Lipid peroxidation and oxidation of several compounds by H2O2 activated metmyoglobin

Activated metmyoglobin (MetMb) by H₂O₂ initiates oxidation of microsomal unsaturated fatty acids, β-carotene and methional but not formate. Lipid peroxidation by activated MetMb was not inhibited by catalase. The activated species which initiaes lipid peroxidation appears to be a porphyrin cation radical, P _∸ ⁺ Fe^IV=O, and not a hydroxyl radical.     

Metals and lipid oxidation. Contemporary issues

Lipid oxidation is now recognized to be a critically important reaction in physiological and toxicological processes as well as in food products. This provides compelling reasons to understand what causes lipid oxidation in order to be able to prevent or control the reactions. Redox-active metals are major factors catalyzing lipid oxidation in biological systems. Classical mechanisms of direct electron transfer to double bonds by higher valence metals and of reduction of hydroperoxides by lower valence metals do not always account for patterns of metal catalysis of lipid oxidation in multiphasic or compartmentalized biological systems. To explain why oxidation kinetics, mechanisms, and products in molecular environments which are both chemically and physically complex often do not follow classical patterns predicted by model system studies, increased consideration must be given to five contemporary issues regarding metal catalysis of lipid oxidation: hypervalent non-heme iron or iron-oxygen complexes, heme catalysis mechanism(s), compartmentalization of reactions and lipid phase reactions of metals, effects of metals on product mixes, and factors affecting the mode of metal catalytic action. Based on a paper presented at the Symposium on Metals and Lipid Oxidation, held at the AOCS Annual Meeting in Baltimore, MD, April, 1990.     

Peroxidation of microsomal membrane protein—Lipid complexes

Nonenzymatic lipid peroxidation was studied using the TBA test on rat liver microsomal fractions, lipid micelles and structural protein-lipid micelle complexes. The kinetics, response to divalent cations, and iron-ascorbate catalysis were alike in the microsomal fraction and in the complex, but different in lipid micelles. The structural protein represented 41% of the total membrane protein, had a S_20,obs of 3.5 and was hydrophobic. The binding of lipid micelles by structural protein proceeded in two steps, with an initial fast rate followed by a slower rate. The binding appeared to involve a hyrophobic association between lipid and protein as evidenced by insensitivity to pH, ionic strength and lack of preference for the individual classes of phospholipid micelles. Deoxycholate caused an increase in the initial peroxidation rate in microsomal fractions. Iron and ascorbate catalyzed lipid peroxidation in both the microsomal fraction and in the complex. Iron catalyzed lipid peroxidation but calcium, cobalt and copper inhibited the reaction in the SP-lipid micelle complex. Lipid peroxidation in microsomal suspensions, therefore, appears to be determined, in part, by the hydrophobic nature of the protein-lipid association found in membranes.     

Lipid oxidation in meat and meat products—A review

Lipid oxidation is a major cause of deterioration in the quality of meat and meat products. Oxidation can occur in either the stored triglycerides or the tissue phospholipids. Ferric heme pigments have been implicated as the major prooxidants in tissue lipid oxidation. Pigment and lipid oxidation are interrelated, and ferric hemes are believed to promote lipid oxidation. The resulting oxidation destroys the hemes. Nonheme iron and ascorbic acid may also function as prooxidants in meat. Sodium chloride accelerates oxidation of the triglycerides, although the mechanism of salt catalysis is not completely known. Cooked meat undergoes rapid deterioration due to tissue lipid oxidation. The meat pigment in the cured pink ferrous form does not promote the rapid oxidation undergone by cooked uncured meat. Refrigerated and frozen fresh meats are also susceptible to lipid oxidation. Protein denaturation and cross-linking may result from lipid oxidation in stored freeze-dried meat. With increased consumption of prepackaged raw meat and precooked convenience meat items, control of oxidation has become increasingly important. Antioxidants and chelating agents are the most effective inhibitors of lipid oxidation. One of 28 papers presented at the Symposium, “Metal-Catalyzed Lipid Oxidation,” ISF-AOCS World Congress, Chicago, September 1970.     

