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Uric acid (or urate) is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. Uric acid is a diprotic acid with pKa1=5.4 and pKa2=10.3[2]. 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 or acid urate ion as its pKa2 is greater than the pKa1 of carbonic acid. As its second ionization is so weak the full urate salts tend to hydrolyse 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 in the same manner as do carboxylic acids. X-Ray diffraction studies on the hydrogen urate ion in crystals of ammomium hydrogen urate, formed in vivo as gouty deposits, revealed that the keto-oxygen in the 2 position of a tautomer of the purine structure existed as a hydroxyl group and that the two flanking nitrogen atoms at the 1 and 3 positions shared the ionic charge in the six membered pi-resonance-stabilized ring.[3] Thus, whereas most organic acids are deprotonated by the ionization of a polar hydrogen-to-oxygen bond, usually accompanied by some form of resonance stabilization (resulting in a carboxylate ion), this 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 upper most NH on the five membered ring is "7", counting counter clockwise around this ring to the lower NH, which is "9".) Solubility of uric acid and its salts Generally the solubilities of uric acid, its Alkali and Alkali Earth Metal salts in water are rather low and all exhibit greater solubility in hot water than cold allowing for easy recrystallization. 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.
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.[4][5][6] Biology Uric acid is produced by xanthine oxidase from xanthine and hypoxanthine, which in turn are produced from purine. Uric acid is more toxic to tissues than either xanthine or hypoxanthine.[citation needed] Uric acid is released in hypoxic conditions.[7] 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.[8] The loss of uricase in higher primates parallels the similar loss of the ability to synthesize ascorbic acid.[9] 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.[7] The Dalmatian dog has a genetic defect in uric acid uptake by the liver, resulting in decreased conversion to allantoin, so this breed excretes uric acid, and not allantoin, in the urine.[10] 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 advantage of reducing water loss.[11] 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.[12] Genetics A proportion of people have mutations in the proteins responsible for the excretion of uric acid by the kidneys. Nine genes have so far been identified: SLC2A9; ABCG2; SLC17A1; SLC22A11; SLC22A12; SLC16A9; GCKR; LRRC16A; and PDZK1. [13] [14] SLC2A9 is known to transport both uric acid and fructose.[12][15] Medicine In human blood plasma, the reference range of uric acid is between 3.6 mg/dL (~214µmol/L) and 8.3 mg/dL (~494µmol/L) (1 mg/dL=59.48 µmol/L).[16] This range is considered normal by the American Medical Association. Uric acid concentrations in blood plasma above and below the normal range are known, respectively, as hyperuricemia and hypouricemia. Similarly, 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. Gout Excess serum accumulation of uric acid can lead to a type of arthritis known as gout.[17] This painful condition is the result of needle-like crystals of uric acid precipitating in joints and capillaries. Elevated serum uric acid (hyperuricemia) can result from high intake of purine-rich foods, and/or impaired excretion by the kidneys. 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 or CT scan. Their presence must be diagnosed by ultrasound for this reason. Very large stones may be detected on x-ray by their displacement of the surrounding kidney tissues. Some patients with gout eventually get uric kidney stones. Gout can occur where serum uric acid levels are as low as 6 mg/dL (~357µmol/L), but an individual can have serum values as high as 9.6 mg/dL (~565µmol/L) and not have gout.[18] One treatment for gout has been administration of Lithium salts; lithium urate is more soluble. Lesch-Nyhan syndrome Lesch-Nyhan syndrome, an extremely rare inherited disorder, is also associated with very high serum uric acid levels.[19] Spasticity, involuntary movement and cognitive retardation as well as manifestations of gout are seen in cases of this syndrome.[20] Cardiovascular disease Although uric acid can act as an antioxidant, excess serum accumulation is often associated with cardiovascular disease. It is not known whether this is causative (e.g., by acting as a prooxidant ) or a protective reaction taking advantage of urate's antioxidant properties.