Singlet oxygen formation by a peroxidase, H2O2 and halide system

Photosensitized Peroxidase Mimicry at the Hierarchical 0D/2D Heterojunction-Like Quasi Metal-Organic Framework Interface for Boosting Biocatalytic Disinfection

Metal-organic frameworks (MOFs) are a versatile toolbox for the bioinspired design of nanozymes for antibacterial applications and beyond, however, designing a nanozyme by the hierarchical quasi-MOF scheme remains largely unpracticed. This work exemplifies the preferential structure-activity correlation of a bimetallic quasi-MOF (Q-MOF_Ce0.5 ) among three series of MOF-derived peroxidase (POD) mimics. The biomimetic quasi-MOF_Ce0.5 nanosheets accommodate both oxygen vacancy-coupled multivalent redox cycles and photosensitive energy band layout, benefiting from the hierarchical heterojunction-like 0D/2D interface featuring isolated nodes-derived CeOCu sites upon the 2D decarboxylated MOF scaffold. These integrated unique merits enable the POD-like Q-MOF_Ce0.5 to generate sustained reactive oxygen species to effectively eradicate the surface-adhered bacteria under visible light, resulting in significant inactivation of Escherichia coli (99.74 %) and Staphylococcus aureus (99.35%) in vitro, and potent disinfection of skin wounds in vivo in safe and on-demand manners. It is hoped that this work can intensify the interventions of MOF nanozymes against the microbial world.

A comprehensive kinetic model for phenol oxidation in seven advanced oxidation processes and considering the effects of halides and carbonate

As one of the most powerful approaches to mechanistically understanding complex chemical reactions and performing simulations or predictions, kinetic modeling has been widely used to investigate advanced oxidation processes (AOPs). However, most of the available models are built based on limited systems or reaction mechanisms so they cannot be readily extended to other systems or reaction conditions. To overcome such limitations, this study developed a comprehensive model on phenol oxidation using over 540 reactions, covering the most common reaction mechanisms in nine AOPs-four peroxymonosulfate (PMS), four peroxydisulfate (PDS), and one H₂O₂ systems-and considering product formation and the effects of co-existing anions (chloride, bromide, and carbonate). Existing models in the literature were first gathered and then revised by correcting inaccurately used reactions and adding other necessary reactions. Extensive model tuning and validation were next conducted by fitting the model against experimental data from both this study and the literature. The effects of anions were found to follow PDS/CuO > H₂O₂/UV > other PDS or PMS systems. Halogenated organic byproducts were mainly observed in the PMS systems in the presence of halides. Most of the 543 reactions were found to be important based on the sensitivity analysis, with some anions-involved reactions being among the most important, which explained why these anions substantially altered some of the reaction systems. With this comprehensive model, a deep understanding and reliable prediction can be made for the oxidation of phenol (and likely other phenolic compounds) in systems containing one or more of the above AOPs.

Effect of thiocyanate on the peroxidase and pseudocatalase activities of Leishmania major ascorbate peroxidase

We report here that the Leishmania major ascorbate peroxidase (LmAPX), having similarity with plant ascorbate peroxidase, catalyzes the oxidation of suboptimal concentration of ascorbate to monodehydroascorbate (MDA) at physiological pH in the presence of added H(2)O(2) with concurrent evolution of O(2). This pseudocatalatic degradation of H(2)O(2) to O(2) is solely dependent on ascorbate and is blocked by a spin trap, alpha-phenyl-n-tert-butyl nitrone (PBN), indicating the involvement of free radical species in the reaction process. LmAPX thus appears to catalyze ascorbate oxidation by its peroxidase activity, first generating MDA and H(2)O with subsequent regeneration of ascorbate by the reduction of MDA with H(2)O(2) evolving O(2) through the intermediate formation of O(2)(-). Interestingly, both peroxidase and ascorbate-dependent pseudocatalatic activity of LmAPX are reversibly inhibited by SCN(-) in a concentration dependent manner. Spectral studies indicate that ascorbate cannot reduce LmAPX compound II to the native enzyme in presence of SCN(-). Further kinetic studies indicate that SCN(-) itself is not oxidized by LmAPX but inhibits both ascorbate and guaiacol oxidation, which suggests that SCN(-) blocks initial peroxidase activity with ascorbate rather than subsequent nonenzymatic pseudocatalatic degradation of H(2)O(2) to O(2). Binding studies by optical difference spectroscopy indicate that SCN(-) binds LmAPX (Kd = 100 +/- 10 mM) near the heme edge. Thus, unlike mammalian peroxidases, SCN(-) acts as an inhibitor for Leishmania peroxidase to block ascorbate oxidation and subsequent pseudocatalase activity.

