Superoxide Dismutase Protects Aerobic Organisms Against Oxygen Toxicity

Transfer of a single electron to O2 generates the potentially damaging superoxide anion free radical (O2-), the destructive effects of which are amplified by its giv ing rise to free radical chain reactions (Chapter 14). The ease with which superoxide can be formed from oxygen in tissues and the occurrence of superoxide dis-mutase, the enzyme responsible for its removal in all aerobic organisms (although not in obligate anaerobes) indicate that the potential toxicity of oxygen is due to its conversion to superoxide.

Superoxide is formed when reduced flavins—present, for example, in xanthine oxidase—are reoxidized univalently by molecular oxygen.

Enz - Flavin - H2 + O2 ^ Enz - Flavin - H + O2' + H+

Superoxide can reduce oxidized cytochrome c

Substrate A-H

Substrate A-H

Figure 11-6. Cytochrome P450 hydroxylase cycle in microsomes. The system shown is typical of steroid hydroxylases of the adrenal cortex. Liver microsomal cytochrome P450 hydroxylase does not require the iron-sulfur protein Fe2S2. Carbon monoxide (CO) inhibits the indicated step.

or be removed by superoxide dismutase.


In this reaction, superoxide acts as both oxidant and reductant. Thus, superoxide dismutase protects aerobic organisms against the potential deleterious effects of superoxide. The enzyme occurs in all major aerobic tissues in the mitochondria and the cytosol. Although exposure of animals to an atmosphere of 100% oxygen causes an adaptive increase in superoxide dismutase, particularly in the lungs, prolonged exposure leads to lung damage and death. Antioxidants, eg, a-tocopherol (vitamin E), act as scavengers of free radicals and reduce the toxicity of oxygen (Chapter 45).

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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