Xanthine oxidase autoxidation

Human ceruloplasmin inhibits lipid autoxidation induced by ascorbate or inorganic Fe It is considered an acute-phase protein with a beneficial effect in inflammation . It was suggested that ceruloplasmin acts as a scavenger of OJ radicals, as it inhibited the reduction of Fe(III)-cytochrome c and of nitroblue tetrazolium in the presence of xanthine oxidase, acetaldehyde, and dioxygen as an OJ-generating system A mechanism without reduction of Cu , similar to that... 

Ascorbic acid reacts with 02" generated from the xanthine oxidase system and may play a role against 02" mediated toxicity. Ascorbic acid quenches the hydroxyl radical (40), Ascorbic acid may protect against free radicals in the lung because ascorbic acid is found in the fluid (39). The toxicity of ozone and oxygen may also be reduced by ascorbic acid (39). Carbon tetrachloride mortality in rats is lowered by ascorbic acid. Autoxidation of ascorbic acid did not generate 02". Reduced glutathione reacts with dehydroascorbic acid (VII) and recycles ascorbic acid. 

Activation of drugs to give toxic products is common. Apart from non-enzymatic activation (e.g., via autoxidation), activation by enzymatic one-electron oxidation or reduction frequently occurs. Several non-specific oxidases and reductases are encountered in mammalian tissues. Enzyme systems that have been studied in detail are peroxidases and microsomal oxidases and reductases. Xanthine oxidase also has received some attention. In many insta .ces the end products of the reaction are critically dependent upon the presence of oxygen in the system. This is because oxygen is an excellent electron acceptor, i.e., it can oxidize donor radicals, forming superoxide in the process. In this way a redox cycle is set up in which the xenobiotic substrate is recovered. The toxic effects of the xenobiotic often can be attributed to the oxidative stress arising from such a cycle. However, it seems that for some substrates, oxidative stress of this kind can be less damaging than anaerobic reduction. Anaerobic reduction can lead to formation of further reduced products with additional toxicity. 

The autoxidation of hydroxylamine to nitrite also involves a radical chain process (Kono, 1978), and the reaction is carried out at high pH. The assay was originally utilized by Elstner and Heupel (1976) who initiated the autoxidation by O2 generated by the xanthine/xanthine oxidase reaction. Nitrite formation was determined colorimetrically at 530 nm by diazo coupling with a-naphthylamine and superoxide dismutase was found to inhibit end product formation. Kono (1978) developed the assay further by utilizing nitroblue tetrazolium as the... 

Xanthine oxidase/xanthine Electrolytic reduction of Oj Photoreduction of riboflavin Autoxidation of pyrogallol Autoxidation of catecholamines >5°) NADH oxidation by phenazine methosul-fate... 

There are overwhelming reports on superoxide anion generation in defined biological systems including redox reactions of small molecules, autoxidable proteins and oxidative enzymes The most prominent enzyme is xanthine oxidase. However, some precautions are neccessary, because the single electron reduction of dioxygen is considered to be an unnatural reactivity of this enzyme, which, under in vivo conditions acts like a dehydrogenase... 

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