Showing papers on "Urea cycle published in 1971"

Urea cycle enzyme adaptation to dietary protein in primates

Abstract: Activities of the five hepatic enzymes of the urea cycle increased two- to threefold in monkeys on a 60 percent protein diet compared with levels in monkeys on an isocaloric 6 percent protein diet. Equal adaptation occurred in rats on the same diets. These enzyme activities in adult humans approximate those of monkeys on a similar protein intake.

Biochemical aspects of intravenous alimentation

Abstract: Biochemical aspects of total parenteral alimentation in 10 patients (two low birth weight newborn infants and eight infants) for periods of 5 to 24 days were studied by: A. Determination of concentration of amino acids and ammonia of two commercially available protein hydrolysates. B. Analyses of blood and urine obtained before, during, and after parenteral alimentation for various biochemical parameters including amino acids, urea nitrogen, glucose, and osmolality. The results showed that the protein hydrolysates contained a very high concentration of ammonia. This inordinate amount of ammonia taxes the Krebs urea cycle and in premature infants enhances acidosis and respiratory distress syndrome. It also explains the abnormal liver function tests and hyperammonemia found concomitantly with infusion of protein hydrolysate. Another effect of infusion of protein hydrolysate, in conventional amount, is persistent hyperaminoacidemia.

The presence of high ornithine-urea cycle enzyme activity in the teleost Opsanus tau

Abstract: 1. 1. Relatively high activities of all five of the ornithine-urea cycle enzymes were found in liver of the teleost Opsanus tau. 2. 2. The substrate and cofactor requirements of these enzymes appear to be the same as those of most other vertebrates. 3. 3. Concentrations of urea and ammonia in the blood of Opsanus tau are low, and the rate of urea excretion is low compared with the rate of ammonia excretion. 4. 4. Thus, there appears to be a lack of correlation between the levels of ornithine-urea cycle enzyme activity and the pattern of nitrogen excretion.

Hereditary Metabolic Disorders of the Urea Cycle

Abstract: Publisher Summary Urea is the main end product of nitrogen metabolism. It is formed from the ammonia arising from the metabolism of the amino acids of protein by a sequence of five reactions, four of which comprise the urea cycle proper. The end result is the conversion of ammonia into urea, with the reformation of the individual reactants of the cycle. Compared with other metabolic pathways, the urea cycle is short, possibly the shortest of all. Defects of the enzymes mediating all four reactions of the urea cycle proper have now been established, and there is some evidence of the existence of a fifth enzyme defect, involving carbamyl phosphate synthetase, mediating the initial reaction of the pathway. One of the most important results of a defect of the biosynthesis of urea is an increased level of blood ammonia. Therefore, it is essential to consider other conditions that might affect indirectly the urea cycle or in some other way raise the blood ammonia. Because lysine can act as a competitive inhibitor of the conversion of arginine to ornithine and urea, an increased level of plasma lysine may therefore inhibit the urea cycle.

Responsiveness of Two Urea Cycle Enzymes in Morris Hepatomas to Metabolic Modulations

Abstract: Arginase and argininosuccinate synthetase activities have been assayed in some 14 hepatomas and the corresponding host livers. The hepatomas exhibited an extremely wide range of activities of the two enzymes of the urea cycle. A good correlation exists between the activities of the two enzymes in the hepatomas. However, the activity of either enzyme in the hepatomas bears no correlation with that in the host livers. Moreover, there is no correlation between the enzyme activity and the growth rate or chromosome number of the hepatomas. Cortisol raised arginase activity to a larger degree in Hepatomas 7800 and 8999 than in the host livers, but the hormone enhanced argininosuccinate synthetase activity more in the host livers than in the two hepatomas. Thyroxine increased arginase activity in Hepatoma 7800 and the host liver of intact rats but decreased it in adrenalectomized rats. The thyroid hormone also elevated the synthetase activity in Hepatoma 7800 but not in the host liver. On the other hand, thyroxine did not affect the activity of these two enzymes in either Hepatoma 9618A or the host liver. While actinomycin D or cycloheximide blocked the hormone-induced increase in arginase activity of Hepatomas 7800 and 8999 and in the synthetase activity of Hepatoma 7800, the antibiotics exerted no inhibitory effect on the enzymes in the host livers. Increasing the protein content of the diet elicited increases in the activity of the two enzymes in Hepatoma 9618A, as well as in the host liver, but not in Hepatoma 8999. These results together with others show an overresponse by enzymes in hepatomas, when compared with the response by the same enzymes in host livers. This overresponsiveness is discussed as a manifestation of deranged control mechanisms in cancer.

The Urea Cycle

Abstract: The presence of 22–30 mg% (3.66–5.0 μmoles/g) of urea in brain was reported by Marshall and Davis as early as 1914(1) Similar amounts were found in cat brain by Tallan et al.(2) The values given for rat brain by Sporn et al. (3) (4.9 μmoles/g) are in good agreement with the amount found by us (4.5 μmoles/g)(4,5) and by Gershenovitch et al. (3.8-.492 μmoles/g).(6) Roberts and Morelos found the level of urea in cerebral cortex to be equal to 5.49 μmoles/g.(7) According to Shaw and Heine,(8) the content of urea in rat brain cerebral hemispheres was 6.56, in midbrain 5.78, in cerebellum 7.68, and in pons medulla 4.76 μmoles/g. The figures presented show that the brain contains a considerable amount of urea, which is almost equal to that found in liver—5.0–6.0 μmoles/g.

