For organisms living in an aerobic environment, exposure to reactive oxygen species (ROS) is continuous and unavoidable. ROS encompass a variety of partially reduced metabolites of oxygen (e.g., superoxide anions, hydrogen peroxide, and hydroxyl radicals) possessing higher reactivities than molecular oxygen. They are generated intracellularly through a variety of processes, for example, as byproducts of normal aerobic metabolism, or as second messengers in various signal transduction pathways. They can also be derived from exogenous sources, either being taken up directly by cells from the extracellular milieu, or produced as a consequence of the cell's exposure to some environmental insult. Transient fluctuations in ROS serve important regulatory functions, but when present at high and/or sustained levels, ROS can cause severe damage to DNA, protein, and lipids.
A number of defense systems have evolved to combat the accumulation of ROS. These include various non-enzymatic molecules (e.g., glutathione, vitamins A, C, and E, and flavenoids) as well as enzymatic scavengers of ROS (e.g., superoxide dismutases (SOD), catalase, and glutathione peroxide). Unfortunately, these defense mechanisms are not always adequate to counteract the production of ROS, resulting in what is termed a state of oxidative stress. Oxidative stress has been implicated in a wide variety of disease processes including atherosclerosis, diabetes, pulmonary fibrosis, neurodegenerative disorders, and arthritis, and is believed to be a major factor in aging.
Oxidative stress is essentially an imbalance between the production of free radicals and the ability of the body to counteract or detoxify their harmful effects through neutralization by antioxidants.
What are free radicals?
A free radicals is an oxygen containing molecule that has one or more unpaired electrons, making it highly reactive with other molecules.
Oxygen by-products are relatively unreactive but some of these can undergo metabolism within the biological system to give rise to these highly reactive oxidants. Not all reactive oxygen species are harmful to the body. Some of them are useful in killing invading pathogens or microbes.
However, free radicals can chemically interact with cell components such as DNA, protein or lipid and steal their electrons in order to become stabilized. This, in turn, destabilizes the cell component molecules which then seek and steal an electron from another molecule, therefore triggering a large chain of free radical reactions.
What are antioxidants?
Every cell that utilizes enzymes and oxygen to perform functions is exposed to oxygen free radical reactions that have the potential to cause serious damage to the cell. Antioxidants are molecules present in cells that prevent these reactions by donating an electron to the free radicals without becoming destabilized themselves. An imbalance between oxidants and antioxidants is the underlying basis of oxidative stress.
Damaged caused by oxidative stress
Oxidative stress leads to many pathophysiological conditions in the body. Some of these include neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease, gene mutations and cancers, chronic fatigue syndrome, fragile X syndrome, heart and blood vessel disorders, atherosclerosis, heart failure, heart attack and inflammatory diseases.
Antioxidants in disease prevention
Antioxidants may be used in disease prevention but the extent of their benefits is not fully understood. The use of antioxidants to treat disease states induced by oxidative stress is controversial.
While studies show that that beta carotene, for example, has a pro-oxidative effect in the lungs of smokers, other epidemiological research has demonstrated an inverse relationship between levels of established antioxidants such as vitamin E and C and disease states such as cancer and cardiovascular disease. The levels of these vitamins are also inversely associated with mortality caused by these diseases.
Most antioxidants are obtained from diet but they can also be taken as supplements in the form of capsules or tablets.
Most fresh fruits and vegetables provide a natural source of antioxidants. Large population studies have shown that regular intake of adequate portions of fruits and vegetables has been seen to protect against numerous cancers, heart diseases and other disorders.
Apart from vitamins such as vitamins E and C, examples of other antioxidants include minerals, resveratrol, beta carotene (a precursor form of vitamin A), selenium, green tea and jiaogulan.
Current antioxidant treatment
Several antioxidants are currently used to treat diseases.
For example, antioxidants may be useful in treating patients after a stroke to protect the nerves and brain cells from oxidative damage and lipid peroxidation. Agents such as superoxide dismutase mimetics, sodium thiopental can be used to treat reperfusion injury that occurs after traumatic brain injury.