Hydroxyl radical formation from the auto-reduction of a ferric citrate complex

Near UV and Visible Light Photodegradation in Solid Formulations: Generation of Carbon Dioxide Radical Anions from Citrate Buffer and Fe(III)

Near UV and visible light photodegradation can target therapeutic proteins during manufacturing and storage. While the underlying photodegradation pathways are frequently not well-understood, one important aspect of consideration is the formulation, specifically the formulation buffer. Citrate is a common buffer for biopharmaceutical formulations, which can complex with transition metals, such as Fe(III). In an aqueous solution, the exposure of such complexes to light leads to the formation of the carbon dioxide radical anion (•CO2-), a powerful reductant. However, few studies have characterized such processes in solid formulations. Here, we show that solid citrate formulations containing Fe(III) lead to the photochemical formation of •CO2-, identified through DMPO spin trapping and HPLC-MS/MS analysis. Factors such as buffers, the availability of oxygen, excipients, and manufacturing processes of solid formulations were evaluated for their effect on the formation of •CO2- and other radicals such as •OH.

Near-UV and Visible Light Degradation of Iron (III)-Containing Citrate Buffer: Formation of Carbon Dioxide Radical Anion via Fragmentation of a Sterically Hindered Alkoxyl Radical

Citrate is a commonly used buffer in pharmaceutical formulations which forms complexes with adventitious metals such as Fe³⁺. Fe³⁺-citrate complexes can act as potent photosensitizers under near-UV and visible light exposure, and recent studies reported evidence for the photo-production of a powerful reductant, carbon dioxide radical anion (^•CO₂^-), from Fe³⁺-citrate complexes (Subelzu, N.; Schöneich, N., Mol. Pharm. 2020, 17, 4163-4179). The mechanisms of ^•CO₂^- formation are currently unknown but must be established to devise strategies against ^•CO₂^- formation in pharmaceutical formulations which rely on the use of citrate buffer. In this study, we first established complementary evidence for the photolytic generation of ^•CO₂^- from Fe³⁺-citrate through spin trapping and electron paramagnetic resonance (EPR) spectroscopy, and subsequently used spin trapping in conjunction with tandem mass spectrometry (MS/MS) for mechanistic studies on the pathways of ^•CO₂^- formation. Experiments with stable isotope-labeled citrate suggest that the central carboxylate group of citrate is the major source of ^•CO₂^-. Competition studies with various inhibitors (alcohols and dimethyl sulfoxide) reveal two mechanisms of ^•CO₂^- formation, where one pathway involves β-cleavage of a sterically hindered alkoxyl radical generated from the hydroxyl group of citrate.

Effects of exogenous organic acids and flooding on root exudates, rhizosphere bacterial community structure, and iron plaque formation in Kandelia obovata seedlings

The rhizosphere of coastal wetland plants is the active interface of iron (Fe) redox transformation. However, coupling mechanism between organic acids (OAs) exuded by plant roots and Fe speciation transformation participated by Fe redox cycling bacteria in the rhizosphere is still unclear. Effects of four common OAs (citric acid, malic acid, tartaric acid, and oxalic acid) on root exudation, rhizosphere bacterial community structure, root Fe plaque, and Fe redox cycling bacterial communities of Kandelia obovata were investigated in this study. Long-term flooding (10 h) was conducive to K. obovata seedlings exuding additional dissolved organic carbon (DOC) and nitrogen and phosphorus organic matter (NH₄⁺-N, NO₃^--N, and dissolved inorganic phosphorus [DIP]) under each OA level. DOC, NH₄⁺-N, NO₃^--N, and DIP in root exudates increased significantly with the increase of exogenous OA level. Notably, long flooding time corresponds to an evidently increasing trend. Exogenous OAs also significantly increased contents of formic and oxalic acids in root exudates. Exogenous OAs and flooding enhanced the rhizosphere effect of K. obovata and significantly enhanced bacterial diversity of the rhizosphere and relative abundance of dominant bacteria in rhizoplane. Bacterial diversity in the rhizosphere of K. obovata seedlings was significantly higher than that in the rhizoplane under the same level of OAs and flooding. Fe plaque content of K. obovata root decreased significantly and the relative abundance of typical Fe-oxidizing bacteria, such as Gallionella, unclassified_f__Gallionellaceae, and Sideroxydans, decreased significantly in the rhizosphere but increased significantly in the rhizoplane with the increase of the treatment level of exogenous OAs. This finding is likely due to the Fe³⁺ reduction caused by acidification of rhizosphere environment after exogenous OA treatment rather than the result of chemotactic colonization of Fe redox cycling bacteria in the rhizoplane.

