Metal-catalyzed oxidation of Escherichia coli glutamine synthetase: correlation of structural and functional changes

In-Depth Characterization of Acidic Variants Induced by Metal-Catalyzed Oxidation in a Recombinant Monoclonal Antibody

Characterization of antibody charge heterogeneity is an important task for antibody drug development. Recently, a correlation between acidic charge heterogeneity and metal-catalyzed oxidation has been observed for antibody drugs. However, to date, the acidic variants induced by metal-catalyzed oxidation have not been elucidated. In addition, it is challenging to satisfactorily explain the induced acidic charge heterogeneity, as the existing analytical workflows, which relied on either untargeted or targeted peptide mapping analysis, could lead to incomplete identification of the acidic variants. In this work, we present a new characterization workflow by combining untargeted and targeted analyses to thoroughly identify and characterize the induced acidic variants in a highly oxidized IgG1 antibody. As a part of this workflow, a tryptic peptide mapping method was also developed for accurate determination of the relative extent of site-specific carbonylation, where a new hydrazone reduction procedure was established to minimize under-quantitation artifacts caused by incomplete reduction of hydrazones during sample preparation. In summary, we identified 28 site-specific oxidation products, which are located on 26 residues and of 11 different modification types, as the sources of the induced acidic charge heterogeneity. Many of the oxidation products were reported for the first time in antibody drugs. More importantly, this study provides new insights to understanding acidic charge heterogeneity of antibody drugs in the biotechnology industry. Additionally, the characterization workflow presented in this study can be applied as a platform approach in the biotechnology industry to better address the need for in-depth characterization of antibody charge variants.

The effects of photodynamic treatment with new methylene blue N on the Candida albicans proteome

Candida albicans is a human pathogenic fungus mainly affecting immunocompromised patients. Resistance to the commonly used fungicides can lead to poor treatment of mucosal infections which, in turn, can result in life-threatening systemic candidiasis. In this scenario, antimicrobial photodynamic treatment (PDT) has emerged as an effective alternative to treat superficial and localized fungal infections. Microbial death in PDT is a consequence of the oxidation of many cellular biomolecules, including proteins. Here, we report a combination of two-dimensional electrophoresis and tandem mass spectrometry to study the protein damage resulting from treating C. albicans with PDT with new methylene blue N and red light. Two-dimensional gels of treated cells showed an increase in acidic spots in a fluence-dependent manner. Amino acid analysis revealed a decrease in the histidine content after PDT, which is one plausible explanation for the observed acidic shift. However, some protein spots remained unchanged. Protein identification by mass spectrometry revealed that both modified and unmodified proteins could be localized to the cytoplasm, ruling out subcellular location as the only explanation for damage selectivity. Therefore, we hypothesize that protein modification by PDT is a consequence of both photosensitizer binding affinity and the degree of exposure of the photooxidizable residues on the protein surface.

The loss of hemoglobin and myoglobin does not minimize oxidative stress in Antarctic icefishes

The unusual pattern of expression of hemoglobin (Hb) and myoglobin (Mb) among Antarctic notothenioid fishes provides an exceptional model system for assessing the impact of these proteins on oxidative stress. We tested the hypothesis that the lack of oxygen-binding proteins may reduce oxidative stress. Levels and activity of pro-oxidants and small-molecule and enzymatic antioxidants, and levels of oxidized lipids and proteins in the liver, oxidative skeletal muscle and heart ventricle were quantified in five species of notothenioid fishes differing in the expression of Hb and Mb. Levels of ubiquitinated proteins and rates of protein degradation by the 20S proteasome were also quantified. Although levels of oxidized proteins and lipids, ubiquitinated proteins, and antioxidants were higher in red-blooded fishes than in Hb-less icefishes in some tissues, this pattern did not persist across all tissues. Expression of Mb was not associated with oxidative damage in the heart ventricle, whereas the activity of citrate synthase and the contents of heme were positively correlated with oxidative damage in most tissues. Despite some tissue differences in levels of protein carbonyls among species, rates of degradation by the 20S proteasome were not markedly different, suggesting either alternative pathways for eliminating oxidized proteins or that redox tone varies among species. Together, our data indicate that the loss of Hb and Mb does not correspond with a clear pattern of either reduced oxidative defense or oxidative damage.

Glutamine Synthetase: Localization Dictates Outcome

Glutamine synthetase (GS) is the adenosine triphosphate (ATP)-dependent enzyme that catalyses the synthesis of glutamine by condensing ammonium to glutamate. In the circulatory system, glutamine carries ammonia from muscle and brain to the kidney and liver. In brain reduction of GS activity has been suggested as a mechanism mediating neurotoxicity in neurodegenerative disorders. In cancer, the delicate balance between glutamine synthesis and catabolism is a critical event. In vitro evidence, confirmed in vivo in some cases, suggests that reduced GS activity in cancer cells associates with a more invasive and aggressive phenotype. However, GS is known to be highly expressed in cells of the tumor microenvironment, such as fibroblasts, adipocytes and immune cells, and their ability to synthesize glutamine is responsible for the acquisition of protumoral phenotypes. This has opened a new window into the complex scenario of the tumor microenvironment, in which the balance of glutamine consumption versus glutamine synthesis influences cellular function. Since GS expression responds to glutamine starvation, a lower glutamine synthesizing power due to the absence of GS in cancer cells might apply a metabolic pressure on stromal cells. This event might push stroma towards a GS-high/protumoral phenotype. When referred to stromal cells, GS expression might acquire a ‘bad’ significance to the point that GS inhibition might be considered a conceivable strategy against cancer metastasis.

Blockade of Glutamine Synthetase Enhances Inflammatory Response in Microglial Cells

AIMS

Microglial cells are brain-resident macrophages engaged in surveillance and maintained in a constant state of relative inactivity. However, their involvement in autoimmune diseases indicates that in pathological conditions microglia gain an inflammatory phenotype. The mechanisms underlying this change in the microglial phenotype are still unclear. Since metabolism is an important modulator of immune cell function, we focused our attention on glutamine synthetase (GS), a modulator of the response to lipopolysaccharide (LPS) activation in other cell types, which is expressed by microglia.

RESULTS

GS inhibition enhances release of inflammatory mediators of LPS-activated microglia in vitro, leading to perturbation of the redox balance and decreased viability of cocultured neurons. GS inhibition also decreases insulin-mediated glucose uptake in microglia. In vivo, microglia-specific GS ablation enhances expression of inflammatory markers upon LPS treatment. In the spinal cords from experimental autoimmune encephalomyelitis (EAE), GS expression levels and glutamine/glutamate ratios are reduced.

INNOVATION

Recently, metabolism has been highlighted as mediator of immune cell function through the discovery of mechanisms that (behind these metabolic changes) modulate the inflammatory response. The present study shows for the first time a metabolic mechanism mediating microglial response to a proinflammatory stimulus, pointing to GS activity as a master modulator of immune cell function and thus unraveling a potential therapeutic target.

CONCLUSIONS

Our study highlights a new role of GS in modulating immune response in microglia, providing insights into the pathogenic mechanisms associated with inflammation and new strategies of therapeutic intervention. Antioxid. Redox Signal. 26, 351-363.

