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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

Measurement of free radicals

This increasing interest in the role of free radicals in the pathogenesis of disease has led to an increased need for techniques to measure free radicals and their reactions in vivo and, most importantly, in the clinical situation.

Several problems arise when considering measurement of free radicals (Cheesman and Slater, 1993): First the ultra-short half-life of these radicals (usually measured in microseconds). Second any free radicals produced in vivo react at or close to their source of formation. Hence, the methods devised to quantitate free radicals are more or less indirect (Slater, 1984; Pryor and Godber, 1991). A third problem is that many of these end products are in themselves reactive although to a lesser degree. Last but not least, the only samples that are readily available for clinical practice are blood, urine and expired breath. It is completely impossible for a free radical that is produced in an internal tissue, having a half-life measured in microseconds to diffuse to the blood. In order to overcome these problems the following methods are used:

Electron spin resonance and radical trapping:

The only analytical technique that directly measures free radicals is electron spin resonance spectrometry. However, it is relatively insensitive and requires steady-state concentrations of free radicals in the micromolecular range; thus it is very limited for use in vivo. Spin trapping involves the addition of a compound known as a spin trap, that reacts rapidly with free radicals to form radical-adducts that are very much more stable and longer lived than the original species (Cheesman and Slater, 1993).

An alternative technique for trapping free radicals produced by cells, organelles and perfused organs is by measuring the non-radical products produced when free radicals attack aromatic compounds. These substances are not produced normally hence their presence is evidence of hydroxyl radical production (Grootveld and Halliwell, 1986). These methods like ESR spin trapping can only produce semi-quantitative data.

Measurement of DNA damage:

Free radical species such as ?OH can produce irreversible DNA modification of DNA bases; theoretically a large number of products are possible but relatively few have been detected in biological systems.

Among these the major products of ?OH with thymine is thymine glycol and guanine is 8-hydroxy-guanine. These DNA products are eliminated by repair enzymes (excision enzymes & glycolases) and are excreted in urine either as the free base or as the nucleoside derivatives, thymidine glycol and 8-hydroxydeoxyguanosine. The latter products can be used as an index of radical attacks against DNA (Cathcart et al. 1984). This method is limited by several considerations including the obscurity of the tissue of origin of the products.

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