Urea cycle enzymes and

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. 

Integrates the latest on regulation of reactions throughout the chapter, with new material on genetic defects in urea cycle enzymes, and updated information on the regulatory function of N-acetylglutamate synthase. 

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... 

Mitochondrial liver (periportal pericentral all urea cycle enzymes) and much less intestine (enzyme not expressed in red or white blood cells or fibroblasts)... 

The liver also contains pathways for both the synthesis and the oxidation of fatty acids as well as the urea cycle enzymes and many enzymes involved in amino acid catabolism and amino add synthesis. Other important overall reactions catalysed by the liver are the formation of ketone bodies, cholesterol and bile acid synthesis, the synthesis and breakdown of triglycerides. 

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. 

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... 

We begin this overview of manganese biochemistry with a brief account of its role in the detoxification of free radicals, before considering the function of a dinuclear Mn(II) active site in the important eukaryotic urea cycle enzyme arginase. We then pass in review a few microbial Mn-containing enzymes involved in intermediary metabolism, and conclude with the very exciting recent results on the structure and function of the catalytic manganese cluster involved in the photosynthetic oxidation of water. 

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...

Figure 10.10 The use of benzoate and phenylacetate to lower the concentration of ammonia in patients with a deficiency of a urea cycle enzyme.

FIGURE 18-14 Treatment for deficiencies in urea cycle enzymes. The aromatic acids benzoate and phenylbutyrate, administered in the diet, are metabolized and combine with glycine and glutamine, respectively. The products are excreted in the urine. Subsequent synthesis of glycine and glutamine to replenish the pool of these intermediates removes ammonia from the bloodstream. 

Symptoms include tremors, slurring of speech, somnolence, vomiting, cerebral edema, and blurring of vision. All inherited deficiencies of urea cycle enzymes cause mental retardation. 

NH3 is the actual substrate for the first reaction in the urea cycle. The overall process requires an energy equivalent of four ATP per molecule of urea formed. The first two reactions in the urea cycle (Figure 20.9) take place in the mitochondria, and the rest takes place in the cytosol. Liver is the only organ that contains all the urea cycle enzymes in sufficient quantity to generate substantial quantities of urea. However, other organs may have individual urea cycle enzymes, so there is an extensive traffic of urea cycle intermediates from one organ to another. 

An adolescent went into delirium after eating a high-protein meal. He had an extremely high blood ammonia level and excreted orotic acid and uracil in the urine. His blood also contained increased glutamine and lysine levels. Despite heroic symptomatic treatment, the patient expired after 2 weeks. His liver tissue showed normal urea cycle enzyme levels, except that for ornithine transcar-bamylase (OTC), whose activity was only 10% that of normal liver. The Km of OTC was normal. Liver carabmylphosphate levels were about 10 times normal. Theoretical computer simulations indicated that urea can be produced at normal rates when liver OTC levels are higher than 0.3% of normal. 

Brusilow SW, Horwich AL Urea cycle enzymes, in Scriver C, Beaudet A, Sly W, et al. (eds) The Metabolic and Molecular Bases of Inherited Disease. 8th ed. McGraw-Hill, New York, 2001, pp. 1909-1963. 

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). 

Ryall J, Nguyen M, Bendayn M, Shore GC. Expression of nuclear genes encoding the urea cycle enzymes, carbamoyl-phosphate synthetase I and ornithine car-bamyl transferase, in rat liver and intestinai mucosa. Eur J Biochem 1985 152 287-92. 

Animal and human studies have shown that an elevated concentration of ammonia (hyperammonemia) exerts toxic effects on the central nervous system. There are several causes, both inherited and acquired, of hyperammonemia. The inherited deficiencies of urea cycle enzymes are the major cause of hyperammonemia in infants. The two major inherited disorders are those involving the metabolism of the dibasic amino acids lysine and ornithine and those involving the metabolism of organic acids, such as propionic acid, methylmalonic acid, isovaleric acid, and others (see Chapter 55). 

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. 

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... 

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