Biochemistry and pathology of radical-mediated protein oxidation

Serum proteins modified by neutrophil-derived oxidants as mediators of neutrophil stimulation

Reactive oxygen intermediates (ROI) released during inflammation may act as important mediators of neutrophil effector functions. The objective of this investigation was to evaluate the influence of ROI generation on neutrophil adhesion molecule regulation and degranulation. Induction of the neutrophil oxidative burst via Fcgamma receptor cross-linking was accompanied by up-regulation of neutrophil surface CD11b, CD35, and CD66b only in the presence of selected serum proteins, such as purified human C4, C5, or human serum albumin (HSA). Scavenging of ROI attenuated protein-dependent receptor regulations. Moreover, exogenous hydrogen peroxide was effective to increase neutrophil CD11b expression in a protein-dependent way. HSA exposed to neutrophil-derived ROI displayed signs of oxidative modification in terms of carbonyl formation. Such modified HSA transferred to resting neutrophils bound readily to the cell surface and effected receptor modulation as well as cellular spreading. In contrast, neither native HSA nor HSA protected against oxidation by the tocopherol analog Trolox exhibited agonistic properties. In conclusion, we demonstrate that neutrophil-derived ROI modify selected serum proteins, which, in turn, act as proinflammatory mediators of neutrophil stimulation.

Albumin is the major plasma protein target of oxidant stress in uremia

BACKGROUND

Patients with uremia are exposed to increased oxidative stress. Examination of the oxidation of individual plasma proteins may be useful in establishing specific pathways of oxidative stress in vivo and in determining functional consequences of oxidant stress exposure. We therefore examined oxidative modification of plasma proteins by carbonyl formation using Western blot immunoassay and enzyme-linked immunosorbent assay (ELISA) techniques in patients with chronic renal failure (CRF) and on chronic hemodialysis therapy (HD).

METHODS

Plasma was obtained from 25 HD, 20 CRF, and 20 healthy volunteers, derivatized with 2,4 dinitrophenylhydrazine (DNP) and electrophoresed on duplicate 4 to 12% gradient sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gels, transferred to nitrocellulose, and stained for DNP for carbonyls and amido black for protein content. Data are recorded as DNP area/protein area and are reported in densitometry units. Total plasma carbonyls were determined by ELISA.

RESULTS

Plasma albumin is substantially more oxidized in HD than in healthy volunteers (1.22 +/- 0.14 densitometry units vs. 0.60 +/- 0.08, P = 0.002). There were no significant differences in oxidation of plasma transferrin, immunoglobulin, and fibrinogen in HD versus healthy volunteers. In CRF patients, plasma albumin is more oxidized compared with normal volunteers (1.36 +/- 0.20 densitometry units vs. 0.94 + 0.08, P = 0.09). There were no differences in oxidation of plasma transferrin, fibrinogen, and immunoglobulin in CRF patients versus healthy volunteers. An increased plasma protein carbonyl concentration in CRF patients compared with healthy volunteers was confirmed by ELISA (0.31 +/- 0.07 vs. 0.04 +/- 0.01 nmol/mg protein (P = 0.001).

CONCLUSION

Albumin is the major plasma protein target of oxidant stress in CRF and HD patients.

Protein oxidation in the brain in Alzheimer's disease

In this study we used immunohistochemistry and two-dimensional fingerprinting of oxidatively modified proteins (two-dimensional Oxyblot) together to investigate protein carbonyl formation in the Alzheimer's disease brain. Increased protein oxidation was detected in sections from the hippocampus and parahippocampal gyrus, superior and middle temporal gyri of six Alzheimer's disease and six age-matched control human subjects, but not in the cerebellum. In two brain regions severely affected by Alzheimer's disease pathology, prominent protein carbonyl immunoreactivity was localized in the cytoplasm of neurons without visual pathomorphological changes and degenerating neurons, suggesting that intracellular proteins might be significantly affected by oxidative modifications. Following two-dimensional electrophoresis the positions of some individual proteins were identified using specific antibodies, and immunoblot analysis for protein carbonyls was performed. These studies demonstrated the presence of protein carbonyl immunoreactivity in beta-tubulin, beta-actin and creatine kinase BB in Alzheimer's disease and control brain extracts. Protein carbonyls were undetectable in spots matching glial fibrillary acidic protein and tau isoforms. Specific protein carbonyl levels in beta-actin and creatine kinase BB were significantly higher in Alzheimer's disease than in control brain extract. beta-Tubulin did not demonstrate a significant increase in specific protein carbonyl content in Alzheimer's disease brains. We suggest that oxidative stress-induced injury may involve the selective modification of different intracellular proteins, including key enzymes and structural proteins, which precedes and may lead to the neurofibrillary degeneration of neurons in the Alzheimer's disease brain.

Antioxidant defense systems of two lipidopteran insect cell lines

Spodoptera frugiperda Sf-9 (Sf-9) and Trichoplusia ni BTI-Tn-5B1-4 (Tn-5B1-4) insect cell lines were found to contain unique assemblages of antioxidant enzymes. Specifically, the Sf-9 insect cell line contained Manganese and Copper-Zinc superoxide dismutase (MnSOD and CuZnSOD) for reducing the superoxide radical (O(2)(*-)) to hydrogen peroxide (H(2)O(2)) and ascorbate peroxidase (APOX) for reducing the resulting H(2)O(2) to H(2)O. Approximately one third of the total SOD activity was found to be MnSOD. The Tn-5B1-4 cells were also found to contain MnSOD (approximately two thirds of the total SOD activity), CuZnSOD and APOX activities. However, the Tn-5B1-4 cell line, in contrast to the Sf-9 cell line, contained catalase (CAT) activity for reducing H(2)O(2) to H(2)O. Both the Sf-9 and Tn-5B1-4 cell lines contained glutathione reductase and dehydroascorbic acid reductase activities for regenerating the reduced forms of glutathione and ascorbic acid, respectively. In addition, both cell lines contained glutathione S-transferase peroxidase activity towards hydroperoxides other than H(2)O(2). Finally, neither cell line contains the glutathione peroxidase activity that is ubiquitous in mammalian cells.