Redox stress and hepatic DNA fragmentation induced by diquat in vivo are not accompanied by increased 8-hydroxydeoxyguanosine contents

Administration of 0.1 mmol/kg of diquat to Fischer-344 rats causes acute hepatic necrosis by mechanisms that appear to involve increased generation of reactive oxygen species, but the critical targets of the proposed oxidations have not been identified. In the present study the effects of diquat-induced redox stresses on hepatic protein thiol status were determined by derivatization of subcellular fractions with monobromobimane and separation of the fluorescent derivatives by SDS-PAGE. No differences in hepatic thiol status were seen in animals 2 or 6 h after diquat, relative to saline-treated controls, despite documentation of injury by elevated plasma transaminase activities. Hepatic DNA fragmentation was increased in diquat-treated animals (24.9±5.1 vs 6.7±0.3% (controls) at 2 h; 57.2±4.1 vs 4.6±0.3% (controls) at 6 h, P<0.001). However, 8-hydroxydeoxyguanosine (8-OHdG) contents in hepatic DNA were not increased by diquat (35.3±6.2 μmol 8-OHdG/mol deoxyguanosine (dG)) over saline-treated controls (28.3±2.6). Plasma NH3 concentrations increased in diquat-treated rats from 49 μM in controls to 170 μM 6 h after treatment with diquat. Hepatic activities of glutamine synthetase (GS) were lower in diquat-treated rats (39.7±13.0 mU/mg protein) than in controls (65.8±13.4, P<0.001), but activities of carbamyl phosphate synthetase-I (CPS-I), were not decreased significantly. The oxidation of proteins to forms reactive with 2,4-dinitrophenylhydrazine (DNPH) was investigated in subcellular fractions by Western blot analyses with a monoclonal antibody to DNP-derivatized bovine serum albumin (BSA). N-terminal sequencing of bands exhibiting reactivity with anti-DNP-BSA antibodies indicated protein carbonyl formation in malate dehydrogenase, protein disulfide isomerase, and glutathione transferase. The functional consequences of oxidation of these proteins are not known but the observation of protein carbonyl formation and no measurable loss of protein thiol content are consistent with iron chelate-mediated oxidation in the transformation critical to expression of tissue damage. The time course data are consistent with DNA fragmentation as a mechanism contributing to the development of cell injury, but the absence of increases in 8-OHdG indicates that direct oxidation of DNA may not be responsible.

Protein oxidation and peroxidation

Proteins are major targets for radicals and two-electron oxidants in biological systems due to their abundance and high rate constants for reaction. With highly reactive radicals damage occurs at multiple side-chain and backbone sites. Less reactive species show greater selectivity with regard to the residues targeted and their spatial location. Modification can result in increased side-chain hydrophilicity, side-chain and backbone fragmentation, aggregation via covalent cross-linking or hydrophobic interactions, protein unfolding and altered conformation, altered interactions with biological partners and modified turnover. In the presence of O2, high yields of peroxyl radicals and peroxides (protein peroxidation) are formed; the latter account for up to 70% of the initial oxidant flux. Protein peroxides can oxidize both proteins and other targets. One-electron reduction results in additional radicals and chain reactions with alcohols and carbonyls as major products; the latter are commonly used markers of protein damage. Direct oxidation of cysteine (and less commonly) methionine residues is a major reaction; this is typically faster than with H2O2, and results in altered protein activity and function. Unlike H2O2, which is rapidly removed by protective enzymes, protein peroxides are only slowly removed, and catabolism is a major fate. Although turnover of modified proteins by proteasomal and lysosomal enzymes, and other proteases (e.g. mitochondrial Lon), can be efficient, protein hydroperoxides inhibit these pathways and this may contribute to the accumulation of modified proteins in cells. Available evidence supports an association between protein oxidation and multiple human pathologies, but whether this link is causal remains to be established.

Elevated protein carbonyl and HIF-1α levels in eyes with proliferative diabetic retinopathy

PURPOSE

  To evaluate the role of protein carbonyls and hypoxia inducible factor-1α (HIF-1α) in diabetic eyes with proliferative diabetic retinopathy (PDR).

METHODS

  Prospective consecutive controlled observational study was performed. Vitreous samples were collected at the start of the 3-ppp vitrectomy. Protein carbonylation analysis was performed by Western blotting with antibody against 2,4-Dinitrophenol (anti-DNP), following derivatization of protein carbonyls with 2,4 Dinitrophenylhydrazine (DNHP). Protein carbonylation was quantified by scanning densitometry analysis and relativized to the total amount of protein into the ponceau staining of membranes. Vitreous HIF-1 α was determined with ELISA in a subgroup of the samples. Thirty-one eyes were operated due to PDR (study group). Of the 189 controls, 39 had nonproliferative diabetic retinopathy (non-PDR), 111 retinal detachment (RD) and 39 macular hole/pucker (MH).

RESULTS

  Comparison of eyes with PDR with controls revealed that the mean vitreous concentrations of protein carbonyls were significantly higher in the eyes affected with PDR being 242±130 (SD) compared with non-PDR controls 180±142, nondiabetic eyes affected with RD 175±131 and MH/pucker 140±95 (p=0.008, one-way anova). Mean HIF-1α values were higher in eyes with PDR compared with controls (RD, MH/pucker); the values being 0.53±0.34 (SEM; n=4) and 0.13±0.04 (SEM; n=19), respectively (p=0.009).

CONCLUSIONS

  Protein carbonyl and HIF-1 α levels were significantly increased in the vitreous fluid of surgically treated eyes with PDR. Our findings suggest an association between increased intravitreal levels of protein carbonyls and the pathogenesis of PDR.

Calcium-induced proline accumulation contributes to amelioration of NaCl injury and expression of glutamine synthetase in greater duckweed (Spirodela polyrhiza L.)

The calcium-mediated proline accumulation is a critical response under NaCl stress and the function of the induced proline as a glutamine synthetase (GS) protectant in greater duckweed was investigated. The plants were treated with solutions containing 100mM NaCl, 200 mM NaCl, 200 mM NaCl plus 10mM CaCl2, or 10mM CaCl2 alone for 4 days. At the end of the experiment, the fronds of inoculum treated with 200 mM NaCl showed the chlorotic effect, higher glutamate dehydrogenase (NADH-GDH) activity and lower GS activity. At the lower salinity, the activities of GS and NADH-GDH were not altered markedly. A significant accumulation of proline was not found under either low or high salinity. The activity of Δ(1)-pyrroline-5-carboxylate reductase (P5CR) was enhanced only at 200 mM NaCl but remained unchanged at 100mM NaCl. The activity of Δ(1)-pyrroline-5-carboxylate synthetase (P5CS) did not change under salinity-stressed. Addition of CaCl2 to the salt stressed plants not only lowered NaCl injury but also showed an elevated level of proline contents in response to the salinity treatment. In addition, both GS activity and corresponding polypeptides were expressed close to the level of control. Exogenous proline protects GS2 and the 32 kDa protein in photosystem II reaction center (D1) from H2O2-induced redox degradation in the chloroplast lysates of duckweed. The results suggest that calcium-induced proline accumulation may play an important role as a GS protectant under NaCl exposure in S. polyrhiza.

