶ M^-1s^-1 were found. Alkyl radicals react at almost diffusion controlled rates and irreversibly with oxygen to form peroxyl radicals, and competition with this reaction is the benchmark for efficient repair in vivo. We consider repair of protein radicals and assume comparable rate constants for the reactions of GSH/ascorbate with peptide alkyl radicals and with alkyl radicals on a protein surface. Given physiological concentrations of oxygen, GSH and ascorbate, protein peroxyl radicals will always be a major product of protein alkyl radicals in vivo. Therefore, if they are formed by oxidative stress, protein alkyl radicals are a probable cause for biological damage.

Physiological Concentrations of Ascorbate Cannot Prevent the Potentially Damaging Reactions of Protein Radicals in Humans

The principal initial biological targets of free radicals formed under conditions of oxidative stress are the proteins. The most common products of the interaction are carbon-centered alkyl radicals which react rapidly with oxygen to form peroxyl radicals and hydroperoxides. All these species are reactive, capable of propagating the free radical damage to enzymes, nucleic acids, lipids, and endogenous antioxidants, leading finally to the pathologies associated with oxidative stress. The best chance of preventing this chain of damage is in early repair of the protein radicals by antioxidants. Estimate of the effectiveness of the physiologically significant antioxidants requires knowledge of the antioxidant tissue concentrations and rate constants of their reaction with protein radicals. Previous studies by pulse radiolysis have shown that only ascorbate can repair the Trp and Tyr protein radicals before they form peroxyl radicals under physiological concentrations of oxygen. We have now extended this work to other protein C-centered radicals generated by hydroxyl radicals because these and many other free radicals formed under oxidative stress can produce secondary radicals on virtually any amino acid residue. Pulse radiolysis identified two classes of rate constants for reactions of protein radicals with ascorbate, a faster one in the range (9-60) × 10⁷ M^-1 s^-1 and a slow one with a range of (0.5-2) × 10⁷ M^-1 s^-1. These results show that ascorbate can prevent further reactions of protein radicals only in the few human tissues where its concentration exceeds about 2.5 mM.

Lipid accumulation in prokaryotic microorganisms from arid habitats

This review shall provide support for the suitability of arid environments as preferred location to search for unknown lipid-accumulative bacteria. Bacterial lipids are attracting more and more attention as sustainable replacement for mineral oil in fuel and plastic production. The development of prokaryotic microorganisms in arid desert habitats is affected by its harsh living conditions. Drought, nutrient limitation, strong radiation, and extreme temperatures necessitate effective adaption mechanisms. Accumulation of storage lipids as energy reserve and source of metabolic water represents a common adaption in desert animals and presumably in desert bacteria and archaea as well. Comparison of corresponding literature resulted in several bacterial species from desert habitats, which had already been described as lipid-accumulative elsewhere. Based on the gathered information, literature on microbial communities in hot desert, cold desert, and humid soil were analyzed on its content of lipid-accumulative bacteria. With more than 50% of the total community size in single studies, hot deserts appear to be more favorable for lipid-accumulative species then humid soil (≤20%) and cold deserts (≤17%). Low bacterial lipid accumulation in cold deserts is assumed to result from the influence of low temperatures on fatty acids and the increased necessity of permanent adaption methods.

Chemical repair of protein carbon-centred radicals: long-distance dynamic factors

The thermodynamic and kinetic study of the repair reactions of three damaged aliphatic amino acids (alanine, valine, and leucine) with dihydrolipoic acid (DHLA) in a polar and a nonpolar solvent is presented in this work. Two simplified protein models were explored in the most common conformations (alpha helix and beta sheet). Calculations are performed at the M06-2X-SMD/6-31++G(d,p) level of theory. DHLA has shown to be an excellent antioxidant repair agent through hydrogen-transfer reaction involving the thiol groups, with rate constants close to diffusion control in most cases. The stability of the initial protein radical is not the most important factor determining the rate of the repair reaction because stabilizing intermolecular interactions involving the protein and the antioxidant can provide additional stability to some transition states accelerating the repair of sites that would otherwise not be so quickly repaired.