The reduction of Cr(VI) to Cr(III) mediated by environmentally relevant carboxylic acids: State-of-the-art and perspectives

The detoxification process mediated by carboxylic acids (CAs) has received considerable spotlights since CAs are clean reagent and ubiquitous in the natural environments and effluents. Here, we present an exhaustive review on surface-bound/dissolved metals-catalyzed Cr(VI) reduction by CAs and CAs-mediated Cr(VI) reduction by many highly/poorly reductive reagents. The overall mechanisms of Cr(VI) reduction are mainly associated with the coordination of CAs with surface-bound/dissolved metals or Cr(VI,V,IV) species and the electron donating abilities of CAs. Additionally, the general decays of intermediate Cr(V,IV) complexes are clearly emerged in the Cr(VI) reduction processes. The performance of various reaction systems for Cr(VI) reduction that is greatly dependent on the operation parameters, including solution pH, reagent concentration, temperature, coexisting ions and gas atmosphere, are also critically commented. From the study survey presented herein, CAs-mediated Cr(VI) reduction processes exhibit good potential for remediation of various Cr(VI)-contaminated waters/sites. However, there is still a need to address the remained bottle-necks and challenges for the remediation of Cr(VI) mediated by CAs in the related natural attenuation cases and the treatment of industrial effluents. Overall, the present review offers the comprehensive understanding of the Cr(VI) reduction mediated by CAs and provide the engineering community with the guidelines for Cr(VI) remediation in the real-world applications.

Hydrogen Peroxide Participates in Perception and Transduction of Cold Stress Signal in Synechocystis

The double mutant ΔkatG/tpx of cyanobacterium Synechocystis sp. strain PCC 6803, defective in the anti-oxidative enzymes catalase (KatG) and thioredoxin peroxidase (Tpx), is unable to grow in the presence of exogenous H2O2. The ΔkatG/tpx mutant is shown to be extremely sensitive to very low concentrations of H2O2, especially when intensified with cold stress. Analysis of gene expression in both wild-type and ΔkatG/tpx mutant cells treated by combined cold/oxidative stress revealed that H2O2 participates in regulation of expression of cold-responsive genes, affecting either signal perception or transduction. The central role of a transmembrane stress-sensing histidine kinase Hik33 in the cold/oxidative signal transduction pathway is discussed.

Labile iron potentiates ascorbate-dependent reduction and mobilization of ferritin iron

Ascorbate mobilizes iron from equine spleen ferritin by two separate processes. Ascorbate alone mobilizes ferritin iron with an apparent K_{m (ascorbate)} ≈1.5mM. Labile iron >2μM, complexed with citrate (10mM), synergises ascorbate-dependent iron mobilization by decreasing the apparent K_{m (ascorbate)} to ≈270μM and raising maximal mobilization rate by ≈5-fold. Catalase reduces the apparent K_m(ascorbate) for both ascorbate and ascorbate+iron dependent mobilization by ≈80%. Iron mobilization by ascorbate alone has a higher activation energy (E_a=45.0±5.5kJ/mole) than when mediated by ascorbate with labile iron (10μM) (E_a=13.7±2.2kJ/mole); also mobilization by iron-ascorbate has a three-fold higher pH sensitivity (pH range 6.0-8.0) than with ascorbate alone. Hydrogen peroxide inhibits ascorbate's iron mobilizing action. EPR and autochemiluminescence studies show that ascorbate and labile iron within ferritin enhances radical formation, whereas ascorbate alone produces negligible radicals. These findings suggest that iron catalysed single electron transfer reactions from ascorbate, involving ascorbate or superoxide and possibly ferroxidase tyrosine radicals, accelerate iron mobilization from the ferroxidase centre more than EPR silent, bi-dentate two-electron transfers. These differing modes of electron transference from ascorbate mirror the known mono and bidentate oxidation reactions of dioxygen and hydrogen peroxide with di-ferrous iron at the ferroxidase centre. This study implies that labile iron, at physiological pH, complexed with citrate, synergises iron mobilization from ferritin by ascorbate (50-4000μM). This autocatalytic process can exacerbate oxidative stress in ferritin-containing inflamed tissue.

