Ascorbic acid oxidation and DNA scission catalyzed by iron and copper chelates

Kinetics of ascorbate and dithiothreitol oxidation by soluble copper, iron, and manganese, and 1,4-naphthoquinone: Influence of the species concentration and the type of fluid

An alternative metric to account for particulate matter (PM) composition-based toxicity is the ability of PM-species to generate reactive oxygen species (ROS) and deplete antioxidants, the so-called oxidative potential (OP). Acellular OP assays are the most used worldwide, mainly those based on ascorbic acid (AA) and dithiothreitol (DTT) depletion; OP values are calculated from AA/DTT concentration over time kinetic curves. Since a great variability in OP-DTT and OP-AA values can be found in the literature, the understanding of those factors affecting the kinetic rate of AA and DTT oxidation in the presence of PM-bound species will improve the interpretation of OP values. In this work, a kinetic study of the oxidation rate of AA and DTT driven by species usually found in PM (transition metals and naphthoquinone (NQ)) was carried out. In particular, the influence of the concentration of Cu(II), Fe(II), Fe(III), Mn(II), Mn(III), and 1,4-NQ, and the type of fluid used in the assay (phosphate buffer (PB), phosphate buffer saline (PBS) and artificial lysosomal fluid (ALF)) is analysed and discussed. The reaction orders with respect to the AA/DTT and the active compound, and the kinetic rate constants were also determined. The results show great variability in OP values among the studied species depending on the fluid used; the OP values were mostly higher in PB0.05 M, followed by PBS1x and ALF. Moreover, different species concentration-responses for OP-DTT/OP-AA were obtained. These differences were explained by the different reaction orders and kinetic rate constants obtained for each active compound in each fluid.

Oxidative Stress, Endothelial Dysfunction, and N-Acetylcysteine in Type 2 Diabetes Mellitus

Significance: Cardiovascular diseases (CVDs) remain the leading cause of morbidity and mortality globally. Endothelial dysfunction is closely associated with the development and progression of CVDs. Patients with diabetes mellitus (DM) especially type 2 DM (T2DM) exhibit a significant endothelial cell (EC) dysfunction with substantially increased risk for CVDs. Recent Advances: Excessive reactive oxygen species (ROS) and oxidative stress are important contributing factors to EC dysfunction and subsequent CVDs. ROS production is significantly increased in DM and is critically involved in the development of endothelial dysfunction in diabetic patients. In this review, efforts are made to discuss the role of excessive ROS and oxidative stress in the pathogenesis of endothelial dysfunction and the mechanisms for excessive ROS production and oxidative stress in T2DM. Critical Issues: Although studies with diabetic animal models have shown that targeting ROS with traditional antioxidant vitamins C and E or other antioxidant supplements provides promising beneficial effects on endothelial function, the cardiovascular outcomes of clinical studies with these antioxidant supplements have been inconsistent in diabetic patients. Future Directions: Preclinical and limited clinical data suggest that N-acetylcysteine (NAC) treatment may improve endothelial function in diabetic patients. However, well-designed clinical studies are needed to determine if NAC supplementation would effectively preserve endothelial function and improve the clinical outcomes of diabetic patients with reduced cardiovascular morbidity and mortality. With better understanding on the mechanisms of ROS generation and ROS-mediated endothelial damages/dysfunction, it is anticipated that new selective ROS-modulating agents and effective personalized strategies will be developed for the management of endothelial dysfunction in DM.

Vitamin C-Sources, Physiological Role, Kinetics, Deficiency, Use, Toxicity, and Determination

Vitamin C (L-ascorbic acid) has been known as an antioxidant for most people. However, its physiological role is much larger and encompasses very different processes ranging from facilitation of iron absorption through involvement in hormones and carnitine synthesis for important roles in epigenetic processes. Contrarily, high doses act as a pro-oxidant than an anti-oxidant. This may also be the reason why plasma levels are meticulously regulated on the level of absorption and excretion in the kidney. Interestingly, most cells contain vitamin C in millimolar concentrations, which is much higher than its plasma concentrations, and compared to other vitamins. The role of vitamin C is well demonstrated by miscellaneous symptoms of its absence-scurvy. The only clinically well-documented indication for vitamin C is scurvy. The effects of vitamin C administration on cancer, cardiovascular diseases, and infections are rather minor or even debatable in the general population. Vitamin C is relatively safe, but caution should be given to the administration of high doses, which can cause overt side effects in some susceptible patients (e.g., oxalate renal stones). Lastly, analytical methods for its determination with advantages and pitfalls are also discussed in this review.

