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Reactive oxygen species in living systems: source, biochemistry, and role in human disease.



(Oxidants and Antioxidants: Pathophysiologic Determinants and Therapeutic Agents)
Authors: Halliwell, Barry
Citation: American Journal of Medicine, Sept 30, 1991 v91 n3C p14S(9)
Subjects: Free radicals (Chemistry) Physiological aspects
Antioxidants Physiological aspects
Diseases Causes and theories of causation
Reference #: A12136862


Author's Abstract: COPYRIGHT Cahners Publishing Company 1991


Reactive oxygen species are constantly formed in the human body and removed by antioxidant defenses. An antioxidant is a substance that, when present at low concentrations compared to that of an oxidizable substrate, significantly delays or prevents oxidation of that substrate. Antioxidants can act by scavenging biologically important reactive oxygen species ([O.sub.2].,[H.sub.2][O.sub.2], .OH, HOCl, ferryl, peroxyl, and alkoxyl), by preventing their formation, or by repairing the damage that they do. One problem with scavenging-type antioxidants is that secondary radicals derived from them can often themselves do biologic damage. These various principles will be illustrated by considering several thiol compounds.


Full Text COPYRIGHT Cahners Publishing Company 1991


It is difficult these days to open a medical journal and not find some paper on the role of "reactive oxygen species" or "free radicals" in human disease. These species have been implicated in over 50 diseases[1]. This large number suggests that radicals are not something esoteric, but that they participate as a fundamental component of tissue injury in most, if not all, human diseases.


What are "free radicals" and "reactive oxygen species"? Do they cause disease? Are they produced in increased amounts as a result of disease and then contribute to further tissue injury? Are they merely an epiphenomenon of no relevance to clinical medicine? This introductory article attempts to answer such questions.




Electrons in atoms occupy regions of space known as orbitals. Each orbital can hold a maximum of two electrons, spinning in opposite directions. A free radical can be defined as any species capable of independent existence that contains one or more unpaired electrons, an unpaired electron being one that is alone in an orbital. Most biologic molecules are nonradicals, containing only paired electrons.


An electron occupying an orbital by itself has two possible directions of spin. Indeed, the technique of measuring electron spin resonance detects radicals by measuring the energy changes that occur as unpaired electrons ~relax' following alignment in response to a magnetic field[2]. Since electrons are more stable when paired together in orbitals, radicals generally are more reactive than nonradicals, although there is a considerable variation in their reactivity. Radicals can react with other molecules in a number of ways[3]. If two radicals meet, they can combine their unpaired electrons (symbolized by.) and join to form a covalent bond (a shared pair of electrons). The hydrogen atom, with one unpaired electron, is a radical and two atoms of hydrogen easily combine to form the diatomic hydrogen molecule:


H. + H. [arrow right] [H.sub.2]


Radicals react with nonradicals in several ways. A radical may donate its unpaired electron to a non-radical (a reducing radical) or it might take an electron from another molecule in order to form a pair (an oxidizing radical). A radical may also join onto a nonradical. Whichever of these three types of reaction occurs, the nonradical species becomes a radical. A feature of the reactions of free radicals with nonradicals is that they tend to proceed as chain reactions, where one radical begets another. For many years, chemists have been interested in free radical reactions. Many plastics, such as polythene, arise by free radical chain polymerization[4]. Combustion is a free radical reaction. The drying and aging of paint also involves free radical reactions. Curators of museums have studied the role of free radical damage in the age-dependent deterioration of paintings and other items[5]. Metabolism of toxins in the human body can produce radicals. For example, carbon tetrachloride ([CCl.sub.4]) is metabolized in the endoplasmic reticulum of the liver to produce the damaging trichloromethyl radical, [CCl.sub.3.][3].