Brain zinc chelation by diethyldithiocarbamate increased the behavioral and mitochondrial damages in zebrafish subjected to hypoxia

The increase in brain levels of chelatable zinc (Zn) in dysfunctions involving oxygen deprivation has stimulated the treatment with Zn chelators, such as diethyldithiocarbamate (DEDTC). However, DEDTC is a redox-active compound and it should be better evaluated during hypoxia. We use the hypoxia model in zebrafish to evaluate DEDTC effects. The exploratory behavior, chelatable Zn content, activities of mitochondrial dehydrogenases, reactive species levels (nitric oxide, superoxide anion, hydroxyl radical scavenger capacity) and cellular antioxidants (sulfhydryl, superoxide dismutase) of zebrafish brain were assessed after recovery, with or without 0.2 mM DEDTC. The increased brain levels of chelatable Zn induced by hypoxia were mitigated by DEDTC. However, the novel tank task indicated that DEDTC did further enhance the exploratory deficit caused by hypoxia. Furthermore, these behavioral impairments caused by DEDTC were more associated with a negative action on mitochondrial activity and brain oxidative balance. Thus, due to apparent pro-oxidant action of DEDTC, our data do not support its use for neuroprotection in neuropathologies involving oxygen deprivation.

Hydrogen peroxide decomposition in bicarbonate solution catalyzed by ferric citrate*This article has a companion paper in this issue (doi: 10.1139/v11-080)

The peroxymonocarbonate mono- and di-anions (HCO4–and CO42–) are known to be generated from H2O2/HCO3–. They are promising oxidants for wood pulp bleaching, but peroxide decomposition catalyzed by ferric complexes can be significant for pulps whose lignin is highly reactive. Dicarboxylates from lignin peroxidation are believed to be the ferric chelators in the pH 8.5 range that is optimum for H2O2/HCO3–. This investigation aimed to see if HCO3–addition caused destabilization of the peroxygen system owing to its partial conversion to HCO4–. This anionic peracid is a much stronger oxidant than H2O2and could lead to a higher rate of Fe(II) oxidation to Fe(III) and (or) Fe(IV). For most free radical chain mechanisms, an increase in Fe(II) oxidation results in a higher rate of peroxide decomposition. Based on the kinetic data that were obtained and theoretical analyses, it was concluded that HCO4–did not significantly destabilize the peroxygen system when citrate was used as a model chelator for Fe(III). Increasing the [HCO3–] fourfold from 0.025 to 0.10 mol/L caused the decomposition rate to increase by only 20%.

Clastogenic effects of food additive citric acid in human peripheral lymphocytes

Clastogenic properties of the food additive citric acid, commonly used as an antioxidant, were analysed in human peripheral blood lymphocytes. Citric acid induced a significant increase of chromosomal aberrations (CAs) at all the concentrations and treatment periods tested. Citric acid significantly decreased mitotic index (MI) at 100 and 200 mug ml(-1) concentrations at 24 h, and in all concentrations at 48 h. However, it did not decrease the replication index (RI) significantly. Citric acid also significantly increased sister chromatid exchanges (SCEs) at 100 and 200 mug ml(-1) concentrations at 24 h, and in all concentrations at 48 h. This chemical significantly increased the micronuclei frequency (MN) compared to the negative control. It also decreased the cytokinesis-block proliferation index (CBPI), but this result was not statistically significant.

Iron-citrate complexes and free radicals generation: Is citric acid an innocent additive in foods and drinks?

The generation of free radicals (Fenton chemistry) from various iron citrate complexes has been studied. Spin trapping methods have been used. The results can question concerning the innocence of added citric acid in foods and cold drinks. We concluded that in absence of pathological situation citric acid is probably not dangerous but it may become dangerous in situation of oxidative stress and/or iron overload.

