Ozone — From Environmental Pollutant to Atherogenic Determinant
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《新英格兰医药杂志》
Molecular oxygen (O2) is abundant in the atmosphere and is essential for most forms of life. A study by Wentworth and colleagues has now implicated one of its derivatives in the highly prevalent vascular disorder atherosclerosis.1
Molecular oxygen engages in a variety of metabolic redox reactions that yield a range of products collectively termed "reactive oxygen species." The addition of one electron converts molecular oxygen to superoxide anion (O2–?), whereas the addition of two electrons yields hydrogen peroxide (H2O2). Hydrogen peroxide, in turn, can react with ferrous iron (Fe2+) to produce the highly reactive hydroxyl radical (?OH) and hydroxide anion (–OH). These reactive oxygen species can interact with many biologically important molecules, often changing their functional properties. Antioxidant enzymes (e.g., superoxide dismutase, catalase, and glutathione peroxidase) and small-molecule antioxidants (e.g., ascorbate, glutathione, and alpha-tocopherol) protect against the adverse effects of reactive oxygen species. When the generation of reactive oxygen species exceeds the antioxidant capacity, oxidant stress results, which is believed to have an important mechanistic role in the pathogenesis of many diseases, including diabetes mellitus and atherosclerosis.
Oxidant stress in the vasculature is an essential feature of atherosclerosis.2 The seminal observations of Steinbrecher and coworkers regarding the oxidation of low-density lipoprotein by endothelial cells3 provided the first definitive evidence that oxidation is involved in atherogenesis, and the results of many subsequent studies have identified the essential features of the oxidative mechanisms underlying atherosclerosis.
Wentworth and colleagues1 have identified yet another reactive oxygen species — ozone — in the atherosclerotic arteries of humans. Known more commonly as an environmental pollutant, ozone is composed of three oxygen atoms (O3) and is formed in the atmosphere by the reaction of molecular oxygen with ultraviolet light. Because ozone is so highly reactive, the authors were able to measure it only indirectly, in the form of a unique cholesterol product of an ozonolysis reaction: 5,6-secosterol (Figure 1A). They showed that this product and its derivatives resulting from aldolization (which they collectively term "atheronals") are cytotoxic to a variety of cells and induce the formation of foam cells on incubation with low-density lipoprotein, indicating their capacity to modify the lipoprotein oxidatively.
Figure 1. Reaction of Ozone (O3) with Cholesterol to Yield the Atheronal 5,6-Secosterol (Panel A) and Proposed Sequence by Which Singlet Oxygen (1O2*) Is Converted to Hydrogen Peroxide (H2O2) and Ozone, Leading to Triplet Oxygen (3O2) and the Hydroxyl Radical (?OH) (Panel B).
Wentworth and colleagues believe that ozone is generated in the atherosclerotic vessel as a by-product of the inflammatory response. Previously, they showed that antibodies catalyze the oxidation of water to hydrogen peroxide by reacting with oxygen in a conserved binding site within their tertiary structure.4 In this reaction, water is used as a source of electrons to reduce the highly reactive singlet form of molecular oxygen (1O2*) to hydrogen peroxide, thereby providing a unique source of this reactive oxygen species and potent bactericidal activity that is not dependent on activated phagocytes. Wentworth and colleagues subsequently suggested that the complex chemical reaction leading to the generation of hydrogen peroxide is fueled by the formation of another trioxygen species, dihydrogen trioxide (H2O3), which can react with the singlet form of molecular oxygen to yield ozone and hydrogen peroxide. Ozone can, in turn, react with hydrogen peroxide to yield the hydrotrioxy radical (?O3H), which disproportionates into triplet oxygen (3O2) and the hydroxyl radical5 (Figure 1B). The latter reaction, if it takes place in vivo, offers a mechanism for the formation of hydroxyl radicals that is independent of transition metals (ferrous ion).
These observations point to a potential source for new biomarkers of atherosclerosis and to the importance of the chemistry of oxygen in the inflamed, atherosclerotic blood vessel. Many questions remain regarding the report of endogenous production of ozone in the vasculature. Did the molecule actually form as a result of the exposure of oxygen in the excised atherosclerotic plaque to ambient light? If ozone is truly produced endogenously, what is the precise source? Are antibodies required, or are other, as yet undefined mechanisms responsible? What is the flux of ozone in an inflamed plaque, and how does it compare with that of other, longer-lived reactive oxygen species? To what extent do products of ozonolysis contribute to the oxidant stress of atherosclerosis? In sum, Wentworth and colleagues have identified a most intriguing component of the panoply of oxidative events that occur in atherosclerosis, although the full impact of their discovery on our understanding of disease pathogenesis and therapy is not yet clear.
Source Information
From Boston University School of Medicine, Boston.
References
Wentworth P Jr, Nieva J, Takeuchi C, et al. Evidence for ozone formation in human atherosclerotic arteries. Science 2003;302:1053-1056.
Goldstein JH, Ho YK, Basu SK, Brown MS. Binding site on macrophages that mediates uptake and degradation of acetylated low density lipoprotein, producing massive cholesterol deposition. Proc Natl Acad Sci U S A 1979;76:333-337.
Steinbrecher UP, Parthasarathy S, Leake DS, Witztum JL, Steinberg D. Modification of low density lipoprotein by endothelial cells involves lipid peroxidation and degradation of low density lipoprotein phospholipids. Proc Natl Acad Sci U S A 1984;81:3883-3887.
Wentworth P Jr, Jones LH, Wentworth AD, et al. Antibody catalysis of the oxidation of water. Science 2001;293:1806-1811.
