Figure 1. Metabolic end products of purine and pyrimidine degradation in humans. Routes of pyrimidine (green) and purine (red) nucleotide degradation showing the structural formula and numbering of the atoms in the respective ring structures of UMP (uridine 5¢-monophosphate) and IMP (inosine 5¢-monophosphate), central intermediates in nucleotide degradation. CMP and UMP are degraded to uridine and largely recycled, since most human cells, except liver, lack URP. A different battery of enzymes degrades TMP (thymidine 5¢-monophosphate) to thymine. The purine nucleotides IMP and GMP are degraded via the corresponding nucleosides to the constituent bases hypoxanthine and guanine, respectively, and recycled at this level. Further catabolism of nonrecycled degradation products in the liver leads to the formation of -alanine and -aminoisobutyric acid for pyrimidines, which enter the CAC, and the metabolic end product uric acid for purines (blue).

Figure 2. Part of a DNA chain. (a) Structural formulae of the four constituent bases, adenine and guanine (red), cytosine and thymine (green), showing that the deoxyribose has a hydrogen atom at the 2¢ position on the pentose ring, instead of the hydroxyl group of ribose. These bases are linked via the 3¢-OH group of the deoxyribose-phosphate moiety to the 5¢-OH group of the next deoxyribose. (b) Schematic representation of the manner in which the above bases are linked on strands making infinite variation possible, depending on the order of these bases. (c) Schematic representation of the role of hydrogen bonding between bases on opposite strands in contributing to the stability of this double-helical structure. The adenine–thymine base pair forms two hydrogen bonds, whereas the guanine–cytosine base pair forms three hydrogen bonds. Colour code for (b) and (c) as for (a).

Figure 3. Structural formula of ATP. Structural formula of adenosine 5¢-triphosphate (ATP), indicating the numbering of the atoms in the ribose, as well as the purine ring.

Figure 4. Fate of oral purine and pyrimidine nucleotides in humans. (a) Up to 50% of dietary purines are degraded to  CO₂ in the gut by bacteria. The remaining purines are degraded by a battery of enzymes present in the intestinal mucosa which includes XDH, thereby ensuring their conversion into uric acid prior to absorption. (b) Dietary pyrimidines are partially degraded to  CO₂ in the gut by bacteria. Because uridine phosphorylase (URP) is not present in the intestinal mucosa in humans, dietary pyrimidines escaping bacterial degradation are absorbed in the form of uridine and thymidine. The pyrimidine nucleosides are then taken up by tissues expressing the relevant kinases. In liver, uridine and thymidine are degraded to uracil and thymine by URP and thymidine phosphorylase (TP), respectively, and are catabolized further in three steps to -alanine and -aminoisobutyric acid, respectively, as shown in Figure 1. DPD, catalysing the first step is ubiquitously expressed in many tissues, but DHP and UP are only expressed in liver and kidney.

Figure 5. Enzymes defective in inborn errors of nucleotide degradation. Metabolic pathways of purine (red) and pyrimidine (green) nucleotide degradation, via the nucleoside and base to the respective metabolic end products (blue), indicating the enzymes (pink) deficient in genetic disorders affecting these pathways. The ‘extra-purine’ origin of adenine (end product of the polyamine pathway) and its degradation to 2,8-dihydroxyadenine by XDH, when the salvage enzyme adenine phosphoribosyltransferase is defective, is indicated in the inset on the left. Abbreviations: ADA, adenosine deaminase; AMPDA, myoadenylate deaminase; BAPAT, -alanine–pyruvate aminotransferase; BAKAT, -alanine-ketoglutarate aminotransferase; BAIBPAT, -amino isobutyrate-pyruvate aminotransferase; CAC, citric acid cycle; DHP, dihydropyrimidinase; DPD, dihydropyrimidine dehydrogenase; PNP, purine nucleoside phosphorylase; TP, thymidine phosphorylase UMPH-1, UMP-hydrolase (or Py5¢-N, pyrimidine 5¢-nucleotidase); UP, ureidopropionase; URP, uridine phosphorylase; XDH, xanthine dehydrogenase.