Influence of ascorbic acid on bonding of peroxide-affected dentin and 4-META/MMA-TBB resin

The purpose of this study was to evaluate the tensile bond strength (TBS) to peroxide-exposed dentin. Furthermore, the effect of ascorbic acid (AA) on the bond strength of peroxide-exposed dentin was investigated. Extracted bovine dentin was exposed to 10% carbamide peroxide, 30% hydrogen peroxide, or distilled water for 30 min, then treated with 10% AA (0, 30, 90, and 180 min), and conditioned with 10% citric acid/3% ferric chloride. The polymethyl-methacrylate (PMMA) rod was bonded to the treated bovine dentin with 4-META/MMA-TBB resin. A minidumbbell-shaped bonded specimen was prepared from these bonded assemblies and the TBS was tested. The fractured surfaces were also observed with a scanning electron microscope. Exposure to peroxide before bonding significantly reduced bond strength. The application of AA to the peroxide-exposed dentin increased bond strength. On the other hand, an adverse effect of AA was found in distilled water-affected dentin. Extended resin fibers were partially seen in the peroxide-exposed dentin. In conclusion, peroxide reduced the bond strength, and the stronger the oxidation, the weaker the obtained bond. Antioxidation with AA recovered the bond strength, and this effect increased the longer the AA was applied.

Mitochondrial DNA damage associated with lipid peroxidation of the mitochondrial membrane induced by Fe2+-citrate

Iron imbalance/accumulation has been implicated in oxidative injury associated with many degenerative diseases such as hereditary hemochromatosis, beta-thalassemia, and Friedreich's ataxia. Mitochondria are particularly sensitive to iron-induced oxidative stress - high loads of iron cause extensive lipid peroxidation and membrane permeabilization in isolated mitochondria. Here we detected and characterized mitochondrial DNA damage in isolated rat liver mitochondria exposed to a Fe2+-citrate complex, a small molecular weight complex. Intense DNA fragmentation was induced after the incubation of mitochondria with the iron complex. The detection of 3' phosphoglycolate ends at the mtDNA strand breaks by a 32P-postlabeling assay, suggested the involvement of hydroxyl radical in the DNA fragmentation induced by Fe2+-citrate. Increased levels of 8-oxo-7,8-dihydro-2'-deoxyguanosine also suggested that Fe2+-citrate-induced oxidative stress causes mitochondrial DNA damage. In conclusion, our results show that iron-mediated lipid peroxidation was associated with intense mtDNA damage derived from the direct attack of reactive oxygen species.

Hydroxyl radical formation resulting from the interaction of nickel complexes of L-histidine, glutathione or L-cysteine and hydrogen peroxide

L-histidine, L-cysteine, reduced glutathione (GSH) and other bioligands, which are ubiquitously present in biological systems, are recognized as antioxidants. Studies have shown that nickel (II) complexed with these ligands catalyzes the disproportionation of H2O2, leading to the generation of hydroxyl radicals (OH radical). However, none of the studies could provide information regarding effective concentrations at which these ligands act either as pro-oxidant or antioxidant. Therefore, the observed paradoxical behaviour of biological antioxidants in nickel-induced oxidative response was evaluated. Benzoic acid (BA) is hydroxylated by OH radical to form highly fluorescent dihydroxy benzoate (OH-BA). We used this model to study the effect of nickel complexes of L-histidine, GSH or L-cysteine on the hydroxylation of BA. The concentration-dependent effect of L-histidine, GSH and L-cysteine, or nickel on the hydroxylation of BA was studied. The hydroxylation of BA was significantly enhanced up to 1:0.5 molar ratio (Ni:hist or GSH). However, beyond 1:0.5 molar ratios, histidine/GSH inhibited the hydroxylation and complete inhibition was observed at 1:1 molar ratios. Sorbitol and caffeic acid, considered as scavengers of hydroxyl radicals, inhibited nickel-induced hydroxylation of BA. The present study demonstrates paradoxical behaviour of these bioligands. They act as pro-oxidant at lower ligand ratios and as antioxidant at higher ligand ratios. The redox properties of nickel complexes with histidine, GSH or cysteine reported here may be crucial for the toxicity of nickel.

