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Formation of Urea and Ornithine

This irreversible reaction is catalyzed by arginase in the cytosol  [Pg.343]

The urea so formed is distributed throughout the body water and excreted. The renal clearance of urea is less than the glomerular filtration rate because of passive tubular back-diffusion. Diffusion of urea in the intestine leads to formation of ammonia, which enters the portal blood and is converted to urea in liver. Reentry of ornithine into mitochondria initiates the next revolution of the urea cycle. Ornithine can be converted to glutamate-y-semialdehyde (which is in equilibrium with its cyclic form A -pyrroline-5-carboxylate) by ornithine aminotransferase and de-carboxylated to putrescine by ornithine decarboxylase. Ornithine is also produced in the arginine-glycine trans-amidinase reaction. [Pg.343]

The availability of substrates (ammonia and amino acids) in the liver determines the amount of urea synthesized. Urea excretion increases with increased protein intake and decreases with decreased protein intake. [Pg.343]


This enzyme [EC 3.5.3.1], also referred to as arginine amidinase and canavanase, catalyzes the hydrolysis of arginine resulting in the formation of urea and ornithine. The enzyme, which uses a manganese ion as a cofactor, will also utilize a-/V-substituted E-arginines and canavan-ine as substrates. [Pg.63]

From these facts, Ejcbs has explained urea formation in the animal body in terms of an ornithine cycle made up of three stages (i.) Formation of citruUine by condensation of one molecule of ammonia and one of carbon dioxide with the 8-amino group of ornithine, (ii.) formation of arginine by condensation of a second molecule of ammonia with the citrulline, (iii.) decomposition of arginine by arginase, with formation of urea and ornithine, which rejoins the cycle. [Pg.386]

Figure 29-9. Reactions and intermediates of urea biosynthesis. The nitrogen-containing groups that contribute to the formation of urea are shaded. Reactions and occur in the matrix of iiver mitochondria and reactions , , and in iiver cytosoi. COj (as bicarbonate), ammonium ion, ornithine, and cit-ruiiine enter the mitochondriai matrix via specific carriers (see heavy dots) present in the inner membrane of iiver mitochondria. Figure 29-9. Reactions and intermediates of urea biosynthesis. The nitrogen-containing groups that contribute to the formation of urea are shaded. Reactions and occur in the matrix of iiver mitochondria and reactions , , and in iiver cytosoi. COj (as bicarbonate), ammonium ion, ornithine, and cit-ruiiine enter the mitochondriai matrix via specific carriers (see heavy dots) present in the inner membrane of iiver mitochondria.
The paper by Krebs and Henseleit (1932), Experiments on the Formation of Urea in Animal Bodies (Klinische Wochenschrift 11,759), contained the phrases... in the synthesis of urea in the living cell, ornithine acts like a catalyst. We therefore draw the conclusion. .. that the primary reaction for the synthesis of urea from ammonia is... [Pg.106]

Cohen and Grisolia then concentrated on the first step in the reaction, obtaining citrulline from ornithine. The reaction appeared to depend on oxygen, a requirement traced to the need for high concentrations of ATP. Physiologically the formation of urea occurs at very low levels of ammonia, which is extremely toxic as it is also lipid soluble and enters cells very easily. Cells are not very effectively buffered against OH. ... [Pg.107]

An anomaly associated with citrulline that became evident when detailed kinetic studies were made in the 1950s (R.B. Fisher and J.R. Bronk) was the irreproducibility of its catalytic activity in liver slices on the formation of urea, despite the clear evidence from Ratner and Petrack of its importance in arginine synthesis. Initially the discrepancy in catalytic activity between ornithine and citrulline was ascribed to the possible impermeability of the liver cell plasma membrane to the latter intermediate, a hypothesis which was rapidly disproved experimentally. Only recently has it been shown that ornithine transcarbamylase is clearly associated with the ornithine/... [Pg.108]

Ornithine and citrulline are amino acids, but they are not used as building blocks of proteins. The formation of NH4 + by glutamate dehydrogenase, its incorporation into carbamoyl phosphate, and the subsequent synthesis of citrulline take place in the mitochondrial matrix. In contrast, the next three reactions of the urea cycle, which lead to the formation of urea, take place in the cytosol. [Pg.960]

Canaline is the product of the hydrolytic cleavage of canavanine with the simultaneous formation of urea. Canaline is an ornithine analogue which also shows neurotoxicity in the adult sexta where it adversely affects central nervous system functions (jj ). It also is a potent inhibitor of vitamin B -containing enzymes (20-22). It forms a stable Schiff base with the pyridoxal phosphate moiety of the enzyme and drastically curtails enzymatic activity. Pyridoxal phosphate-containing enzymes are vital to insects because they function in many essential transamination and decarboxylation reactions. Ornithine is an important metabolic precursor for insect production of glutamic acid and proline (23). [Pg.123]

