Reactive Oxygen Species

The most relevant free radicals in biological systems are derivatives of oxygen. The superoxide free radical anion ("superoxide") is produced as a result of oxygen reduction by transfer of a single electron to it:

While, hydrogen peroxide is generated either by two-electron reduction of oxygen:

or via the generation of superoxide where two superoxide molecules can react together to form hydrogen peroxide and oxygen:

The breaking down of hydrogen peroxide in the presence of transition metal ions such as iron will generate the most reactive and damaging of the oxygen free radicals, the hydroxyl radical (OH):

In addition, hydroxyl radical produced by reaction of superoxide directly with hydrogen peroxide:

In biological system, ROS can be formed in three ways [17]:

1. By the homolytic cleavage of a covalent bond of a normal molecule, with each fragment retaining one of the paired electrons (X: Y ^ X^ + V).

2. By the loss of a single electron from a normal molecule (X: Y ^ X :- +Y+).

3. By the addition of a single electron to a normal molecule (A + e- ^ A-^.

ROS are generated during the normal enzymatic reactions of inter- and intracellular signaling [15]. Spermatozoa have the ability to produce many types of ROS aerobically, such as, hydrogen peroxide (H2O2), the superoxide anion fO2 -), the hydroxyl radical (Off), hypochlorite radical (OHCT) , nitric oxide, peroxyl, ozone, lipid peroxides, nitrous oxide, peroxynitrite, and nitroxyl ion [18]. Binding of these compounds with other cellular structures can cause oxidation and ending with cellular damage [18, 19] The presence of high ROS levels has been reported in the semen of 25-40% of infertile men [20].

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