Biochemistry - The Chemical Reactions of Living Cells, Volume 3 - D. Metzler 1980

Metabolism of Nitrogenous Compounds
Metabolism of Pyrimidines and Purines
Oxidative Purine Metabolism

As shown in Fig. 14-33, free adenine produced during nucleic acid Catabolism can be hydrolytically deaminated to hypoxanthine. Similarly, guanine can be deaminated to xanthine. Xanthine oxidase, a molybdenum-containing enzyme (Chapter 8, Section B.6), oxidizes hypoxanthine to xanthine and subsequently to uric acid. Another reaction of xanthine in certain plants is its conversion into caffeine, which is a trimethyl derivative.

While uric acid is the end product of purine METABOLISM in humans, many other species possess the copper-containing enzyme urate oxidase, which converts uric acid to allantoin—the primary excretory product of most mammals, with the exception of primates. In many fish, allantoin is hydrolyzed to allantoic acid, and some species excrete this compound as their end product. However, in most fish, the Hydrolysis process goes further, yielding urea and glyoxylate as products. In certain invertebrates, urea may be further hydrolyzed to ammonia.

In organisms that hydrolyze uric acid to urea or ammonia, this pathway is used exclusively for The breakdown of Purines derived from nucleotide turnover. Excess nitrogen resulting from Amino Acid Catabolism is excreted either directly as ammonia or by being converted into urea (Section B.2).

¹⁾ This same ribonucleotide is believed to serve as a precursor for the pyrimidine ring of thiamine, although the exact biosynthetic pathway remains unclear [155a].

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FIG. 14-33. Degradation of purines yielding uric acid and other excretory products.