Pyrimidines are ultimately catabolized (degraded) to CO, HO, and urea. Cytosine can be broken down to uracil, which can be further broken down to N-carbamoyl-β-alanine, and then to beta-alanine, CO, and ammonia by beta-ureidopropionase. Thymine is broken down into β-aminoisobutyrate which can be further broken down into intermediates eventually leading into the citric acid cycle.
β-aminoisobutyrate acts as a rough indicator for rate of DNA turnover.