Uric acid

Uric acid

Crystals of urate in polarized light
Names
Systematic IUPAC name
7,9-Dihydro-1H-purine-2,6,8(3H)-trione
Other names
2,6,8-Trioxypurine; 2,6,8-Trihydroxypurine; 2,6,8-Trioxopurine; 1H-Purine-2,6,8-triol
Identifiers
69-93-2 YesY
3D model (Jmol) Interactive image
Interactive image
3DMet B00094
156158
ChEBI CHEBI:27226 N
ChEMBL ChEMBL792 YesY
ChemSpider 1142 YesY
DrugBank DB01696 YesY
ECHA InfoCard 100.000.655
EC Number 200-720-7
4731
KEGG C00366 N
MeSH Uric+Acid
PubChem 1175
UNII 268B43MJ25 YesY
Properties
C5H4N4O3
Molar mass 168.11 g·mol−1
Appearance White crystals
Melting point 300 °C (572 °F; 573 K)
0.6 mg/100 mL (at 20 °C)
log P −1.107
Acidity (pKa) 5.6
Basicity (pKb) 8.4
Thermochemistry
166.15 J K−1 mol−1 (at 24.0 °C)
173.2 J K−1 mol−1
−619.69 – −617.93 kJ mol−1
−1921.2 – −1919.56 kJ mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YesYN ?)
Infobox references

Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen 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 nucleotides, and it is a normal component of urine. High blood concentrations of uric acid can lead to gout and are associated with other medical conditions including diabetes and the formation of ammonium acid urate kidney stones.

Chemistry

Uric acid is a diprotic acid with pKa1 = 5.4 and pKa2 = 10.3.[1] Thus in strong alkali at high pH, it forms the dually-charged full urate ion, but at biological pH or in the presence of carbonic acid or carbonate ions, it forms the singly-charged hydrogen urate or acid urate ion, as its pKa1 is lower than the pKa1 of carbonic acid. As its second ionization is so weak, the full urate salts tend to hydrolyze back to hydrogen urate salts and free base at pH values around neutral. It is aromatic because of the purine functional group.

As a bicyclic, heterocyclic purine derivative, uric acid does not protonate as an oxygen (−OH) like carboxylic acids do. X-ray diffraction studies on the hydrogen urate ion in crystals of ammonium hydrogen urate, formed in vivo as gouty deposits, reveal the keto oxygen in the 2 position of a tautomer of the purine structure exists as a hydroxyl group and the two flanking nitrogen atoms at the 1 and 3 positions share the ionic charge in the six-membered pi-resonance-stabilized ring.[2]

Thus, while most organic acids are deprotonated by the ionization of a polar hydrogen–oxygen bond, usually accompanied by some form of resonance stabilization (resulting in a carboxylate ion), uric acid is deprotonated at a nitrogen atom and uses a tautomeric keto/hydroxy group as an electron-withdrawing group to increase the pK1 value. The five-membered ring also possesses a keto group (in the 8 position), flanked by two secondary amino groups (in the 7 and 9 positions), and deprotonation of one of these at high pH could explain the pK2 and behavior as a diprotic acid. Similar tautomeric rearrangement and pi-resonance stabilization would then give the ion some degree of stability.

(On the structure shown at the upper-right, the NH at the upper-right on the six-membered ring is "1", counting clockwise around the six-membered ring to "6" for the keto carbon at the top of the six-membered ring. The uppermost NH on the five-membered ring is "7", counting counter-clockwise around this ring to the lower NH, which is "9".)

Uric acid was first isolated from kidney stones in 1776 by the Swedish chemist Carl Wilhelm Scheele.[3] In 1882, the Ukrainian chemist Ivan Horbaczewski first synthesized uric acid by melting urea with glycine.[4]

Solubility

In general, the water solubility of uric acid and its alkali metal and alkaline earth 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 is very low or negligible. In ethanol/water mixtures, the solubilities are somewhere between the end values for pure ethanol and pure water.

