Oxygen, in its non-reactive form, is an element crucial to the survival of many organisms. However, during normal metabolic processes and as a result of numerous environmental pollutants, oxygen gains an extra electron by colliding into an electron carrier molecule to become highly toxic and lethal intermediates capable of destroying us. These intermediates, which include the hydroxyl radical, hydrogen peroxide, and the superoxide radical, are referred to as active oxygen species and free radicals, with the latter being a unique species able to exist with unpaired electrons. The hydroxyl radical, which is produced by the separation of the O-H bonds in water, is among the most toxic due to its ability to act as a powerful one-electron oxidant that absorbs electrons from a large number of compounds. Since it seems to be produced in a cell’s nucleus, the hydroxyl radical destroys and mutates cell DNA in a variety of ways, with one of the most common products of its attack on DNA bases being 8-hydroxyguanine, a miscoding lesion leading to G to T transversions. In addition, this deadly radical ruins the delicate sulphur atoms and metal ions in virtually all proteins and introduces a free radical chain reaction called lipid peroxidation to weaken various membranes throughout the organism. Hydrogen peroxide and the superoxide radical are not as deadly as the one mentioned above, but they are still threats to life as superoxide radicals are directly toxic and hydrogen peroxide reacts with iron or copper ions to produce the ominous hydroxyl radical.
Our bodies undergo oxidative stress when these free radicals and active oxygen species reach overly high levels due to malnutrition or stress, among other things. Though our cells can tolerate mild oxidative stress and usually respond by increasing the synthesis of antioxidant defence enzymes, severe oxidative stress can cause cellular physiological dysfunction and death. In mammals, it causes increases in free calcium ions and free iron within the cells, which leads to the formation of hydroxyl radicals and activates endonucleases to start DNA fragmentation. Other biological consequences of oxidative damage include peroxidation of membrane lipids, loss of organelle function, mutations, enzyme inactivation, and reduced metabolic efficiency. In addition, free radicals and derivatives have been the cause of aging, cancer, emphysema, and immunologic impairments.
Fortunately, our cells make a number of antioxidant enzymes to prevent oxidative stress from occurring. Among them are superoxide dismutase, which converts superoxide radicals into hydrogen peroxide, and catalase, which converts hydrogen peroxide into water and oxygen gas. Many catalase molecules patrol the cells, counteracting the hydrogen peroxide resulting from everyday metabolism and keeping it at a safe level. Catalases are also some of the most efficient enzymes found in cells as each catalase molecule is capable of decomposing millions of hydrogen peroxide molecules per second.