Modulation of oxidative neurometabolism in ischemia/reperfusion by nitrite

Nitrite has been viewed essentially as an inert metabolic endpoint of nitric oxide (^•NO). However, under certain conditions, nitrite can be a source of ^•NO. In the brain, this alternative source of ^•NO production independent of nitric oxide synthase activity may be particularly relevant in ischemia/reperfusion (I/R), where low oxygen availability limits enzymatic production of ^•NO. Notably, in vivo concentration of nitrite can be easily increased with diet, through the ingestion of nitrate-rich foods, opening the window for a therapeutic intervention based on diet. Considering the modulation of mitochondrial respiration by ^•NO, we have hypothesized that the protective action of nitrite in I/R may also result from modulation of mitochondrial function. We used high-resolution respirometry to evaluate the effects of nitrite in two in vitro models of I/R. In both cases, an increase in oxygen flux was observed following reoxygenation, a phenomenon that has been coined "oxidative burst". The amplitude of this "oxidative burst" was decreased by nitrite in a concentration-dependent manner. Additionally, a pilot in vivo study in which animals received a nitrate-rich diet as a strategy to increase circulating and tissue levels of nitrite also revealed that the "oxidative burst" was decreased in the nitrate-treated animals. These results may provide mechanistic support to the observation of a protective effect of nitrite in situations of brain ischemia.

In vivo hydrogen peroxide diffusivity in brain tissue supports volume signaling activity

Hydrogen peroxide is a major redox signaling molecule underlying a novel paradigm of cell function and communication. A role for H₂O₂ as an intercellular signaling molecule and neuromodulator in the brain has become increasingly apparent, with evidence showing this biological oxidant to regulate neuronal polarity, connectivity, synaptic transmission and tuning of neuronal networks. This notion is supported by its ability to diffuse in the extracellular space, from source of production to target. It is, thus, crucial to understand extracellular H₂O₂ concentration dynamics in the living brain and the factors which shape its diffusion pattern and half-life. To address this issue, we have used a novel microsensor to measure H₂O₂ concentration dynamics in the brain extracellular matrix both in an ex vivo model using rodent brain slices and in vivo. We found that exogenously applied H₂O₂ is removed from the extracellular space with an average half-life of t_1/2 = 2.2 s in vivo. We determined the in vivo effective diffusion coefficient of H₂O₂ to be D* = 2.5 × 10⁻⁵ cm² s⁻¹. This allows it to diffuse over 100 μm in the extracellular space within its half-life. Considering this, we can tentatively place H₂O₂ within the class of volume neurotransmitters, connecting all cell types within the complex network of brain tissue, regardless of whether they are physically connected. These quantitative details of H₂O₂ diffusion and half-life in the brain allow us to interpret the physiology of the redox signal and lay the pavement to then address dysregulation in redox homeostasis associated with disease processes.

A High Fat/Cholesterol Diet Recapitulates Some Alzheimer's Disease-Like Features in Mice: Focus on Hippocampal Mitochondrial Dysfunction

BACKGROUND

Ample evidence from clinical and pre-clinical studies suggests mid-life hypercholesterolemia as a risk factor for developing Alzheimer's disease (AD) at a later age. Hypercholesterolemia induced by dietary habits can lead to vascular perturbations that increase the risk of developing sporadic AD.

OBJECTIVE

To investigate the effects of a high fat/cholesterol diet (HFCD) as a risk factor for AD by using a rodent model of AD and its correspondent control (healthy animals).

METHODS

We compared the effect of a HFCD in normal mice (non-transgenic mice, NTg) and the triple transgenic mouse model of AD (3xTgAD). We evaluated cognitive performance in relation to changes in oxidative metabolism and neuron-derived nitric oxide (•NO) concentration dynamics in hippocampal slices as well as histochemical staining of markers of the neurovascular unit.

RESULTS

In NTg, the HFCD produced only moderate hypercholesterolemia but significant decline in spatial memory was observed. A tendency for decrease in •NO production was accompanied by compromised mitochondrial function with decrease in spare respiratory capacity. In 3xTgAD mice, a robust increase in plasma cholesterol levels with the HFCD did not worsen cognitive performance but did induce compromise of mitochondrial function and significantly decreased •NO production. We found increased staining of biomarkers for astrocyte endfeet and endothelial cells in 3xTgAD hippocampi, which was further increased by the HFCD.

