Environmental Stressors in Health and Disease

Jürgen Fuchs, Jurgen Fuchs

• Publisher: CRC Press
• Year: 2001
• Print ISBN: 9780824705305
• eBook ISBN: 9781135551261
• Topic: Medicine
• Subtopic: Pharmacology

This volume illustrates the impact of environmental oxidants on the tissues of the eyes, lungs and skin, as well as on the immune system - highlighting common illnesses, injuries and pathologies induced by pro-oxidant environmental xenobiotics such as inflammation, immune response, signal transduction, regulation of gene expression, and carcinogenesis. It provides clinical presentations and discusses the effects of environmental oxidants on target organs.

Environmental Stressors in Health and Disease

1
Free Radical Reactions in Human Disease

BARRY HALLIWELL

National University of Singapore, Singapore

I. INTRODUCTION

The biomedical literature continues to resound with suggestions that “free radicals” and other “reactive species” are involved in different human diseases. They have been implicated in over 100 disorders, ranging from scleroderma and hemorrhagic shock to cardiomyopathy and cystic fibrosis to gastrointestinal ischemia, AIDS, hearing loss (1–7) and even male pattern baldness (8). The various chapters that follow provide further illustrations. This wide range of disorders implies that free radicals are not something esoteric, but that their increased formation accompanies tissue injury in most, if not all, human diseases (9). Reasons for this are summarized in Figure 1. Sometimes free radicals make a significant contribution to disease pathology; at other times they may not, as Figure 2 illustrates. A major task of researchers in this field is to distinguish between these scenarios so as to be able to create useful therapies: there is no therapeutic advantage to inhibiting free radical damage when it is merely an accompaniment to tissue injury. As Table 1 summarizes, demonstrating that free radicals are important in any disease involves much more than a mere demonstration of their formation in increased amounts. The same comment applies to nitric oxide, cytokines, leukotrienes, and any of the other potential mediators of tissue injury.

A clear understanding of some basic definitions and principles is necessary to guide us in evaluating the role of free radicals in human disease and in designing antioxidants for therapeutic use. This chapter provides those definitions and then considers how best to evaluate the role of free radicals and other reactive species in disease and how to evaluate the effects of therapeutic antioxidants in vivo.

Figure 1 Some of the resone why tissue injury (by any mechanism) causes oxidative stress.

Figure 2 Free radicals: cause and consequence.

Table 1 Criteria for Implicating Reactive Species (RS) as a Significant Mechanism of Tissue Injury in Human Disease

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1. The RS should always be present at the site of injury.
2. Its time course of formation should be consistent with the time course of tissue injury, preceding or accompanying it.
3. Direct application of the RS to the tissue at concentrations within the range found in vivo should reproduce most or all of the damage observed.
4. Removing the RS or inhibiting its formation should diminish the injury to an extent related to the degree of removal of the RS or inhibition of its formation.

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These criteria apply equally well to other agents (e.g., nitric oxide, prostaglandins, leukotrienes, and cytokines). All of these agents have been implicated in multiple diseases, but their importance has been proven only infrequently.

II. BASIC DEFINITIONS

A. Free Radical

Electrons in atoms and molecules move within regions of space known as orbitals. Each orbital holds a maximum of two electrons, since no two electrons can have the same four quantum numbers. A free radical is 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. Examples of free radicals are atomic hydrogen (H^•) and atomic sodium (Na^•). Others include the oxygen-centered radicals such as superoxide (O₂^•−) and hydroxyl (OH^•), the sulfur-centered radicals such as thiyl (RS^•), and carbon-centred radicals such as trichloromethyl (CCl₃^•). Nitric oxide (NO^•) and nitrogen dioxide (NO₂^•) are free radicals in which the unpaired electron is delocalized between different atoms. A superscript dot is used to designate a free radical.

Most biological molecules are nonradicals, containing only paired electrons. A free radical can be made by loss of one electron from a nonradical (X),

X→X^•++e⁻,

by a gain of one electron,

X+e⁻→X^•−,

by homolytic fission of a covalent bond (splitting of the electron pair forming the bond so as to leave one electron on each of the originally bonded atoms),

X−Y→X^•+Y^•,

or by abstraction of a hydrogen atom. Since H^• has one electron, its removal must leave behind an unpaired electron on the atom to which the H^• was covalently bonded. Some examples,

CH₄+Cl^•→ CH3^•+HCl,

Lipid-H+OH^•→lipid^•+H₂O,

RSH+RO₂^•→RS^•+RO₂H.

Antioxidants are usually not free radicals; thus when an antioxidant is acting as a free radical scavenger, an antioxidant-derived radical will be created (radicals beget radicals).

Similarly, if a free radical loses or gains a single electron, it usually ceases to be a free radical. Thus, for nitric oxide,

NO^•→e−+NO+(nitrosyl cation),

NO^•+e−→NO−(nitroxyl anion).

