Showing papers on "Urea cycle published in 1989"

Urea excretion as a strategy for survival in a fish living in a very alkaline environment.

Abstract: Ammonia is toxic to all vertebrates. It can be converted to the less toxic urea, but this is a metabolically expensive process found only in terrestrial vertebrates that cannot readily excrete ammonia and marine fish that use urea as an osmotic filler. Freshwater fish mostly excrete ammonia with only a small quantity of urea. It seems the ornithine cycle for urea production has been suppressed in all freshwater teleosts except for some airbreathers which, when exposed to air, increase urea synthesis via the cycle. Here we show that the tilapia fish Oreochromis alcalicus grahami, the only fish living in Lake Magadi, an alkaline soda lake (pH = 9.6-10) in the Kenyan Rift Valley, excretes exclusively urea and has ornithine-urea cycle enzymes in its liver. A closely related species that lives in water at pH 7.1 lacks these enzymes and excretes mainly ammonia with small amounts of urea produced via uricolysis. It dies within 60 min when placed in water from Lake Magadi. We suggest that urea production via the ornithine-urea cycle permits O. a. grahami to survive the very alkaline conditions in Lake Magadi.

Evolutionary aspects of urea cycle enzyme genes.

Abstract: The functions and expression pattern of urea cycle enzymes have undergone considerable changes during the course of evolution. Sequence analyses shows that urea cycle enzymes from mammals are homologous to microbial enzymes of the arginine-metabolic pathway. Recently, an unexpected relationship was found between argininosuccinate lyase (EC 4.3.2.1), the fourth enzyme of the cycle, and delta-crystallin, a lens structural protein of birds and reptiles.

A fourth case of fumarase deficiency.

Abstract: Fumarate hydratase (fumarase, EC 4.2.1.2) has a key role in cellular energy production: in the mitochondria it is a component of the tricarboxylic acid cycle; in the cytoplasm it is presumably a source of malate for the malate-aspartate shuttle, with an important source of fumarate being the urea cycle. Three cases of proven fumarase deficiency (McKusick 13686) have been reported

Excretion of hippuric acid during sodium benzoate therapy in patients with hyperglycinaemia or hyperammonaemia.

Abstract: In patients with non-ketotic hyperglycinaemia and two patients with urea cycle disorders treated with varying doses of sodium benzoate there was linear correlation between intake of benzoate and excretion of hippurate. Patients with non-ketotic hyperglycinaemia excreted significantly more benzoate in the form of hippurate than patients with urea cycle disorders (74 +/- 7.0 vs 41 +/- 3.6%). The plasma concentration of glycine decreased following benzoate treatment only in the patients with non-ketotic hyperglycinaemia. The observed difference between the two groups in the excretion of hippurate seems to support the concept that glycine availability may be limiting in benzoate therapy for some patients.

Orotic aciduria due to arginine deprivation: changes in the levels of carbamoyl phosphate and of other urea cycle intermediates in mouse liver.

Abstract: The orotic aciduria induced in mammals by arginine deprivation is believed to result from accumulation of carbamoyl phosphate in liver, but this accumulation has never been demonstrated in vivo during arginine deprivation. There has been disagreement even on the basal levels of carbamoyl phosphate. In this report we show, using an improved assay, that the hepatic level of carbamoyl phosphate is very low (less than 1.3 nmol/g) in the fasted mouse or after a meal containing a mixture of amino acids including arginine, and that it increases dramatically (up to 180 nmol/g liver) after a meal without arginine. We estimated a fast turnover for carbamoyl phosphate, and we found a marked correlation between liver carbamoyl phosphate and urinary orotate, and also between urinary orotate and intake of an arginine-free diet. These results support the hypothesis that accumulation of carbamoyl phosphate in liver mitochondria, its efflux from this organelle and its utilization by the cytosolic pyrimidine pathway cause the orotic aciduria of arginine deprivation. We assayed liver acetylglutamate (the activator of carbamoyl phosphate synthesis) and several intermediates of the urea cycle and found that low levels of ornithine partly explain the accumulation of carbamoyl phosphate during arginine deprivation. However, acetylglutamate and citrulline were increased, and the potential significance of these changes is discussed.

