Role of oxidative stress and nitric oxide in atherothrombosis

Abstract

During the last decade basic and clinical research has highlighted the central role of reactive oxygen species (ROS) in cardiovascular disease. Enhanced production or attenuated degradation of ROS leads to oxidative stress, a process that affects endothelial and vascular function, and contributes to vascular disease. Nitric oxide (NO), a product of the normal endothelium, is a principal determinant of normal endothelial and vascular function. In states of inflammation, NO production by the vasculature increases considerably and, in conjunction with other ROS, contributes to oxidative stress. This review examines the role of oxidative stress and NO in mechanisms of endothelial and vascular dysfunction with an emphasis on atherothrombosis.

Figure 1

Source of Superoxide Anion. The membrane-associated NAD(P)H oxidase enzyme complex catalyze the one-electron reduction of molecular oxygen using NAD(P)H as an electron donor, generating superoxide anion (O₂^−•). The catalytic b₅₅₈-type cytochrome Nox subunit is bound to p22^phox in the plasma membrane, and they stabilize each other. The cytosolic subunits shown may also be required for full and substained activation of the complex in vascular cells. Among the other sources of O₂^−• are the xanthine oxidoreductase enzyme system and ‘uncoupled’ eNOS. The mitochondrial electron transport chain produces O₂^−• by incomplete reduction of oxygen (O₂), mainly at complex I (NADH coenzyme Q reductase) and complex III (ubiquinol cytochrome c reductase).

Figure 2

Biochemical Reactions of Reactive Oxygen Species. Superoxide anion reacts with itself to form hydrogen peroxide (H₂O₂) and oxygen (O₂) by spontaneous (k=∼5 × 10⁵ M⁻¹ • s⁻¹ at pH 7.4) and enzymatic dismutation reactions (k=∼2 × 10⁹ M⁻¹ • s⁻¹). Basal levels of H₂O₂ appear to regulate some signal transduction pathways, or are reduced to water (H₂O) by peroxiredoxins, glutathione peroxidases, or catalase. By the metal-catalyzed Fenton reaction, H₂O₂ forms the highly reactive hydroxyl radical (^•OH), which is the strongest oxidizing agent known. Superoxide anion reacts also with nitric oxide (NO) to from peroxynitrite (ONOO⁻) at the diffusion limit with a rate (k=∼6.7 × 10⁹ M⁻¹ • s⁻¹) that is faster than that of the superoxide dismutase (SOD) reaction.

Figure 3

Common Chemical Reactions of Nitric Oxide. Synthesized from nitric oxide synthase (NOS), using L-arginine, nitric oxide (NO) reacts with a variety of targets, such as the ferrous iron (Fe²⁺) in heme moieties. In hemoglobin (Hb²⁺), the conversion to ferric iron (Fe³⁺) forms methemoglobin (Hb³⁺). Nitric oxide may also activate soluble guanylyl cyclase (sGC) and react with transition metals (M) to alter their valence (x). Nitric oxide also reacts with thiol groups (RSH) to produce S-nitrosothiols (RSNO). At rapid rates, NO reacts with superoxide anion (O₂^−•) to form peroxynitrite (ONOO⁻/ONOOH), which can form to nitrate (NO₃⁻). Nitric oxide can be reduced to nitrous oxide (N₂O) or oxidized to nitrite (NO₂⁻). Nitrite can react rapidly with oxygen, yielding nitrogen dioxide radical (^•NO₂), which exists in equilibrium with the potent nitrosating agents dinitrogen trioxide (N₂O₃) and dinitrogen tetroxide (N₂O₄).

Figure 4

Biological Consequences of Peroxynitrite Formation. Peroxynitrite formation has two important biological consequences: loss of bioactive nitric oxide (NO) and direct cytotoxic effects. Peroxynitrite and its conjugate acid can oxidize a variety of biomolecules with the consequence of protein modification or inactivation of ion channels. Peroxynitrite inactivates Mn-SOD, thereby increasing the flux of superoxide anion (O₂^−•) available to react with nitric oxide and establishing an autocatalytic spiral of increasing mitochondrial peroxynitrite formation.