Ammonia deamination

Urea is dehydrated to cyanamide which trimerizes to melamine in an atmosphere of ammonia to suppress the formation of deamination products. The ammonium carbamate [1111-78-0] also formed is recycled and converted to urea. For this reason the manufacture of melamine is usually integrated with much larger facilities making ammonia and urea. 

While ammonia, derived mainly from the a-amino nitrogen of amino acids, is highly toxic, tissues convert ammonia to the amide nitrogen of nontoxic glutamine. Subsequent deamination of glutamine in the liver releases ammonia, which is then converted to nontoxic urea. If liver function is compromised, as in cirrhosis or hepatitis, elevated blood ammonia levels generate clinical signs and symptoms. Rare metabolic disorders involve each of the five urea cycle enzymes. 

Urea biosynthesis occurs in four stages (1) transamination, (2) oxidative deamination of glutamate, (3) ammonia transport, and (4) reactions of the urea cycle (Figure 29-2). 

Phenylalanine ammonia-lyase (PAL EC 4.3.1.5) is a pivotal enzyme in controlling flow of carbon from aromatic amino acids to secondary aromatic compounds (Figure 1) (28). PAL primarily deaminates phenylalanine to form t-cinnamic acid, however, in many species, it also less efficiently deaminates tyrosine to form -coumaric acid. Because PAL is restricted to plants and is an important enzyme in plant development, Jangaard (29) suggested that PAL inhibitors might make safe and effective herbicides, however, in his screen of several herbicides, he found no compound to have a specific effect on PAL. This was also the case in studies by Hoagland and Duke (30, 31.) in which 16 herbicides were screened. 

Two-step synthetic routes to poly(/i-aminoborazines) from /i-chloroborazines involve initial nucleophilic reaction of the /i-chloroborazine with appropriate linking reagents followed by a deamination reaction of the as-obtained /i-aminoborazine. The 5-tiichloroborazine undergoes nucleophilic attack by ammonia or amine derivatives on the boron atom linked to chlorine atoms. For the same reasons previously quoted a tertiary amine (e.g., Et3N) must be added to precipitate the corresponding hydrochloride. 

Nitrogen is dumped into the urea cycle by transamination to make Asp or Glu or by deamination to make ammonia. 

The key reaction that links primary and secondary metabolism is provided by the enzyme phenylalanine ammonia lyase (PAL) which catalyzes the deamination of l-phenylalanine to form iran.v-cinnamic acid with the release of NH3 (see Fig. 3.3). Tyrosine is similarly deaminated by tyrosine ammonia lyase (TAL) to produce 4-hydroxycinnamic acid and NH3. The released NH3 is probably fixed by the glutamine synthetase reaction. These deaminations initiate the main phenylpropanoid pathway. 

In the first enzymatic step, phenylalanine ammonia lyase (PAL) converts phenylalanine to trans cinnamate, via a deamination reaction liberating ammonia. PAL can also convert tyrosine to p-coumarate, albeit at lower efficiency (MacDonald and D Cunha 2007). PAL functions as a tetramer of identical subunits, with two subunits combining to form one active site (Stafford 1990 MacDonald and D Cunha 2007). 

Deficiency of the muscle-specific myoadenylate deaminase (MADA) is a frequent cause of exercise-related myopathy and is thought to be the most common cause of metabolic myopathy. MADA catalyzes the deamination of AMP to IMP in skeletal muscle and is critical in the purine nucleotide cycle. It is estimated that about 1-2% of all muscle biopsies submitted to medical centers for pathologic examination are deficient in AMP deaminase enzyme activity. MADA is 10 times higher in skeletal muscle than in any other tissue. Increase in plasma ammonia (relative to lactate) after ischemic exercise of the forearm may be low in this disorder, which is a useful clinical diagnostic test in patients with exercise-induced myalgia... 

The experiments described above indicated amino acids were oxidatively deaminated in liver and their a-amino groups converted to urea. A start on investigations of the mechanism of urea biosynthesis was made by Schultzen and Nenki (1869) who concluded that amino acids gave rise to cyanate which might combine with ammonia from proteins to produce urea. Von Knieren (1873) demonstrated that when he drank an ammonium chloride solution, or gave it to a dog, there was an increase in the formation of urea, without any rise in urinary ammonia. His results were consistent with the cyanate theory but did not eliminate the possibility that urea arose from ammonium carbonate which could be dehydrated to urea ... 

Besides investigating the reactions by which ammonia was converted to urea, Krebs also turned his attention to the origins of the ammonia. An oxidase was discovered (1932-1933) which catalyzed oxidative deamination ... 

The 2-oxoglutarate produced is recycled for transamination or may enter the TCA cycle. The ammonia liberated by oxidative deamination is used to form glutamine (from glutamate, catalysed by glutamine synthase) prior to export from the muscle cell ... 