Lipid peroxidation in rat tissue homogenates: Interaction of iron and ascorbic acid as the normal catalytic mechanism

Iron and ascorbic acid appear to be the normal catalytic components responsible for the lipid peroxidation reaction in aerobically incubated rat tissue homogenates. The amounts of each present in the catalytically-active fractions of rat liver, brain, testis, and kidney are appropriate to explain the lipid peroxidation reaction measured. Utilization of ascorbic acid as part of the normal catalytic mechanism is indicated by the following: The catalytic activity of the tissue soluble phase occurs only in the small molecule fraction eluted from Sephadex, and ascorbic acid occurs only in this fraction; the extent of catalysis by the small molecule fractions of the soluble phases from several tissues is proportional to their ascorbic acid content; and pH effect on lipid peroxidation is the same for both soluble-phase and ascorbic acid catalysis. Utilization of iron as part of the normal catalytic mechanism is indicated by EDTA inhibition studies and by measurements of pH effects. Previous studies have demonstrated the lack of catalytic activity by cations other than iron for the lipid peroxidation reaction in homogenates. Lipid peroxidation is inhibited at high tissue concentration and the inhibition is due to components occurring in the large molecule fraction of the soluble phase.     

Effect of heme compounds on lipid oxidation

By polarographic oxygen analyzer, the rate of oxidation of linoleate solutions was catalyzed by low concentrations of heme and heme-proteins and inhibited by higher concentrations. High concentrations of these substances also inhibited lipoxygenase catalysis of linoleate oxidation. Manganese and cobalt salts inhibited heme-catalyzed linoleate oxidation. These combined effects may reflect the oxidative synthesis of antioxidants from heme compounds. One of 28 papers presented at the Symposium, “Metal-Catalyzed Lipid Oxidation,” ISF-AOCS World Congress, Chicago, September 1970.     

Non-heme Fe(IV)-oxo intermediates

High-valent non-heme iron-oxo intermediates have been proposed for decades as the key intermediates in numerous biological oxidation reactions. In the past three years, the first direct characterization of such intermediates has been provided by studies of several alphaKG-dependent oxygenases that catalyze either hydroxylation or halogenation of their substrates. In each case, the Fe(IV)-oxo intermediate is implicated in cleavage of the aliphatic C-H bond to initiate hydroxylation or halogenation. The observation of non-heme Fe(IV)-oxo intermediates and Fe(II)-containing product(s) complexes with almost identical spectroscopic parameters in the reactions of two distantly related alphaKG-dependent hydroxylases suggests that members of this subfamily follow a conserved mechanism for substrate hydroxylation. In contrast, for the alphaKG-dependent non-heme iron halogenase, CytC3, two distinct Fe(IV) complexes form and decay together, suggesting that they are in rapid equilibrium. The existence of two distinct conformers of the Fe site may be the key factor accounting for the divergence of the halogenase reaction from the more usual hydroxylation pathway after C-H bond cleavage. Distinct transformations catalyzed by other mononuclear non-heme enzymes are likely also to involve initial C-H bond cleavage by Fe(IV)-oxo complexes, followed by diverging reactivities of the resulting Fe(III)-hydroxo/substrate radical intermediates.     

Thermal activation of peroxidase as a lipid oxidation catalyst

Lipid oxidation catalysis with peroxidase heat treated in vitro was strongly influenced by the pH at which the treatment was carried out, particularly in the range 5.5–6.5. Below pH 5 no increase in lipid oxidation activity occurred due to masking of the heme groups. Electron microscopy studies showed differences in size and shape of the thermal aggregates produced at pH 7.2 and 4.9. Increased lipid oxidation activity of peroxidase on heat treatment at pH 6.5 was almost exclusively associated with the aggregates, which were separated by gel chromatography from nonaggregated material containing the residual enzyme activity. Heme migration during heat treatment led to relatively higher heme content of the aggregates, thus increasing the number of catalytic sites. Thermal destruction of the heme group decreased its lipid oxidation activity.     

