Showing papers on "Urea cycle published in 1976"

Studies on functional and metabolic role of urea cycle intermediates in brain

Abstract: — The distribution of argininosuccinate synthetase, argininosuccinase and arginase, and the synthesis of urea in cerebullum. cerebral cortex and brain stem have been studied. Cerebral cortex had high levels of argininosuccinate synthetase and argininosuccinase. and a high ability to synthesize urea from aspartic acid and citrulline. Of the three regions, cerebullum had the highest arginase activity. The activities of the enzymes transamidinase and ornithine aminotransferase in the metabolism of arginine and ornithine in pathways other than urea formation have been studied in the three regions of the rat brain. The activity of creatine phosphokinase in all regions was the same: carbamylphosphatase activity was highest in cerebullum. Cerebral cortex had a high activity of aspartic acid transcarba-mylase. The brain stem, among the three regions, had the lowest activities of glutamine synthetase and glutaminase. The activities of these enzymes in the different regions are discussed in relation to urea production and the utilization of the urea cycle intermediates. Intraperitoneal injection of high amounts of citrulline brought about a rise in the glutamine synthetase activity of cerebellum and brain stem and a rise in ornithine aminotransferase in cerebral cortex and liver. These results are discussed in relation to the mechanism of action of citrulline in alleviating the toxicity in hyperammonaemic states.

Effect of ornithine and lactate on urea synthesis in isolated hepatocytes.

Abstract: 1. In hepatocytes isolated from 24 h-starved rats, urea production from ammonia was stimulated by addition of lactate, in both the presence and the absence of ornithine. The relationship of lactate concentration to the rate of urea synthesis was hyperbolic. 2. Other glucose precursors also stimulated urea production to varying degrees, but none more than lactate. Added oleate and butyrate did not stimulate urea synthesis. 3. Citrulline accumulation was largely dependent on ornithine concentration. As ornithine was increased from 0 to 40 mM, the rate of citrulline accumulation increased hyperbolically, and was half-maximal when ornithine was 8-12 mM. 4. The rate of citrulline accumulation was independent of the presence of lactate, but with pyruvate the rate increased. 5. The rate of urea production continued to increase as ornithine was varied from 0 to 40 mM. 6. It was concluded that intermediates provided by both ornithine and lactate are limiting for urea production from ammonia in isolated liver cells. It was suggested that the stimulatory effect of lactate lies in increased availability of cytosolic aspartate for condensation with citrulline.

Effect of fatty acids on the disposition of ammonia.

Abstract: Subcoma doses of fatty acids and ammonium salts injected intraperitoneally at the same time into rats or cats act synergistically to produce coma. Under these circumstances, the blood ammonia, is more than double that when the NH4+ is given alone. After these observations a rat liver homogenate system was utilized to study the effect of fatty acids on ammonia utilization in urea, glutamate and glutamine synthesis in vitro. Acetylglutamate-catalyzed urea synthesis was completely inhibited by 45 mM octanoate and was depressed 46% by 9.5 mM octanoate. Citrulline synthesis was similarly inhibited 86 and 28%, respectively. The concentration of liver octanoate at the moment of occurrence of coma after an in vivo injection was approximately 10 mM. The inhibitory effect of fatty acids on the utilization of NH4+ in the urea cycle was greater the longer the fatty acid chain. The critical step in this interference with ammonia metabolism was the inhibition of carbamyl phosphate synthetase. Argininosuccinate synthetase activity was also inhibited to a lesser degree, but ornithine transcarbamylase, argininosuccinate lyase and arginase were unaffected. Glutamate dehydrogenase was likewise inhibited in liver (83%) and brain (43%) by 13 mM octanoate, whereas glutamine synthetase was unaffected. Thus, the two main processes whereby ammonia is metabolized were inhibited by fatty acids at concentrations that exist pathologically, which accounts, at least in part, for the rise in blood ammonia in vivo.

Propionic acidemia and hyperlysinemia in a case with ornithine transcarbamylase (OTC) deficiency

Abstract: A female infant with episodic hyperammonemia due to a disorder of the urea cycle and who had hyperlysinemia and an unusual elevation of short chain fatty acids, mainly propionate, is described. Both occurred apparently only during attacks of hyperammonemia. Propionic acidemia was ruled out by enzyme studies. OTC deficiency was diagnosed on the basis of: 1) decreased enzyme activity in leukocytes;2) hyperammonemia in response to protein intakes in excess of 2.0 g/kg/day; 3) orotic aciduria in the patient and her asymptomatic mother; 4) suggestive evidence of x-linked dominant inheritance; and 5) exclusion of citrullinemia, argininosuccinic aciduria, argininemia, and disorders of lysine metabolism that are associated with hyperammonemia. Homocitrullinuria, presence of epsilon-N-acetyl-l-lysine in urine, and absence of saccharopine indicate deficiency of the saccharopine pathway of lysine degradation. However, alpha-ketoglutarate reductase was normal in fibroblasts. Since these metabolites were observed only in conjunction with hyperammonemia but not after a lysine load, we suggest that there was competition between ammonia and lysine for alpha-ketoglutarate. The link between disorders of the urea cycle and short chain fatty acid metabolism remains unexplained..