New cofactors and inhibitors for a DNA-cleaving DNAzyme: superoxide anion and hydrogen peroxide mediated an oxidative cleavage process

Herein, we investigated the effects of new cofactors and inhibitors on an oxidative cleavage of DNA catalysis, known as a pistol-like DNAzyme (PLDz), to discuss its catalytic mechanism. PLDz performed its catalytic activity in the presence of ascorbic acid (AA), in which Cu²⁺ promoted, whereas Fe²⁺ significantly inhibited the catalytic function. Since Fe²⁺/AA-generated hydroxyl radicals are efficient on DNA damage, implying that oxidative cleavage of PLDz had no relation with hydroxyl radical. Subsequently, we used Fe²⁺/H₂O₂ and Cu²⁺/H₂O₂ to identify the role of hydroxyl radicals in PLDz catalysis. Data showed that PLDz lost its activity with Fe²⁺/H₂O₂, but exhibited significant cleavage with Cu²⁺/H₂O₂. Because Fe²⁺/H₂O₂ and Cu²⁺/H₂O₂ are popular reagents to generate hydroxyl radicals and the latter also produces superoxide anions, we excluded the possibility that hydroxyl radical participated in oxidative cleavage and confirmed that superoxide anion was involved in PLDz catalysis. Moreover, pyrogallol, riboflavin and hypoxanthine/xanthine oxidase with superoxide anion and hydrogen peroxide generation also induced self-cleavage of PLDz, where catalase inhibited but superoxide dismutase promoted the catalysis, suggesting that hydrogen peroxide played an essential role in PLDz catalysis. Therefore, we proposed a catalytic mechanism of PLDz in which superoxide anion and hydrogen peroxide mediated an oxidative cleavage process.

Dynamics of counterion binding during acoustic nebulisation of surfactant solutions

A metal ion (Cu(2+)) and a complex copper species, copper (II) bis-bipyridine, were used as alternate counterions in an aqueous surfactant solution of sodium dodecylbenzenesulfonate (SDBS) to investigate the dynamics of counterion interactions in an acoustic field. Sonoluminescence spectral studies showed that such counterions were able to replace sodium ions at the interface, even when the interface was rapidly oscillating under the acoustic field. Ultrasound induced nebulisation was then used to probe the interfacial profile of surfactant and bound counterions in a dynamic environment. At low bulk concentrations, the copper (II) bis-bipyridine cation was more effective at enhancing the loading of the dodecylbenzenesulfonate anion on the interface, due to its documented greater binding ability. However, at higher bulk concentrations, the movement of this cation is limited by its larger size and the smaller Cu(2+) cation is more effective in enhancing the loading of the dodecylbenzenesulfonate anion. The results show that under dynamic conditions, the surface concentrations are governed by mass transfer kinetics rather than equilibrium thermodynamics.

Sequence-specific DNA cleavage mediated by bipyridine polyamide conjugates

The design of molecules that damage a selected DNA sequence provides a formidable opportunity for basic and applied biology. For example, such molecules offer new prospects for controlled manipulation of the genome. The conjugation of DNA-code reading molecules such as polyamides to reagents that induce DNA damages provides an approach to reach this goal. In this work, we showed that a bipyridine conjugate of polyamides was able to induce sequence-specific DNA breaks in cells. We synthesized compounds based on two polyamide parts linked to bipyridine at different positions. Bipyridine conjugates of polyamides were found to have a high affinity for the DNA target and one of them produced a specific and high-yield cleavage in vitro and in cultured cells. The bipyridine conjugate studied here, also presents cell penetrating properties since it is active when directly added to cell culture medium. Harnessing DNA damaging molecules such as bipyridine to predetermined genomic sites, as achieved here, provides an attractive strategy for targeted genome modification and DNA repair studies.

Protective effect of low dose of ascorbic acid on hepatobiliary function in hepatic ischemia/reperfusion in rats

BACKGROUND/AIMS

The aim of this study was to investigate the effect of ascorbic acid (AA) an alterations in hepatic secretory and microsomal functions during hepatic ischemia and reperfusion.

METHODS

Rats were subjected to 60 min of hepatic ischemia, and 1 and 5 h of reperfusion. Five minutes prior to ischemia, the animals were administered either vehicle or ascorbic acid (AA) (30, 100, 300, and 1000 mg/kg) intravenously.