Wentworth P Jr, Wentworth AD, Zhu X, et al. Evidence for the production of trioxygen species during antibody-catalyzed chemical modification of antigens. Proc Natl Acad Sci U S A 2003;100:1490-1493.(Joseph Loscalzo, M.D., Ph)
Molecular oxygen engages in a variety of metabolic redox reactions that yield a range of products collectively termed "reactive oxygen species." The addition of one electron converts molecular oxygen to superoxide anion (O2–?), whereas the addition of two electrons yields hydrogen peroxide (H2O2). Hydrogen peroxide, in turn, can react with ferrous iron (Fe2+) to produce the highly reactive hydroxyl radical (?OH) and hydroxide anion (–OH). These reactive oxygen species can interact with many biologically important molecules, often changing their functional properties. Antioxidant enzymes (e.g., superoxide dismutase, catalase, and glutathione peroxidase) and small-molecule antioxidants (e.g., ascorbate, glutathione, and alpha-tocopherol) protect against the adverse effects of reactive oxygen species. When the generation of reactive oxygen species exceeds the antioxidant capacity, oxidant stress results, which is believed to have an important mechanistic role in the pathogenesis of many diseases, including diabetes mellitus and atherosclerosis.
Oxidant stress in the vasculature is an essential feature of atherosclerosis.2 The seminal observations of Steinbrecher and coworkers regarding the oxidation of low-density lipoprotein by endothelial cells3 provided the first definitive evidence that oxidation is involved in atherogenesis, and the results of many subsequent studies have identified the essential features of the oxidative mechanisms underlying atherosclerosis.
Wentworth and colleagues1 have identified yet another reactive oxygen species — ozone — in the atherosclerotic arteries of humans. Known more commonly as an environmental pollutant, ozone is composed of three oxygen atoms (O3) and is formed in the atmosphere by the reaction of molecular oxygen with ultraviolet light. Because ozone is so highly reactive, the authors were able to measure it only indirectly, in the form of a unique cholesterol product of an ozonolysis reaction: 5,6-secosterol (Figure 1A). They showed that this product and its derivatives resulting from aldolization (which they collectively term "atheronals") are cytotoxic to a variety of cells and induce the formation of foam cells on incubation with low-density lipoprotein, indicating their capacity to modify the lipoprotein oxidatively.
Figure 1. Reaction of Ozone (O3) with Cholesterol to Yield the Atheronal 5,6-Secosterol (Panel A) and Proposed Sequence by Which Singlet Oxygen (1O2*) Is Converted to Hydrogen Peroxide (H2O2) and Ozone, Leading to Triplet Oxygen (3O2) and the Hydroxyl Radical (?OH) (Panel B).
Wentworth and colleagues believe that ozone is generated in the atherosclerotic vessel as a by-product of the inflammatory response. Previously, they showed that antibodies catalyze the oxidation of water to hydrogen peroxide by reacting with oxygen in a conserved binding site within their tertiary structure.4 In this reaction, water is used as a source of electrons to reduce the highly reactive singlet form of molecular oxygen (1O2*) to hydrogen peroxide, thereby providing a unique source of this reactive oxygen species and potent bactericidal activity that is not dependent on activated phagocytes. Wentworth and colleagues subsequently suggested that the complex chemical reaction leading to the generation of hydrogen peroxide is fueled by the formation of another trioxygen species, dihydrogen trioxide (H2O3), which can react with the singlet form of molecular oxygen to yield ozone and hydrogen peroxide. Ozone can, in turn, react with hydrogen peroxide to yield the hydrotrioxy radical (?O3H), which disproportionates into triplet oxygen (3O2) and the hydroxyl radical5 (Figure 1B). The latter reaction, if it takes place in vivo, offers a mechanism for the formation of hydroxyl radicals that is independent of transition metals (ferrous ion).
These observations point to a potential source for new biomarkers of atherosclerosis and to the importance of the chemistry of oxygen in the inflamed, atherosclerotic blood vessel. Many questions remain regarding the report of endogenous production of ozone in the vasculature. Did the molecule actually form as a result of the exposure of oxygen in the excised atherosclerotic plaque to ambient light? If ozone is truly produced endogenously, what is the precise source? Are antibodies required, or are other, as yet undefined mechanisms responsible? What is the flux of ozone in an inflamed plaque, and how does it compare with that of other, longer-lived reactive oxygen species? To what extent do products of ozonolysis contribute to the oxidant stress of atherosclerosis? In sum, Wentworth and colleagues have identified a most intriguing component of the panoply of oxidative events that occur in atherosclerosis, although the full impact of their discovery on our understanding of disease pathogenesis and therapy is not yet clear.
Source Information
From Boston University School of Medicine, Boston.
References
Wentworth P Jr, Nieva J, Takeuchi C, et al. Evidence for ozone formation in human atherosclerotic arteries. Science 2003;302:1053-1056.
Goldstein JH, Ho YK, Basu SK, Brown MS. Binding site on macrophages that mediates uptake and degradation of acetylated low density lipoprotein, producing massive cholesterol deposition. Proc Natl Acad Sci U S A 1979;76:333-337.
Steinbrecher UP, Parthasarathy S, Leake DS, Witztum JL, Steinberg D. Modification of low density lipoprotein by endothelial cells involves lipid peroxidation and degradation of low density lipoprotein phospholipids. Proc Natl Acad Sci U S A 1984;81:3883-3887.
Wentworth P Jr, Jones LH, Wentworth AD, et al. Antibody catalysis of the oxidation of water. Science 2001;293:1806-1811.
Wentworth P Jr, Wentworth AD, Zhu X, et al. Evidence for the production of trioxygen species during antibody-catalyzed chemical modification of antigens. Proc Natl Acad Sci U S A 2003;100:1490-1493.(Joseph Loscalzo, M.D., Ph)