The human body excretes nitrogen in the form of urea, NH2CONH2. The key biochemical step in urea formation is the reaction of water with arginine to produce urea and ornithine ... [Pg.107]

Arginine is synthesized from aspartate and ornithine during the formation of urea. Argininosuccinate synthetase and arginino succinate lyase catalyze the condensation and cleavage reactions, respectively, that result in the formation of arginine. [Pg.441]

The so-called ureide plants use allantoin and allantoic acid as a nitrogen store from which ammonia may be liberated by further degradation (E 2.2), In the liver of lungfish a glycine-allantoin cycle (Fig. 182) causes the formation of urea (cf. the formation of urea via L-ornithine derivatives, D 19). [Pg.321]

Ethyl carbamate in wine is formed (mostly at the end of fermentation) from urea. The intermediates of its degradation are probably cyanates and cyanic acid (HO-C=N), also known as hydrogen cyanate, which may isomerise to isocyanic acid (H-N=C=0). Iso-cyanic acid can also arise by protonation of the cyanate anion and nucleophilic addition of ethanol to isocyanic acid yields ethyl carbamate. Isocyanic acid also reacts with other nucleophilic reagents, such as water (with formation of ammonia and carbon dioxide), thiols and amino groups of proteins. By catalysis with ornithinecar-bamoyl transferase, citrulline is transformed into ornithine and carbamoyl phosphate, the ethanolysis of which yields ethyl carbamate (Figure 12.39). [Pg.952]

London and his colleagues (1937) were unable to confirm the operation of the Krebs cycle in the living animal, and report negative results following the perfusion with ornithine and citrulline. Ikeda (1938) claims that perfusion of dog liver by blood containing citrulline and ammonia results in formation of urea. [Pg.387]

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. [Pg.251]

Urea formation in the liver starts with the multistep conversion of ornithine to arginine (fig. 22.7). This is followed by the breakdown of arginine into ornithine and urea. The cyclic nature of this pathway was first appreciated by Hans Krebs and Henseleit in 1932. In subsequent years the impor-... [Pg.517]

In some cases, the function of the metal ion is more to deactivate alternative sites of reaction than to activate a particular atom towards attack by an electrophile. A good example of this is seen in the transamination reaction of ornithine (5.12) with urea. Co-ordination of the ornithine to copper(n) results in the formation of a five-membered chelate ring, leaving the amino group of the 3-aminopropyl substituent as the most nucleophilic site in the complex. Reaction of this complex with urea results in a transamination process and the formation of the copper(n) complex of the substituted urea, which is the amino acid citrulline (5.13) (Fig. 5-20). The complex may be demetallated to yield the free amino acid in respectable yields. [Pg.100]

Urea is synthesized via the urea cycle (Fig. 18-1). In 1932, Krebs and Henseleit pubEshed data demonstrating that ornithine stimulates the synthesis of urea without stoichiometric consumption of this intermediate. This apparent catalytic function was determined to be the result of the cycEc nature of the pathway. This was a revolutionary idea since metabolic pathways were conceptualized as purely linear prior to the pubEcation of these observations. In the foUowing sections, we discuss the biochemical processes involved in urea formation. [Pg.198]

Lysinoalanine formation is not restricted to alkaline conditions—it can also be formed by prolonged heat treatment. Any factor favoring lower pH and less drastic heat treatment will reduce the formation of lysinoalanine. Hurrell (1984) found that dried whole milk and UHT milk contained no lysinoalanine and that evaporated and sterilized milk contained 1,000 ppm. More severe treatment with alkali can decompose arginine into ornithine and urea. Ornithine can combine with dehydroalanine in a reaction similar to the one giving lysinoalanine and, in this case, omithinoalanine is formed. [Pg.100]


See other pages where Formation of Urea and Ornithine is mentioned: [Pg.58]    [Pg.332]    [Pg.343]    [Pg.58]    [Pg.332]    [Pg.343]    [Pg.678]    [Pg.105]    [Pg.68]    [Pg.667]    [Pg.1376]    [Pg.519]    [Pg.402]    [Pg.280]    [Pg.71]    [Pg.350]    [Pg.667]    [Pg.571]    [Pg.456]    [Pg.25]    [Pg.84]    [Pg.589]    [Pg.307]    [Pg.43]    [Pg.515]    [Pg.4]    [Pg.14]    [Pg.104]    [Pg.105]    [Pg.843]    [Pg.383]   


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