New noncellular fluorescence microplate screening assay for scavenging activity against singlet oxygen

In the present study, a new fluorescence microplate screening assay for evaluating scavenging activity against singlet oxygen (1O2) was implemented. The chemical generation of 1O2 was promoted using the thermodissociable endoperoxide of disodium 3,3'-(1,4-naphthalene)bispropionate (NDPO2). The detection of 1O2 was achieved using dihydrorhodamine 123 (DHR), a nonfluorescent molecule that is oxidizable to the fluorescent form rhodamine 123 (RH). The combined use of a 1O2-selective generator and a highly sensitive probe (DHR) was then successfully applied to perform a screening assay of the 1O2 scavenging activities of ascorbic acid, penicillamine, cysteine, N-acetylcysteine (NAC), methionine, reduced glutathione (GSH), dihydrolipoic acid, lipoic acid, and sodium azide. All of these antioxidants exhibited concentration-dependent 1O2 scavenging capacities. They could be ranked according to observed activity: ascorbic acid>cysteine>penicillamine>dihydrolipoic acid>GSH>NAC>sodium azide>lipoic acid (IC50 values of 3.0+/-0.2, 8.0+/-0.7, 10.9+/-0.8, 25.2+/-4.5, 57.4+/-5.9, 138+/-13, 1124+/-128, 2775+/-359 microM, mean+/-SEM, respectively)>methionine (35% of scavenging effect at 10 mM). In conclusion, the use of NDPO2 as a selective generator for 1O2 and its fluorescence detection by the highly sensitive probe DHR is shown to be a reliable and resourceful analytical alternative means to implement a microplate screening assay for scavenging activity against 1O2.

Determination of H2O2-dependent generation of singlet oxygen from human saliva with a novel chemiluminescence probe

Singlet oxygen (1O2) has been shown to play an important role in salivary defense system, but its generation process and level from human saliva remain uncertain due to the lack of a reliable detection method. We have previously reported 4,4'(5')-bis[2-(9-anthryloxy)ethylthio]tetrathiafulvalene (BAET) as a novel chemiluminescence probe for 1O2. In this work, the probe is successfully used to characterize H2O2-dependent generation of 1O2 from saliva in real time. However, the yield of 1O2 is found to be very low, for example, being about 0.13 nmol from 200 microL saliva in the presence of 1 mM of hydrogen peroxide over a 5-s reaction period. The result is also compared with that obtained with another 1O2 probe 2-methyl-6-phenyl-3,7-dihydroimidazo[1,2-a]pyrazin-3-one (CLA), demonstrating that, besides 1O2, the other reactive oxygen species such as hydroxyl radical may also be involved in the reaction of saliva with H2O2. Furthermore, the present study shows that the selectivity of BAET for 1O2 is much higher than that of CLA and thus BAET is highly suited for the detection of 1O2 in the presence of other reactive oxygen species in biological systems.

