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Case Study- Urea cycle disorders
A 6 month- old Infant began to vomit occasionally and ceased to gain weight. At 9 months of age he was readmitted to the hospital.Routine examination and laboratory tests were normal but after one week he became drowsy, his temperature rose to 39.4 ° C, his pulse was elevated, and his liver was enlarged. The Electro Encephalogram (EEG) was grossly abnormal. Since the infant could not retain milk by tube feeding, Intravenous glucose was administered. Urine analysis showed abnormally high amount of Glutamine and Uracil. This suggested high amount of Ammonia concentration, which was confirmed by laboratory test.
What is the cause of hyper ammonemia in this patient?
Why were the urine glutamine and Uracil levels elevated?
How can such a patient be treated?
Case Discussion-The child is most probably suffering from hyperammonemia due to impaired urea formation. Hyperammonemia in a new born or very young infant is the characteristic sign of inherited defect in a gene for urea cycle enzymes. The enzyme affected in this patient seems to be Ornithine Transcarbamoylase as apparent from the enhanced excretion of Uracil. Excessive excretion of Uracil or its precursor Orotic acid, results from an accumulation of Carbamoyl phosphate in the mitochondria. In the absence of Ornithine Transcarbamoylase, Carbamoyl phosphate accumulates and leaks in to the cytoplasm, where it can be used to make Carbamoyl Aspartate, the first intermediate in the pathway of pyrimidine nucleotide biosynthesis. This case is unusual in that the symptoms took longer to appear. Urine glutamine excretion has increased because it is excreted in compensation for the inoperative urea cycle. Free ammonia is toxic to brain. It is detoxified by conversion of Glutamate to glutamine. High Glutamine level indicates hyperammonemia that may be due to any liver pathology or may be due to defective urea cycle enzymes.
Ornithine Transcarbamoylase deficiency
The disease is characterized as X linked dominant because most females are also somewhat affected. Females usually respond well to treatment. A significant number of carrier females have hyperammonemia and neurologic compromise. The risk for hyperammonemia is particularly high in pregnancy and the postpartum period. The disease is much more severe in males than in females. The enzyme activity can range from 0% to 30% of the normal.
The urea cycle is the sole source of endogenous production of arginine and it is the principal mechanism for the clearance of waste nitrogen resulting from protein turnover and dietary intake. This extra nitrogen is converted into ammonia (NH3) and transported to the liver where it is processed. The urea cycle disorders (UCD) result from inherited molecular defects which compromise this clearance.
Reactions of urea cycle
Synthesis of 1 mol of urea requires 3 mol of ATP plus 1 mol each of ammonium ion and of the α-amino nitrogen of aspartate. Five enzymes catalyze the reactions of urea cycle (See Figure). Of the six participating amino acids, N-acetyl glutamate functions solely as an enzyme activator. The others serve as carriers of the atoms that ultimately become urea. The major metabolic role of Ornithine, Citrulline, and argininosuccinate in mammals is urea synthesis. Urea synthesis is a cyclic process. Since the Ornithine consumed in 2nd reaction and is regenerated in last reaction, so there is no net loss or gain of Ornithine, Citrulline, argininosuccinate, or arginine. Ammonium ion, CO2, ATP, and aspartate are, however, consumed. Some reactions of urea synthesis occur in the matrix of the mitochondrion, other reactions in the cytosol (See Figure).
Reaction 1- Carbamoyl Phosphate Synthase I Initiates Urea Biosynthesis
Condensation of CO2, ammonia, and ATP to form Carbamoyl phosphate is catalyzed by mitochondrial Carbamoyl phosphate synthase I (CPS-1), A cytosolic form of this enzyme, Carbamoyl phosphate synthase II, uses glutamine rather than ammonia as the nitrogen donor and functions in pyrimidine biosynthesis. Carbamoyl phosphate synthase I, the rate-limiting enzyme of the urea cycle, is active only in the presence of its allosteric activator N-acetyl glutamate, which enhances the affinity of the synthase for ATP. Formation of Carbamoyl phosphate requires 2 mol of ATP, one of which serves as a phosphoryl donor.
Reaction -2-Carbamoyl Phosphate Plus Ornithine Forms Citrulline
L-Ornithine Transcarbamoylase (OTC) catalyzes transfer of the Carbamoyl group of Carbamoyl phosphate to Ornithine, forming Citrulline and orthophosphate. While the reaction occurs in the mitochondrial matrix, both the formation of Ornithine and the subsequent metabolism of Citrulline take place in the cytosol. Entry of Ornithine into mitochondria and exodus of Citrulline from mitochondria therefore involve mitochondrial inner membrane transport systems.