The flavoprotein amino acid oxidases (AAOs) catalyze an essentially irreversible oxidative deamination of an amino acid. Molecular oxygen is the oxidant and the products are ammonia, the oxoacid, and H2O2 (Equation (1)). 

Similar to the AAOs, the aaDHs catalyze oxidative deamination, forming an oxoacid and ammonia. However, rather than using enzyme-hound FAD as the oxidant, followed hy O2, these enzymes employ nicotinamide cofactors, NAD or NADP, in free solution (Equation (3)). 

Eubacterium acidaminophilum not only reductively deaminates glycine to ammonia and acetate but also expresses enzymes capable of reductively deaminating sarcosine and betaine when cells are cultured in the presence of formate (Hormann and Andreesen 1989). One would expect that the enzymes catalyzing the latter two reactions might be similar to GR. In fact, the substrate-specific protein B for sarcosine reductase was purified and found to be similar to GR protein B from C. sticklandii (Meyer et al. 1995). Apparently, this organism has evolved means to use different amino acids as electron acceptors and preferentially expresses each in response to conditions in the environment. 

The fate of the glutamate is re-formation of oxoglntarate in the deamination reaction catalysed by glntamate dehydrogenase, in which the NH2 gronp in glntamate is removed as ammonia and the oxoglntarate is formed, as follows ... 

This combination of reactions is known as transdeamination and is the mechanism for deamination of a number of amino acids (Table 8.9). The role of this process in catabolism is shown in Figure 8.10. The ammonia that is prodnced is converted, almost exclnsively, to urea for excretion. Becanse of the biochemical and clinical significance of ammonia, a whole chapter is devoted to it and to urea formation. 

The physiological relevance together with chnical importance of transamination and deamination is wide-ranging. As an aid to understanding the somewhat complex nature of amino acid metabolism, it can be considered (or imagined) as a metabolic box (represented in Figure 8.13). Some pathways feed oxoacids into the box whereas others remove oxoacids and the ammonia that is released is removed to form urea. The box illustrates the role of transdeamination as central to a considerable amount of the overall metabolism in the liver cell (i.e. protein, carbohydrate and fat metabohsm, see below). 

Within a cell, a nncleotidase catalyses the hydrolysis of either a ribonncleotide or deoxyribonucleotide (Fignre 10.8). The qnantitatively important pathway for degradation of AMP in liver and mnscle involves deamination to IMP, catalysed by AMP deaminase, producing ammonia, and snbseqnent hydrolysis of IMP to inosine. This may be an important sonrce of inosine for synthesis of phosphati-dylinositol, a key phospholipid in membranes. 

Attempts to establish the structure of the initial adduct by NMR-spectroscopy failed because of the low solubility of 27. This makes it impossible to draw a clear conclusion as to whether the ammonia adds to C-6 (as occurs in the case of the A-methylpyrimidinium salts) or at C-2. Since NMR spectroscopy of a solution of 4,6-diphenylpyrimidine in potassium amide/liquid ammonia strongly supports the formation of an anionic C-2 adduct (75UP1], it is justified to assume that also in the deamination of 27 by liquid ammonia, a C-2 adduct 28 is involved (Scheme III. 16). It is evident that the major part of the deamination (73%) does not involve a ringopening reaction the main deamination reaction occurs by an Sn2 attack of ammonia on the A-amino group in 27. A similar mechanism has also been postulated in the deoxygenation of pyrimidine A-oxides, when they are heated with liquid ammonia (Scheme III.16) [77UP2]. 

Deamination into 2,4,6-trimethylpyrimidine, together with a simultaneous formation of 3,5-dimethyl-l,2,4-triazole, occurs when A-amino-2,4,6-trimethylpyrimidinium mesitylenesulfonate reacts with liquid ammonia... 

Amino-l,5-diphenyltriazole is deaminated by diazotization in ethanolic solution and warming. 1-Aminotriazoles are deaminated in high yield by treatment with nitrous acid. Removal of a toluene-p-sulfonamido group can be accomplished in two steps by acid hydrolysis followed by diazotization, or in one step by treatment of the 1-toluene-p-sulfonamide derivative with sodium in liquid ammonia. ... 

Oxidative deamination basically resembles the dealkylation of tertiary amines, beginning with the formation of a hydroxylamine that then decomposes into ammonia and the corresponding aldehyde. The latter is partly reduced to an alcohol and partly oxidized to a carboxylic acid. 

Dichloro-9-[2-benzoyloxyethoxymethyl]purine was prepared as a key intermediate, which upon selective substitution of the 6-chloro group by ammonia followed by deamination and then displacement of the 2-chloro group gave 9-(2-hydroxyethoxymethyl)guanine. The use of the same synthetic procedure led to a variety of analogs. Of these ACV was found to... 

This enzyme [EC 3.5.3.13] catalyzes the hydrolytic deamination of iV-formimino-L-glutamate to yield iV-formyl-L-glutamate and ammonia. 

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