2-Ethylhexanoic acid inhibits urea synthesis and stimulates carnitine acetyltransferase activity in rat liver mitochondria.

Abstract: Adult male 3-month-old Wistar rats were given 0, 100 mg/l, 1, 5 or 10 g/l 2-ethylhexanoic acid in their drinking water for 20 days. Their daily consumption of contaminated water was measured and compared with the free acid found in their 24-h urine samples. The excretion was dose and time dependent. At the termination of the experiment, liver mitochondrial carnitine acetyltransferase activity was induced dose dependently and the citrulline synthesis in the urea cycle inhibited. Our results compare very well with the toxicity of a structural congener of the 2-ethylhexanoic acid, i.e. valproate, an antiepileptic drug.

Plasma ammonia, plasma, brain and liver amino acids and urea cycle enzyme activities in rats fed ammonium acetate.

Abstract: Male Sprague-Dawley rats were trained to eat a 6% casein diet within a 3-h period each day. They were then fed a 6% casein diet for 10-16 d before they were fed either the same 6% casein diet or the 6% casein diet supplemented with 15% ammonium acetate for 1 or 7 d. During the absorptive period, plasma ammonia, plasma amino acids and brain amino acids were measured on d 1 and d 7 after feeding ammonium acetate. Food intake of rats fed 15% ammonium acetate was depressed on d 1 and increased to approximately 75% of the intake of the 6% casein-fed group by d 7. On d 1 plasma ammonia of the rats fed 5% ammonium acetate was 101 microM as compared to 56 microM for the rats fed 6% casein (P less than 0.05). On d 7, plasma ammonia of the rats fed 15% ammonium acetate was 240 microM (P less than 0.05) as compared to 44 microM for the rats fed 6% casein. In rats fed 15% ammonium acetate, after 7 d ornithine transcarbamylase and arginase activities were higher and argininosuccinate synthetase activity was lower (P less than 0.05) while carbamyl phosphate synthetase activity tended to be higher than that of rats fed 6% casein. The results suggest that rats adapt to ingestion of 15% ammonium acetate by some unknown neural mechanism rather than by increases in all urea cycle enzyme activities. Feeding ammonium acetate causes changes in plasma, brain and liver amino acid concentrations.

Benzoate stimulates glutamate release from perfused rat liver.

Abstract: In isolated perfused rat liver, benzoate addition to the influent perfusate led to a dose-dependent, rapid and reversible stimulation of glutamate output from the liver. This was accompanied by a decrease in glutamate and 2-oxoglutarate tissue levels and a net K+ release from the liver; withdrawal of benzoate was followed by re-uptake of K+. Benzoate-induced glutamate efflux from the liver was not dependent on the concentration (0-1 mM) of ammonia (NH3 + NH4+) in the influent perfusate, but was significantly increased after inhibition of glutamine synthetase by methionine sulphoximine or during the metabolism of added glutamine (5 mM). Maximal rates of benzoate-stimulated glutamate efflux were 0.8-0.9 mumol/min per g, and the effect of benzoate was half-maximal (K0.5) at 0.8 mM. Similar Vmax. values of glutamate efflux were obtained with 4-methyl-2-oxopentanoate, ketomethionine (4-methylthio-2-oxobutyrate) and phenylpyruvate; their respective K0.5 values were 1.2 mM, 3.0 mM and 3.8 mM. Benzoate decreased hepatic net ammonia uptake and synthesis of both urea and glutamine from added NH4Cl. Accordingly, the benzoate-induced shift of detoxication from urea and glutamine synthesis to glutamate formation and release was accompanied by a decreased hepatic ammonia uptake. The data show that benzoate exerts profound effects on hepatic glutamate and ammonia metabolism, providing a new insight into benzoate action in the treatment of hyperammonaemic syndromes.