Protein oxidation and proteolysis by the nonradical oxidants singlet oxygen or peroxynitrite

Exposure of proteins to oxidants leads to increased oxidation followed by preferential degradation by the proteasomal system. The role of the biologically occurring oxidants singlet oxygen and peroxynitrite in oxidation of proteins in living cells and enhanced degradation of these proteins was examined in this study. Subsequent to treatment of an isolated model protein, ferritin, with singlet oxygen or peroxynitrite, there was enhanced degradation by the isolated 20S proteasome. Treatment of clone 9 liver cells (normal liver epithelia) with two different singlet oxygen-generating systems or peroxynitrite leads to a concentration-dependent increase in cellular protein turnover. At high concentrations of these oxidants, the protein turnover decreases without significant loss of cell viability and proteasome activity. To compare the increase of intracellular protein turnover with that obtained with other oxidants, cells were exposed to hydrogen peroxide or xanthine/xanthine oxidase. The maximal increase in protein turnover was similar with the various oxidants. The oxidized protein moieties were removed by enhanced protein turnover. Removal of singlet oxygen- or peroxynitrite-damaged proteins is dependent on the proteasomal system, as suggested by the sensitivity to lactacystin. Our results provide evidence that the proteasomal system is able to selectively recognize and degrade proteins modified by singlet oxygen or peroxynitrite in vitro as well as in living cells.

Reaction of human myoglobin and H2O2. Electron transfer between tyrosine 103 phenoxyl radical and cysteine 110 yields a protein-thiyl radical

The sequence of human myoglobin (Mb) is similar to that of other species except for a unique cysteine at position 110 (Cys(110)). Adding hydrogen peroxide (H(2)O(2)) to human Mb affords Trp(14)-peroxyl, Tyr(103)-phenoxyl, and Cys(110)-thiyl radicals and coupling of Cys(110)-thiyl radicals yields a homodimer through intermolecular disulfide bond formation (Witting, P. K., Douglas, D. J., and Mauk, A. G. (2000) J. Biol. Chem. 275, 20391-20398). Treating a solution of wild type Mb and H(2)O(2) with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) at DMPO:protein </= 10 mol/mol yields DMPO-Cys(110) adducts as determined by EPR. At DMPO:protein ratios (25-50 mol/mol), both DMPO-Tyr(103) and DMPO-Cys(110) adducts were detected, whereas at DMPO:protein >/= 100 mol/mol only DMPO-Tyr(103) radicals were present. The DMPO-dependent decrease in DMPO-Cys(110) was matched by a near 1:1 stoichiometric increase in DMPO-Tyr(103). In contrast, reaction of the Y103F human Mb with H(2)O(2) gave no DMPO-Cys(110) at DMPO:protein </= 10 mol/mol, and only trace DMPO-Cys(110) at DMPO:protein >/= 100 mol/mol (i.e. conditions that consistently gave DMPO-Tyr(103) in the case of wild type Mb). No detectable homodimer was formed by incubation of the Y103F variant with H(2)O(2). However, the homodimer was detected in a mixture of both the Y103F and C110A variants of human Mb upon treatment with H(2)O(2) (C110A:Y103F:H(2)O(2) 2:1:5 mol/mol/mol); the yield of this homodimer increased with increasing ratios of C110A:Y103F. Together, these data suggest that addition of H(2)O(2) to human Mb can produce Cys(110)-thiyl radicals through an intermolecular electron transfer reaction from Cys(110) to a Tyr(103)-phenoxyl radical.

Dialysis-induced oxidative stress: biological aspects, clinical consequences, and therapy

Oxidative stress, which results from a rupture in the natural balance between pro- and antioxidant systems, is considered as a major factor in dialysis-associated morbidity and mortality. Emerging pharmacologic and dialytic antioxidant therapeutic and dialysis strategies should enable us to reduce the harmful consequences of oxidative stress in dialysis patients. Moreover, since there is increasing evidence of oxidative stress long before the initiation of maintenance dialysis, antioxidant therapeutic strategies should probably be developed very early in the course of renal failure.

Comparing the effect on protein stability of methionine oxidation versus mutagenesis: steps toward engineering oxidative resistance in proteins

The biological activity of some proteins is known to be sensitive to oxidative damage caused by a variety of oxidants. The model protein staphylococcal nuclease was used to explore the effect on protein structural stability of oxidizing methionine to the sulfoxide form. These effects were compared with the effects of substituting methionines with isoleucine and leucine, a potential strategy for stabilizing proteins against oxidative damage. Wild-type nuclease and various mutants were oxidized with hydrogen peroxide. Stabilities of both oxidized and unoxidized proteins were determined by guanidine hydrochloride denaturation. Oxidation destabilized the wild-type protein by over 4 kcal/mol. This large loss of stability supports the idea that in some cases loss of biological activity is linked to disruption of the protein native state. Comparison of mutant protein's stability losses upon oxidation showed that methionines 65 and 98 had a much greater destabilizing effect when oxidized than methionines 26 or 32. While substitution of methionine 98 carried as great an energetic penalty as oxidation, substitution at position 65 was less disruptive than oxidation. Thus a simple substitution mutagenesis strategy to protect a protein against oxidative destabilization is practical for some methionine residues.

Protein oxidation and proteolysis in RAW264.7 macrophages: effects of PMA activation

Macrophages are stimulable cells able to increase the production of reactive oxygen and nitrogen species dramatically for a short period of time. Free radicals and other oxidants are able to oxidize the intracellular protein pool. These oxidized proteins are selectively recognized and degraded by the intracellular proteasomal system. We used the mouse macrophage-like cell line RAW264.7 to test whether macrophagial cells are able to increase their protein turnover after oxidative stress and whether this is accompanied by an increased protein oxidation. Macrophagial cells are particularly susceptible to bolus additions of hydrogen peroxide and peroxynitrite. In further experiments we activated RAW264.7 cells with PMA to test whether the production of endogenous oxidants has analogous effects. A clear dependence of the protein turnover and protein oxidation on the oxidative burst could be measured. In further experiments the role of the proteasomal system in the selective removal of oxidized proteins could be revealed exploring the proteasome specific inhibitor lactacystin. Therefore, although oxidants are able to attack the intracellular protein pool in macrophages, these cells are able to remove oxidized proteins selectively and protect the intracellular protein pool from oxidation.

