Oxidant Stress

Oxidant, or oxidative, stress is a pro-oxidant shift in the oxidant-antioxidant balance caused by a relative or absolute deficiency of antioxidants (Figure 1). A pro-oxidant shift promotes damaging oxidative changes to important cellular constituents, and this may, in turn, lead to cellular dysfunction and, ultimately, to aging, disability, and disease.

Molecular oxygen is relatively unreactive in its ground state. However, molecular oxygen can be reduced in several ways within the body to produce more reactive species (Table 1). These species include radical and nonradical forms of oxygen, some of which contain nitrogen or chlorine. A 'free radical' is capable of independent existence and has a single (unpaired) electron in an orbital. Electrons

stabilize as pairs with opposing spins within an orbital. An unpaired electron seeks a partner for stability, and this increases the reactivity of the radical. A partner electron can be obtained by removing ('abstracting') an electron from another species or co-reactant. The result of this interaction may be either quenching by reduction (electron addition) of the radical with the production of a new radical by oxidation (electron loss) of the reductive ('antioxi-dant') co-reactant or quenching of two radicals if the co-reactant is also a radical (one quenched by reduction (electron addition) and one by oxidation (electron removal)).

Free radicals produced in vivo include superoxide, the hydroxyl radical, nitric oxide, oxygen-centered organic radicals such as peroxyl and alkoxyl radicals, and sulfur-centered thiyl radicals. Other oxygen-containing reactive species that are not radicals are also formed. These include hydrogen peroxide, peroxynitrite, and hypochlorous acid. While these are not radical species, they are actually or potentially damaging oxidants. The collective term ROS is often used to describe both radical and nonradical species.

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