Biochemistry and pathology of radical-mediated protein oxidation

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.

Correction of long-term diet-induced hypertension and nitrotyrosine accumulation by diet modification

Several recent studies have demonstrated that various forms of hypertension are associated with enhanced reactive oxygen species (ROS) activity. We have recently shown that long-term consumption of a diet similar to that ingested in westernized societies, containing high saturated fat and refined carbohydrate, induces oxidative stress and hypertension in normal rats. We hypothesized that diet modification may reverse diet-induced hypertension via (among other mechanisms) decreased ROS activity and improved nitric oxide (NO) availability. To test this hypothesis, female Fischer rats were placed on either a high-fat (primarily saturated), refined carbohydrate (sucrose) diet (HFS) or low-fat, complex-carbohydrate diet (LFCC) starting at 2 months of age. After 2 years when hypertension was well established, a group of HFS rats was converted to the LFCC diet (HFS/LFCC group) for a period of 2 months. Plasma malondialdehyde, a marker of lipid peroxidation by ROS, was elevated in the HFS group. Hypertension was present in the HFS group at 2 years as was a significant accumulation, in various tissues, of nitrotyrosine, which is the footprint of NO inactivation by ROS. Conversion from the HFS to the LFCC diet for 2 months led to normalization of blood pressure and reduced nitrotyrosine accumulation in the absence of caloric restriction. These results demonstrate that oxidative stress and hypertension induced by long-term consumption of an HFS diet are reversible with implementation of a low-fat, unrefined carbohydrate diet. The effects of the HFS diet and subsequent conversion to the LFCC diet on blood pressure appear to be, in part, mediated by changes in NO availability.

Flavones from Scutellaria baicalensis Georgi attenuate apoptosis and protein oxidation in neuronal cell lines

The oxidative modification of proteins plays a major role in a number of human diseases including Alzheimer's disease (AD). Flavones in extracts of Scutellaria baicalensis (SbE) have been reported to have exceptional antioxidant properties. We examined the effects of SbE on neuronal cells exposed to oxidative stress. Neuronal HT-22 cells were exposed to low levels of H(2)O(2) generated from glucose oxidase (GO) under conditions that caused cell death in 24 h. The mechanism of cell death was shown to occur via apoptosis. Flavone extracts (50 microg/ml) protected cells and increased viability to 85+/-5% (P<0.001). The flavones also increased the content of Bcl-2 in the cell, resulted in its phosphorylation, and in contrast decreased the Bax levels. Furthermore, the oxidative-stress-induced protein carbonyl formation was reduced nearly two-fold when cells were pretreated with the flavone extract. Two-dimensional electrophoresis (2-DE) showed that less than 15% of the total visible proteins were oxidized and that the oxidation was specific for certain oxidation-sensitive proteins. These data support the idea that flavones in SbE can attenuate oxidant stress and protect cells from lethal oxidant damage.

The mitochondrial DNA polymerase as a target of oxidative damage

The mitochondrial respiratory chain is a source of reactive oxygen species (ROS) that are responsible for oxidative modification of biomolecules, including proteins. Due to its association with mitochondrial DNA, DNA polymerase gamma (pol gamma) is in an environment to be oxidized by hydrogen peroxide and hydroxyl radicals that may be generated in the presence of iron ions associated with DNA. We tested whether human pol gamma was a possible target of ROS with H2O2 and investigated the effect on the polymerase activities and DNA binding efficiency. A 1 h treatment with 250 microM H2O2 significantly inhibited DNA polymerase activity of the p140 subunit and lowered its DNA binding efficiency. Addition of p55 to the p140 catalytic subunit prior to H2O2 treatment offered protection from oxidative inactivation. Oxidatively modified amino acid residues in pol gamma resulting from H2O2 treatment were observed in vitro as well as in vivo, in SV40-transfected human fibroblasts. Pol gamma was detected as one of the major oxidized mitochondrial matrix proteins, with a detectable decline in polymerase activity. These results suggest pol gamma as a target of oxidative damage, which may result in a reduction in mitochondrial DNA replication and repair capacities.

Lipid peroxidation and protein oxidation in Alzheimer's disease brain: potential causes and consequences involving amyloid beta-peptide-associated free radical oxidative stress

Amyloid beta-peptide (A(beta)) is heavily deposited in the brains of Alzheimer's disease (AD) patients, and free radical oxidative stress, particularly of neuronal lipids and proteins, is extensive. Recent research suggests that these two observations may be linked by A(beta)-induced oxidative stress in AD brain. This review summarizes current knowledge on phospholipid peroxidation and protein oxidation in AD brain, one potential cause of this oxidative stress, and consequences of A(beta)-induced lipid peroxidation and protein oxidation in AD brain.

Protein turnover by the proteasome in aging and disease

A significant body of evidence supports a key role for free radicals in causing cumulative damage to cellular macromolecules, thereby contributing to senescence/aging, and a number of age-related disorders. Proteins are recognized as major targets for oxidative damage (in addition to DNA and lipids) and the accumulation of oxidized proteins has been reported for many experimental aging models, as measured by several markers for protein oxidation. In young and healthy individuals, moderately oxidized soluble cell proteins are selectively and rapidly degraded by the proteasome. However, severely oxidized, cross-linked proteins are poor substrates for degradation and actually inhibit the proteasome. Considerable evidence now indicates that proteasome activity declines during aging, as the protease is progressively inhibited by binding to ever increasing levels of oxidized and cross-linked protein aggregates. Cellular aging probably involves both an increase in the generation of reactive oxygen species and a progressive decline in proteasome activity, resulting in the progressive accumulation of oxidatively damaged protein aggregates that eventually contribute to cellular dysfunction and senescence.

Beta-scission of side-chain alkoxyl radicals on peptides and proteins results in the loss of side-chains as aldehydes and ketones

Exposure of proteins to radicals in the presence of O(2) results in side-chain oxidation and backbone fragmentation; the interrelationship between these processes is not fully understood. Recently, initial attack on Ala side-chains was shown to give alpha-carbon radicals (and hence backbone cleavage) and formaldehyde, via the formation and subsequent beta-scission, of C-3 alkoxyl radicals. We now show that this side-chain to backbone damage transfer, is a general mechanism for aliphatic side-chains. Oxidation of Val, Leu, and Asp residues by HO(*)/O(2) results in the release of a family of carbonyls (including formaldehyde, acetone, isobutyraldehyde, and glyoxylic acid) via the formation, and subsequent beta-scission of alkoxyl radicals. The concentration of these products increases with the HO(*) flux. The release of multiple carbonyls confirms the occurrence of oxidation at C-3 and C-4 for Val, and these sites, plus C-5, for Leu. The detection of glyoxylic acid and CO(2)(-*) from Asp demonstrates the occurrence of competing beta-scission processes for the Asp C-3 alkoxyl radical. The yield of hydroperoxides and released carbonyls account for 10-145% of the initial HO(*). The greater than 100% yields confirm the occurrence of chain reactions in peptide/protein oxidation, with more than one residue being damaged per initiating radical.