Nutritional Therapy for Selected Inborn Errors of Metabolism

Abstract: Nutritional approaches are available for the management of several different classes of inborn metabolism errors. In phenylketonuria (PKU), phenylalanine is not properly metabolized; and its accumulation leads to neurologic dysfunction and metal retardation. Altering the diet to limit phenylalanine intake led to remarkable improvement in children with PKU. It was later found that instituting dietary therapy immediately after identification of the disorder in newborns prevented mental retardation. Throughout the 1960s nutritional therapies were found for other inborn disorders, including galactosemia, maple syrup urine disease, and homocystinuria. For the group of disorders associated with defects in the urea cycle, leading to profound hyperammonemia, therapy based on the concept of waste nitrogen excretion (i.e., by increasing excretion of urea cycle intermediates in the urine, nitrogen that would otherwise recycle as ammonia can be eliminated) dramatically produced better control of hyperammonemia and its consequences. Some inborn errors of metabolism respond to vitamin therapy. Biotin-related multiple carboxylase synthetase deficiency can be produced by either of two enzyme defects--holocarboxylase synthetase deficiency or biotinidase deficiency. Both are treatable with biotin supplementation. The symptoms of multiple carboxylase deficiency can also occur after intestinal resection or ingestion of raw eggs. Multiple carboxylase deficiency has been treated successfully in utero by giving the mother biotin supplements. Peroxisomal disorders may respond to dietary management. Liver disease in hereditary tyrosinemia may be accentuated by hypermethioninemia and treated by controlling the blood methionine level. Glycogen storage disease Type I, which causes hypoglycemia, can be controlled by oral administration of cornstarch.

Effect of Vitamin A Deficiency on Nitrogen Balance and Hepatic Urea Cycle Enzymes and Intermediates in Rats

Abstract: After 12 wk of feeding a 4-d nitrogen balance was carried out in 8 vitamin A--deficient and 8 pair-fed control rats to understand the effect of vitamin A deficiency on protein metabolism. Urea nitrogen (UN) was lower and amino nitrogen (AN) was higher in plasma of deficient animals than in pair-fed controls. No significant alteration in the nitrogen retention or in the general pattern of other nitrogen metabolites in plasma and urine was observed in vitamin A--deficient rats as compared to controls. However, there was a significant increase in the excretion of urinary ammonium nitrogen (Am-N) in relation to creatinine (CR). Activities of hepatic carbamoylphosphate synthase-I (CPS-I) and ornithine transcarbamoylase (OTC) were significantly lower in vitamin A--deficient animals than in control rats at the end of 13 wk of feeding. While the liver levels of ornithine and polyamines were significantly greater, that of glutamine was lower in vitamin A--deficient rats than in pair-fed controls. The results suggest that vitamin A deficiency leads to a reduced efficiency of urea synthesis pathway, thus accounting for the increased Am-N excretion seen in vitamin A deficiency.

The role of amino acids and their transport systems in the regulation of ureogenesis in the primary culture of adult rat hepatocytes.

Abstract: NAGAO, M., MORI, T., TSUCHIYAMA, A. and OYANAGI, K. The Role of Amino Acids and Their Transport Systems in the Regulation of Ureogenesis in the Primary Culture of Adult Rat Hepatocytes. Tohoku J. Exp. Med., 1989, 158 (4), 309-316-The biosynthesis of urea, the metabolism of ammonia and the transport of amino acids were studied using the primary culture of hepatocytes from adult rats. The urea synthesis and ammonia detoxication were affected by the amino acids of urea cycle intermediates, such as ornithine, arginine and aspartate. When the hepatocytes were incubated in the medium containing 1mM ammonium chloride, the transport activity of system-A, which was determined by the uptake of specific substrate methyl-2-amino isobutyric acid (MeAIB), did not change compared with the control level. However, the transport activity of ornithine was increased to a maximum after 4hr of incubation with ammonia, and then decreased gradually to twice the control level. The activity of ornithine transcarbamylase (OTC) increased to twice the control. These results indicated that the amino acids of urea cycle intermediates, especially ornithine, can be the important regulators of ureogenesis.

