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# Pyrimidine degradation and β-alanine

Cytosine, uracil and thymine are broken down to soluble products such as β-alanine; one enzyme of this route decides whether 5-fluorouracil is safe.

Updated Nov 12, 2026

## Key facts

- Cytosine is deaminated to uracil; dihydropyrimidine dehydrogenase (DPD) then reduces uracil and thymine, the first step and the one that sets the pace.
- The ring is opened and split: uracil ends as β-alanine, CO₂ and ammonia; thymine ends as β-aminoisobutyrate.
- These end products are water-soluble, so unlike uric acid from purines they do not crystallise or cause gout.
- β-Alanine is used to make carnosine (a buffer dipeptide in muscle) and is part of pantothenic acid and so of coenzyme A.
- β-Aminoisobutyrate is excreted in urine or converted to succinyl-CoA.

## In detail

- Urinary β-aminoisobutyrate rises when DNA turnover is high (leukaemia, radiotherapy, chemotherapy); some healthy people are genetic high excretors.
- β-Alanine can also be broken down to acetyl-CoA by way of malonate semialdehyde.
- Defects of the later enzymes (dihydropyrimidinase, β-ureidopropionase) are rare and can cause neurological problems.

**Also called**
Pyrimidine catabolism, β-Aminoisobutyric acid

## Clinical notes

- Dihydropyrimidine dehydrogenase deficiency causes severe, sometimes fatal toxicity from 5-fluorouracil and capecitabine, which this enzyme normally clears; DPYD testing before treatment is recommended in many countries.
- Pyrimidine breakdown products need no special treatment in tumour lysis; the uric acid from purines is what causes kidney injury.

## Test yourself

Think of the answer, then tap the question to see it.

## Which enzyme is rate-limiting in pyrimidine degradation, and why does it matter in oncology?

Dihydropyrimidine dehydrogenase (DPD); its deficiency makes 5-fluorouracil and capecitabine highly toxic.

## Why does pyrimidine breakdown not cause gout?

Its end products (β-alanine, β-aminoisobutyrate) are water-soluble, unlike uric acid.

## Which two molecules contain β-alanine?

Carnosine and pantothenic acid (and so coenzyme A).

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## Connections

## Related 4

- [Pyrimidine synthesis](https://durgical.com/en/biochemistry/pathways/pyrimidine-synthesis/)
- [Purine degradation and salvage](https://durgical.com/en/biochemistry/pathways/purine-degradation-and-salvage/)
- [Pantothenic acid (vitamin B5)](https://durgical.com/en/biochemistry/molecules/pantothenic-acid/)
- [Odd-chain fatty acids and propionate metabolism](https://durgical.com/en/biochemistry/topics/odd-chain-fatty-acids-and-propionate-metabolism/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Pyrimidine catabolism](https://reactome.org/PathwayBrowser/#/R-HSA-73621) (Reactome, 2026); [Dihydropyrimidine dehydrogenase deficiency: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/dihydropyrimidine-dehydrogenase-deficiency/) (MedlinePlus, US National Library of Medicine, 2026); [Beta-ureidopropionase deficiency: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/beta-ureidopropionase-deficiency/) (MedlinePlus, US National Library of Medicine, 2026).

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