Changes in thiol content and expression of glutathione redox system genes in the hippocampus and cerebellum in Alzheimer's disease

In this report, we compared total and protein-bound thiol levels in the hippocampus and cerebellum of six Alzheimer's disease (AD) patients and six age-matched control subjects. Total level of sulfhydryl (SH) groups, determined using the 5,5'-dithiobis(2-nitrobenzoic acid) method, was not significantly altered in the hippocampus and cerebellum of AD patients. The level of protein-bound SH groups, determined by labeling with 3-(N-maleimido-propionyl) biocytin, was decreased in the AD hippocampus compared with controls. Reverse transcription-polymerase chain reaction analysis of the expression of key glutathione redox system genes demonstrated the induction of glutathione reductase and glutathione peroxidase messages in the AD hippocampus. Levels of glutathione transferase mu and A4-4 messages were unchanged. This study suggests that protective antioxidant gene responses are insufficient to counteract the increased oxidative damage of proteins in a vulnerable region of the AD brain.

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.

The importance of proteins in defense against oxidation

Free radicals are a normal feature of cellular oxygen metabolism. However, free radical-associated damage is an important factor in many pathological and toxicological processes. For a long time, lipid peroxidation, mediated by oxygen-derived free radicals, was probably the most extensively investigated process. From more recent studies, it has become evident that proteins are also the targets of free radicals, and this has important implication for their activity, unfolding, and degradation, as well as in cell functioning. After giving a brief overview of the key role of proteins in the overall antioxidant defense, this review examines their role as targets of oxidation reactions, taking into account the reactivity of amino acid residues and some of their oxidation products. In light of recent data, we then consider the specific role of sulfur-containing amino acids in protein degradation and their possible interplay with the reversal of limited oxidative lesions. The participation of proteins in the overall antioxidant defense is also discussed, specifically the role of metallothionein as an intracellular antioxidant and that of albumin as a circulating antioxidant.

How long should telomeres be?

What began as a study of the "end-replication problem" took on a new dimension as it became clear that telomeres are a "molecular clock" of replication in human somatic cells. Here we review the biology of telomeres in vitro and in vivo, in mice and humans. We suggest that, in humans, telomeres are involved in the biology of aging and pathobiology of disorders of aging, including cancer and cardiovascular disease. We also propose that the underlying dynamics of telomere biology is in line with broad principles of evolutionary theories.

Dopamine induced protein damage in mitochondrial-synaptosomal fraction of rat brain

Dopamine during in vitro oxidation induced covalent cross-linking of membrane proteins in rat brain crude mitochondrial-synaptosomal fraction. The process is not inhibited by hydroxyl radical scavengers, lipid soluble anti-oxidants, metal-chelator or catalase, but reduced glutathione produced a dramatic inhibition of cross-linking. The protein cross-linking mediated by dopamine is not associated with any detectable membrane lipid peroxidation but significant formation of protein bound quinone takes place during incubation. Our results indicate that reactive quinones rather than oxygen free radicals are involved in dopamine induced protein cross-linking in rat brain membrane fraction.

Importance of oxidatively modified proteins in chronic renal failure

Considerable evidence has accumulated that chronic uremia is associated with a multifactorial immunoinflammatory syndrome, which occurs early in the course of renal failure, is accentuated with the progression of uremia, and culminates in maintenance dialysis therapy. We previously described the presence of a circulating oxidized plasma protein named advanced oxidation protein products (AOPPs). Beyond evidence that AOPPs represent an exquisite marker of oxidative stress, their role(s) in the pathophysiology of chronic renal failure and dialysis-related complications might be of great importance. Regarding the mechanisms of generation of AOPP, we underscore the importance of the chlorinated oxidants, previously solely considered as microbicidal agents, in the generation of AOPP. Indeed, AOPPs appear to act as true inflammatory mediators since they are able to trigger the oxidative burst in neutrophils as well as in monocytes. Thus, it is hypothesized that the AOPPs, which arise from the reaction between chlorinated oxidants and plasma proteins, constitute a new molecular basis for the deleterious activity of oxidants, and they could be considered to be true mediators of the proinflammatory effect of oxidative stress in uremia.

Effects of zinc on lipids of erythrocytes from carp (Cyprinus carpio L.) acclimated to different temperatures

We compared the effect of zinc (0.01, 0.1, 0.5 and 1 mM) at two temperatures (5 and 20 degrees C) on erythrocytes from summer and winter acclimatised carp. An increase in temperature from 5 to 20 degrees C increased the unsaturation index (UI) and relative proportion (UI/SFA) of unsaturated to saturated fatty acids in total lipids of the red cells. At 5 degrees C, the unsaturation index of phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidylethanolamine (PE) and phosphatidylinositol (PI) decreased (30-40%) in the presence of 1 mM zinc. The change in unsaturation of phospholipids in the presence of zinc at 5 degrees C is probably responsible for the alteration in structural integrity of erythrocyte membrane as observed by hemolysis and the decreased thiol group content in the erythrocytes. In light of this result, zinc may be considered an environmental hazard for these fish at low temperatures.

Oxidative protein damage in early stage Type 1 diabetic patients

To examine the influence of oxidative stress on oxidative protein damage, we studied 51 young Type 1 diabetic patients clinically free of complications and 48 healthy normolipidaemic age-matched controls. We determined: (1) plasma carbonyl (PCO), plasma total thiol (T-SH), and nitrotyrosine (NT) levels as markers of oxidative protein damage; (2) plasma lipid hydroperoxide (LHP), and nitric oxide (NO) levels as markers of oxidative stress; (3) plasma total antioxidant capacity (TAO), ceruloplasmin (Cp), transferrin (TRF), unsaturated iron binding capacity (UIBC), erythrocyte glutathione (GSH), and erythrocyte superoxide dismutase (SOD) as markers of free radical scavengers. There were no significant differences in the levels of these markers between prepubertal diabetic patients and the controls. The levels of both of PCO and LHP were increased in adolescent and young adult Type 1 diabetic patients with respect to their controls. In the adolescent group, patient versus control values for PCO were 1.04+/-0.067 versus 0.67+/-0.0274 nmol/mg and for LHP they were 2. 10+/-1.09 versus 1.00+/-0.4 nmol/mg. In the young adult group, patient versus control values for PCO were 0.99+/-0.054 versus 0. 66+/-0.02 nmol/mg and for LHP they were 1.96+/-0.78 versus 1.15+/-0. 4 nmol/mg. TAO levels were significantly decreased in adolescent diabetic patients compared to their controls (0.92+/-0.27 vs. 1. 86+/-0.37) and in young adult diabetic patients compared to their controls (0.80+/-0.27 vs. 2.11+/-0.54 nmol/mg). T-SH was not different between diabetic patients and the controls. Serum NT, NO, and erythrocyte SOD levels were not different either between three groups of diabetic patients or between the patients and their controls. We attribute this lack of difference to limited disease duration. Changes in markers of oxidative stress other than NT, NO, and SOD observed in adolescent and young adult early stage Type 1 diabetic patients contribute to the imbalance in the redox status of the plasma. We attribute this imbalance to metal-catalyzed protein oxidation in both groups of Type 1 diabetic patients clinically free from complications.