Chemists and biologists have examined in detail the role of free radical reactions in the damage done to living cells by high-energy radiation. When tissues are exposed to, for example, gamma radiation, most of the energy taken up is absorbed by the cell water, largely because there is more water there than any other molecule. The radiation causes one of the oxygen - hydrogen covalent bonds in water to split, leaving a single electron on hydrogen and one on oxygen, thus creating two radicals:


[Mathematical Expression Omitted]


H. is a hydrogen radical (or hydrogen atom), and .OH is a hydroxyl radical. The latter is the most reactive radical known to chemistry. It can attack and damage almost every molecule found in living cells at a diffusion-controlled rate, i.e., .OH reacts as soon as it comes into contact with another molecule in solution. Since it is so reactive, .OH generated in vivo does not persist for even a microsecond and rapidly combines with molecules in its immediate vicinity. Reactions of .OH with biologic molecules, must of which are nonradicals, set off chain reactions[1]. Reactions of .OH include its ability to interact with the purine and pyrimidine bases of DNA, leading to radicals that have a number of possible chemical fates[6]. .OH can also abstract hydrogen atoms from many biologic molecules, including thiols:

R - SH + .OH [arrow right] RS. + [H.sub.2.O]


The resulting sulfur radicals (thiyl radicals) have many interesting chemical properties. They can combine with oxygen to generate oxysulfur radicals, such as [RSO.sub.2.] and RSO., a number of which damage biologic molecules[7-9]. For example, sulfur-containing radicals derived from the drug penicillamine are able to attack and damage certain proteins[10]. When discussing the use of thiol compounds as free radical scavengers, it is essential to ask what may happen to the resulting sulfur radicals in biologic systems[11]. Perhaps the best-characterized biologic damage caused by .OH is its ability to stimulate the free radical chain reaction known as lipid peroxidation. This occurs when the .OH is generated close to membranes and attacks the fatty acid side chains of the membrane phospholipids. It preferentially attacks polyunsaturated fatty acid side chains, such as arachidonic acid. The .OH abstracts an atom of hydrogen from one of the carbon atoms in the side chain and combines with it to form water:


CH - + .OH [arrow right] - C + [H.sub.2.O]


Reaction (4) removes the .OH, but leaves behind a carbon-centered radical ( - C - ) in the membrane. Carbon-centered radicals formed from polyunsaturated fatty acid side chains usually undergo molecular rearrangement to give conjugated diene structures, which can have various fates. Thus, if two such radicals collided in the membrane, cross-linking of fatty acid side chains could occur as the two electrons joined to form a covalent bond. Reaction with membrane proteins is also a possibility. However, under physiologic conditions, the most likely fate of carbon-centered radicals is to combine with oxygen, creating yet another radical, the peroxyl radical (sometimes abbreviated to the peroxy radical):


[Mathematical Expression Omitted]


Peroxyl radicals are reactive enough to attack adjacent fatty acid side chains, abstracting hydrogen:


[Mathematical Expression Omitted]


Another carbon-centered radical is generated, and so the chain reaction [equations (5) and (6)] continues. One .OH can result in the conversion of many hundred fatty acid side chains into lipid hydroperoxides. Accumulation of lipid hydroperoxides in a membrane disrupts its function and can cause it to collapse. Lipid hydroperoxides can also decompose to yield a range of highly cytotoxic products, among the most unpleasant of which are aldehydes[12]. A great deal of attention in the literature has been focused on malonaldehyde (malondialdehyde), but this is much less noxious than such products as 4-hydroxynonenal[12,13]. Peroxyl radicals and cytotoxic aldehydes can also cause severe damage to membrane proteins, inactivating receptors and membrane-bound enzymes[14].




Biochemists (apart from those with a special interest in "background" free radical generation in vivo, due to exposure to ionizing radiation) became interested in radicals only in the 1970s. This interest followed from the discovery in 1968 of superoxide dismutase (SOD), and enzyme specific for a free radical substrate[15]. SOD removes superoxide radical, a species that is formed by adding an extra electron onto the oxygen molecule:


[Mathematical Expression Omitted]


SOD removes O2 by catalyzing a dismutation reaction, involving oxidation of the O2 to oxygen and reduction of another O2 to hydrogen peroxide:


[Mathematical Expression Omitted]


In the absence of SOD, reaction (8) occurs nonezymically but at a rate approximately four orders of magnitude less at pH 7.4.


