Inactivation of Escherichia coli glutamine synthetase by xanthine oxidase, nicotinate hydroxylase, horseradish peroxidase, or glucose oxidase: effects of ferredoxin, putidaredoxin, and menadione

Effect of Humid Air Exposed to IR Radiation on Enzyme Activity

Water vapor absorbs well in the infra-red region of the electromagnetic spectrum. Absorption of radiant energy by water or water droplets leads to formation of exclusion zone water that possesses peculiar physico-chemical properties. In the course of this study, normally functioning and damaged alkaline phosphatase, horseradish peroxidase and catalase were treated with humid air irradiated with infrared light with a wavelength in the range of 1270 nm and referred to as coherent humidity (CoHu). One-minute long treatment with CoHu helped to partially protect enzymes from heat inactivation, mixed function oxidation, and loss of activity due to partial unfolding. Authors suggest that a possible mechanism underlying the observed effects involves altering the physicochemical properties of aqueous media while treatment of the objects with CoHu where CoHu acts as an intermediary.

Protein oxidation by the cytochrome P450 mixed-function oxidation system

This mini-review summarizes results of studies on the oxidation of proteins and low-density lipoprotein (LDL) by various mixed-function oxidation (MFO) systems. Oxidation of LDL by the O2/FeCl3/H2O2/ascorbate MFO system is dependent on all four components and is much greater when reactions are carried out in the presence of a physiological bicarbonate/CO2 buffer system as compared to phosphate buffer. However, FeCl3 in this system could be replaced by hemin or the heme-containing protein, hemoglobin, or cytochrome c. Oxidation of LDL by the O2/cytochrome P450 cytochrome c reductase/NADPH/FeCl3 MFO system is only slightly higher (25%) in the bicarbonate/CO2 buffer as compared to phosphate buffer, but is dependent on all components except FeCl3. Omission of FeCl3 led to a 60% loss of activity. These results suggest that peroxymonobicarbonate and/or free radical derivatives of bicarbonate ion and/or CO2 might contribute to LDL oxidation by these MFO systems.

Reactive oxygen species and protein oxidation in aging: a look back, a look ahead

The existence of free radicals, as chemical entities, was inferred 100 years ago but not universally accepted for some 30-40 years. The existence and importance of free radicals in biological systems was not recognized until the mid 1950s, by a small number of visionary scientists who can be credited with founding the field of reactive oxygen biochemistry. For most of the remaining 20th century, reactive oxygen species (ROS) were considered a type of biochemical "rusting agent" that caused stochastic tissue damage and disease. As we enter the 21st century, reactive oxygen biochemistry is maturing as a discipline and establishing its importance among the biomedical sciences. It is now recognized that virtually every disease state involves some degree of oxidative stress. Moreover, we are now beginning to recognize that ROS are produced in a well-regulated manner to help maintain homeostasis on the cellular level in normal, healthy tissue. This review summarizes the history of reactive oxygen biochemistry, outlining major paradigm shifts that the field has undergone and continues to experience. The contributions of Earl Stadtman to the recent history of the field (1980-present) are especially highlighted. The role of ROS in signal transduction is presented in some detail as central to the latest paradigm shift. Emerging technologies, particularly proteomic technologies, are discussed that will facilitate further evolution in the field of reactive oxygen biochemistry.

Parameters of dietary selenium and vitamin E deficiency in growing rabbits

4 x 5 growing female rabbits (New Zealand White) with an initial live weight of 610 +/- 62 g were fed a torula yeast based semisynthetic diet low in selenium (<0.03 mg/kg diet) and containing <2 mg alpha-tocopherol per kg (group I). Group II received a vitamin E supplementation of 150 mg alpha-tocopherylacetate per kg diet, whereas for group III 0.40 mg Se as Na-selenite and for group IV both supplements were added. Selenium status and parameters of tissue damage were analyzed after 10 weeks on experiment (live weight 2,355 +/- 145 g). Selenium depletion of the Se deficient rabbits (groups I and II) was indicated by a significantly lower plasma Se content (group I: 38.3 +/- 6.23 microg Se/mL plasma, group II: 42.6 +/- 9.77, group III: 149 +/- 33.4, group IV: 126 +/- 6.45) and a significantly lower liver Se content (group I: 89.4 +/- 18.2 microg/kg fresh matter, group II: 111 +/- 26.2) as compared to the Se supplemented groups III (983 +/- 204) and IV (926 +/- 73.9). After 5 weeks on the experimental diets differences in the development of plasma glutathione peroxidase were observed. As compared to the initial status group (45.2 +/- 4.50) pGPx activity in mU/mg protein was decreased in group I (19.1 +/- 7.08), remained almost stable in the vitamin E supplemented group II (46.3 +/- 11.2) whereas an elevated enzyme activity was measured in the Se supplemented groups III (62.4 +/- 23.9) and IV (106 +/- 19.9). In the rabbit organs investigated 10 weeks of Se deficiency caused a significant loss of Se dependent cellular glutathione peroxidase activity (GPx1) of 94% (liver), 80% (kidney), 50% (heart muscle) and 60% (musculus longissimus dorsi) in comparison to Se supplemented control animals. Damage of cellular lipids and proteins in the liver was due to either Se or vitamin E deficiency. However damage was most severe under conditions of a combined Se and vitamin E deficiency. It can be concluded that the activity of plasma glutathione peroxidase is a sensitive indicator of Se deficiency in rabbits. The loss of GPx1 activity indicates the selenium depletion in various rabbit organs. Both selenium and vitamin E are essential and highly efficient antioxidants which protect rabbits against lipid and protein oxidation.