Simple and sensitive high-performance liquid chromatographic method for the investigation of dynamic changes in the redox state of rat serum albumin

Serum albumin is a mixture of mercaptalbumin (reduced form) and non-mercaptalbumin (oxidized form), i.e. a protein redox couple in serum. To investigate dynamic changes in the redox state of rat serum albumin (RSA), we developed a simple and sensitive high-performance liquid chromatographic (HPLC) system using an ion-exchange column with a linear gradient of ethanol concentration. Furthermore, we applied this HPLC system to examine dynamic changes in the redox state of RSA caused by severe oxidative stress such as exhaustive physical exercise. Using this system, we successfully separated RSA to rat mercaptalbumin (MA(r)) and rat non-mercaptalbumin (NA(r)), and also found the best conditions for the clear separation of RSA. In the experiments with exhaustive exercise, mean values for the MA(r) fraction in control and exercise groups were 76.2+/-1.8 and 69.0+/-3.5%, respectively. The MA(r) in the exercise group was significantly oxidized compared with that of the control group (P<0.01). These results suggested that RSA might act as one of the major scavengers in extracellular fluids under severe oxidative stress.

Practicalities of glutathione supplementation in nutritional support

New nutraceutical products for nutritional support and antioxidant therapy such as glutathione require practical advice and information on the indications, methods and routes of administration, dosing (therapeutic drug monitoring), stability and physicochemical compatibility. This review is based on recent clinical and experimental publications in which glutathione has been used as a drug.

Suicide inactivation of peroxidases and the challenge of engineering more robust enzymes

As the number of industrial applications for proteins continues to expand, the exploitation of protein engineering becomes critical. It is predicted that protein engineering can generate enzymes with new catalytic properties and create desirable, high-value, products at lower production costs. Peroxidases are ubiquitous enzymes that catalyze a variety of oxygen-transfer reactions and are thus potentially useful for industrial and biomedical applications. However, peroxidases are unstable and are readily inactivated by their substrate, hydrogen peroxide. Researchers rely on the powerful tools of molecular biology to improve the stability of these enzymes, either by protecting residues sensitive to oxidation or by devising more efficient intramolecular pathways for free-radical allocation. Here, we discuss the catalytic cycle of peroxidases and the mechanism of the suicide inactivation process to establish a broad knowledge base for future rational protein engineering.

Comparative time-courses of copper-ion-mediated protein and lipid oxidation in low-density lipoprotein

Free radicals damage both lipids and proteins and evidence has accumulated for the presence of both oxidised lipids and proteins in aged tissue samples as well as those from a variety of pathologies including atherosclerosis, diabetes, and Parkinson's disease. Oxidation of the protein and lipid moieties of low-density lipoprotein is of particular interest due to its potential role in the unregulated uptake of lipids and cholesterol by macrophages; this may contribute to the initial stage of foam cell formation in atherosclerosis. In the study reported here, we examined the comparative time-courses of lipid and protein oxidation during copper-ion-mediated oxidation of low-density lipoprotein. We show that there is an early, lipid-mediated loss of 40-50% of the Trp residues of the apoB100 protein. There is no comparable loss over an identical period during the copper-ion-mediated oxidation of lipid-free BSA. Concomitant with Trp loss, the antioxidant alpha-tocopherol is consumed with subsequent extensive lipid peroxidation. Further changes to the protein, including the copper-ion-dependent 3.5-fold increase in 3,4-dihydroxyphenylalanine and the copper-ion-independent 3-5-fold increase in o-tyrosine, oxidation products of Tyr and Phe, respectively, only occur after maximal lipid peroxidation. Long incubation periods result in depletion of 3,4-dihydroxyphenylalanine, presumably reflecting further oxidative changes. Overall, copper-ion-mediated oxidation of LDL appears to proceed initially by lipid radical-dependent processes, even though some of the earliest detectable changes occur on the apoB100 protein. This is followed by extensive lipid peroxidation and subsequent additional oxidation of aromatic residues on apoB100, though it is not yet clear whether this late protein oxidation is lipid-dependent or occurs as a result of direct radical attack.

Metal-catalyzed oxidation of extracellular matrix proteins promotes human mesangial cell apoptosis and is associated with enhanced expression of Bax and caspase activation

Oxidative injury in glomerular disease may oxidize extracellular matrix proteins which might modulate their interaction with mesangial cells and thereby account for the hypocellularity seen in advanced glomerulosclerosis. In this study we evaluated whether oxidation of extracellular matrix could modulate human mesangial cell apoptosis. Human mesangial cells were seeded onto plates coated with unmodified (control) or oxidized Matrigel, laminin, or type IV collagen. Mesangial cell apoptosis was increased on oxidized Matrigel as well as on oxidized laminin and type IV collagen. Mesangial cells behaved similarly on plates coated with control and oxidized forms of the integrin ligand-containing peptide GRGDSP. Cells on oxidized matrix demonstrated enhanced expression of Bax, increased fragmentation of PARP, and diminished apoptosis in the presence of the interleukin-1 beta converting enzyme inhibitor Ac-Tyr-Val-Ala-Asp-aldehyde. These data suggest that oxidation of extracellular matrix proteins may enhance human mesangial cell apoptosis via a mechanism that appears to involve enhanced expression of Bax and caspase activation. This may account for irreversible mesangial hypocellularity in glomerulosclerosis.

Impairment of mineralocorticoid receptor (MR)-dependent biological response by oxidative stress and aging: correlation with post-translational modification of MR and decreased ADP-ribosylatable level of elongating factor 2 in kidney cells

Acute and chronic treatments of mice with the glutathione-depleting agent, L-buthionine-(SR)-sulfoximine (BSO), impaired the mineralocorticoid receptor (MR)-dependent biological response by inhibiting aldosterone binding. This steroid-binding inhibition was fully reversed when reducing agents were added to kidney cytosol obtained from mice treated for 5 h, but it was only partially reversed in cytosol obtained from mice treated for 10 days. Although the oligomeric structure of the MR-hsp90 heterocomplex was always unaffected, a decreased amount of MR protein was evidenced after the long term treatment. Such a deleterious effect was correlated with a post-translational modification of MR, as demonstrated by an increased level of receptor carbonylation. In addition, a failure at the elongation/termination step was also observed during the receptor translation process in a reticulocyte lysate system. Thus, a high polyribosomes/monomers ratio and both increased proteolysis and decreased ADP-ribosylatable concentration of elongation factor 2 (EF-2) were shown. Importantly, similar observations were also performed in vivo after depletion of glutathione. Notwithstanding the EF-2 functional disruption, not all renal proteins were equally affected as the MR. Interestingly, both EF-2 and MR expressed in old mice were similarly affected as in L-buthionine-(SR)-sulfoximine-treated young mice. We therefore propose that a dramatic depletion of glutathione in kidney cells mimics the cumulative effect of aging which, at the end, may lead to a renal mineralocorticoid dysfunction.