The effect of the administration of L-tryptophan on synthesis of urea and gluconeogenesis in man.

Abstract: Inhibition of phosphoenolpyruvate carboxykinase (EC 4.1.1.32) after L-. tryptohan administration has previously been indicated in animal experiments. To see if a similar phenomenon would be detected in man and if such an inhibition would result in decreased glucose and urea production by the liver, experiments were performed with 3 healthy subjects. Tryptophan was given by stomach tube located in the duodenum to 3 volunteers kept on a carbohydrate-free diet for 3 days. On the 4th day a mixture of L-amino acids was infused continuously for 3 hours. Glucose and urea production from the liver were found to be decreased following tryptophan administration. Concurrent measurements of pyruvate, lactate, α-ketoglutarate, acetoacetate, s-hydroxybutyrate, aspartate and glutamate in arterial and liver vein blood indicated that phosphoenolpyruvate carboxykinase was inhibited. This inhibition was judged to be the direct cause of the reduction in glucose production and indirectly the cause for the fall in urea production.

Studies on the role of ornithine cycle and purine catabolism in urea biosynthesis in the fresh-water crayfish, Orconectes rusticus

Abstract: 1. 1. The fresh-water crayfish, Orconectes rusticus , when acclimated to 50% sea water, excretes more urea. 2. 2. The activities of arginase and uricase, allantoinase, allantoicase and urease were measured to investigate the possible role of the ornithine cycle and/or purine catabolism in the synthesis of this extra urea. 3. 3. The excess urea excreted on acclimation to the salt water medium does not seem to be synthesized either via the ornithine cycle or by purine degradation.

A reinvestigation of the status of the ornithine—Urea cycle in adult Ascaris lumbricoides

Abstract: 1 1 The ornithine—urea cycle has been investigated in Ascaris lumbricoides kept in large volumes of saline or in a U-tube 2 2 In neither case was a functional urea cycle detected, although ornithine transcarbamylase and arginase were present 3 3 It is concluded that the ornithine—urea cycle is not functional in A lumbricoides or in parasitic helminths in general

Complete ornithine transcarbamylase deficiency: A cause of lethal neonatal hyperammonemia

Abstract: Hyperammonemia secondary to deficiency of one of the enzymes of the urea cycle causes infantile somatic and mental retardation, but has not, bitherto, been noted to cause death in the newborn period. A term infant, born to healthy parents after an uneventful pregnancy and delivery, thrived for three days, then lapsed rapidly into deep coma. Becuase a previous sibling had died under identical circumstances, an inherited metabolic derangement was sought. The blood ammonia concentration was 1208 μg% (normal <150 μg%). The blood urea nitrogen was 7 mg% and numerous other studies of plasma and urinary amino or organic acids were unrevealing. Despite a protein free diet, enemas, antibiotic therapy and an exchange transfusion, the blood ammonia remained about 1200 μg% and the child expired on the fifth day of life. Hepatic assays of the five enzymes of the urea cycle revealed absence of ornithine transcarbamylase (OCT) activity. No OCT acitivity was restored by changes in substrte concentration, enzyme concentration or pH, and mixing experiments excluded the presence of an inhibitor of OCT in the patient's cells. Activity of the other four urea cycle enzymes was int he range noted in other age-matche, autopsy-control livers. These findings document complete OCT deficiency for the first time and emphasize the lethality of this enzymatic defect. Hyperammonemia must be considered in a newborn with coma, particularly if there is a family history of neonatal death. In such situations, unrestricted dietary protein ingestion will have disastrous consequences.

[261a] Clinical aspects of arginase☆☆☆

Abstract: Publisher Summary This chapter discusses the clinical aspects of arginase. Arginase catalyzes the hydrolysis of L-arginine to form ornithine and urea. This enzyme, which mediates the final step in the ornithinecitrulline- urea cycle, is found in animals primarily in liver and is localized exclusively in the 105,000 g supernatant of the cell. The arginase activity of human erythrocytes is 200 times greater than that of serum and that of neutrophilic leucocytes 10,000 times greater. Human liver arginase has been separated by chromatography on CM-cellulose columns into two distinct isozymes. Methods for measuring arginase activity have involved measuring one of the products of the reaction or the substrate remaining after hydrolysis. Ornithine has been measured by titration, arginine by the Sakaguchi reaction or by ultraviolet absorption, and urea by treatment with urease and measurement of either CO 2 or NH 3 formed during the reaction or by direct colorimetric methods for urea. The unit is defined as the amount of enzyme that will catalyze the formation of 1 micromole of urea per minute under the conditions of the assay. These conditions are pH 9.5, 0.06 M arginine and incubation at 37°.

Activities of urea cycle enzymes in tumor-bearing mice

Abstract: The hepatic arginase activity was found to be elevated significantly in tumor­ bearing mice a t the terminal stage. Furthermore , other enzymes in the urea cycle, ornithin e transcarbamylase, arginine synthet ase (over all reaction J and argininosuccinate cleavage enzyme, also showed elevated activity with a con­ comitant increase in urinary excretion of urea. The present experiments seem to show that the enhanced activities of these enzymes and the in creas ed urea excretion both are related to protein cat abolism under adrenal hyperfunction in tumor -bearing mice at the t erminal stage. Pos­ sible explanations for the development of cachexia in tumor-bearing hosts were made.