Free Radicals and Antioxidants Lecture PDF

Free Radicals and Antioxidants Biochem 474 By Dr. Mai Madkour Basics of redox chemistry Oxidation ▪ Gain in oxygen ▪ Loss of hydrogen or electrons Reduction ▪ Loss of oxygen ▪ Gain of hydrogen or electrons Oxidant ▪ Oxidize another chemical compound by...

Free Radicals and Antioxidants Biochem 474 By Dr. Mai Madkour Basics of redox chemistry Oxidation ▪ Gain in oxygen ▪ Loss of hydrogen or electrons Reduction ▪ Loss of oxygen ▪ Gain of hydrogen or electrons Oxidant ▪ Oxidize another chemical compound by accepting electrons, hydrogen Reductant ▪ Reduces another chemical compound by donating electrons, hydrogen  Free radicals ▪ A free radical is defined as an atom or molecule that contains one or more unpaired electrons in its outer orbital rather than the usual paired electrons that spin in opposite directions ▪ It is an electron-defiecient species ▪ It is represented by a superscript dot to the right R. ▪ Examples: - Hydroxyl radical (HO·), a molecule that has one unpaired electron on the oxygen atom Free radical nomenclature A free radical is donated by a superscript dot to the the oxygen or carbon e.g.,.OH, NO.,. CH3 If a free radical is a charged species, the dot is put and then the charge e.g., O2-. Characteristics of Free radicals Free radicals are ▪ Highly reactive and react quickly with other compounds ▪ Unstable and try to become stable ▪ Short life span (short lived ) as they tend to catch electron from other molecules ▪ Generation of new free radicals by chain reaction ▪ Damage to various tissues Radicals can be formed by 1. The loss of a single electron from a non-radical, or by the gain of a single electron by a non-radical 2. The breakage of covalent bond “homolytic fission’ - Covalent bond breakage in which the shared electrons is split evenly between the products - In the presence of heat/light ▪ Therefore if two free radicals react, they neutralize each other. ▪ However, if the free radicals react with stable molecules, there is generation of more free radials. ▪ This character enables the free radicals to participate in autocatalytic chain reactions. - Molecules are themselves converted to free radicals to propagate the chain of damages. Function of free radical ▪ Some of free radicals arise normally during metabolism ▪ Sometimes the body’s immune system create them to neutralize viruses and bacteria - Respiratory burst in WBC - NO signaling ▪ Some free radicals at low levels are signaling molecules, i.e. they are responsible for turning on and off of genes. ▪ Some free radicals kill cancer cells. ▪ Normally, the body can handle free radicals, but if antioxidants are unavailable, or if the free radicals production becomes excessive, damage can occur Non-radicals ▪ Species that have strong oxidizing potential ▪ These nonradicals molecules can produce oxidation “per se” or can also be converted into free radicals. Hydrogen peroxide ▪ Examples - Hydrogen peroxide (H₂O₂) Hypochlrous acid - Hypochlorus acid (HClO) Ozone - Ozone (O₃) - Singlet oxygen (1O2 ) Singlet oxygen - Peroxynitrite (ONOO−) - Transition metals Peroxynitrite Ex: manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) Source of free radicals Exogenous source of free radical Ionizing radiation Ultraviolet radiation Chemicals, smoking Pollution Diet Endogenous source of free radical ▪ Free radical formation occurs continuously in the cells as a consequence of both enzymatic and nonenzymatic reactions ▪ Enzymatic reactions, which serve as source of free radicals, include those involved in the respiratory chain, phagocytosis, prostaglandin synthesis, and in the cytochrome P-450 system ▪ Free radicals can also be formed in nonenzymatic reactions of oxygen with organic compounds as well as those initiated by ionizing reactions (x-ray and UV) which lyse water leading to formation of hydroxyl radical ▪ Transition metal ions including Cu +2, Co +2, Ni +2 and Fe +2 Can react nonenzymatically with oxygen ▪ or react with hydrogen peroxide leading to the formation of hydroxyl radicals Examples ▪ Cells (neutrophil, eosinophil,……) ▪ Inflammation ▪ Phagocytosis ▪ Enzymes (Nitric oxide synthase, xanthine oxidase, NADPH oxidase) ▪ Mitochondria ▪ Endoplasmic reticulum oxidation ▪ Cytochrome P450  Source of free radical ▪ Endogenous free radical ▪ Exogenous free radicals E.g. ionizing radiation Types of free radicals Reactive oxygen species (ROS): - Hydrogen peroxide (H2O2) - Hypochlorous acid (HOCl) - Singlet oxygen ( 1O2 ) - Superoxide anion (O2-.) - Hydroxyl radical (.OH) Reactive nitrogen species (ROS): - Nitric oxide ( NO. ) - Peroxy nitrite (ONOO-) - Peroxy nitrate (O2NOO-) Other reactive species - Lipid peroxyl radical (LOO.) - Lipid hydroperoxide (LOOH)  Activation of oxygen ▪ Atmospheric oxygen in its ground-state is distinctive among the gaseous elements because it is a biradical, or in other words it has two unpaired electrons. This feature makes oxygen paramagnetic ▪ The high reactivity of atmospheric oxygen is due to its biradical state (the two unpaired electrons in oxygen has parallel spins) Activation of oxygen may occur by two different mechanisms: ▪ Absorption of sufficient energy to reverse the spin on one of the unpaired electrons, or monovalent reduction. ▪ singlet oxygen is much more reactive towards organic molecules than its triplet oxygen A singlet state refers to a system in which all the electrons are paired. Whereas, the triplet state of a system describes that the system has two unpaired electrons. ▪ The second mechanism of activation is by the stepwise monovalent reduction of oxygen to form superoxide (O-2), hydrogen peroxide (H2O2), hydroxyl radical (.OH) and finally water ▪ The first step in the reduction of oxygen forming superoxide is endothermic but subsequent reductions are exothermic. The activation states of oxygen