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Hydrogen peroxide (H 2 O 2) is one of the most extensively studied reactive oxygen species (ROS) in biology and medicine. It is generated constitutively from various cellular processes either directly via two-electron reduction of molecular oxygen indirectly via dismutation of superoxide. The notable direct cellular sources for H 2 O 2 include xanthine oxidoreductase, monoamine oxidase, endoplasmic reticulum oxicireductin 1, oxidases in peroxisomes, and possibly certain members of the NOX/DUOX family. Because of the high activation energy, H 2 O 2 reacts poorly with most cellular constituents. However, it may oxidize the thiol groups in certain proteins and enzymes, including these involved in cell signaling transduction. The potential of H 2 O 2 to cause oxidative stress and tissue injury primarily results from its reactions with other molecules to form secondary reactive species, including hydroxyl radical and hypochlorous acid. While the tightly controlled production of H 2 O 2 plays important roles in various physiological responses, overproduction of this ROS contributes to the pathophysiology of a variety of disease processes and related conditions, including cardiovascular diseases, diabetes, neurodegeneration, cancer, and aging, among many others.

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