Oxidative stress, free radicals and protein peroxides

Primary free radicals generated under oxidative stress in cells and tissues produce a cascade of reactive secondary radicals, which attack biomolecules with efficiency determined by the reaction rate constants and target concentration. Proteins are prominent targets because they constitute the bulk of the organic content of cells and tissues and react readily with many of the secondary radicals. The reactions commonly lead to the formation of carbon-centered radicals, which generally convert in vivo to peroxyl radicals and finally to semistable hydroperoxides. All of these intermediates can initiate biological damage. This article outlines the advantages of the application of ionizing radiations to studies of radicals, with particular reference to the generation of desired radicals, studies of the kinetics of their reactions and correlating the results with events in biological systems. In one such application, formation of protein hydroperoxides in irradiated cells was inhibited by the intracellular ascorbate and glutathione.

Modelling the chemical repair of protein carbon-centered radicals formed via oxidative damage with dihydrolipoic acid

Dihydrolipoic acid repairs carbon-centred radicals at diffusion-controlled rates via HAT mechanism.

2,2'-Azobis(isobutyronitrile)-derived alkylperoxyl radical scavenging activity assay of hydrophilic antioxidants by employing EPR spin trap method

As interest in the study of antioxidant intake from foods and other agricultural products increases, methods for performing radical scavenging activity assays based on the electron paramagnetic resonance spectroscopic method, in which there is no interference from the sample color and turbidity, are required. In this study, we have developed a rapid and simple electron paramagnetic resonance based assay to evaluate the alkylperoxyl radical scavenging activity of several antioxidants. The alkylperoxyl radical species was generated by the photolysis of azo-radical initiator 2,2'-azobis(isobutyronitrile), in which the radical generation rate and period were controlled by the illumination light. The relative alkylperoxyl radical scavenging activity was obtained by a simple formula of competing reaction of antioxidant and spin trap toward the oxygen radical. The scavenging activities toward alkylperoxyl radical and alkoxy radical species were evaluated in six antioxidants. Although quercetin showed the highest activity toward both radicals, the order of the relative activities in the other antioxidants was different mutually between the alkylperoxyl radical and the alkoxyl radical. This alkylperoxyl radical scavenging activity assay based on electron paramagnetic resonance spectroscopy is useful for evaluation of colored and turbid food samples.

Method for the simultaneous determination of free/protein malondialdehyde and lipid/protein hydroperoxides

A simple and sensitive method is presented for the simultaneous quantification (spectrophotometric and spectrofluorimetric) of the main lipid and protein peroxidation products after their initial fractionation: free malondialdehyde (FrMDA), protein-bound malondialdehyde (PrMDA), total hydroperoxides (LOOH), and protein hydroperoxides (PrOOH). FrMDA and PrMDA (released from proteins by alkaline hydrolysis) are measured after the reaction of MDA with thiobarbituric acid (TBA) under acidic conditions, by the specific fluorimetric quantification of the resulting MDA-(TBA)2 adduct chromophore. The measurement of LOOH and PrOOH is based on the reaction of Fe(3+) (resulting from the reaction of LOOH and PrOOH with Fe(2+)) with xylenol orange (XO) and the photometric quantification of the resulting XO-Fe complex. The sensitivity of the assays for FrMDA/PrMDA and LOOH/PrOOH is 20 and 100pmol, respectively. The method was applied successfully on human plasma and can be used for the evaluation of oxidative stress in both basic and clinical research.