Role of free radicals in the neurodegenerative diseases: therapeutic implications for antioxidant treatment

Free radicals and other so-called 'reactive species' are constantly produced in the brain in vivo. Some arise by 'accidents of chemistry', an example of which may be the leakage of electrons from the mitochondrial electron transport chain to generate superoxide radical (O2*-). Others are generated for useful purposes, such as the role of nitric oxide in neurotransmission and the production of O2*- by activated microglia. Because of its high ATP demand, the brain consumes O2 rapidly, and is thus susceptible to interference with mitochondrial function, which can in turn lead to increased O2*- formation. The brain contains multiple antioxidant defences, of which the mitochondrial manganese-containing superoxide dismutase and reduced glutathione seem especially important. Iron is a powerful promoter of free radical damage, able to catalyse generation of highly reactive hydroxyl, alkoxyl and peroxyl radicals from hydrogen peroxide and lipid peroxides, respectively. Although most iron in the brain is stored in ferritin, 'catalytic' iron is readily mobilised from injured brain tissue. Increased levels of oxidative damage to DNA, lipids and proteins have been detected by a range of assays in post-mortem tissues from patients with Parkinson's disease, Alzheimer's disease and amyotrophic lateral sclerosis, and at least some of these changes may occur early in disease progression. The accumulation and precipitation of proteins that occur in these diseases may be aggravated by oxidative damage, and may in turn cause more oxidative damage by interfering with the function of the proteasome. Indeed, it has been shown that proteasomal inhibition increases levels of oxidative damage not only to proteins but also to other biomolecules. Hence, there are many attempts to develop antioxidants that can cross the blood-brain barrier and decrease oxidative damage. Natural antioxidants such as vitamin E (tocopherol), carotenoids and flavonoids do not readily enter the brain in the adult, and the lazaroid antioxidant tirilazad (U-74006F) appears to localise in the blood-brain barrier. Other antioxidants under development include modified spin traps and low molecular mass scavengers of O2*-. One possible source of lead compounds is the use of traditional remedies claimed to improve brain function. Little is known about the impact of dietary antioxidants upon the development and progression of neurodegenerative diseases, especially Alzheimer's disease. Several agents already in therapeutic use might exert some of their effects by antioxidant action, including selegiline (deprenyl), apomorphine and nitecapone.

Inhibition of glyceraldehyde-3-phosphate dehydrogenase by peptide and protein peroxides generated by singlet oxygen attack

Reaction of certain peptides and proteins with singlet oxygen (generated by visible light in the presence of rose bengal dye) yields long-lived peptide and protein peroxides. Incubation of these peroxides with glyceraldehyde-3-phosphate dehydrogenase, in the absence of added metal ions, results in loss of enzymatic activity. Comparative studies with a range of peroxides have shown that this inhibition is concentration, peroxide, and time dependent, with H2O2 less efficient than some peptide peroxides. Enzyme inhibition correlates with loss of both the peroxide and enzyme thiol residues, with a stoichiometry of two thiols lost per peroxide consumed. Blocking the thiol residues prevents reaction with the peroxide. This stoichiometry, the lack of metal-ion dependence, and the absence of electron paramagnetic resonance (EPR)-detectable species, is consistent with a molecular (nonradical) reaction between the active-site thiol of the enzyme and the peroxide. A number of low-molecular-mass compounds including thiols and ascorbate, but not Trolox C, can prevent inhibition by removing the initial peroxide, or species derived from it. In contrast, glutathione reductase and lactate dehydrogenase are poorly inhibited by these peroxides in the absence of added Fe2+-EDTA. The presence of this metal-ion complex enhanced the inhibition observed with these enzymes consistent with the occurrence of radical-mediated reactions. Overall, these studies demonstrate that singlet oxygen-mediated damage to an initial target protein can result in selective subsequent damage to other proteins, as evidenced by loss of enzymatic activity, via the formation and subsequent reactions of protein peroxides. These reactions may be important in the development of cellular dysfunction as a result of photo-oxidation.

Smoking has no effect on the amino acid composition of apolipoprotein B100 of LDL while directly influencing the antioxidant status

Previous studies have demonstrated increased plasma levels of oxidised low-density lipoprotein (oxLDL) in chronic smokers, which has been associated with the extent of endothelial dysfunction. In this study we examine the relationship between the amino acid composition of apolipoprotein B100 (apo B) of low-density lipoprotein (LDL), by reverse phase HPLC after precolumn derivatisation, between smokers (> or =40 cigarettes/day) and nonsmokers in relation to their plasma and LDL antioxidant status. While there was a significant difference in the levels of plasma vitamin C and alpha-tocopherol between female smokers and nonsmokers, as well as in the levels of LDL alpha-tocopherol, there was no significant difference in the amino acid composition of apo B between the two groups.

Theoretical studies of the cross-linking mechanisms between cytosine and tyrosine

DNA-protein cross-linking is one of the many DNA lesions mediated by hydroxyl radicals, the most damaging among the reactive oxygen species in biological systems. Density functional theory methods are employed to investigate the complex reaction mechanisms of the formation of cytosine-tyrosine cross-links as observed in gamma-irradiated aqueous solutions of cytosine and tyrosine, as well as in gamma-irradiated nucleohistone. The majority of the radical addition mechanisms considered are found to have significant barriers and therefore to be thermodynamically unfavorable for the formation of the initial cross-linked product. Our calculated reaction potential energy surfaces suggest that a feasible complete mechanism consists of radical combination forming the initial cross-linked product, a hydrogen shuffle within the initial cross-linked product, and an acid-catalyzed dehydration reaction. Water and hydrogen-bonding interactions are suggested to play a key role in catalyzing the hydrogen-transfer step of the reaction.

Signatures of hippocampal oxidative stress in aged spatial learning-impaired rodents

Neurons and glia within the hippocampus of aged, spatial learning-impaired Long-Evans rats exhibit uniquely altered gene expression profiles, and we have postulated oxidative stress as the basis for this. To test this hypothesis we quantitated the extent of protein and nucleic acid oxidative damage, evaluated the status of mitochondrial DNA integrity, and examined several signaling entities and molecular indicators frequently associated with oxidative stress and gliosis. Immunoblotting demonstrated elevated heme oxygenase-1 in the aged-impaired hippocampus and immunocytochemistry suggested that heme oxygenase-1 is largely cytosolic and at least partly neuronal in nature. In the aged-impaired group, immunoreactivity to 8-hydroxy-2'-deoxyguanosine, an oxidative nucleic acid adduct, was found to be elevated in the dentate gyrus and in area CA1 of the hippocampal formation. Isolated mitochondrial DNA was found to be significantly damaged in the aged-impaired group. In the aged learning-impaired rats only, proteins in a 65-kDa band were found to contain excessive levels of carbonyl residues. Glial activation was examined by in situ hybridization histochemistry to tumor necrosis factor alpha and by immunocytochemistry with OX-6, which detects activated microglia. White matter in aged brains exhibited a modest up-regulation of tumor necrosis factor alpha mRNA and OX-6 immunoreactivity, but the hippocampal formation expressed tumor necrosis factor alpha mRNA equivalent to young animals and few OX-6-positive microglia. The mRNA for manganese-dependent superoxide dismutase, which is elevated in the aged hippocampus, was found preferentially expressed in neurons. We conclude that aged hippocampal neurons appear to be under oxidative stress and this is more severe in the learning-impaired subjects, suggesting a possible basis for age-induced cognitive decline.