Krebs Cycle Intermediates Modulate Thiobarbituric Acid Reactive Species (TBARS) Production in Rat Brain In Vitro

The aim of this study was to investigate the effect of Krebs cycle intermediates on basal and quinolinic acid (QA)- or iron-induced TBARS production in brain membranes. Oxaloacetate, citrate, succinate and malate reduced significantly the basal and QA-induced TBARS production. The potency for basal TBARS inhibition was in the order (IC50 is given in parenthesis as mM) citrate (0.37) > oxaloacetate (1.33) = succinate (1.91) > > malate (12.74). alpha-Ketoglutarate caused an increase in TBARS production without modifying the QA-induced TBARS production. Cyanide (CN-) did not modify the basal or QA-induced TBARS production; however, CN- abolished the antioxidant effects of succinate. QA-induced TBARS production was enhanced by iron ions, and abolished by desferrioxamine (DFO). The intermediates used in this study, except for alpha-ketoglutarate, prevented iron-induced TBARS production. Oxaloacetate, citrate, alpha-ketoglutarate and malate, but no succinate and QA, exhibited significantly iron-chelating properties. Only alpha-ketoglutarate and oxaloacetate protected against hydrogen peroxide-induced deoxyribose degradation, while succinate and malate showed a modest effect against Fe2+/H2O2-induced deoxyribose degradation. Using heat-treated preparations citrate, malate and oxaloacetate protected against basal or QA-induced TBARS production, whereas alpha-ketoglutarate induced TBARS production. Succinate did not offer protection against basal or QA-induced TBARS production. These results suggest that oxaloacetate, malate, succinate, and citrate are effective antioxidants against basal and iron or QA-induced TBARS production, while alpha-ketoglutarate stimulates TBARS production. The mechanism through which Krebs cycle intermediates offer protection against TBARS production is distinct depending on the intermediate used. Thus, under pathological conditions such as ischemia, where citrate concentrations vary it can assume an important role as a modulator of oxidative stress associated with such situations.

Genetic analysis of iron citrate toxicity in yeast: implications for mammalian iron homeostasis

Deletion of the yeast homologue of frataxin, YFH1, results in mitochondrial iron accumulation and respiratory deficiency (petite formation). We used a genetic screen to identify mutants that modify iron-associated defects in respiratory activity in Deltayfh1 cells. A deletion in the peroxisomal citrate synthase CIT2 in Deltayfh1 cells decreased the rate of petite formation. Conversely, overexpression of CIT2 in Deltayfh1 cells increased the rate of respiratory loss. Citrate toxicity in Deltayfh1 cells was dependent on iron but was independent of mitochondrial respiration. Citrate toxicity was not restricted to iron-laden mitochondria but also occurred when iron accumulated in cytosol because of impaired vacuolar iron storage. These results suggest that high levels of citrate may promote iron-mediated tissue damage.

Kinetics of phosphate-mediated oxidation of ferrous iron and formation of 8-oxo-2'-deoxyguanosine in solutions of free 2'-deoxyguanosine and calf thymus DNA

Formation of 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dG) in solutions of free 2'-deoxyguanosine (dG) and calf thymus DNA (DNA) was compared for the diffusion-dependent and localised production of oxygen radicals from phosphate-mediated oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+). The oxidation of Fe2+ to Fe3+ was followed at 304 nm at pH 7.2 under aerobic conditions. Given that the concentration of Fe2+ >or=phosphate concentration, the rate of Fe2+ oxidation was significantly higher in DNA-phosphate as compared for the same concentration of inorganic phosphate. Phosphate catalysed oxidation of ferrous ions in solutions of dG or DNA led through the production of reactive oxygen species to the formation of 8-oxo-dG. The yield of 8-oxo-dG in solutions of dG or DNA correlated positively with the inorganic-/DNA-phosphate concentrations as well as with the concentrations of ferrous ions added. The yield of 8-oxo-dG per unit oxidised Fe2+ were similar for dG and DNA; thus, it differed markedly from radiation-induced 8-oxo-dG, where the yield in DNA was several fold higher. For DNA in solution, the localisation of the phosphate ferrous iron complex relative to the target is an important factor for the yield of 8-oxo-dG. This was supported from the observation that the yield of 8-oxo-dG in solutions of dG was significantly increased over that in DNA only when Fe2+ was oxidised in a high excess of inorganic phosphate (50 mM) and from the lower protection of DNA damage by the radical scavenger (hydroxymethyl)aminomethane (Tris)-HCl.

