Uric acid is a heterocyclic compound of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

, and

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

with the formula C₅H₄N₄O₃. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the metabolic breakdown of

purine Purine is a heterocyclic aromatic organic compound that consists of two rings ( pyrimidine and imidazole) fused together. It is water-soluble. Purine also gives its name to the wider class of molecules, purines, which include substituted purines ...

nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecu ...

s, and it is a normal component of

urine Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra. Cellul ...

. High blood concentrations of uric acid can lead to

gout Gout ( ) is a form of inflammatory arthritis characterized by recurrent attacks of a red, tender, hot and swollen joint, caused by deposition of monosodium urate monohydrate crystals. Pain typically comes on rapidly, reaching maximal intens ...

and are associated with other medical conditions, including

diabetes Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ...

and the formation of ammonium acid urate kidney stones.

Chemistry

Uric acid was first isolated from kidney stones in 1776 by Swedish chemist

Carl Wilhelm Scheele Carl Wilhelm Scheele (, ; 9 December 1742 – 21 May 1786) was a Swedish German pharmaceutical chemist. Scheele discovered oxygen (although Joseph Priestley published his findings first), and identified molybdenum, tungsten, barium, hydr ...

. In 1882, the Ukrainian chemist Ivan Horbaczewski first synthesized uric acid by melting urea with glycine. Uric acid displays lactam–lactim tautomerism (also often described as keto–enol tautomerism). Although the lactim form is expected to possess some degree of

aromaticity In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

, uric acid crystallizes in the lactam form, with computational chemistry also indicating that tautomer to be the most stable. Uric acid is a diprotic acid with p''K''_a1 = 5.4 and p''K''_a2 = 10.3, thus at physiological pH, it predominately exists as the monoionic urate ion. :

Water solubility

In general, the water

solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

of uric acid and its

alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

and

alkaline earth The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are all ...

salts is rather low. All these salts exhibit greater solubility in hot water than cold, allowing for easy recrystallization. This low solubility is significant for the etiology of gout. The solubility of the acid and its salts in

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

is very low or negligible. In ethanol/water mixtures, the solubilities are somewhere between the end values for pure ethanol and pure water. : The figures given indicate what mass of water is required to dissolve a unit mass of compound indicated. The lower the number, the more soluble the substance in the said solvent.

Biochemistry

Xanthine oxidase (found in mammals primarily as xanthine dehydrogenase and rarely as an oxidase) is an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

which catalyzes the formation of uric acid from

xanthine Xanthine ( or ; archaically xanthic acid; systematic name 3,7-dihydropurine-2,6-dione) is a purine base found in most human body tissues and fluids, as well as in other organisms. Several stimulants are derived from xanthine, including caffein ...

and

hypoxanthine Hypoxanthine is a naturally occurring purine derivative. It is occasionally found as a constituent of nucleic acids, where it is present in the anticodon of tRNA in the form of its nucleoside inosine. It has a tautomer known as 6-hydroxypurine. Hyp ...

, which in turn are produced from other

s. Xanthine oxidase is a large enzyme whose active site consists of the metal molybdenum bound to

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

and oxygen. Within cells, xanthine oxidase can exist as xanthine dehydrogenase and xanthine oxireductase, which has also been purified from bovine milk and

spleen The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .

extracts. Uric acid is released in hypoxic conditions (low oxygen saturation).

Genetic and physiological diversity

Primates

humans" \n\n\n\n\nThe robots exclusion standard, also known as the robots exclusion protocol or simply robots.txt, is a standard used by websites to indicate to visiting web crawlers and other web robots which portions of the site they are allowed to visi ...

and other great apes, uric acid (actually hydrogen urate ion) is the final

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...

(breakdown) product of

purine metabolism Purine metabolism refers to the metabolic pathways to synthesize and break down purines that are present in many organisms. Biosynthesis Purines are biologically synthesized as nucleotides and in particular as ribotides, i.e. bases attached to ...

and is excreted in urine, whereas in most other mammals, the enzyme uricase further oxidizes uric acid to allantoin. The loss of uricase in higher primates parallels the similar loss of the ability to synthesize ascorbic acid, leading to the suggestion that urate may partially substitute for ascorbate in such species. Both uric acid and ascorbic acid are strong

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth met ...

s ( electron donors) and potent antioxidants. In humans, over half the antioxidant capacity of

blood plasma Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but contains proteins and other constituents of whole blood in suspension. It makes up about 55% of the body's total blood volume. It is the int ...

comes from hydrogen urate ion.