The discovery of SOD led to the realization that [O.sup.-.sub.2] is formed in vivo in living organisms, and SOD removes it. Some of the O2 formed in vivo arises from a chemical accident. For example, when mitochondria are functioning, some of the electrons passing through the respiratory chain leak from the electron carriers and pass directly onto oxygen, reducing it to O2[15,16]. Many molecules oxidize on contact with oxygen, e.g., and epinephrine solution left on the bench "goes off" and eventually forms a pink product. The first stage in this oxidation is transfer of an electron from the epinephrine to [O.sub.2], forming O2. Such oxidations undoubtedly proceed in vivo as well[1]. For example, several sugars, including glucose, interact with proteins to produce oxygen radicals. It has been suggested that decades of exposure of body tissues to elevated blood glucose can result in diabetic patients suffering "oxidative stress" that may contribute to the side effects of hyperglycemia[17]. Glycation of proteins involves not only direct reaction with the sugar but also free radical reactions[17].


Thiols can also be oxidized in the presence of oxygen, generating sulfur-containing radicals as well as O2 and .OH. Thiol oxidation is favored by alkaline pH values and by the presence of transition metal ions, especially copper ions[18]. Thus, mixtures of copper ions and thiols can be cytotoxic, as shown for cysteine[19]. Iron ions can also promote free radical generation from thiols under certain circumstances[20]. Attempts to use thiols as anti-oxidants in systems containing iron or copper ions may even result in stimulation of oxidative damage.


Superoxide and Phagocyte Action


Some of the O2 production in vivo may be accidental but much is functional. Activated phagocytic cells generate O2 as shown for monocytes, neutrophils, eosinophils, and macrophages of all types[21]. Radical production is important in allowing phagocytes to kill some of the bacterial strains that they engulf. This can be illustrated by examining patients with chronic granulomatous disease, a series of inborn conditions in which the membrane-bound reduced nictotinamide adenine dinucleotide phosphate (NADPH) oxidase system in phagocytes that makes the O2 fails to work[21]. Such patients have phagocytes that engulf and process bacteria normally, but several bacterial strains are not killed and are released in viable form when the phagocytes die. Thus, patients suffer sever, persistent, and multiple infections with such organisms as Staphylococcus aureus. Another killing mechanism used by neutrophils (but not by macrophages) is the enzyme myeloperoxidase[22]. It uses [H.sub.2.O.sub.2] produced by dismutation of O2 to oxidize chloride ions into hypochlorous acid (HOCl), a powerful anti-bacterial agent:


[H.sub.2.O.sub.2] + [Cl.sup.- arrow right] HOCl + [OH.sup.-]


Thiol groups are easily oxidized by HOCl. Hence, low molecular mass thiol compounds such as glutathione (GSH), N-acetylcysteine, and mercaptopropionylglycine are very effective at protecting, for example, proteins against oxidative damage by HOCl[23.24]. Superoxice formed in vivo, whether functionally or accidentally, is disposed of by SOD [equation (8)]. Recent studies using genetic engineering techniques to manipulate SOD levels of organisms, or to delete the genes encoding SOD, provide further evidence of the importance of SOD[25]. It is interesting to note that no complete inborn deficiencies of SOD have been reported in humans, perhaps because they would be lethal mutations.


Reactive Oxygen Species


SOD removes O2 by converting it into hydrogen peroxide ([H.sub.2.O.sub.2]) and [O.sub.2] [equation (8)]. [H.sub.2.sub.2] itself can be quite toxic to cells. For example, incubation of cells with [H.sub.2.O.sub.2] causes deoxyribonucleic acid (DNA) damage, membrane disruption, and release of [Ca.sup.2+] ions within the cells, leading to activation of [Ca.sup.2+]-dependent proteases and nucleases[26]. At least some of this damage may be mediated by a reaction of [H.sub.2.O.sub.2] with O2 in the presence of iron or copper ions, to form highly reactive radicals, one of which .OH. This reaction proceeds in a number of stages, but the overall process is summarized by


[Mathematical Expression Omitted]


Thus, removal of [H.sub.2.O.sub.2], as well as of O2, is biologically advantageous[27].