CONCLUSION

A short term (8 weeks) intervention with HFCD can produce an AD-like phenotype even in the absence of overt systemic hypercholesterolemia and highlights mitochondrial dysfunction as a link between hypercholesterolemia and sporadic AD.

The bioactivity of neuronal-derived nitric oxide in aging and neurodegeneration: Switching signaling to degeneration

The small and diffusible free radical nitric oxide (^•NO) has fascinated biological and medical scientists since it was promoted from atmospheric air pollutant to biological ubiquitous signaling molecule. Its unique physical chemical properties expand beyond its radical nature to include fast diffusion in aqueous and lipid environments and selective reactivity in a biological setting determined by bioavailability and reaction rate constants with biomolecules. In the brain, ^•NO is recognized as a key player in numerous physiological processes ranging from neurotransmission/neuromodulation to neurovascular coupling and immune response. Furthermore, changes in its bioactivity are central to the molecular pathways associated with brain aging and neurodegeneration. The understanding of ^•NO bioactivity in the brain, however, requires the knowledge of its concentration dynamics with high spatial and temporal resolution upon stimulation of its synthesis. Here we revise our current understanding of the role of neuronal-derived ^•NO in brain physiology, aging and degeneration, focused on changes in the extracellular concentration dynamics of this free radical and the regulation of bioenergetic metabolism and neurovascular coupling.

Evaluation of Mitochondrial Function in the CNS of Rodent Models of Alzheimer's Disease - High Resolution Respirometry Applied to Acute Hippocampal Slices

Alzheimer's disease (AD) is a multifactorial disease characterized by extracellular deposits of amyloid plaques and intracellular neurofibrillary tangles. These hallmark alterations are preceded by synaptic deterioration, changes in neuromolecular plasticity phenomena, mitochondrial dysfunction, increase in oxidative damage to cellular constituents and decreased energy metabolism. The hippocampus is a structure of the temporal medial lobe implicated in specific forms of memory processes. It is also one of the first and most affected regions of the CNS in AD. Here we present a novel approach to the study if mitochondrial function/disfunction in 2 rodent models of AD: an acute rat model obtained by intracerebroventricular injection of the toxin streptozotocin (STZ) and a progressive triple transgenic mouse model (3TgAD) harboring PS1M146V, APPSwe, and tauP301L transgenes. Mitochondrial dysfunction has classically been assessed in such models by isolating mitochondria, synaptossoms or working with cell cultures. Anyone of these approaches destroys the intricate intercellular connectivity and cytoarchitecture of neuronal tissue. We used acute hippocampal slices obtained from the 2 models of AD and evaluated changes in mitochondrial function as a function of disease and/or age. Mitochondrial stress test were performed on the high resolution respirometry (Oroboros 2K Oxymeter). Upon analysis of oxygen consumption rates (OCR) we observed significant decreases in basal OCR, maximal respiratory capacity, ATP turnover and a tendency for decrease in sparing capacity in the STZ rat model compared to shame injected animals. Regarding the 3TgAD model we observed an age-dependent decrease in all parameters evaluated in the mitochondrial stress test, in both 3TgAD and NTg animals. However, although a tendency towards decreased OCR was observed when comparing 3TgAD and age-matched NTg animals, no statistically significant difference was observed.

The pattern of glutamate-induced nitric oxide dynamics in vivo and its correlation with nNOS expression in rat hippocampus, cerebral cortex and striatum

Nitric oxide (NO) is a diffusible intercellular messenger, acting via volume signaling in the brain and, therefore, the knowledge of its temporal dynamics is determinant to the understanding of its neurobiological role. However, such an analysis in vivo is challenging and indirect or static approaches are mostly used to infer NO bioactivity. In the present work we measured the glutamate-dependent NO temporal dynamics in vivo in the hippocampus (CA1, CA3 and DG subregions), cerebral cortex and striatum, using NO selective microelectrodes. Concurrently, the immunolocalization of nNOS was evaluated in each region. A transitory increase in NO levels occurred at higher amplitudes in the striatum and hippocampus relatively to the cortex. In the hippocampus, subtle differences in the profiles of NO signals were observed along the trisynaptic loop, with CA1 exhibiting the largest signals. The topography of NO temporal dynamics did not fully overlap with the pattern of the density of nNOS expression, suggesting that, complementary to the distribution of nNOS, the local regulation of NO synthesis as well as the decay pathways critically determine the effective NO concentration sensed by a target within the diffusional spread of this free radical. In sum, the rate and pattern of NO changes here shown, by incorporating regulatory mechanisms and processes that affect NO synthesis and decay, provide refined information critical for the understanding of NO multiple actions in the brain.