The toxic effects of diethyl phthalate on the activity of glutamine synthetase in greater duckweed (Spirodela polyrhiza L.)

The toxic effects of diethyl phthalate (DEP), a potent allelochemical, on the enzyme activity and polypeptide accumulation of glutamine synthetase (GS) in greater duckweed were investigated. In our previous studies, DEP induced oxidative responses at concentrations from 0.5 to 2 mM in greater duckweed and the antioxidant enzymes played important roles in the defense strategy against DEP stress. In this study, DAB-H(2)O(2) and NBT stain for superoxide radicals (O(2)(·-)), lipid peroxidation, HSP70, and ammonia accumulation in DEP-treated duckweed tissues revealed adverse effect of DEP in plant growth. Biochemical analysis and physiological methods were combined to investigate GS activity and polypeptide accumulation under DEP-induced stress. The results showed that GS activity was reduced with the increasing concentration of DEP, indicative of enhanced toxic effect. Immunoblot analysis with chloroplast soluble fractions indicated that the chloroplastic GS (GS2) polypeptide from greater duckweed was degraded under DEP stress conditions. The response of GS2 to the DEP stress may be modulated by means of redox change in plant tissues, chloroplasts, and chloroplast lysates. The results suggest that DEP is toxic to the greater duckweed by inhibition of the GS isoenzymes in nitrogen assimilation and the GS2 plays important roles in the adaptation strategy against DEP toxicity.

Quantitation of protein carbonylation by dot blot

Protein carbonylation is the most commonly used measure of oxidative modification of proteins. It is frequently measured spectrophotometrically or immunochemically by derivatizing proteins with the classical carbonyl reagent, 2,4-dinitrophenylhydrazine. We developed an immunochemical dot blot method for quantitation of protein carbonylation in homogenates or purified proteins. Dimethyl sulfoxide was employed as the solvent because it very efficiently extracts proteins from tissues and keeps them soluble. It also readily dissolves 2,4-dinitrophenylhydrazine and wets polyvinylidene difluoride (PVDF) membranes. The detection limit is 0.19 ± 0.04 pmol of carbonyl, and 60 ng of protein is sufficient to measure protein carbonyl content. This level of sensitivity allowed measurement of protein carbonylation in individual Drosophila.

HSP70 mediates dissociation and reassociation of the 26S proteasome during adaptation to oxidative stress

We report an entirely new role for the HSP70 chaperone in dissociating 26S proteasome complexes (into free 20S proteasomes and bound 19S regulators), preserving 19S regulators, and reconstituting 26S proteasomes in the first 1-3h after mild oxidative stress. These responses, coupled with direct 20S proteasome activation by poly(ADP ribose) polymerase in the nucleus and by PA28αβ in the cytoplasm, instantly provide cells with increased capacity to degrade oxidatively damaged proteins and to survive the initial effects of stress exposure. Subsequent adaptive (hormetic) processes (3-24h after stress exposure), mediated by several signal transduction pathways and involving increased transcription/translation of 20S proteasomes, immunoproteasomes, and PA28αβ, abrogate the need for 26S proteasome dissociation. During this adaptive period, HSP70 releases its bound 19S regulators, 26S proteasomes are reconstituted, and ATP-stimulated proteolysis is restored. The 26S proteasome-dependent, and ATP-stimulated, turnover of ubiquitinylated proteins is essential for normal cell metabolism, and its restoration is required for successful stress adaptation.

Oxidative stress and reduced glutamine synthetase activity in the absence of inflammation in the cortex of mice with experimental allergic encephalomyelitis

Pathological changes occur in areas of CNS tissue remote from inflammatory lesions in multiple sclerosis (MS) and its animal model experimental allergic encephalomyelitis (EAE). To determine if oxidative stress is a significant contributor to this non-inflammatory pathology, cortex tissues from mice with clinical signs of EAE were examined for evidence of inflammation and oxidative stress. Histology and gene expression analysis showed little evidence of immune/inflammatory cell invasion but reductions in natural antioxidant levels and increased protein oxidation that paralleled disease severity. Two-dimensional oxyblots and mass-spectrometry-based protein fingerprinting identified glutamine synthetase (GS) as a particular target of oxidation. Oxidation of GS was associated with reductions in enzyme activity and increased glutamate/glutamine levels. The possibility that this may cause neurodegeneration through glutamate excitotoxicity is supported by evidence of increasing cortical Ca(2+) levels in cortex extracts from animals with greater disease severity. These findings indicate that oxidative stress occurs in brain areas that are not actively undergoing inflammation in EAE and that this can lead to a neurodegenerative process due to the susceptibility of GS to oxidative inactivation.

Exposure of rice seedlings to heat shock protects against subsequent Cd-induced decrease in glutamine synthetase activity and increase in specific protease activity in leaves

In the present study, we investigated the effect of heat shock (HS) on the subsequent Cd-induced decrease in the activity of glutamine synthetase (GS) and increase in the specific activity of protease in rice leaves. HS exposure of rice seedlings for 3h in the dark was effective in reducing subsequent Cd-induced decrease in the activity of glutamine synthetase and increase in the specific activity of protease. The effect of HS can be mimicked by pretreatment of rice seedlings with exogenous H(2)O(2) or reduced glutathione (GSH) under non-HS conditions. We also found that HS protected against subsequent Cd-induced decrease in the activity of GS and increase in the specific activity of protease can be counteracted by imidazole, a NADPH oxidase inhibitor. Pretreatment with buthione sulfoximine (a GSH synthesis inhibitor) under HS conditions enhanced subsequent Cd effects on the activity of GS and the specific activity of protease. Moreover, the effect of BSO can be reversed by the addition of GSH. The mechanisms of the protective effect of HS effect against subsequent Cd effects are discussed.

HIV proteins (gp120 and Tat) and methamphetamine in oxidative stress-induced damage in the brain: potential role of the thiol antioxidant N-acetylcysteine amide

An increased risk of HIV-1 associated dementia (HAD) has been observed in patients abusing methamphetamine (METH). Since both HIV viral proteins (gp120, Tat) and METH induce oxidative stress, drug abusing patients are at a greater risk of oxidative stress-induced damage. The objective of this study was to determine if N-acetylcysteine amide (NACA) protects the blood brain barrier (BBB) from oxidative stress-induced damage in animals exposed to gp120, Tat and METH. To study this, CD-1 mice pre-treated with NACA/saline, received injections of gp120, Tat, gp120+Tat or saline for 5days, followed by three injections of METH/saline on the fifth day, and sacrificed 24h after the final injection. Various oxidative stress parameters were measured, and animals treated with gp120+Tat+Meth were found to be the most challenged group, as indicated by their GSH and MDA levels. Treatment with NACA significantly rescued the animals from oxidative stress. Further, NACA-treated animals had significantly higher expression of TJ proteins and BBB permeability as compared to the group treated with gp120+Tat+METH alone, indicating that NACA can protect the BBB from oxidative stress-induced damage in gp120, Tat and METH exposed animals, and thus could be a viable therapeutic option for patients with HAD.