Protein kinases activities in erythrocyte membranes of asphyxiated newborns

OBJECTIVES

Perinatal asphyxia represents a major cause of acute brain impairment and mortality in neonates. To develop the effective therapies able to reduce post-asphyxial damages, the understanding of biochemical processes accompanying asphyxia appears to be of the great relevance.

DESIGN AND METHODS

The activities of protein kinases A and C, and tyrosine kinases in erythrocyte membranes of healthy and asphyxiated neonatals were compared. Using monoclonal antibodies the band 3 presence and its phosphotyrosine levels were assayed.

RESULTS

In asphyxiated erythrocyte membranes the activities of PKA and tyrosine kinases increased, whereas the activity of PKC was reduced in relation to healthy newborns. Under asphyxia the band 3 has been overphosphorylated; however, its amount decreased.

CONCLUSION

These findings may provide some evidence for a potential role of asphyxia in disturbance of phosphorylation processes in erythrocytes, as reflected by altered protein kinases activities. The diminished band 3 presence may be partially responsible for the impairment of erythrocyte function.

Changes in the expression and the enzymic properties of the 20S proteasome in sugar-starved maize roots. evidence for an in vivo oxidation of the proteasome

The 20S proteasome (multicatalytic proteinase) was purified from maize (Zea mays L. cv DEA 1992) roots through a five-step procedure. After biochemical characterization, it was shown to be similar to most eukaryotic proteasomes. We investigated the involvement of the 20S proteasome in the response to carbon starvation in excised maize root tips. Using polyclonal antibodies, we showed that the amount of proteasome increased in 24-h-carbon-starved root tips compared with freshly excised tips, whereas the mRNA levels of alpha 3 and beta 6 subunits of 20S proteasome decreased. Moreover, in carbon-starved tissues, chymotrypsin-like and caseinolytic activities of the 20S proteasome were found to increase, whereas trypsin-like activities decreased. The measurement of specific activities and kinetic parameters of 20S proteasome purified from 24-h-starved root tips suggested that it was subjected to posttranslational modifications. Using dinitrophenylhydrazine, a carbonyl-specific reagent, we observed an increase in carbonyl residues in 20S proteasome purified from starved root tips. This means that 20S proteasome was oxidized during starvation treatment. Moreover, an in vitro mild oxidative treatment of 20S proteasome from non-starved material resulted in the activation of chymotrypsin-like, peptidyl-glutamyl-peptide hydrolase and caseinolytic-specific activities and in the inhibition of trypsin-like specific activities, similar to that observed for proteasome from starved root tips. Our results provide the first evidence, to our knowledge, for an in vivo carbonylation of the 20S proteasome. They suggest that sugar deprivation induces an oxidative stress, and that oxidized 20S proteasome could be associated to the degradation of oxidatively damaged proteins in carbon starvation situations.

Proteolytic response to oxidative stress in mammalian cells

Oxidative stress in mammalian cells is an inevitable consequence of their aerobic metabolism. The production of reactive oxygen and nitric oxide species causes oxidative modifications of proteins often combined with a loss of their biological function. Like most partially denatured proteins, moderately oxidized proteins are more sensitive to proteolytic attack by proteases. The diverse cellular proteolytic systems are an important secondary defense against oxidative stress by degrading oxidized and damaged proteins, thereby preventing their intracellular accumulation. In mammalian cells, a range of proteases exists which are distributed throughout the cell. In this review we summarize the function of the cytosolic (proteasome and calpains), the lysosomal, the mitochondrial and the nuclear proteolytic pathways in response to oxidative stress. Particular emphasis is given to the proteasomal system, since this pathway appears to be the most important proteolytic system involved in the removal of oxidatively modified or damaged proteins.

Oxidative stress and dopamine deficiency in a genetic mouse model of Lesch-Nyhan disease

Lesch-Nyhan disease, a neurogenetic disorder caused by congenital deficiency of the purine salvage enzyme hypoxanthine guanine phosphoribosyl transferase, is associated with a prominent loss of striatal dopamine. The current studies address the hypothesis that oxidant stress causes damage or dysfunction of nigrostriatal dopamine neurons in a knockout mouse model of the disease, by assessing several markers of oxidative damage and free radical scavenging systems. Some of these measures provided evidence for an increase in oxidative stress in the mutant mice (aconitase activity, oxidized glutathione, and lipid peroxides), but others did not (superoxide dismutase, protein thiol content, carbonyl protein content, total glutathione, glutathione peroxidase, catalase, and thiobarbituric reducing substances). Immunolocalization of heme-oxygenase 1 provided no evidence for oxidative stress restricted to specific elements of the striatum or midbrain in the mutants. Striatal dopamine systems of the mutant mice were more vulnerable to a challenge with the neurotoxin 6-hydroxydopamine, but they were not protected by cross-breeding the mutants with transgenic mice over-expressing superoxide dismutase. Overall, these data provide evidence for increased oxidative stress, but the failure to protect the knockout mice by over-expressing SOD1 argues that oxidative stress is not the sole process responsible for the loss of striatal dopamine.

Identification in Saccharomyces cerevisiae of a new stable variant of alkyl hydroperoxide reductase 1 (Ahp1) induced by oxidative stress

Yeasts lacking cytoplasmic superoxide dismutase (Cu,Zn-SOD) activity are permanently subjected to oxidative stress. We used two-dimensional PAGE to examine the proteome pattern of Saccharomyces cerevisiae strains lacking Cu,Zn-SOD. We found a new stable form of alkyl hydroperoxide reductase 1 (Ahp1) with a lower isoelectric point. This form was also present in wild type strains after treatment with tert-butyl hydroperoxide. In vitro enzyme assays showed that Ahp1p had lower specific activity in strains lacking Cu,Zn-SOD. We studied three mutants presenting a reduced production of the low pI variant under oxidative stress conditions. Two of the mutants (C62S and S59D) were totally inactive, thus suggesting that the acidic form of Ahp1p may only appear when the enzyme is functional. The other mutant (S59A) was active in vitro and was more resistant to inactivation by tert-butyl hydroperoxide than the wild type enzyme. Furthermore, the inactivation of Ahp1p in vitro is correlated with its conversion to the low pI form. These results suggest that in vivo during some particular oxidative stress (alkyl hydroperoxide treatment or lack of Cu,Zn-SOD activity but not hydrogen peroxide treatment), the catalytic cysteine of Ahp1p is more oxidized than cysteine-sulfenic acid (a natural occurring intermediate of the enzymatic reaction) and that cysteine-sulfinic acid or cysteine-sulfonic acid variant may be inactive.

