|oxidants and antioxidants|
ion vs radical, oxidation vs
Oxidants are compounds which oxidize other molecules. In the biological context, oxidants are molecules which are relatively strong oxidizing agents, strong enough to do damage if allowed to accumulate unchecked.
is a charged particle. In a molecule this means that the
total number of electrons and protons are unequal. Ions
may be cations or anions.
Oxidation is loss of electrons. When a molecule or atom is oxidized, it loses electrons, and vice versa. Reduction is gain of electrons.
important oxidants that are known to be formed during
enzymatic reactions in the body are:
Not all of
these are radicals.
Normal roles of oxidants. Oxidants are there for a purpose, and are obviously not produced by the cells merely to cause harm to the body. They are byproducts of some metabolic processes, in particular during the metabolism of respiratory oxygen. Their main function appears to be to kill germs. In infections, by a process called a "respiratory burst", large amounts of oxidants are produced, especially in the white blood cells. This huge production of oxidants helps to kill micro-organisms. In diseases in which there is a deficiency of enzymes that produce these oxidants, immunity is depressed. Oxidants are produced in other metabolic processes in the body as well.
Although oxidants have their uses, sometimes the oxidants may escape the normal protective backup devices and attack proteins, carbohydrates and fats to cause harm.
Excessive inflammation. Oxidants may stimulate white cells to release hormones and other chemicals in such large quantities that there can be inflammation in the entire body. Such a situation is called the Systemic Inflammatory Response Syndrome, SIRS, and is a very dangerous condition.
Several diseases. Oxidative damage to cells has been implicated in the causation of several conditions, including heart disease, atherosclerosis, amyotrophic lateral sclerosis, stomach ulcers, birth defects (interestingly, thalidomide, a drug which was associated with birth defects, is a oxidant inducer), sperm defects, infertility, and also several eye diseases including cataracts and macular degeneration.
Aging. There is evidence that prolonged oxidant attacks over a period of years causes aging. Changes in brains from oxidation are similar to those that are present in aging brains. Animal species with low levels of antioxidant enzymes and low rates of oxygen radical production in mitochondria live longest, and within a given species the same applies.
Cancer. Oxidants may attack DNA in the nuclei of cells, causing changes that may eventually lead to the development of cancer. Carcinogens such as cigarette smoke, asbestos, gamma and UV rays, nitrosamines (present in burnt food) and hydrocarbons such as benzanthracene, all produce oxidants. It was shown that women whose DNA is getting oxidized tend to develop breast cancer after some years. Epidemiological and case control studies indicate that high intake of ascorbic acid, alpha tocopherol, selenium, beta carotene, vegetables containing A, C, E may lower mortality rates for certain cancers in humans.
Once an oxidizing molecule is produced, the body recognizes that it can do harm. It therefore makes efforts to neutralize the oxidants with different anti-oxidants. Some protective compounds in made by the body are superoxide dismutase, glutathione, catalase, melatonin and antiproteinases. There are many others, not produced by the body. These include Vit A, C, E, some drugs (eg carvedilol used in heart disease), catechins in tea (especially green tea), and some compounds in honey. The oxidants can be reduced, or incorporated into large molecules within which their oxidizing capacity becomes greatly decreases..
Antioxidants have been studied in great detail, but information is mostly indirect. They, and their metabolites, can be useful in
antioxidants be used clinically? The answer is probably
yes. Vitamin C is safe at levels of supplementation up to
600 mg/d, and higher levels, up to 2000 mg/d, are without
risk. Vitamin E has a very low human toxicity and an
intake of 30 mg/ day is the recommended daily allowance
(RDA). Some studies could not detect vitamin E toxicity
even at doses as high as 1000 mg/d.
Sood, MBBS (AIIMS), MS (AIIMS), MAMS, is a
practising surgeon attached to Dharamshila Cancer
Hospital, Sir Ganga Ram Hospital and to Noida Medicare
Center. Formerly Professor of Surgery, Himalayan
Institute of Medical Sciences, Dehradun, Dr Sood has a
special interest in gastrointestinal surgery. He has had
an active academic career, has published several papers
in national and international journals, and is the Editor
(with Dr Anurag Krishna) of a widely acclaimed book
titled Surgical Diseases in Tropical Countries.
Last revised: May 12, 2000