Oxidation free radicals formation

Oxidative stress refers to an imbalance between the generation of ROS and the activity of the antioxidant defenses in the body (Aruoma, 1998). Severe oxidative stress can cause cell damage and death. Oxidative stress may occur as a consequence of pollution (cigarette smoke, ozone and nitrogen oxides). Free radical formation may also be the side effect of certain drugs or disease treatments (radiation therapy) (Halliwell, 1997). An excess of free radicals may induce changes that ultimately lead to the development of various diseases. 

There are numerous synthetic and natural compounds called antioxidants which regulate or block oxidative reactions by quenching free radicals or by preventing free-radical formation. Vitamins A, C, and E and the mineral selenium are common antioxidants occurring naturally in foods (104,105). A broad range of flavonoid or phenoHc compounds have been found to be functional antioxidants in numerous test systems (106—108). The antioxidant properties of tea flavonoids have been characterized using models of chemical and biological oxidation reactions. 

The reactive species that iaitiate free-radical oxidatioa are preseat ia trace amouats. Exteasive studies (11) of the autoxidatioa mechanism have clearly estabUshed that the most reactive materials are thiols and disulfides, heterocycHc nitrogen compounds, diolefins, furans, and certain aromatic-olefin compounds. Because free-radical formation is accelerated by metal ions of copper, cobalt, and even iron (12), the presence of metals further compHcates the control of oxidation. It is difficult to avoid some metals, particularly iron, ia fuel systems. 

The oxidation of polymers is most commonly depicted in terms of the kinetic scheme developed by BoUand [14]. The scheme is summarized in Figure 15.1. The key to the process is the initial formation of a free-radical species. At high temperatures and at large shear forces, it is likely that free-radical formation takes place by cleavage of C-C and C-H bonds. 

The coupling of two DIT molecules to form T4—or of an MIT and DIT to form T3—occurs within the thyroglobulin molecule. A separate coupfing enzyme has not been found, and since this is an oxidative process it is assumed that the same thyroperoxidase catalyzes this reaction by stimulating free radical formation of iodotyrosine. This hypothesis is supported by the observation that the same drugs which inhibit H oxidation also inhibit coupfing. The formed thyroid hor-... 

A discussion of ligand exchange reactions of organometallic compounds associated with oxidation-reduction processes leading to free-radical formation will be found in Volume 14 (Free-radical polymerization). 

The initiating action of ozone on hydrocarbon oxidation was demonstrated in the case of oxidation of paraffin wax [110] and isodecane [111]. The results of these experiments were described in a monograph [109]. The detailed kinetic study of cyclohexane and cumene oxidation by a mixture of dioxygen and ozone was performed by Komissarov [112]. Ozone is known to be a very active oxidizing agent [113 116]. Ozone reacts with C—H bonds of hydrocarbons and other organic compounds with free radical formation, which was proved by different experimental methods. 

Since for an endothermic reaction the activation energy E > AH, all such reactions cannot explain the experimental value of the activation energy (see Chapter 4). The following mechanism seems to be the most probable now. Hydrogen peroxide is protonized in a polar alcohol solution. Protonization of H202 intensifies its oxidizing reactivity. Protonized hydrogen peroxide reacts with alcohol with free radical formation. 

Free radical formation in oxidized organic compounds occurs through a few reactions of oxygen bimolecular and trimolecular reactions with the weakest C—H bond and double bond (see Chapter 4). The study of free radical generation in polymers (PE, PP) proved that free radicals are produced by the reaction with dioxigen. The rate of initiation was found to be proportional to the partial pressure of oxygen [6,97]. This rate in a polymer solution is proportional to the product [PH] x [02]. The values of the apparent rate constants (/ti0) of free radical formation by the reaction of dioxygen (v 0 = k 0[PH][O2]) are collected in Table 13.8. 

Due to the high reactivity of cumene, the reaction of the peroxyl macroradical with cumene occurs more rapidly than the intramolecular reaction and the formed POOH is only from the single hydroperoxyl groups. Such POOH decomposes with free radical formation much more slowly than POOH produced in PP oxidation in the solution and solid state. 

Hence, the copper surface catalyzes the following reactions (a) decomposition of hydroperoxide to free radicals, (b) generation of free radicals by dioxygen, (c) reaction of hydroperoxide with amine, and (d) heterogeneous reaction of dioxygen with amine with free radical formation. All these reactions occur homolytically [13]. The products of amines oxidation additionally retard the oxidation of hydrocarbons after induction period. The kinetic characteristics of these reactions (T-6, T = 398 K, [13]) are presented below. 

High antioxidative activity carvedilol has been shown in isolated rat heart mitochondria [297] and in the protection against myocardial injury in postischemic rat hearts [281]. Carvedilol also preserved tissue GSL content and diminished peroxynitrite-induced tissue injury in hypercholesterolemic rabbits [298]. Habon et al. [299] showed that carvedilol significantly decreased the ischemia-reperfusion-stimulated free radical formation and lipid peroxidation in rat hearts. Very small I50 values have been obtained for the metabolite of carvedilol SB 211475 in the iron-ascorbate-initiated lipid peroxidation of brain homogenate (0.28 pmol D1), mouse macrophage-stimulated LDL oxidation (0.043 pmol I 1), the hydroxyl-initiated lipid peroxidation of bovine pulmonary artery endothelial cells (0.15 pmol U1), the cell damage measured by LDL release (0.16 pmol l-1), and the promotion of cell survival (0.13 pmol l-1) [300]. SB 211475 also inhibited superoxide production by PMA-stimulated human neutrophils. 

