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
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
Related conditions
Related
- Dihydroorotate dehydrogenase (DHODH)
- UMP synthase
- CAD (carbamoyl phosphate synthetase II)
- PRPP (phosphoribosyl pyrophosphate)
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.
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) (PubChem, NCBI, 2026) · Public domain; Pyrimidine biosynthesis (Reactome, 2026); UMPS: Uridine 5'-monophosphate synthase (human) (UniProt, 2026).
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Representative model, for educational purposes only.