Proteins urea cycle

All defects in urea synthesis result in ammonia intoxication. Intoxication is more severe when the metabolic block occurs at reactions 1 or 2 since some covalent linking of ammonia to carbon has already occurred if citrulline can be synthesized. Clinical symptoms common to all urea cycle disorders include vomiting, avoidance of high-protein foods, intermittent ataxia, irritability, lethargy, and mental retardation. The clinical features and treatment of all five disorders discussed below are similar. Significant improvement and minimization of brain damage accompany a low-protein diet ingested as frequent small meals to avoid sudden increases in blood ammonia levels. 

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 cycle defects Failure to convert ammonia to urea via urea cycle (Fig. 40-5). Coma, convulsions, vomiting, respiratoryfailure in neonate. Often mistaken for sepsis of the newborn. Mental retardation, failure to thrive, lethargy, ataxia and coma in the older child. Associated with hyperammonemia and abnormalities of blood aminogram Low protein diet Acylation therapy (sodium benzoate, sodium phenylacetate) Arginine therapy in selected syndromes Hepatic transplantation... 

Treatment of aminoacidurias with a low-protein diet may influence brain chemistry. It should be emphasized that the treatment of the patient with an aminoaciduria may affect brain chemistry, perhaps in an adverse manner. Nearly all patients receive a low-protein diet. Indeed, undiagnosed patients sometimes avoid consumption of protein, which they feel intuitively can cause lethargy, headache, nausea and mental confusion. As dietary protein declines, the intake of carbohydrate frequently increases. The concomitant rise of endogenous insulin secretion favors an increase in the ratio of the concentration of blood tryptophan to that of other amino acids, thereby promoting the entry of tryptophan to the brain. The latter amino acid is precursor to brain serotonin, which tends to increase. This physiology is known to be operative in patients with urea cycle defects. 

Cheung, C.W., Cohen, N.G., Raijman, L. (1989). Channeling of urea cycle intermediates in situ in permeabilized hepatocytes. J. Biol. Chem. 264,4038-4044. Cohen, P.P. (1954). Nitrogen metabolism of amino acids. In Chemical Pathways in Metabolism (Greenberg, D.M., Ed.), Vol. 2, pp. 1-46. Academic Press, New York. Fisher, R.B. (1954). Protein Metabolism. Methuen, London. 

An increase in the protein content of the diet in rats increases the maximnm activities of all the enzymes of the cycle in the liver. It is assnmed that this represents increased amonnts of these enzymes in the liver (Table 10.4). Since a chronic increase in the protein in the diet in hnmans increases urea production over a long period and also a decrease in protein in the diet decreases urea production, it is assnmed that, as in the rat, this is due to changes in the concentrations and therefore activities of urea cycle enzymes. 

Table 10.4 Chronic effects of high and zero protein diets on maximum activities of urea cycle enzymes in the liver of the rat...

Only a few important representatives of the non-proteinogenic amino acids are mentioned here. The basic amino acid ornithine is an analogue of lysine with a shortened side chain. Transfer of a carbamoyl residue to ornithine yields citrulline. Both of these amino acids are intermediates in the urea cycle (see p.l82). Dopa (an acronym of 3,4-dihydroxy-phenylalanine) is synthesized by hydroxyla-tion of tyrosine. It is an intermediate in the biosynthesis of catecholamines (see p.352) and of melanin. It is in clinical use in the treatment of Parkinson s disease. Selenocys-teine, a cysteine analogue, occurs as a component of a few proteins—e.g., in the enzyme glutathione peroxidase (see p.284). 

Ornithine is a metabolically quite active amino acid, and the important precursor of pyrrolidine nucleus, which is found in pyrrolizidine alkaloids. Ornithine itself is a non-protein amino acid formed mainly from L-glumate in plants, and synthesized from the urea cycle in animals as a result of the reaction catalyzed by enzymes in arginine. 

A series of supernatant transfer studies uncovered the fact that HC also produce NO, and in relatively large quantities (Curran et al., 1989). A supernatant generated from KC 8 hr after exposure to LPS and interferon-gamma (IFN-y) stimulated HC -NO biosynthesis and caused a simultaneous decrease in protein synthesis. As seen in the coculture model, a 6- to 8-hr delay in measurable NOj + N03 synthesis was seen in the HC exposed to conditioned KC supernatant. However, unlike KC or KC + HC where equimolar concentrations of citrulline were released into the culture supernatant, no increase in citrulline release paralleled the N02 + N03 synthesis by stimulated HC. It is most likely that the citrulline enters the HC urea cycle and is not secreted. 

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. 

FIGURE 3-8 Uncommon amino acids, (a) Some uncommon amino acids found in proteins. All are derived from common amino acids. Extra functional groups added by modification reactions are shown in red. Desmosine is formed from four Lys residues (the four carbon backbones are shaded in yellow). Note the use of either numbers or Creek letters to identify the carbon atoms in these structures, (b) Ornithine and citrulline, which are not found in proteins, are intermediates in the biosynthesis of arginine and in the urea cycle. 