Hydrogen peroxide induced oxidation of peroxisomal malate synthase and catalase

Peroxisomes contain oxidases that produce H(2)O(2), which can result in protein oxidation. To test the vulnerability of peroxisomal proteins to oxidation in vivo the organelles were isolated from castor bean endosperm incubated with H(2)O(2). When peroxisomes were exposed to H(2)O(2)in vivo, the peroxisomal proteins exhibited an increase in carbonylation as detected in avidin blots of biotin hydrazide derivatized samples. Biotin-tagged peptides from trypsin digests of the proteins were analyzed by mass spectroscopy and compared to the masses of peptides from the same protein that had not been biotin-tagged and from proteins not exposed to excess H(2)O(2). H(2)O(2) exposure was found to increase the activity of catalase (CAT), and to increase the number of oxidized peptides found in CAT and malate synthase (MS). CAT had 10 peptides that were affected by in vivo exposure to H(2)O(2) and MS had 8. These sites of oxidation have definable locations within the proteins' structures.

Protein carbonylation: avoiding pitfalls in the 2,4-dinitrophenylhydrazine assay

Protein carbonyl content is widely used as both a marker for oxidative stress and a measure of oxidative damage. Widely used methods for determination of protein carbonylation utilize the reaction of carbonyl groups with 2,4-dinitrophenylhydrazine (DNPH) to form protein-bound 2,4-dinitrophenylhydrazones. Hydrazones can be quantitated spectrophotometrically or, for greater sensitivity, detected immunochemically with anti-dinitrophenyl antibodies. Attention to methodology is important to avoid artifactual elevation in protein carbonyl measurements. We studied extracts of Escherichia coli to identify and eliminate such effects. Nucleic acid contamination caused serious artifactual increases in the protein carbonyl content determined by spectrophotometric techniques. Both in vitro synthesized DNA oligonucleotides and purified chromosomal DNA reacted strongly with 2,4-DNPH. Treatment of cell extracts with DNase+RNase or with streptomycin sulfate to precipitate nucleic acids dramatically reduced the apparent carbonyl, while exposure to proteinase K did not. The commercial kit for immunochemical detection of protein carbonylation (OxyBlot from Chemicon/Millipore) recommends a high concentration of thiol in the homogenizing buffer. We found this recommendation leads to an artifactual doubling of the protein carbonyl, perhaps due to a thiol-stimulated Fenton reaction. Avoiding oxidizing conditions, removal of nucleic acids, and prompt assay of samples can prevent artifactual effects on protein carbonyl measurements.

Increased oxidation, glycoxidation, and lipoxidation of brain proteins in prion disease

The basic molecular underpinnings of the pathological changes that unfold in prion disease remain elusive. A key role of increased oxidative stress has been hypothesized. Given the transient nature of most intermediate molecules implicated, increased oxidative stress is better assessed by quantitating the damage it causes to macromolecules. We used mass spectrometry-based methods to measure specific products of protein oxidation, glycoxidation, and lipoxidation in brains from patients suffering from Creutzfeldt-Jakob disease and Syrian hamsters affected by scrapie. In both cases, increased amounts of glutamic and aminoadipic semialdehydes, products of metal-catalyzed oxidation, malondialdehydelysine (a product of lipoxidation), N-epsilon-carboxyethyllysine (a product of glycoxidation), and N-epsilon-carboxymethyllysine (generated by lipoxidation and glycoxidation) were measured. PrP(Sc), the infectious isoform of the prion protein that accumulates in prion disease, was itself shown to be a target of increased oxidative modification. These changes were accompanied by alterations in fatty acid composition and increased phosphorylation of ERK(1/2) and p38, protein kinases known to respond to increased flows of ROS. These data support an important role of oxidative damage in the pathology of prion disease.

Elevated Ras/protein kinase A activity in Saccharomyces cerevisiae reduces proliferation rate and lifespan by two different reactive oxygen species-dependent routes

Cells with overactive RAS/protein kinase A (PKA) signaling, such as RAS2(Val19) cells, exhibit reduced proliferation rates and accelerated replicative senescence. We show here that the extended generation time of RAS2(Val19)cells is the result of abrogated ATP/ADP carrier activity of the mitochondria. Both PKA-dependent and independent routes are responsible for inhibiting ATP/ADP exchange in the RAS-overactive cells. The reduced carrier activity is due, at least in part, to elevated levels of reactive oxygen species (ROS), which also cause a proteolysis-dependent fragmentation of the Aac2p carrier both in vivo and on isolated mitochondria. Attenuated carrier activity is suppressed by overproducing the superoxide dismutase, Sod1p, and this enhances both the proliferation rate and the replicative longevity of RAS2(Val19) cells. In contrast, overproducing functional Aac2p restored proliferation but not longevity of RAS2(Val19) cells. Thus, Ras signaling affects proliferation rate and replicative lifespan by two different, ROS-dependent, routes. While the reduction in generation time is linked to the inactivation, specifically, of the mitochondrial nucleotide carrier, longevity is affected by other, and hitherto unknown, target(s) of ROS attack.

The participation of hydrogen peroxide in methyl jasmonate-induced NH(4)(+) accumulation in rice leaves

Ammonium is a central intermediate in the nitrogen metabolism of plants. We have previously shown that methyl jasmonate (MJ) not only increases the content of H(2)O(2), but also causes NH(4)(+) accumulation in rice leaves. More recently, H(2)O(2) is thought to constitute a general signal molecule participating in the recognition of and the response to stress factors. In this study, we examined the role of H(2)O(2) as a link between MJ and subsequent NH(4)(+) accumulation in detached rice leaves. MJ treatment resulted in an accumulation of NH(4)(+) in detached rice leaves, which was preceded by a decrease in the activity of glutamine synthetase (GS) and an increase in the specific activities of protease and phenylalanine ammonia-lyase (PAL). GS, PAL, and protease appear to be the enzymes responsible for the accumulation of NH(4)(+) in MJ-treated detached rice leaves. Dimethylthiourea (DMTU), a chemical trap for H(2)O(2), was observed to be effective in inhibiting MJ-induced NH(4)(+) accumulation in detached rice leaves. Scavengers of free radicals (sodium benzoate, SB, and glutathione, GSH), nitric oxide donor (N-tert-butyl-alpha-phenylnitrone, PBN), the inhibitors of NADPH oxidase (diphenyleneiodonium chloride, DPI, and imidazole, IMD), and inhibitors of phosphatidylinositol 3-kinase (wortmannin, WM, and LY 294002, LY), which have previously been shown to prevent MJ-induced H(2)O(2) production in detached rice leaves, inhibited MJ-induced NH(4)(+) accumulation. Similarly, changes in enzymes responsible for NH(4)(+) accumulation induced by MJ were observed to be inhibited by DMTU, SB, GSH, PBN DPI, IMD, WM, or LY. Seedlings of rice cultivar Taichung Native 1 (TN1) are jasmonic acid (JA)-sensitive and those of cultivar Tainung 67 (TNG67) are JA-insensitive. On treatment with JA, H(2)O(2) accumulated in the leaves of TN1 seedlings but not in the leaves of TNG67. Ethylene action inhibitor, silver thiosulfate, was observed to inhibit MJ- and abscisic acid-induced accumulation of NH(4)(+) and changes in enzymes responsible for NH(4)(+) accumulation in detached rice leaves, suggesting that the action of MJ and ABA is ethylene dependent.