Sub-micromolar concentrations of extramitochondrial Ca2+ stimulate the rate of citrulline synthesis by rat liver mitochondria.

Abstract: 1. In the presence of physiological concentrations of Na+ and Mg2+, the rate of citrulline synthesis by isolated rat liver mitochondria respiring on a range of substrates was stimulated by up to 60% when the extramitochondrial Ca2+ concentration was raised from 130 pM to 770 nM. 2. Our findings suggest that hormonal stimulation of the urea cycle may be mediated by Ca2+.

Ammonia in Liver and Extrahepatic Tissues: An Overview of Metabolism and Toxicity in Mammals

Abstract: Ammonia is a major byproduct of systemic and cerebral nitrogen metabolism and is generated in at least 20 enzymatic reactions within the major organs of the body. Ammonia is thought to be generated in the gastrointestinal tract by the action of bacteria on nitrogenous substrates and by deamidation of glutamine in the large and small intestine. Substantial amounts of ammonia are generated in the liver from glutamate and in the kidney by deamidation of glutamine. The principal fate of systemic blood ammonia, in the brain and other organs, is incorporation into glutamine (amide). Portal vein ammonia (which is present at a much higher concentration (~0.5–1.0 mM) than in the peripheral arterial or venous (20–110 μM) blood), on the other hand, is largely detoxified as urea in the liver. The glutamine derived from brain, muscle and other tissues acts as an energy source for the gut and at the same time releases ammonia for urea synthesis. Thus, ultimately, most extrahepatic ammonia is incorporated into urea by temporary storage in glutamine (amide).

Effect of Portacaval Anastomosis on Ammonia Metabolism in Brain and Liver

Abstract: Construction of an end-to-side portacaval anastomosis (PCA) in the rat results in liver atrophy and increased blood and brain ammonia. Since brain is devoid of an effective urea cycle, removal of ammonia involves glutamine synthesis and PCA results in increased brain glutamine. Glutamine synthetase (GS) activities are decreased by 15% in cerebral cortex following PCA but are unchanged in brainstem. Administration of ammonium acetate to rats with a PCA results in severe encephalopathy progressing to coma. Glutamine content of brainstem of comatose rats is increased a further 2-fold whereas that of cerebral cortex is unchanged. Consequently, ammonia levels in cerebral cortex rise rapidly to attain levels of the order of 5 mM. These findings suggest a limitation in the capacity of cerebral cortex to remove additional blood-borne ammonia following PCA. GS activities of liver are decreased by 80% following PCA. This latter effect probably results from the selective loss of perivenous hepatocytes following PCA.

Effects of Dietary Polyamine Precursors on the Metabolism and Tissue Concentrations of Amino Acids in the Rat

Abstract: Experiments were conducted to determine the effects of dietary supplements of amino acid precursors of polyamines on amino acid metabolism in the rat in order to better understand comparative aspects of polyamine metabolism. Rats were fed isonitrogenous combinations of methionine, ornithine, arginine and 2-difluoromethylornithine in casein-based diets. It was observed that hepatic concentrations of methionine, 5'-deoxy-5'-methylthioadenosine and decarboxylated S-adenosylmethionine were more easily influenced by diet than were arginine and metabolites. It was concluded that rats may be more refractory to exogenous polyamine precursor amino acids than are chicks because of the presence of a functional urea cycle.