Human colon adenocarcinoma HT-29 cell: electrochemistry and nicotine stimulation

Recently, it was demonstrated that colorectal cancer HT-29 cells can secrete epinephrine (adrenaline) in an autocrine manner to auto-stimulate cellular growth by adrenoreceptors activation, and that this secretion is enhanced by nicotine, showing an indirect relation between colorectal cancer and tobacco. The electrochemical behaviour of human colon adenocarcinoma HT-29 cells from a colorectal adenocarcinoma cell line, the hormone and neurotransmitter epinephrine, and nicotine, were investigated by cyclic voltammetry, using indium tin oxide (ITO), glassy carbon (GC) and screen printed carbon (SPC) electrodes. The oxidation of the HT-29 cells, previously grown onto ITO or SPC surfaces, followed an irreversible oxidation process that involved the formation of a main oxidation product that undergoes irreversible reduction, as in the epinephrine oxidation mechanism. The effect of nicotine stimulation of the HT-29 cells was also investigated. Nicotine, at different concentration levels 1, 2 and 15 mM, was introduced in the culture medium and an increase with incubation time, 0 to 3h and 30 min, of the HT-29 cells oxidation and reduction peaks was observed. The interaction of nicotine with the HT-29 cells stimulated the epinephrine secretion causing an increase in epinephrine release concentration, and enabling the conclusion that epinephrine and nicotine play an important role in the colorectal tumour growth.

Critical role of hydrogen peroxide signaling in the sequential activation of p38 MAPK and eNOS in laminar shear stress

Laminar shear stress (LSS) is a protective hemodynamic regulator of endothelial function and limits the development of atherosclerosis and other vascular wall diseases related to pathophysiological generation of reactive oxygen species. LSS activates several endothelial signaling responses, including the activation of MAPKs and eNOS. Here, we explored the mechanisms of activation of these key endothelial signaling pathways. Using the cone/plate model we found that LSS (12 dyn/cm(2)) rapidly promotes endothelial intracellular generation of superoxide and hydrogen peroxide (H(2)O(2)). Physiological concentrations of H(2)O(2) (flux of 0.1 nM/min and 15 μM added extracellularly) significantly activated both eNOS and p38 MAPK. Pharmacological inhibition of NADPH oxidases (NOXs) and specific knockdown of NOX4 decreased LSS-induced p38 MAPK activation. Whereas the absence of eNOS did not alter LSS-induced p38 MAPK activation, pharmacological inhibition and knockdown of p38α MAPK blocked H(2)O(2)- and LSS-induced eNOS phosphorylation and reduced (•)NO levels. We propose a model in which LSS promotes the formation of signaling levels of H(2)O(2), which in turn activate p38α MAPK and then stimulate eNOS, leading to increased (•)NO generation and protection of endothelial function.

Modulation of cellular respiration by endogenously produced nitric oxide in rat hippocampal slices

Nitric oxide (•NO) is a ubiquitous signaling molecule that participates in neuromolecular phenomena associated with memory formation as well as in excitotoxicity. In the hippocampus, neuronal •NO production is coupled to the activation of the NMDA-type of glutamate receptor. More recently, Cytochrome c oxidase has emerged as a novel target for •NO, which competes with O 2 for binding to this mitochondrial complex. This reaction establishes •NO not only as a regulator of cellular metabolism but possibly also as a regulator of mitochondrial production of reactive oxygen species which participate in cellular signaling. A major gap in the understanding of •NO bioactivity, namely, in the hippocampus, has been the lack of knowledge of its concentration dynamics. Here, we present a detailed description of the simultaneous recording of •NO and O2 concentration dynamics in rat hippocampal slices. Carbon fi ber microelectrodes are fabricated and applied for real-time measurements of both gases in a system close to in vivo models. This approach allows for a better understanding of the current paradigm by which an intricate interplay between •NO and O 2 regulates cellular respiration.