A novel peroxiredoxin activity is located within the C-terminal end of Rhodospirillum rubrum adenylyltransferase

Adenylyltransferase (GlnE) catalyzes the reversible adenylylation of glutamine synthetase. In this report we present, for the first time, evidence for a peroxiredoxin activity of Rhodospirillum rubrum GlnE, through the carboxyl-terminal AhpC/thiol-specific antioxidant (TSA) domain. The combination of GlnE and AhpC/TSA domains within the same polypeptide constitutes a unique domain architecture that has not previously been identified among proteobacteria.

Metal-Catalyzed Oxidation of Protein-Bound Dopamine

Dopamine (DA) is an unstable neurotransmitter that readily oxidizes to the DA quinone and forms reactive oxygen species, such as superoxide and hydrogen peroxide. The oxidized dopamine also forms thiol conjugates with sulfhydryl groups on cysteine, glutathione, and proteins. In the present study, we determined the redox potential of the protein-bound DA and established a novel mechanism for the oxidative modification of the protein, in which the DA-cysteine adduct generated in the DA-modified protein causes oxidative modification of the DA-bound protein in the presence of Cu2+. Exposure of a sulfhydryl enzyme, glyceraldehyde-3-phosphate dehydrogenase, to DA resulted in a significant loss of sulfhydryl groups and the formation of the DA-cysteine adduct. When the DA-modified protein was incubated with Cu2+, we observed aggregation and degradation of the DA-bound protein and concomitant formation of a protein carbonyl, a marker of an oxidatively modified protein. Furthermore, we analyzed the carbonyl products generated during the Cu2+-catalyzed oxidation of the DA-modified protein and revealed the production of glutamic and aminoadipic semialdehydes, consisting of the protein carbonyls generated. The cysteinyl-DA residue generated in the DA-modified protein was suggested to represent a redox-active adduct, based on the observations that the cysteinyl-DA adduct, 5-S-cysteinyldopamine, produced by the reaction of cysteine with DA, gave rise to the oxidative modification of bovine serum albumin in the presence of Cu2+. These data suggest that the DA-modified protein may be involved in redox alteration under oxidative stress, whereby DA covalently binds to cysteine residues, generating the redox-active cysteinyl-DA adduct that causes the metal-catalyzed oxidation of protein.

Susceptibility of amyloid beta peptide degrading enzymes to oxidative damage: a potential Alzheimer's disease spiral

Insulysin (IDE) and neprilysin (NEP) were found to be inactivated by oxidation with hydrogen peroxide, an iron-ascorbate oxidation system, and by treatment with 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH). In each case reaction led to the introduction of protein carbonyl groups as judged by reaction with 2,4-dintrophenylhydrazine. IDE was inactivated by reaction with 4-hydroxy-2-nonenal (HNE) with the concomitant formation of protein adducts. NEP was not inactivated to a significant extent by HNE, but some HNE-adduct formation did occur. Prior reaction with hydrogen peroxide or AAPH led to enhanced formation of HNE adducts. Treatment of IDE with AAHP or hydrogen peroxide increased its susceptibility to proteolysis, while treatment of NEP with iron/ascorbate or hydrogen peroxide increased its susceptibility to proteolysis. Since IDE and NEP play a prominent role in the clearance of amyloid beta peptides, their oxidative inactivation and enhanced proteolysis can contribute to the onset and/or progression of Alzheimer's disease.

Quantitation of protein on gels and blots by infrared fluorescence of Coomassie blue and Fast Green

Coomassie blue staining of gels and blots is commonly employed for detection and quantitation of proteins by densitometry. We found that Coomassie blue or Fast Green FCF bound to protein fluoresces in the near infrared. We took advantage of this property to develop a rapid and sensitive method for detection and quantitation of proteins in gels and on blots. The fluorescence response is quantitative for protein content between 10 ng and 20 microg per band or spot. Staining and destaining require only 30 min, and the method is compatible with subsequent immunodetection.

Hydrogen peroxide is required for abscisic acid-induced NH4+ accumulation in rice leaves

The role of H2O2 in abscisic acid (ABA)-induced NH4+ accumulation in rice leaves was investigated. ABA treatment resulted in an accumulation of NH4+ in rice leaves, which was preceded by a decrease in the activity of glutamine synthetase (GS) and an increase in the specific activities of protease and phenylalanine ammonia-lyase (PAL). GS, PAL, and protease seem to be the enzymes responsible for the accumulation of NH4+ in ABA-treated rice leaves. Dimethylthiourea (DMTU), a chemical trap for H2O2, was observed to be effective in inhibiting ABA-induced accumulation of NH4+ in rice Leaves. Inhibitors of NADPH oxidase, diphenyleneiodonium chloride (DPI) and imidazole (IMD), and nitric oxide donor (N-tert-butyl-alpha-phenylnitrone, PBN), which have previously been shown to prevent ABA-induced increase in H2O2 contents in rice leaves, inhibited ABA-induced increase in the content of NH4+. Similarly, the changes of enzymes responsible for NH4+ accumulation induced by ABA were observed to be inhibited by DMTU, DPI, IMD, and PBN. Exogenous application of H2O2 was found to increase NH4+ content, decrease GS activity, and increase protease and PAL-specific activities in rice leaves. Our results suggest that H2O2 is involved in ABA-induced NH4+ accumulation in rice leaves.

Patterns of protein oxidation in Arabidopsis seeds and during germination

Increased cellular levels of reactive oxygen species are known to occur during seed development and germination, but the consequences in terms of protein degradation are poorly characterized. In this work, protein carbonylation, which is an irreversible oxidation process leading to a loss of function of the modified proteins, has been analyzed by a proteomic approach during the first stages of Arabidopsis (Arabidopsis thaliana) seed germination. In the dry mature seeds, the legumin-type globulins (12S cruciferins) were the major targets. However, the acidic alpha-cruciferin subunits were carbonylated to a much higher extent than the basic (beta) ones, consistent with a model in which the beta-subunits are buried within the cruciferin molecules and the alpha-subunits are more exposed to the outside. During imbibition, various carbonylated proteins accumulated. This oxidation damage was not evenly distributed among seed proteins and targeted specific proteins as glycolytic enzymes, mitochondrial ATP synthase, chloroplastic ribulose bisphosphate carboxylase large chain, aldose reductase, methionine synthase, translation factors, and several molecular chaperones. Although accumulation of carbonylated proteins is usually considered in the context of aging in a variety of model systems, this was clearly not the case for the Arabidopsis seeds since they germinated at a high rate and yielded vigorous plantlets. The results indicate that the observed specific changes in protein carbonylation patterns are probably required for counteracting and/or utilizing the production of reactive oxygen species caused by recovery of metabolic activity in the germinating seeds.