Dithiothreitol induces the sacrificial antioxidant property of human serum albumin in a metal-catalyzed oxidation and gamma-irradiation system

The species *OH or H2O2 are produced by both metal-catalyzed oxidation (MCO) of reducing equivalents and gamma-irradiation. Intact or Cys-34-modified human serum albumin (HSA) was significantly degraded in the MCO system containing dithiothreitol (DTT) as electron donor, but as long as it lasted, HSA prohibited *OH or H2O2 from initiating molecular damage of DNA. However, in the GSH and ascorbate (nonthiol) MCO system, HSA was not sacrificially degraded, and indeed accelerated the formation of DNA strand breaks. In the y-irradiation system producing *OH from H2O, only DTT attenuated the generation of DNA strand breaks by HSA. It did not degrade more H2O2 in the presence of reduced GSH (thiol-linked peroxidase) than in its absence. Therefore it would seem that in an MCO system, the antioxidant activity of HSA depends on the effectiveness of reducing equivalents to induce exposure of a functional group scavenging the *OH or H2O2 species, by reduction of its disulfide-bonds. In the presence of DTT, disulfide bonds in HSA were quantitatively reduced to cysteinyl residues but not significantly reduced by ascorbate or GSH. In conclusion, the antioxidant activity of HSA in the D

Manganese: a transition metal protects nigrostriatal neurons from oxidative stress in the iron-induced animal model of parkinsonism

It has been suggested that transition metals such as iron and manganese produce oxidative injury to the dopaminergic nigrostriatal system. which may play a critical role in the pathogenesis of Parkinson's disease. Intranigral infusion of ferrous citrate (0 to 8.4 nmol, i.n.) acutely increased lipid peroxidation in the substantia nigra and dopamine turnover in the caudate nucleus. Subsequently, it caused a severe depletion of dopamine levels in the rat caudate nucleus. In contrast to iron's pro-oxidant effect, manganese (up to 30 nmol, i.n.) causes neither lipid peroxidation nor nigral injury/dopamine depletion. Manganese (1.05 to 4.2 nmol, i.n.) dose-dependently protected nigral neurons from iron-induced oxidative injury and dopamine depletion. Manganese also suppressed acute increase in dopamine turnover and contralateral turning behaviour induced by iron. In brain homogenates manganese (0 to 10 microM) concentration-dependently inhibited propagation of lipid peroxidation caused by iron (0 to 5 microM). Without the contribution of manganese-superoxide dismutase manganese was still effective in sodium azide and/or heat-pretreated brain homogenates. Surprisingly, iron but not manganese, catalysed the Fenton reaction or the conversion of hydrogen peroxide to hydroxyl radicals. The results indicate that iron and manganese are two transition metals mediating opposite effects in the nigrostriatal system, as pro-oxidant and antioxidant, respectively. In conclusion, intranigral infusion of iron, but not manganese, provides an animal model for studying the pathophysiological role of oxidant and oxidative stress in nigrostriatal degeneration and Parkinsonism. The present results further suggest that the atypical antioxidative properties of manganese may protect substantia nigra compacta neurons from iron-induced oxidative stress.

Reactive oxygen-mediated protein oxidation in aging and disease

Highly reactive oxygen species that are formed during normal metabolism and under conditions of oxidative stress are able to oxidize proteins or convert lipid and carbohydrate derivatives to compounds that react with functional groups on proteins. Among other changes, these ROS-mediated reactions lead to the formation of protein carbonyl derivatives, which serves as a marker of ROS-mediated protein damage. On the basis of this marker, it is established that oxidatively damaged protein is associated with aging and some diseases. The accumulation of oxidatively damaged protein reflects the balance among a myriad of factors that govern the rates of ROS generation and the rate at which damaged protein is degraded. Peroxynitrite, which is formed under normal physiological conditions, is able to oxidize methionine residues in proteins and to nitrate tyrosine residues; however, its ability to do so is dependent on the availability of CO2, which stimulates the nitration of tyrosine residues but inhibits the oxidation of methionine residues. Nitration of tyrosine residues may contribute to peroxynitrite toxicity, as nitration precludes the phosphorylation or nucleotidylation of tyrosine residues and thereby seriously compromises one of the most important mechanisms of cellular regulation and signal transduction.

Recombinant human O6-alkylguanine-DNA alkyltransferase (AGT), Cys145-alkylated AGT and Cys145 --> Met145 mutant AGT: comparison by isoelectric focusing, CD and time-resolved fluorescence spectroscopy

Isoelectric focusing, CD, steady-state and time-resolved fluorescence spectroscopy were used to compare the native recombinant human DNA-repair protein O6-alkylguanine-DNA alkyltransferase (AGT) with AGT derivatives methylated or benzylated on Cys145 or modified by site-directed mutagenesis at the active centre (Met145 mutant). The AGT protein is approximately spherical with highly constrained Trp residues, but is not stabilized by disulphide bridges. In contrast with native AGT, alkylated AGT precipitated at 25 degrees C but remained monomeric at 4 degrees C. As revealed by isoelectric focusing, pI changed from 8.2 (AGT) to 8. 4 (Cys145-methylated AGT) and 8.6 (Cys145-benzylated AGT). The alpha-helical content of the Met145 mutant was decreased by approx. 5% and Trp residues were partially liberated. Although non-covalent binding of O6-benzylguanine did not alter the secondary structure of AGT, its alpha-helical content was increased by approx. 2% on methylation and by approx. 4% on benzylation, altogether indicating a small conformational change in AGT on undergoing alkylation. No signal sequences have been found in AGT that mark it for polyubiquitination. Therefore the signal for AGT degradation remains to be discovered.

Oxidative inactivation of glutamine synthetase from the cyanobacterium Anabaena variabilis

In crude extracts of the cyanobacterium Anabaena variabilis, glutamine synthetase (GS) could be effectively inactivated by the addition of NADH. GS inactivation was completed within 30 min. Both the inactivated GS and the active enzyme were isolated. No difference between the two enzyme forms was seen in sodium dodecyl sulfate-gels, and only minor differences were detectable by UV spectra, which excludes modification by a nucleotide. Mass spectrometry revealed that the molecular masses of active and inactive GS are equal. While the Km values of the substrates were unchanged, the Vmax values of the inactive GS were lower, reflecting the inactivation factor in the crude extract. This result indicates that the active site was affected. From the crude extract, a fraction mediating GS inactivation could be enriched by ammonium sulfate precipitation and gel filtration. GS inactivation by this fraction required the presence of NAD(P)H, Fe3+, and oxygen. In the absence of the GS-inactivating fraction, GS could be inactivated by Fe2+ and H2O2. The GS-inactivating fraction produced Fe2+ and H2O2, using NADPH, Fe3+, and oxygen. Accordingly, the inactivating fraction was inhibited by catalase and EDTA. This GS-inactivating system of Anabaena is similar to that described for oxidative GS inactivation in Escherichia coli. We conclude that GS inactivation by NAD(P)H is caused by irreversible oxidative damage and is not due to a regulatory mechanism of nitrogen assimilation.