The use of the methods of radiolysis to explore the mechanisms of free radical modifications in proteins

The method of radiolysis is based upon the interaction of ionising radiation with the solvent (water). One can form the same free radicals as in conditions of oxidative stress ((•)OH, O2(•)(-), NO2(•)…). Moreover, the quantity of reactive oxygen (ROS) or nitrogen (RNS) species formed in the irradiated medium can be calculated knowing the dose and the radiation chemical yield, G, thus this method is quantitative. The use of the method of radiolysis has provided a wealth of data, especially about the kinetics of the oxidation by various free radicals and their mechanisms, the identification of transients formed, their lifetimes and the possibility to repair them by the so-called antioxidants. In this review we have collected the most recent data about protein oxidation that might be useful to a proteomic approach. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Role of Mn2+ and compatible solutes in the radiation resistance of thermophilic bacteria and archaea

Radiation-resistant bacteria have garnered a great deal of attention from scientists seeking to expose the mechanisms underlying their incredible survival abilities. Recent analyses showed that the resistance to ionizing radiation (IR) in the archaeon Halobacterium salinarum is dependent upon Mn-antioxidant complexes responsible for the scavenging of reactive oxygen species (ROS) generated by radiation. Here we examined the role of the compatible solutes trehalose, mannosylglycerate, and di-myo-inositol phosphate in the radiation resistance of aerobic and anaerobic thermophiles. We found that the IR resistance of the thermophilic bacteria Rubrobacter xylanophilus and Rubrobacter radiotolerans was highly correlated to the accumulation of high intracellular concentration of trehalose in association with Mn, supporting the model of Mn²⁺-dependent ROS scavenging in the aerobes. In contrast, the hyperthermophilic archaea Thermococcus gammatolerans and Pyrococcus furiosus did not contain significant amounts of intracellular Mn, and we found no significant antioxidant activity from mannosylglycerate and di-myo-inositol phosphate in vitro. We therefore propose that the low levels of IR-generated ROS under anaerobic conditions combined with highly constitutively expressed detoxification systems in these anaerobes are key to their radiation resistance and circumvent the need for the accumulation of Mn-antioxidant complexes in the cell.

The nitroxide TEMPO is an efficient scavenger of protein radicals: cellular and kinetic studies

Protein oxidation occurs during multiple human pathologies, and protein radicals are known to induce damage to other cell components. Such damage may be modulated by agents that scavenge protein radicals. In this study, the potential protective reactions of the nitroxide TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl radical) against Tyr- and Trp-derived radicals (TyrO./TrpN.) have been investigated. Pretreatment of macrophage cells with TEMPO provided protection against photo-oxidation-induced loss of cell viability and Tyr oxidation, with the nitroxide more effective than the hydroxylamine or parent amine. Pulse radiolysis was employed to determine rate constants, k, for the reaction of TEMPO with TyrO. and TrpN. generated on N-Ac-Tyr-amide and N-Ac-Trp-amide, with values of k~10(8) and 7×10(6)M(-1)s(-1), respectively, determined. Analogous studies with lysozyme, chymotrypsin, and pepsin yielded k for TEMPO reacting with TrpN. ranging from 1.5×10(7) (lysozyme) to 1.1×10(8) (pepsin)M(-1)s(-1). Pepsin-derived TyrO. reacted with TEMPO with k~4×10(7)M(-1)s(-1); analogous reactions for lysozyme and chymotrypsin TyrO. were much slower. These data indicate that TEMPO can inhibit secondary reactions of both TyrO. and TrpN., though this is protein dependent. Such protein radical scavenging may contribute to the positive biological effects of nitroxides.

Efficient depletion of ascorbate by amino acid and protein radicals under oxidative stress

Ascorbate levels decrease in organisms subjected to oxidative stress, but the responsible reactions have not been identified. Our earlier studies have shown that protein C-centered radicals react rapidly with ascorbate. In aerobes, these radicals can react with oxygen to form peroxyl radicals. To estimate the relative probabilities of the reactions of ascorbate with protein C- and O-centered radicals, we measured by pulse radiolysis the rate constants of the reactions of C-centered radicals in Gly, Ala, and Pro with O₂ and of the resultant peroxyl radicals with ascorbate. Calculations based on the concentrations of ascorbate and oxygen in human tissues show that the relative probabilities of reactions of the C-centered amino acid radicals with O₂ and ascorbate vary between 1:2.6 for the pituitary gland and 1:0.02 for plasma, with intermediate ratios for other tissues. The high frequency of occurrence of Gly, Ala, and Pro in proteins and the similar reaction rate constants of their C-centered radicals with O₂ and their peroxo-radicals with ascorbate suggest that our results are also valid for proteins. Thus, the formation of protein C- or O-centered radicals in vivo can account for the loss of ascorbate in organisms under oxidative stress.

