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Free Radicals, Types, Sources and Damaging Reactions

Submitted by Dr. Tamer Fouad, M.D.


Free radicals are a chemical species that possess an unpaired electron in the outer shell of the molecule.


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Oxidative mechanisms in carcinogenesis

Types of Free Radicals in the body (continued)


It is produced by the reaction of nitric oxide with superoxide. The result is a radical-radical reaction in which peroxynitrite (ONOO-) is formed:

O2?- + NO? = ONOO-

Hypochlorous acid

Activated polymorphonuclear cells produce HOCl as a major bactericidal agent. It is generated by the action of myeloperoxidase on chloride ions in the presence of H2O2.

H2O2 + Cl- ? HOCl + OH-

This reaction occurs in the neutrophils phagocytic vacuole after fusion with the myeloperoxidase-containing lysosomal vesicles.

Hypochlorous acid can cross cell membranes and, in the presence of transitional metal ions, generate hydroxyl radicals (Aruoma, 1994). Highly reactive hydroxyl radicals can be formed from HOCl/OCl- on reaction with reductants that are one-electron donors. Important examples include superoxide radicals and ferrous iron:

HOCl + O2?- ? ?OH + Cl- + O2

HOCl + Fe2+ ? ?OH + Cl- + Fe3+

HOCl has been shown to be capable of initiating lipid peroxidation (Panasenko et al. 1995), combining with H2O2 to damage DNA and DNA repair processes (VanRensburg et al. 1992) and altering intracellular free Ca2+ and pH (Kuroda et al. 1995). It may contribute to tissue damage during the inflammatory process. This latter event may result from the activation of collagenases or the inactivation of alpha-1 antiproteinase (Ching et al. 1994).

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