Role of oxidative carbonylation in protein quality control and senescence

Proteins can become modified by a large number of reactions involving reactive oxygen species. Among these reactions, carbonylation has attracted a great deal of attention due to its irreversible and unrepairable nature. Carbonylated proteins are marked for proteolysis by the proteasome and the Lon protease but can escape degradation and form high-molecular-weight aggregates that accumulate with age. Such carbonylated aggregates can become cytotoxic and have been associated with a large number of age-related disorders, including Parkinson's disease, Alzheimer's disease, and cancer. This review focuses on the generation of and defence against protein carbonyls and speculates on the potential role of carbonylation in protein quality control, cellular deterioration, and senescence.

Metal-induced oxidative damage in cultured hepatocytes and hepatic lysosomal fraction: beneficial effect of a curcumin/absinthium compound

OBJECTIVE

Metals undergo redox cycling and there is increasing evidence of free radical generation and oxidative injury in the pathogenesis of liver injury and fibrosis in metal storage diseases. The aim of the present study was to test a natural hepatoprotective compound in metal-induced liver injury.

METHODS

Hepatocytes were isolated from Wistar rats by collagenase perfusion method and cultured as such and also with alpha-linolenic acid (LNA)-bovine serum albumin (BSA). Hepatocytes were then cultured with a graded dilution of PN-M001 (100 microg/mL and 200 microg/mL), which is a curcuma/absinthium-containing compound, or sylibin (100 microg/mL) dissolved in dimethyl sulfoxide for 10 min before the addition of metallic salts (iron, copper and vanadium). Lysosomal fractions were prepared for lysosome fragility tests in which beta-galactosidase activity and lactate dehydrogenase (LDH) leakage were measured, as well as oxidative damage tests in the presence of hydrophilic and lipophilic free radical generators. Quenching activity by 1,1-diphenyl-2-picrylhydrazyl (DPPH) was also assessed.

RESULTS

Malonildialdehyde accumulation in the medium showed a direct time-course increase with incubation time. Both PN-M001 and sylibin showed a significant protective effect against all challenge metal ions, as expressed by the half inhibition concentration (IC(50)) against lipid peroxidation. However, on a molar ratio, sylibin seemed to be more effective than PN-M001 in Fe-induced peroxidative damage (P < 0.05). Both test compounds, irrespective of the concentration, significantly reduced the LDH and beta-galactosidase concentration in the lysosomal fractions. As compared with untreated lysosomal fractions challenged with the two peroxide radicals generators, either PN-M001 or sylibin exerted significant protection However, PN-M001 was significantly better than sylibin in suppressing acid phosphatase enzyme activity. Both compounds showed comparable and significant DPPH radical-scavenging activity.

CONCLUSION

These data support the potential clinical application of curcumin-containing compounds.

Activation of chaperone-mediated autophagy during oxidative stress

Oxidatively damaged proteins accumulate with age in almost all cell types and tissues. The activity of chaperone-mediated autophagy (CMA), a selective pathway for the degradation of cytosolic proteins in lysosomes, decreases with age. We have analyzed the possible participation of CMA in the removal of oxidized proteins in rat liver and cultured mouse fibroblasts. Added to the fact that CMA substrates, when oxidized, are more efficiently internalized into lysosomes, we have found a constitutive activation of CMA during oxidative stress. Oxidation-induced activation of CMA correlates with higher levels of several components of the lysosomal translocation complex, but in particular of the lumenal chaperone, required for substrate uptake, and of the lysosomal membrane protein (lamp) type 2a, previously identified as a receptor for this pathway. In contrast with the well characterized mechanism of CMA activation during nutritional stress, which does not require de novo synthesis of the receptor, oxidation-induced activation of CMA is attained through transcriptional up-regulation of lamp2a. We conclude that CMA is activated during oxidative stress and that the higher activity of this pathway under these conditions, along with the higher susceptibility of the oxidized proteins to be taken up by lysosomes, both contribute to the efficient removal of oxidized proteins.

Identification of a Heme-sensing Domain in Iron Regulatory Protein 2

Iron regulatory protein 2 coordinates the cellular regulation of iron metabolism by binding to iron-responsive elements in mRNA. The protein is synthesized constitutively but is rapidly degraded when iron stores are replete. The mechanisms that prevent degradation during iron deficiency or promote degradation during iron sufficiency are not delineated. Iron regulatory protein 2 contains a domain not present in the closely related iron regulatory protein 1, and we found that this domain binds heme with high affinity. A cysteine within the domain is axially liganded to the heme, as occurs in cytochrome P450. The protein-bound heme reacts with molecular oxygen to mediate the oxidation of cysteine, including beta-elimination of the sulfur to yield alanine. This covalent modification may thus mark the protein molecule for degradation by the proteasome system, providing another mechanism by which heme can regulate the level of iron regulatory protein 2.

Progression and Specificity of Protein Oxidation in the Life Cycle of Arabidopsis thaliana

Protein carbonylation is an irreversible oxidative process leading to a loss of function of the modified proteins, and in a variety of model systems, including worms, flies, and mammals, carbonyl levels gradually increase with age. In contrast, we report here that in Arabidopsis thaliana an initial increase in protein oxidation during the first 20 days of the life cycle of the plant is followed by a drastic reduction in protein carbonyls prior to bolting and flower development. Protein carbonylation prior to the transition to flowering targets specific proteins such as Hsp70, ATP synthases, the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and proteins involved in light harvesting/energy transfer and the C2 oxidative photosynthetic carbon cycle. The precipitous fall in protein carbonyl levels is due to the specific reduction in the levels of oxidized proteins rather than an overall loss of chlorophyll and Rubisco associated with the senescence syndrome. The results are discussed in light of contemporary theories of aging in animals.

Iron regulatory protein 2 as iron sensor. Iron-dependent oxidative modification of cysteine

Iron regulatory protein 2 coordinates cellular regulation of iron metabolism by binding to iron responsive elements in mRNA. The protein is synthesized constitutively but is rapidly degraded when iron stores are replete. This iron-dependent degradation requires the presence of a 73-residue degradation domain, but its functions have not yet been established. We now show that the domain can act as an iron sensor, mediating its own covalent modification. The domain forms an iron-binding site with three cysteine residues located in the middle of the domain. It then reacts with molecular oxygen to generate a reactive oxidizing species at the iron-binding site. One cysteine residue is oxidized to dehydrocysteine and other products. This covalent modification may thus mark the protein molecule for degradation by the proteasome system.

