Activities of the Urea Cycle Enzymes

Activity of Urea Cycle Enzymes in Normal Human Liver  

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Factors Affecting the Activities of the Urea Cycle Enzymes... 

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. 

The activity of the urea cycle is regulated at the level of enzyme synthesis and by allosteric regulation of the enzyme that catalyzes the formation of carbamoyl phosphate. 

The major enzyme involved in the formation of ammonia in the liver, brain, muscle, and kidney is glutamate dehydrogenase, which catalyzes the reaction in which ammonia is condensed with 2-oxoglutarate to form glutamate (Sec. 15.1). Small amounts of ammonia are produced from important amine metabolites such as epinephrine, norepinephrine, and histamine via amine oxidase reactions. It is also produced in the degradation of purines and pyrimidines (Sec. 15.6) and in the small intestine from the hydrolysis of glutamine. The concentration of ammonia is regulated within narrow limits the upper limit of normal in the blood in humans is 70/tmol L-1. It is toxic to most cells at quite low concentrations hence there are specific chemical mechanisms for its removal. The reasons for ammonia toxicity are still not understood. The activity of the urea cycle in the liver maintains the concentration of ammonia in peripheral blood at 20/ molL. ... 

The accumulation of any of these amino acids could be due to reduced activity of their respective enzymes in the urea cycle (Sec. 15.5), resulting in decreased overall activity of the cycle. Inborn errors of metabolism are known for deficiencies in these enzymes. Decreased activity of the urea cycle results in elevated levels of ammonia in the blood, a condition known as hyperammonemia that causes nausea, vomiting and even coma. 

Hyperammonemia occurs in biotin deficiency and the functional deficiency associated with lack of holocarboxylase synthetase (Section 11.2.2.1) and bio-tinidase (Section 11.2.3.1). In deficient rats, the activity of ornithine carbamyl-transferase is two - thirds of that in control animals, as a result of decreased gene expression, although the activities of other urea cycle enzymes are unaffected (Maeda etal., 1996). 

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... 

In addition to the decreased intestinal absorption of calcium in uremia, calcium ATPase activity is also low (S6). Many gastrointestinal hormones are found in increased concentrations in uremia. These include gastrin (OlO), cholecystokinin (OlO), pepsinogen I (II), gastric inhibitory peptide (LI), amylase, and trypsin (R17). There is a decrease in the conjugation of cholic to deoxycholic acid (G13) and in the pancreatic secretion of bicarbonate (Oil). Hepatic output of urea is decreased, as is the activity of some urea cycle enzymes e.g., ornithine tran-scarbamylase (T9). The clinical significance of these hormonal and enzymatic perturbations remains to be elucidated. 

Condensation of CO2, ammonia, and ATP to form carbamoyl phosphate is catalyzed by mitochondrial carbamoyl phosphate synthase I (reaction 1, Figure 29-9). A cytosolic form of this enzyme, carbamoyl phosphate synthase II, uses glutamine rather than ammonia as the nitrogen donor and functions in pyrimidine biosynthesis (see Chapter 34). Carbamoyl phosphate synthase I, the rate-hmiting enzyme of the urea cycle, is active only in the presence of its allosteric activator JV-acetylglutamate, which enhances the affinity of the synthase for ATP. Formation of carbamoyl phosphate requires 2 mol of ATP, one of which serves as a phosphate donor. Conversion of the second ATP to AMP and pyrophosphate, coupled to the hydrolysis of pyrophosphate to orthophosphate, provides the driving... 

The activity of carbamoyl phosphate synthase I is determined by A -acetylglutamate, whose steady-state level is dictated by its rate of synthesis from acetyl-CoA and glutamate and its rate of hydrolysis to acetate and glutamate. These reactions are catalyzed by A -acetylglu-tamate synthase and A -acetylglutamate hydrolase, respectively. Major changes in diet can increase the concentrations of individual urea cycle enzymes 10-fold to 20-fold. Starvation, for example, elevates enzyme levels, presumably to cope with the increased production... 

Gene therapy for rectification of defects in the enzymes of the urea cycle is an area of active investigation. Encouraging preliminary results have been obtained, for example, in animal models using an adenoviral vector to treat citrullinemia. 

Note that total starvation of rats increases the activities of all the urea cycle enzymes. 

Glucagon stimulates the adenylate cyclase system in the liver and thereby the formation of cAMP, which gives rise to important metabolic changes, (s. tab. 3.10) Furthermore, there is a consequent decline in cholesterol synthesis, improvement in alanine membrane transport, activation of the enzymes of the urea cycle and stimulation of amino acid degradation. 

Although the liver is the principal organ for the conversion of ammonia to urea, it has been demonstrated by Sporn et al. (S15) that this process can occur also in the brain, although the activity of the cycle is small. The urea cycle enzymes were later demonstrated in cerebral tissue by Tomlinson and Westall (T6), but the activities were very small, less than 1%, compared with the liver (R5). In vivo experiments in cats showed that N-labeled ammonia as ammonium acetate injected in the brain was found largely in glutamine. That injected in the body was found mostly as urea and free NH3 in the liver (B3). This suggests that glutamine is more important for the removal of ammonia in the brain whereas in the liver the urea cycle is more important. 

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