When free radicals react together, there can also be a net loss of radicals. Radical-radical termination reactions are often beneficial by disposing of reactive radicals. However, this is not always true; sometimes the products are damaging. For example, reaction of O₂^•− and NO• generates peroxynitrite, ONOO⁻ (16),

O2^•−+NO^•→ONOO⁻

and self-reactions of peroxyl radicals can generate singlet O₂ (17).

B. Reactive Species

The term reactive oxygen species (ROS), often used in the free radical field, is a collective one that includes not only oxygen-centered radicals such as O₂^•− and OH^•, but also some nonradical derivatives of oxygen, such as hydrogen peroxide (H₂O₂), singlet oxygen ¹Δg, and hypochlorous acid (HOCl) (Table 2). A similar term, reactive nitrogen species, has diffused into the literature over the past 3 years. The term reactive chlorine species has recently made an appearance (Table 2).

C. Oxidative Stress

The term oxidative stress is widely used in the literature, but is often vaguely defined. In essence, it refers to a serious imbalance between production of reactive species and antioxidant defense. Sies introduced the term in 1985, and defined it in 1991 (19) as a disturbance in the prooxidant-antioxidant balance in favor of the former, leading to potential damage.

Oxidative stress can result from:

1. Diminished levels of antioxidants (e.g., mutations affecting the activities of antioxidant defense enzymes such as CuZnSOD, MnSOD, and glutathione peroxidase, or toxins that deplete antioxidant defenses). For example, many xenobiotics are metabolized by conjugation with GSH; high doses can deplete GSH and cause oxidative stress even if the xenobiotic is not itself a generator of reactive species (3). Deficiencies in dietary antioxidants and other essential dietary constituents can also lead to oxidative stress (1,3).

2. Increased production of reactive species [(e.g., by exposure of cells or organisms to elevated levels of O₂), the presence of toxins that are themselves reactive species (e.g., NO₂^•) or are metabolized to generate reactive species (e.g., paraquat), or excessive activation of “natural” systems producing such species (e.g., inappropriate activation of phagocytic cells in chronic inflammatory diseases, such as rheumatoid arthritis and ulcerative colitis)]. Mechanism 2 is usually assumed to be more relevant as a source of oxidative stress in human diseases and is frequently the target of attempted therapeutic intervention, but rarely is much attention paid to the antioxidant nutritional status of sick patients, which can often be very poor (20,21).

Oxidative stress can result in:

1. Adaptation of the cell or organism by upregulation of defense systems, which may (a) completely protect against damage; (b) protect against damage to some extent, but not completely; and (c) “overprotect,” (e.g., the cell is then resistant to higher levels of oxidative stress imposed subsequently).

Table 2 Definition and Nomenclature of Reactive Species

As an example of (b), if adult rats are gradually acclimatized to elevated O₂, they can tolerate pure O₂ for much longer than control rats, apparently due to increased synthesis of antioxidant defense enzymes and of GSH in the lung. Although the damage is slowed, it is not prevented (22). As an example of (c), treatment of E. coli with low levels of H₂O₂ increases transcription of genes regulated by the oxyR protein and renders the bacteria resistant to higher H₂O₂ levels (23). Examples of type (c) adaptation in animals are rarer, but one may be provided by ischemic preconditioning (24). A brief period of ischemia in pig hearts led to depression of contractile function, and administration of antioxidants offered protection against this “myocardial stunning.” However, repeated periods of ischemia led to quicker return of contractile function, but this adaptive response was blocked in the antioxidant-treated animals. Hence, reactive species produced by ischemia/reperfusion were initially damaging, but also led to a response protective against subsequent insult.

Figure 3 Interacting and overlapping methods of cell injury by oxidative stress.

2. Tissue injury. This can involve damage to any or all molecular targets: lipids, DNA, carbohydrates, and proteins (including lipoproteins and nucleoproteins). When measurements of such damage are made after injury has started, it may not be clear which biomolecules were attacked first, since injury mechanisms are interrelated in complex ways (Fig. 3). Indeed, depending on the cell or tissue under study, the primary molecular target of oxidative stress may vary (3,25). DNA is an important early target of damage when H₂O₂ is added to many mammalian cells (26), and often increased DNA damage (measured as strand breakage or formation of modified DNA bases) occurs before detectable lipid peroxidation or oxidative protein damage. The word detectable is emphasized because such conclusions are obviously dependent on the assays used to measure oxidative damage. For example, measurement of protein carbonyls (27) would not detect oxidative protein damage occurring by oxidation of essential -SH groups on membrane ion transporters or the conversion of methionine to its sulfoxide.

Figure 3 may seem complex, but it is an oversimplification. In particular, it omits the effects of reactive species on signal transduction systems. Overactivation of such systems by mild oxidation, or inhibition of them by excessive oxidation, may also contribute to cell injury (28–30). Effects on the transport/levels of ions other than Ca²⁺, iron, and copper (31) have also been omitted.

3. Cell death from multiple mechanisms, such as the rupture of membrane blebs (32). Excessive activation of poly(ADP ribose) polymerase (PARP) can so deplete intracellular NAD⁺ and NADH levels that the cell cannot make AT...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Series Introduction
• Preface
• Contributors
• Environmental Stressors in Health and Disease