Hydrogen peroxide-mediated inactivation of microsomal cytochrome P450 during monooxygenase reactions

Cytochrome P450 can undergo inactivation following monooxygenase reactions in liver microsomes of untreated, phenobarbital and 3-methylcholanthrene-treated rats and rabbits. The acceleration of cytochrome P450 loss in the presence of catalase inhibitors (sodium azide, hydroxylamine) indicates that hydrogen peroxide is involved in hemoprotein degradation. It was revealed that cytochrome P450 is inactivated mainly by H2O2 formed through peroxy complex breakdown, whereas H2O2 formed via the dismutation of superoxide anions produces a slight inactivating effect. The hydrogen peroxide added outside or formed by a glucose-glucose oxidase system has less of an inactivating effect than H2O2 produced within the cytochrome P450 active center. Self-inactivation of cytochrome P450 during oxygenase reactions is highly specific. Other components of the monooxygenase system, such as cytochrome b5, NADH- and NADPH-specific flavoproteins, undergo no inactivation. The alterations in phospholipid content and in the rate of lipid peroxidation were not observed as well. The inactivation of cytochrome P450 by H2O2 is the result of heme loss or destruction without cytochrome P420 formation. Such a mechanism operates with different substrates and cytochrome P450 species catalyzing the partially coupled monooxygenase reactions.

Effects of EUK-8, a synthetic catalytic superoxide scavenger, on hypoxia- and acidosis-induced damage in hippocampal slices

Anoxia produces deleterious effects on synaptic transmission in the hippocampal slice preparation. A proposed source of damage is the superoxide radical (.O2-) produced during the earlier period of reoxygenation. The present study tested the effects of a synthetic, catalytic superoxide radical scavenger (EUK-8) on CA1 pyramidal cell responses elicited by electrical stimulation of the Schaffer-commissural pathway after severe anoxic episodes. Following reoxygenation, slices incubated with EUK-8 (50 microM) exhibited significantly better recovery of excitatory postsynaptic potentials (EPSPs) than control slices. In addition, repeated episodes of anoxia produced irreversible loss of synaptic transmission in the majority of control slices (93 +/- 7%, n = 15), compared to a small fraction in EUK-8-incubated slices (27 +/- 12%, n = 15). A thiobarbituric acid (TBA) test was used to assess the effect of EUK-8 on lipid peroxidation elicited in hippocampal slices by acidosis and lactic acid (pH 5.0 and 30 mM lactic acid). Incubation in the presence of EUK-8 totally prevented the increase in lipid peroxidation produced by acidosis and lactic acid in both the incubation medium and the slice homogenates. These results indicate that a superoxide scavenger like EUK-8 prevents damage produced by acidosis and anoxia in hippocampal slices and suggest the possibility of using this type of molecule under various pathological conditions.

The oxidative inactivation of cytochrome P450 in monooxygenase reactions

Possible mechanisms of cytochrome P450 self-inactivation during catalytic turnover have been considered. Two ways of hemoprotein inactivation are so far known. The first, studied extensively by many authors, is the formation of active substrate intermediates, capable of modifying heme and apoenzyme. The second way, revealed quite recently and resulting from uncoupled cytochrome P450-catalyzed monooxygenase reactions, is yet to be clarified. Briefly, it involves formation of hydrogen peroxide in the hemoprotein active center, which interacts with the enzyme associated Fe2+, thereby generating hydroxyl radicals that bleach the heme and modify the apoenzyme. This mechanism operates with substrates and cytochrome P450 forms with partially coupled monooxygenase reactions, thus causing the formation of hydrogen peroxide as a byproduct.

Protein modification in aging

During aging a number of enzymes accumulate as catalytically inactive or less active forms. The age-related changes in catalytic activity are due in part to reactions of the protein with "active" oxygen species such as ozone, singlet oxygen, or with oxygen free radicals as are produced during exposure to ionizing radiation or to metal ion catalyzed oxidation (MCO) systems. The levels of oxidized proteins in cultured human fibroblasts from individuals of various ages and in liver and brain extracts of rats of different ages increase progressively with age, and in old rats can represent 30-50% of the total cellular protein. The age-related increase in oxidized protein in rat liver and brain tissue is accompanied by a loss of glutamine synthetase (GS) and glucose-6-P dehydrogenase (G-6-PDH) activities, and to a decrease in the level of cytosolic neutral protease activity which is responsible for the degradation of oxidized (denatured) protein. Of particular significance are the results of experiments showing that similar age-related changes occur in the gerbil brain and that these changes are accompanied by a loss of short-term memory as measured by the radial arm maze technique. Chronic treatment (intraperitoneal injections) of old animals with the free radical spin-trap reagent, N-tert-butyl-alpha-phenylnitrone (PBN) resulted in normalization of the several biochemical parameters to those characteristic of the young animals; coincidentally, the short-term memory index was restored to the young animal values. These results provide the first evidence that there is likely a linkage between the age-dependent accumulation of oxidized enzymes and the loss of physiological function.

