LECTURE I: OXIDATIVE STRESS AND ANTIOXIDANT PROTECTION

Reactive oxygen species (ROS) and vitamin E

A small amount of all oxygen passing through the respiratory system in the mitochondria is released as reactive oxygen. Reactive oxygen species (ROS) may have beneficial effects as they are part of cellular signalling systems and are used by the immune system to kill invading bacteria. However, reactive oxygen species produced in excess may lead to oxidation damage of cell components.
The free radical superoxide, •O₂^- may leak out from the mitochondria during reduction of O₂ along the electron transport chain. Superoxide is converted by the protective enzyme superoxide dismutase (SOD) to the less toxic hydrogen peroxide H₂O₂, which is then broken down by other protective enzymes. Otherwise hydrogen peroxide is a substrate for the Fenton reaction producing free hydroxyl radical, •OH. Hydroxyl radical is extremely reactive and attacks DNA, proteins and lipids, causing oxidative damage. This damage may contribute to cardiovascular disease, cancer and other serious diseases, and it is also thought to be a factor in aging. Cells have potent antioxidant defences, augmented by antioxidant micronutrients from the diet, including vitamin E. Vitamin E (α-tocopherol) is lipid-soluble, is transported in plasma lipoproteins, is an integral part of cell membranes, and accumulates in fat-stores.

Figure
Formation of some ROS species from molecular oxygen:
-the free radical superoxide, •O₂^-
-hydrogen peroxide, H₂O₂
-hydroxyl radical, •OH