BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 10. NUCLEIC ACID METABOLISM

10.4. Catabolism of Purine Bases

The breakdown of purine bases proceeds in such a way that the purine ring remains intact and is converted into uric acid or its reaction products, which are then excreted from the body.

The process begins with the hydrolytic deamination of purine bases by specific deaminases. Adenine is converted into hypoxanthine by adenine deaminase, and guanine is converted into xanthine by guanine deaminase (Fig. 10.5).

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Fig. 10.4. Catabolism of pyrimidine bases:

1 - dihydropyrimidine dehydrogenase; 2 - dihydropyrimidinase (dihydropyrimidine cyclohydrolase); 3 — ureidopropionase

Fig. 10.5. Catabolism of Purine bases

It should be noted that the subsequent oxidation of hypoxanthine to xanthine and further to uric acid can be catalyzed either by xanthine oxidase, which oxidizes Purines using molecular oxygen with The formation of hydrogen peroxide, or by xanthine dehydrogenase, which oxidizes the substrate via the reduction of NAD.

During The oxidation of the respective substrates in Reactions Catalyzed by xanthine oxidase, hydrogen peroxide is formed, whose toxic effect is neutralized by peroxidase Enzymes.

The End products of purine base catabolism vary among different organisms. Thus, in humans, birds, primates, and certain reptiles (snakes, lizards), the end product of purine catabolism is uric acid, which is excreted from the body primarily in the urine. Other mammals, turtles, and some insects possess the enzyme urate oxidase, which degrades uric acid to allantoin, which is also excreted. In some fish (teleosts), allantoin is hydrated by allantoinase to allantoic acid, whereas in chondrostean fish and amphibians, this acid is cleaved into urea and glyoxylate by allantoicase. Finally, in plants, crustaceans, most marine invertebrates, and microorganisms, purine bases are degraded into the simplest compounds—ammonia, carbon dioxide, Glycine, and formic acid.