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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

For many years the existence of free radicals in biological systems was dismissed as either non-existent or simply an unimportant curiosity. However, more recently due to improved investigational techniques, this view has changed rather dramatically. Currently, free radicals have found a place in the aetiology of many diseases and there is a great deal of enthusiasm regarding the role of free radicals in many previously unexplained disease phenomena. To name a few important areas; free radicals have found a role in the rheumatoid arthritis, Alzheimer's disease, hypertension, myocardial ischemia, liver cell injury and carcinogenesis. The reason as to why the role of free radicals has been so ambiguous is probably due to their ultra-short half-life. However, free radicals have finally come into existence through the use of more sophisticated methods of assay.

Definition of free radicals

They are a chemical species that possess an unpaired electron in the outer (valence) shell of the molecule. This is the key factor in the structure of this species (Greenwald, 1991; Halliwell, 1995) and is the reason why they are highly reactive. This species is in reality composed of a group of molecular fragments that are capable of independent existence. (Cheeseman and Slater, 1993).

The fact that they are highly reactive means that they have low chemical specificity; i.e. they can react with most molecules in its vicinity. This includes proteins, lipids, carbohydrates and DNA. It also means that in trying to gain stability by capturing the needed electron they don't survive in their original state for very long and quickly react with their surroundings. Hence, free radicals attack the nearest stable molecule, "stealing" its electron. When the "attacked" molecule loses its electron, it becomes a free radical itself, beginning a chain reaction. Once the process is started, it can cascade, finally resulting in the disruption of a living cell.

Free radicals are produced continuously in cells either as by-products of metabolism or deliberately as in phagocytosis (Cheeseman and Slater, 1993).

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