Sequential Injection Analysis with Chemiluminescence Detection for the Antioxidative Activity against Singlet Oxygen

A sequential injection analysis (SIA) with chemiluminescence (CL) detection was developed for the measurement of antioxidative activity against singlet oxygen ((1)O2). Lactoperoxidase-hydrogen peroxide-bromide ion system was used for the generation of (1)O2. When a 100 mM sodium acetate buffer (pH 4.5) was used as a carrier solution, the SIA-CL system could be optimized with respect to the flow-rate of the carrier, concentration of reagents and their aspiration order. The antioxidative activity was expressed as an attenuation of luminol CL due to the quenching of (1)O2 by an antioxidant. The relative standard deviations of antioxidative activity (n=3) against (1)O2 for within- and between-day analyses were < or = 1.6% (20 microM Trolox). The system was successfully applied to the assay of antioxidative activities of various antioxidants including vitamin supplements at a rate of 10 samples within 15 min. The proposed SIA-CL system was rapid and reproducible with minimum consumption of the sample and of reagents, and thus was useful for the screening of compounds possessing antioxidative activity against (1)O2.

Chemiluminescence and reactive oxygen scavenging activities of the hydrogen peroxide/gallic acid/lactoperoxidase system

Photon emission in the hydrogen peroxide/gallic acid/lactoperoxidase system was studied using visible region chemiluminescence. The photon emission intensity showed a pH-dependence curve with a maximum at pH 4.5-5.0. The spectral analysis showed to be at 510 nm and its energy was 61.0 kcal/mol at either pH 4.5 or pH 7.0. In addition, near-infrared spectral analysis at 1270 nm suggested that this photon emission from the hydrogen peroxide/gallic acid/lactoperoxidase system was produced without generation of (1)O(2). The gallic acid/lactoperoxidase system, based on the chemiluminescence system, has superoxide-, hydroxyl radical-scavenging activities and antioxidative activity. These results are strong evidence that the gallic acid/lactoperoxidase system is one of the reactive oxygen scavenging systems.

Detection of superoxide anion radical in phospholipid liposomal membrane by fluorescence quenching method using 1,3-diphenylisobenzofuran

Utilization of a fluorescence dye, 1,3-diphenylisobenzofuran (DPBF) as a detector of superoxide anion radical (O2*-) was examined. The fluorescence intensity of DPBF incorporated in phospholipid liposomes consisting of phosphatidylcholine (PC) and phosphatidylserine (PS) is effectively quenched by incubation with xanthine/xanthine oxidase system. On the other hand, xanthine or xanthine oxidase alone did not induce quenching of the DPBF fluorescence in the liposomes. Xanthine/xanthine oxidase-induced fluorescence quenching of DPBF-labeled liposomes was almost completely protected by the addition of superoxide dismutase (SOD, 1 U/ml), but not by heat-denatured SOD (10 min boiling) at the same concentration. On the other hand, catalase (1 U/ml), and hydroxyl radical and singlet oxygen scavengers (10 mM sodium benzoate, 300 mM mannitol, 1 mM tryptophan and 1 mM sodium azide) did not protect xanthine/xanthine oxidase-induced fluorescence quenching of DPBF-labeled liposomes. The concentration dependence profiles of xanthine oxidase on the DPBF fluorescence quenching and O2*- generation showed that there is a good correlation between these parameters. Under the present experimental conditions, approximately 7 microM H(2)O(2)/30 min were produced, but the addition of H(2)O(2) (1 mM) to DPBF-labeled liposomes did not quench the dye fluorescence in the liposomes. Temperature dependence profiles of the DPBF fluorescence quenching induced by xanthine/xanthine oxidase treatment and the excimer fluorescence formation of pyrene molecules embedded in the liposomal membrane suggested that the quenching efficiency of the DPBF fluorescence is largely dependent on their lipid dynamics. Based on these results, we proposed the possibility that DPBF fluorescence quenching method is able to be used as a simple method for detecting O2*- inside the membrane lipid layer and that DPBF fluorescence quenching by O2*- is controlled by the physical state of membrane lipids.