Protective effects of Mangifera indica L. extract, mangiferin and selected antioxidants against TPA-induced biomolecules oxidation and peritoneal macrophage activation in mice

We compared the protective abilities of Mangifera indica L. stem bark extract (Vimang) 50-250 mgkg(-1), mangiferin 50 mgkg(-1), vitamin C 100 mgkg(-1), vitamin E 100 mgkg(-1)and beta -carotene 50 mgkg(-1)against the 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced oxidative damage in serum, liver, brain as well as in the hyper-production of reactive oxygen species (ROS) by peritoneal macrophages. The treatment of mice with Vimang, vitamin E and mangiferin reduced the TPA-induced production of ROS by the peritoneal macrophages by 70, 17 and 44%, respectively. Similarly, the H(2)O(2)levels were reduced by 55-73, 37 and 40%, respectively, when compared to the control group. The TPA-induced sulfhydryl group loss in liver homogenates was attenuated by all the tested antioxidants. Vimang, mangiferin, vitamin C plus E and beta -carotene decreased TPA-induced DNA fragmentation by 46-52, 35, 42 and 17%, respectively, in hepatic tissues, and by 29-34, 22, 41 and 17%, in brain tissues. Similar results were observed in respect to lipid peroxidation in serum, in hepatic mitochondria and microsomes, and in brain homogenate supernatants. Vimang exhibited a dose-dependent inhibition of TPA-induced biomolecule oxidation and of H(2)O(2)production by peritoneal macrophages. Even if Vimang, as well as other antioxidants, provided significant protection against TPA-induced oxidative damage, the former lead to better protection when compared with the other antioxidants at the used doses. Furthermore, the results indicated that Vimang is bioavailable for some vital target organs, including liver and brain tissues, peritoneal exudate cells and serum. Therefore, we conclude that Vimang could be useful to prevent the production of ROS and the oxidative tissue damages in vivo.

Reactive oxygen species activate mitogen-activated protein kinases in pancreatic acinar cells

It has been recently reported that kinases that belong to the mitogen-activated protein kinase (MAPK) family are rapidly activated by cholecystokinin (CCK) in rat pancreas both in vitro and in vivo. It is known that reactive oxygen species (ROS) play an important role in the pathogenesis of acute pancreatitis induced by supraphysiologic stimulation with CCK analogue, cerulein. The aim of our study was to evaluate whether MAPKs are activated by ROS in pancreatic acini. The activity of MAPK, c-Jun amino-terminal kinase (JNK), and p38 MAPK was determined in isolated rat pancreatic acinar cells by means of Western blotting, with the use of specific antibody that recognizes active, dually phosphorylated kinases. Incubation of acini with ROS donors, hydrogen peroxide (H2O2) and/or menadione (MND), strongly activated all three kinases. Activation of these kinases by ROS, but not by CCK, was substantially inhibited by pretreatment of acini with antioxidant N-acetylo-L-cysteine (NAC). Whereas CCK-induced activation of MAPK or JNK was totally or partially blocked by protein kinase C (PKC) inhibitor GF-109203X, ROS-induced activation of MAPK, JNK, and p38 MAPK was PKC independent. In conclusion, ROS strongly activate MAPK, JNK, and p38 MAPK in pancreatic acinar cells. It may be of importance in acute pancreatitis, because ROS are involved in the pathogenesis of this disease.

Histone carbonylation in vivo and in vitro

Non-enzymic damage to nuclear proteins has potentially severe consequences for the maintenance of genomic integrity. Introduction of carbonyl groups into histones in vivo and in vitro was assessed by Western blot immunoassay and reductive incorporation of tritium from radiolabelled NaBH(4) (sodium borohydride). Histone H1 extracted from bovine thymus, liver and spleen was found to contain significantly elevated amounts of protein-bound carbonyl groups as compared with core histones. The carbonyl content of nuclear proteins of rat pheochromocytoma cells (PC12 cells) was not greatly increased following oxidative stress induced by H(2)O(2), but was significantly increased following alkylating stress induced by N-methyl-N'-nitro-N-nitrosoguanidine or by combined oxidative and alkylating stress. Free ADP-ribose, a reducing sugar generated in the nucleus in proportion to DNA strand breaks, was shown to be a potent histone H1 carbonylating agent in isolated PC12 cell nuclei. Studies of the mechanism of histone H1 modification by ADP-ribose indicate that carbonylation involves formation of a stable acyclic ketoamine. Our results demonstrate preferential histone H1 carbonylation in vivo, with potentially important consequences for chromatin structure and function.

Chlorophyllin as an effective antioxidant against membrane damage in vitro and ex vivo

Chlorophyllin (CHL), the sodium-copper salt and the water-soluble analogue of the ubiquitous green pigment chlorophyll, has been attributed to have several beneficial properties. Its antioxidant ability, however, has not been examined in detail. Using rat liver mitochondria as model system and various sources for the generation of reactive oxygen species (ROS) we have examined the membrane-protective properties of CHL both under in vitro and ex vivo conditions. Oxidative damage to proteins was assessed as inactivation of the enzymes, cytochrome c oxidase and succinic dehydrogenase besides formation of protein carbonyls. Damage to membrane lipids was measured by formation of lipid hydroperoxides and thiobarbituric acid reactive substances. The effect of this compound on the antioxidant defense system was studied by estimating the level of glutathione and superoxide dismutase. ROS were generated by gamma-radiation, photosensitization, ascorbate-Fe(2+), NADPH-ADP-Fe(3+) and the peroxyl radical generating agent, azobis-amidopropane hydrochloride. Our results show that CHL is highly effective in protecting mitochondria, even at a low concentration of 10 microM. The antioxidant ability, at equimolar concentration, was more than that observed with ascorbic acid, glutathione, mannitol and tert-butanol. When CHL was fed to mice at a dose of 1% in drinking water, there was a significant reduction in the potential for oxidative damage in cell suspensions from liver, brain and testis. To examine the possible mechanisms responsible for the observed antioxidant ability we have studied the reaction of CHL with the potent ROS in the form of hydroxyl radical and singlet oxygen. The compound shows a fairly high rate constant with singlet oxygen, in the order of 1.3x10(8) M(-1) s(-1). In conclusion, our studies showed that CHL is a highly effective antioxidant, capable of protecting mitochondria against oxidative damage induced by various ROS.

