Carbamyl phosphate cycle

Hyperammonemia has occurred during parenteral nutrition as a component of therapy for renal insufficiency (905). The hyperammonemia presented as a change in mental status, developing about 3 weeks after initiation of parenteral nutrition therapy in most cases the episodes are of increasing duration and paroxysmal. In three of the patients, serum amino acid analysis in the acute phase showed reduced concentrations of ornithine and citrulline (the respective substrate and product of condensation with carbamyl phosphate at its entry into the urea cycle). Concentrations of arginine, the precursor to ornithine, were raised. 

Production of urea by cestodes suggests the existence of the urea (Krebs-Henseleit) cycle, which is shown in Fig. 6.11. One of the key enzymes, arginase, has been widely reported in cestodes (796, 185-187). However, some of the other enzymes, notably carbamyl phosphate synthetase and ornithine transcarbamyl, are either absent or present in only low amounts (39) and it is doubtful if a complete cycle operates in cestodes. It is likely that the urea excreted by tapeworms comes from the activity of arginase alone. The uric acid produced and excreted by cestodes probably arises from the breakdown of purines (39). 

Fig. 6.11. The (Krebs-Henseleit) ornithine cycle. Numbers refer to enzymes as follows. (1) Carbamyl phosphate synthetase (E.C.2.7.2.a). (2) Ornithine transcarbamylase (E.C.2.1.3.3). (3) Arginino-succinate synthetase (E.C.6.3.4.5). (4) Arginino-succinate lyase (E.C.4.3.2.1). (5) Arginase (E.C.3.5.3.1). (After Smyth, 1969.)...

Biosynthesis and Utilization of Acetyl Phosphate, Formyl Phosphate, and Carbamyl Phosphate and their Relations to the Urea Cycle... 

The uiea cycle may be considered to be a mitochondrial pathway, as carbamyl phosphate synthase and ornithine transcarbamylase are mitochondrial enzymes however, the enzymes catalyzing subsequent steps of the pathway arc cytosolic-The steps leading to conversion of citrulline to ornithine occur in the cytosol. Hence, the pathway is shared by the mitochondrial and cytosolic compartments. The fumarate produced by the urea cycle is converted to malate by a cytoplasmic form of fumarase. Mittxihondrial fumarase is part of the Krebs cycle. Cytoplasmic malate can enter the mitochondrion by means of a transport system, such as the malate/phosphate exchanger or the ma ate/a-ketoglutaratc exchanger. These transport systems are membrane-bound proteins. 

Citrulline is an endogenous amino acid involved in the urea cycle. Clinically, it can be used as an arginine substitute in the treatment of inborn errors of urea synthesis, including carbamyl phosphate synthetase and ornithine transcarbamylase. It is also a diuretic. 

Figure 24-2 The urea cycle pathway. CPS I, Carbamyl phosphate synthetase I N-acetyigiutamate as positive allosteric effector OTC, ornithine transcarbamyiase MS, argininosuccinate synthetase Ai, argininosuccinate iyase AR, arginase ADP, adenosine diphosphate, ATf adenosine triphosphate, P, inorganic phosphate.

Fia. 2. Pathways of ammonia uptake, including urea cycle. CP, carbamyl phosphate ASA, argininosuccinate KG, ketoglutarate. 

Defects of the enzymes mediating all four reactions of the urea cycle proper have now been established, and there is some evidence of the existence of a fifth enzyme defect, involving carbamyl phosphate synthetase, mediating the initial reaction of the pathway. As the first report of a metabolic disorder involving the urea cycle was only in 1958, it is not surprising that there have been very few reviews of this topic, that of Efron (El) being the most complete to date. 

Fig. 5. Diagrammatio representation of intracellular localization of urea cycle pathway showing diffusion pattern. CPS, carbamyl phosphate synthetase ASL, argininosuccinate lyase ASA, argininosucoinate.

It must be noted that only few results have been obtained on fresh biopsy specimens most have been from specimens which have been stored in the frozen state for some time or from specimens of liver which have been removed at necropsy at varying unstated periods after death and kept deep frozen at —15°C for various periods of time before analysis. There is some evidence from our results that at least two, carbamyl phosphate synthetase and ornithine transcarbamylase, of the urea cycle enzyme activities fall off on storage at — 15°C for even 1 day, and this decrease continues over longer periods. Thus carbamyl phosphate synthetase activity in fresh mouse liver is in our experience appreciably higher than in liver kept frozen for some days or weeks. This is borne out by a comparison of the enzyme activities found in human liver obtained by biopsy, measured immediately, after storage at —15°C, and finally in liver obtained at necropsy (Fig. 6). Ornithine transcarbamylase activity in a human biopsy specimen of liver is greater when assayed immediately than when it is kept frozen even a short time or... 

None of these cases can be considered as established examples of an isolated carbamyl phosphate synthetase deficiency. Although in the first the clinical history and the presence of severe hyperammonemia support the diagnosis of a defect of urea synthesis, the normal finding of levels of plasma amino acids, apart from glycine, is against it. No actual numerical data on the level of activity of the urea cycle enzymes are given. 

One of the two —NH2 groups comes in from ammonia, which reacts with CO2 and ATP to form carbamyl phosphate, NH2.COO-phosphate, one of the feed compounds for the cycle. This then reacts with ornithine, shedding the phosphate and forming another new amino acid, citmlline. The second —NH2 comes in, not from ammonia but from aspartic acid (see Topic 23). Reaction of aspartic acid with cit-rulline and another ATP gives argininosuccinic acid (do not worry about either the... 

Rosenthal (1972) was able to show that in crude leaf extracts if sufficient carbamyl phosphate was present, canavanine was continuously synthesized and then degraded to canaline, he proposed a cycle shown in Fig. 7. Such a cycle would require six ATP molecules to drive it to allow for the reassimilation of the ammonia evolved. It is unlikely that the cycle operates in vivo, but that normally within the cell the enzymes are localized within different compartments, probably involved either in canavanine breakdown or synthesis. Whichever section of the cycle operates will depend on the physiological age and nature of the tissue. 

The primary step in the urea cycle is the synthesis of carbamyl phosphate from ammonia and carbon dioxide (11.76). This first stage, and the later stage of synthesis of arginosuccinic acid from citrulline and aspartic acid, both require the transfer of energy from ATP hydrolysis. The pyrophosphate formed in the latter reaction is itself hydrolysed which, together with the former reaction. 

The synthesis of nucleotide triphosphates required for polynucleotide chain building is a complex process which will not be considered in full detail here. The biosynthetic routes for purine and pyrimidine nucleosides are somewhat different and commence with 5 phosphoribosyl-l-pyrophos-phate and carbamyl phosphate, respectively. These two materials undergo successive enzyme-catalysed reactions, linking at times with compounds encountered in other biochemical cycles, and utilising ATP in several stages. Polynucleotides can be synthesised by purely chemical means in the laboratory (Chapter 10.4). 

The st example concerns the intracellulEir localization of the reactions of the ornithine cycle. The enzymes catalyzing two of the reactions, carbamyl phosphate synthetase and ornithine transcarbamylase, Eire found in the mitochondrial matrix while the other three enzymes are in the cjrtosol. As a result, the mitochondrial membrane is interposed as a barrier to the operation of the cycle, necessitating two addi-... 

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