Degradation of Pyrimidine Nucleotides

The catabolism of pyrimidine nucleotides, like that of purine nucleotides, involves dephosphorylation, deamination, and glycosidic bond cleavage. However, in contrast to purine catabolism, the pyrimidine bases in most organisms are subjected to reduction rather than oxidation. Reductive pyrimidine base catabolism occurs in most microorganisms, plants, and animals. In plants, the pyrimidine bases, uracil, and thymine, derived from uridine monophosphate and deoxythymidine-5'-monophosphate are directly catabolized by a reductive degradation pathway. The catabolism of cytidine-5'-monophosphate must take place after conversion of cytidine to uridine by cytidine deaminase, which is followed by uracil formation. In plants, a dual function of pyrimidine metabolism has been proposed. In addition to the pyrimidine salvage for nucleotides and nucleic acid synthesis, a degradation product of uracil, p-alanine, is used for pantothenic acid (vitamin B5) synthesis. Hence, a portion of pyrimidine ring catabolites is recovered as amino acid-related compounds.