Redox cycles of caffeic acid, alpha-tocopherol, and ascorbate: implications for protection of low-density lipoproteins against oxidation

This study addresses the dynamic interactions among alpha-tocopherol, caffeic acid, and ascorbate in terms of a sequence of redox cycles aimed at accomplishing optimal synergistic antioxidant protection. Several experimental models were designed to examine these interactions: UV irradiation of alpha-tocopherol-containing sodium dodecyl sulfate micelles, one-electron oxidations catalyzed by the hypervalent state of myoglobin, ferrylmyoglobin, and autoxidation at appropriate pHs. These models were assessed by ultraviolet (UV) and electron paramagnetic resonance (EPR), entailing direct- and continuous-flow experiments, spectroscopy and by separation and identification of products by HPLC. The alpha-tocopheroxyl radical EPR signal generated by UV irradiation of alpha-tocopherol-containing micelles was suppressed by caffeic acid and ascorbate; in the former case, no other EPR signal was observed at pH 7.4, whereas in the latter case, the alpha-tocopheroxyl radical EPR signal was replaced by a doublet EPR spectrum corresponding to the ascorbyl radical (A*-). The potential interactions between caffeic acid and ascorbate were further analyzed by assessing, on the one hand, the ability of ascorbate to reduce the caffeic acid o-semiquinone (generated by oxidation of caffeic acid by ferrylmyoglobin) and, on the other hand, the ability of caffeic acid to reduce ascorbyl radical (generated by autoxidation or oxidation of ascorbate by ferrylmyoglobin). The data presented indicate that the reductive decay of ascorbyl radical (A*-) and caffeic acid o-semiquinone (Caf-O*) can be accomplished by caffeic acid (Caf-OH) and ascorbate (AH-), respectively, thus pointing to the reversibility of the reaction Caf-O* + AH- <--> Caf-OH + A*-. Continuous-flow EPR measurements of mixtures containing ferrylmyoglobin, alpha-tocopherol-containing micelles, caffeic acid, and ascorbate revealed that ascorbate is the ultimate electron donor in the sequence encompassing transfer of the radical character from the micellar phase to the phase. In independent experiments, the effects of caffeic acid and ascorbate on the oxidation of two low-density lipoprotein (LDL) populations, control and alpha-tocopherol-enriched, were studied and results indicated that alpha-tocopherol, caffeic acid, and ascorbate acted synergistically to afford optimal protection of LDL against oxidation. These results are analyzed for each individual antioxidant in terms of three domains: its localization and that of the antioxidant-derived radical, its reduction potential, and the predominant decay pathways for the antioxidant-derived radical, that exert kinetic control on the process.

Cholesteryl ester hydroperoxide formation in myoglobin-catalyzed low density lipoprotein oxidation: concerted antioxidant activity of caffeic and p-coumaric acids with ascorbate

Two diet-derived phenolic acids, caffeic and p-coumaric acids, interplayed with ascorbate in the protection of low density lipoproteins (LDL) from oxidation promoted by ferrylmyoglobin. Ferrylmyoglobin, a two-electron oxidation product from the reaction of metmyoglobin and H2O2, was able to oxidize LDL, degrading free cholesterol and cholesteryl esters. Upon exposure to ferrylmyoglobin, LDL became rapidly depleted of cholesteryl arachidonate and linoleate, which turn into the corresponding hydroperoxides. Cholesteryl oleate and cholesterol were, comparatively, more resistant to oxidation. Caffeic (2 microM) and p-coumaric (12 microM) acids efficiently delayed oxidations, as reflected by an increase in the lag times required for linoleate hydroperoxide and 7-ketocholesterol formation as well as for cholesteryl linoleate consumption. At the same concentration, ascorbate, a standard water-soluble antioxidant, was less efficient than the phenolic acids. Additionally, phenolic acids afforded a protection to LDL that, conversely to ascorbate, extends along the time, as inferred from the high levels of cholesteryl linoleate and cholesteryl arachidonate left after 22 hr of oxidation challenging. Significantly, the coincubation of LDL with ascorbate and each of the phenolic acids resulted in a synergistic protection from oxidation. This was inferred from the lag phases of cholesteryl linoleate hydroperoxide (the major peroxide found in LDL) formation in the presence of mixtures of ascorbate with phenolic acids longer than the sum of individual lag phases of ascorbate and the phenolic acids. A similar description could be drawn for the accumulation of a late product of oxidation, 7-ketocholesterol. It is concluded that ferrylmyoglobin induces a typical pattern of LDL lipid peroxidation, the oxidation rate of cholesteryl esters being a function of unsaturation; furthermore, there is a synergistic antioxidant activity of diet-derived phenolic acids with ascorbate in the protection of LDL from oxidation, a finding of putative physiological relevance.