[17] Diabetes The association of high serum uric acid with insulin resistance has been known since the early part of the 20th century, nevertheless, recognition of high serum uric acid as a risk factor for diabetes has been a matter of debate. In fact, hyperuricemia has always been presumed to be a consequence of insulin resistance rather than its precursor.[21] However, it was shown in a prospective follow-up study that high serum uric acid is associated with higher risk of type 2 diabetes independent of obesity, dyslipidemia, and hypertension.[22] Metabolic syndrome Hyperuricemia is associated with components of metabolic syndrome and it has been debated for a while to be a component of it. It has been shown in a recent study that fructose-induced hyperuricemia may play a pathogenic role in the metabolic syndrome. 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.[23] Uric acid stone formation 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 promotor 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).[24] Low uric acid Multiple sclerosis Lower serum values of uric acid have been associated with Multiple Sclerosis. Multiple sclerosis (MS) patients have been found to have serum levels ~194µmol/L, with patients in relapse averaging ~160µmol/L and patients in remission averaging ~230µmol/L. Serum uric acid in healthy controls was ~290µmol/L.[25] Conversion factor: 1 mg/dL=59.48 µmol/L[16] A 1998 study completed a statistical analysis of 20 million patient records, comparing serum uric acid values in patients with gout and patients with multiple sclerosis. Almost no overlap between the groups was found.[26] Uric acid has been successfully used in the treatment and prevention of the animal (murine) model of MS. A 2006 study found that elevation of serum uric acid values in multiple sclerosis patients, by oral supplementation with inosine, resulted in lower relapse rates, and no adverse effects.[27] Oxidative stress Uric acid may be a marker of oxidative stress,[28] and may have a potential therapeutic role as an antioxidant.[29] On the other hand, like other strong reducing substances such as ascorbate, uric acid can also act as a prooxidant,[30] particularly at elevated levels. 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.[31][32] For example, some researchers propose that hyperuricemia-induced oxidative stress is a cause of metabolic syndrome.[23][33] On the other hand, plasma uric acid levels correlate with longevity in primates and other mammals.[34] This is presumably a function of urate's antioxidant properties.[citation needed] Sources of uric acid * In humans Purines are excreted as uric acid. Purines are found in high amounts in animal food products, such as liver and sardines.[35] A moderate amount of purine is also contained in beef, pork, poultry, fish and seafood, asparagus, cauliflower, spinach, mushrooms, green peas, lentils, dried peas, beans, oatmeal, wheat bran and wheat germ.[36] * Examples of high purine sources include: sweetbreads, anchovies, sardines, liver, beef kidneys, brains, meat extracts (e.g., Oxo, Bovril), herring, mackerel, scallops, game meats, and gravy.[citation needed] * Moderate intake of purine-containing food is not associated with an increased risk of gout.[37] * In many instances, people have elevated uric acid levels for hereditary reasons. Diet may also be a factor; eating large amounts of sea salt can cause increased levels of uric acid.[citation needed] (Medical consultation is recommended before using large quantities of sea salt in daily cooking.[citation needed])Serum uric acid can be elevated due to reduced excretion by the kidneys, and or high intake of dietary purine.[citation needed]
Low uric acid (hypouricemia) can have numerous causes.[citation needed] Low dietary zinc intakes cause lower uric acid levels. This effect can be even more pronounced in women taking oral contraceptive medication.[38] Sevelamer, a drug indicated for prevention of hyperphosphataemia in patients with chronic renal failure, can significantly reduce serum uric acid.[39] Normalizing low uric acid Correcting low or deficient zinc levels can help elevate serum uric acid [40] Inosine can be used to elevate uric acid levels.[25] Other uric acid facts The high nitrogen content of uric acid makes guano a useful agricultural fertilizer.[citation needed] The crystalline form of uric acid is used as a reflector in certain species of fireflies.[citation needed] The uric acid in urine can also dry in a baby's diaper to form a pinkish powder that is harmless.[citation needed] Urate is being researched for its possible antioxidant properties in treating Parkinson's disease.[41] References 1. ^ "Uric Acid." Biological Magnetic Resonance Data Bank. Indicator Information Retrieved on 18 February 2008. External links Retrieved from "http://en.wikipedia.org/" |
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