Apoptosis in skeletal muscle with aging

Sarcopenia may be partly due to a loss in total fiber number by apoptosis. We have investigated age-related alterations in the mitochondria-mediated pathway leading to apoptosis in the gastrocnemius muscle from 6-mo-old and 24-mo-old male Fisher 344 rats. Apoptosis (mono- and oligonucleosome fragmentation) in the gastrocnemius muscle was increased by 50% in the old rats compared with the adult animals. Furthermore, there was a significant correlation between cytosolic cytochrome c and caspase-3 activity, although neither cytochrome c nor caspase-3 activity increased significantly with age. Furthermore, there was a significant correlation between caspase-3 activity and mono- and oligonucleosome fragmentation in the old rats only. Mitochondrial Bcl-2 and Bax were not altered with age. In vitro experiments demonstrated that activation of the caspase cascade in skeletal muscle might be limited by procaspase-9 activation. This is the first study to explore the role of apoptosis in sarcopenia and suggests that subtle changes in apoptosis are involved.

Gas-phase reactions of charged phenyl radicals with neutral biomolecules evaporated by laser-induced acoustic desorption

A generally applicable method for the study of phenyl radicals' reactions with neutral biomolecules in the gas phase is demonstrated. Neutral biomolecules were evaporated into a Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR) by means of laser-induced acoustic desorption (LIAD) and subsequently reacted with trapped charged phenyl radicals. The structural integrity of the evaporated alanylalanine molecules was verified by reaction with dichlorophosphenium ions. Examination of the reactions of charged phenyl radicals with alanylalanine and thymidine evaporated via LIAD revealed hydrogen atom abstraction for both alanylalanine and thymidine as well as an addition/elimination product for the reaction with thymidine. These reactions are consistent with the results obtained by others in solution. Further, a previously unstudied reaction of the nucleotide of thymine (T1) with charged phenyl radical was found to yield analogous products as the reaction with thymidine.

Characterization of the metal-binding site of bovine growth hormone through site-specific metal-catalyzed oxidation and high-performance liquid chromatography-tandem mass spectrometry

Metal-catalyzed oxidation was used to identify metal-binding His residues in bovine growth hormone (bGH), which has not been characterized well crystallographically due to a high propensity of bGH to aggregate. bGH was exposed to Cu(2+) and ascorbate (ascorbate/Cu(2+)/O(2)). 2-Oxo-His formation was identified by HPLC-tandem mass spectrometry (MS/MS) analysis of a tryptic digest. Two 2-oxo-His-containing fragments were detected, T2(O) (MH(2+)(2) = 748.8) and T20(O) (MH(+) = 528.3), both masses corresponding to the addition of only one oxygen atom (+16 amu) to the respective native fragments, T2 and T20. T2 contains (20)His and (22)His, and T20 contains (170)His. Quantitative HPLC-MS/MS analysis shows the following order of reactivity: (170)His >> (22)His > (20)His. Solvent-accessible surface area calculations determined (22)His and (170)His to be 26 and 35% solvent exposed, respectively, while (20)His is 65% solvent exposed. The presence of an analogous metal-binding site in human growth hormone, which is located in the hydrophobic core, and our experimental finding that oxidation was greatest for (22)His and (170)His in bGH suggests that (22)His and (170)His of bGH participate in metal binding. This result is supported by a previously predicted tertiary structure of bGH and compared with the location of metal-binding His residues of human growth hormone.

Oxidation of proteinaceous cysteine residues by dopamine-derived H2O2 in PC12 cells

Cellular metabolism of dopamine (DA) generates H2O2, which is further reduced to hydroxyl radicals in the presence of iron. Cellular damage inflicted by DA-derived hydroxyl radicals is thought to contribute to Parkinson's disease. We have previously developed procedures for detecting proteins that contain H2O2-sensitive cysteine (or selenocysteine) residues. Using these procedures, we identified ERP72 and ERP60, two members of the protein disulfide isomerase family, creatine kinase, glyceraldehyde-3-phosphate dehydrogenase, phospholipase C-gamma1, and thioredoxin reductase as the targets of DA-derived H2O2. Experiments with purified enzymes identified the essential Cys residues of creatine kinase and glyceraldehyde-3-phosphate dehydrogenase, that are specifically oxidized by H2O2. Although the identified proteins represent only a fraction of the targets of DA-derived H2O2, functional impairment of these proteins has previously been associated with cell death. The oxidation of proteins that contain reactive Cys residues by DA-derived H2O2 is therefore proposed both to be largely responsible for DA-induced apoptosis in neuronal cells and to play an important role in the pathogenesis of Parkinson's disease.

Effect of drinking green tea on age-associated accumulation of Maillard-type fluorescence and carbonyl groups in rat aortic and skin collagen

Tea catechins and other flavonoids have been shown to potentially protect against chronic cardiovascular diseases such as coronary heart disease and atherosclerosis. In this study, 6-month-old female Sprague-Dawley rats were fed green tea extract (50 mg/100 ml in drinking water) up to the age of 22 months, and the age-associated changes in Maillard-type fluorescence and carbonyl groups in the aortic and skin collagen were compared with those occurring in the water-fed control animals. Collagen-linked Maillard-type fluorescence was found to increase in both the aortic and skin tissues as animals aged. The age-associated increase in the fluorescence in the aortic collagen was remarkably inhibited by the green tea extract treatment, while that occurring in the skin collagen was not significantly inhibited by the treatment. The collagen carbonyl content also increased in both the aortic and skin tissues as animals aged. In contrast with the case of Maillard-type fluorescence, however, the age-associated increase in the carbonyl content was not inhibited by the green tea extract treatment either in the aortic or skin collagen. These results suggest that the inhibition of AGE formation in collagen is an important mechanism for the protective effects of tea catechins against cardiovascular diseases.

Rabbit brain glucose-6-phosphate dehydrogenase: biochemical properties and inactivation by free radicals and 4-hydroxy-2-nonenal

Glucose-6-phosphate dehydrogenase (G6PD) purified from rabbit brain is composed of two identical subunits of 56 kDa. The enzyme exhibits biphasic pH curve, linear Arrhenius plot and elevated susceptibility to inactivation by metal catalyzed oxidation and thiol binding reagents. 4-Hydroxy-2-nonenal (HNE) is able to inactivate the enzyme after only a few minutes of incubation. Since reactive oxygen species and G6PD-HNE adducts form easily in brain under conditions of oxidative stress, these findings have important implications in the loss of active G6PD molecules in vivo, a process which lowers the antioxidant protection and may be critical for neuron survival.

Evidence of oxidative damage in Alzheimer's disease brain: central role for amyloid beta-peptide

Amyloid beta-peptide (Abeta) is heavily deposited in the brains of Alzheimer's disease (AD) patients. Free-radical oxidative stress, particularly of neuronal lipids, proteins and DNA, is extensive in those AD brain areas in which Abeta is abundant. Recent research suggests that these observations might be linked, and it is postulated that Abeta-induced oxidative stress leads to neurodegeneration in AD brain. Consonant with this postulate, Abeta leads to neuronal lipid peroxidation, protein oxidation and DNA oxidation by means that are inhibited by free-radical antioxidants. Here, we summarize current research on phospholipid peroxidation, as well as protein and DNA oxidation, in AD brain, and discuss the potential role of Abeta in this oxidative stress.