Humans

The normal concentration range of uric acid (or hydrogen urate ion) in human blood is 25 to 80 mg/L for men and 15 to 60 mg/L for women (but see below for slightly different values). An individual can have serum values as high as 96 mg/L and not have gout. In humans, about 70% of daily uric acid disposal occurs via the kidneys, and in 5–25% of humans, impaired renal (kidney) excretion leads to hyperuricemia. Normal excretion of uric acid in the urine is 270 to 360 mg per day (concentration of 270 to 360 mg/L if one litre of urine is produced per day – higher than the solubility of uric acid because it is in the form of dissolved acid urates), roughly 1% as much as the daily excretion of urea.

Dogs

The Dalmatian has a genetic defect in uric acid uptake by the

liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...

and

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...

s, resulting in decreased conversion to allantoin, so this breed excretes uric acid, and not allantoin, in the urine.

Birds and reptiles

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s and reptiles, and in some desert-dwelling mammals (such as the

kangaroo rat Kangaroo rats, small mostly nocturnal rodents of genus ''Dipodomys'', are native to arid areas of western North America. The common name derives from their bipedal form. They hop in a manner similar to the much larger kangaroo, but developed ...

), uric acid also is the end product of purine metabolism, but it is excreted in feces as a dry mass. This involves a complex

metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical ...

that is energetically costly in comparison to processing of other nitrogenous wastes such as urea (from the

urea cycle The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions that produces urea (NH2)2CO from ammonia (NH3). Animals that use this cycle, mainly amphibians and mammals, are called ureotelic. The urea cycle converts h ...

) or

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...

, but has the advantages of reducing water loss and preventing dehydration.

Invertebrates

'' Platynereis dumerilii'', a marine polychaete worm, uses uric acid as a sexual pheromone. The female of the species releases uric acid into the water during mating, to induce males to release sperm.

Genetics

Although foods such as meat and seafood can elevate serum urate levels, genetic variation is a much greater contributor to high serum urate. A proportion of people have mutations in the urate transport proteins responsible for the excretion of uric acid by the kidneys. Variants of a number of genes, linked to serum urate, have so far been identified: ''

SLC2A9 Solute carrier family 2, facilitated glucose transporter member 9 is a protein that in humans is encoded by the ''SLC2A9'' gene. This gene encodes a member of the SLC2A facilitative glucose transporter family. Members of this family play a signif ...

''; ''

ABCG2 ATP-binding cassette super-family G member 2 is a protein that in humans is encoded by the ''ABCG2'' gene. ABCG2 has also been designated as CDw338 (cluster of differentiation w338). ABCG2 is a translocation protein used to actively pump drugs ...

''; '' SLC17A1''; '' SLC22A11''; ''

SLC22A12 Solute carrier family 22 (organic anion/cation transporter), member 12, also known as SLC22A12 and URAT1, is a protein which in humans is encoded by the ''SLC22A12'' gene. Function The protein encoded by this gene is a urate transporter and ur ...

''; '' SLC16A9''; '' GCKR''; '' LRRC16A''; and '' PDZK1''. GLUT9, encoded by the ''SLC2A9'' gene, is known to transport both uric acid and fructose. Myogenic hyperuricemia, as a result of the Purine Nucleotide Cycle running when ATP reservoirs in muscle cells are low, is a common pathophysiologic feature of glycogenoses such as GSD-III, GSD-V and GSD-VII, as they are metabolic myopathies which impair the ability of ATP (energy) production for the muscle cells to use.Mineo I, Kono N, Hara N, Shimizu T, Yamada Y, Kawachi M, Kiyokawa H, Wang YL, Tarui S. Myogenic hyperuricemia. A common pathophysiologic feature of glycogenosis types III, V, and VII. N Engl J Med. 1987 Jul 9;317(2):75-80. doi: 10.1056/NEJM198707093170203. PMID: 3473284. In these metabolic myopathies, myogenic hyperuricemia is exercise-induced; inosine, hypoxanthine and uric acid increase in plasma after exercise and decrease over hours with rest. Excess IMP (inosine monophosphate) is converted into uric acid. IMP → Inosine → Hypoxanthine → Xanthine → Uric Acid

Clinical significance and research

In human

, the reference range of uric acid is typically 3.4–7.2 mg per 100 mL(200–430 μmol/L) for men, and 2.4–6.1 mg per 100 mL for women (140–360 μmol/L). Uric acid concentrations in blood plasma above and below the normal range are known as, respectively, hyperuricemia and hypouricemia. Likewise, uric acid concentrations in urine above and below normal are known as hyperuricosuria and hypouricosuria. Uric acid levels in saliva may be associated with blood uric acid levels.