SOD therefore works in conjunction with two enzymes. catalase and glutathione peroxidase[27], that remove [H.sub.2.O.sub.2] in human cells. The study of inborn errors of metabolism suggests that glutathione peroxidase (GSH-Px) is the more important of the two in removing [H.sub.2.O.sub.2], probably because it is located in the same subcellular compartments (cytosol and mitochondria) as SOD. GSH-Px has the distinction of being the only human enzyme known requiring the element selenium for its activity; a selecnocysteine residue (side chain -SeH instead of -SH, as in normal cysteine) is present at its active site. However, it is unlikely that the sole function of selenium in humans is to act as a cofactor for GSH-Px[28]. GSH-Px removes [H.sub.2.O.sub.2] by suisng it to oxidize reduced glutathione (GSH) into oxidized glutathione (GSSG):


2GSH + [H.sub.2.O.sub.2 arrow right] GSSG + [2H.sub.2O


[H.sub.2.O.sub.2] has no unpaired electrons and does not qualify as a radical. Hence, the term reactive oxygen species has been introduced to describe collectively not only O2 and .OH (radicals) but also [H.sub.2.O.sub.2] (nonradical). Hypochlorous acid (HOCl) produced by myeloperoxidase is also a nonradical, having no unpaired electrons. [H.sub.2.O.sub.2.], .OH, and HOCl are sometimes collectively called "oxidants." This is valid description of [H.sub.2.O.sub.2], .OH, and HOCl, which are oxidizing agents. However, O2 has both oxidizing and reducing properties. The latter property is used in a popular assay for O2, the SOD-inhibitable reduction of cytochrome c, often applied to measure [O.sup.-.sbu.2.] production by phagocytes:


cyt c (Fe.sup.3+] + [O.sup.-.sub.2 [arrow right] O.sub.2] + cyt c (Fe.sup.2+




Many transition metals have variable oxidation numbers, e.g., iron has [Fe.sup.2+] and [Fe.sup.3+] ions and copper has [Cu.sup.+] and [Cu.sup.2+] ions. Changing between oxidation states involves accepting and donating single electrons, e.g.,


[Mathematical Expression Omitted]


Transition metal ions are remarkably good promoters of free radical reactions[29]. Polymer scientists and food chemists have been aware of this for years[4,30], and biochemist are learning it too [1,17-20,26,31-34]. It has already been noted that copper ions promote oxidation of thiols:


R-SH + [Cu.sub.2+ arrow right R-S.] + [Cu.sup.+] + [H.sup.+]


and that [Fe.sup.2+] ions reduce [H.sub.2.O.sub.2] to give * OH [equation (10)].


Transition Metals and Lipid Peroxidation


Transition metal ions are involved in lipid peroxidation in two ways. They can participate in first-chain initiation, which involves attack by any species capable of abstracting a hydrogen atom. .OH, which has this property, is produced by the reaction of O2 and [H.sub.2.O.sub.2] with iron ion catalysis [equation (10)]. It is also be produced by reaction of [H.sub.2.O.sub.2] with copper ions, probably in addition to oxidizing copper(III)-oxygen complexes[26,31]. Several iron ion-oxygen complexes, such as perferryl, ferryl, or [Fe.sup.2+]/[Fe.sup.3+]/[O.sup.2] complexes, have also been claimed to initiate peroxidation[32], although their ability to do so is uncertain[33,34].