Dynamics of iron-ascorbate-induced lipid peroxidation in charged and uncharged phospholipid vesicles

Peroxidation of egg yolk phosphatidylcholine (egg PC) liposomes was induced by addition of ascorbic acid (AsA) and Fe(II) in the presence of a trace of autoxidized egg PC (PC−OOH), but not in the absence of PC−OOH. PC−OOH was degraded upon addition of AsA and Fe(II) but not of either one alone. The results suggest that PC−OOH is necessary to initiate lipid peroxidation by AsA/Fe(II). AsA oxidation in the bulk water phase was also associated with an increase in lipid peroxidation by AsA/Fe(II) in the presence of PC−OOH, but not in the absence of PC−OOH. Furthermore, the spin probe 12-NS [12-(N-oxyl-4,4′-dimethyloxazolidin-2-yl)stearic acid], which labels the hydrophobic region of dimyristoyl phosphatidylcholine (DMPC) liposomal membranes, was degraded upon addition of AsA and Fe(II) in the presence of PC−OOH, but not in the absence of PC−OOH. These results indicate that the “induction message” that is associated with decreases of PC−OOH and AsA in the initiation step of lipid peroxidation must be transferred from the membrane surface to the inner hydrophobic membrane region. AsA in the bulk phase was oxidized faster and more extensively upon its addition together with Fe(II) to egg PC liposomes than to DMPC liposomes, though the initial content of PC−OOH in the former was 5–10 times lower than in the latter. This suggests that, in egg PC liposomes, the OOH-groups of new PC−OOH generated in the inner membrane regions must become accessible from the surface, enabling reaction with AsA/Fe(II) which in turn would result in an extensive decrease in AsA. By contrast, in DMPC liposomes, that do not generate PC−OOH, AsA is only oxidized slightly in connection with the degradation of the PC−OOH initially present. The effect of surface charges on the membrane surface was also studied to obtain further information on the initiation step of lipid peroxidation. The rate of lipid peroxidation by AsA/Fe(II) or Fe(III) decreased in the order, egg PC liposomes ≫negatively charged egg PC liposomes containing dicetylphosphate>positively charge egg PC liposomes containing stearylamine. The rate of associated AsA oxidation was in the order, egg PC liposomes≫egg PC/stearylamine liposomes>egg PC/dicetylphosphate liposomes. However, in DMPC liposomes that do not generate PC−OOH, the rates of AsA oxidation associated with the reductive cleavage of PC−OOH by AsA/Fe(II) and coupled with the reduction of Fe(III) to Fe(II) were in the order, DMPC liposomes =DMPC/stearylamine liposomes≫DMPC/dicetylphosphate liposomes. These differences in the rates of lipid peroxidation, depending on differences in membrane charge, are discussed in relation to two properties of AsA: (i) its antioxidant property through trapping of lipid radicals and (ii) its prooxidant properties (a) by being an effective iron chelator thus altering the reactivity of iron with oxygen and peroxides and (b) by being an iron reductant and providing a source of Fe(II).     

Quinolinic Acid: Neurotoxin or Oxidative Stress Modulator?