Solubility of urate salts (grams of water per gram of compound)
Compound Cold water Boiling water
Uric acid 15,000 2,000
Ammonium hydrogen urate – 1,600
Lithium hydrogen urate 370 39
Sodium hydrogen urate 1,175 124
Potassium hydrogen urate 790 75
Magnesium dihydrogen diurate 3,750 160
Calcium dihydrogen diurate 603 276
Disodium urate 77 —
Dipotassium urate 44 35
Calcium urate 1,500 1,440
Strontium urate 4,300 1,790
Barium urate 7,900 2,700

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.[5][6][7]

Biology

The enzyme xanthine oxidase catalyzes formation of uric acid from xanthine and hypoxanthine, which in turn are produced from other purines. Xanthine oxidase is a large enzyme whose active site consists of the metal molybdenum bound to sulfur and oxygen.[8] Within cells, xanthine oxidase can exist as xanthine dehydrogenase and xanthine oxireductase, which has also been purified from bovine milk and spleen extracts.[9] Uric acid is released in hypoxic conditions.[10]

In humans and higher primates, uric acid is the final oxidation (breakdown) product of purine metabolism and is excreted in urine. In most other mammals, the enzyme uricase further oxidizes uric acid to allantoin.[11] 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.[12] Both uric acid and ascorbic acid are strong reducing agents (electron donors) and potent antioxidants. In humans, over half the antioxidant capacity of blood plasma comes from uric acid.[13]

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.[14]

The Dalmatian dog has a genetic defect in uric acid uptake by the liver and kidneys, resulting in decreased conversion to allantoin, so this breed excretes uric acid, and not allantoin, in the urine.[15]

In birds and reptiles, and in some desert dwelling mammals (e.g., the kangaroo rat), 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 that is energetically costly in comparison to processing of other nitrogenous wastes such as urea (from urea cycle) or ammonia, but has the advantages of reducing water loss and preventing dehydration.[16]

Platynereis dumerilii, a marine Polychaete worm, uses uric acid as a sexual pheromone released into the water by females during mating to induce males to release sperm.[17]

Genetics

A proportion of people have mutations in the proteins responsible for the excretion of uric acid by the kidneys. Variants within a number of genes have so far been identified: SLC2A9; ABCG2; SLC17A1; SLC22A11; SLC22A12; SLC16A9; GCKR; LRRC16A; and PDZK1.[18][19][20] SLC2A9 is known to transport both uric acid and fructose.[14][21]

Medicine

In human blood plasma, the reference range of uric acid is typically 3.4–7.2 mg/dL (200–430 µmol/L) for men, and 2.4–6.1 mg/dL for women (140–360 µmol/L)[22] – one milligram per decilitre (mg/dL) equals 59.48 micromoles/litre (µmol/L). However, blood test results should always be interpreted using the range provided by the laboratory that performed the test. 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. Such abnormal concentrations of uric acid are not medical conditions, but are associated with a variety of medical conditions.

High uric acid

Hyperuricemia (high levels of uric acid) can lead to gout.

Causes of high uric acid

Gout

Excess serum accumulation of uric acid in the blood can lead to gout, which is a type of arthritis.[27] This painful condition is the result of needle-like crystals of uric acid precipitating in joints, capillaries, skin, and other tissues.[28] Kidney stones can also form through the process of formation and deposition of sodium urate microcrystals.[29][30]

A study found that men who drink two or more sugar-sweetened beverages a day have an 85% higher chance of developing gout than those who drank such beverages infrequently.[31]

Gout can occur where serum uric acid levels are as low as 6 mg/dL (~357 µmol/L), but an individual can have serum values as high as 9.6 mg/dL (~565 µmol/L) and not have gout.[32]

One treatment for gout, in the 19th century, had been administration of lithium salts;[33] 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.[34] Allopurinol, which weakly inhibits xanthine oxidase, is an analog of hypoxanthine that is hydroxylated by xanthine oxidoreductase at the 2-position to give oxipurinol.[35] Oxipurinol has been supposed to bind tightly to the reduced molybdenum ion in the enzyme and, thus, inhibits uric acid synthesis.[36]