Mechanism of lysozyme inactivation and degradation by iron

The site-specific lysozyme damage by iron and by iron-catalysed oxygen radicals was investigated. A solution of purified lysozyme was inactivated by Fe(II) at pH 7.4 in phosphate buffer, as tested on cleavage of Micrococcus lysodeikticus cells; this inactivation was time- and iron concentration-dependent and was associated with a loss of tryptophan fluorescence. In addition, it was reversible at pH 4, as demonstrated by lysozyme reactivation and by the intensity of the 14.4-kD-band on SDS-PAGE. Desferal (1 mM) and Detapac (1 mM) added before iron, prevented lysozyme inactivation, while catalase (100 micrograms/ml), superoxide dismutase (100 micrograms/ml) and bovine serum albumin (100 micrograms/ml) gave about 30 to 40% protection by competing with lysozyme for iron binding. The denaturing effect of iron on lysozyme was studied in the presence of H2O2 (1 mM) and ascorbate (1 mM); under these conditions the enzyme underwent partly irreversible inactivation and degradation different to that produced by gamma radiolysis-generated .OH. Catalase almost fully protected lysozyme; in contrast, mannitol (10 mM), benzoate (10 mM), and formate (10 mM) provided no protection because of their inability to access the site at which damaging species are generated. In this system, radical species were formed in a site-specific manner, and they reacted essentially with lysozyme at the site of their formation, causing inactivation and degradation differently than the hydroxyl radical.

Protein oxidation and aging

A number of systems that generate oxygen free radicals catalyze the oxidative modification of proteins. Such modifications mark enzymes for degradation by cytosolic neutral alkaline proteases. Protein oxidation contributes to the pool of damaged enzymes, which increases in size during aging and in various pathological states. The age-related increase in amounts of oxidized protein may reflect the age-dependent accumulation of unrepaired DNA damage that, in a random manner, affects the concentrations or activities of numerous factors that govern the rates of protein oxidation and the degradation of oxidized protein.

Role of oxygen in oxidation of lipid and protein during ischemia/reperfusion in isolated perfused rat lung

Considerable evidence has accumulated that oxygen free radicals play a major role in ischemic injury, particularly when followed by reperfusion. Few reports have demonstrated the occurrence of oxidative damage during the ischemic period, itself. Our laboratory has demonstrated that events occurring during an ischemic period with adequate oxygen supply can mimic the "oxygen paradox," using lipid peroxidation as an index of oxidative stress and lung edema as an index of tissue injury. The present study compares lipid peroxidation and oxidation of soluble (100,000g supernatant) protein during ischemia and reperfusion in isolated rat lung model perfused with artificial medium and ventilated with varying alveolar oxygen tension. Protein oxidation was determined by a modified dinitrophenylhydrazine (DNPH) method using Sephadex G-25 column chromatography to isolate the DNPH bound proteins. Global ischemia was produced by discontinuing perfusion while ventilation continued with gas mixtures containing 5% CO2 and a fixed oxygen concentration between 0 and 95%. After 1 h ischemia in the isolated rat lung ventilated with 20% oxygen, protein carbonyls and thiobarbituric acid reactive substances (TBARS) increased significantly compared with controls. These changes were more pronounced after 60 min of reperfusion with 95% oxygen in the ventilation gas. With 0% oxygen (95% nitrogen and 5% CO2) content of the ventilating gas during ischemia, TBARS and protein carbonyls remained at the control level. The wet/dry weight ratio showed changes parallel to the indices of tissue oxidation. The presence of 5,8,11,14-eicosatetraynoic, an inhibitor of cyclooxygenase and lipoxygenase pathways, in the perfusate had no effect on the generation of protein carbonyls although inhibition of lipid peroxidation was demonstrated. This implies that the oxidation of soluble protein is not mediated by the eicosanoid metabolic cascade. These data indicate that oxidative processes occur during ischemia and are dependent on the alveolar oxygen concentration. Oxidation of soluble protein can be used as an index of oxidative damage during lung ischemia and reperfusion.

The inactivation of mitochondrial F1 ATPase by H2O2 is mediated by iron ions not tightly bound in the protein

Exposure to purified mitochondrial F1 ATPase to continuous flux of H2O2 resulted in significant loss (up to 60%) of the ATP hydrolytic activity. The presence of chelating agents including desferrioxamine or previous selective removal of the iron ions not tightly bound in the protein completely prevented the inactivation, whereas re-loading of the enzyme with F3+ restored the sensitivity to H2O2. A marked protective effect was provided as well by mannitol or by Cu,Zn superoxide dismutase. The results indicated the decomposition of H2O2 by redox-active iron-protein adducts as responsible for the enzyme inactivation, probably through site-directed generation of more highly reactive oxygen species. A possible role for iron associated to F1 component in the oxidation, aging and turnover of ATP synthase complex in vivo may be suggested on the basis on these results.

Excess brain protein oxidation and enzyme dysfunction in normal aging and in Alzheimer disease

The relationship between Alzheimer disease (AD) and aging is not currently known. In this study, postmortem frontal- and occipital-pole brain samples were obtained from 16 subjects with AD, 8 age-matched controls, and 5 young controls. These samples were analyzed both for protein oxidation products (carbonyl) and the activities of two enzymes vulnerable to mixed-function oxidation, glutamine synthetase and creatine kinase. Glutamine synthetase is more sensitive to mixed-function oxidation than creatine kinase. Carbonyl content rises exponentially with age, at double the rate in the frontal pole compared with the occipital pole. Compared with young controls, both aged groups (AD and age-matched controls) have increased carbonyl content and decreased glutamine synthetase and creatine kinase activities, which are more marked in the frontal than occipital pole in all instances. We conclude that protein oxidation products accumulate in the brain and that oxidation-vulnerable enzyme activities decrease with aging in the same regional pattern (frontal more affected than occipital). However, only glutamine synthetase activity distinguishes AD from age-matched controls: Because glutamine synthetase activity is differentially reduced in the frontal pole in AD, we suggest that AD may represent a specific brain vulnerability to age-related oxidation.