Formation of a protein-bound pyrazinium free radical cation during glycation of histone H1

Glycation, the nonenzymatic reaction between protein amino groups and reducing sugars, induces protein damage that has been linked to several pathological conditions, especially diabetes, and general aging. Here we describe the direct identification of a protein-bound free radical formed during early glycation of histone H1 in vitro. Earlier EPR analysis of thermal browning reactions between free amino acids and reducing sugars has implicated the sugar fragmentation product glycolaldehyde in the generation of a 1,4-disubstituted pyrazinium free radical cation. In order to evaluate the potential formation of this radical in vivo, the early glycation of BSA, lysozyme, and histone H1 by several sugars (D-glucose, D-ribose, ADP-ribose, glycolaldehyde) under conditions of physiological pH and temperature was examined by EPR. The pyrazinium free radical cation was identified on histone H1 glycated by glycolaldehyde (g = 2.00539, aN = 8.01 [2N], aH = 5.26 [4H], aH = 2.72 [4H]), or ADP-ribose. Reaction of glycoaldehyde with poly-L-lysine produced an identical signal, whereas reaction with BSA or lysozyme produced only a minor unresolved singlet signal. In the absence of oxygen the signal was stable over several days. Our results raise the possibility that pyrazinium radicals may form during glycation of histone H1 in vivo.

Free-radical-induced inactivation of lysozyme and carbonyl residue generation in protein are not necessarily associated

The 2,2'-azobis(2-amidinopropane) (AAPH)-induced inactivation and oxidative modification of lysozyme, as determined by the loss of tryptophan-associated fluorescence (TAF) and the increase in dinitrophenylhydrazine-reactive carbonyl groups (CO), were studied in the absence and in the presence of antioxidants. AAPH induced a progressive inactivation of the enzyme and a parallel decrease of its TAF. Both changes were closely correlated (R2 = 0.97); however, the inactivation was only partially associated with an increase in CO. The latter reached maximal values at times half those needed to attain maximal losses in both lysozyme activity and TAF. A stoichiometric comparison reveals that whereas over 74% of the enzyme molecules had lost their activity, only 5% exhibited an increment in CO. CO formation was affected differentially by boldine and trolox. Both antioxidants fully protected against the early inactivation and loss of TAF; however, the increase in CO was completely unaffected by trolox. Exposure of lysozyme to Fe3+/ascorbate induced no loss of activity or TAF, but it led to an accumulation of CO similar to that induced by AAPH. Results indicate that CO formation and lysozyme inactivation are two mechanistically dissociable events and that changes in the former parameter can perfectly occur in the absence of changes in the latter.

Relative reactivity of lysine and other peptide-bound amino acids to oxidation by hypochlorite

Antibacterial and inflammatory responses of neutrophils and macrophages produce hypochlorite as a major oxidant. Numerous side chains of amino acids found in extracellular proteins can be modified by hypochlorite, including His, Arg, Tyr, Lys, Trp, and Met. We studied the relative reactivity of each of these amino acid residues in short N-blocked peptides, where other residues in the peptide were highly resistant to hypochlorite attack. Hypochlorite treatment led to modified peptides in each case, which were detected by changes in retention on reversed-phase HPLC. A distinct single product, consuming two equivalents of hypochlorite per equivalent of peptide, was obtained from the Lys-containing peptides. UV spectroscopy, nuclear magnetic resonance (NMR), and electrospray/mass spectroscopy identified this product as the dichloramine at the epsilon-amino group of the Lys side chain. The dichloramine at Lys did not decompose to form a detectable amount of carbonyl reactive with dinitrophenylhydrazine. The dichloramine at Lys did however quantitatively revert back to Lys during HCl digestion of the tetrapeptide for amino acid analysis, with simultaneous modification of the adjacent Phe residue. The formation of the dichloramine at Lys was not blocked by peptides or acetylated amino acids that contained Tyr, His, or Arg. In contrast, the presence of equimolar Met-containing peptide, or N-Acetyl-Trp, both inhibited the formation of the dichloramine at Lys. Thus, Met and Trp side chains of proteins might be able to protect Lys from chloramine formation under some circumstances, but this interpretation must consider that Met and Trp are typically found in relatively inaccessible hydrophobic sites, whereas lysine is typically exposed on the protein surface. The hierarchy of amino acid reactivities examined here will aid in the prediction of residues in biological samples most likely to be modified by hypochlorite.

Modulating cellular aging in vitro: hormetic effects of repeated mild heat stress on protein oxidation and glycation

Intracellular and extracellular proteins are subject to a variety of spontaneous non-enzymatic modifications which affect their structure, function and stability. Protein oxidation and glycation are tightly linked and are implicated in the development of many pathological consequences of aging. Although multiple endogenous pathways in the cell can prevent the formation of oxidized and glycated proteins, and repair and degrade abnormal proteins, such abnormal proteins do accumulate during aging. The heat shock response involving the family of stress-proteins or the so-called heat shock proteins (HSP), represents the quickest and highly conserved response to proteotoxic insults. Since repeated mild heat stress is able to prevent the onset of various age-related changes during cellular aging in vitro, we suggest that treatments which increase HSP expression should reduce the extent of accumulation of abnormal proteins during aging. Such modulation of aging is an example of hormesis, which is characterized by the beneficial effects resulting from the cellular responses to mild repeated stress.

Metabolism of protein-bound DOPA in mammals

Protein-bound 3,4-dihydroxyphenylalanine (DOPA) can be generated in mammalian cells by both controlled enzymatic pathways, and by uncontrolled radical reactions. Protein-bound DOPA (PB-DOPA) has reducing activity and the capacity to inflict secondary damage on other important biomolecules such as DNA. This may be mediated through replenishment of transition metals or from catechol-quinone-catechol redox cycles in the presence of cellular components such as ascorbate or cysteine, resulting in amplification of radical damaging events. The generation of PB-DOPA confers on protein the ability to chelate transition metals generating protein 'oxychelates'; this may be amongst the factors, which localise such damage. Tissue levels of PB-DOPA are increased in a number of age-related pathologies such as atherosclerosis and cataract formation. We discuss the detoxification, and the subsequent proteolysis and excretion of components of PB-DOPA. We contrast the fact that in marine organisms, and particularly in extracellular proteins, PB-DOPA and other DOPA-polymers can play important functional roles in adhesion and the provision of tensile properties.