Lesch-Nyhan syndrome

Lesch-Nyhan syndrome, an extremely rare inherited disorder, is also associated with very high serum uric acid levels.[37] Spasticity, involuntary movement, and cognitive retardation as well as manifestations of gout are seen in cases of this syndrome.[38]

Cardiovascular disease

Hyperuricemia may be associated with an increase in risk factors for cardiovascular disease.[39]

Type 2 diabetes

The association of high serum uric acid with insulin resistance has been known since the early part of the 20th century, but the hypothesis that high serum uric acid is a risk factor for diabetes has long been a matter of debate. In fact, hyperuricemia was presumed to be a consequence of insulin resistance rather than its precursor.[40] However, a prospective follow-up study showed high serum uric acid is associated with higher risk of type 2 diabetes, independent of obesity, dyslipidemia, and hypertension.[41]

Metabolic syndrome

Hyperuricemia is associated with components of metabolic syndrome. A study has suggested fructose-induced hyperuricemia may play a pathogenic role in the metabolic syndrome.[42] This is consistent with the increased consumption in recent decades of fructose-containing beverages (such as fruit juices and soft drinks sweetened with sugar and high-fructose corn syrup) and the epidemic of diabetes and obesity.[31]

Uric acid stone formation

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 and so do not appear on an abdominal plain X-ray, and thus their presence must be diagnosed by ultrasound for this reason or stone protocol CT. Very large stones may be detected on X-ray by their displacement of the surrounding kidney tissues.

Uric acid stones, which form in the absence of secondary causes such as chronic diarrhea, vigorous exercise, dehydration, and animal protein loading, are felt to be secondary to obesity and insulin resistance seen in metabolic syndrome. Increased dietary acid leads to increased endogenous acid production in the liver and muscles, which in turn leads to an increased acid load to the kidneys. This load is handled more poorly because of renal fat infiltration and insulin resistance, which are felt to impair ammonia excretion (a buffer). The urine is, therefore, quite acidic, and uric acid becomes insoluble, crystallizes and stones form. In addition, naturally present promoter and inhibitor factors may be affected. This explains the high prevalence of uric stones and unusually acidic urine seen in patients with type 2 diabetes. Uric acid crystals can also promote the formation of calcium oxalate stones, acting as "seed crystals" (heterogeneous nucleation).[43]

Low uric acid

Causes of low uric acid

Low uric acid (hypouricemia) can have numerous causes.

Low dietary zinc intakes cause lower uric acid levels. This effect can be even more pronounced in women taking oral contraceptive medication.[44]

Xanthine oxidase (XO) is an iron–molybdenum enzyme, so people with iron deficiency (the most common cause of anemia in young women) or molybdenum deficiency can experience hypouricemia.

Xanthine oxidase loses its function and gains ascorbase function when some of the iron atoms in XO are replaced with copper atoms. As such, people with high copper/iron can experience hypouricemia and vitamin C deficiency, resulting in oxidative damage. Since estrogen increases the half-life of copper, people with very high estrogen levels and intense blood loss during menstruation are likely to have high copper/iron and present with hypouricemia.

Sevelamer, a drug indicated for prevention of hyperphosphataemia in patients with chronic renal failure, can significantly reduce serum uric acid.[45]

But the main cause of congenitally low uric acid, sometimes as low as zero, remains the molybdenum cofactor deficiency.

Multiple sclerosis

In one preliminary study, lower serum values of uric acid were somewhat associated with multiple sclerosis in which people with the disease had serum levels of ~194 µmol/L, people in relapse averaged ~160 µmol/L, and people in remission had ~230 µmol/L. Serum uric acid in healthy people was ~290 µmol/L.[46]

Normalizing low uric acid

Correcting low or deficient zinc levels can help elevate serum uric acid.[47] Inosine can be used to elevate uric acid levels.[46] Zinc inhibits copper absorption, helping to reduce the high copper/iron in some people with hypouricemia. Iron supplements can ensure adequate iron reserves (ferritin above 25 ng/dl), also correcting the high copper/iron.

