Nitrogen excretory products

Another form of detoxified ammonia that is used in nitrogen excretion is uric acid. Uric acid is the predominant nitrogen excretory product in birds and terrestrial reptiles (turtles excrete urea, whereas alligators excrete ammonia unless they are dehydrated, in which case they, too, excrete uric acid). Uric acid formed as a product of amino acid catabolism involves the de novo pathway of purine biosynthesis therefore, its formation from NH3 liberated in amino acid catabolism is described elsewhere (see chapter 23). In mammals, uric acid is exclusively an intermediate in purine... 

Table 6.7. Nitrogen excretory products of cestodes. (Data from Smyth, 1969)... 

Although urea is the major nitrogenous excretory product, nitrogen may also be excreted as NH4+, uric acid, and creatinine. 

Purines are degraded to form uric acid, a nitrogenous excretory product. 

Nitrogenous excretory products are removed from the body mainly in the urine. In renal failure, these products will be retained. Blood urea nitrogen (BUN), creatinine, and uric acid levels will rise. 

Fig. 1. Summary of amino acid metabolism. Dietary proteins are digested to amino acids in the stomach and intestine, which are absorbed by the intestinal epithelium, transferred to the circulation, and taken up by cells. Amino acids are used to synthesize proteins and other nitrogen-containing compounds. The carbon skeletons of amino acids are also oxidized for energy, and the nitrogen is converted to urea and other nitrogenous excretory products.

The normal human adult is in nitrogen balance that is, the amount of nitrogen ingested each day, mainly in the form of dietary protein, is equal to the amount of nitrogen excreted. The major nitrogenous excretory product is urea, which exits from the body in the urine. This innocuous compound, produced mainly in the liver by the urea cycle, serves as the disposal form of ammonia, which is toxic, particularly to the brain and central nervous system. Normally, little ammonia (or NH4 ) is present in the blood. The concentration ranges between... 

Olsen KH (1986b) Modification of conspecific chemoattraction in Arctic charr (Salvelinus alpinus (L.)), by nitrogenous excretory products. Comp Biochem Physiol 85A 77-81... 

Uric acid, 2,6,S-trihydroxypmnu an excretory product of purine metabolism in most animals. In some animals, known as uricotelic organisms (birds, reptiles, many insects), it is the main nitrogenous excretory product. U.a. (Af, 168.1, m.p. 400°C) was discovered in urine in 1776 by Scheele, and it can be isolated in quantity from bird excrements (guano) its salts are called urates Humans and the great apes usually excrete U. a. unchanged. In the adult human, 1-3 % of urinary nitrogen is represented by U.a. 

Actually they assumed 6.72 Cal per g of protein instead of 4.2, since the exact heat of combustion of protein and of the nitrogenous excretory products was not known at that time. 

Purines.—The biological purines are simple hydroxy, amino, or methyl derivatives of a parent purine ring, which does not occur free in nature. Hydroxy purines, hypoxanthim and xanthine, occur in tissues and tissue fluids of many animals uric acid, a trihydroxy purine, is the chief nitrogenous excretory product of birds and snakes, and is invariably present in mammalian urine. 

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