Fast repair of protein radicals by urate

The repair of tryptophan and tyrosine radicals in proteins by urate was studied by pulse radiolysis. In chymotrypsin, urate repairs tryptophan radicals efficiently with a rate constant of 2.7 × 10(8)M(-1)s(-1), ca. 14 times higher than the rate constant derived for N-acetyltryptophan amide, 1.9 × 10(7)M(-1)s(-1). In contrast, no repair of tryptophan radicals was observed in pepsin, which indicates a rate constant smaller than 6 × 10(7)M(-1)s(-1). Urate repairs tyrosine radicals in pepsin with a rate constant of 3 × 10(8)M(-1)s(-1)-ca. 12 times smaller than the rate constant reported for free tyrosine-but not in chymotrypsin, which implies an upper limit of 1 × 10(6)M(-1)s(-1) for the corresponding rate constant. Intra- and intermolecular electron transfer from tyrosine residues to tryptophan radicals is observed in both proteins, however, to different extents and with different rate constants. Urate inhibits electron transfer in chymotrypsin but not in pepsin. Our results suggest that urate repairs the first step on the long path to protein modification and prevents damage in vivo. It may prove to be a very important repair agent in tissue compartments where its concentration is higher than that of ascorbate. The product of such repair, the urate radical, can be reduced by ascorbate. Loss of ascorbate is then expected to be the net result, whereas urate is conserved.

Oxidative stress and cardiac hypertrophy: a review

Cardiac hypertrophy (CH) is an adaptive response of the heart to pressure overload. It is a common pathological feature in the natural course of some major cardiovascular diseases, like, hypertension and myocardial infarction. Cardiac hypertrophy is strongly associated with an increased risk of heart failure and sudden cardiac death. The complex and dynamic pathophysiological mechanisms of CH has been the focus of intense scientific investigation, in an effort to design preventive and curative strategies. Oxidative stress has been identified as one of the key contributing factors in the development of cardiac hypertrophy. In this review, evidences supporting the oxidative stress as a cause of cardiac hypertrophy with emphasis on mitochondrial oxidative stress and possible options for pharmacological interventions have been discussed. Reactive oxygen species (ROS) also activate a broad variety of hypertrophy signaling kinases and transcription factors, like, MAP kinase, NF K-B, etc. In addition to profound alteration of cellular function, ROS modulate the extracellular matrix function, evidenced by increased interstitial and perivascular fibrosis. Translocator protein (TSPO) present in the outer mitochondrial membrane is known to be involved in oxidative stress and cardiovascular pathology. Recently, its role in cardiac hypertrophy has been reported by us. All these evidences strongly provide support to beneficial role of drugs which selectively interfere with the generation of free radicals or augment endogenous antioxidants in cardiac hypertrophy.

Intracellular GSH and ascorbate inhibit radical-induced protein chain peroxidation in HL-60 cells

The results of this study suggest that the well-documented loss of GSH and ascorbate in organisms under oxidative stress may be mainly due to their reactions with protein radicals and/or peroxides. Protein hydroperoxides were generated in HL-60 cells exposed to radiation-generated hydroxyl radicals. We found for the first time evidence of chain peroxidation of the proteins in cells, with each hydroxyl radical leading to the formation of about 10 hydroperoxides. Protein peroxidation showed a lag, probably due to the endogenous antioxidant enzymes, with simultaneous loss of the intracellular GSH. Enhancement of the GSH levels by N-acetylcysteine decreased the formation of hydroperoxides, while treatment with l-buthionine sulfoximine had the opposite effect. The effect of variation of GSH levels on the formation of the peroxidized proteins is explained primarily by reduction of the protein hydroperoxides by GSH. Loading of the cells with ascorbate resulted in reduction of the amounts of protein hydroperoxides generated by the radiation, which was proportional to the intracellular ascorbate concentration. In contrast to the GSH, inhibition of protein hydroperoxide formation in the presence of the high (mM) intracellular ascorbate levels achieved was mainly due to the direct scavenging of hydroxyl radicals by the vitamin.