Effects of molecular oxygen, oxidation-reduction potential, and antioxidants upon in vitro replication of Treponema pallidum subsp. pallidum

The effects of various concentrations of dithiothreitol, molecular oxygen, and several antioxidants upon the in vitro replication of Treponema pallidum were studied. The optimal dithiothreitol concentration was between 0.65 and 1.62 mM, and the optimum oxygen concentration was 3.0% +/- 0.5% in both the presence and absence of additional antioxidants. It was discovered that the reduced sulfhydryl concentration and the oxidation-reduction potential of the medium were stabilized after 5 days. The water-soluble antioxidants cobalt chloride, cocarboxylase, mannitol, and histidine were individually tested for their ability to increase treponemal growth in vitro. The optimum concentrations for these antioxidants were 21 nM, 4.3 nM, 0.55 mM, and 0.23 mM, respectively. When combined at these concentrations, the mixture of antioxidants stimulated the in vitro replication of T. pallidum. The number of treponemes in cultures with the antioxidants averaged a 59-fold increase, compared with a 43-fold increase in cultures lacking the antioxidants. It was further demonstrated that histidine and mannitol were the most critical components of this mixture. Catalase and superoxide dismutase were investigated for their ability to promote the growth and maintain viability of T. pallidum in tissue culture. The optimum concentrations for these enzymes were 10,000 U/liter and 25,000 U/liter, respectively. When these enzymes and the above antioxidants were combined and added to a chemically reduced modified Eagle medium, the treponemes increased an average of 70-fold, compared with an average of 35-fold in cultures lacking them. Furthermore, this medium, T. pallidum culture medium, supported the replication of T. pallidum at oxygen concentrations from 5 to 7% with little loss in yield or viability. The lipid-soluble antioxidants vitamin A and vitamin E acetate were also shown to enhance the in vitro growth of T. pallidum in this medium.

Role of site-specific, metal-catalyzed oxidation in lens aging and cataract: a hypothesis

The evidence reviewed here supports the hypothesis that metal catalyzed oxidation reactions occur in the lens and may make a significant contribution to the changes seen in the lens with age and in cataract formation. The major support for this hypothesis is as follows. (1) All of the components of the non-enzymic metal catalyzed oxidation systems are present in the lens normally. Ascorbate, glutathione and oxygen are present in much lower concentrations. Although, even at low concentrations, the reactions could occur over many years with significant consequences. Components of some of the enzymic systems are also present, although primarily in the epithelial layer and outer cortical region. Copper and iron levels may be increased in some cataracts. (2) Protein carbonyl derivatives are increased in both aging and cataractous lenses. Amino acid-derived protein carbonyl derivatives have only been demonstrated in oxidative reactions derived from oxygen radical generation, particularly those catalyzed by metal-catalyzed oxidation systems. (3) Treatment of isolated bovine crystallins with metal catalyzed oxidation systems generates modifications similar to those found in vivo. The proposed mechanism of site-specific metal catalyzed oxidation appears to be a feasible mechanism of oxidation in the lens, and verification of the mechanism requires further study. Although the focus of this manuscript has been on the oxidative modification induced in proteins,m oxidative damage to DNA or membrane resulting from similar mechanisms may also play an important role in alteration of lens function during aging and cataractogenesis.

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

Metal-catalyzed oxidation of proteins has been implicated in a variety of biological processes, particularly in the marking of proteins for subsequent proteolytic degradation. The metal-catalyzed oxidation of bacterial glutamine synthetase causes conformational, covalent, and functional alterations in the protein. To understand the structural basis of the functional changes, the time course of oxidative modification of glutamine synthetase was studied utilizing a nonenzymic model oxidation system consisting of ascorbate, oxygen, and iron. The structural modifications induced included: decreased thermal stability; weakening of subunit interactions; decrease in isoelectric point; introduction of carbonyl groups into amino acid side chains; and loss of two histidine residues. These changes did not denature the protein, but instead induced relatively subtle changes. Indeed, even the most extensively modified protein had a sedimentation velocity which was identical to that of the native enzyme. Comparison of the time courses of the structural and functional changes established that: (i) Loss of the metal binding site and of catalytic activity occurred with loss of one histidine per subunit; (ii) increased susceptibility to proteolysis occurred with loss of two histidine residues per subunit. Thus, oxidation at one site suffices to inactivate the enzyme, but two sites must be modified to induce susceptibility to proteolysis. The limited and specific changes induced by metal-catalyzed oxidation are consistent with a site-specific free radical mechanism.

Manganese-dependent disproportionation of hydrogen peroxide in bicarbonate buffer

At physiological concentrations of HCO3- and CO2, Mn(II) catalyzes disproportionation of H2O2. This catalase-like activity is directly proportional to the concentrations of Mn(II) and H2O2, and it increases exponentially with increases in pH. The effect of increasing pH is almost completely attributable to the concomitant increase in HCO3- concentration. The rate is proportional to the third power of the HCO3- concentration, suggesting that 3 equivalents of HCO3- combine with 1 equivalent of Mn(II) to form the catalytic complex. It is presumed that the redox potential of the Mn(II) in equilibrium with Mn(III) couple in such a complex permits H2O2 to carry out facile reactions with Mn(II) comparable to those that occur with Fe(III) and Cu(II) chelate complexes, in which OH. and O2-. are established intermediates. The Mn-catalyzed disproportionation of H2O2 does not occur at physiological pH in the absence of HCO3-. Hepes, inorganic phosphate, and inorganic pyrophosphate inhibit the reaction catalyzed by the Mn/HCO3- system. These results are similar to those of Sychev et al. [Sychev, A.Y., Pfannmeller, U. & Isak, V.G. (1983) Russ. J. Phys. Chem. 57, 1690-1693]. The catalase-like activity of Mn(II)-bicarbonate complexes reported here, together with the superoxide dismutase activity of Mn complexes demonstrated by Archibald and Fridovich [Archibald, F.S. & Fridovich, I. (1982) Arch. Biochem. Biophys. 214, 452-463], strengthen the proposition that Mn may play an important role in the protection of cells against oxygen radical-mediated damage.