Oxidative stress

Uric acid may be a marker of oxidative stress,[48] and may have a potential therapeutic role as an antioxidant.[49] On the other hand, like other strong reducing substances such as ascorbate, uric acid can also act as a prooxidant. Thus, it is unclear whether elevated levels of uric acid in diseases associated with oxidative stress such as stroke and atherosclerosis are a protective response or a primary cause.[50]

For example, some researchers propose hyperuricemia-induced oxidative stress is a cause of metabolic syndrome.[42][51] On the other hand, plasma uric acid levels correlate with longevity in primates and other mammals.[52] This is presumably a function of urate's antioxidant properties.[53]

Sources

See also

References

  1. ↑ McCrudden, Francis H. (2008). Uric Acid. BiblioBazaar.
  2. ↑ European Powder Diffraction Conference, EPDIC-9
  3. ↑ Scheele, C. W. (1776). "Examen Chemicum Calculi Urinari" [A chemical examiniation of kidney stones]. Opuscula. 2: 73.
  4. ↑ Horbaczewski, Johann (1882). "Synthese der Harnsäure" [Synthesis of uric acid]. Monatshefte für Chemie und verwandte Teile anderer Wissenschaften. 3: 796–797.
  5. ↑ CRC Handbook of Chemistry and Physics (62nd ed.).
  6. ↑ Merck Index (9th ed.).
  7. ↑ McCrudden, Francis H. Uric acid. p. 58.
  8. ↑ Hille, R. (2005). "Molybdenum-containing hydroxylases". Archives of Biochemistry and Biophysics. 433 (1): 107–116. doi:10.1016/j.abb.2004.08.012. PMID 15581570.
  9. ↑ Hori, N.; Uehara, K.; Mikami, Y. (1992). "Enzymatic Synthesis of 5-Methyluridine from Adenosine and Thymine with High Efficiency". Biosci., Biotechnol., Biochem. 56 (4): 580–582. doi:10.1271/bbb.56.580.
  10. ↑ Baillie, J. K.; Bates, M. G.; Thompson, A. A.; Waring, W. S.; Partridge, R. W.; Schnopp, M. F.; Simpson, A.; Gulliver-Sloan, F.; Maxwell, S. R.; Webb, D. J. (May 2007). "Endogenous urate production augments plasma antioxidant capacity in healthy lowland subjects exposed to high altitude". Chest. 131 (5): 1473–1478. doi:10.1378/chest.06-2235. PMID 17494796.
  11. ↑ Angstadt, Carol N. (4 December 1997). "Purine and Pyrimidine Metabolism: Purine Catabolism". NetBiochem.
  12. ↑ Proctor, P. (November 1970). "Similar functions of uric acid and ascorbate in man?". Nature. 228 (5274): 868. Bibcode:1970Natur.228..868P. doi:10.1038/228868a0. PMID 5477017.
  13. ↑ Maxwell, S. R. J.; Thomason, H.; Sandler, D.; Leguen, C.; Baxter, M. A.; Thorpe, G. H. G.; Jones, A. F.; Barnett, A. H. (1997). "Antioxidant status in patients with uncomplicated insulin-dependent and non-insulin-dependent diabetes mellitus". European Journal of Clinical Investigation. 27 (6): 484–490. doi:10.1046/j.1365-2362.1997.1390687.x. PMID 9229228.
  14. 1 2 Vitart, V.; Rudan, I.; Hayward, C.; et al. (April 2008). "SLC2A9 is a newly identified urate transporter influencing serum urate concentration, urate excretion and gout". Nature Genetics. 40 (4): 437–442. doi:10.1038/ng.106. PMID 18327257.