Hypochlorite- and hypobromite-mediated radical formation and its role in cell lysis

Activated leukocytes generate the potent oxidants HOCl and HOBr via the formation of H(2)O(2) and the release of peroxidase enzymes (myeloperoxidase, eosinophil peroxidase). HOCl and HOBr are potent microbiocidal agents, but excessive or misplaced production can cause tissue damage and cell lysis. In this study it is shown that HOBr induces red blood cell lysis at approximately 10-fold lower concentrations than HOCl, whereas with monocyte (THP1) and macrophage (J774) cells HOCl and HOBr induce lysis at similar concentrations. The role of radical formation during lysis has been investigated by EPR spin trapping, and it is shown that reaction of both oxidants with each cell type generates cell-derived radicals. Red blood cells exposed to nonlytic doses of HOCl generate novel nitrogen-centered radicals whose formation is GSH dependent. In contrast, HOBr gives rise to nitrogen-centered, membrane-derived protein radicals. With lytic doses of either oxidant, protein (probably hemoglobin)-derived, nitrogen-centered radicals are observed. Unlike the red blood cells, treatment of monocytes and macrophages with HOCl gives significant radical formation only under conditions where cell lysis occurs concurrently. These radicals are nitrogen-centered, cell-protein-derived species and have parameters identical to those detected with red blood cells and HOBr. Exposure of these cells to HOBr did not give detectable radicals. Overall these experiments demonstrate that HOCl and HOBr react with different selectivity with cellular targets, and that this can result in radical formation. This radical generation can precede, and may play a role in, cell lysis.

Selectivity of protein oxidative damage during aging in Drosophila melanogaster

The purpose of the present study was to determine whether oxidation of various proteins during the aging process occurs selectively or randomly, and whether the same proteins are damaged in different species. Protein oxidative damage to the proteins, present in the matrix of mitochondria in the flight muscles of Drosophila melanogaster and manifested as carbonyl modifications, was detected immunochemically with anti-dinitrophenyl-group antibodies. Aconitase was found to be the only protein in the mitochondrial matrix that exhibited an age-associated increase in carbonylation. The accrual of oxidative damage was accompanied by an approx. 50% loss in aconitase activity. An increase in ambient temperature, which elevates the rate of metabolism and shortens the life span of flies, caused an elevation in the amount of aconitase carbonylation and an accelerated loss in its activity. Exposure to 100% ambient oxygen showed that aconitase was highly susceptible to undergo oxidative damage and loss of activity under oxidative stress. Administration of fluoroacetate, a competitive inhibitor of aconitase activity, resulted in a dose-dependent decrease in the life span of the flies. Results of the present study demonstrate that protein oxidative damage during aging is a selective phenomenon, and might constitute a mechanism by which oxidative stress causes age-associated losses in specific biochemical functions.

NADPH oxidase of neutrophils elevates o,o'-dityrosine cross-links in proteins and urine during inflammation

Reactive intermediates generated by phagocytic white blood cells are of central importance in destroying microorganisms, but they may also damage normal tissue at sites of inflammation. To investigate the potential role of such oxidants in tissue injury, we used gas chromatography/mass spectrometry to quantify levels of o,o'-dityrosine in mouse peritoneal neutrophils and urine. In wild-type animals, neutrophils markedly increased their content of protein-bound dityrosine when they were activated in vivo. This increase failed to occur in mice that were deficient in the phagocyte NADPH oxidase. Levels of o,o'-dityrosine in urine mirrored those in neutrophil proteins. When o,o'-[(14)C]dityrosine was injected intravenously into mice, the radiolabel was not metabolized or incorporated into tissue proteins: instead, it was recovered in urine with near-quantitative yield. Patients with sepsis markedly increased their output of o,o'-dityrosine into urine, suggesting that systemic inflammation also may be a potent source of oxidative stress in humans. These observations demonstrate that activated neutrophils produce o,o'-dityrosine cross-links in tissue proteins, which may subsequently be degraded into free amino acids and excreted into urine. Our results indicate that mouse phagocytes use oxidants produced by the NADPH oxidase to create o,o'-dityrosine cross-links in vivo and raise the possibility that reactive intermediates produced by this pathway promote inflammatory tissue damage in humans.

3-Hydroxy-(4H)-benzopyran-4-ones as potential iron chelating agents in vivo

Increasing evidence suggests that iron plays an important role in tissue damage both during chronic iron overload diseases (i.e., hemochromatosis) and when, in the absence of actual tissue iron overload, iron is delocalised from specific carriers or intracellular sites (inflammation, neurodegenerative diseases, post-ischaemic reperfusion, etc.). In order to be used for therapeutical purposes in vivo, a reliable iron chelator should be capable of preventing the undesired effects that follow the electrochemical activation of iron (see below). Bearing in mind the molecular structure of some flavonols that are able to chelate iron, we synthesised a new oral iron-chelator, 2-methyl-3-hydroxy-4H-benzopyran-4-one (MCOH). We demonstrate that MCOH chelates iron in a 2:1 ratio showing a stability constant of approximately 10(10). MCOH is able to cross cell membranes (erythrocytes, ascite tumour cells) in both directions. Following intraperitoneal administration to rats, it is quickly taken up by the liver and excreted in the urine within 24h. A similar behaviour has been documented after oral administration. We propose that MCOH may represent the prototype of a new class of iron chelating agents to be developed for iron-removal therapy in vivo with the goal of preventing tissue damage caused by the iron redox cycle.

Effect of Mangifera indica L. extract (QF808) on protein and hepatic microsome peroxidation

The antioxidant activities of QF808, a steam bark extract of Mangifera indica L., were studied on hydroxyl-mediated oxidation of bovine serum albumin (BSA) and in a hepatic microsome system. The extract was effective in reducing the oxidation of BSA, since its half- maximal inhibition concentration (IC(50)) was 0.0049% w/v in the inhibition of carbonyl group formation and lower than 0.0025% w/v in the inhibition of sulfhydryl group loss. QF808 inhibited lipid peroxidation which was initiated enzymatically by reduced nicotinamide adenine dinucleotide phosphate (NADPH), IC(50)= 0.00075% w/v, or non-enzymatically by ascorbic acid, IC(50) = 0.0126% w/v. The extract tested did not inhibit NADPH-dependent cytochrome P-450 reductase activity, since it had no effect on the oxidation rate of NADPH. These results suggest that QF808 has an antioxidant activity, probably due to its ability to scavenge free radicals involved in microsome lipid peroxidation. In addition, QF808 antioxidant profile in vitro is probably similar to its principal polyphenolic component, mangiferin, a glycosylated xanthone.

