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Reactive oxygen species (ROS) are chemically reactive molecules containing oxygen. Examples include peroxides, superoxide, hydroxyl radical, and singlet oxygen. In a biological context, ROS are formed as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling and homeostasis. However, during times of environmental stress (e.g., UV or heat exposure), ROS levels can increase dramatically.〔 This may result in significant damage to cell structures. Cumulatively, this is known as oxidative stress. ROS are also generated by exogenous sources such as ionizing radiation. == Formation and decomposition == The reduction of molecular oxygen (O2) produces superoxide (·O2−) and is the precursor of most other reactive oxygen species: * O2 + e− → ·O2− Dismutation of superoxide produces hydrogen peroxide (H2O2):〔 * 2H+ + ·O2− + ·O2− → H2O2 + O2 Hydrogen peroxide in turn may be partially reduced to hydroxyl radical (·OH) or fully reduced to water:〔 * H2O2 → HO · + ·OH * H2O2 + H2O2 → H2O + H2O + O2 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Reactive oxygen species」の詳細全文を読む スポンサード リンク
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