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[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Orotic acid

Orotic acid is the newly made pyrimidine ring before it joins the sugar; in urine it signals a block in the pathway or the urea cycle.

Updated 2026-11-01

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## Key facts

- Dihydroorotate dehydrogenase makes it in mitochondria.
- UMP synthase joins it to PRPP to form OMP.
- High urinary orotic acid + normal ammonia: hereditary orotic aciduria. High + hyperammonaemia: OTC deficiency.
- It was once called «vitamin B13», but it is not a vitamin: we make it.

**Also called**
Orotate

**Formula**
C₅H₄N₂O₄

**PubChem**
[CID 967](https://pubchem.ncbi.nlm.nih.gov/compound/967)

## Structure

- Uracil with a carboxyl group at C6 (uracil-6-carboxylic acid).

## Clinical notes

- In excess it crystallises in urine and can obstruct the urinary tract.
- In OTC deficiency, carbamoyl phosphate that cannot enter the urea cycle leaks to the cytosol and is diverted to orotate.

## Pathways

- [Pyrimidine synthesis](https://durgical.com/en/biochemistry/pathways/pyrimidine-synthesis/)

## Related conditions

- [Hereditary orotic aciduria](https://durgical.com/en/conditions/hereditary-orotic-aciduria/)

## Related

- [Dihydroorotate dehydrogenase (DHODH)](https://durgical.com/en/biochemistry/enzymes/dihydroorotate-dehydrogenase/)
- [UMP synthase](https://durgical.com/en/biochemistry/enzymes/ump-synthase/)
- [CAD (carbamoyl phosphate synthetase II)](https://durgical.com/en/biochemistry/enzymes/cad/)
- [PRPP (phosphoribosyl pyrophosphate)](https://durgical.com/en/biochemistry/molecules/prpp/)

## Test yourself

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

## Which enzyme turns orotate into a nucleotide?

UMP synthase (OPRT activity), which joins it to PRPP to form OMP.

## How is OTC deficiency told apart from hereditary orotic aciduria?

Both raise orotic acid, but only OTC deficiency has hyperammonaemia.

## Is orotic acid a vitamin?

No: the body makes it; the name «vitamin B13» is historical.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

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

Sources: [Orotic Acid (CID 967)](https://pubchem.ncbi.nlm.nih.gov/compound/967) (PubChem, NCBI, 2026) · Public domain; [Pyrimidine biosynthesis](https://reactome.org/PathwayBrowser/#/R-HSA-500753) (Reactome, 2026); [UMPS: Uridine 5'-monophosphate synthase (human)](https://www.uniprot.org/uniprotkb/P11172/entry) (UniProt, 2026).

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Source: https://durgical.com/en/biochemistry/molecules/orotic-acid/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/molecules/orotic-acid/