Photo-oxidation of proteins

Photo-induced damage to proteins occurs via multiple pathways. Direct damage induced by UVB (λ 280-320 nm) and UVA radiation (λ 320-400 nm) is limited to a small number of amino acid residues, principally tryptophan (Trp), tyrosine (Tyr), histidine (His) and disulfide (cystine) residues, with this occurring via both excited state species and radicals. Indirect protein damage can occur via singlet oxygen ((1)O(2)(1)Δ(g)), with this resulting in damage to Trp, Tyr, His, cystine, cysteine (Cys) and methionine (Met) residues. Although initial damage is limited to these residues multiple secondary processes, that occur both during and after radiation exposure, can result in damage to other intra- and inter-molecular sites. Secondary damage can arise via radicals (e.g. Trp, Tyr and Cys radicals), from reactive intermediates generated by (1)O(2) (e.g. Trp, Tyr and His peroxides) and via molecular reactions of photo-products (e.g. reactive carbonyls). These processes can result in protein fragmentation, aggregation, altered physical and chemical properties (e.g. hydrophobicity and charge) and modulated biological turnover. Accumulating evidence implicates these events in cellular and tissue dysfunction (e.g. apoptosis, necrosis and altered cell signaling), and multiple human pathologies.

Tryptophan from human milk induces oxidative stress and upregulates the Nrf-2-mediated stress response in human intestinal cell lines

Chemical screening of digested human milk protein using the oxygen radical absorbance capacity (ORAC(FL)) antioxidant assay confirmed the presence of a peptide fraction (PF23) with high antioxidant activity [5.53 mmol Trolox equivalents (TE)/g] that contained tryptophan as a main component. We evaluated the effects of both PF23 and tryptophan alone on the modulation of oxidative stress in cultured intestinal cells using a dichlorofluorescein diacetate probe. Despite the high ORAC(FL) value, PF23 enhanced (P < 0.05) 2, 2'-azobis (2-amidinopropane) dihydrochloride (peroxyl radical generator)-induced intracellular oxidation in the Caco-2 human adenocarcinoma cell line, suggesting prooxidant activity. Compared to selected peptide fractions with relatively lower ORAC(FL) values, PF23 induced oxidative stress more than all other peptide fractions tested (P < 0.05) and contained more tryptophan than the others (P < 0.05). Similar prooxidant activity was observed for tryptophan when it was added to culture medium for both the Caco-2 cells and FHs 74 Int primary fetal enterocytes, while also exhibiting a high ORAC(FL) value (9.69 mmol TE/g). The effect of tryptophan that involves activation of the Nrf-2 pathway and transcription of antioxidant enzymes was therefore investigated in FHs 74 Int cells. Exposure of infant intestinal cells to tryptophan resulted in Nrf-2 activation and an increase in the gene transcript level of glutathione peroxidase 2. We conclude that tryptophan-induced oxidative stress associated with tryptophan-containing milk peptides induces an adaptive response that involves the activation of the antioxidant responsive signaling pathway in intestinal cells.

The protective function of hydrogen sulfide for lysozyme against riboflavin-sensitized photo-oxidation

Hydrogen sulfide is the third endogenous signaling gasotransmitter, following nitric oxide and carbon monoxide. Recent studies showed that hydrogen sulfide could alleviate many diseases which were related to the oxidative damage of tissues. It reminded us that hydrogen sulfide might serve as an antioxidant to reduce oxidative pressure. This study showed that hydrogen sulfide protected lysozyme from photo-oxidation induced by riboflavin (RF). Laser flash photolysis was used to explore the mechanisms of antioxidant activity of hydrogen sulfide. The scavenging effects of hydrogen sulfide on the triplet state of riboflavin (³RF(*)) and radicals of tryptophan and tyrosine (TyrO· and TrpN·) were attributed to the protection of lysozyme from photo-oxidation. The results suggested that hydrogen sulfide could serve as an antioxidant in alleviation of oxidative pressure.