  15. ↑ Friedman, Meyer & Byers, Sanford O. (1 September 1948). "Observations concerning the causes of the excess excretion of uric acid in the Dalmatian dog". The Journal of Biological Chemistry. 175 (2): 727–735. PMID 18880769.
  16. ↑ Hazard, Lisa C. (2004). Sodium and Potassium Secretion by Iguana Salt Glands. Iguanas: Biology and Conservation. University of California Press. pp. 84–85. ISBN 978-0-520-23854-1.
  17. ↑ Zeeck, Erich; Harder, Tilman; Beckmann, Manfred (1998). "Uric acid: the sperm-release pheromone of the marine polychaete Platynereis dumerilii". Journal of Chemical Ecology. 24 (1): 13–22. doi:10.1023/A:1022328610423.
  18. ↑ Aringer, M; Graessler, J. (December 2008). "Understanding deficient elimination of uric acid". Lancet. 372 (9654): 1929–1930. doi:10.1016/S0140-6736(08)61344-6. PMID 18834627.
  19. ↑ Kolz, M.; Johnson, T.; et al. (June 2009). Allison, David B., ed. "Meta-analysis of 28,141 individuals identifies common variants within five new loci that influence uric acid concentrations". PLoS Genet. 5 (6): e1000504. doi:10.1371/journal.pgen.1000504. PMC 2683940Freely accessible. PMID 19503597.
  20. ↑ Köttgen, A.; et al. (February 2013). "Genome-wide association analyses identify 18 new loci associated with serum urate concentrations". Nature Genetics. 45 (2): 145–154. doi:10.1038/ng.2500. PMC 3663712Freely accessible. PMID 23263486.
  21. ↑ Döring, A.; Gieger, C.; Mehta, D.; et al. (April 2008). "SLC2A9 influences uric acid concentrations with pronounced sex-specific effects". Nature Genetics. 40 (4): 430–436. doi:10.1038/ng.107. PMID 18327256.
  22. ↑ "Harmonisation of Reference Intervals" (PDF). Pathology Harmony (UK). Retrieved 13 August 2013.
  23. ↑ Cirillo, P.; Sato, W.; Reungjui, S.; et al. (December 2006). "Uric acid, the metabolic syndrome, and renal disease". J. Am. Soc. Nephrol. 17 (12 Suppl. 3): S165–S168. doi:10.1681/ASN.2006080909. PMID 17130256.
  24. ↑ Angelopoulos, Theodore J.; Lowndes, Joshua; Zukley, Linda; Melanson, Kathleen J.; Nguyen, Von; Huffman, Anik; Rippe, James M. (June 2009). "The Effect of High-Fructose Corn Syrup Consumption on Triglycerides and Uric Acid". J. Nutr. 139 (6): 1242S–1245S. doi:10.3945/jn.108.098194. PMID 19403709.
  25. ↑ Mayo Clinic staff (11 September 2010). "High uric acid level". Mayo Clinic. Retrieved 2011-04-24.
  26. ↑ "Diuretic-Related Side Effects: Development and Treatment". Medscape. Retrieved 17 May 2013.
  27. ↑ Heinig, M.; Johnson, R. J. (December 2006). "Role of uric acid in hypertension, renal disease, and metabolic syndrome". Cleveland Clinic Journal of Medicine. 73 (12): 1059–1064. doi:10.3949/ccjm.73.12.1059. PMID 17190309.
  28. ↑ Richette, P.; Bardin, T. (January 2010). "Gout". Lancet. 375 (9711): 318–328. doi:10.1016/S0140-6736(09)60883-7. PMID 19692116.
  29. ↑ Banach, K.; Bojarska, E.; Kazimierczuk, Z.; Magnowska, L.; Bzowska, A. (2005). "Kinetic Model of Oxidation Catalyzed by Xanthine Oxidase—The Final Enzyme in Degradation of Purine Nucleosides and Nucleotides". Nucleic Acids. 24: 465–469.
  30. ↑ "What is Gout: What Causes Gout?". MedicalBug. 6 January 2012. Retrieved 6 May 2012.