Quinolinic acid (2,3-pyridinedicarboxylic acid, QUIN) is a well-known neurotoxin. Consequently, QUIN could produce reactive oxygen species (ROS). ROS are generated in reactions catalyzed by transition metals, especially iron (Fe). QUIN can form coordination complexes with iron. A combination of differential pulse voltammetry, deoxyribose degradation and Fe(II) autoxidation assays was used for explorating ROS formation in redox reactions that are catalyzed by iron in QUIN-Fe complexes. Differential pulse voltammetry showed an anodic shift of the iron redox potential if iron was liganded by QUIN. In the H₂O₂/FeCl₃/ascorbic acid variant of the deoxyribose degradation assay, the dose-response curve was U-shaped. In the FeCl₃/ascorbic acid variant, QUIN unambiguously showed antioxidant effects. In the Fe(II) autoxidation assay, QUIN decreased the rate of ROS production caused by Fe(II) oxidation. Our study confirms that QUIN toxicity may be caused by ROS generation via the Fenton reaction. This, however, applies only for unnaturally high concentrations that were used in attempts to provide support for the neurotoxic effect. In lower concentrations, we show that by liganding iron, QUIN affects the Fe(II)/Fe(III) ratios that are beneficial to homeostasis. Our results support the notion that redox chemistry can contribute to explaining the hormetic dose-response effects.     

Non‐Heme Imine‐Based Iron Complexes as Catalysts for Oxidative Processes

Non‐heme iron complexes are emerging as powerful and versatile catalysts in several oxidative transformations. The most investigated iron complex structures are based on aminopyridine ligands, but a number of imine‐based ligands have been also tested. In this review a collection of recent results obtained in oxidation catalysis with non‐heme imine‐based iron complexes is presented. Their catalytic performances in C H, CC and  S oxidation are spread over a wide range of efficiency, going from very low to quite high. Such performances are discussed, whenever possible, in light of the operating reaction mechanisms and of catalyst stability. In order to facilitate the discussion, an initial survey of the most useful mechanistic tools widely applied to distinguish a metal‐based oxidation from a radical‐chain process is also reported. Imine‐based catalysts are divided into two classes: (i) salen‐Fe complexes, and (ii) imine‐Fe complexes. In some cases clues for free‐radical oxidation mechanisms have been reported while in other cases evidence for metal‐based mechanisms has been collected. The preferred mechanistic pathway is shown to be a function of catalyst structure and features. Interestingly, some imine‐based iron complexes are able to perform stereospecific oxidation reactions, demonstrating that the imine functionality can be incorporated in ligands designed for oxidation catalysis.

     

Naturally occurring hydroxy napthoquinones and their iron complexes as modulators of radiation induced lipid peroxidation in synaptosomes

The modulation of radiation induced lipid peroxidation in synaptosomes by iron (II) and iron (III) complexes of two naturally occurring and therapeutically relevant naphthoquinones viz. 5,hydroxy-1,4 naphthoquinone; juglone and 2,hydroxy-1,4 naphthoquinone; lawsone, have been studied. At lower concentrations the complexes enhance lipid peroxidation predominantly through redox cycling as observed for Fe(II)- juglonate while at higher concentrations the complexes tend to limit lipid peroxidation through fast recombinations.     

Acceleration and inhibition of lipid oxidation by heme compounds

The acceleration and inhibition of unsaturated fatty acid oxidation by heme compounds was followed in model systems with an oxygen analyzer. The linoleate to heme molar ratios for maximum catalysis were 100 for hemin and catalase, 250 for metmyoglobin, 400 for cytochrome c and 500 for methemoglobin. With heme concentrations of 2 to 4 times the optimum catalytic amount, no oxidation occurred. Rapid heme destruction was observed with catalyzing ratios of lipid to heme, but with inhibitory ratios a stable red compound formed, believed to be a lipid hydroperoxide derivative of the heme. The ratios of lipid to metmyoglobin for maximum acceleration varied with the pH. Linolenate was much less sensitive to heme catalysis than linoleate. Colorless products of heme degradation had a marked antioxidant effect. A possible mechanism for the antioxidant effect of hemes is advanced, based on the formation of stable heme peroxide complexes or stable heme radicals, or both, during the early stages of oxidation. Prooxidant activity is believed to occur only when the peroxide to heme ratio is so high that the oxidation of the hemes goes beyond the initial stages.     

