The purines (adenine and guanine) and the pyrimidines (uracil and cytosine) are involved in many intracellular reactions when high-energy di- and triphosphates have been added. These compounds also form the building blocks of DNA and RNA. Purines are heterocyclic double-ring compounds synthesized with phosphoribosylpyrophosphate (PRPP) sugar as a base to which the amide N of glutamine is added, followed by attachment of a glycine molecule, a methylene group from tetrahydrofolate, and an amide N from another glutamine to form the imidazole ring. Then CO2 is added, followed by the amino N of aspartic acid and another carbon to form the final ring to produce inosine monophosphate (IMP)—a purine attached to a ribose phosphate sugar. The other purines, adenine and guanine, are formed from inosine monophosphate by addition of a glutamine amide N or aspartate amino N to make guanosine monophosphate (GMP) or adenosine monophosphate (AMP), respectively. These compounds can be phosphorylated to high-energy di- and triphosphate forms: ADP, ATP, GDP, and GTP.
In contrast to purines, pyrimidines are not synthesized after attachment to a ribose sugar. The amide N of glutamine is condensed with CO 2 to form carbamoyl phosphate, which is further condensed with aspartic acid to make orotic acid—the pyrimidine's heterocyclic 6-member ring. The enzyme forming carbamoyl phosphate is present in many tissues for pyrimidine synthesis but is not the hepatic enzyme that makes urea ( Fig 2.3). However, a block in the urea cycle causing a lack of adequate amounts of arginine to prime the urea synthesis cycle in the liver will result in diversion of unused carbamoyl phosphate to orotic acid and pyrimidine synthesis (41). Uracil is synthesized as uridine monophosphate by forming orotidine monophosphate from orotic acid followed by decarboxylation. Cytosine is formed by adding the amide group of glutamine to uridine triphosphate to form cytidine triphosphate.
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