Transition metal ions also affect lipid peroxidation by decomposing peroxides. Commercial fatty acids are heavily contaminated with peroxides[34]. Cell disruption to isolate membrane fractions increases rates of nonenzymic free radical reactions and activates enzymes (cyclooxygenases and lipooxygenases) that produce peroxides (Figure 1). When transition metal ions are added to lipid systems already containing peroxides, their main action is to decompose these peroxides into peroxyl and alkoxyl (lipid-O *) radicals that in turn abstract hydrogen and perpetuate the chain reaction of lipid peroxidation[34]. This may be represented by the following simplified equations, in which lipid * symbolises a carbon-centered radical


[Mathematical Expression Omitted]


lipid O. + lipid-H [arrow right] lipid-OH + lipid.


lipid-OO. + lipid-H [arrow right] lipid-OOH + lipid.


lipid. + [O.sub.2] [arrow right] lipid-OO.


Reducing agents, such as ascorbic acid or [O.sup.2-*], accelerate these metal ion-dependent peroxidation reactions because [Cu.sup.+] and [Fe.sup.2+] ions seem to react with peroxides faster than do [Cu.sup.2+] and [Fe.sup.3+] respectively. The end products of these complex metal ion-catalyzed breakdowns of lipid hydroperoxides include the cytotoxic aldehydes mentioned previously (malonaldehyde, 4-hydroxynonenal), as well as hydrocarbon gases such as ethane and pentane[1]. Some thiyl compounds can also reduce metal ions and accelerate peroxidation of lipids, e.g., cysteine[35]. It has been suggested that some thiyl radicals (RS.] initiate peroxidation by abstracting hydrogen atoms from lipids [36]. Different thiols behave differently in peroxidizing lipid systems, presumably depending on their metal ion-reducing ability and the reactivity of their thiyl radicals.




Organisms use superoxide dismutases, catalase, and glutathione peroxidase as protection against generation of reactive oxygen species. Organisms also keep as many iron and copper ions as possible safely bound in storage or transport proteins[37-39]. There is three times as much transferrin iron-binding capacity in plasma as iron needing to be transported, so that there are essentially no free iron ions in the plasma[38]. Iron ions bound to transferrin cannot stimulate lipid peroxidation or formation of free .OH radicals. The same is true of copper ions bound to the plasma proteins ceruloplasmin or albumin[37-40]. The value of this sequestration is shown by an inspection of the pathology suffered by patients with iron-overload disease, in whom iron ion-citrate chelates circulate in the blood[40]. These patients can suffer liver damage, diabetes, joint inflammation, and hepatoma, among other problems[41]. Metal ion sequestration is an important antioxidant defense. For example, recent papers have referred to ascorbic acid as a major antioxidant in plasma. However, ascorbate can only exert antioxidant properties in the absence of transition metal ions[11].




As well as the primary defenses (scavenger enzymes and metal-ion sequestration), secondary defenses are also present. The cell membranes and plasma lipoproteins contain [alpha]-tocopherol, a lipidsoluble molecule that functions as a chain-breaking antioxidant. Attached to the hydrophobic structure of [alpha]-tocopherol is an -OH group whose hydrogen atom is easily removed. Hence, peroxyl and alkoxyl radicals generated during lipid peroxidation preferentially combine with the antioxidant, e.g.,


[Mathematical Expression Omitted]


instead of with an adjacent fatty acid side chain. This therefore terminates the chain reaction, whence the term chain-breaking antioxidant. It also converts the [alpha]-tocopherol into a new radical, tocopherol-O., which is poorly reactive and unable to attack adjacent fatty acid side chains, consequently stopping the chain reaction. Evidence exists[43,44] that the tocopherol radical can migrate to the membrane surface and reconvert to [alpha]-tocopherol by reaction with ascorbic acid (vitamin C). Both vitamin C and [alpha]-tocopherol seem to minimize the consequences of lipid peroxidation in lipoproteins and in membranes, should this process begin. Some thiol compounds, such as GSH, might also be involved in regenerating [alpha]-tocopherol from its radical in vivo[44].