The Haber-Weiss reaction and mechanisms of toxicity

The concept that the highly reactive hydroxyl radical (HO) could be generated from an interaction between superoxide (O(2)(-)) and hydrogen peroxide (H(2)O(2)) was proposed (with Joseph Weiss) in Professor Haber's final paper published in 1934. Until it was recognized that free radicals are produced in biological systems, this finding seemed to have no relevance to biology. However, following the discovery that O(2)(-) was a normal cellular metabolite, it was quickly recognized that the Haber-Weiss reaction (O(2)(-)+H(2)O(2) -->HO+O(2)+HO(-)) might provide a means to generate more toxic radicals. Although the basic reaction has a second order rate constant of zero in aqueous solution and thus cannot occur in biological systems, the ability of iron salts to serve as catalysts was discussed by these authors. Because transition metal ions, particularly iron, are present at low levels in biological systems, this pathway (commonly referred to as the iron-catalyzed Haber-Weiss reaction) has been widely postulated to account for the in vivo generation of the highly reactive HO. Recent data documenting the importance of redox regulation of various cellular signaling pathways makes it clear that free radicals are essential for normal cellular function. However, this also makes it obvious that disruptions of free radical production or defenses at many different levels can lead to adverse effects on cells. While the generation of HO, which is by far the most reactive oxygen species, is generally indicative of an overtly toxic event, it is through studies at this level that we have reached a better understanding of free radicals as both signaling molecules and toxic species.

Regulation of protein function by S-glutathiolation in response to oxidative and nitrosative stress

Protein S-glutathiolation, the reversible covalent addition of glutathione to cysteine residues on target proteins, is emerging as a candidate mechanism by which both changes in the intracellular redox state and the generation of reactive oxygen and nitrogen species may be transduced into a functional response. This review will provide an introduction to the concepts of oxidative and nitrosative stress and outline the molecular mechanisms of protein regulation by oxidative and nitrosative thiol-group modifications. Special attention will be paid to recently published work supporting a role for S-glutathiolation in stress signalling pathways and in the adaptive cellular response to oxidative and nitrosative stress. Finally, novel insights into the molecular mechanisms of S-glutathiolation as well as methodological problems related to the interpretation of the biological relevance of this post-translational protein modification will be discussed.

Quantification and significance of protein oxidation in biological samples

Protein oxidation is defined here as the covalent modification of a protein induced either directly by reactive oxygen species or indirectly by reaction with secondary by-products of oxidative stress. Oxidative modification of proteins can be induced experimentally by a wide array of prooxidant agents and occurs in vivo during aging and in certain disease conditions. Oxidative changes to proteins can lead to diverse functional consequences, such as inhibition of enzymatic and binding activities, increased susceptibility to aggregation and proteolysis, increased or decreased uptake by cells, and altered immunogenicity. There are numerous types of protein oxidative modification and these can be measured with a variety of methods. Protein oxidation serves as a useful marker for assessing oxidative stress in vivo. There are both advantages and disadvantages to using proteins for this purpose compared to lipids and DNA. Finally, it is important to monitor the degree of oxidative modification of therapeutic proteins manufactured for commercial use. This review will examine various aspects of protein oxidation, with emphasis on using proteins as markers of oxidative stress in biological samples.

Antioxidant properties of Asparagus racemosus against damage induced by gamma-radiation in rat liver mitochondria

The possible antioxidant effects of crude extract and a purified aqueous fraction of Asparagus racemosus against membrane damage induced by the free radicals generated during gamma-radiation were examined in rat liver mitochondria. gamma-Radiation, in the dose range of 75-900 Gy, induced lipid peroxidation as assessed by the formation of thiobarbituric acid reactive substances (TBARS) and lipid hydroperoxides (LOOH). Using an effective dose of 450 Gy, antioxidant effects of A. racemosus extract were studied against oxidative damage in terms of protection against lipid peroxidation, protein oxidation, depletion of protein thiols and the levels of the antioxidant enzyme, superoxide dismutase. An active fraction consisting of polysaccharides (termed as P3) was effective even at a low concentration of 10 microg/ml. Both the crude extract as well as the P3 fraction significantly inhibited lipid peroxidation and protein oxidation. The antioxidant effect of P3 fraction was more pronounced against lipid peroxidation, as assessed by TBARS formation, while that of the crude extract was more effective in inhibiting protein oxidation. Both the crude extract and P3 fraction also partly protects against radiation-induced loss of protein thiols and inactivation of superoxide dismutase. The inhibitory effects of these active principles, at the concentration of 10 microg/ml, are comparable to that of the established antioxidants glutathione and ascorbic acid. Hence our results indicate that extracts from A. racemosus have potent antioxidant properties in vitro in mitochondrial membranes of rat liver.

Increased intracellular reactive oxygen species in patients with end-stage renal failure: effect of hemodialysis

BACKGROUND

Reactive oxygen species (ROS) have been implicated in various forms of cellular injury. ROS may cause cell damage and are involved in the pathophysiology of several diseases, including atherosclerosis and chronic inflammation.

METHODS

Disturbances of intracellular ROS levels were investigated in 28 patients with end-stage renal failure. The intracellular ROS levels were measured in lymphocytes before and after hemodialysis using biocompatible membranes and were compared with those from 11 patients with end-stage renal failure, not yet on renal replacement therapy, and 27 healthy control subjects. ROS levels were measured spectrophotometrically using the intracellular dye dichlorofluorescin diacetate.

RESULTS

The spontaneous production of ROS was significantly higher in lymphocytes from patients with end-stage renal failure compared with healthy control subjects (P < 0.01). The addition of 100 nmol/L phorbol-myristate-acetate (PMA) produced a significant increase of ROS, both in lymphocytes from patients with end-stage renal failure and healthy control subjects. The PMA-induced ROS increase was significantly higher in lymphocytes from patients with end-stage renal failure compared with healthy control subjects (P < 0.01). In patients with end-stage renal failure, not yet on renal replacement therapy, the PMA-induced ROS was also significantly higher compared with healthy control subjects. The PMA-induced ROS increases were significantly inhibited by catalase, but not by superoxide dismutase or the superoxide dismutase mimetic, tempol. PMA-induced ROS was significantly reduced by tyrphostin A51 in lymphocytes from patients with end-stage renal failure and from healthy control subjects (each P < 0.01), indicating the involvement of a tyrosine kinase-dependent pathway. In patients with end-stage renal failure, the spontaneous and the PMA-induced production of ROS was not significantly different before and after hemodialysis.