Hydrogen exchange equilibria in glutathione radicals: rate constants

The reduction of oxidized glutathione GSSG by hydrated electrons and hydrogen atoms to form GSSG^•− is quantitative. The radical anion dissociates into GS^• and GS⁻, and the S-centered radical subsequently abstracts a hydrogen intramolecularly. We observe sequential development of UV absorbance signatures that indicate the formation of both α- and β-carbon-centered radicals. From experiments performed at pH 2 and pH 11.8, we determined forward and reverse rate constants for the overall equilibrium between sulfur-centered and carbon-centered radicals: k_forward = 3·10⁵ s⁻¹, k_reverse = 7·10⁵ s⁻¹, and K = 0.4. Furthermore, on the basis of the differences between the kinetics traces at 240 and 280 nm, we estimate that α- and β-carbon-centered radicals are formed at a surprising ratio of 1:3. The ratios found at pH 2 also apply to pH 7, with the conclusion that the equilibrium ratio of S-centered:β-centered:α-centered radicals is, very approximately, 8:3:1. The formation of carbon-centered radicals could lead to irreversible damage in proteins via the formation of carbon−carbon bonds or backbone fragmentation.

Real-time measurement of cytosolic free calcium concentration in DEM-treated HL-60 cells during static magnetic field exposure and activation by ATP

This study investigated whether glutathione depletion affected the sensitivity of HL-60 cells to static magnetic fields. The effect of Diethylmaleate (DEM) on static magnetic field induced changes in cytosolic free calcium concentration ([Ca(2+)](c)) was examined. Cells were loaded with a fluorescent dye and exposed to a uniform static magnetic field at a strength of 0 mT (sham) or 100 mT. [Ca(2+)](c) was monitored during field and sham exposure using a ratiometric fluorescence spectroscopy system. Cells were activated by the addition of ATP. Metrics extracted from the [Ca(2+)](c) time series included: average [Ca(2+)](c) during the Pre-Field and Field Conditions, peak [Ca(2+)](c) following ATP activation and the full width at half maximum (FWHM) of the peak ATP response. Comparison of each calcium metric between the sham and 100 mT experiments revealed the following results: average [Ca(2+)](c) measured during the Field condition was 53 +/- 2 nM and 58 +/- 2 nM for sham and 100 mT groups, respectively. Average FWHM was 51 +/- 3 s and 54 +/- 3 s for sham and 100 mT groups, respectively. An effect of experimental order on the peak [Ca(2+)](c) response to ATP in sham/sham experiments complicated the statistical analysis and did not allow pooling of the first and second order experiments. No statistically significant difference between the sham and 100 mT groups was observed for any of the calcium metrics. These data suggested that manipulation of free radical buffering capacity in HL-60 cells did not affect the sensitivity of the cells to a 100 mT static magnetic field.

Efficient repair of protein radicals by ascorbate

Protein radicals were selectively generated by reaction with azide radicals on Trp and Tyr residues in insulin, beta-lactoglobulin, pepsin, chymotrypsin, and bovine serum albumin at rate constants in the range (2.9-19)x10(8) M(-1) s(-1). Monohydrogen ascorbate reduced tryptophanyl radicals in chymotrypsin and pepsin with rate constants in the narrow range of (1.6-1.8)x10(8) M(-1) s(-1), whereas beta-lactoglobulin tryptophanyl radicals reacted almost 10 times slower. The corresponding values for the protein tyrosyl radicals were about an order of magnitude smaller. Comparison of the rate constants of reactions of free and protein-bound tryptophanyl and tyrosyl radicals showed that, in most cases, the location of the radicals in the protein chain did not constitute a major barrier to the reaction with monohydrogen ascorbate. The results suggest that, under physiological concentrations of dioxygen, monohydrogen ascorbate is likely to be a significant target of protein radicals. It seems likely, therefore, that reaction with protein radicals may be responsible for much of the well-documented loss of ascorbate in living organisms subjected to oxidative stress.