Enzymes of Arginine Metabolism in the Scorpion Palamneus phipsoni: Presence of a Partial Urea Cycle and Conversion of Arginine to Proline and Glutamate

Abstract: Scorpion (Palamneus phipsoni) tissues were assayed for the activities of the enzymes of the urea cycle and arginine catabolism. In the hepatopancreas, appreciable levels of arginase and ornithine carbamoyltransferase were present, but arginine synthetase activity could not be detected. None of the three enzymes was found in claw muscle and telson. The results suggest that a functional urea cycle is absent in this guanotelic species. Ornithine aminotransferase, Δ¹-pyrroline-5-carboxylate dehydrogenase, Δ¹-pyrroline-5-carboxylate reductase, and proline oxidase, which catalyze the interconversions of ornithine, glutamate, and proline, were present in scorpion tissues. The metabolic significance of these results has been evaluated in relation to arginine nutrition and ornithine catabolism in the scorpion.

Perturbations in nitrogen metabolism of brain and liver of rat following repeated benthiocarb administration.

Abstract: Effects of repeated administration of benthiocarb on the nitrogen metabolism of hepatic and neuronal systems have been studied. Repeated benthiocarb treatment was associated with significant decrease in proteins with a concomitant increase in free amino acids (FAA) and specific activity levels of proteases suggesting impaired protein synthesis or elevated proteolysis. The glycogenic aminotransferases showed a significant elevation in both the tissues indicating high feeding of ketoacids into oxidative pathway for efficient operation of TCA cycle to combat energy crisis during induced benthiocarb stress. However, the activity levels of branched-chain aminotransferases decreased suggesting their reduced contribution of intermediates to TCA cycle. A comparative evaluation of the activity levels of ammonogenic enzymes, AMP deaminase, adenosine deaminase and glutamate dehydrogenase (GDH) indicated that ammonia was mostly contributed by nucleotide deamination rather than by oxidative deamination. GDH exhibited reduced activity due to low availability of glutamate. In accordance with increased levels of urea, the activity levels of arginase, a terminal enzyme of urea cycle was increased suggesting increased urea cycle operation in order to combat the increased ammonia content. As the presence of urea cycle in the brain is rather doubtful, the conversion of ammonia to glutamine for the synthesis of GABA is envisaged in brain whereas in liver, excess ammonia was converted to urea through ornithine-arginine reacting system. The increased glutaminase activity observed during benthiocarb intoxication is accounted for counteracting acidosis or maintenance of metabolic homeostasis. Arginase, a terminal enzyme of ornithine cycle showed increased activity denoting the efficient potentiality of tissues to avert ammonia toxicity. The changes observed in tissues of rat administered with benthiocarb reflects a shift in nitrogen metabolism for efficient mobilization of end products of protein catabolism.

An unusual aminoacidopathy associated with mitochondrial encephalomyopathy.

Abstract: Five patients from two unrelated pedigrees are affected by an inherited form or forms of mitochondrial encephalomyopathy in which the exact site of the block in the respiratory chain has yet to be identified. All five patients regularly exhibit an unusual aminoacidopathy evident both in fasting plasma and in CSF. Alanine concentrations are elevated, reflecting high tissue pyruvate and lactate levels. Concentrations of the four essential amino acids threonine, methionine, tryptophan and lysine are substantially reduced, as are those of citrulline, ornithine and arginine. This pattern of amino-acid deficiency is apparently not due to failure to absorb the dibasic amino acids, to any abnormality of the urea cycle, to excessive synthesis and turnover of creatine, or to protein malnutrition. The aminoacidopathy presumably is a metabolic consequence of one or more impairments in the electron transport chain in mitochondria. A detailed explanation of its aetiology needs to be sought.

Hyperargininemia, epilepsy and the metabolism of guanidino compounds.

Abstract: Patients with hyperargininemia have an arginase deficiency which leads to blockade of the urea cycle in the last step with several clinical symptoms. Owing to the arginase deficiency this patients accumulate arginine which leads to eventual alternative yet unknown pathways of arginine metabolism. These clinical and biochemical findings intensified the research on guanidino compounds. It has been shown that most of the guanidino compounds are epileptogenic. Therefore we investigated the levels of 12 guanidino compounds in serum and brain of audiogenic sensitive rats. The changes of some guanidino compounds in serum and brain will be discussed.