Uric acid free base, oxidation

Xanthine oxidase, mol wt ca 275,000, present in milk, Hver, and intestinal mucosa (131), is required in the cataboHsm of nucleotides. The free bases guanine and hypoxanthine from the nucleotides are converted to uric acid and xanthine in the intermediate. Xanthine oxidase cataly2es oxidation of hypoxanthine to xanthine and xanthine to uric acid. In these processes and in the oxidations cataly2ed by aldehyde oxidase, molecular oxygen is reduced to H2O2 (133). Xanthine oxidase is also involved in iron metaboHsm. Release of iron from ferritin requires reduction of Fe " to Fe " and reduced xanthine oxidase participates in this conversion (133). 

Degradation of dietary nucleic acids occurs in the small intestine, where a family of pancreatic enzymes hydrolyzes the nucleotides to nucleosides and free bases. Inside cells, purine nucleotides are sequentially degraded by specific enzymes, with uric acid as the end product of this pathway. [Note Mammals other than primates oxidize uric acid further to allantoin, which, in some animals other than mammals, may be further degraded to urea or ammonia.]... 

Aerobic P. d. The amino groups of adenine and guanine are removed hydrolytically by specific deaminases, which attack the free bases, the nucleosides or the nucleotides (Fig.). Uric acid is then produced by the action of xanthine oxidase (EC 1.2.3.2), which is the key enzyme of aerobic P.d. In humans and apes, the uric acid is excreted largely unchanged. In most reptiles and mammals, it is oxidized to allantoin by uricase (EC 1.73.3) (uricolysis). 

Uric Add Formation. In vertebrates purines are oxidized to uric acid. This reaction is catalyzed by xanthine oxidase (or dehydrogenase), which attacks both hypoxanthine and xanthine.Since adenine and guanine nucleotides can give rise to the hydroxylated purines either as the nucleotide, nucleoside, or free base, all of the naturally occurring purines of animals can be converted to uric acid. Adenine may also be oxidized to 2,8-dihydroxy-4-aminopurine, which is excreted in the urine. The formation of uric acid from any of its precursors is followed conveniently spectrophotometrically (Fig. 29). ... 

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