Inactivation of chosen dehydrogenases by the products of water radiolysis and secondary albumin and haemoglobin radicals

PURPOSE

Inactivation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), alcohol dehydrogenase (ADH) and lactate dehydrogenase (LDH) by products of water radiolysis and by secondary radicals localized on haemoglobin (Hb) and human albumin (HSA) was studied.

MATERIALS AND METHODS

Aqueous solutions of ADH, GAPDH and LDH were irradiated under air and under nitrous oxide (N2O) in the absence and in the presence of Hb or HSA. In order to determine the effectiveness of inactivation of the enzymes by radicals localized on Hb and HSA, the inactivation efficiency determined experimentally was compared with that calculated under assumption that only hydroxyl radicals are responsible for the enzyme inactivation.

RESULTS

In the absence of other proteins, under air, GAPDH showed the highest radiation sensitivity, followed by ADH and LDH. The sequence was reverse under anaerobic atmosphere. Oxygen increased considerably the inactivation of GAPDH and ADH. Secondary albumin and haemoglobin radicals brought about considerable inactivation of GAPGH and ADH. Albumin radicals (HSA) generated under N2O inactivated GAPDH and ADH more effectively than haemoglobin radicals (Hb). Under air, however, inactivation of GAPDH and ADH by haemoglobin peroxyl radicals was higher than by albumin peroxyl radicals. LDH was resistant to inactivation by haemoglobin and albumin radicals, and peroxides of these proteins.

CONCLUSIONS

In the light of these results and literature data, the observed differences in the effectiveness of inactivation of the dehydrogenases studied by secondary protein radicals depend on the amino acid residues present at the active site and in its close neighborhood and on the number of amino acid residues available on the protein surface.

Mitochondrial dysfunction in SOD1G93A-bearing astrocytes promotes motor neuron degeneration: prevention by mitochondrial-targeted antioxidants

Mitochondrial dysfunction and oxidative stress contribute to motor neuron degeneration in amyotrophic lateral sclerosis (ALS). Recent reports indicate that astrocytes expressing the mutations of superoxide dismutase-1 (SOD1) may contribute to motor neuron injury in ALS. Here, we provide evidence that mitochondrial dysfunction in SOD1(G93A) rat astrocytes causes astrocytes to induce apoptosis of motor neurons. Mitochondria from SOD1(G93A) rat astrocytes displayed a defective respiratory function, including decreased oxygen consumption, lack of ADP-dependent respiratory control, and decreased membrane potential. Protein 3-nitrotyrosine was detected immunochemically in mitochondrial proteins from SOD1(G93A) astrocytes, suggesting that mitochondrial defects were associated with nitroxidative damage. Furthermore, superoxide radical formation in mitochondria was increased in SOD1(G93A) astrocytes. Similar defects were found in mitochondria isolated from the spinal cord of SOD1(G93A) rats, and pretreatment of animals with the spin trap 5,5-dimethyl-1-pyrroline N-oxide restored mitochondrial function, forming adducts with mitochondrial proteins in vivo. As shown previously, SOD1(G93A) astrocytes induced death of motor neurons in cocultures, compared with nontransgenic ones. This behavior was recapitulated when nontransgenic astrocytes were treated with mitochondrial inhibitors. Remarkably, motor neuron loss was prevented by preincubation of SOD1(G93A) astrocytes with antioxidants and nitric oxide synthase inhibitors. In particular, low concentrations (approximately 10 nm) of two mitochondrial-targeted antioxidants, ubiquinone and carboxy-proxyl nitroxide, each covalently coupled to a triphenylphosphonium cation (Mito-Q and Mito-CP, respectively), prevented mitochondrial dysfunction, reduced superoxide production in SOD1(G93A) astrocytes, and restored motor neuron survival. Together, our results indicate that mitochondrial dysfunction in astrocytes critically influences motor neuron survival and support the potential pharmacological utility of mitochondrial-targeted antioxidants in ALS treatment.