Hydrogen peroxide-induced luminescence and evolution of molecular oxygen in human saliva

Low-level chemiluminescence (CL) appeared on addition of hydrogen peroxide to human saliva and then decayed slowly. Azide (10 microM) inhibited CL by about 50% and deuterium oxide (99.75%) enhanced it about twofold. 1,4-Diazabicyclo[2,2,2]octane (50 mM) and tryptophan (10 mM) were also enhancing, which suggests that singlet oxygen participates in this CL. The optimal pH for CL was around 8.5. Molecular oxygen was produced on addition of hydrogen peroxide to human saliva with a time course similar to that of CL; the optimal pH for oxygen evolution was around 8.0. The levels of SCN- and OSCN- at first decreased and increased, respectively, on addition of hydrogen peroxide and then remained constant as long as the induced CL could be detected. Dithiothreitol (1 mM) and mercaptoethanol (1 mM) completely suppressed CL. Induced CL was observed in saliva dialysed against 10 mM sodium phosphate (pH 7.5). Its intensity was increased by NaSCN, reaching a maximum around 0.1 mM NaSCN in the presence of 0.2 mM hydrogen peroxide. These results suggest that part of the molecular oxygen evolved on addition of hydrogen peroxide to human saliva is in a singlet state and that molecular oxygen is evolved by oxidation of hydrogen peroxide, which may be catalysed by OSCN- bound to salivary peroxidase.

Thiocyanate, a plausible physiological electron donor of gastric peroxidase

Gastric peroxidase (GPO) was purified to apparent homogeneity to characterize its major physiological electron donor. The enzyme (RZ = 0.7), with a subunit molecular mass of 50 kDa, is a glycoprotein, with a relative abundance of aspartic and glutamic acid over arginine and lysine. It has a Soret maximum at 412 nm, which is shifted to 426 nm by H2O2 due to formation of compound II. Although the physiological electron donors I-, Br- and SCN-, but not Cl-, are oxidized by GPO optimally at acid pH, only I- and SCN- are oxidized appreciably at physiological pH. Considering that the I- concentration in stomach is less than 1 microM, whereas the SCN- concentration is about 250 microM, SCN- may act as a major electron donor for GPO. Moreover, SCN- oxidation remains unaltered in the presence of physiological concentrations of other halides. The second-order rate constant for the reaction of GPO with H2O2 (k1) and compound I with SCN- (k2) at pH 7 was found to be 8 x 10(7) M-1.s-1 and 2 x 10(5) M-1.s-1 respectively. GPO has significant pseudocatalase activity also in the presence of I- or Br-, but it is blocked by SCN-. The SCN- oxidation product OSCN- may be reduced back to SCN- by cellular GSH, and GSSG may be reduced back to GSH by glutathione reductase and NADPH. In a system reconstituted with pure glutathione reductase, NADPH, GSH, SCN- and H2O2. GPO-catalysed SCN- oxidation could be coupled to NADPH oxidation. This system where GPO utilizes SCN- as the major physiological electron donor may operate efficiently to scavenge intracellular H2O2.

Hydrogen peroxide-dependent generation of singlet molecular oxygen by human saliva: its detection by chemiluminescence from a Cypridina luciferin analog

By the addition of hydrogen peroxide to human saliva, chemiluminescence from Cypridina luciferin analog (CCLA) and oxygen evolution were observed. Chemiluminescence was inhibited by inhibitors of salivary peroxidase, azide and cyanide and by a singlet oxygen quencher, crocin. Deuterium oxide (99.75%) stimulated the initial increase of CCLA by 15-50% and the integrated CCLA 2.1-3.6-fold. The results suggest that the generation of singlet oxygen by peroxidase in human saliva depends on hydrogen peroxide.

A new fluorescence method to detect singlet oxygen inside phospholipid model membranes

A fluorescence method for detecting singlet oxygen (1O2) in model membranes is proposed. 1O2 was generated by hydrogen peroxide/sodium hypochlorite system. 1,3-Diphenylisobenzofuran (DPBF), a specific 1O2 trap, dissolved in organic solvents gives a strong fluorescence spectrum when excited at 410 nm. A similar spectrum, with a maximum at 455 nm, is obtained when DPBF is incorporated in unilamellar dipalmitoylphosphatidylcholine liposomes. The intensity of fluorescence spectrum decreases when DPBF-labeled liposomes are exposed to singlet oxygen. This decrease is sensitive to 1O2 traps and quenchers like tryptophan and sodium azide, to lipid membrane fluidity and to the concentration of sodium hypochlorite and hydrogen peroxide.