Promotion of copper excretion from the isolated rat heart attenuates postischemic cardiac oxidative injury

This study examined the role of Cu as a mediator of cardiac postischemic oxidative injury. Isolated rat hearts were subjected to 20 min of normothermic global ischemia, followed by 30 min of reperfusion; after 20 min of preischemic loading with Krebs-Henseleit buffer +/- 20 or 30 microM zinc-bis-histidinate (Zn-His2), 0.5 mM deferoxamine (DEF) or 42 microM neocuproine (NEO). Postischemic developed systolic pressure and rate-pressure product were highest and postischemic end-diastolic pressure was lowest in hearts treated with 20 or 30 microM Zn-His2 and 0.5 mM DEF. Cu efflux was significantly increased by 225 and 290% (end of preischemic loading), and 325 and 375% (immediate postischemic period) of control basal rates in hearts treated with 30 microM Zn-His2 and 0.5 mM DEF, respectively. NEO did not effect any of these parameters. By the end of ischemia, protein carbonyls were lowest in Zn-His2-treated hearts and highest in DEF-treated hearts when compared with control hearts. The results of this study suggest that removal of redox-active Cu before ischemia has beneficial effects, indicating a mediatory role in postischemic cardiac oxidative injury.

Neuroprotection by nitric oxide against hydroxyl radical-induced nigral neurotoxicity

We investigated the effects of nitric oxide on an in vitro and in vivo generation of hydroxyl radicals, and in vivo neurotoxicity caused by intranigral infusion of ferrous citrate in rats. The formation of hydroxyl radicals in vitro, without exogenous hydrogen peroxide, was dose-dependent. Some nitric oxide donors (e.g. sodium nitroprusside) stimulated, while others (nitroglycerin, diethylamine/nitric oxide, nitric oxide in Ringer's solution) suppressed hydroxyl radical generation in vitro. A significant increase in extra-cellular hydroxyl radicals was detected in a brain microdialysis study. Intranigral infusion of ferrous citrate caused long-lasting lipid peroxidation and dopamine depletion in the ipsilateral nigral region and striatum, respectively. Sub-acute dopamine depletion in the striatum was positively correlated with acute lipid peroxidation in substantia nigra. Intranigral administration of nitric oxide did not affect striatal dopamine. Interestingly, nitric oxide in Ringer's protected nigral neurones against the oxidative injury. The results demonstrate that a regional increase in the levels of iron can result in hydroxyl radical generation and lipid peroxidation leading to neurotoxicity. It also demonstrates that exogenous nitric oxide can act as hydroxyl radical scavenger and protect neurones from oxidative injury.

Reactive iron species in biological fluids activate the iron-sulphur cluster of aconitase

Low molecular mass iron (LMrFe) can appear in plasma when the transferrin becomes fully iron loaded. Such iron poses a risk factor for oxidative damage, and for microbial virulence. A previous novel approach to the detection and measurement of LMrFe in plasma was the use of the iron-binding properties of the glycopeptide antitumour antibiotic bleomycin and its ability to degrade DNA in the presence of oxygen, bound iron, and an iron reducing agent. Since bleomycin is a non-physiological ligand with iron-binding and redox cycling properties, it has been suggested that it may not be a valid biological model for detecting and measuring LMrFe. To address these concerns we have developed a biological approach to the detection and measurement of LMrFe based on the activation of iron-requiring aconitase. Parallel measurements, in a variety of clinical conditions in which there was a complete saturation of the plasma transferrin, showed that the bleomycin assay and the aconitase assay can give similar results for LMrFe.

Iron binding and autoreduction by citrate: are these involved in signalling by iron regulatory protein-1?

Ferric ions bind to citrate and undergo an autoreduction to form a ferrous-citrate complex, greatly increasing the redox activity of the iron complex. Ferrous ions and citrate are also essential for the enzymic activity of aconitase. Aconitase, with its iron-sulphur cluster has a versatile structure which allows it to act as an iron regulatory protein (IRP-1). The purpose of this study was to see whether iron binding, and its autoreduction by citrate, could play a physiological signalling role in iron regulation. Significant amounts of ferrous ions were associated with citrate, when measured using ferrozine, however, these did not appear to activate iron-requiring aconitase.