Proteomic identification of oxidatively modified proteins in Alzheimer's disease brain. Part I: creatine kinase BB, glutamine synthase, and ubiquitin carboxy-terminal hydrolase L-1

Oxidative alterations of proteins by reactive oxygen species (ROS) have been implicated in the progression of aging and age-related neurodegenerative disorders such as Alzheimer's disease (AD). Protein carbonyls, a marker of protein oxidation, are increased in AD brain, indicating that oxidative modification of proteins is relevant in AD. Oxidative damage can lead to several events such as loss in specific protein function, abnormal protein clearance, depletion of the cellular redox-balance and interference with the cell cycle, and, ultimately, to neuronal death. Identification of specific targets of protein oxidation represents a crucial step in establishing a relationship between oxidative modification and neuronal death in AD, and was partially achieved previously in our laboratory through immunochemical detection of creatine kinase BB and beta-actin as specifically oxidized proteins in AD brain versus control brain. However, this process is laborious, requires the availability of specific antibodies, and, most importantly, requires a reasonable guess as to the identity of the protein in the first place. In this study, we present the first proteomics approach to identify specifically oxidized proteins in AD, by coupling 2D fingerprinting with immunological detection of carbonyls and identification of proteins by mass spectrometry. The powerful techniques, emerging from application of proteomics to neurodegenerative disease, reveal the presence of specific targets of protein oxidation in Alzheimer's disease (AD) brain: creatine kinase BB, glutamine synthase, and ubiquitin carboxy-terminal hydrolase L-1. These results are discussed with reference to potential involvement of these oxidatively modified proteins in neurodegeneration in AD brain. Proteomics offers a rapid means of identifying oxidatively modified proteins in aging and age-related neurodegenerative disorders without the limitations of the immunochemical detection method.

The role of cysteine residues in the oxidation of ferritin

We have shown that ferritin is oxidized during iron loading using its own ferroxidase activity and that this oxidation results in its aggregation (Welch et al., Free Radic. Biol. Med. 31:999-1006; 2001). In this study we determined the role of cysteine residues in the oxidation of ferritin. Loading iron into recombinant human ferritin by its own ferroxidase activity decreased its conjugation by a cysteine specific spin label, indicating that cysteine residues were altered during iron loading. Using LC/MS, we demonstrated that tryptic peptides of ferritin that contained cysteine residues were susceptible to modification as a result of iron loading. To assess the role of cysteine residues in the oxidation of ferritin, we used site-directed mutagenesis to engineer variants of human ferritin H chain homomers where the cysteines were substituted with other amino acids. The cysteine at position 90, which is located at the end of the BC-loop, appeared to be critical for the formation of ferritin aggregates during iron loading. We also provide evidence that dityrosine moieties are formed during iron loading into ferritin by its own ferroxidase activity and that the dityrosine formation is dependent upon the oxidation of cysteine residues, especially cysteine 90. In conclusion, cysteine residues play an integral role in the oxidation of ferritin and are essential for the formation of ferritin aggregates.

Carbonyl modified proteins in cellular regulation, aging, and disease

The oxidative modification of proteins by reactive species is implicated in the etiology or progression of a panoply of disorders and diseases. The level of these modified molecules can be quantitated by measurement of the protein carbonyl content, which has been shown to increase in a variety of diseases and processes, notably during aging. For the most part, oxidatively modified proteins are not repaired and must be removed by proteolytic degradation, a process which normally proceeds very efficiently, from microorganisms to mammals. In eukaryotes, removal is usually carried out by the proteosome, which selectively degrades oxidatively modified proteins, whether they be damaged by reactive oxygen species or specifically oxidized by cellular regulatory processes. The molecular deficiencies that cause accumulation of oxidatively modified proteins are not identified, but regardless of cause, the accumulation is likely to disrupt normal cellular function.

Specific and reversible inactivation of Phycomyces blakesleeanus isocitrate lyase by ascorbate-iron: role of two redox-active cysteines

Phycomyces blakesleeanus isocitrate lyase (EC 4.1.3.1) is in vivo reversibly inactivated by hydrogen peroxide. The purified enzyme showed reversible inactivation by an ascorbate plus Fe(2+) system under aerobic conditions. Inactivation requires hydrogen peroxide; was prevented by catalase, EDTA, Mg(2+), isocitrate, GSH, DTT, or cysteine; and was reversed by thiols. The ascorbate served as a source of hydrogen peroxide and also reduced the Fe(3+) ions produced in a "site-specific" Fenton reaction. Two redox-active cysteine residues per enzyme subunit are targets of oxidative modification; one of them is located at the catalytic site and the other at the metal regulatory site. The oxidized enzyme showed covalent and conformational changes that led to inactivation, decreased thermal stability, and also increased inactivation by trypsin. These results represent an example of redox regulation of an enzymatic activity, which may play a role as a sensor of redox cellular status.

Regulation of glutamine synthetase by metal-catalyzed oxidative modification in the marine oxyphotobacterium Prochlorococcus

The inactivation of glutamine synthetase (GS; EC 6.3.1.2) by metal-catalyzed oxidation (MCO) systems was studied in several Prochlorococcus strains, including the axenic PCC 9511. GS was inactivated in the presence of various oxidative systems, either enzymatic (as NAD(P)H+NAD(P)H-oxidase+Fe(3+)+O(2)) or non-enzymatic (as ascorbate+Fe(3+)+O(2)). This process required the presence of oxygen and a metal cation, and is prevented under anaerobic conditions. Catalase and peroxidase, but not superoxide dismutase, effectively protected the enzyme against inactivation, suggesting that hydrogen peroxide mediates this mechanism, although it is not directly responsible for the reaction. Addition of azide (an inhibitor of both catalase and peroxidase) to the MCO systems enhanced the inactivation. Different thiols induced the inactivation of the enzyme, even in the absence of added metals. However, this inactivation could not be reverted by addition of strong oxidants, as hydrogen peroxide or oxidized glutathione. After studying the effect of addition of the physiological substrates and products of GS on the inactivation mechanism, we could detect a protective effect in the case of inorganic phosphate and glutamine. Immunochemical determinations showed that the concentration of GS protein significantly decreased by effect of the MCO systems, indicating that inactivation precedes the degradation of the enzyme.

Proteasome inhibition in glyoxal-treated fibroblasts and resistance of glycated glucose-6-phosphate dehydrogenase to 20 S proteasome degradation in vitro

Glycation and glycoxidation protein products are formed upon binding of sugars to NH(2) groups of lysine and arginine residues and have been shown to accumulate during aging and in pathologies such as Alzheimer's disease and diabetes. Because the proteasome is the major intracellular proteolytic system involved in the removal of altered proteins, the effect of intracellular glycation on proteasome function has been analyzed in human dermal fibroblasts subjected to treatment with glyoxal that promotes the formation of N epsilon-carboxymethyl-lysine adducts on proteins. The three proteasome peptidase activities were decreased in glyoxal-treated cells as compared with control cells, and glyoxal was also found to inhibit these peptidase activities in vitro. In addition, the activity of glucose-6-phosphate dehydrogenase, a crucial enzyme for the regulation of the intracellular redox status, was dramatically reduced in glyoxal-treated cells. Further analysis was performed to determine whether glycated proteins are substrates for proteasome degradation. In contrast to the oxidized glucose-6-phosphate dehydrogenase, both N epsilon-carboxymethyl-lysine- and fluorescent-glycated enzymes were resistant to degradation by the 20 S proteasome in vitro, and this resistance was correlated with an increased conformational stability of the glycated proteins. These results provide one explanation for why glycated proteins build up both as a function of disease and aging. Finally, N epsilon-carboxymethyl-lysine-modified proteins were found to be ubiquitinated in glyoxal-treated cells suggesting a potential mechanism by which these modified proteins may be marked for degradation.