The improvement of boron-doped diamond anode system in electrochemical degradation of p-nitrophenol by zero-valent iron

Boron-doped diamond (BDD) electrodes are promising anode materials in electrochemical treatment of wastewaters containing bio-refractory organic compounds due to their strong oxidation capability and remarkable corrosion stability. In order to further improve the performance of BDD anode system, electrochemical degradation of p-nitrophenol were initially investigated at the BDD anode in the presence of zero-valent iron (ZVI). The results showed that under acidic condition, the performance of BDD anode system containing zero-valent iron (BDD-ZVI system) could be improved with the joint actions of electrochemical oxidation at the BDD anode (39.1%), Fenton's reaction (28.5%), oxidation–reduction at zero-valent iron (17.8%) and coagulation of iron hydroxides (14.6%). Moreover, it was found that under alkaline condition the performance of BDD-ZVI system was significantly enhanced, mainly due to the accelerated release of Fe(II) ions from ZVI and the enhanced oxidation of Fe(II) ions. The dissolved oxygen concentration was significantly reduced by reduction at the cathode, and consequently zero-valent iron corroded to Fe(II) ions in anaerobic highly alkaline environments. Furthermore, the oxidation of released Fe(II) ions to Fe(III) ions and high-valent iron species (e.g., FeO²⁺, FeO₄²⁻) was enhanced by direct electrochemical oxidation at BDD anode.     

Mechanism and kinetics analysis of the phenol degradation by Fe(Ⅱ)-EDTA/K2S2O8 process

考察了Fe(Ⅱ)-EDTA/K2S2O8体系降解苯酚的影响因素,并对体系中苯酚的降解机理和反应动力学进行了初步分析。结果表明,铁离子、氧化剂形态以及紫外光辐照、螯合剂的存在对氧化体系作用效果影响显著。EDTA剂量为0.5 mmol/L、Fe(Ⅱ)剂量为1.0 mmol/L、K2S2O8剂量为2.0 mmol/L、溶液初始pH值为7.0、温度为60 ℃的条件下反应60 min,浓度为100 mg/L的苯酚降解率为71.48%,同样条件下紫外光辐照时降解率可达89.38%。降解机理与动力学分析表明,苯酚在Fe(Ⅱ)-EDTA/K2S2O8体系中降解途径以与Fe4+络合降解为主。     

Effect of pH on the Oxidation of Ferrous Ion and Immobilization Technology of Iron Hydr(oxide) in Fluidized Bed Reactor

Abstract

New, efficient, and a cheap method for the removal of ferrous ion from aqueous in a fluidized bed reactor was developed. Different from the adsorption process in the treatment of iron species, the immobilization of iron oxide on support media in a fluidized bed never reaches saturation. Furthermore, the immobilized iron oxide is reusable in catalysis and adsorption. Silica sand (Si) and iron oxide (SiG) were employed as support media to remove Fe(II) from aqueous in a fluidized bed reactor. The oxidation rate of Fe(II) and the immobilization rate of iron oxide are strongly depend on pH value so the variation of solution pH is considered to be the major parameter. Furthermore, the aeration effect on iron removal efficiency is investigated. 97% of total iron was removed at pH 8 in the presence of SiG and 87% of total iron was removed at pH 6 in the presence of Si. When the initial pH was adjusted to 6 and was not adjusted during the reactions, the optimum total iron removal efficiency (97%) was found. The air aeration was provided to keep the dissolved oxygen in constant. Aeration air accelerates the oxidation of ferrous ion, but does not improve the total iron removal efficiency.     

