oxygen toxicity

Oxygen toxicity

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.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.

oxygen toxicity

[′äk·sə·jən täk′sis·əd·ē]

(physiology)

Harmful effects of breathing oxygen at pressures greater than atmospheric.

A toxic effect in a living organism caused by a species of oxygen-containing reactive intermediate produced during the reduction of dioxygen.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.