What Causes Oxidative Stress

Oxygen is essential not only for energy metabolism and respiration; it also plays a role in degenerative processes (Marx 1987). It is a biradical, which means that it has two unpaired electrons with parallel spins (see Table 4.1). While oxygen does have unpaired electrons, which would normally make it rather reactive, the fact that

Table 4.1 Nomenclature of the different oxygen Species

Formula species

Triplet oxygen (ground state) O-O'

Singlet oxygen Superoxide Perhydroxide radical Hydrogen peroxide Hydroxy radical Hydroxide ion Water

these electrons have parallel spins (known as the "triplet state") actually makes it difficult for oxygen to participate in reactions with organic molecules. Such reactions become easier if the biradical is activated. Activation can occur if enough energy is absorbed by triplet oxygen to "flip" the spin of one of its unpaired electrons (i.e. making the spins antiparallel - the "singlet state"). Singlet oxygen is much more reactive towards organic molecules than triplet oxygen.

Another way to activate oxygen is through the stepwise monovalent reduction of oxygen to superoxide, superoxide to hydrogen peroxide, and hydrogen peroxide to hydroxide radical. The final product of this series of reductions is water:

Hydrogen peroxide is a very important metabolite, because it can diffuse across membranes and is not dispersed in cells. Peroxidase enzymes (POX) use hydrogen peroxide as a substrate in oxidative reactions, primarily during the complex synthesis of organic molecules.

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