RESULTS

The serum aminotransferase level and lipid peroxidation were markedly higher as a result of ischemia/reperfusion. These increases were significantly attenuated by AA doses of 30 and 100 mg/kg but were augmented by dose of 1000 mg/kg. Bile flow and cholate output were markedly decreased by ischemia/reperfusion. AA doses of 30 and 100 mg/kg restored but dose of 1000 mg/kg inhibited their secretion. Both the cytochrome P(450) content and aminopyrine N-demethylase activity were decreased by ischemia/reperfusion, which were prevented by AA doses of 30 and 100 mg/kg but were aggravated by dose of 1000 mg/kg. Aniline p-hydroxylase activity was elevated by ischemia/reperfusion, and this was prevented by AA doses of 100, 300 and 1000 mg/kg.

CONCLUSIONS

Ischemia/reperfusion diminishes the hepatic secretory and microsomal functions. AA has both antioxidant and pro-oxidant effects, depending upon the dose.

The involvement of singlet oxygen in copper-phenanthroline/H2O2-induced DNA base damage: a chemiluminescent study

Copper in the presence of excess 1,10-phenanthroline, a reducing agent, and H2O2 causes DNA base damage as well as strand breakage. We have reported in previous work that a strong chemiluminescence was followed by DNA base damage in this system, which is characteristic of guanine. In the present work, the mechanism of the chemiluminescence was studied. Results show that the luminescence was inhibited by all three classes of reactive oxygen species (*OH, O2-, (1)O2) scavengers to different degrees. Singlet oxygen scavengers showed the most powerful inhibition while the other two classes of scavengers were relatively weaker. The emission intensity in D2O was 3-fold that in H2O. Comparing the effect of scavengers on the luminescence of DNA with that of dGMP, the ratio of inhibition was similar. On the other hand, DNA breakage analysis showed that inhibition by the singlet oxygen scavenger NaN3 of strand breakage was strong and comparable to that of the scavengers of the two oxygen radicals. The results suggest that singlet oxygen may be a major factor for the chemiluminescence of guanine, while DNA strand breakage may be caused by many active species.

Reactive oxygen species generated from the reaction of copper(II) complexes with biological reductants cause DNA strand scission

The generation of hydroxyl radicals (.OH) from the reaction of Cu(II) complexes with biological reductants such as ascorbic acid, glutathione, acetylcysteine, and hydroquinone was confirmed by spin-trapping experiments using electron spin resonance (ESR). The following Cu(II) complexes were used: Cu(II)-(CyHH)2 (CyHH, cyclo(L-histidyl-L-histidyl)), Cu(II)(OP)2 (OP, o-phenanthroline), Cu(II)(HGG) (HGG, L-histidyl-glycylglycine), and Cu(II)(en)2 (en, ethylenediamine). The methyl radical adduct of alpha-(pyridyl-4-N-oxide)-N-tert-butylnitrone (POBN-CH3) was obtained from the reaction of ascorbic acid with all Cu(II) complexes used here in the presence of a spin trap, POBN, and dimethyl sulfoxide, indicating the generation of .OH. Glutathione, N-acetylcysteine, and hydroquinone reacted with both Cu(II)(CyHH)2 and Cu(II)(OP)2 to generate POBN-CH3, while these reductants did not react with either Cu(II)(HGG) or Cu(II)(en)2. Interestingly, the formation of POBN-CH3 in the reaction of Cu(II)-(CyHH)2 with glutathione or N-acetylcysteine was found only at a Cu(II)(CyHH)2/glutathione or Cu(II)(CyHH)2/N-acetylcysteine ratio of 1. The DNA strand scission caused by reaction mixtures of Cu(II) complexes with reductants was investigated under the same conditions as the ESR spin-trapping experiments. Addition of ascorbic acid to mixtures of these four Cu(II) complexes and DNA resulted in DNA strand breakage. Hydroquinone plus Cu(II)(CyHH)2 also caused DNA strand scission. In addition, DNA strand breakage was observed with the reaction of Cu(II)(OP)2 with glutathione, N-acetylcysteine, and hydroquinone. In contrast, reaction mixtures of glutathione, N-acetylcysteine, or hydroquinone with Cu(II)-(HGG) or Cu(II)(en)2 did not cause DNA strand scission within the concentration range used. The results obtained here suggest that there is a good correlation between POBN-CH3 formation and DNA strand scission. Thus, DNA strand scission may be caused by .OH generated from the reaction of some Cu(II) complexes with biological reductants under aerobic conditions. Since ascorbic acid, glutathione, and N-acetylcysteine are present in living cells, some Cu(II) complexes may be capable of initiating DNA damage in the presence of these reductants.