Phagomimetic action of antimicrobial agents

A wide variety of extracted and synthesised drug molecules have electron transfer capabilities which allow them to generate reactive oxygen species (ROS). In particular, many antibiotics that kill or inhibit bacteria, yeasts and cancer cells readily transfer electrons to oxygen making superoxide and hydrogen peroxide in the process. When suitable redox active forms of iron are available, Fenton chemistry occurs generating the highly damaging hydroxyl radical. This type of chemistry is very similar to that which evolved within phagocytic cells as part of their microbial killing armoury. Many antibiotics, when used in model systems, have well defined pharmacological actions against key cellular functions, but their clinical usefulness is also often demonstrable at concentrations in vivo well below their in vitro minimum inhibitory concentrations. These observations have led us to propose that a common mechanism exists whereby phagocytic cells and antibiotics exploit the use of ROS for microbial killing.

Lipid peroxidation: the role of Ca2+ and protection by calcinine

When calcinine (A-23187) (2 microM), a known Ca2+ ionophore, is present, a significant protection is observed to a mitochondrial suspension undergoing lipid peroxidation by Fe(2+)-citrate complex. A-23187 can remove Ca2+, which seems to have an important role in the lipid peroxidation process, from its 'lesive sites' and consequently preventing the damage. This information has importance in terms of knowing the mechanisms and avoiding the damages of lipid peroxidation that occur in some pathological cases such as tumor promotion and hemochromatosis.

Iron supplementation generates hydroxyl radical in vivo. An ESR spin-trapping investigation

Electron spin resonance (ESR) spectroscopy has been used to investigate hydroxyl radical generation in rats with chronic dietary iron loading. A secondary radical spin-trapping technique was used where hydroxyl radical forms methyl radical upon reaction with DMSO. The methyl radical was then detected by ESR spectroscopy as its adduct with the spin trap alpha-phenyl-N-t-butylnitrone (PBN). This adduct was detected in the bile of rats 10 wk after being fed an iron-loading diet and 40 min after the i.p. injection of the spin trap PBN dissolved in DMSO. Bile samples were collected into a solution of the ferrous stabilizing chelator 2,2'-dipyridyl in order to prevent the generation of radical adducts ex vivo during bile collection. Identification of the ESR spectrum of the major radical adduct as that of PBN/.CH3 provides evidence for the generation of the hydroxyl radical during iron supplementation. Desferal completely inhibited in vivo hydroxyl radical generation stimulated by high dietary iron intake. No radical adducts were detected in rats which were fed the control diet for the same period of time. This is the first evidence of hydroxyl radical generation in chronic iron-loaded rats.

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.

Ferrous-citrate complex and nigral degeneration: evidence for free-radical formation and lipid peroxidation

Increased nigral iron content in the parkinsonian brain is now well documented and is implicated in the pathogenesis of this movement disorder. Free iron in the pigmented DA-containing neurons catalyze DA autoxidation and Fenton reaction to produce cytotoxic .OH, initiating lipid peroxidation and consequent cell damage. The present results clearly demonstrate that a regional increase in the levels of the "labile iron pool" can result in the degeneration of dopaminergic nigral neurons as reflected by a significant inhibition in the expression of tyrosine hydroxylase mRNA and DA depletion. Iron-complex-induced damage of dopaminergic neurons in the substantia nigra, might have resulted from a sequence of cytotoxic events including the .OH generation and lipid peroxidation as demonstrated in this study. This free-radical-induced oxidative nigral injury may be a reliable free-radical model for studying parkinsonism and may be relevant to idiopathic Parkinson's disease. This apparent nigral injury stimulated by Fe(2+)-citrate is more severe than that produced by ferric iron and its citrate complex. Moreover, these data indicate that Fe(2+)-citrate is as potent as MPP+ in causing oxidative injury to the substantia nigral neurons. However, the nigral toxicity of MPTP and its congeners are not progressive, while Fe(2+)-citrate complex may produce a progressive degeneration of the nigrostriatal neurons which is similar to the progression of ideopathic Parkinson's disease. Thus, this unique Fe(2+)-citrate complex animal model could be used for studying neuroprotective treatments for retarding or halting the progressive nigrostriatal degeneration caused by free radicals in the iron-rich basal ganglia.

