Pyrimidines are degraded to b-alanine, NH₃ e CO₂

Animals cells degrade pyrimidine nucleotides to their component bases. These ractions occur through dephosphorylation, deamination and glycosidic bond cleavages. The resulting uracil and thymine are then broken down in the liver through reduction rather than by oxidation as occurs in purine catabolism.

5'-nucleotidase can dephosphorylate dUMP to deoxyuridine. Uridine or deoxyuridine than formed can be degraded by uridine phosphorilase to uracil and ribose-1-P or deoxyribose-1-P. Thymine nucleotides also are dephosphorylated by 5'-nucleotidase and then cleaved by thymidine phosphorylase.

Some mammals liver enzymes can act on both thymine and uracil.