Age-related changes in protein oxidation and proteolysis in mammalian cells

Reactive oxygen species generated as by-products of oxidative metabolism, or from environmental sources, frequently damage cellular macromolecules. Proteins are recognized as major targets of oxidative modification, and the accumulation of oxidized proteins is a characteristic feature of aging cells. An increase in the amount of oxidized proteins has been reported in many experimental aging models, as measured by the level of intracellular protein carbonyls or dityrosine, or by the accumulation of protein-containing pigments such as lipofuscin and ceroid bodies. In younger individuals, moderately oxidized soluble cell proteins appear to be selectively recognized and rapidly degraded by the proteasome. An age-related accumulation of oxidized proteins could, therefore, be a result of declining activity of the proteasome. Previous research to investigate the notion of an age-related decline in the content and/or activity of the proteasome has generated contradictory results. The latest evidence, including our own recent findings, indicates that proteasome activity does, indeed, decline during aging as the enzyme complex is progressively inhibited by oxidized and cross-linked protein aggregates. We propose that cellular aging involves both an increase in (mitochondrial) oxidant production and a progressive decline in proteasome activity. Eventually so much proteasome is inactivated that oxidized proteins begin to accumulate rapidly and contribute to cellular dysfunction and senescence.

"Mangifera indica L. extract (QF808) reduces ischaemia-induced neuronal loss and oxidative damage in the gerbil brain"

The effect of oral administration of Mangifera indica L. extract (QF808) on ischemia-reperfusion-induced neuronal death in the gerbil hippocampal CA1 sector was examined. Oral administration of QF808 for 7 days dose-dependently protected against neuronal cell death following transient ischaemia and reperfusion as assessed by histopathology. In addition, locomotor activity assessment prior to ischaemia and 7 days after correlated well with the histological results. To evaluate redox alterations by reactive oxygen species, total sulfhydryl, non-protein sulfhydryl groups (NPSH), malondialdehyde + 4-hydroxyalkenals and total nitrogen oxide levels were assayed in hippocampus and cortex homogenates. QF808 treatment attenuated NPSH loss, nitrogen oxide levels and lipid peroxidation in the hippocampus. These results suggest that orally administered QF808 is absorbed across the blood-brain barrier and attenuates neuronal death of the hippocampal CA1 area after ischaemia-reperfusion. These protective effects are most likely due to the antioxidant activity of QF808.

Oxidation of recombinant human interleukin-2 by potassium peroxodisulfate

PURPOSE

The oxidation of recombinant human interleukin-2 (rhlL-2) by potassium peroxodisulfate (KPS) with or without N,N,N',N'-tetramethylethylenediamine (TEMED), which are used for the preparation of dextran-based hydrogels, was investigated.

METHODS

The oxidation of (derivatives of) methionine. tryptophan, histidine and tyrosine, as well as rhlL-2 was investigated. Both the oxidation kinetics (RP-HPLC) and the nature of the oxidation products (mass spectrometry) were studied as a function of the KPS and TEMED concentration, and the presence of a competitive antioxidant, methionine.

RESULTS

Under conditions relevant for the preparation of rhIL-2 loaded hydrogels, only methionine and tryptophan derivatives were susceptible to oxidation by KPS. The oxidation of these compounds was inhibited once TEMED was present, suggesting that the peroxodisulfate anion, rather than the radicals formed in the presence of TEMED, is the oxidative species. KPS only induced oxidation of the four methionines present in rhIL-2, whereas the tryptophan residue remained unaffected. The radicals, formed after KPS decomposition by TEMED, induced some dimerization of rhIL-2. The oxidation of rhIL-2 could be substantially reduced by the addition of methionine, or by pre-incubation of KPS with TEMED.

CONCLUSIONS

Only the methionine residues in rhlL-2 are oxidized by KPS. The extent of oxidation can be minimized by a proper selection of the reaction conditions.

Lipid peroxidation associated protein damage in rat brain crude synaptosomal fraction mediated by iron and ascorbate

In crude synaptosomal fractions from rat brain exposed to iron and ascorbate, enhanced lipid peroxidation (more than 3-fold compared to control), loss of protein thiols up to the extent of 40% compared to control, increased incorporation of carbonyl groups into proteins (more than 4.5-fold compared to control) and non-disulphide covalent cross-linking of membrane proteins have been observed. The phenomena are not inhibited by catalase or hydroxyl radical scavengers like mannitol or dimethyl sulphoxide. However, chain breaking antioxidants like alpha-tocopherol and butylated hydroxytoluene prevent both lipid peroxidation and accompanying protein oxidation. It is suggested that in this system lipid peroxidation propagated by the decomposition of preformed lipid hydroperoxides by iron and ascorbate is the primary event and products of the peroxidation process cause secondary protein damage. In view of high ascorbate content of brain and availability of several transition metals, such ascorbate mediated oxidative damage may be relevant in the aetiopathogenesis of several neurodegenerative disorders as well as ageing of brain.

Novel intra- and inter-molecular sulfinamide bonds in S100A8 produced by hypochlorite oxidation

Hypochlorite is a major oxidant generated when neutrophils and macrophages are activated at inflammatory sites, such as in atherosclerotic lesions. Murine S100A8 (A8) is a major cytoplasmic protein in neutrophils and is secreted by macrophages in response to inflammatory stimuli. After incubation with reagent HOCl for 10 min, approximately 85% of A8 was converted to 4 oxidation products, with electrospay ionization mass spectrometry masses of m/z 10354, 10388, 10354 +/- 1, and 20707 +/- 3. All were resistant to reduction by dithiothreitol. Initial formation of a reactive Cys sulfenic acid intermediate was demonstrated by the rapid conjugation of 5,5-dimethyl-1,3-cyclohexanedione (dimedone) to HOCl-treated A8 to form stable adducts. Matrix-assisted laser desorption-reflectron time of flight peptide mass fingerprinting of isolated oxidation products confirmed the mass additions observed in the full-length proteins. Both Met(36/73) were converted to Met(36/73) sulfoxides. An additional product with an unusual mass addition of m/z 14 (+/-0.2) was identified and corresponded to the addition of oxygen to Cys(41), conjugation to various epsilon-amines of Lys(6), Lys(34/35), or Lys(87) with loss of dihydrogen and formation of stable intra- or inter-molecular sulfinamide cross-links. Specific fragmentations identified in matrix-assisted laser desorption-post source decay spectra and low energy collisional-induced dissociation tandem mass spectroscopy spectra of sulfinamide-containing digest peptides confirmed Lys(34/35) to Cys(41) sulfinamide bonds. HOCl oxidation of mutants lacking Cys(41) (Ala(41)S100A8) or specific Lys residues (e.g. Lys(34/35), Ala(34/35)S100A8) did not form sulfinamide cross-links. HOCl generated by myeloperoxidase and H(2)O(2) and by phorbol 12-myristate 13-acetate-activated neutrophils also formed these products(.) In contrast to the disulfide-linked dimer, oxidized monomer retained normal chemotactic activity for neutrophils. Sulfinamide bond formation represents a novel oxidative cross-linking process between thiols and amines and may be a general consequence of HOCl protein oxidation in inflammation not identified previously. Similar modifications in other proteins could potentially regulate normal and pathological processes during aging, atherogenesis, fibrosis, and neurogenerative diseases.

