biochemistry (free radicals and antioxidants)

free radical

an atom or a group of atoms that has at least one unpaired electron in its outer orbit. they are highly reactive and can initiate chain reactions

superoxide anion radical (O2-)

formed from free oxygen by the donation of an electron to another free radical

hydrogen peroxide (H2O2)

not actually free radical, but it is the main source of hydroxyl radicals. involved in the production of hypochlorous acid. produced in biological systems

hydroxyl radical (OH+ superscript dots)

extremely reactive oxidizing radical. can damage all types of biomolecules as carbohydrate, protein and nucleic acid

singlet oxygen

differs from normal oxygen as it has extra energy. it is formed mainly by radiation and sunlight

nitric oxide (NO+superscript dots)

free radical produced endogenously by nitric oxide synthase

xanthine oxidase enzyme

generates superoxide and hydrogen peroxide during conversion of hypoxanthine to xanthine then to uric acid

autoxidation of some cellular components

could generates superoxide as oxidation of ascorbic and flavin coenzymes

oxidative stress

imbalance between production of free radicals and antioxidant

antioxidants

substances that protect cells from the damage caused by the free radicals

superoxidase dismutase (SOD)

metal-containing enzymes that depend on manganese, copper or zinc for their antioxidant activity. considered as the first line of defense against oxygen activity

catalase

heme-containing enzyme, present mainly in the erythrocytes. present in variable amounts in the liver, spleen and kidney

glutathione peroxidase

selenium-dependent enzyme and responsible for the removal of the H2O2. it is also present in different tissue such as RBC

vitamin E

very important in the protection against lipid peroxidation. it is major lipid-soluble antioxidant in cell membranes and also terminates the radical chain reactions

vitamin C (ascorbic acid)

acts as a scavenger of superoxide and hydroxyl radicals and also donates electrons to free radicals in two steps

carotenoids and vitamin A

converts ROO into inactive product

selenium

cofactor for glutathione peroxidase. it also involved in the elimination of ROS