Biochemistry - The Chemical Reactions of Living Cells, Volume 3 - D. Metzler 1980

Metabolism of Nitrogenous Compounds
Pyrimidine and Purine Metabolism
Catabolism of Pyrimidine Nucleotides and Nucleosides

In Cells (as well as in the digestive tract), Nucleic Acids are constantly subjected to attack by various Nucleases. For example, a crucial factor in the Regulation of Protein Synthesis is the degradation—typically quite rapid—of messenger RNAs. Although DNA itself is highly stable, nucleases serve to excise damaged segments from single strands, which is a vital part of the DNA Repair process (Chap. 15, Sec. 3.2). Thus, there is an active Cleavage of polynucleotides into mononucleotides, which are subsequently hydrolyzed by Phosphatases to nucleosides. Nucleosides are converted into free bases by the action of nucleoside phosphorylases [Equation (14-52)].

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Further degradation of cytosine begins with its deamination to uracil via the reaction discussed in the previous section. The Catabolism of uracil involves reduction by NADPH as the first step, according to Equation (14-53). The final product of the reaction is ß-Alanine, which can be converted by oxidative cleavage into malonate semialdehyde and malonyl-CoA (Fig. 9-6): it can also be utilized as a precursor in The Biosynthesis of pantothenic acid and coenzyme A (Fig. 14-10), as well as in the biosynthesis of the Peptides carnosine1) (ß-alanylhistidine) and its N2-methyl derivative, anserine. High concentrations of these peptides are found in Muscles. Their Functions remain unknown, but they may be related to the strong buffering action of Histidine derivatives within the pH range of 6 to 7.

The degradation pathway of thymine is analogous to that shown in Equation (14-53), but it is accompanied by The formation of ß-aminoisobutyrate. The latter can be oxidatively converted into methylmalonate [Equation (14-54)], which can then be utilized in the reactions of The Methylmalonyl Pathway (Fig. 9-6).