Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
A toxic effect in a living organism caused by a species of oxygen. Oxygen has two aspects, one benign and the other malignant. Those organisms that avail themselves of the enormous metabolic advantages provided by dioxygen (O2) must defend themselves against its toxicity. The complete reduction of one molecule of O2 to two of water (H2O) requires four electrons; therefore, intermediates must be encountered during the reduction of O2 by the univalent pathway. The intermediates of O2 reduction, in the order of their production, are the superoxide radical (O2-), hydrogen peroxide (H2O2), and the hydroxyl radical (HO·).
The intermediates of oxygen reduction, rather than O2, itself, are probably the primary cause of oxygen toxicity. It follows that defensive measures must deal with these intermediates. The superoxide radical is eliminated by enzymes that catalyze the following reaction. These enzymes, known as superoxide dismutases, have been isolated from a wide variety of living things.
Hydrogen peroxide (H2O2) must also be eliminated, and this is achieved by two enzymatic mechanisms. The first of these is the dismutation of H2O2 into water and oxygen, a process catalyzed by catalases. The second is the reduction of H2O2 into two molecules of water at the expense of a variety of reductants, a process catalyzed by peroxidases. See Enzyme
The multiplicity of superoxide dismutases, catalases, and peroxidases, and the great catalytic efficiency of these enzymes, provides a formidable defense against O2- and H2O2. If these first two intermediates of O2 reduction are eliminated, the third (HO·) will not be produced. No defense is perfect, however, and some HO· is produced; therefore its deleterious effects must be minimized. This is achieved to a large extent by antioxidants, which prevent free-radical chain reactions from propagating. See Antioxidant, Peptide
The apparent comfort in which aerobic organisms live in the presence of an atmosphere that is 20% O2 is due to a complex and effective system of defenses against this peculiar gas. Indeed, these defenses are easily overwhelmed, and overt symptoms of oxygen toxicity become apparent when organisms are exposed to 100% O2. For example, a rat maintained in 100% O2 will die in 2 to 3 days.