Ascorbic acid in the 21st century - more than a simple antioxidant

Ascorbic acid (AA) is an essential micronutrient for man, with many biological roles. It is a powerful antioxidant both directly via scavenging of reactive oxygen species and indirectly through regeneration of other antioxidant systems. Paradoxically, under certain conditions (low concentration in vitro, presence of metal ions) it can exert a pro-oxidant effect, increasing oxidative damage to lipids, DNA and protein. Herein, the effects of vitamin C both in vitro and in vivo are addressed in terms of modulation of oxidative DNA damage, gene expression and protein oxidation. The view of AA as a simple scavenger is outdated, where the arrival of new bioinformatic techniques, heralds a new dawning in our understanding of ascorbate as a potential direct or indirect modulator of gene expression.

Protein oxidation and ageing

Organisms produce reactive oxygen species (ROS) throughout their lives. The activities of a number of key antioxidant enzymes, such as catalase, superoxide dismutase and glutathione peroxidase, which protect against the damaging effects of ROS, have been reported to decrease with increasing age, though this is not unequivocal. In contrast, sacrificial antioxidants such as ascorbate, thiols and tocopherol do not appear to decrease with increasing age. It is also possible that ROS production increases with age as a result of poorer coupling of electron transport components, and an increased level of redox-active metal ions that could catalyse oxidant formation. As a result of this decrease in antioxidant defences, and increased rate of ROS formation, it is possible that the impact of ROS increases with age. ROS are known to oxidise biological macromolecules, with proteins an important target. If the argument that the impact of ROS increases with age is true, then proteins would be expected to accumulate oxidised materials with age, and the rate of such accumulation should increase with time, reflecting impaired inefficiency of homeostasis. Here we review the evidence for the accumulation of oxidised, or modified, extra- and intra-cellular proteins in vivo.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Novel inter-protein cross-link identified in the GGH-ecotin D137Y dimer

In the presence of a suitable oxidizing agent, the Ni(II) complex of glycyl-glycyl-histidine (GGH) mediates efficient and specific oxidative protein cross-linking. The fusion of GGH to the N terminus of a protein allows for the cross-linking reagent to be delivered in a site-specific fashion, making this system extremely useful for analyzing protein-protein contacts in complicated mixtures of biomolecules. Tyrosine residues have been postulated to be the primary amino acid target of this reaction, and using the dimeric serine protease inhibitor ecotin, we previously demonstrated that engineering a tyrosine at the protein interface of a dimer dramatically increased cross-linking efficiency. Cross-linking increased four-fold for GGH-ecotin D137Y in comparison to wild-type GGH-ecotin, presumably through bityrosine formation at the dimer interface. Here we report the first complete structural analysis of the cross-linked GGH-ecotin D137Y dimer. Using a combination of mass spectrometric and chemical derivatization methods, a sole novel cross-link between the N-terminal glycine residues and the engineered tyrosine at position 137 has been characterized. The dimer cross-link is localized to a single site without other protein modifications, but different reaction pathways produce structurally related products. We propose a mechanism that involves covalent bond formation between the protein backbone and a dopaquinone moiety derived from a specific tyrosine residue. This finding establishes that it is not necessary to have two tyrosine residues within close proximity in the protein interface to obtain high protein cross-linking yields, and suggests that the cross-linking reagent may be of more general utility than previously thought.

BW373U86, a delta opioid agonist, partially mediates delayed cardioprotection via a free radical mechanism that is independent of opioid receptor stimulation

Opioids have been shown to produce both an early and delayed phase of cardioprotection; however, the signaling pathways involved, particularly in the delayed response, have not been well defined. Therefore, we investigated the potential of BW373U86 (BW), a potent delta opioid agonist, to produce delayed cardioprotection and characterized the role of opioid receptors and oxygen-derived free radicals (OFRs) in this delayed response. All rats underwent 30 min of ischemia followed by 2 h of reperfusion. The rats were divided into four groups. First, rats were pretreated with selective opioid receptor antagonists or the antioxidant, 2-mercaptopropionyl glycine (2-MPG), in the presence of BW and allowed to recover for 24 h before the ischemia-reperfusion protocol. Second, rats were pretreated with BW, allowed to recover for 24 h, and subsequently treated with either opioid antagonists or 2-MPG, 10 min prior to the ischemia-reperfusion protocol. Third, rats underwent ischemic preconditioning (IPC) (1x5 min occlusion) both with and without 2-MPG to determine the role of OFRs in acute cardioprotection. Fourth, rats were pretreated with TAN-67, an opioid agonist known to signal through the delta1 opioid receptor in the presence and absence of 2-MPG. Control rats were injected with saline and allowed to recover for 24 h. BW produced a bell-shaped dose-related reduction in infarct size with a maximal reduction observed at 0.1 mg/kg v control (16+/-3%v 60+/-3%, P<0.001). Surprisingly, the delayed protection induced by BW was only partially blocked by pretreatment with the delta1-selective antagonist, BNTX; however, it was completely blocked by pretreatment with 2-MPG (47+/-5%, P<0.001). Only naloxone given acutely inhibited the protective effects of BW; however, at the dose used, 2-MPG partially reduced the protective effect of acute IPC. TAN-67 (0.1 mg/kg) also produced a significant reduction in infarct size compared to control (18+/-4%v 60+/-3%, P<0.001). This protection was blocked by pretreatment with 2-MPG (42+/-4%, P<0.001). These data suggest that BW and TAN-67 mediate delayed cardioprotection via a free radical mechanism that appears to be only partially dependent on delta opioid receptor stimulation. Furthermore, it is the early burst in OFRs that is crucial to initiating the protective effect.

Photocytotoxicity of lipofuscin in human retinal pigment epithelial cells

Lipofuscin accumulates with age in a variety of highly metabolically active cells, including the retinal pigment epithelium (RPE) of the eye, where its photoreactivity has the potential for cellular damage. The aim of this study was to assess the phototoxic potential of lipofuscin in the retina. RPE cell cultures were fed isolated lipofuscin granules and maintained in basal medium for 7 d. Control cells lacking granules were cultured in an identical manner. Cultures were either maintained in the dark or exposed to visible light (2.8 mWcm2) at 37 degrees C for up to 48 h. Cells were subsequently assessed for alterations in cell morphology, cell viability, lysosomal stability, lipid peroxidation, and protein oxidation. Exposure of lipofuscin-fed cells to short wavelength visible light (390-550 nm) caused lipid peroxidation (increased levels of malondialdehyde and 4-hydroxy-nonenal), protein oxidation (protein carbonyl formation), loss of lysosomal integrity, cytoplasmic vacuolation, and membrane blebbing culminating in cell death. This effect was wavelength-dependent because light exposure at 550 to 800 nm had no adverse effect on lipofuscin-loaded cells. These results confirm the photoxicity of lipofuscin in a cellular system and implicate it in cell dysfunction such as occurs in ageing and retinal diseases.