Superoxide radicals are normally produced by the enzyme NADPH oxidase in order to activate click here the defense mechanisms against invading pathogens (Halliwell and Gutteridge, 2007). Superoxide is produced by the electron transport chain from oxygen occupying the final position and acting as the terminal electron acceptor. Some electrons can randomly “leak” from the electron transport chain (Campian et al., 2004) and interact with oxygen

to produce superoxide radicals. Thus under physiological conditions, about 1–3% of the oxygen molecules in the mitochondria are converted into superoxide radicals. Superoxide radical is normally present mainly in the form of an anion radical and is removed by a dismutation reaction (Liochev and Fridovich, 2000): equation(1) 2O2−·+2H+⟶SODH2O2+O2 While without SOD this reaction Doxorubicin clinical trial proceeds very slowly (k ∼ 0.2 M−1 s−1), the reaction becomes biologically relevant

when it is catalyzed by the SOD. The kinetic constant of the SOD-catalyzed superoxide depletion dismutation reaction has been estimated to be 2.5 × 109 M−1 s−1 ( Liochev and Fridovich, 2003). A mutual link between superoxide radicals and iron shows, that under in vivo stress conditions, an excess of superoxide releases “free iron” from iron-containing molecules (e.g. ferritin). The release of iron by superoxide has also been demonstrated for the [4Fe–4S] cluster-containing enzymes. Inactivation of these enzymes by O2− is a rapid process that leads to oxidation of the iron-sulphur cluster. The native clusters contain two Fe(II) and two Fe(III) ions, and the oxidation [one Fe(II) is oxidized to Fe(III)] may be denoted as follows (Liochev and Fridovich, 1994): equation(2) [2Fe(II) 2Fe(III)–4S]2+ + O2−  + 2H+ → [Fe(II) 3Fe(III)–4S]3+ + H2O2 The rate constant for reaction Clostridium perfringens alpha toxin (2) has been estimated in the range of 108 to 109 M−1 s−1. Since the oxidized protein binds the Fe(III) more firmly, Fe(II) ions are released from protein

according to the following reaction: equation(3) [Fe(II) 3Fe(III)–4S]3+ → [3Fe(III)–4S]+ + Fe(II) The released Fe(II) can participate in the Fenton reaction, generating highly reactive hydroxyl radicals ( OH) (Prousek, 2007) equation(4) Fe(II) + H2O2 → Fe(III) +  OH + OH−  (Fenton reaction) The Fenton reaction has its in vivo significance mainly under state of an organisms overloaded by iron (as in the conditions of hemochromatosis, b-thalassemia, hemodialysis). Thus high amounts of “free available iron” can have deleterious effects (Kakhlon and Cabantchik, 2002). The superoxide radical participates in the Haber–Weiss reaction (Liochev and Fridovich, 2002): equation(5) O2−  + H2O2 → O2 +  OH + OH−which is a combination of Fenton reaction and the reduction of Fe(III) by superoxide: equation(6) Fe(III) + O2−  → Fe(II) + O2 The hydroxyl radical is highly reactive with a half-life in aqueous solution of less than 1 ns (Pastor et al., 2000).