High uric acid

Hyperuricemia (high levels of uric acid), which induces

, has various potential origins: * Diet may be a factor. High intake of dietary

, high-fructose corn syrup, and sucrose can increase levels of uric acid. * Serum uric acid can be elevated by reduced excretion via the

s. * Fasting or rapid weight loss can temporarily elevate uric acid levels. * Certain drugs, such as thiazide diuretics, can increase blood uric acid levels by interfering with

renal clearance In pharmacology, clearance is a pharmacokinetic measurement of the volume of plasma from which a substance is completely removed per unit time. Usually, clearance is measured in L/h or mL/min. The quantity reflects the rate of drug elimination div ...

. *

Tumor lysis syndrome Tumor lysis syndrome (TLS) is a group of metabolic abnormalities that can occur as a complication from the treatment of cancer, where large amounts of tumor cells are killed off ( lysed) from the treatment, releasing their contents into the blo ...

, a metabolic complication of certain cancers or chemotherapy, due to

nucleobase Nucleobases, also known as ''nitrogenous bases'' or often simply ''bases'', are nitrogen-containing biological compounds that form nucleosides, which, in turn, are components of nucleotides, with all of these monomers constituting the basi ...

and potassium release into the plasma. * Low ATP (energy) reservoirs activating the Purine Nucleotide Cycle to produce ATP from adenine nucleotides (AMP, IMP, and S-AMP).Johnson TA, Jinnah HA, Kamatani N. Shortage of Cellular ATP as a Cause of Diseases and Strategies to Enhance ATP. Front Pharmacol. 2019 Feb 19;10:98. doi: 10.3389/fphar.2019.00098. PMID: 30837873; PMCID: PMC6390775. Low ATP reservoirs can be due to strenuous exercise, fasting or starvation, or impairment of ATP production such as metabolic myopathies,

, scurvy (Vitamin C deficiency),Travica N, Ried K, Sali A, Scholey A, Hudson I, Pipingas A. Vitamin C Status and Cognitive Function: A Systematic Review. Nutrients. 2017 Aug 30;9(9):960. doi: 10.3390/nu9090960. PMID: 28867798; PMCID: PMC5622720.Pietrzak I, Opala G. Rola karnityny w przemianie lipidowej u człowieka he role of carnitine in human lipid metabolism Wiad Lek. 1998;51(1-2):71-5. Polish. PMID: 9608835. and excessive alcohol consumption.Zakhari S. Overview: how is alcohol metabolized by the body? Alcohol Res Health. 2006;29(4):245-54. PMID: 17718403; PMCID: PMC6527027. Excess IMP (inosine monophosphate) is converted into uric acid. IMP → Inosine → Hypoxanthine → Xanthine → Uric Acid

Gout

A 2011 survey in the United States indicated that 3.9% of the population had gout, whereas 21.4% had hyperuricemia without having symptoms. Excess blood uric acid can induce

, a painful condition resulting from needle-like crystals of uric acid precipitating in joints, capillaries,

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...

, and other tissues. Gout can occur where serum uric acid levels are as low as 6 mg per 100 mL (357 μmol/L), but an individual can have serum values as high as 9.6 mg per 100 mL (565 μmol/L) and not have gout. In humans, purines are metabolized into uric acid, which is then excreted in the urine. Consumption of large amounts of some types of purine-rich foods, particularly meat and seafood, increases gout risk. Purine-rich foods include liver, kidney, and sweetbreads, and certain types of seafood, including anchovies, herring, sardines, mussels, scallops, trout, haddock, mackerel, and tuna. Moderate intake of purine-rich vegetables, however, is not associated with an increased risk of gout. One treatment for gout in the 19th century was administration of

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense soli ...

salts; lithium urate is more soluble. Today, inflammation during attacks is more commonly treated with NSAIDs, colchicine, or corticosteroids, and urate levels are managed with allopurinol. Allopurinol, which weakly inhibits xanthine oxidase, is an analog of hypoxanthine that is hydroxylated by xanthine oxidoreductase at the 2-position to give oxipurinol.

Tumor lysis syndrome

, an emergency condition that may result from blood cancers, produces high uric acid levels in blood when tumor cells release their contents into the blood, either spontaneously or following

chemotherapy Chemotherapy (often abbreviated to chemo and sometimes CTX or CTx) is a type of cancer treatment that uses one or more anti-cancer drugs (chemotherapeutic agents or alkylating agents) as part of a standardized chemotherapy regimen. Chemother ...

. Tumor lysis syndrome may lead to acute kidney injury when uric acid crystals are deposited in the kidneys. Treatment includes

hyperhydration Water intoxication, also known as water poisoning, hyperhydration, overhydration, or water toxemia, is a potentially fatal disturbance in brain functions that results when the normal balance of electrolytes in the body is pushed outside safe l ...

to dilute and excrete uric acid via

, rasburicase to reduce levels of poorly soluble uric acid in blood, or allopurinol to inhibit

catabolism from adding to uric acid levels.