Changes in the activities of the enzymes of urea synthesis caused by dexamethasone and dibutyryladenosine 3':5'-cyclic monophosphate in foetal rat liver maintained in organ culture.

Abstract: Liver explants from 19-day foetal rats were maintained in organ culture, in a defined medium, for up to 48h. Both 6-N,2'-O-dibutyryl cyclic AMP, in the presence of theophylline, and dexamethasone caused an increase in the activities of carbamoyl phosphate synthase, argininosuccinate synthetase, argininosuccinate lyase and arginase. These increases could be abolished by simultaneously incubating the explants with cycloheximide. No change in the activity of ornithine transcarbamoylase was found with either hormone. Previous work has shown that injection of corticosteroids into 19.5-day foetal rats in utero did not cause an increase in the arginine synthetase system. Present results suggest that this lack of effect is not due to any incompetence of the foetal rat liver at this stage to respond to this agent. The observations on ornithine transcarbamoylase activity suggest that this enzyme is induced in the liver of the perinatal rat by neither corticosteroids nor hormones acting via cyclic AMP, and it may be that all the enzymes of the urea cycle are induced physiologically by an agent or agents as yet unidentified.

Hyperdibasicaminoaciduria and hyperammonemia in familial protein intolerance

Abstract: • A 3-year-old boy with hyperdibasic-aminoaciduria and hyperammonemia showed characteristics of familial protein intolerance (FPI). Oral loading tests of lysine and arginine disclosed a remarkably reduced capability for intestinal absorption of these amino acids. Because urinary excretion and renal clearance of dibasic amino acids were only moderately elevated in the patient, the conspicuously decreased serum concentration of lysine, arginine, and ornithine was attributed to the defect in intestinal absorption. A possible explanation for elevated blood ammonia levels in FPI is that it is due to a deficiency of arginine and ornithine in the urea cycle that in turn results from a severe impairment in absorption of the amino acids by the gut mucosa. ( Am J Dis Child 130:1340-1344, 1976)

Intravenous loading with arginine-hydrochloride and ornithine-aspartate in siblings of two families, presenting a familial neurological syndrome associated with cystinuria.

Abstract: 1. Arginine-hydrochloride and ornithine-aspartate solutions have been infused intravenously to children of two families. Three children of the WOL. family are affected with hyperargininemia and hyperammonemia, due to a lack of arginase. They present a secondary cystine-lysinuria. The three WIL. siblings are suffering from muscular hypotonia, dwarfism, incomplete renal tubular acidosis and primary cystinuria. 2. The aim was to verify how and to what extent the artificial rise of one serum amino acid could influence the serum concentrations and the urinary losses of the other amino acids. The results found for the serum have been submitted to a statistical analysis of variance. 3. The variations observed for the amino acids of the urea cycle can be interpreted as being the reflections of known metabolic pathways. 4. Additional remarks are made on a paradox in the lysinemia-lysinuria relation after arginine infusion, with a simultaneous rise of this essential amino acid in serum and urine.

Effects of arginine infusion in infants: increased urea synthesis associated with unchanged ammonia blood levels.

Abstract: Infusion of L-arginine hydrochloride in infants and children (ages ranging from 1 day to 12 yr) at a dosage of 0.5 g/kg body weight resulted in a dramatic increase in the arginine plasma concentration, with highest values of approximately 7 mmole/liter immediately after the end of the infusion; 120 min later the mean plasma level of the amino acid had decreased to mean values of 1 mmole/liter. These fluctuations were paralleled by increased ornithine concentrations, although the mean plasma levels of this amino acid remained far below those of arginine, i.e., 0.73 and 0.22 mmole/liter after 30 and 90 min, respectively. When expressed on a molar basis, arginine administration resulted in an almost stoichiometric rise in urinary urea excretion. These findings indicate that arginine is rapidly metabolized via urea and ornithine, the latter being transformed to glucose, as evidenced by a significant rise in the blood glucose concentration. Blood gas analyses and serum urea and blood ammonia concentrations determined after the load showed no significant deviations from preinfusion levels. Thus, in contrast to the effects to be expected form studies with tissue culture homogenates, even when administered to newborn infants, arginine does not impair the turnover of the urea cycle.