Molecular oxygen in biological system is relatively unreactive and plays an important role in the proper functioning of many biological processes (11). However, metabolic pathways in the cell can reduce oxygen incompletely to produce reactive oxygen species (ROS), which are unstable compared to molecular oxygen, and can react with a number of biological mac-romolecules and disturb the cellular homeostasis (11-17). ROS are constantly produced in aerobic organisms both enzymatically and nonenzymatically (Figure 10.2). One electron reduction of molecular oxygen in biological systems results in the formation of the superoxide anion which is the precursor of most ROS (24). Biological dismutation of the superoxide anion produces the peroxide radical, which can undergo a complete reduction to form water, or an incomplete reduction to form the highly reactive hydroxyl radical (24) (Figure 10.2). Mitochondrial leakage of electrons or direct transfer of electrons to oxygen via the electron transport chain is another important source of ROS in cellular systems (25,26). Many other sources for the formation of ROS have been identified in cellular systems of living organisms (25,26). Superoxide is also formed upon one electron reduction of oxygen mediated by enzymes such as NADPH oxidase located on the cell membrane of polymorphonuclear cells, macrophages, and endothelial cells; from xanthine oxidase or from the respiratory chain (27-30). Superoxide radicals can also be formed from the electron cycling carried out by the
A Myeloperoxidase Singlet oxygen
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