The terms "[alpha]-tocopherol" and "vitamin E" are often used synonymously, which is not strictly correct. Vitamin E is defined nutritionally as a factor needed in the diet of pregnant female rats to prevent resorption of the fetus[45] and compounds other than [alpha]-tocopherol (e.g., [beta-, gamma-, and delta-tocopherols) have some effect in this assay. However, [alpha]-tocopherol is the most effective, and it seems to be the most important lipid-soluble chainbreaking antioxidant in vivo in humans[46]. The content of [alpha]-tocopherol in circulating low-density lipoproteins helps to determine their resistance to lipid peroxidation and thus may affect the development of atherosclerosis, a disease in which lipid peroxidation is involved[47]. Low plasma levels of f [alpha]-tocopherol and vitamin C correlate with an increased incidence of myocardial infarction and of some forms of cancer[47].


Other Antioxidants and Repair Systems


Some other compounds may also function as antioxidants in vivo, such as uric acid, ubiquinol, and bilirubin (reviewed in[11].). Antioxidant defenses are not quite perfect. Cells contain systems that can repair DNA after attack by radicals[48], degrade proteins damages by radicals[49], and metabolize lipid hydroperoxides[1].




What Is an Antioxidant?

"Antioxidant" can be define in various ways. Often, the term is implicitly restricted to chainbreaking antioxidant inhibitors of lipid peroxidation, such as vitamin E. However, the author prefers a broader definition - an antioxidant is any substance that, when present at low concentrations compared with those of an oxidizable substrate, significantly delays or prevents oxidation of that substrate[1]. The term "oxidizable substrate" includes almost everything found in living cells, including proteins, lipids, carbohydrates, and DNA.


Antioxidants act in many different ways (Table I). In proposing antioxidants for use in human disease, it is important to note the following: (a) the precise role played in the disease pathology by reactive oxygen species; and (b) the molecular targets of oxidative damage that need protecting. Thus, oxidative stress can damage a multiplicity of targets in living cells and the initial damage to one target can then affect others[26]. Figure 2 attempts to illustrate some of the complex interacting mechanisms by which express production of reactive oxygen species can produce cell damage. If, for example, the primary event in damage to DNA, then an inhibitor of lipid peroxidation might offer little or no protection. !!! TABLE TABLE I Questions to Ask When Evaluating the Proposed Role of an "Antioxidant" In Vivo

1. What biomolecule is the compound supposed to protect? An inhibitor of lipid peroxidation is unlikely to be useful if the oxidative damage is mediated by an attack on proteins or DNA.

2. Is the compound present in vivo at or near that biomolecule at sufficient concentration? For example, many compounds have been suggested to act as .OH scavengers in vivo. In order to compete with biologic molecules for .OH, a scavenger must be present in at least millimolar concentrations in vivo. Most drugs never achieve this sort of concentration.

3. How does it protect: by scavenging reactive oxygen species, by preventing their formation, or by repairing damage?

4. For naturally occurring antioxidants, is antioxidant protection the primary biologic role of the molecule or a secondary one? For example, SOD has probably evolved as an antioxidant enzyme. By contrast, transferrin has probably evolved as an iron transport protein, although the binding of iron ions to transferrin prevents them from accelerating radical reactions, giving this protein an important secondary role in extracellular antioxidant defense.

5. If the antioxidant acts by scavenging a reactive oxygen species, can the antioxidant-derived radicals themselves do biologic damage?

6. Can the antioxidant cause damage in biologic systems different from those in which it exerts protection?


Free Radicals and Human Disease: Causation or Consequence?

Does increased formation of free radicals and other reactive oxygen species cause any human disease? Radiation-induced carcinogenesis be initiated by free radical damage[48]. The signs produced by chronic dietary deficiencies of selenium (Keshan disease) or of vitamin E (neurologic disorders seen in patients with inborn errors in the mechanism of intestinal fat absorption) could also be mediated by reactive oxygen species[28,50]. In the premature infant, exposure of the incompletely vascularized retina to elevated concentrations of oxygen can lead to retinopathy of prematurity, which in its most severe forms can result in blindness. Several controlled clinical trials have documented the efficiency of [alpha]- tocopherol in minimizing the retinopathy[51], suggesting a role for lipid per-oxidation.