Ferrous ion formation by ferrioxamine prepared from aged desferrioxamine: a potential prooxidant property

The siderophore desferrioxamine (DEFOM) binds ferric ions in a 1:1 ratio resulting in a ferrioxamine (FOM) complex. When DEFOM is stored or heat degraded, the resulting FOMD undergoes an autoreduction with the transfer of electrons to the bound ferric ions forming ferrous ions which react with Ferrozine to yield a pink-coloured complex absorbing at 562 nm. Heat-aged DEFOM forms a FOMD complex with an absorption maxima changing from 432 nm to 441 nm. When the autoreduced FOMD complex is placed in a phosphate buffer at pH 7.4, ferrous ions autoxidise transferring electrons to molecular oxygen to form superoxide and hydrogen peroxide. Fenton chemistry leading to the formation of hydroxyl radicals can then occur. Studies with a variety of reactive oxygen scavengers support a role for the hydroxyl radical in damage to the detector molecule deoxyribose. However, when EDTA is present, damage to deoxyribose is decreased and the radicals causing deoxyribose degradation no longer appear to be characteristic of the hydroxyl radical.

New roles for quin2: powerful transition-metal ion chelator that inhibits copper-, but potentiates iron-driven, Fenton-type reactions

The objective of this study was to investigate whether quin2, through its metal chelating properties, could affect copper- or iron-driven Fenton reactions. Chelation of ferric ion with quin2 uniformly strongly enhanced the formation of oxidizing species, detected with the DMSO and deoxyribose assays, both by H2O2 and a mixture of superoxide/hydrogen peroxide produced by hypoxanthine/xanthine oxidase. Fe(3+)-EDTA gave the same effects, but lacked reactivity with bolus H2O2 as detected with the DMSO assay. Whereas the formation of oxidizing species with Fe(3+)-EDTA and ferric ions alone were strongly inhibited by superoxide dismutase both in the bolus H2O2 and hypoxanthine/xanthine oxidase systems, such formation in the presence of Fe(3+)-quin2 either did not decrease or decreased only moderately. Fe(3+)-quin2 also strongly enhanced plasmid DNA strand breakage in the presence of H2O2. Our findings suggest that quin2 as chelator of ferric ion may be a more powerful enhancer of oxidant formation than other chelators so far tested. The formation of oxidizing species from copper ions and bolus H2O2 was found to be fundamentally dependent on the choice of buffer system. We could only detect significant amounts of oxidants in both assays in Hepes buffer, but not in the phosphate, cacodylate or unbuffered systems, which all gave low reactivity in the DMSO assay compared to the deoxyribose assay. Quin2 chelation of cupric ion effectively inhibited the formation of oxidants as well as plasmid DNA strand breakage.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of zinc on superoxide-dependent hydroxyl radical production in vitro

Trace elements play an important role in oxygen metabolism and therefore in the formation of free radicals. Whereas iron and copper are usually the main enhancers of free radical formation, other trace elements, such as zinc and selenium, protect against the harmful effects of these radicals. To investigate the different protective mechanisms of zinc on radical formation, we examined the effects of added zinc and copper on superoxide dismutase activity. We also studied the effects of copper and iron on xanthine oxidase activity and on the Haber-Weiss cycle (iron, superoxide, and hydrogen peroxide), which generates hydroxyl radicals in vitro. The hypoxanthine/xanthine oxidase radical generating system contained a variety of different physiological ligands for binding the iron. This study confirmed the inhibitory effect of copper on xanthine oxidase activity. Moreover, it demonstrated that zinc inhibited hydroxyl radical formation when this formation was catalyzed by a citrate-iron complex in the hypoxanthine/xanthine oxidase reaction. Finally, human blood plasma inhibited citrate-iron-dependent hydroxyl radical formation under the same conditions. Although trace elements seemed responsible for this antioxidant activity of plasma, it is likely that zinc played no role as a plasma antioxidant. Indeed, calcium appeared to be responsible for most of this effect under our experimental conditions.