Nitrogen excretion urea cycle

Ureoteles and uricoteles use pre-existing, very ancient biochemical mechanisms for nitrogen excretion urea synthesis by the urea cycle, which first evolved for the synthesis and degradation of arginine and... 

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. 

The urea cycle is essential for the detoxification of ammonia 678 Urea cycle defects cause a variety of clinical syndromes, including a metabolic crisis in the newborn infant 679 Urea cycle defects sometimes result from the congenital absence of a transporter for an enzyme or amino acid involved in the urea cycle 680 Successful management of urea cycle defects involves a low-protein diet to minimize ammonia production as well as medications that enable the excretion of ammonia nitrogen in forms other than urea 680... 

Urea is a colorless, odorless crystalline substance discovered by Hilaire Marin Rouelle (1718—1779) in 1773, who obtained urea by boiling urine. Urea is an important biochemical compound and also has numerous industrial applications. It is the primary nitrogen product of protein (nitrogen) metabolism in humans and other mammals. The breakdown of amino acids results in ammonia, NH3, which is extremely toxic to mammals. To remove ammonia from the body, ammonia is converted to urea in the liver in a process called the urea cycle. The urea in the blood moves to the kidney where it is concentrated and excreted with urine. 

Metabolism of nitrogen in a patient with a deficiency in the urea cycle enzyme carbamoyl phosphate synthetase I. Treatment with phenylbutyrate converts nitrogenous waste to a form that can be excreted. 

Correct answer = D. The amino nitrogen of dietary protein is excreted as urea. The two nitrogens enter the urea cycle as ammonia and aspartate. Urea is produced by the hydrolysis of arginine. The cleavage of argininosucdnate does not require ATP. The urea cycle occurs partly in the mitochondria. 

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). 

One of the major products of amino acid metabolism is ammonia (NLI3), a molecule known to be highly toxic to higher organisms. In the liver, ammonia and carbon dioxide are used to produce a water-soluble form of nitrogen, urea, via the urea cycle. The liver passes this urea to the blood, which carries it to the kidneys to be filtered out and excreted in the urine. Since one function of the kidney is to collect and excrete urea, increases in the concentration of this compound in the blood are an indicator of poor kidney function. Since urea is formed in the liver, low blood urea nitrogen is often the consequence of impaired liver function due to disease or as the result of infection (hepatitis). 

The urea cycle is responsible for the excretion of some 80% of the body s excreted nitrogen in the form of urea this is generated in the liver (Figure 6.10). 

Sodium benzoate Sodium benzoate has already been successfully used to treat congenital enzymatic defects of the urea synthesis (M. L. Batshaw et at, 1981). In HE, the administration of sodium benzoate or sodium phenylacetate (each 10 g/day) achieved clinical success equivalent to lactulose therapy in ca. 80% of cases. (122, 144, 159) These substances (which are cheap and practically free of side effects) lead to an increased excretion of nitrogen through the conjugation of glycine and glutamine, whereby ammonium is bound as hippuric acid and excreted renally (bypassing the urea cycle). 

A26. About 80% of the excreted nitrogen is in the form of urea, which is also largely made in the liver, in a series of reactions that are distributed between the mitochondrial matrix and the cytosol. The series of reactions that form urea is known as the Urea Cycle or the Krebs-Henseleit Cycle. 

A-28. The urea cycle operates only to eliminate excess nitrogen. On high-protein diets the carbon skeletons of the amino acids are oxidized for energy or stored as fat and glycogen, but the amino nitrogen must be excreted. 

To facilitate this process, enzymes of the urea cycle are controlled at the gene level. When dietary proteins increase significantly, enzyme concentrations rise. On return to a balanced diet, enzyme levels decline. Under conditions of starvation, enzyme levels rise as proteins are degraded and amino acid carbon skeletons are used to provide energy, thus increasing the quantity of nitrogen that must be excreted. 

FJGURE, 20 Eliminiitian of ammonium ions via the urea cycle or via direct excretion. With protein catabolism, the excretion of wafste nitrogen via the urea cycle results in net prod union of acid in the body however, excretion of ammonium ions by the kidney into the urine does not result in this production of acid in the body. 

In mammals, hepatic arginase is the terminal enzyme of the urea cycle, which represents the major end-product of nitrogen metabolism — the average adult human excretes some 10 kg of urea per year. The enzyme is not restricted to the liver, since ornithine is a precursor of the nonessential amino acid proUne, and a biosynthetic precursor of polyamines, required for rapidly dividing tissues. Arginine is also the precursor of the important messenger in many vertebrate signal-transduction pathways nitric oxide, NO (Scheme 16.1), of which more shortly. 

The fact that relatively large amounts of urea can be produced by subjects in whom an enzyme defect of the urea cycle has been proved has perplexed many investigators. Despite the fact that the activity of the rate-limiting enzyme has been reduced even to zero as measured by a sophisticated method, urea production still continues. It must therefore be concluded that, in all these cases, urea production is only impaired, not abolished. In all normal circumstances, all but a small fraction of the nitrogen in excess of tissue protein requirements is still excreted as urea and/or as the amino acid whose further metabolism is blocked. The impairment shows itself in the elevated levels of blood ammonia and consequently of glutamine, which vary according to the stress placed upon the urea cycle by varying the rates of protein intake. 

Urea cycle The series of reactions that occur in the liver to synthesize urea for the excretion of nitrogen. The two nitrogen atoms present in urea arise from ammonium ion and the a-amino group of aspartate. The cycle also requires CO (HCO3) and the expenditure of four high-energy phosphate bonds and produces fumarate. 

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