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Free Radicals, Types, Sources and
Damaging Reactions
Submitted by Dr. Tamer Fouad, M.D.
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Free radicals are a chemical
species that possess an unpaired electron in the outer shell of
the molecule. |
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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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