Uric acid

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16 atoms, 17 bonds.

IUPAC name: 7,9-dihydro-3H-purine-2,6,8-trione
Molar mass: 168.112(18) g/mol
Atoms: 16
SMILES: C12=C(NC(=O)N1)NC(=O)NC2=O
InChIKey: LEHOTFFKMJEONL-UHFFFAOYSA-N
PubChem CID: 1175

Made of

Structural formula of uric acid. Each unlabelled corner is a carbon atom. It shows which atoms are joined to which, not the shape. The model above shows the shape.

When cells die, the purines inside them are broken down, and in a human body that breakdown ends at uric acid. MedlinePlus calls it a normal waste product. Purines also arrive in certain foods and drinks. The NCI Thesaurus, the US National Cancer Institute’s reference vocabulary, describes uric acid as white, tasteless, odourless crystals. Blood and urine carry it, and the organs hold traces.

A mole of C₅H₄N₄O₃ weighs 168.11 g, built from carbon, hydrogen, nitrogen and oxygen. The IUPAC (International Union of Pure and Applied Chemistry) name is 7,9-dihydro-3H-purine-2,6,8-trione; its 2, 6 and 8 number the ring positions of the three oxygens. Dissolved in blood it is often called urate, and a high level of urate there is hyperuricaemia.

Structure and bonding

Five carbons, four nitrogens, three oxygens and four hydrogens make up the model: sixteen atoms held by seventeen bonds. The carbons and nitrogens form two rings, one of six atoms and one of five, fused along a shared C=C double bond. The ChEBI database (Chemical Entities of Biological Interest) classes uric acid as an oxopurine: a purine ring with oxo groups, C=O, at positions 2, 6 and 8. Every nitrogen carries one hydrogen, and no hydrogen touches a carbon.

In the Ball-and-stick view the three red oxygens point outward from the rim. Each carbon has three neighbours and one double bond, to an oxygen or to its partner at the ring junction. In Chem1’s account such a carbon holds its three partners in a flat trigonal arrangement. Seen edge-on, PubChem’s computed model is a flat sheet, and the Space-filling view makes it a thin plate.

That sheet is one tautomer, the trione; tautomers share their atoms but place hydrogens differently. ChEBI lists uric acid as a tautomer of a 2,6-dihydroxy form, three purine-2,6,8-triols and a second trione. The dihydroxy form and the triols carry O–H groups where the model has C=O and N–H.

Where it turns up

For birds, the US Geological Survey (USGS) calls uric acid the main nitrogen end product, passed in the faeces; in humans it is minor beside urea. Bird guano is probably the primary source of the uric acid on tree islands in the Everglades, in the USGS’s judgement. At Joshua Tree National Park, reptiles need no water for excretion, the park service notes; their nitrogen waste leaves as solid uric acid.

In poultry houses, microbes mineralise urea and uric acid in the litter, breaking them down to ammonia. Holding that ammonia in the litter keeps its value as fertiliser, according to the US Agricultural Research Service. The LOTUS natural-products database reports uric acid in the pomegranate, Punica granatum.

Why it matters

Most urate the kidneys filter returns to the blood, helped by the protein GLUT9; the rest leaves in urine. In the blood, urate works as an antioxidant that shields cells from free radicals, unstable molecules that cause damage. A protein made from the ABCG2 gene releases urate into the gut. Gout-associated gene changes tip that balance: in SLC2A9, the gene for GLUT9, they raise reabsorption and cut release into urine; in ABCG2 they weaken release into the gut.

The NIDDK, the US institute for diabetes, digestive and kidney diseases, counts uric acid stones among four main types, with calcium stones the most common. A uric acid stone can form when urine is too acidic. In about 15 per cent of people with gout, urate collects in the kidneys as stones.

In Lesch–Nyhan syndrome, cells lack HGprt, the enzyme that recycles purines. The purines are broken down instead, and uric acid builds up until gouty arthritis follows. The first sign is usually an orange colour in a baby’s nappy from uric acid deposits in the urine.