Lesch–Nyhan syndrome

Lesch–Nyhan syndrome, a rare inherited disorder, is also associated with high serum uric acid levels. Spasticity, involuntary movement, and cognitive retardation as well as manifestations of gout are seen in this syndrome.

Cardiovascular disease

Hyperuricemia is associated with an increase in risk factors for cardiovascular disease. It is also possible that high levels of uric acid may have a causal role in the development of atherosclerotic cardiovascular disease, but this is controversial and the data are conflicting.

Type 2 diabetes

Hyperuricemia may be a consequence of insulin resistance in

rather than its precursor. One study showed high serum uric acid was associated with higher risk of type 2 diabetes, independent of

obesity Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's ...

, dyslipidemia, and hypertension. Hyperuricemia is associated with components of metabolic syndrome, including in children.

Uric acid stone formation

Kidney stones can form through deposits of sodium urate microcrystals. Saturation levels of uric acid in blood may result in one form of kidney stones when the urate crystallizes in the kidney. These uric acid stones are

radiolucent Radiodensity (or radiopacity) is opacity to the radio wave and X-ray portion of the electromagnetic spectrum: that is, the relative inability of those kinds of electromagnetic radiation to pass through a particular material. Radiolucency or hypod ...

, so do not appear on an abdominal plain

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

. Uric acid crystals can also promote the formation of calcium oxalate stones, acting as "seed crystals".

Metabolic Myopathies

Metabolic myopathies, such as glycogenoses, impair the ability of the muscle cell to create ATP (energy). When ATP reservoirs are low, ATP is produced via the Purine Nucleotide Cycle. In these metabolic myopathies, myogenic hyperuricemia is exercise-induced; inosine, hypoxanthine and uric acid increase in plasma after exercise and decrease over hours with rest. Excess IMP (inosine monophosphate) is converted into uric acid. IMP → Inosine → Hypoxanthine → Xanthine → Uric Acid

Excessive Alcohol Consumption

Hyperuricemia can be caused by excessive alcohol consumption by converting

→ acetaldehyde → acetate decreasing the amount of NAD⁺ available. NAD⁺ is needed for the Krebs Cycle to produce energy via oxidative phosphorylation. With impairment of the Krebs Cycle, this leads the ATP reservoir to become low, activating the Purine Nucleotide Cycle. Excess IMP (inosine monophosphate) is converted into uric acid. IMP → Inosine → Hypoxanthine → Xanthine → Uric Acid

Scurvy (Vitamin C Deficiency)

Vitamin C acts as a reducing agent, donating electrons in various enzymatic and non-enzymatic reactions. It is a cofactor for at least eight enzymatic reactions involved in key bodily processes including the production of collagen, preventing harmful genetic mutations, protecting white blood cells and the production of

carnitine Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria. In support of energy metabolism, carnitine transports long-chain fatty acids into mitochondria to be oxidized for energy production, an ...

, vital for energy. Carnitine plays a crucial role in lipid metabolism. As scurvy reduces the production of carnitine, the ability of the body to utilize fats for energy is impaired, leading to a low ATP reservoir. When ATP reservoirs are low, ATP is produced via the Purine Nucleotide Cycle. Excess IMP (inosine monophosphate) is converted into uric acid. IMP → Inosine → Hypoxanthine → Xanthine → Uric Acid

Low uric acid

Low uric acid ( hypouricemia) can have numerous causes. Low dietary

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

intakes cause lower uric acid levels. This effect can be even more pronounced in women taking oral contraceptive medication. Sevelamer, a drug indicated for prevention of hyperphosphataemia in people with

chronic kidney failure Chronic kidney disease (CKD) is a type of kidney disease in which a gradual loss of kidney function occurs over a period of months to years. Initially generally no symptoms are seen, but later symptoms may include leg swelling, feeling tired, v ...

, can significantly reduce serum uric acid.

Multiple sclerosis

Meta-analysis A meta-analysis is a statistical analysis that combines the results of multiple scientific studies. Meta-analyses can be performed when there are multiple scientific studies addressing the same question, with each individual study reporting m ...

of 10 case-control studies found that the serum uric acid levels of patients with multiple sclerosis were significantly lower compared to those of healthy controls, possibly indicating a diagnostic biomarker for multiple sclerosis.

Normalizing low uric acid

Correcting low or deficient zinc levels can help elevate

serum Serum may refer to: * Serum (blood), plasma from which the clotting proteins have been removed **Antiserum, blood serum with specific antibodies for passive immunity * Serous fluid, any clear bodily fluid *Truth serum, a drug that is likely to mak ...

uric acid.

References

External links

Uric acid blood test – MedlinePlus
{{DEFAULTSORT:Uric Acid Nitrogen cycle Organic acids Triketones