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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Types of Free Radicals in the body (continued)

Hydroxyl radical:

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 peroxide).

Around 1933, Fritz Haber and Joseph Weiss first proposed that hydroxyl free radicals (·OH) were produced when superoxide and hydrogen peroxide react together:

O₂·^- + H₂O₂ ® O₂ + ·OH + OH^- (1)

This formula was coined the Haber-Weiss reaction.

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⁺:

H₂O₂ + Fe²⁺ ® ·OH + OH⁻ + Fe³⁺ (2)

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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