Metal ion-catalyzed oxidation of proteins: biochemical mechanism and biological consequences

In the presence of O2, Fe(III) or Cu(II), and an appropriate electron donor, a number of enzymic and nonenzymic oxygen free radical-generating systems are able to catalyze the oxidative modification of proteins. Whereas random, global modification of many different amino acid residues and extensive fragmentation occurs when proteins are exposed to oxygen radicals produced by high energy radiation, only one or a few amino acid residues are modified and relatively little peptide bond cleavage occurs when proteins are exposed to metal-catalyzed oxidation (MCO) systems. The available evidence indicates that the MCO systems catalyze the reduction of Fe(III) to Fe(II) and of O2 to H2O2 and that these products react at metal-binding sites on the protein to produce active oxygen (free radical?) species (viz; OH, ferryl ion) which attack the side chains of amino acid residues at the metal-binding site. Among other modifications, carbonyl derivatives of some amino acid residues are formed; prolyl and arginyl residues are converted to glutamylsemialdehyde residues, lysyl residues are likely converted to 2-amino-adipylsemialdehyde residues; histidyl residues are converted to asparagine and/or aspartyl residues; prolyl residues are converted to glutamyl or pyroglutamyl residues; methionyl residues are converted to methionylsulfoxide residues; and cysteinyl residues to mixed-disulfide derivatives. The biological significance of these metal ion-catalyzed reactions is highlighted by the demonstration: (i) that oxidative modification of proteins "marks" them for degradation by most common proteases and especially by the cytosolic multicatalytic proteinase from mammalian cells; (ii) protein oxidation contributes substantially to the intracellular pool of catalytically inactive and less active, thermolabile forms of enzymes which accumulate in cells during aging, oxidative stress, and in various pathological states, including premature aging diseases (progeria, Werner's syndrome), muscular dystrophy, rheumatoid arthritis, cataractogenesis, chronic alcohol toxicity, pulmonary emphysema, and during tissue injury provoked by ischemia-reperfusion. Furthermore, the metal ion-catalyzed protein oxidation is the basis of biological mechanisms for regulating changes in enzyme levels in response to shifts from anaerobic to aerobic metabolism, and probably from one nutritional state to another. It is also involved in the killing of bacteria by neutrophils and in the loss of neutrophil function following repeated cycles of respiratory burst activity.

Manganese(II) catalyzes the bicarbonate-dependent oxidation of amino acids by hydrogen peroxide and the amino acid-facilitated dismutation of hydrogen peroxide

In bicarbonate/CO2 buffer, Mn(II) and Fe(II) catalyze the oxidation of amino acids by H2O2 and the dismutation of H2O2. As the Mn(II)/Fe(II) ratio is increased, the yield of carbonyl compounds per mole of leucine oxidized is essentially constant, but the ratio of alpha-ketoisocaproate to isovaleraldehyde formed increases, and the fraction of H2O2 converted to O2 increases. In the absence of Fe(II), the rate of Mn(II)-catalyzed leucine oxidation is directly proportional to the H2O2, Mn(II), and amino acid concentrations and is proportional to the square of the HCO3- concentration. The rate of Mn(II)-catalyzed O2 production in the presence of 50 mM alanine or leucine is about 4-fold the rate observed in the absence of amino acids and accounts for about half of the H2O2 consumed; the other half of the H2O2 is consumed in the oxidation of the amino acids. In contrast, O2 production is increased nearly 18-fold by the presence of alpha-methylalanine and accounts for about 90% of the H2O2 consumed. The data are consistent with the view that H2O2 decomposition is an inner sphere (cage-like) process catalyzed by a Mn coordination complex of the composition Mn(II), amino acid, (HCO3-)2. Oxidation of the amino acid in this complex most likely proceeds by a free radical mechanism involving hydrogen abstraction from the alpha-carbon as a critical step. The results demonstrate that at physiological concentrations of HCO3- and CO2, Mn(II) is able to facilitate Fenton-type reactions.

Protein oxidation and proteolysis during aging and oxidative stress

Previous studies in this laboratory have shown that glutamine synthetase (GS) and other key metabolic enzymes are inactivated by metal-catalyzed oxidation reactions in vitro. Oxidative inactivation renders these proteins highly susceptible to proteolysis, especially to a class of newly identified alkaline proteases which exhibit little or no activity against the native enzymes. These studies have suggested that oxidative inactivation may be an important marking step for intracellular protein degradation. Because many of the enzymes which have been shown to accumulate as inactive or less active forms during aging are readily inactivated by metal-catalyzed oxidation reactions in vitro, we have investigated the possible relationship between protein oxidation and proteolysis during aging and oxidative stress in vivo. Oxidized proteins accumulate in hepatocytes of rats exposed to 100% oxygen during the first 48 h of oxygen treatment. In the interval between 48 and 54 h the levels of oxidized proteins decline sharply. The specific activities of at least two liver enzymes, glutamine synthetase and glucose-6-phosphate dehydrogenase (G-6-PDH), decrease during the 54-h experiment. GS and G-6-PDH specific immunological cross-reactivity remains high during the first 48 h of oxygen treatment and then declines in the interval between 48 and 54 h. During this same interval the levels of alkaline proteases which degrade oxidized proteins increase, indicating that these activities are induced or activated in response to oxidative stress and subsequently degrade the proteins which have become oxidized during the initial phase of oxygen treatment. Oxidized proteins accumulate progressively during aging in hepatocytes from rats 3 to 26 months old, with the largest incremental increase between 20 and 26 months. The increase in protein oxidation is correlated with a loss of specific activity of GS and G-6-PDH without a concomitant loss of immunological cross-reactivity. The levels of alkaline proteases which degrade oxidized proteins in hepatocytes from 26-month-old rats is only 20% that of 3-month-old rats, suggesting that oxidized proteins accumulate in hepatocytes from old rats, in part, because the proteases which degrade them are deficient or defective. moreover, when old rats are subjected to treatment with 100% oxygen, the levels of oxidized proteins continue to increase and the alkaline protease activity remains low, indicating that these protease activities are not increased in response to oxidative stress in old rats.

