CHAPTER ONE

1.1 INTRODUCTION

1.1.1 Pro-oxidant Effect Pro-oxidants are chemicals that induce oxidative stress, either by generating reactive oxygen species or by inhibiting antioxidant systems.

The oxidative stress produced by these chemicals can damage cells and tissues, for example an overdose of the analgesic paracetamol (acetaminophen) can fatally damage the liver, partly through its production of reactive oxygen species. Some substances can serve as either antioxidants or pro-oxidants, depending on conditions. Some of the conditions that are important include the concentration of the chemical and if oxygen or transition metals are present. While thermodynamically very favored, reduction of molecular oxygen or peroxide to superoxide or hydroxyl radical respectively is fortunately spin forbidden. This greatly reduces the rates of these reactions, thus allowing aerobic life to exist. As a result, the reduction of oxygen typically involves either the initial formation of singlet oxygen, or spin-orbit coupling through a reduction of a transition-series metal such as manganese, iron, or copper. This reduced metal then transfers the single electron to molecular oxygen or peroxide.

1.1.2 Antioxidant Lipid Peroxidation Antioxidants are molecules that inhibit the oxidation of other molecules.

Oxidation is a chemical reaction that can produce free radicals, leading to chain reactions that may damage cells. Antioxidants such as thiols or ascorbic acid (vitamin C) terminate these chain reactions. The term "antioxidant" is mainly used for two different groups of substances: industrial chemicals which are added to products to prevent oxidation, and natural chemicals found in foods and body tissue which are said to have beneficial health effects. To balance the oxidative state, plants and animals maintain complex systems of overlapping antioxidants, such as glutathione and enzymes (e.g., catalase and superoxide dismutase) produced internally or the dietary antioxidants: vitamin A, vitamin C, and vitamin E. Antioxidant dietary supplements have not been shown to improve health in humans, or to be effective in preventing diseases. This includes supplements of beta-carotene, vitamin A, and vitamin E having no effect on mortality rate or cancer risk. Supplementation with selenium or vitamin E does not reduce the risk of cardiovascular disease.

1.1.3 Lipid Lipids Are Classically Divided into Two Groups: Apolar and Polar.

Triglycerides (apolar), stored in various cells, but especially in adipose (fat) tissue, are usually the main form of energy storage in mammals. Polar lipids are structural components of cell membranes, where they participate in the formation of the permeability barrier of cells and subcellular organelles in the form of a lipid bilayer. The major lipid type defining this bilayer in almost all membranes is glycerol-based phospholipid. The importance of the membrane lipid physical (phase) state is evidenced by the fact that lipids may control the physiological state of a membrane organelle by modifying its biophysical aspects, such as the polarity and permeability. Lipids also have a key role in biology as signaling molecules.

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