single electron to or from a normal molecule, respectively. Of these three processes, electron transfer is the most common. It generally requires high energy input, and may be instigated by such factors as high temperature, UV light, and ionizing radiation (1:482).
Four species which are particularly active in free radical biochemistry include oxygen, superoxide, hydrogen peroxide, and transition metal ions. These substances react to produce the most damaging of the oxygen free radicals, the hydroxyl radical (1:483).
Thus, of the important free radicals, most are derivatives of oxygen. The compounds often result from electron "leakage" by electron transport systems and oxidases. For example, transfer of a single electron to the "di radical" oxygen, produces a superoxide free radical anion, otherwise known as "superoxide" (O2 + e > O2 .). In addition, the transfer of two electrons to an oxygen molecule produces hydrogen peroxide (O2 + 2e + 2H+ > H2O2). Another mechanism by which hydrogen peroxide can be formed involves the combination of two superoxide molecules to form hydrogen peroxide and oxygen (2O2 . + 2H+ > H2O2 + O2) (1:483).
Others sources of electrons in free radical formation include the transition metals (2:441). For instance, in the presence of iron, hydrogen peroxide breaks down to produce the hydroxyl radical (H2O2 + Fe2+ > .OH + OH + Fe3+). This reaction is known as the iron catalyzed Haber Weiss reaction (3:354). Although the non catalyzed Haber Weiss reaction (O2 . + H2O2 > .OH + OH + O2) may also occur, low levels of the reactants in biological systems make it less common.
In addition, reversible redox reactions can also generate free radicals through the autoxidation of reduced transition metal ions. Spontaneous electron transfer from either iron or copper, for instance, produces superoxide (e.g., Fe2+ + O2 > Fe3+ + O2 . and Cu+ + O2 > Cu2+ + O2 .) (1:484)...