Copper(II) interactions with an experimental antiviral agent, I-deoxynojirimycin, and oxygen activation by resulting complexes

1-deoxynojirimycin (DNJ), a 5-imino analog of 1-deoxyglucose, is a potent inhibitor of alpha-glucosidase 1. DNJ and its derivatives have been considered as experimental drugs against human HIV-1 and hepatitis B viruses. Since amino and imino ligands have a high affinity for copper, it seems possible that biological activity of DNJ may be, at least in part, modulated by tissue copper. To test this possibility, potentiometric and spectroscopic studies of the complexation of DNJ by cupric ions were performed in order to obtain thermodynamic and structural background for further pharmacologic investigations. The effect of histidine, a major tissue copper carrier, on coordination equilibria was also studied. Results indicate that DNJ and Cu(II) form two stable complexes at physiological pH, CuH-1(DNJ)2+ and CuH-2(DNJ)2, involving Cu(II) chelation by the N-5 and O-6 donor atoms. In the presence of histidine, ternary complexes are also formed, of which the CuDNJHis+ species is stable in the physiological pH range. Binary Cu(II)-DNJ complexes are extremely effective mediators of in vitro oxidation of the guanine moiety in both 2'-deoxyguanosine (dG) and DNA to 8-oxoguanine (8-oxo-dG) and of DNA double strand scission by ambient O2 or H2O2. This mediation is suppressed by histidine in dG, but not in DNA. The results suggest that tissue Cu(II) may greatly enhance nonspecific cytotoxic effects of systemically administered DNJ through oxidative damage mechanisms, and therefore the prospective use of DNJ for therapeutic purposes must be developed with caution. On the other hand, however, the expected high genotoxic potential of synthetic Cu(II)-DNJ complexes may be used against viruses by means of targeted delivery of these complexes to the infected cells.

Cupric ion/ascorbate/hydrogen peroxide-induced DNA damage: DNA-bound copper ion primarily induces base modifications

The kinetics of frank DNA strand breaks and DNA base modifications produced by Cu(II)/ascorbate/H2O2 were simultaneously determined in purified human genomic DNA in vitro. Modified bases were determined by cleavage with Escherichia coli enzymes Nth protein (modified pyrimidines) and Fpg protein (modified purines). Single-stranded lesion frequency before (frank strand breaks) and after (modified bases) Nth or Fpg protein digestion was quantified by neutral glyoxal gel electrophoresis. Dialysis of EDTA-treated genomic DNA purified by standard proteinase K digestion/phenol extraction was necessary to remove low molecular weight species, probably transition metal ions and metal ion chelators, which supported frank strand breaks in the presence of ascorbate + H2O2 without supplemental copper ions. We then established a kinetic model of the DNA-damaging reactions caused by Cu(II) + ascorbate + H2O2. The principal new assumption in our model was that DNA base modifications were caused exclusively by DNA-bound Cu(I) and frank strand breaks by non-DNA-bound Cu(I). The model was simulated by computer using published rate constants. The computer simulation quantitatively predicted: (1) the rate of H2O2 degradation, which was measured using an H2O2-sensitive electrode, (2) the linearity of accumulation of DNA strand breaks and modified bases over the reaction period, (3) the rate of modified base accumulation, and (4) the dependence of modified base and frank strand production on initial Cu(II) concentration. The simulation significantly overestimated the rate of frank strand break accumulation, suggesting either that the ultimate oxidizing species that attacks the sugar-phosphate backbone is a less-reactive species than the hydroxyl radical used in the model and/or an unidentified hydroxyl radical-scavenging species was present in the reactions. Our experimental data are consistent with a model of copper ion-DNA interaction in which DNA-bound Cu(I) primarily mediates DNA base modifications and nonbound Cu(I) primarily mediates frank strand break production.