CONCLUSIONS

Regular hemodialysis sessions using biocompatible membranes have no effect on the elevated intracellular ROS in patients with end-stage renal failure.

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.

Superoxides from mitochondrial complex III: the role of manganese superoxide dismutase

In this report we show that ubiquinone cytochrome c reductase (complex III) from isolated rat heart mitochondria when inhibited with antimycin A, produces a large amount of superoxide as measured by the chemiluminescent probe coelenterazine. When mitochondria are inhibited with myxothiazol or stigmatellin, there is no detectable formation of superoxide. The antimycin A-sensitive free radical production can be dramatically reduced using either myxothiazol or stigmatellin. This suggests that the antimycin A-sensitive generation of superoxides originates primarily from the Q(o) semiubiquinone. When manganese superoxide dismutase depleted submitochondrial particles (SMP) were inhibited with myxothiazol or stigmatellin, a large superoxide signal was observed. These two inhibitors likely increase the concentration of the Q(i) semiquinone at the N center. The antimycin A-sensitive signal can, in the case of both the mitochondria and the SMP, be dissipated by the addition of copper zinc superoxide dismutase, suggesting that the measured coelenterazine signal was a result of superoxide production. Taken together, this data suggests that free radicals generated from the Q(i) species are more effectively eliminated by MnSOD in intact mitochondria.

Reaction of human myoglobin and H2O2. Involvement of a thiyl radical produced at cysteine 110

The human myoglobin (Mb) sequence is similar to other mammalian Mb sequences, except for a unique cysteine at position 110. Reaction of wild-type recombinant human Mb, the C110A variant of human Mb, or horse heart Mb with H(2)O(2) (protein/H(2)O(2) = 1:1.2 mol/mol) resulted in formation of tryptophan peroxyl (Trp-OO( small middle dot)) and tyrosine phenoxyl radicals as detected by EPR spectroscopy at 77 K. For wild-type human Mb, a second radical (g approximately 2. 036) was detected after decay of Trp-OO( small middle dot) that was not observed for the C110A variant or horse heart Mb. When the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was included in the reaction mixture at protein/DMPO ratios </=1:10 mol/mol, a DMPO adduct exhibiting broad absorptions was detected. Hyperfine couplings of this radical indicated a DMPO-thiyl radical. Incubation of wild-type human Mb with thiol-blocking reagents prior to reaction with peroxide inhibited DMPO adduct formation, whereas at protein/DMPO ratios >/=1:25 mol/mol, DMPO-tyrosyl radical adducts were detected. Mass spectrometry of wild-type human Mb following reaction with H(2)O(2) demonstrated the formation of a homodimer (mass of 34,107 +/- 5 atomic mass units) sensitive to reducing conditions. The human Mb C110A variant afforded no dimer under identical conditions. Together, these data indicate that reaction of wild-type human Mb and H(2)O(2) differs from the corresponding reaction of other myoglobin species by formation of thiyl radicals that lead to a homodimer through intermolecular disulfide bond formation.

Induction of oxidative stress by glutathione depletion causes severe hypertension in normal rats

Several recent studies have shown that certain forms of genetic or acquired hypertension are associated with oxidative stress and that animals with those types of hypertension respond favorably to antioxidant therapy. We hypothesize that oxidative stress may cause hypertension via (among other mechanisms) enhanced oxidation and inactivation of nitric oxide (NO). To test this hypothesis, Sprague-Dawley rats were subjected to oxidative stress by glutathione (GSH) depletion by means of the GSH synthase inhibitor buthionine sulfoximine (BSO, 30 mmol/L in drinking water) for 2 weeks. The control group was given drug-free drinking water. In parallel experiments, subgroups of animals were provided vitamin E-fortified chow and vitamin C-supplemented drinking water. The BSO-treated group showed a 3-fold decrease in tissue GSH content, a marked elevation in blood pressure, and a significant reduction in the urinary excretion of the NO metabolite nitrate plus nitrite, which suggests depressed NO availability. These characteristics were associated with a significant accumulation in various tissues of nitrotyrosine, which is the footprint of NO inactivation by reactive oxygen species. Administration of vitamin E plus vitamin C ameliorated hypertension, improved urinary nitrate-plus-nitrite excretion, and mitigated nitrotyrosine accumulation (despite GSH depletion) in the BSO-treated animals but had no effect in the control group. In conclusion, GSH depletion resulted in perturbation of the NO system and severe hypertension in normal animals. The effects of BSO were mitigated by concomitant antioxidant therapy despite GSH depletion, which supports the notion that oxidative stress was involved in the pathogenesis of hypertension in this model.

Production of immunoreactive thyroglobulin C-terminal fragments during thyroid hormone synthesis

Here, we studied the fragmentation of the prothyroid hormone, thyroglobulin (Tg), which occurs during thyroid hormone synthesis, a process which involves iodide, thyroperoxidase, and the H2O2-generating system, consisting of glucose and glucose oxidase. Various peptides were found to be immunoreactive to autoantibodies to Tg from patients and monoclonal antibodies directed against the immunodominant region of Tg. The smallest peptide (40 kDa) bore thyroid hormones and was identified at the C-terminal end of the Tg molecule, which shows homologies with acetylcholinesterase. Similar peptides were obtained by performing metal-mediated oxidation of Tg via a Fenton reaction. It was concluded that the oxidative stress induced during hormone synthesis generates free radicals, which, in turn, cleave Tg into immunoreactive peptides.