Effect of dietary phenolic compounds on apoptosis of human cultured endothelial cells induced by oxidized LDL

1. Oxidized low density lipoproteins (LDL) are toxic to cultured endothelial cells. Mildly oxidized LDL, characterized by relatively low levels of TBARS and only minor modifications of apoB, were obtained by using 2 experimental model systems of oxidation, namely oxidation by u.v. radiation or ferrylmyoglobin (a two electron oxidation product from the reaction of metmyoglobin with H2O2). 2. Toxic concentrations of mildly oxidized LDL induce apoptosis (programmed cell death) of cultured endothelial cells, as shown by typical morphological features, by the in situ TUNEL procedure and by DNA fragmentation revealed on gel electrophoresis. This apoptosis is calcium-dependent and subsequent to the intense and sustained cytosolic [Ca2+]i peak elicited by oxidized LDL. 3. Five naturally occurring phenolic compounds present in food and beverages were able to prevent, in a concentration-dependent manner, the apoptosis of endothelial cells induced by oxidized LDL. Among the compounds tested, caffeic acid was the most effective. Under the conditions used, the protective effect of caffeic acid (IC50 8.3+/-2.1 micromol l[-1]) in the prevention of apoptosis induced by oxidized LDL was significantly higher than that of the other compounds tested (IC50s were 12.4+/-3.2, 14.1+/-4.1, 20.4+/-4.4 and 72.6+/-9.2 micromol l(-1) for ferulic, protocatechuic, ellagic and p-coumaric acids, respectively). 4. The anti-apoptotic effect of caffeic acid results from the addition of two effects, (i) the antioxidant effect which prevents LDL oxidation and subsequent toxicity ('indirect' protective effect); (ii) a 'direct' cytoprotective effect, acting at the cellular level. 5. Effective concentrations of caffeic acid acted at the cellular level by blocking the intense and sustained cytosolic [Ca2+]i rise elicited by oxidized LDL. 6. In conclusion, phenolic acids (caffeic and ferulic acids being the most potent of the compounds tested under the conditions used) exhibit a potent cytoprotective effect of cultured endothelial cells against oxidized LDL. In addition to antioxidant effect delaying LDL oxidation, caffeic acid acts as a cytoprotective agent, probably by blocking the intracellular signalling triggered by oxidized LDL and culminating in the sustained calcium rise which is involved in oxidized LDL-induced apoptosis.

Inhibition of metmyoglobin/H2O2-dependent low density lipoprotein lipid peroxidation by naturally occurring phenolic acids

The ferrylmyoglobin <==> metmyoglobin redox transitions promoted by hydrogen peroxide and dietary phenolic acids and their potential role in the oxidation of LDL were studied. The use of parinaric acid incorporated in LDL as a probe for radicals (detected by fluorescence quenching of the probe) revealed an oxidative stress inside LDL shortly ( < 1 min) after addition of hydrogen peroxide to metmyoglobin in the aqueous phase outside the particle, reflecting an efficient access of the oxidant to LDL lipids. However, the propagation step of peroxidation only occurs after a lag phase, as detected by the kinetics of oxygen consumption. Triton X-100 decreases but does not suppress the lag phase of oxidation. Addition of metmyoglobin (without peroxide) to LDL was not followed by significant oxidation during the time of the experiment, unless Triton X-100 was present in the medium. When dietary phenolic acids were present in the medium before peroxide addition, an inhibition of parinaric acid fluorescence quenching and oxygen consumption was recorded as a function of concentration and substitution pattern on the phenol ring of the phenolic acids. This was associated with a conversion of ferrylmyoglobin to metmyoglobin. The results indicate that the naturally occurring phenolic acids prevent ferrylmyoglobin-dependent LDL oxidation in a way strongly dependent on the substitution pattern on the phenol ring. Among the phenolic compounds studied, the o-dihydroxy derivatives of cinnamic and benzoic acids (caffeic, chlorogenic, and protocatechuic acids), in a molar ratio of 1 to metmyoglobin, efficiently blocked LDL oxidation initiated by ferrylmyoglobin. Replacement of one OH group from catecholic structure with an H (p-coumaric acid) or methoxy group (ferulic acid) decreased the antioxidant activity. Also, the catechol structure fused in heterocyclic rings with adjacent carbonyl groups (ellagic acid) resulted in decreased antioxidant activity. These observations correlate with the efficiency of phenolic acids to reduce ferrylmyoglobin to metmyoglobin. Therefore, the protection of LDL against oxidation is assigned to the reduction of the oxoferryl moiety of the hemoprotein to the ferric form. Additionally, it is suggested that an access constraint of oxidants plays a minor role in the ferrylmyoglobin-induced oxidation against LDL.

