Formation of Organic Ion-Radicals in Living Organisms

One-electron transfer reactions are typical in living organisms. Ion-radicals are acting participants of metabolism. Of course, such ion-radicals are instantly included in further biotransformations. Therefore, it is reasonable to consider the problem of ion-radical formation together with the data on their behavior in biosystems. Chapter 3 contains a special section covering this topic. However, the issue of competition for an electron during ion-radical formation deserves to be mentioned here. 

Sometimes, mesonidazole and AF-2 medication are administered jointly. Mesinidazole is l-(2-nitroimidazol-l-yl)-3-methoxypropan-2-ol and AF-2 is 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide. It is clear that the latter is a stronger acceptor than the former. When anion-radicals of mesonidazole are formed, they pass their unpaired electrons to molecules of AF-2. One medicament cancels the action of the other (Clarke et al. 1984a,b). 

The drugs most frequently used in the treatment of Chagas disease are nifurtimox and benzni-dazole. But both of them have very low antiparasitic activity in the chronic stage of the disease and 

Electron-transfer chains in plants differ in several striking aspects from their mammalian counterparts. Plant mitochondria are well known to contain alternative oxidase that couples oxidation of hydroquinones (e.g., ubiquinol) directly to reduction of oxygen. Semiquinones (anion-radicals) and superoxide ions are formed in such reactions. The alternative oxidase thus provides a bypass to the conventional cytochrome electron-transfer pathway and allows plants to respire in the presence of compounds such as cyanides and carbon monoxide. There are a number of studies on this problem (e.g., see Affourtit et al. 2000, references therein). 

Besides the enzyme, the superoxide ion can also be an electron donor. The ion arises as a result of detoxication of xenobiotics (xenobiotics are outsiders, which are involved in the chain of metabolism). Xenobiotics yield anion-radicals by the neutralizing influence of redox proteins. Oxygen (inhaled with air) takes an unpaired electron off from a part of these anion radicals and forms the superoxide ion. The superoxide ion plays its own active role in biochemical reactions. 

---