CHAPTER ONE

1.1     Introduction

Antioxidant enzymes are the main line of defence against free radicals in animals and plant cells (Gutteridge, 1999).

Uncontrolled generation of reactive oxygen species is involved in a number of human disease states, including diabetes and cancer due to disturbance in cellular and molecular processes including cell growth, differentiation and proliferation. When cells are exposed to oxidative stress, a defense system endorses the expression and regulation of a number of antioxidant enzymes as defense mechanism to protect them from damage induced by free radicals (Hurst, 2007).

Aerobic organisms posses antioxidant defense system that deals with reactive oxygen species produced as a consequence of aerobic respiration and substrate oxidation. Small amount of reactive oxygen species, including hydroxyl radicals (OH), superoxide anions(o-) and hydroxide (H₂O₂) are constantly generated in aerobic organisms in response to both external and internal stimuli (Mills, 1998).

Low level of reactive oxygen species are indispensable in many biochemical processes, including intracellular messaging in the cell differentiation and cell progression or the arrest of growth, apoptosis, immunity, and defense against micro-organisms (Lee et al., 1998).

In contrast, high doses and/or inadequate removal of reactive  oxygen species result in oxidative stress , which may cause severe  metabolic  malfunction and damage to macromolecules (Chopra and Wallace,  2008).

The natural occurring antioxidants in low – density lipoprotein  (LDLs) and plasma protect cells from oxidation. The prevention of lipid peroxidation is an essential process in aerobic organisms, as lipid peroxidation product can cause DNA damage and directly inhibits proteins such as sodium ion / potassium ion AT_pase (Na+/K+ AT_pase).

Increased lipid peroxidation and decreased antioxidant protection generate epoxides that may spontaneously react with nucleophilic centers in the cell and thereby covalently bind to DNA, RNA and protein. Such a reaction may lead to cytotoxicity, allergy, mutagenicty and carcinogenicity depending on the properties of the epoxide in question (Leone, 1998).

The enzymatic and non enzymatic antioxidant defenses include superoxide dismutase (SOD), glutathione peroxidase (GPX), catalase (Cat), glutathione GSH , ascorbic acid (vitamin C) and vitamin A which can be evaluated using easy photometric assays (Stahl et al., 2008).

There are balance between both the activities and the intracellular levels of these antioxidants that are essential for the survival of organisms and their health (Mills, 1998).