Figure 1427

Production of the hydroxyl radical: the Haber-Weiss reaction. Superoxide is converted to hydrogen peroxide by superoxide dismutase. Superoxide and hydrogen peroxide per se are not highly reactive and cytotoxic. However, hydrogen peroxide can be converted to the highly reactive and injurious hydroxyl radical by an iron-catalyzed reaction that requires the presence of free reduced iron. The availability of free "catalytic iron" is a critical determinant of hydroxyl radical production. In addition to providing a source of hydroxyl radical, superoxide potentiates hydroxyl radical production in two ways: by releasing free iron from iron stores such as ferritin and by reducing ferric iron and recycling the available free iron back to the ferrous form. The heme moiety of hemoglobin, myoglobin, or cytochrome present in normal cells can be oxidized to metheme (Fe3+). The further oxidation of metheme results in the production of an oxyferryl moiety (Fe4+=O), which is a long-lived, strong oxidant which likely plays a role in the cellular injury associated with hemoglobinuria and myoglobinuria.

Activated leukocytes produce superoxide and hydrogen peroxide via the activity of a membrane-bound enzyme NADPH oxidase. This superoxide and hydrogen peroxide can be converted to hydroxyl radical via the Haber-Weiss reaction. Also, the enzyme myeloper-oxidase, which is specific to leukocytes, converts hydrogen peroxide to another highly reactive and injurious oxidant, hypochlorous acid.

...Large Gibbs energy

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