Effects of anti-free radical interventions on phosphatidylcholine hydroperoxide in plasma after ischemia-reperfusion in the liver of rats

The present study set out to investigate whether plasma phosphatidylcholine hydroperoxide (PCOOH) levels could accurately reflect lipid peroxidation linking to liver damage due to ischemia--reperfusion. PCOOH is a primary peroxidative product of phosphatidylcholine (PC), which is the most important functional lipid in the hepatocellular membrane, and may mediate oxidative stress. We quantified PCOOH and PC in the plasma and liver of rats subjected to hepatic ischemia-reperfusion by chemiluminescence detecting HPLC (CL-HPLC) method. Plasma PCOOH levels showed no significant rise in either the ischemia only group or in the sham-operation group, compared to controls (0.7 nmol/mL plasma). At 60 min subsequent to reperfusion, the PCOOH levels in plasma and liver, as well as the levels of several serum markers of liver injury [lactic dehydrogenase (LDH), glutamic-oxalacetic transaminase (GOT), glutamic-pyruvic transaminase (GPT)] increased in proportion to the duration of ischemia (up to 60 min). During periods of reperfusion following 30 min of ischemia, plasma PCOOH increased biphasically (2 nmol/mL; 12-24 hr duration of reperfusion), and generally ran parallel to that in the liver after more than 60 min of reperfusion. Dose-dependent protective effects against warm ischemia (30 min)-reperfusion (12 hr) injury were clearly demonstrated in the groups treated with allopurinol, diclofenac Na, ascorbic acid (V.C), alpha-tocopherol and coenzyme Q10, but not in those treated with r-h-superoxide dismutase or betamethasone. The rises in plasma PCOOH and serum GOT, GPT and LDH of the ischemia-reperfused rats were ameliorated most in the group pretreated with diclofenac Na, and next most in the group pretreated with V.C. These results indicate that the plasma PCOOH levels are a useful index both for liver cell damage induced by oxygen free radicals generated during ischemia-reperfusion, and to investigate the efficacy of drugs against oxidative stress.

Reversal by nickel(II) of inhibitory effects of some scavengers of active oxygen species upon hydroxylation of 2'-deoxyguanosine in vitro

Effects of ethanol (EtOH), mannitol (Man), L-histidine (His) and glutathione (GSH) on the oxidation of 2'-deoxyguanosine (dG) to its 8-hydroxy derivative (8-OH-dG) with H2O2 plus L-ascorbic acid (Ascb) in the absence and presence of Ni(II) were investigated in order to unveil the nature of active oxygen species involved in that oxidation. In the absence of Ni(II), production of 8-OH-dG was inhibited by His much greater than GSH greater than or equal to GSSG (oxidized glutathione) much greater than EtOH, but not by Man. The latter tended to enhance the production of 8-OH-dG. In the presence of Ni(II), the inhibition by His, GSH and GSSG, but not EtOH, was prevented. The results indicate involvement of a 'crypto-hydroxyl' radical as the dG oxidizing species in both the absence and presence of Ni(II). Also, the results provide evidence that Ni(II) complexes with His, GSH and GSSG may lack antioxidant capacity. Moreover, the Ni(II) complex with His was found capable of enhancing 8-OH-dG production by the Ascb+H2O2 system to a greater extent than Ni(II) alone. Likewise, although to a lesser extent, the formation of 8-OH-dG was enhanced by the combination of Ni(II) and Man which do not form complexes at pH 7.4. Since His is a major Ni(II) carrier in animal tissues, the dG oxidation enhancing capacity of the Ni(II) complex with His may contribute to the toxic and carcinogenic effects of Ni(II).

Synergistic effects of Cu(II) and dimethylammonium 2,4-dichlorophenoxyacetate (U46 D fluid) on PM2 DNA and mechanism of DNA damage

Dimethylammonium 2,4-dichlorophenoxyacetate (2,4-D . DMA) induced strand breaks in PM2 DNA when incubated with CuCl2, whereas 2,4-D . DMA alone or CuCl2 alone did not show any or only a negligible effect. The formation of single strand breaks increased linearly with time and concentration of 2,4-D . DMA. Neocuproine, a specific Cu(I) chelator totally prevented strand break formation. So did catalase (up to 100 mM 2,4-D . DMA), but DMSO had only a small protective effect. 2,4-Dichlorophenol, CO2 and formaldehyde were detected as reaction products of 2,4-D and CuCl2. From these results a redox reaction of Cu(II) and 2,4-D is proposed, which could explain the DNA damaging properties of CuCl2/2,4-D . DMA.