Effect of propagators and inhibitors on the ultraweak luminescence from maize roots

This study has investigated the kinetics and mechanism of ultraweak luminescence in maize roots. Mannitol induced the second maximum and enhanced the main maximum of the relative intensity of luminescence from the roots. Hydroquinone and quinone enhanced the relative intensity of the luminescence. Catalase enhanced the maximum of the luminescence and changed the kinetics of the light emission. The effect of catalase on the kinetics was abolished by superoxide dismutase. Ascorbate in the presence of catalase on the kinetics was abolished by superoxide dismutase. Ascorbate in the presence of catalase reduced the luminescence maximum, but did not alter the kinetics. In the presence of catalase only, or in the combination with superoxide dismutase, or ascorbate, the luminescence intensity in the stationary phase was significantly lower compared to the control. The results support the participation of superoxide-radical, singlet oxygen, electron transfer and the role of peroxidase in the reactions generating ultraweak luminescence in the roots. Ascorbate, catalase and superoxide dismutase have a protective role in the luminescent reactions.

Singlet oxygen production by biological systems

Singlet oxygen (1 delta g) is a highly reactive, short-lived intermediate which readily oxidizes a variety of biological molecules. The biochemical production of singlet oxygen has been proposed to contribute to the destructive effects seen in a number of biological processes. Several model biochemical systems have been shown to produce singlet oxygen. These systems include the peroxidase-catalyzed oxidations of halide ions, the peroxidase-catalyzed oxidations of indole-3-acetic acid, the lipoxygenase-catalyzed oxidation of unsaturated long chain fatty acids and the bleomycin-catalyzed decomposition of hydroperoxides. Results from these model systems should not be uncritically extrapolated to living systems. Recently, however, an intact cell, the human eosinophil, was shown to generate detectable amounts of singlet oxygen. This result suggests that singlet oxygen may be shown to be a significant biochemical intermediate in a few biological processes.

Oxygen radicals, inflammation, and tissue injury

Inflammatory reactions often result in the activation and recruitment of phagocytic cells (e.g., neutrophils and/or tissue macrophages) whose products result in injury to the tissue. In killing of endothelial cells by activated neutrophils as well as in lung injury produced by either activated neutrophils or activated macrophages there is evidence that H2O2 and iron play a role. HO. may be a key oxygen product related to the process of injury. Endothelial cells in some vascular compartments may be susceptible to neutrophil mediated injury in a manner that is independent of oxygen radicals. On the basis of in vitro observations, a synergy exits between platelets and neutrophils, resulting in enhanced oxygen radical formation by the latter. Finally, the cytokines, interleukin 1 and tumor necrosis factor, released from macrophages have both direct stimulatory effects on oxygen radical formation in neutrophils and can "prime" macrophages for enhanced oxygen radical responses to other agonists. Cytokines may also alter endothelial cells rendering them more susceptible to oxygen radical mediated injury by neutrophils. This suggests a complex network of interactions between phagocytic cells and peptide mediators, the result of which is acute, oxygen radical mediated tissue injury.

