Pyrimidine redox properties

An almost complete description of both OH radical-mediated and one-electron oxidation reactions of the thymine moiety (3) of DNA and related model compounds is now possible on the basis of detailed studies of the final oxidation products and their radical precursors. Relevant information on the structure and redox properties of transient pyrimidine radicals is available from pulse radiolysis measurements that in most cases have involved the use of the redox titration technique. It may be noted that most of the rate constants implicating the formation and the fate of the latter radicals have been also assessed. This has been completed by the isolation and characterization of the main thymine and thymidine hydroperoxides that arise from the fate of the pyrimidine radicals in aerated aqueous solutions. Information is also available on the formation of thymine hydroperoxides as the result of initial addition of radiation-induced reductive species including H" atom and solvated electron. 

These enzymes catalyze the two-electron oxidation of purines, aldehydes and pyrimidines, sulfite, formate and nicotinic acid in the general reaction shown in equation (49). These enzymes show some differences in properties. Xanthine oxidase, xanthine dehydrogenase and aldehyde oxidase all have relatively low redox potentials and a unique cyanolyzable sulfur atom, and so will be discussed together. 

XOR accelerates the hydroxylation of purines, pyrimidines, pterins and aldehydes [132]. In humans, the enzyme catalyzes the last two steps of purine catabolism the oxidation of hypoxanthine to xanthine and of the latter to uric acid. An unusual property of this, but not aU XOR enzymes [133], is its interconversion between xanthine dehydrogenase and xanthine oxidase activities which implies a switch between NAD" and molecular oxygen being used as the final electron acceptor [134]. Structural studies suggest that this switch, that can be irreversibly induced by proteolysis [135], results from conformational changes that lead to both restricted access to the NAD cofactor to its binding site and changes in the redox potential of the FAD cofactor [136],... 

Compared with other methods, electrochemical ones have a wider range of application, which makes it possible to study the details of the reaction s mechanism. They are suitable for unique syntheses and for the solution of analytical problems. The use of electrochemical methods made it possible to obtain detailed information about the thermodynamics (redox potentials), kinetics (number of electrons, etc.) and mechanism of reactions with the participation of heterocyclic nitrogen compounds (purines, pyrimidines, porphyrines, etc.). [For more details see 2]. Capacity measurements provided important information [see, for example 3] on the adsorption properties of low-molecular and high-molecular bio-logically-active compounds (proteins, DNA, RNA). 

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