Many organisms including humans use oxygen in their metabolism to generate energy from food. Though essential to life, oxygen as a molecule is highly dangerous - it oxidises biochemicals and metals and is toxic in high concentrations because of its high reactivity. One only has to observe the devastating effect oxygen has on solid iron - rust!
During aerobic metabolism, reactive oxygen species, also known as free radicals, are generated by the body. Free radicals are the more harmful forms of oxygen, which can attack and damage almost all cellular compounds including DNA, proteins and lipids (fats). For example, as part of normal metabolism, your body generates hypocholorous acid (HClO) and hydrogen peroxide, which is often used as a bleach. The potential for free radicals to cause cellular damage, if left unchecked, is known as oxidative stress.
Animals and humans have developed inbuilt ways of reducing or eliminating the potential damage caused by oxidative stress. These defence mechanisms involve using specific enzymes or antioxidants to convert the free radicals into less dangerous molecules, such as water (see fig. 1). The antioxidants can also serve as sacrifices, being oxidised by the free radicals instead of important cell components (in the same way that a bodyguard would protect someone by diving in front of a bullet). The major cellular antioxidant, glutathione, is essential in this process.
Primary antioxidants inhibit oxidisation by transferring an electron to the free radical species, thus nullifying its oxidative effects. Secondary antioxidants, by contrast, decompose a specific class of free radicals - known as hydroperoxides - into their non-reactive form. In all cases, antioxidants are responsible for stopping the 'cascade effect' of self-multiplying free radicals.
Unfortunately, the body's mechanism for dealing with oxidative stress becomes less efficient as we get older as fewer antioxidants are produced. This decline in function is associated with over a hundred human diseases, as well as the process of ageing.