The role of hydroxyl radicals in irreversible inactivation of lactoperoxidase by excess H2O2. A spin-trapping/ESR and absorption spectroscopy study

H2O2 is catalytically metabolized by ferric lactoperoxidase (LPO)----compound (cpd) I----cpd II----ferric LPO cycles. An excess of the substrate, however, is degraded by a ferric LPO----cpd I----cpd II----cpd III----ferrous LPO----ferric LPO cycle. This latter pathway leads to the partial or total irreversible inactivation of the enzyme depending on the excess of H2O2 (H. Jenzer, W. Jones, and H. Kohler (1986) J. Biol. Chem. 261, 15550-15556). Spin-trapping/ESR data indicate that in the course of the reaction superoxide (HO2./O2-) and hydroxyl radicals (OH.) are formed. Since many substances known to scavenge radicals, such as a spin trap (e.g., 5,5-dimethyl-1-pyrroline-N-oxide) desferrioxamine, albumin, or mannitol, do not prevent enzyme inactivation, we conclude that OH. generation is a site-specific reaction at or near the active center of LPO where bulky scavenger molecules may not be able to penetrate. We suggest the formation of OH. by a Fenton-like reaction between H2O2 and the intermediate ferrous state of the enzyme, which substitutes for Fe2+ in the Fenton reaction. OH. is a powerful oxidant which in turn may attack rapidly the nearest partner available, either H2O2 to produce HO2. and H2O, or the prosthetic group to give rise to oxidative cleavage of the porphyrin ring structure of the heme moiety of LPO and thus to the liberation of iron.

The role of compound III in reversible and irreversible inactivation of lactoperoxidase

In the presence of iodide (I-, 10 mM) and hydrogen peroxide in a large excess (H2O2, 0.1-10 mM) catalytic amounts of lactoperoxidase (2 nM) are very rapidly irreversibly inactivated without forming compound III (cpd III). In contrast, in the absence of I- cpd III is formed and inactivation proceeds very slowly. Increasing the enzyme concentration up to the micromolar range significantly accelerates the rate of inactivation. The present data reveal that irreversible inactivation of the enzyme involves cleavage of the prosthetic group and liberation of heme iron. The rate of enzyme destruction is well correlated with the production of molecular oxygen (O2), which originates from the oxidation of excess H2O2. Since H2O2 and O2 per se do not affect the heme moiety of the peroxidase, we suggest that the damaging species may be a primary intermediate of the H2O2 oxidation, such as oxygen in its excited singlet state (1 delta gO2), superoxide radicals (O-.2), or consequently formed hydroxyl radicals (OH.).

A novel oxidation of the carcinogen N-hydroxy-N-2-fluorenylacetamide catalyzed by peroxidase/H2O2/Br-

N-Hydroxy-N-2-fluorenylacetamide, a proximate carcinogenic metabolite of N-2-fluorenylacetamide, is oxidized largely to 2-nitrosofluorene by lactoperoxidase or extract of peroxidative activity of rat uterus in an H2O2- and Br- -dependent reaction. Evidence is presented that the oxidizing species includes OBr- (HOBr). This novel oxidation may be involved in carcinogenesis by N-arylhydroxamic acids.

Pseudo-catalytic degradation of hydrogen peroxide in the lactoperoxidase/H2O2/iodide system

Non-stoichiometric (excessive) consumption of hydrogen peroxide (H2O2), which was observed in various lactoperoxidase-catalysed reactions, was tested in the lactoperoxidase/H2O2/iodide system. In preliminary experiments the suitability of the system was tested with special regard to the triiodide (I-3) absorption and the I2/I-3 equilibrium. Triiodide equilibrium concentrations evaluated theoretically and experimentally were compared after adding a known amount of iodine (I2) to solutions containing variable I- concentrations. A close fit of the two methods was only obtained if experiments were carried out in pure aqueous or 0.001 M H2SO4 medium. The presence of various anions, e.g. OH- and Cl-, led to a measurable decrease in I-3 and I2 equilibrium concentrations. These ions are able to displace competitively I- in forming association products with I+ and I2. When I+ and I2 were generated enzymatically by lactoperoxidase and hydrogen peroxide, additional interactions with H2O2 were observed. Depending on the enzyme and iodide concentrations, variable amounts of H2O2 disappeared nonproductively. Due to its ambivalent redox reactivity, part of the H2O2 is not reduced to H2O in the enzyme-catalysed generation of iodine, but undergoes oxidation to O2 by an oxidized iodine compound. This suggests a pseudo-catalytic side reaction which can competitively interfere with the I2/I-3 generation or (and) the iodination reaction.

