Reactive oxygen , superoxide anion radical produced from

An example of free radical formation is molecular oxygen, which can accept electrons from a variety of sources to produce reactive oxygen species (ROS) such as the superoxide radical, the hydroxyl radical, and the nitric oxide radical. The superoxide anion radical is formed when one electron is taken up by one of the 2p orbitals of molecular oxygen. Certain drugs and other xenobiotics have the capacity to undergo so-called redox cycles, whereby they provide electrons to molecular oxygen and form super oxide. 

Fig. 3. Production of reactive species. (A) ROS can be produced from the weak radical oxygen in the mitochondria and endoplasmic reticulum, by various enzymatic reactions, and from oxyhemoglobin. Normally, nontoxic hydrogen peroxide can give rise to the powerful hydroxyl radical in the presence of transition metals (R5). Oxygen can also be induced to react with biomolecules by transition metals and enzymes. RNS can be produced by reaction of superoxide anion radical with the weak radical nitric oxide. These can react to form the powerful oxidant peroxynitrite/peroxynitrous acid, which can cause formation of other radicals, some with longer lives. See the text for details. SOD, superoxide dismutase. (B) Myeloperoxidase in leukocytes can produce the reactive species hypochlorous acid and tyrosyl radical. Unpaired electrons are indicated by the dense dots and paired electrons by the light ones.

The T)/pe I reaction often results in the formation of superoxide anion via electron transfer from the excited triplet PS to molecular oxygen. Though superoxide is not particularly damaging to the cell, it can react with itself, producing hydrogen peroxide and oxygen in the enzymatic reaction catalyzed by superoxide dismutase, or it can produce highly reactive hydroxyl radical (HO ). 

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