Fe（Ⅱ）-EDTA/K2S2O8降解苯酚及反应动力学分析

考察了Fe(Ⅱ)-EDTA/K2S2O8体系降解苯酚的影响因素,并对体系中苯酚的降解机理和反应动力学进行了初步分析。结果表明,铁离子、氧化剂形态以及紫外光辐照、螯合剂的存在对氧化体系作用效果影响显著。EDTA剂量为0.5 mmol/L、Fe(Ⅱ)剂量为1.0 mmol/L、K2S2O8剂量为2.0 mmol/L、溶液初始pH值为7.0、温度为60℃的条件下反应60 min,浓度为100 mg/L的苯酚降解率为71.48%,同样条件下紫外光辐照时降解率可达89.38%。降解机理与动力学分析表明,苯酚在Fe(Ⅱ)-EDTA/K2S2O8体系中降解途径以与Fe4+络合降解为主。     

The role of Fe on PtPd oxidation catalyst prepared by simultaneous and sequential incipient wetnesses

The roles and effects of Fe on the catalytic performance and physicochemical properties of a PtPd diesel oxidation catalyst prepared by three different methods were investigated by CO oxidation reaction, X-ray diffraction, temperature-programmed reduction (TPR), temperature-programmed oxidation, and BET surface area. It was found that the roles of Fe depended strongly on the sequential order of Fe introduction during the preparation of the PtPd catalyst. The Fe/PtPd/Al₂O₃ catalyst was prepared by introducing Fe onto the PtPd/Al₂O₃, and the PtPd/Fe/Al₂O₃ catalyst was obtained by loading the PtPd onto the Fe/Al₂O₃. The former had a superior activity. From the TPR results, the catalytic activity of CO oxidation was correlated with the oxygen mobility of the iron oxides. For PtPd/Fe/Al₂O₃, the iron interacted preferentially with the alumina support forming FeAlO₃, which resulted in the stabilization of the support and a reduction in the surface area. The major role of Fe was to promote the enhancement of the catalytic activity of PtPd through an intimate interaction between the PtPd and iron oxides, which had lattice oxygens to generate oxygen with oxidation abilities.     

活性艳橙-KR的脱色和光催化氧化二步处理

对活性染料活性艳橙-KR的脱色、絮凝条件及光催化的条件进行了研究。活性艳橙-KR经过铁粉处理后,调节pH,絮凝沉降后,再经过光催化氧化降解,其COD的去除率可达到85％～92％,由于铁粉的加入．溶液中加入了铁离子,使得TiO2作为光催化氧化催化剂的催化作用增强,这是一种有效的降解方法,对实际生产有实用价值。     

Quantification of Antioxidant Ability Against Lipid Peroxidation with an ‘Area Under Curve’ Approach

When oxygen is passed through a linoleic acid (LA) emulsion containing copper(II), primary (hydroperoxides) and secondary oxidation products (aldehydes and ketones) are formed, monitored by ferric thiocyanate (Fe(III)‐SCN) and thiobarbituric acid reactive substances (TBARS) colorimetry, respectively. As total antioxidant capacity (TAC) against lipid peroxidation was not quantified before, both methods were adapted to an ‘area under curve (AUC)’ approach. LA peroxidation followed pseudo‐first order kinetics in aerated emulsions. Absorbance changes as a function of incubation time exhibited sigmoidal curves, enabling the calculation of ‘area under curve’ (AUC) and net AUC = AUC_blank?AUC_sample, standard calibration curve as net AUC versus concentration, and trolox‐equivalent antioxidant capacity of the tested compounds. Garlic extract showed an antioxidative effect on hydroperoxide formation, but a prooxidative effect on TBARS. Although inhibition of lipid peroxidation was described qualitatively before, it was not evaluated quantitatively, e.g., the trolox‐equivalent antioxidant capacities (TEAC values) of antioxidants with respect to their inhibitive effect against lipid peroxidation were not calculated and compared. Additionally, real‐time monitoring of lipid oxidation products requires highly sophisticated but costly instrumental techniques, but no single oxidation product is a direct measure of lipid oxidation or its antioxidative prevention. The AUC approach is the first quantitative method measuring antioxidant protection against lipid oxidation, with a slightly different order of antioxidative effectiveness from reductive assays because of interfacial effects.