Conversion of ribulose-1,5-bisphosphate carboxylase to an acidic and catalytically inactive form by extracts of osmotically stressed Lemna minor fronds

The fronds of Lemna minor L. respond to a number of stresses, and in particular to an osmotic stress, by producing an enzyme system which catalyzes the oxidation of ribulose-1,5-bisphosphate carboxylase (RuBPCase; EC 4.1.1.39) to an acidic and catalytically inactive form. During the first 24 h of osmotic stress the induced oxidase system does not seem to exert a significant in-vivo effect on RuBPCase, presumably because of compartmentation. Subsequently, the oxidase system gains access to the enzyme and converts it to the acid and catalytically inactive form and eventually the oxidase system declines in activity.A number of partially acidified forms of RuBPCase are formed during oxidation, and this process appears to be correlated with the disappearance of varying numbers of SH residues. The number of-SH residues in RuBPCase from Lemna has been estimated at 89. However, RuBPCase isolated from 24-h osmotically stressed fronds showed a reduction in the number of-SH residues per molecule from 89 to 54. It seems likely that the oxidation of-SH groups is causally related to the acidification of RuBPCase which occurs during osmotic stress.

Effect of osmotic stress on protein turnover in Lemna minor fronds

Evidence is presented that although many proteins from the fronds of Lemna minor L. undergo enhanced degradation during osmotic stress, ribulose-1,5-bisphosphate carboxylase (RuBPCase) is not degraded. Instead RuBPCase is converted in a series of steps to a very high-molecular-weight form. The first step involves the induction of an oxidase system which after 24 h of stress converts RuBPCase to an acidic and catalytically inactive form. Subsequently, the oxidised RuBPCase protein is gradually polymerized to a number of very large aggregates (molecular weight of several million).The conversion of RuBPCase to a high-molecular-weight form appears to be correlated with (i) a reduction in the number of-SH residues and (ii) the susceptibility to in-vitro proteolysis. Indeed, the number of-SH groups per RuBPCase molecule decreases from 89 in the native enzyme to 54 and 22 in the oxidised and polymerized forms, respectively. On the other hand, the oxidised enzyme is more susceptible to in-vitro proteolysis than the native form. However, it is the polymerized form of RuBPCase which is particularly susceptible to in-vitro proteolysis.Western-blotting experiments and anti-ubiquitin antibodies were used to detect the presence of ubiquitin conjugates in extracts from osmotically stressed Lemna fronds. The possible involvement of ubiquitin in the formation of the aggregates is discussed.

Oxidation of Neurospora crassa NADP-specific glutamate dehydrogenase by activated oxygen species

The glutamine synthetase and the NADP-specific glutamate dehydrogenase activities of Neurospora crassa were lost in a culture without carbon source only when in the presence of air. Glutamine synthetase was previously reported to be liable to in vitro and in vivo inactivation by activated oxygen species. Here we report that NADP-specific glutamate dehydrogenase was remarkably stable in the presence of activated oxygen species but was rendered susceptible to oxidative inactivation when chelated iron was bound to the enzyme and either ascorbate or H2O2 reacted on the bound iron. This reaction gave rise to further modifications of the enzyme monomers by activated oxygen species, to partial dissociation of the oligomeric structure, and to precipitation and fragmentation of the enzyme. The in vitro oxidation reaction was affected by pH, temperature, and binding to the enzyme of NADPH. Heterogeneity in total charge was observed in the purified and immunoprecipitated enzymes, and the relative amounts of enzyme monomers with different isoelectric points changes with time of the oxidizing reaction.

Different selectivities of oxidants during oxidation of methionine residues in the alpha-1-proteinase inhibitor

Oxidation of the reactive site methionine (Met) in alpha-1-proteinase inhibitor (alpha-1-PI) to methionine sulfoxide (Met(O] is known to cause depletion of its elastase inhibitory activity. To estimate the selectivity of different oxidants in converting Met to Met(O) in alpha-1-PI, we measured the molar ratio Met(O)/alpha-1-PI at total inactivation. This ratio was determined to be 1.2 for both the myeloperoxidase/H2O2/chloride system and the related compound NH2Cl. With taurine monochloramine, another myeloperoxidase-related oxidant, 1.05 mol Met(O) were generated per mol alpha-1-PI during inactivation. These oxidants attack preferentially one Met residue in alpha-1-PI, which is identical with Met 358, as concluded from the parallelism of loss of elastase inhibitory activity and oxidation of Met. A similar high specificity for Met oxidation was determined for the xanthine oxidase-derived oxidants. In contrast, the ratio found for ozone and m-chloroperoxybenzoic acid was 6.0 and 5.0, respectively, indicating oxidation of additional Met residues besides the relative site Met in alpha-1-PI, i.e. unselective action of these oxidants. Further studies were performed on the efficiency of oxidants for total depletion of the elastase inhibitory capacity of alpha-1-PI. Ozone and m-chloroperoxybenzoic acid were 10-fold less effective and the superoxide anion/hydroxyl radicals were 30-50-fold less effective to inactivate the elastase inhibitory activity as compared to the myeloperoxidase-derived oxidants. The myeloperoxidase-related oxidants are discussed as important regulators of alpha-1-PI activity in vivo.

Mechanism of spectrin degradation induced by phenylhydrazine in intact human erythrocytes

The exposure of human erythrocytes to phenylhydrazine results in the degradation of both monomers of spectrin, a major cytoskeleton membrane protein. The degradative process, characterized by a loss of spectrin without the appearance of high-molecular-weight products, either under reducing conditions or not, is almost complete in 10 min when a 5% erythrocyte suspension is treated with 1 mM phenylhydrazine. Under these conditions, we found a loss of 62.3 and 48.5% for the alpha and beta monomer, respectively. A similar degradative extent was obtained when the membrane ghost plus cellular free extracts, were dialyzed, and the membrane ghost plus hemoglobin was exposed to 1 mM phenylhydrazine for 10 min. The presence of different proteinase inhibitors and effectors, such as EDTA, diethylenetriaminepentaacetic acid, EGTA, leupeptin, aprotinin, phenylmethylsulfonyl fluoride, pepstatin, Ca2+ and ATP plus Mg2+, in the membrane ghost plus cellular free extract system (undialyzed) did not affect the degree of the spectrin-degradative process induced by phenylhydrazine. In addition, a purified spectrin tetramer preparation exposed to 1 mM phenylhydrazine in the presence of hemoglobin was degraded to an extent comparable to that with intact cells. Our data suggest that the initial degradative step of spectrin induced by phenylhydrazine in intact erythrocytes may be ascribed more to a direct oxidative breakdown, probably involving main-chain cleavage and side-chain cleavage processes, than to an eventual proteolytic system.

