chapter  27
4 Pages

Overview of Oxidative Stress

OS occurs when there is a higher concentration of oxidants than of the opposing antioxidants, i.e., there is a shift in the ratio of oxidants to antioxidants to greater than 1. Basically, our body uses oxygen for the maintenance of life. Oxygen is used in the oxidation of carbohydrates to create energy to live and in the creation of

oxidized

lipids and nucleic acids to produce the four bases that form the code of life. Substances that protect us from infection are derived from oxygen molecules in one form or another. However, utilization of oxygen produces a long-term risk. This risk is related to attack on our cellular macromolecules, e.g., protein, DNA, RNA, and lipid, from the myriad of different oxygenlinked substances. Some of these are quite toxic depending on the form of oxygen and the atoms linked to it, which create new and possibly more toxic compounds. For example, oxygen can exist in a ground state or, if it absorbs energy, in two different excited states, epsilon and delta. When an electron is added to oxygen, superoxide ( ) is produced. When it becomes further reduced, it can form hydrogen peroxide (H

O

). It can exist in a highly toxic and energetic state called hydroxyl free radical (OH

·

), which is formed in the presence of reduced iron ion (Fe

) and hydrogen peroxide in a Fenton reaction. Additionally, if superoxide reacts with nitric oxide it can form (OONO

), or peroxynitrite. Another strong oxidizing entity is formed by the reaction of hypochlorite and hydrogen peroxide to produce an oxidizing peroxide, possibly peroxyhypochlorite (OOClO

). See Tables 27.1 and 27.2 for half-lives of oxygen species and serum concentrations of antioxidants.