  31. 1 2 Malik, V. S.; Popkin, B. M.; Bray, G. A.; Després, J. P.; Willett, W. C.; Hu, F. B. (2010). "Sugar-sweetened beverages and risk of metabolic syndrome and type 2 diabetes: a meta-analysis". Diabetes Care. 33 (11): 2477–2483. doi:10.2337/dc10-1079. PMC 2963518Freely accessible. PMID 20693348.
  32. ↑ Tausche, A. K.; Unger, S.; Richter, K.; et al. (May 2006). "Hyperurikämie und Gicht" [Hyperuricemia and gout: diagnosis and therapy]. Der Internist (in German). 47 (5): 509–521. doi:10.1007/s00108-006-1578-y. PMID 16586130.
  33. ↑ Schrauzer, Gerhard N. (2002). "Lithium: Occurrence, Dietary Intakes, Nutritional Essentiality". Journal of the American College of Nutrition. 21 (1): 14–21. doi:10.1080/07315724.2002.10719188. PMID 11838882.
  34. ↑ "NHS Clinical Knowledge Summaries". UK National Health Service. Archived from the original on 4 March 2012.
  35. ↑ Pacher, P; Nivorozhkin, A; Szabó, C (2006). "Therapeutic effects of xanthine oxidase inhibitors: Renaissance half a century after the discovery of allopurinol". Pharmacological Reviews. 58 (1): 87–114. doi:10.1124/pr.58.1.6. PMC 2233605Freely accessible. PMID 16507884.
  36. ↑ Okamoto, K.; Eger, B. T.; Nishino, T.; Pai, E. F.; Nishino, T. (2008). "Mechanism of inhibition of xanthine oxidoreductase by allopurinol: Crystal structure of reduced bovine milk xanthine oxidoreductase bound with oxipurinol". Nucleosides, Nucleotides & Nucleic Acids. 27 (6): 888–893. doi:10.1080/15257770802146577. PMID 18600558.
  37. ↑ Luo, Y. C.; Do, J. S.; Liu, C. C. (October 2006). "An amperometric uric acid biosensor based on modified Ir–C electrode". Biosensors & Bioelectronics. 22 (4): 482–488. doi:10.1016/j.bios.2006.07.013. PMID 16908130.
  38. ↑ Nyhan, W. L. (March 2005). "Lesch-Nyhan Disease". Journal of the History of the Neurosciences. 14 (1): 1–10. doi:10.1080/096470490512490. PMID 15804753.
  39. ↑ Borghi, C.; Verardi, F. M.; Pareo, I.; Bentivenga, C.; Cicero, A. F. (2014). "Hyperuricemia and cardiovascular disease risk". Expert. Rev. Cardiovasc. Ther. 12 (10): 1219–1225. doi:10.1586/14779072.2014.957675. PMID 25192804.
  40. ↑ Cappuccio, F. P.; Strazzullo, P.; Farinaro, E.; Trevisan, M. (July 1993). "Uric acid metabolism and tubular sodium handling. Results from a population-based study". J. Am. Med. Assoc. 270 (3): 354–359. doi:10.1001/jama.270.3.354. PMID 8315780.
  41. ↑ Dehghan, A.; van Hoek, M.; Sijbrands E. J., Hofman A.; Witteman, J. C. (February 2008). "High serum uric acid as a novel risk factor for type 2 diabetes". Diabetes Care. 31 (2): 361–362. doi:10.2337/dc07-1276. PMID 17977935.
  42. 1 2 Nakagawa, T.; Hu, H.; Zharikov, S; et al. (March 2006). "A causal role for uric acid in fructose-induced metabolic syndrome". American Journal of Physiology: Renal Physiology. 290 (3): F625–F631. doi:10.1152/ajprenal.00140.2005. PMID 16234313.
  43. ↑ Pak, C. Y. (September 2008). "Medical stone management: 35 years of advances". The Journal of Urology. 180 (3): 813–819. doi:10.1016/j.juro.2008.05.048. PMID 18635234.