For most human diseases, increased formation of reactive oxygen species is secondary to the primary disease process. For example, activated neutrophils produce [O.sub.2-], and HOCl in order to kill bacteria. If a large number of phagocytes become activated in a localized area, they can produce tissue damage. The synovial fluid in the swollen knee joints of rheumatoid patients swarm with activated neutrophils. There is evidence that reactive oxygen species and other products derived from neutrophils contribute to the joint injury. Whether this is a major or a minor contribution to joint damage remains to be established[52]. In some forms of adult respiratory distress syndrome (ARDS), lung damage seems to be mediated by an influx of neutrophils into the lung, where they become activated to produce prostaglandins, leukotrienes, proteolytic enzymes such as elastase, and reactive oxygen species[53]. Among other effects, reactive oxygen species inactivate proteins (such as [alpha 1]-antiproteinase) within the lung that normally inhibit the action of elastase and prevent it from attacking lung elastic fibers. The precise contribution of oxidative damage to lung injury in ARDS is unknown, but deserves investigation in view of the high mortality rate.


In both ARDS and in rheumatoid arthritis, increased generation of reactive oxygen species is secondary to the processes that cause neutrophil infiltration, but they then may make an additional detrimental contribution to tissue injury.


There are several examples in which injury, by a nonradical mechanism, leads to increased free radical reactions. Mechanical (e.g., crushing) or chemical injury to tissues can cause cells to rupture and release their contents, including transition metal ions (Figure 1), into the surrounding area. Administration of cytotoxic drugs to patients with acute myeloid leukemia has been shown to create a temporary "iron-overload" state, probably due to extensive drug-induced lysis of the leukemic cells. This increased iron availability could contribute to the side effects of cytotoxic chemotherapy[54].


Perhaps the greatest interest in this area lies in the sequelae of traumatic or ischemic injury to the brain. Some Areas of the human brain are rich in iron. Cerebrospinal fluid has no significant ironbinding capacity, since its content of transferrin is low. It has been proposed[55] that injury to the brain by mechanical means (trauma) or by oxygen deprivation (stroke) can result in release of iron ions into the surrounding area. These ions facilitate further damage to the surrounding areas by accelerating free radical reactions. This proposal has been given some support from animal studies, using antioxidants such as chelating agents that bind iron ions and prevent from catalyzing radical reactions. Promising results have been obtained with amino-steroid-based antioxidants. Thus, one such "lazaroid," U74006F, has been observed to decrease the effects of reperfusion injury upon the brain of cats[56] to decrease post-traumatic spinal cord degeneration in cats[57] and to minimize neurologic damage after head injury in mice[58].


Free Radicals in Human Disease: A Triviality?

Tissue destruction and degeneration can result in increased oxidative damage, by such processes as metal-ion release, phagocyte activation, lipoxygenase activation, and disruption of mitochondrial electron transport chains, so that more electrons "escape" to oxygen to form [O.sub.2-] . (Figure 1). If follows that almost any disease is likely to be accompanied by increased formation of reactive oxygen species. It is not therefore surprising that the list of diseases in which their formation has been implicated is long and is growing longer[1]. For atherosclerosis[43,59], rheumatoid arthritis[52], some forms of ARDS, reoxygeneration injury[60,61], and traumatic or ischemic damage to the central nervous system, there is reasonable evidence to suggest that free radical reactions make a significant detrimental contribution to the pathologic process. As previously stressed[62], it is equally likely that in some (perhaps most) diseases, the increased ROS formation is an epiphenomenon, making no significant contribution to the progression of the disease. Each proposal must be subject to stringent examination, because the likely clinical value of "antioxidant therapy" will depend on how well the exact role of reactive oxygen species is known.




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