These changes in demand for urea cycle activity are met over the long term by regulation of the rates of synthesis of the four urea cycle enzymes and carbamoyl phosphate synthetase I in the liver. All five enzymes are synthesized at higher rates in starving animals and in animals on veiy-high-protein diets than in well-fed animals eating primarily carbohydrates and fats. Animals on protein-free diets produce lower levels of urea cycle enzymes. 

Formation of citrulline Ornithine and citrulline are basic amino acids that participate in the urea cycle. [Note They are not incorporated into cellular proteins, because there are no codons for these amino acids (see p. 429).] Ornithine is regenerated with each turn of the urea cycle, much in the same way that oxaloacetate is regenerated by the reactions of the citric acid cycle (see p 109). The release of the high-energy phosphateof carbamoyl phosphate as inorganic phosphate drives the reaction in the forward direction. The reaction product, citrulline, is trans ported to the cytosol. 

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. 

Correct answer = D. Methionine is the precursor of cysteine. An increase in gluconeogenesis releases increased ammonia and results in increased urea production. The essential amino acids leucine and lysine are ketogenic. Ornithine and citrulline are amino acids that are intermediates in the urea cycle, but are not found in tissue proteins. 

An acceleration of protein turnover by thyroxine also has been shown, implying that the hormone may alter various processes by a specific effect on synthesis of certain key proteins Involved in enzymatic reactions, Thus, not only does thyroxine increase the rate of formation of new protein material, hut it also may be responsible for the transformation of non-en/.ymalically active protein Into protein with enzymatic activity. The hormone has also been shown to be capable of acceleration of the synthesis of urea cycle enzymes and probably is essential for the production of a... 

The complete urea cycle as it occurs in the mammalian liver requires five enzymes Argininosuccinate synthase, arginase, and argininosuccinate lyase (which function in the cytosol), and ornithine transcarbamoylase, and carbamoyl phosphate synthase (which function in the mitochondria). Additional specific transport proteins are required for the mitochondrial uptake of L-ornithine, NH3, and HC03 and for the release of L-citrulline. 

The oxidative phosphorylation system contains over 80 polypeptides. Only 13 of them are encoded by mtDNA, which is contained within mitochondria, and all the other proteins that reside in the mitochondrion are nuclear gene products. Mitochondria depend on nuclear genes for the synthesis and assembly of the enzymes for mtDNA replication, transcription, translation, and repair (Tl). The proteins involved in heme synthesis, substrate oxidation by TCA cycle, degradation of fatty acids by /i-oxidalion, part of the urea cycle, and regulation of apoptosis that occurs in mitochondria are all made by the genes in nuclear DNA. 

Urea cycle This pathway converts ammonia, a toxic nitrogen-containing waste product of protein metabolism, into another, less toxic molecule called urea, which can be eliminated from the body as urine. 

One of the nitrogen atoms of urea comes from ammonia, the other is transferred from the amino acid aspartate, while the carbon atom comes from C02. Ornithine, an amino acid that is not in the standard set of 20 amino acids and is not found in proteins, is the carrier of these nitrogen and carbon atoms. Five enzymatic reactions are involved in the urea cycle (Fig. 1), the first two of which take place in mitochondria, the other three in the cytosol ... 

In recent studies(23,26) attempts were made to normalize the tumor-induced abnormalities in the levels of hepatic enzymes. It is known that catalysts of the urea cycle are induced by high protein diet, while high carbohydrate intake, decreasing the levels of alanine aminotransferase, increases those of malic enzyme and pyruvate kinase. In each case, the same percent change was seen in tumor-bearing and normal adult animals (Table V), so that the tumor-induced defiency of such enzymes seen in rats on a normal diet was also apparent on these altered diets. Hormone treatments,... 

The entry of activated ammonia into the urea cycle occurs by the ornithine transcarbamoylase reaction where the carbamoyl group is transferred to the side chain amino group of the non-protein amino acid, ornithine. Ornithine has five carbons its carbon chain therefore has the same length as that of arginine. The product of the ornithine transcarbamoylase reaction is the amino acid citrulline. 

The most abundant amino add in the human organism does not occur in proteins and does not have a carboxyl group. Its addic residue is the sulfonate group, and its name is taurine (N+H3-CH2-CH2-S03 ). It occurs in the free state (exact function often unknown) and in bile salts, in which it plays an important role in fat digestion and absorption (see Chapters 9 and 19). Other amino acids that do not occur in proteins are ornithine and citrulline. They are important intermediates in the urea cycle described in Chapter 20. 

A number of amino acid transport disorders may be associated with one or several of the systems described in Table 20.4. These are characterized by the excretion of amino acids in the urine but no increase in amino acid levels in the bloodstream. They are usually of hereditary origin. The most common disorder is cystinuria, characterized by the excretion of cystine. Because cystine is only slightly water soluble, cystinuria is often accompanied by the deposition of cystine-containing stones in the genitourinary tract. Cystinuria is apparently caused by a defect in the cationic amino acid transport system. Another disease that affects this system is lysinuric protein intolerance, which is associated with a failure to transport lysine, ornithine, arginine, and citrulline across membranes. Citrulline and ornithine are urea cycle intermediates (see later), and a disruption of their interorgan traffic results in hyperammonemia. 

---