  44. ↑ Hess, F. M.; King, J. C.; Margen, S. (1 December 1977). "Effect of low zinc intake and oral contraceptive agents on nitrogen utilization and clinical findings in young women". The Journal of Nutrition. 107 (12): 2219–2227. PMID 925768.
  45. ↑ Garg, J. P.; Chasan-Taber, S.; Blair, A.; et al. (January 2005). "Effects of sevelamer and calcium-based phosphate binders on uric acid concentrations in patients undergoing hemodialysis: a randomized clinical trial". Arthritis and Rheumatism. 52 (1): 290–295. doi:10.1002/art.20781. PMID 15641045.
  46. 1 2 Toncev, G.; Milicic, B.; Toncev, S.; Samardzic, G. (May 2002). "Serum uric acid levels in multiple sclerosis patients correlate with activity of disease and blood–brain barrier dysfunction". European Journal of Neurology. 9 (3): 221–226. doi:10.1046/j.1468-1331.2002.00384.x. PMID 11985629.
  47. ↑ Umeki, S; Ohga, R.; Konishi, Y.; Yasuda, T.; Morimoto, K.; Terao, A. (November 1986). "Oral zinc therapy normalizes serum uric acid level in Wilson's disease patients". The American Journal of the Medical Sciences. 292 (5): 289–292. doi:10.1097/00000441-198611000-00007. PMID 3777013.
  48. ↑ Becker BF (June 1993). "Towards the physiological function of uric acid". Free Radical Biology & Medicine. 14 (6): 615–631. doi:10.1016/0891-5849(93)90143-I. PMID 8325534.
  49. ↑ Glantzounis, G. K.; Tsimoyiannis, E. C.; Kappas, A. M.; Galaris, D. A. (2005). "Uric acid and oxidative stress". Current Pharmaceutical Design. 11 (32): 4145–4151. doi:10.2174/138161205774913255. PMID 16375736.
  50. ↑ Proctor, P. H. (May 2008). "Uric acid: neuroprotective or neurotoxic?". Stroke. 39 (5): e88–e89. doi:10.1161/STROKEAHA.107.513242. PMID 18369163.
  51. ↑ Hayden, M. R.; Tyagi, S. C. (2004). "Uric acid: A new look at an old risk marker for cardiovascular disease, metabolic syndrome, and type 2 diabetes mellitus: The urate redox shuttle". Nutrition & Metabolism. 1 (1): 10. doi:10.1186/1743-7075-1-10. PMC 529248Freely accessible. PMID 15507132.
  52. ↑ Cutler, R. G. (December 1984). "Urate and ascorbate: their possible roles as antioxidants in determining longevity of mammalian species". Archives of Gerontology and Geriatrics. 3 (4): 321–348. doi:10.1016/0167-4943(84)90033-5. PMID 6532339.
  53. ↑ Ames, B. N.; et al. (November 1981). "Uric acid provides an antioxidant defense in humans against oxidant- and radical-caused aging and cancer: a hypothesis". Proc. Natl. Acad. Sci. USA. 78 (11): 6858–6862. Bibcode:1981PNAS...78.6858A. doi:10.1073/pnas.78.11.6858. PMC 349151Freely accessible. PMID 6947260.
  54. ↑ Gout Causes: List of Diet/Food Sources High or Low in Purine Content
  55. ↑ Gout Diet / Low Purine Diet – Limit High Purine foods
  56. ↑ Choi, H. K.; Atkinson, K.; Karlson, E. W.; Willett, W.; Curhan, G. (March 2004). "Purine-rich foods, dairy and protein intake, and the risk of gout in men". The New England Journal of Medicine. 350 (11): 1093–1103. doi:10.1056/NEJMoa035700. PMID 15014182.

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