Behavior of Anion-Radicals in Living Organisms

The majority of the enzyme-catalyzed reactions discussed so far are oxidative ones. However, reductive electron transfer reactions take place as well. Diaphorase, xanteneoxidase, and other enzymes as well as intestinal flora, aquatic, and skin bacteria—all of them can act as electron donors. Another source of an electron is the superoxide ion. It arises after detoxification of xenobiotics, which are involved in the metabolic chain. Under the neutralizing influence of redox proteins, xenobiotics yield anion-radicals. Oxygen, which is inhaled with air, strips unpaired electrons from these anion-radicals and gives the superoxide ions (Mason and Chignell 1982). 

Some biologically important o-quinones can react with the superoxide ion giving catechol derivatives, which may play a role in many diseases. For example, compounds bearing a nitro-catechol moiety have been claimed to be efficient catechol-0-methyl transferase inhibitors (Suzuki et al. 1992, Perez et al. 1992). The transferase is the first enzyme in the metabolism of catecholamine a hyperactivity of this enzyme leads to Parkinson s disease. Therefore, prediction of biological activity and antioxidant properties of quinones is an important challenge for researchers. 

Smertenko et al. (2000) have proposed a special index for this purpose based on differences in redox potentials between a quinone and oxygen. This index takes into account the peak merging for electrochemical reductions of the quinone and oxygen and, according to first estimations, works well. 

The biological oxidant P-450 can also catalyze reduction, cycling between the Fe and Fe forms (Guengerich and Macdonald 1993). The preponderance of oxidation reactions is not surprising, because in the sequence of oxygen activation steps, ferrous P-450 binds O2 rapidly and tightly. 

Reductive dehalogenation reactions catalyzed by P-450 have been studied extensively, primarily because of the interest in compounds such as anesthetics, pesticides, and potentially toxic industrial solvents. An anesthetic named halothane gives anion-radical that undergoes dehalogenation according to the following equation  

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