Photo-oxidation of methionine-containing peptides by the 4-carboxybenzophenone triplet state in aqueous solution. Competition between intramolecular two-centered three-electron bonded (S...S)+ and (S...N)+ formation

Quantum yields for the formation of transients were measured following the quenching of triplet 4-carboxy-benzophenone (3CB*) by methionine-containing peptides in aqueous solutions. Ketyl radicals (CBH.), ketyl radical anions (CB.-) and various sulfur radical cations were identified following the triplet-quenching events. The presence of these intermediates indicated that the triplet-quenching mechanism can be characterized as mainly electron-transfer in nature. The quenching rate constants were of the order of 2 x 10(9) M-1 s-1. There were small, but significant, differences in the triplet-quenching rate constants, and these trends indicate the existence of multiple sulfur targets in the quenchers. The absorption of the transient products was followed in detail by using spectral-resolution analysis. From the absorption data, quantum yields were estimated for the formation of the various transients. There were differences found in the yields of the transient products between the experiments, where the quenchers were the "mixed" stereoisomers of methionylmethionine (L,D and D,L) and experiments where the quenchers were L,L and D,D stereoisomers. Triplet-quenching data from several other methionine-containing small oligopeptides were analyzed in an analogous manner. Systematic variations were observed, and these patterns were discussed in terms of competitive donation of protons to the CB.- within the charge-transfer complex. The competition was between protons on carbons adjacent to the sulfur-radical center and protons on the protonated amino groups of the radical cation. In addition, there was a competition between the two intramolecular two-centered, three-electron bonded species (S therefore S)+ and (S therefore N)+ that play roles in the secondary kinetics.

Vanillin as an antioxidant in rat liver mitochondria: inhibition of protein oxidation and lipid peroxidation induced by photosensitization

Using rat liver mitochondria, as model systems, we have examined the ability of the natural compound and the food-flavoring agent, vanillin to protect membranes against oxidative damage induced by photosensitization at concentrations normally used in food preparations. Vanillin, at a concentration of 2.5 mmol/L, has afforded significant protection against protein oxidation and lipid peroxidation in hepatic mitochondria induced by photosensitization with methylene blue plus light. The effect observed was both time- and concentration-dependent. The inhibitory effect is similar to ascorbic acid and the singlet oxygen quencher, diazabicyclo[2.2.2]octane (DABCO) but less effective than sodium azide and glutathione. Examination of possible mechanisms responsible for the observed protection, showed that vanillin has a significant ability to quench singlet oxygen (1O2), a reactive species responsible for damage induced during photosensitization by Type II mechanism. Hence, this flavoring compound, due to its antioxidant ability, may have potential to prevent oxidative damage to membranes in mammalian tissues and thereby the ensuing diseased states.

Role of oxidative stress in cardiovascular diseases

OBJECTIVES

In view of the critical role of intracellular Ca2 overload in the genesis of myocyte dysfunction and the ability of reactive oxygen species (ROS) to induce the intracellular Ca2+-overload, this article is concerned with analysis of the existing literature with respect to the role of oxidative stress in different types of cardiovascular diseases.

OBSERVATIONS

Oxidative stress in cardiac and vascular myocytes describes the injury caused to cells resulting from increased formation of ROS and/or decreased antioxidant reserve. The increase in the generation of ROS seems to be due to impaired mitochondrial reduction of molecular oxygen, secretion of ROS by white blood cells, endothelial dysfunction, auto-oxidation of catecholamines, as well as exposure to radiation or air pollution. On the other hand, depression in the antioxidant reserve, which serves as a defense mechanism in cardiac and vascular myocytes, appears to be due to the exhaustion and/or changes in gene expression. The deleterious effects of ROS are mainly due to abilities of ROS to produce changes in subcellular organelles, and induce intracellular Ca2+-overload. Although the cause-effect relationship of oxidative stress with any of the cardiovascular diseases still remains to be established, increased formation of ROS indicating the presence of oxidative stress has been observed in a wide variety of experimental and clinical conditions. Furthermore, antioxidant therapy has been shown to exert beneficial effects in hypertension, atherosclerosis, ischemic heart disease, cardiomyopathies and congestive heart failure.

CONCLUSIONS

The existing evidence support the view that oxidative stress may play a crucial role in cardiac and vascular abnormalities in different types of cardiovascular diseases and that the antioxidant therapy may prove beneficial in combating these problems.

Correlation of low-density lipoprotein modification by myeloperoxidase with hypochlorous acid formation

Myeloperoxidase is an enzyme in phagocytes which catalyzes several redox reactions. A major product is hypochlorous acid which appears to be important in inflammatory processes such as atherosclerosis. The aim of this study was to investigate whether the kinetics of low-density lipoprotein modification by the myeloperoxidase/hydrogen peroxide/chloride system in vitro conform to the established kinetics of hypochlorous acid formation and to compare the results with known in vivo data. The absorbance at 234 nm was applied to study the kinetics of the modification of low-density lipoprotein. Variation of the concentration of low-density lipoprotein, hydrogen peroxide, and chloride, respectively, had a biphasic effect on the maximal rate of low-density lipoprotein modification. Increasing the substrates up to certain threshold levels resulted in increased modification, however, further increases caused inhibition of low-density lipoprotein modification. The inhibitory effect of higher low-density lipoprotein concentrations might be relevant, since these concentrations occur in the human aortic intima. Furthermore, a positive correlation was found between the maximal rate of low-density lipoprotein modification and the acidity of the medium. In summary, low-density lipoprotein modification is affected by the myeloperoxidase/hydrogen peroxide/chloride system in a similar manner to hypochlorous acid production. We conclude that myeloperoxidase, which has been detected in atherosclerotic lesions, is able to modify low-density lipoprotein into the form which is taken up by macrophages in an uncontrolled manner.

Differential impairment of 20S and 26S proteasome activities in human hematopoietic K562 cells during oxidative stress

The 20S proteasome and the 26S proteasome are major components of the cytosolic and nuclear proteasomal proteolytic systems. Since proteins are known to be highly susceptible targets for reactive oxygen species, the effect of H(2)O(2) treatment of K562 human hematopoietic cells toward the activities of 20S and 26S proteasomes was investigated. While the ATP-independent degradation of the fluorogenic peptide suc-LLVY-MCA was not affected by H(2)O(2) concentrations of up to 5 mM, the ATP-stimulated degradation of suc-LLVY-MCA by the 26S proteasome began to decline at 400 microM and was completely abolished at 1 mM oxidant treatment. A combination of nondenaturing electrophoresis and Western blotting let us believe that the high oxidant susceptibility of the 26S proteasome is due to oxidation of essential amino acids in the proteasome activator PA 700 which mediates the ATP-dependent proteolysis of the 26S-proteasome. The activity of the 26S-proteasome could be recovered within 24 h after exposure of cells to 1 mM H(2)O(2) but not after 2 mM H(2)O(2). In view of the specific functions of the 26S proteasome in cell cycle control and other important physiological functions, the consequences of the higher susceptibility of this protease toward oxidative stress needs to be considered.