Two related phenolic antioxidants with opposite effects on vitamin E content in low density lipoproteins oxidized by ferrylmyoglobin: consumption vs regeneration

Endogenous alpha-tocopherol of low density lipoprotein (LDL) particles exposed to ferrylmyoglobin (iron in the form of FeIV = O) vanishes as a function of myoglobin concentration. After alpha-tocopherol depletion, subsequent heavy lipid peroxidation is prevented by caffeic and p-coumaric acids, i.e., phenolic acids present in foods and beverages, by a mechanism involving the one-electron transfer reaction between the phenols and the ferrylmyoglobin, with formation of metmyoglobin and the corresponding phenoxyl radicals from caffeic and p-coumaric acids, as previously discussed. Caffeic acid delays alpha-tocopherol consumption when present before oxidation challenging and restores alpha-tocopherol when added halfway during the reaction. Conversely, p-coumaric acid accelerates the rate of alpha-tocopherol consumption when added either before or during the oxidation reaction. In LDL enriched with alpha-tocopherol, caffeic acid induces an inhibition period of oxidation longer than that expected from the sum of discrete periods characteristic of the phenolic acid and alpha-tocopherol. Surprisingly, p-coumaric acid decreases the peroxidation chain rate. Similar effects of these phenolic acids on alpha-tocopherol consumption were observed in a Triton X-100 micellar system, i.e., in the absence of a peroxidation chain reaction. Results suggest that caffeic acid acts synergistically with alpha-tocopherol, extending the antioxidant capacity of LDL by recycling alpha-tocopherol from the alpha-tocopherol radical (i.e., alpha-tocopheroxyl radical). By contrast, the phenoxyl radical from p-coumaric acid (produced by electron-transfer reaction between phenolic acid and ferrylmyoglobin) oxidizes alpha-tocopherol. However, in spite of alpha-tocopherol consumption, the exchange reaction recycling p-coumaric acid can still afford an antioxidant protection to LDL on basis of the chain-breaking activity of p-coumaric acid. These results emphasize the biological relevance of small structural modifications of phenols on the interaction with alpha-tocopherol in LDL. The significance of these results in the context of atherosclerosis is discussed.

Reduction of ferrylmyoglobin by dietary phenolic acid derivatives of cinnamic acid

The reaction of dietary phenolic acid derivatives of cinnamic acid with high valence states of horse myoglobin was studied. When metmyoglobin was oxidized by hydrogen peroxide (H2O2) in the presence of the phenolic acids, ferrylmyoglobin was reduced to metmyoglobin. However, addition of the phenolic acids to a ferrylmyoglobin solution resulted in a modified metmyoglobin spectrum characterized mainly by a blue shift of the 631 nm peak and a new isosbestic point at 613 nm suggesting an irreversible modification of the hemeprotein. The efficiency and the kinetic profile of ferrylmyoglobin reduction were dependent on both the concentration and the structure of the phenolic acid. Electron-donating substituent groups in the phenol ring increased the efficiency of ferrylmyoglobin reduction whereas electron-withdrawing groups decreased it. The phenolic acids exhibiting a catechol structure were the most efficient in reducing ferrylmyoglobin. Caffeic and chlorogenic acids react faster than trolox, and caffeic acid faster than ascorbate. During the electron-transfer reactions, the phenolic acids were oxidized to quinoid forms and, in some cases, to polymer products as indicated by comparison of the ultraviolet (UV) spectra obtained in the presence of metmyoglobin/H2O2 with those recorded after oxidation of phenolic acids by horseradish peroxidase/H2O2 and catechol oxidase. The results suggest that dietary phenolic derivatives of cinnamic acid may counteract deleterious oxidations initiated by ferrylmyoglobin.