Synergistic effects of U46 D fluid (dimethylammonium salt of 2,4-D) and CuCl2 on cytotoxicity and DNA repair in human fibroblasts

The cytotoxicity of U46 D Fluid was tested in human fibroblasts after pretreatment with non-toxic or slightly toxic concentrations of CuCl2. While cell survival, colony-forming ability and protein synthesis were not affected by pretreatment with CuCl2, the inhibition of cell growth was enhanced as was inhibition of DNA synthesis. Synergistic effects of CuCl2 and U46 D Fluid were also detected on the induction of DNA repair measured by unscheduled DNA synthesis. While neither U46 D Fluid nor CuCl2 alone induced DNA repair, preincubation with CuCl2 followed by treatment with U46 D Fluid strongly provoked DNA repair.

Bactericidal activity of catecholamine copper complexes

Washed or growing E. coli cells are killed by epinephrine, norepinephrine or dopamine in the presence of non lethal concentrations of Cu(II). Killing is enhanced by anoxia and by sublethal concentrations of H2O2. The rate of killing is proportional to the rate of catecholamine oxidation. The copper epinephrine complex binds to E. coli cells, induces membrane damage and depletion of the cellular ATP pool. The cells may be partially protected by SOD or catalase but not by OH radical scavengers. Addition of H2O2 to cells which were sensitized by preincubation with the epinephrine-copper complex, causes rapid killing and DNA degradation. Sensitized cells are not protected by BSA.

Hydrogen peroxide dependent oxidative degradation of DNA by copper epinephrine

The hydrogen peroxide dependent oxidation of the epinephrine-copper complex to adrenochrome is mediated by free copper ions. The oxidation is enhanced by chloride ions and by the presence of serum albumin. The reaction is not inhibited by SOD or by hydroxyl radical scavengers. The 2:1 epinephrine or dopamine:Cu(II) complexes are able to bind to DNA and to catalyze its oxidative destruction in the presence of hydrogen peroxide. The DNA-epinephrine-Cu(II) terenary complex has characteristic spectral properties. It has the capacity to catalyze the reduction of oxygen or H2O2 and it preserves the capacity over a wide range of complex:DNA ratios. The rate of DNA clevage is proportional to the rate of epinephrine oxidation and the rate determining step of the reaction seems to be the reduction of free Cu(II) ions. The ability to form redox active stable DNA ternary complexes, suggests that under specific physiological conditions, when "free" copper ions are available, catecholamines may induce oxidative degradation of DNA and other biological macromolecules.

The protective role of neocuproine against cardiac damage in isolated perfused rat hearts

The effect of neocuproine on cardiac injury was studied using retrogradely perfused isolated rat hearts in two experimental systems. In the first system, where hydrogen peroxide-induced damage was studied, neocuproine at the range of 40-175 microM provided protection at the level of 70-85%, as demonstrated by the reduced loss in the peak systolic pressure (P), in +dP/dt and in -dP/dt. In the second system, where ischemia/reperfusion-induced arrhythmias were studied, neocuproine (42 microM) provided a marked protection against cardiac injury as demonstrated by the lowering of the incidence in irreversible ventricular fibrillation, by decreasing the duration of ventricular fibrillation and by the concomitant increase of the duration of normal sinus rhythm, and by improving the post-ischemic recovery of P, +dP/dt and -dP/dt. Free radicals have already been implicated as causative agents in cardiac injury resulting from either hydrogen peroxide or ischemia followed by reperfusion. Additionally, iron and copper have already been shown to drastically exacerbate the injurious effects of free radicals. Thus, the results reported here with neocuproine, a highly effective chelator for both iron and copper, as well as with adventitious copper and with the combination of neocuproine and copper, are in accord with the mediatory role of transition metals in enhancing the deleterious effects induced by free radicals.

Haemoglobin and myoglobin as inhibitors of hydroxyl radical generation in a model system of "iron redox" cycle

Methionine was oxidized to ethylene by an "Iron Redox" system containing H2O2, Fe-EDTA and ascorbate, generating hydroxyl radicals or another species of similar reactivity. Oxy or met forms of haemoglobin and myoglobin were found to inhibit methionine oxidation. Methionine oxidation was elevated in the "Iron Redox" system by increasing ascorbic acid concentration. However, in the presence of metmyoglobin or methaemoglobin, the increases in ascorbic acid did not lower the haemproteins' inhibitory effects but rather increased them. The pro-oxidative or anti-oxidative activities of haemproteins in biological oxidative reactions seem to be dependent on compartmentalization and on the presence and concentrations of reducing compounds and H2O2.