At present, numerous free radical studies related to many pathologies have been carried out. The amount of these studies is really enormous and many of them are too far from the scope of this book. The main topics of this chapter will be confined to the mechanism of free radical formation and oxidative processes under pathophysiological conditions. We will consider the possible role of free radicals in cardiovascular disorders, cancer, anemias, inflammation, diabetes mellitus, rheumatoid arthritis, and some other diseases. Furthermore, the possibilities of antioxidant and chelating therapies will be discussed. 

Cystic fibrosis is the most common lethal autosomal-recessive disease, in which oxidative stress takes place at the airway surface [274]. This disease is characterized by chronic infection and inflammation. Enhanced free radical formation in cystic fibrosis has been shown as early as 1989 [275] and was confirmed in many following studies (see references in Ref. [274]). Contemporary studies also confirm the importance of oxidative stress in the development of cystic fibrosis. Ciabattoni et al. [276] demonstrated the enhanced in vivo lipid peroxidation and platelet activation in this disease. These authors found that urinary excretion of the products of nonenzymatic lipid peroxidation PGF2 and TXB2 was significantly higher in cystic fibrotic patients than in control subjects. It is of importance that vitamin E supplementation resulted in the reduction of the levels of these products of peroxidation. Exhaled ethane, a noninvasive marker of oxidative stress, has also been shown to increase in cystic fibrosis patients [277]. 

Some other examples of free radical formation in various pathologies are discussed below. (Of course, they are only few examples among many others, which can be found in literature.) Mitochondrial diseases are associated with superoxide overproduction [428] and cytochrome c release [429], For example, mitochondrial superoxide production apparently contributes to hippocampal pathology produced by kainate [430]. It has been found that erythrocytes from iron deficiency anemia are more susceptible to oxidative stress than normal cells but have a good capacity for recovery [431]. The beneficial effects of treatment of iron deficiency anemia with iron dextran and iron polymaltose complexes have been shown [432,433]. 

FIGURE 32-7 Sources of free radical formation which may contribute to injury during ischemia-reperfusion. Nitric oxide synthase, the mitochondrial electron-transport chain and metabolism of arachidonic acid are among the likely contributors. CaM, calcium/calmodulin FAD, flavin adenine dinucleotide FMN, flavin mononucleotide HtT, tetrahydrobiopterin HETES, hydroxyeicosatetraenoic acids L, lipid alkoxyl radical LOO, lipid peroxyl radical NO, nitric oxide 0 "2, superoxide radical. 

Dermal toxicity due to JP-8 may also be attributed to increased free radical formation that may be involved in the development of skin sensitization [45], Following dermal exposure to JP-8, increasing levels of oxidative species are observed [46], In vitro studies with rat lung epithelial cells demonstrated that JP-8-induced cell death is inhibited by exogenous glutathione or the thiol-containing antioxidant N-acetyl-cysteine... 

How does UV-induced free radical formation activate immune suppression Some have suggested that UV-induced cytokine production is involved. Because both DNA damage and oxidative stress can activate transcription of the cytokines that activate immune suppression,23>24one of the problems faced by investigators in the field was to divorce the effects of DNA damage from membrane oxidation. One approach was to look at the activation of transcription factors in UV-irradiated enucleated cells. Devary and colleagues25 observed that both NF-kB and AP-1 were activated in enucleated... 

As mentioned above, PAF and PAF-like molecules are rapidly synthesized by keratinocytes following UV exposure. We suggest that two mechanisms are involved. UV-induced free radical formation leads to membrane oxidation and the formation of oxidized phosphatidylcholine. The PAF-like molecules bind to PAF receptors in either a paracrine or autocrine fashion. This induces the release of arachidonic acid from the membrane, activates PI.A2 and promotes the synthesis of bona fide PAF.55 The newly synthesized PAF then binds to PAF receptors, which upregulates the production of more PAF and downstream biological modifiers such as eicosanoids and cytokines. Ultimately this activates the cascade of events that leads to immune suppression. 

Transition metals (iron, copper, nickel and cobalt) catalyse oxidation by shortening the induction period, and by promoting free radical formation [60]. Hong et al. [61] reported on the oxidation of a substimted a-hydroxyamine in an intravenous formulation. The kinetic investigations showed that the molecule underwent a one-electron transfer oxidative mechanism, which was catalysed by transition metals. This yielded two oxidative degradants 4-hydroxybenzalde-hyde and 4-hydroxy-4-phenylpiperidine. It has been previously shown that a-hydroxyamines are good metal ion chelators [62], and that this can induce oxidative attack on the a-hydroxy functionality. 

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