Catalatic activity of lactoperoxidase in the presence of SCN-

Lactoperoxidase catalyzed the catalatic decomposition of H2O2 in the presence of SCN-. The pH optimum for O2 evolution was 8.5, while the enzyme activity as disclosed by the rate of H2O2 disappearance was optimal at 4.5. Since the catalatic activity of lactoperoxidase was SCN- dependent, and no O2 was evolved, when H2O2 was added to OSCN- in the absence of lactoperoxidase, an enzyme-OSCN complex may be assumed to be an intermediate in the catalatic activity of lactoperoxidase.

Initiation of lipid peroxidation by a peroxidase/hydrogen peroxide/halide system

A lactoperoxidase/H2O2/halide system caused the initiation of linoleate peroxidation as indicated by diene conjugation. Coupled lipid peroxidation was accelerated by iodide, chloride and bromide ions at pH 4.0 and 6.2. No peroxidation occurred in the presence of H2O2 or lactoperoxidase alone. The rate of linoleate peroxidation by lactoperoxidase in the presence of chloride depended on the concentration of H2O2. Linoleate peroxidation by the enzymatic system was inhibited by high concentration of H2O2 by methionine, tryptophan and BHT. Oxygen was absorbed during peroxidation and the major products were the 13-hydroperoxides. The mechanisms of the initiation of lipid peroxidation by a peroxidase/H2O2/halide system are discussed.

Lipid deterioration: beta-carotene destruction and oxygen evolution in a system containing lactoperoxidase, hydrogen peroxide and halides

A model system containing lactoperoxidase/H2O2/halide decomposed beta-carotene in a reaction greatly affected by the concentration of H2O2. The optimal concentrations of H2O2 for activation of iodide and bromide were 2 mM and 10 microM, respectively. The oxidation of chloride by a lactoperoxidase, using beta-carotene destruction as a sensitive method to determine the activity of the enzyme, is reported herein. In the presence of optimal amounts of H2O2, the rate of beta-carotene destruction increases slowly until a critical concentration of the halides, followed by a rapid increase in the rate when halide concentrations were further increased. A lactoperoxidase/H2O2/iodide and/or bromide system generates oxygen in the presence of high H2O2 and halide concentrations. beta-Carotene inhibited the evolution of oxygen. A possible mechanism of beta-carotene destruction and triplet unexcited oxygen evolution by a lactoperoxidase/H2O2/halide system are proposed.

Initiation of oxidative changes in foods

Initiation of lipid peroxidation in foods may be accomplished by a variety of mechanisms. Two principal initiation reactions involve homolytic scission of preformed peroxides as catalyzed by metal ions and heme proteins and the reaction of activated oxygen species with the lipid substrate to yield peroxides and free radicals. Copper and cytochromes in the milk fat globule membrane may serve as focal points for initiation of lipid peroxidation by catalyzing homolytic scission of peroxides. Activated oxygen species which may be important in initiating oxidative changes in foods include singlet oxygen, hydroxyl radical, ozone, superoxide anion (perhydroxyl radical at low pH), and hydrogen peroxide. Chemical and enzymic reactions in biological materials can generate singlet oxygen, hydroxyl radical, superoxide anion, and hydrogen peroxide. Ozone is primarily a product of photoreactions in polluted air. Reactions involving singlet oxygen, hydroxyl radical, and ozone with food constituents ultimately can yield peroxides which decompose to initiate oxidative chain reactions. Superoxide anion and hydrogen peroxide are relatively inert toward organic molecules but can decompose to produce the more reactive singlet oxygen and hydroxyl radical. Inhibition of reactions initiated by reactive oxygen species in foods should be very important in preserving the oxidative stability of foods. This paper presents a brief review of possible initiation reactions for lipid peroxidation and inhibition of reactions of activated oxygen species that are of importance in food systems.