Properties

Properties of uric acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 1175.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₅H₄N₄O₃
Molar mass	168.112(18) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	5
Hydrogen atoms	4
Nitrogen atoms	4
Oxygen atoms	3
Bonds	17
Single bonds	13
Double bonds	4
PubChem molecular weight	168.11 g/mol	g/mol	PubChem

Safety

As a laboratory powder, uric acid is close to harmless: in 30 of 31 company reports it met no hazard criteria of the Globally Harmonized System (GHS). The PubChem Laboratory Chemical Safety Summary for uric acid is the authoritative sheet.

Questions

What does a uric acid molecule look like?

A flat double ring. In PubChem's computed 3D model of C₅H₄N₄O₃, all sixteen atoms lie within 0.002 Å of one plane: a ring of six atoms and a ring of five, fused along one shared bond. The three C=O bonds measure 1.22 to 1.23 Å, shorter than the ring's carbon–nitrogen bonds, which run from 1.36 to 1.43 Å.

What causes gout?

Gout develops when hyperuricaemia, a high level of urate in the blood, leaves urate crystals in the joints. The immune system reacts with inflammation, and the first flare usually strikes the big toe, foot or ankle. Genes and environment both contribute: purines broken down from food add urate, and after menopause falling oestrogen, which helps clear urate, lets levels rise in older women. Only about a quarter of people with hyperuricaemia develop gout.

How does the body make uric acid?

It starts from purines, which come from the body's own cells as they die and from certain foods and drinks. The enzyme xanthine dehydrogenase performs the last two steps, turning hypoxanthine into xanthine and then xanthine into uric acid. That enzyme is sometimes converted into a second form, xanthine oxidase. In hereditary xanthinuria type I the xanthine is not converted, so blood and urine carry plenty of xanthine and very little uric acid.

Why can't humans break down uric acid like other mammals?

Humans have no urate oxidase, the National Cancer Institute notes, while most other mammals use that enzyme to oxidise uric acid further, to allantoin. For humans and other primates, MeSH (a US medical vocabulary) lists uric acid as the final oxidation product of purine breakdown. Allantoin dissolves in water 5 to 10 times more readily. The drug rasburicase is a urate oxidase made by a genetically modified strain of the yeast Saccharomyces cerevisiae.

Made of the same elements

C₅H₅N₅O Guanine 16 atoms, 17 bonds.
C₄H₄N₂O₂ Uracil 12 atoms, 12 bonds.
C₄H₅N₃O Cytosine 13 atoms, 13 bonds.
C₅H₆N₂O₂ Thymine 15 atoms, 15 bonds.
C₂H₅NO₂ Glycine 10 atoms, 9 bonds.
CO(NH₂)₂ Urea 8 atoms, 7 bonds.
C₃H₇NO₂ Alanine 13 atoms, 12 bonds.
C₅H₉NO₄ Glutamic acid 19 atoms, 18 bonds.

All 25 compounds made of carbon, hydrogen, nitrogen and oxygen

C₈H₁₀N₄O₂ Caffeine 24 atoms, 25 bonds.
C₈H₉NO₂ Acetaminophen 20 atoms, 20 bonds.
C₈H₁₁NO₂ Dopamine 22 atoms, 22 bonds.
C₆H₁₃NO₂ Isoleucine 22 atoms, 21 bonds.
C₆H₁₃NO₂ Leucine 22 atoms, 21 bonds.
C₉H₁₁NO₃ Tyrosine 24 atoms, 24 bonds.
C₉H₁₁NO₂ Phenylalanine 23 atoms, 23 bonds.
C₉H₁₃NO₃ Epinephrine 26 atoms, 26 bonds.
C₁₀H₁₂N₂O Serotonin 25 atoms, 26 bonds.
C₁₁H₁₂N₂O₂ Tryptophan 27 atoms, 28 bonds.
C₁₃H₁₆N₂O₂ Melatonin 33 atoms, 34 bonds.
C₁₂H₁₇NO DEET 31 atoms, 31 bonds.
C₁₄H₁₈N₂O₅ Aspartame 39 atoms, 39 bonds.
C₁₃H₂₁NO₃ Albuterol 38 atoms, 38 bonds.
C₁₄H₂₂N₂O Lidocaine 39 atoms, 39 bonds.
C₁₇H₂₁NO Diphenhydramine 40 atoms, 41 bonds.
C₂₀H₂₄N₂O₂ Quinine 48 atoms, 51 bonds.