الفهرس 
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Abstract
Reactive oxygen species (ROS) are oxidizing agents produced in an body (Halliwell, 1991 a). They include superoxide anion (02- ), ;ydrogen peroxide (H202) , hydroxyl-radical (OR), hypocWorous acid lOCI), peroxynitrite and singlet oxygen C02) (Andreoli, 1995). Several powerfull oxidants are produced during the course of !metabolism, in both blood cells and most other cells of the body. is formed in the red blood cell by the autoxidation of hemoglobin to methemoglobin (Murray, 1996). Likewise, membrane bound cytochromes (P450-and b5) located in the endoplasmic reticulum generate 0,.- by autoxidation (Baud and Ardaillou,1986). Activated phagocytes also deliberately generate superoxide as part of their bactericidal role (Babior, 1978). When stimulated by contact with bacteria, neutrophils exhibit a respiratory burst and produce superoxide in reaction catalyzed by NADPH-oxidase (Murray, 1996).Cellular enzymes, other than the electron transport chain and NADPH oxidase system, that are implicated in ROS production are : xanthine oxidase, cyclooxygenase, NO synthase, and lipoxygenase (Marsden et al, 1996). Oxidative damage induced by ROS affects major classes of biomolecules: nucleic acids, protein, and lipids (Marsden et al, 1996). lipid peroxidation is a reaction between polyunsaturated fatty acids and reactive oxygen species (Schafer, 1990). Lipid peroxidation in biological membranes leads to decrease fluidity, increased leakiness to ions and enzymes (Amado et al., 1984). ROS attack nucleic acids, producing base modification and strand breaks. Strand breaks in DNA result from OH¬attack of hydroxyl groups on the sugar portion of DNA. Strand repair may