The events of human pyrimidine metabolism are conveniently, if somewhat arbitrarily, grouped into four pathways: de novo synthesis of the pyrimidine ring and its conversion to uridine 5'-monophosphate (UMP), the biosynthesis of other pyrimidine ribo- and deoxyribonucleotides, pyrimidine salvage reactions, and pyrimidine catabolism (Loffler et al. 2005).
De novo synthesis of the pyrimidine ring and its conversion to UMP. The pyrimidine base orotate is synthesized from glutamine, bicarbonate, and aspartate. Orotate reacts with 5-phospho-alpha-D-ribose 1-diphosphate (PRPP) and is then decarboxylated to form the pyrimidine nucleotide UMP.
Pyrimidine biosynthesis [interconversion]. Pyrimidine ribo- and deoxyribonucleotide di- and triphosphates are synthesized, both from UMP and from pyrimidine ribonucleotide monophosphates generated in salvage reactions.
Pyrimidine salvage reactions. Pyrimidine nucleosides and free bases generated by DNA and RNA breakdown are converted to nucleotide monophosphates.
Pyrimidine catabolism. The pyrimidine bases thymine and uracil are degraded to beta-aminoisobutyrate and beta-alanine, respectively, which are excreted from the body.