Molecular Mechanisms Involving Free Radical Reactions of Antioxidants and Radioprotectors

摘要

Aerobic organisms produce a number of reactive free radicals (molecules or atoms having unpaired electrons) continuously in cells during respiration, metabolism and phagocytes. Out of these, the most important source of free radicals being the respiratory chain where approx 1 to 2 percent oxygen is converted into superoxide radicals (O_2~-). While superoxide radical is not so reactive and may not be able to cause any direct damage to cells, its reaction product hydrogen peroxide in presence of trace metal ions, is converted to more powerful hydroxyl radicals (OH), which can oxidize most of the biornolecules. Organic substrates (RH) or lipids (LH) on reaction with hydroxy! radicals in presence of oxygen are converted in to peroxyl radicals (ROO)/(LOO), which are known to undergo chain reactions, and thereby multiplying the damage. Thus, free radicals formed within the ceils can induce multiple chemical changes in cellular organelies like membrane Iipids, DNA and proteins, which can eventually lead to cell nitric oxide, nitrogen dioxide radicals as well as for non-radical reactive intermediates like hydrogen peroxide (H_2O_2) and peroxynitrite (DNOO~-) etc, and their excessive -production, termed as "oxidative stress" has been implicated in many pathological disorders like heart disease, cancer and ageing.