Excretion of Excess Nitrogen

Animals, such as fish, that live in an aquatic environment excrete nitrogen as ammonia they are protected from the toxic effects of high concentrations of ammonia not only by the removal of ammonia from their bodies but also by rapid dilution of the excreted ammonia by the water in the environment. The principal waste product of nitrogen metabolism in terrestrial animals is urea (a water-soluble compound) its reactions provide some interesting comparisons with the citric acid cycle. Birds excrete nitrogen in the form of uric acid, which is insoluble in water. They do not have to carry the excess weight of water, which could hamper flight, to rid themselves of waste products. 

Ammonia gas is toxic to most organisms and must usually be disposed of rapidly. In a certain sense, one can almost guess the mechanism of nitrogen-waste disposal if one knows the amount of water available to the organism in question. For example, bacteria 

urea becomes a major by-product of nitrogen metabolism in mammals. 

Reptiles and other desert animals do not usually have much  

Uric acid, the typical waste product from purines, can cause problems in primates due to its marginal water solubility. Deposits of uric acid in the joints and extremities cause gout (Section 23.8). Other mammals do not have a problem with uric acid because they convert it to allantoin, which is very water-soluble. 

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M.p. 222 C. Soluble in water, insoluble in alcohol. Citrulline is an intermediate in the urea cycle in the excretion of excess nitrogen from the body. 

A common amino acid in the body is ornithine. It is involved in the excretion of excess nitrogen into the urine. The structural formula of ornithine is... 

In addition to the pathways for synthesis and degradation of nitrogenous substances, many organisms have specialized metabolism for incorporation of excess nitrogen into relatively nontoxic excretion products. All of these aspects of nitrogen metabolism will be dealt with in this and the following chapter. 

The organic molecule urea is normally made in your kidneys and excreted in urine to dispose of excess nitrogen. Wohler had produced urea in the laboratory from ammonia and cyanic acid. He showed that it is possible to take lifeless molecules and produce one of the molecules of life in the laboratory. Today, it is possible to synthesize artificially many thousands of complex biomolecules. However, living cells still are the most efficient laboratories, and it can take months or years for chemists to synthesize a large molecule that a cell can make in seconds or minutes. 

The catabolism of amino acids results in the release of nitrogen. Many parasites excrete the excess nitrogen in the form of ammonia. In protozoa there is no evidence for a complete urea cycle which would allow the conversion of ammonia into urea, and urea released by some species (49,50) is likely to be formed solely as a product of arginine catabolism. Urea makes up between 2 and 10% of the total nitrogenous end-product of helminths, but evidence for a functional urea cycle in parasitic species has been equivocal (1). Arginase and ornithine carbamoyltransferase do occur widely but the other urea cycle enzymes do not. Small quantities of uric acid are excreted by some cestodes and trematodes. 

Because ostriches don t fly, one could argue that they would excrete their excess nitrogen as urea. On the other hand, they are birds, and as such probably have the same metabolism of their lighter counterparts, and might likely... 

Amino acids may be used as sources of energy when in excess or when other forms of energy are deficient. The process involves excretion of the nitrogen as urea and is relatively inefficient. 

Nitrogen compounds commonly determined are creatinine, urea, and uric acid. Creatinine is an end product of the energy process occurring within the muscles, and is thus related to muscle mass. Creatinine in urine is commonly used as an indicator and correction factor of dilution in urine. Creatinine in serum is an indicator of the filtration capacity of the kidney. Urea is the end product of the nitrogen luea cycle, starting with carbon dioxide and ammonia, and is the bulk compoimd of urine. The production of uric acid is associated with the disease gout. In some cases, it appears that the excess of uric acid is a consequence of impaired renal excretion of this substance. 

Amino groups released by deamination reactions form ammonium ion (NH " ), which must not escape into the peripheral blood. An elevated concentration of ammonium ion in the blood, hyperammonemia, has toxic effects in the brain (cerebral edema, convulsions, coma, and death). Most tissues add excess nitrogen to the blood as glutamine. Muscle sends nitrogen to the liver as alanine and smaller quantities of other amino acids, in addition to glutamine. Figure I-17-1 summarizes the flow of nitrogen from tissues to either the liver or kidney for excretion. The reactions catalyzed by four major enzymes or classes of enzymes involved in this process are summarized in Table T17-1. 

These conditions can be treated with a low protein diet and administration of sodium benzoate or phenylpyruvate to provide an alternative route for capturing and excreting excess nitrogen. 

The formation and excretion of urea is the primary mechanism by which excess nitrogen, in the form of ammonia, is removed from the body. Surprisingly, it was found that the actual rate of urea synthesis exceeded considerably the rate of excretion of the urea. The interesting question, therefore, is what is the fate of this lost urea The answer is that urea enters the large intestine, where it is degraded by microorganisms that possess the enzyme urease, which catalyses the reaction ... 

Proteins are constantly being lost via the intestine and, to a lesser extent, via the kidneys. To balance these inevitable losses, at least 30 g of protein have to be taken up with food every day. Although this minimum value is barely reached in some countries, in the industrial nations the protein content of food is usually much higher than necessary. As it is not possible to store amino acids, up to 100 g of excess amino acids per day are used for biosynthesis or degraded in the liver in this situation. The nitrogen from this excess is converted into urea (see p. 182) and excreted in the urine in this form. The carbon skeletons are used to synthesize carbohydrates or lipids (see p. 180), or are used to form ATP. 

Figure 25-18 Pathways of catabolism of purine nucleotides, nucleosides, and free bases. Spiders excrete xanthine while mammals and birds excrete uric acid. Spiders and birds convert all of their excess nitrogen via the de novo pathway of Fig. 25-15 into purines. Many animals excrete allantoin, urea, or NH4+. Some legumes utilize the pathway marked by green arrows in their nitrogen transport via ureides.

Allantoin is the excretory product in most mammals other than primates. Most fish hydrolyze allantoin to allantoic acid, and some excrete that compound as an end product. However, most continue the hydrolysis to form urea and glyoxylate using peroxisomal enzymes.336 In some invertebrates the urea may be hydrolyzed further to ammonia. In organisms that hydrolyze uric acid to urea or ammonia, this pathway is used only for degradation of purines from nucleotides. Excess nitrogen from catabolism of amino acids either is excreted directly as ammonia or is converted to urea by the urea cycle (Fig. 24-10). 

Ureas are diamides of carbonic acid, with two nitrogen atoms bonded to the carbonyl group. The unsubstituted urea, simply called urea, is the waste product excreted by mammals from the metabolism of excess protein. 

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