Purines free-radical attack

Allantoin is the catabolic end product of purines in most mammals. It is formed by the action of the enzyme uricase on urate. Humans and other primates lack uricase and excrete urate as the final product of purine metabolism. However, small amounts of allantoin are present in human serum. Some authors demonstrated that free radical attack on urate generates allantoin. Therefore small amounts of allantoin detected in human serum may provide a marker of free radical activity in vivo. 

Hydroxyl radicals are the most reactive free-radical species known and have the ability to react with a wide number of cellular constituents including amino-acid residues, and purine and pyrimidine bases of DNA, as well as attacking membrane lipids to initiate a free-radical chain reaction known as lipid peroxidation. 

Visscher KJ, de Haas MP, Loman H, Vojnovic B, Warman JM (1987) Fast protonation of adenosine and of its radical anion formed by hydrated electron attack a nanosecond optical and dc-conduc-tivity pulse radiolysis study. Int J Radiat Biol 52 745-753 Visscher KJ, Spoelder HJW, Loman H, Hummel A, Horn ML (1988) Kinetics and mechanism of electron transfer between purines and pyrimidines, their dinucleotides and polynucleotides after reaction with hydrated electrons a pulse radiolysis study. Int J Radiat Biol 54 787-802 von Sonntag C (1980) Free radical reactions of carbohydrates as studied by radiation techniques. Adv Carbohydr Chem Biochem 37 7-77... 

The increase in protonation sites in the oxopurines with increasing oxygen functions extends the possibilities for Ai"-alkylation sites. Normally in the case of monooxo- and dioxo-purines, with the exception of reactions with diazoalkanes, many of which are probably free radical in nature, alkylating agents even in alkaline solutions produce only Af-alkyl derivatives. Trioxopurines such as uric acid derivatives are an exception. The nature of the alkylation and the site(s) of attack nevertheless still vary according to the solvent, pH and temperature. 

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