uric acid(redirected from Lithic acid)
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uric acid(yo͝or`ĭk), white, odorless, tasteless crystalline substance formed as a result of purinepurine,
type of organic base found in the nucleotides and nucleic acids of plant and animal tissue. The German chemist Emil Fischer did much of the basic work on purines and introduced the term into the chemical literature in the early 20th cent.
..... Click the link for more information. degradation in man, other primates, dalmatians, birds, snakes, and lizards. The last three groups of animals also channel all amino acidamino acid
, any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins.
..... Click the link for more information. degradation into the formation of glycineglycine
, organic compound, one of the 20 amino acids commonly found in animal proteins. Glycine is the only one of these amino acids that is not optically active, i.e., it does not have d- and
..... Click the link for more information. , aspartic acidaspartic acid
, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer participates in the biosynthesis of proteins.
..... Click the link for more information. , and glutamineglutamine
, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer occurs in mammalian protein.
..... Click the link for more information. , which combine to form purines and finally uric acid; these so-called uricotelic organisms thus excrete uric acid as the major end-product of the metabolism of all nitrogen-containing compounds. Uric acid is a very weak organic acid that is barely soluble in water and insoluble in alcohol and ether. The urates are its salts. Uric acid is present in human urine only in extremely small amounts but constitutes a large part of the body waste matter of birds (see guanoguano
, dried excrement of sea birds and bats found principally on the coastal islands of Peru, Africa, Chile, and the West Indies. It contains about 6% phosphorus, 9% nitrogen, 2% potassium, and moisture. Guano is found mixed with feathers and bones and is used as a fertilizer.
..... Click the link for more information. ) and of reptiles. It collects sometimes in the human kidneys or bladder in calculi, or stones, and is responsible, when present in tissues or deposited upon bones in the form of urates, for gouty conditions (see goutgout,
condition that manifests itself as recurrent attacks of acute arthritis, which may become chronic and deforming. It results from deposits of uric acid crystals in connective tissue or joints.
..... Click the link for more information. ). It occurs also in normal human blood. The pure acid is obtained from guano and other similar substances. Upon decomposition urea is obtained. A common test for the presence of the acid in urine depends upon the formation of murexide (an ammonium salt), which is an intense reddish purple. Nitric acid is added to the urine, which is then evaporated. If uric acid is present, murexide is formed when ammonia is added to the residue.
The main excretory end product of protein metabolism in certain species of birds and reptiles. In mammals, uric acid is derived from purines; in higher primates, including humans, it is excreted as such and is not oxidized to allantoin, the main excretory purine metabolism product of most species. In humans, uric acid levels are increased following excessive intake of dietary purines, primary synthesis in certain diseases (gout, Lesch-Nyhan syndrome), endogenous nucleic acid metabolism (leukemia, an abnormal number of erythrocytes in blood, chemotherapy-induced tumor lysis), and restricted renal excretion (renal diseases, ketoacidosis, lacticidosis, diuretics). Uric acid levels are lowered by the use of drugs causing increased uric acid excretion, and by renal tubular defects. See Liver, Nucleic acid, Protein metabolism, Purine
(2,6,8-trioxypurine), a colorless, crystalline solid that is very slightly soluble in water and that decomposes below the melting point. Uric acid, which was discovered in 1776 by K. Scheele as a component of urine, can exist in two forms— keto and enol:
In man and primates, uric acid is the end product of purine metabolism, forming as a result of the enzymatic oxidation of xanthine; in all other mammals, uric acid is converted into allantoin. The brain, liver, and blood of all mammals and man, as well as the urine and sweat, contain small quantities of uric acid. Some metabolic disorders are characterized by an accumulation in the body of uric acid and of uric acid salts (called urates); gouty deposits and the formation of stones in the kidneys and urinary bladder often accompany these metabolic disorders.
In birds, some reptiles, and most terrestrial insects, uric acid is the end product not only of purine metabolism but also of protein metabolism. Because an organism requires only a minimal amount of water in order to eliminate uric acid from the body (uric acid can even be eliminated as a solid), these animals, which have a limited water balance, use the uric acid biosynthetic pathway as the primary mechanism for neutralizing ammonia. (The neutralization is advantageous because ammonia is a more toxic product of nitrogen metabolism than uric acid.) This mechanism is in contrast to the mechanism found in most vertebrates, which have a more complete water balance and thus use the urea biosynthetic pathway as a means of neutralizing ammonia. (Urea requires much water to be safely eliminated from the body.)
Up to 25 percent of avian excrement, or guano, is uric acid; thus, the excrement serves as a source for obtaining uric acid. Uric acid is also found in a number of plants. It is the starting material for the synthesis of caffeine.
REFERENCESProsser, C. L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. (Translated from English.)
E. N. SAFONOVA