Question

Compare how components of pyrimidine and purine bases are excreted from the mammalian organism. What has to happen to each for excretion?

Answer 1

Those pyrimidine rings can be degraded completely to CO2 and urea excretion, Then about purine excretion,purine rings are degraded to the metabolically inert uric acid.Degradation in humans, however, is only complete for pyrimidines (C, T, U), but not purines (G, A), which are excreted from the body in form of uric acid . De novosynthesis of pyrimidines and purines follows two different pathways. Pyrimidines are synthesized first from aspartate and carbamoyl-phosphate in the cytoplasm to the common precursor ring structure orotic acid ,onto which a phosphorylated ribosyl unit is covalently linked. Purines, however, are first synthesized from the sugar template onto which the ring synthesis occurs.

Guanine which will deaminate to xanthine ,in turn is oxidized to uric acid. This last reaction is irreversible. Similarly, AMP is deaminated to IMP from which the ribose unit is removed to form hypoxanthine. Hypoxanthine is oxidized to xanthine and finally to uric acid. Instead of uric acid secretion, guanine and IMP can be used for recycling purposes and nucleic acid synthesis in the presence of PRPP and aspartate.