Xanthine, biological oxidation

In the case of the methylated xanthines, particularly theophylline, theobromine and caffeine, the preponderance of data on the metabolism of these compounds in man suggests that a methylated uric acid is the principal product. However, the data presented earlier proposes at best a 77 per cent accounting of the methylated xanthine administered. The question can be raised as to whether the final products observed upon electrochemical oxidation of these compounds aids these studies. Very recently studies of metabolism of caffeine have revealed that 3,6,8-trimethylallantoin is a metabolite of caffeine 48>. This methylated allantoin is, of course, a major product observed electrochemically. The mechanism developed for the electrochemical oxidation seems to nicely rationalize the observed products and electrochemical behavior. The mechanism of biological oxidation could well be very similar, although insufficient work has yet been performed to come to any definite conclusions. There is however, one major difference between the electrochemical and biological reactions which is concerned with the fact that in the former situation no demethylation occurs whereas in the latter systems considerable demethylation appears to take place. 

The initial electrochemical and biological oxidation with xanthine oxidase are essentially identical. However, electrochemically 2,8-dioxyadenine the final product in the presence of xanthine oxidase is much more readily oxidizable than adenine 59) so that considerable further oxidation occurs. To the authors knowledge, 2,8-dioxyadenine is not a major metabolite of adenine in man or other higher organisms. Accordingly, it is likely that other enzymes accomplish further degradation of 2,8-dioxyadenine. The relationship between the products so formed and the mechanism of the reaction to the related electrochemical processes has yet to be studied. 

Because purified xanthine oxidase constitutes a simple model for the study of biological oxidation, many laboratories have tried to elucidate its mechanism of action. A detailed description of the various mechanisms that have been proposed is beyond the scope of this text. However, the use of modern physicochemical techniques will probably be helpful in the study of xanthine oxidase s mechanism of action. Examination of xanthine oxidase frozen in liquid air by the electron spin resonance technique and the use of chemoluminescent substance to detect free radicals have demonstrated that the reduced enzyme is oxidized in the presence of oxygen in a sequence of steps during which free radicals are formed. 

At the present time, the greatest importance of covalent hydration in biology seems to lie in the direction of understanding the action of enzymes. In this connection, the enzyme known as xanthine oxidase has been extensively investigated.This enzyme catalyzes the oxidation of aldehydes to acids, purines to hydroxypurines, and pteridines to hydroxypteridines. The only structural feature which these three substituents have in common is a secondary alcoholic group present in the covalently hydrated forms. Therefore it was logical to conceive of this group as the point of attack by the enzyme. 

As noted earlier, peroxynitrite is formed with a diffusion-controlled rate from superoxide and nitric oxide (Reaction 10). As both these radicals are ubiquitous species, which present practically in all cells and tissues, peroxynitrite can be the most important species responsible for free radical-mediated damage in biological systems. Moreover, it is now known that NO synthases are capable of producing superoxide and nitric oxide simultaneously (see Chapter 22), greatly increasing the possible rate of peroxynitrite production. In addition, another enzyme xanthine dehydrogenase is also able to produce peroxynitrite in the presence of nitrite... 

Another route to the formation of H00./02. is UV irradiation of HOOH in aqueous solutions. Aerobic organisms produce minor fluxes of superoxide ion (02, and thereby HOO.) during respiration and oxidative metabolism for example, possibly up to 10-15% of the O2 reduced by cytochrome c oxidase and by xanthine oxidase passes through the H00./02. state. Thus, the chemistry of HOO. (and of 02 ) may be important to an understanding of oxygen toxicity in biological systems. In aprotic media, the rate of protonation of 02. is proportional to the acidity of the associated Bronsted acids (equation 96). Electrolytic oxidation of... 

Mitochondria are the site of ROS production during ischemia. Arachidonic acid, eNOS, NADPH oxidase and xanthine oxidase are sources of reactive oxygen species at reperfusion. Cytochrome P450 monooxygenases (CYPs) have also been implicated. CYP catalyzes arachidonic acid oxidation to a variety of biologically active eicosanoids and generates reactive oxygen species.1 3... 

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