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.
Types of Free Radicals in the body (continued)
The hydroxyl radical is an
extremely reactive oxidising radical that will react to most
biomolecules at diffusion controlled rates (Cheeseman
and Slater, 1993), which means that reactions will occur
immediately with biomolecules. The hydroxyl free radical is
important in radiobiological damage and is several orders of
magnitude more reactive towards cellular constituents than
superoxide radicals (and many orders more reactive than hydrogen
Around 1933, Fritz Haber and
Joseph Weiss first proposed that hydroxyl free radicals (?OH) were
produced when superoxide and hydrogen peroxide react together:
? O2 + ?OH + OH- (1)
This formula was coined the
About 100 years ago, Henry
Fenton had observed that the reducing agent, ferrous iron (Fe2+),
together with hydrogen peroxide could oxidize some organic
compounds. The mechanism is now known to involve hydroxyl radicals,
with a key step analogous to reaction (1) but with the electron
donor, O2?- replaced by Fe2+:
? ?OH + OH⁻
The above reaction is more
complicated than is stated above and is most commonly referred to as
the iron catalysed Haber-Weiss reaction or the superoxide-driven
Fenton reaction (Halliway
and Gutteridge, 1992).
In the body the pool of free
iron that is available to catalyse the reaction from H2O2
to the hydroxyl ion is, under normal conditions, extremely limited (Borg
and Schaich, 1987). Red blood cells contain much of the iron in
the body. Fortunately, several iron transporters are available to
prevent inadvertent release and therefore limit the availability of
free iron to catalyse the Haber-Weiss reaction.
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