Oxidative modification of proteins during aging

Accumulating experimental evidence supports the proposal that many of the changes which occur during aging are a consequence of oxidative damage. Reactive oxygen species react with all three of the major cellular macromolecules, nucleic acids, lipids, and proteins. This minireview focuses on proteins as targets of oxidizing species during aging. Many of the reactions mediated by these oxidizing species result in the introduction of carbonyl groups into proteins. The steady-state level of carbonyl-bearing proteins increases exponentially during the last third of lifespan in animals ranging from C. elegans to man. Genetic and non-genetic manipulations which lengthen lifespan cause a decrease in the level of protein carbonyl while those which shorten lifespan increase the level. Oxidized proteins bearing carbonyl groups are generally dysfunctional, and in the last third of lifespan the content of these oxidized proteins rises to a level likely to cause substantial disruption of cellular function.

Inactivation of catalase and superoxide dismutase by singlet oxygen derived from photoactivated dye

Both superoxide dismutase (SOD) and catalase are key enzymes in the antioxidant system of the cells that work to maintain low steady-state concentrations of the reactive oxygen species. When exposed to a singlet oxygen-producing system composed of dye, such as methylene blue or rose bengal, and visible light both SOD and catalase were susceptible to oxidative modification and damage as indicated by the loss of activity, fragmentation and aggregation of peptide as well as by the formation of carbonyl groups. Histidine, a powerful quenching agent for singlet oxygen, and the polyamines, such as spermine and spermidine, were effective at protecting the activity loss mediated by illuminated dye, whereas spin traps were only mildly effective. The structural alterations of modified enzymes were indicated by the increase in susceptibility to proteases, the change in absorption spectra and in fluorescence spectra. The singlet oxygen-mediated damage to SOD and catalase may result in the perturbation of cellular antioxidant defense mechanisms and subsequently lead to a pro-oxidant condition.

Constitutive overexpression of cytosolic glutamine synthetase (GS1) gene in transgenic alfalfa demonstrates that GS1 may be regulated at the level of RNA stability and protein turnover

Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of NH4+ with glutanate to yield glutamine. Gene constructs consisting of the cauliflower mosaic virus (CaMV) 35S promoter driving a cytosolic isoform of GS (GS1) gene have been introduced into alfalfa (Medicago sativa). Although transcripts for the transgene were shown to accumulate to high levels in the leaves, they were undetectable in the nodules. However, significant amounts of beta-glucuronidase activity could be detected in nodules of plants containing the CaMV 35S promoter-beta-glucuronidase gene construct, suggesting that the transcript for the GS1 transgene is not stable in the root nodules. Leaves of alfalfa plants with the CaMV 35S promoter-GS1 gene showed high levels of accumulation of the transcript for the transgene when grown under low-nitrogen conditions and showed a significant drop in the level of GS1 transcripts when fed with high levels of NO3-. However, no increase in GS activity or polypeptide level was detected in the leaves of transgenic plants. The results suggest that GS1 is regulated at the level of RNA stability and protein turnover.

Generation and propagation of radical reactions on proteins

The oxidation of proteins by free radicals is thought to play a major role in many oxidative processes within cells and is implicated in a number of human diseases as well as ageing. This review summarises information on the formation of radicals on peptides and proteins and how radical damage may be propagated and transferred within protein structures. The emphasis of this article is primarily on the deleterious actions of radicals generated on proteins, and their mechanisms of action, rather than on enzymatic systems where radicals are deliberately formed as transient intermediates. The final section of this review examines the control of protein oxidation and how such damage might be limited by antioxidants.

Glutamic and aminoadipic semialdehydes are the main carbonyl products of metal-catalyzed oxidation of proteins

Metal-catalyzed oxidation results in loss of function and structural alteration of proteins. The oxidative process affects a variety of side amino acid groups, some of which are converted to carbonyl compounds. Spectrophotometric measurement of these moieties, after their reaction with 2,4-dinitrophenylhydrazine, is a simple, accurate technique that has been widely used to reveal increased levels of protein carbonyls in aging and disease. We have initiated studies aimed at elucidating the chemical nature of protein carbonyls. Methods based on gas chromatography/mass spectrometry with isotopic dilution were developed for the quantitation of glutamic and aminoadipic semialdehydes after their reduction to hydroxyaminovaleric and hydroxyaminocaproic acids. Analysis of model proteins oxidized in vitro by Cu2+/ascorbate revealed that these two compounds constitute the majority of protein carbonyls generated. Glutamic and aminoadipic semialdehydes were also detected in rat liver proteins, where they constitute approximately 60% of the total protein carbonyl value. Aminoadipic semialdehyde was also measured in protein extracts from HeLa cells, and its level increased as a consequence of oxidative stress to cell cultures. These results indicate that glutamic and aminoadipic semialdehydes are the main carbonyl products of metal-catalyzed oxidation of proteins, and that this reaction is a major route leading to the generation of protein carbonyls in biological samples.

Glutamic and aminoadipic semialdehydes are the main carbonyl products of metal-catalyzed oxidation of proteins

Metal-catalyzed oxidation results in loss of function and structural alteration of proteins. The oxidative process affects a variety of side amino acid groups, some of which are converted to carbonyl compounds. Spectrophotometric measurement of these moieties, after their reaction with 2,4-dinitrophenylhydrazine, is a simple, accurate technique that has been widely used to reveal increased levels of protein carbonyls in aging and disease. We have initiated studies aimed at elucidating the chemical nature of protein carbonyls. Methods based on gas chromatography/mass spectrometry with isotopic dilution were developed for the quantitation of glutamic and aminoadipic semialdehydes after their reduction to hydroxyaminovaleric and hydroxyaminocaproic acids. Analysis of model proteins oxidized in vitro by Cu2+/ascorbate revealed that these two compounds constitute the majority of protein carbonyls generated. Glutamic and aminoadipic semialdehydes were also detected in rat liver proteins, where they constitute approximately 60% of the total protein carbonyl value. Aminoadipic semialdehyde was also measured in protein extracts from HeLa cells, and its level increased as a consequence of oxidative stress to cell cultures. These results indicate that glutamic and aminoadipic semialdehydes are the main carbonyl products of metal-catalyzed oxidation of proteins, and that this reaction is a major route leading to the generation of protein carbonyls in biological samples.

Protein oxidation

The oxidative modification of proteins by reactive species, especially reactive oxygen species, is implicated in the etiology or progression of a panoply of disorders and diseases. These reactive species form through a large number of physiological and non-physiological reactions. An increase in the rate of their production or a decrease in their rate of scavenging will increase the oxidative modification of cellular molecules, including proteins. For the most part, oxidatively modified proteins are not repaired and must be removed by proteolytic degradation, and a decrease in the efficiency of proteolysis will cause an increase in the cellular content of oxidatively modified proteins. The level of these modified molecules can be quantitated by measurement of the protein carbonyl content, which has been shown to increase in a variety of diseases and processes, most notably during aging. Accumulation of modified proteins disrupts cellular function either by loss of catalytic and structural integrity or by interruption of regulatory pathways.