Macroxyproteinase (M.O.P.): a 670 kDa proteinase complex that degrades oxidatively denatured proteins in red blood cells

Erythrocytes and reticulocytes are shown to undergo rapid rates of protein degradation following exposure to oxidative stress. Experiments with ATP depletion revealed that, unlike the proteolysis of many other abnormal proteins, the degradation of oxidatively modified proteins is an ATP-independent process. Ion exchange chromatography (DEAE Sepharose CL-6B), ammonium sulfate precipitation, gel filtration chromatography (Sephacryl S-300 or Sepharose CL-6B), and a second ion exchange step were used to resolve the activity responsible for degrading oxidatively modified proteins from (dialyzed) cell-free extracts of erythrocytes and reticulocytes. Gel filtration studies revealed that some 70-80% of the activity in erythrocytes, and some 60-70% of the activity in reticulocytes, is expressed by a 670 kDa proteinase complex that is not stimulated by ATP (in fact, ATP is slightly inhibitory). This proteinase complex is inhibited by sulfhydryl reagents, serine reagents, and transition metal chelators, and has a pH optimum of 7.8. We propose the trivial name "macroxyproteinase" or "M.O.P." (abbreviated from Macro-Oxy-Proteinase) for the complex because of its large size, substrate preference (oxidatively modified proteins), and inhibitor profile (which indicates multiple catalytic sites). Electrophoresis studies of the 670 kDa M.O.P. complex revealed the presence of 8 distinct polypeptide subunits with the following apparent molecular sizes: 21.5, 25.3, 26.2, 28.1, 30.0, 31.9, 33.3, and 35.7 kDa. The large molecular size of the M.O.P. complex, its ATP- and ubiquitin-independence, its inhibitor profile, its distinctive subunit banding pattern in denaturing electrophoresis gels, its pH optimum, and its proteolytic profile with fluorogenic peptide substrates all indicate that M.O.P. is identical to 600-700 kDa neutral/alkaline proteinase complexes that have been isolated from a wide variety of eucaryotic cells and tissues, but for which no function has previously been clear. We propose that macroxyproteinase is responsible for catalyzing most of the selective degradation of oxidatively denatured proteins in red blood cells. We further suggest that M.O.P. may perform the same function in other eucaryotic cells and tissues.

Mitochondria contain a proteolytic system which can recognize and degrade oxidatively-denatured proteins

When incubated with mitochondria in an air atmosphere, menadione and doxorubicin (which redox cycle with the respiratory chain to produce oxygen radicals), as well as xanthine oxidase plus xanthine (which generate superoxide and H2O2), stimulated the degradation of newly-synthesized [( 3H]leucine-labelled) mitochondrial polypeptides. No stimulation was observed in an N2 atmosphere, ATP was not required, and xanthine oxidase was not effective without xanthine. Various forms of oxidative stress induced varying degrees of protein cross-linking, protein fragmentation and proteolysis, as judged by gel electrophoresis and amino acid analysis. To learn more about the proteolytic enzymes involved in degradation, we undertook studies with purified protein substrates which had been exposed to oxidative stress (OH or H2O2) in vitro. Despite mitochondrial contamination with acid proteases of lysosomal (and other) origin, pH profiles revealed distinct proteolytic activities at both pH 4 and pH 8. The pH 8 activity preferentially degraded the oxidatively-denatured forms of haemoglobin, albumin and superoxide dismutase; was unaffected by digitonin; and exhibited a several-fold increase in activity upon mitochondrial disruption (highest activity being found in the matrix). In contrast, the pH 4 activity was dramatically decreased by digitonin treatment (to reduce lysosomal contamination); was unaffected by mitochondrial disruption; and showed no preference for oxidatively-denatured proteins. The pH 8 activity was not stimulated by ATP, but was inhibited by EDTA, haemin and phenylmethylsulphonyl fluoride. In contrast, the contaminating pH 4 activity was only inhibited by pepstatin and leupeptin. Thus, our experiments reveal a distinct mitochondrial (matrix) proteolytic pathway which can preferentially degrade oxidatively-denatured proteins.

Biochemical markers of aging

It is the purpose of this report to identify possible metabolic deficiencies that might serve as biochemical markers of aging. It is proposed that the multiplicity of physical and physiological changes associated with aging could be most readily explained by alterations in the regulation and/or the activities of enzymes that occupy central positions in metabolism. Specifically, a search for metabolic markers of aging might include efforts to determine if there are age-related changes in the following enzymes or enzyme systems: (a) allosteric enzymes that catalyze reactions in highly branched metabolic pathways; (b) enzymes that catalyze opposing reactions between metabolites that are common intermediates in biosynthetic and biodegradative pathways (reactions which in the absence of final control would lead to futile substrate cycling); (c) enzymes that catalyze bimolecular reactions in which one member of a coenzyme pair is a cosubstrate (e.g., reactions involving NAD+ or NADH); (d) enzymes that are regulated by phosphorylation/dephosphorylation cycles; and (e) G-protein-dependent enzyme systems. It is also emphasized that changes in the concentrations and ratios of coenzyme substrate pairs (e.g., [NAD]/[NADH], [CoA]/[acyl CoA]) and the energy charge ratio [ATP] + 0.5 [ADP]/[ATP] + [ADP] + [AMP] may signal deviations from normal metabolism and therefore might be reliable markers of aging. In addition, because of their critical roles in metabolism, changes in the concentration of GTP, GDP and the second messengers, c-AMP, c-GMP should be monitored. Finally, it is noted that the accumulation of the altered forms of some enzymes which occurs during aging reflects imbalance between posttranslational modification of the enzymes and the degradation of the altered enzyme forms. The biological mechanisms involved and the genetic implications are discussed.