Enzymes urea cycle

The use of some additional enzyme tests has been limited by the lack of available commercially prepared reagents or methodologies unsuitable for the current generation of automated biochemical analyzers. Older manual methods, which were less precise, have largely been discarded, and they are not commonly used in human medicine. There are four broad effects on plasma enzymes in hepatotoxicity  

In animals (and humans), plasma ALT is an effective marker of liver damage and it should always be measured, although it is not entirely specific for liver (see 

Chapter 2). Plasma ALT can increase or decrease following microsomal enzyme induction effects in the rat and the dog (Amacher, Schomaker, and Burkhardt 1998 Amacher et al. 2001) and when there is a heavy fatty infiltration of hepatic cells, where plasma enzymes may reflect the displacement of cytoplasm as the fat load increases. This enzyme may also be increased in biliary toxicity and following bile duct obstruction through the effects of bile salts on the neighboring hepatocyte cell membranes. ALT can be affected by food intake and stress (Chapter 11). Plasma ALT is the most useful enzyme for detecting hepatocellular injury in most laboratory animal species. 

Plasma AST measurements are a useful adjunct to ALT, and increased AST can be an indication of mitochondrial and cytoplasmic injury, although this enzyme is less specific for hepatotoxicity compared to ALT. Sustained increases of the plasma aminotransferase levels can indicate a progressive injury, but the aminotransferases may not be elevated during acute necrosis if the timing of the sample collection has allowed the increased circulating plasma enzymes to be cleared. Plasma ALT and AST may be decreased when the enzyme cofactor pyridoxal phosphate is reduced in vivo (Dhami et al. 1979 also see Chapter 2). 

The term transaminitis has been use in human medicine to describe mild elevations of the aminotransferases in the absence of other abnormal clinical laboratory findings in asymptomatic individuals, with apparently no supporting evidence from preclinical studies (Balazs, Farber, and Feuer 1978 Amacher 1998). 

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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... 

Deficiency of a Urea Cycle Enzyme Results in Excretion of Pyrimidine Precursors... 

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...

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

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.

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. 

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. 

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. 

Metabolism of nitrogen in a patient with a deficiency in the urea cycle enzyme carbamoyl phosphate synthetase I. Treatment with phenylbutyrate converts nitrogenous waste to a form that can be excreted. 

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

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... 

Christmann R. Valproate-induced coma in a patient with urea cycle enzyme deficiency. Epilepsia 1990 31 228. 

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). 

Inhibitor design. Compound A has been synthesized as a potential inhibitor of a urea-cycle enzyme. Which enzyme do you think compound A might inhibit ... 

Todo, S., Starzl, T.E., Tzakis, A., Benkov, K.J., Kalousek, F., Saheki, T., Tanikawa, K., Fenton, W.A. Orthotopic liver transplantation for urea cycle enzyme deficiency. Hepatology 1992 15 419—422... 

Cause deficiencies of urea cycle enzymes, almost exclusively seen in children. 

Schimke, R, T. (1962). Adaptive characteristics of urea cycle enzymes in the rat. J. Biol Otem. 237,459 68. 

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