Reaction -3 Citrulline plus Aspartate Forms Argininosuccinate
Argininosuccinate synthase (ASS) links L- Aspartate and Citrulline via the amino group of aspartate and provides the second nitrogen of urea. The reaction requires ATP and involves an intermediate formation of citrullyl-AMP. Subsequent displacement of AMP by aspartate then forms Argininosuccinate.
Reaction -4-Cleavage of Argininosuccinate Forms Arginine & Fumarate
Cleavage of argininosuccinate catalyzed by argininosuccinate lyase (ASL), proceeds with retention of nitrogen in arginine and release of the aspartate skeleton as fumarate. Addition of water to fumarate forms L-malate, and subsequent NAD+-dependent oxidation of malate forms oxaloacetate. These two reactions are analogous to reactions of the citric acid cycle but are catalyzed by cytosolic Fumarase and malate dehydrogenase. Transamination of oxaloacetate by glutamate aminotransferase then re-forms aspartate. (See Figure 2)The carbon skeleton of aspartate-fumarate thus acts as a carrier of the nitrogen of glutamate into a precursor of urea.
Reaction -5-Cleavage of Arginine Releases Urea & Re-Forms Ornithine
Hydrolytic cleavage of the guanidino group of arginine, catalyzed by liver arginase (ARG1) releases urea, the other product, Ornithine, reenters liver mitochondria for additional rounds of urea synthesis. Ornithine and lysine are potent inhibitors of arginase, competitive with arginine. Arginine also serves as the precursor of the potent muscle relaxant nitric oxide (NO) in a Ca2+-dependent reaction catalyzed by NO synthase.
Regulation of Urea formation
Carbamoyl Phosphate Synthase I Is the Pacemaker Enzyme of the Urea Cycle
The activity of Carbamoyl phosphate synthase I is determined by N-acetyl glutamate, 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 N-acetyl glutamate synthase and N-acetyl glutamate Hydrolase, respectively. Major changes in diet can increase the concentrations of individual urea cycle enzymes 10- to 20-fold. Starvation, for example, elevates enzyme levels, presumably to cope with the increased production of ammonia that accompanies enhanced protein degradation.
Figure-1- showing reactions of urea cycle
Figure -2 showing the relationship of Urea cycle to TCA
Fate of Urea- Urea formed in the liver is transported through circulation to kidneys for excretion through urine. It is also transported to intestine where it is decomposed by Urease produced by microbial action. Ammonia liberated by this activity is transported by portal circulation to liver where it is detoxified back to urea. A fraction of ammonia goes to systemic circulation. (See figure -3
Figure-3 Showing the fate of urea
Urea cycle disorders
1) Carbamoyl Phosphate synthetase (CPS-1) deficiency
Along with OTC deficiency, deficiency of CPSI is the most severe of the urea cycle disorders. Individuals with complete CPS-I deficiency rapidly develop hyperammonemia in the newborn period. Children who are successfully rescued from crisis are chronically at risk for repeated bouts of hyperammonemia.
2) Ornithine Transcarbamoylase deficiency (OTC deficiency)
Absence of OTC activity in males is as severe as CPSI deficiency. Approximately 15% of carrier females develop hyperammonemia during their lifetime and many require chronic medical management
3) Citrullinemia type I (ASS deficiency)
The hyperammonemia in this disorder is quite severe. Affected individuals are able to incorporate some waste nitrogen into urea cycle intermediates, which makes treatment slightly easier.
4) Argininosuccinic aciduria (ASL deficiency)
This disorder also presents with rapid-onset hyperammonemia in the newborn period. This enzyme defect is past the point in the metabolic pathway at which all the waste nitrogen has been incorporated into the cycle. Treatment of affected individuals often requires only supplementation of arginine. ASL deficiency is marked by chronic hepatic enlargement and elevation of transaminases. Biopsy of the liver shows enlarged hepatocytes, which may over time progress to fibrosis, the etiology of which is unclear. Affected individuals can also develop trichorrhexis nodosa, a node-like appearance of fragile hair, which usually responds to arginine supplementation. Affected individuals who have never had prolonged coma but nevertheless have significant developmental disabilities have been reported.
5) Arginase deficiency (hyperargininemia; ARG deficiency)
This disorder is not typically characterized by rapid-onset hyperammonemia. Affected individuals develop progressive spasticity and can also develop tremor, ataxia, and choreoathetosis. Growth is affected
6) NAG Synthase deficiency. Deficiency of this enzyme has been described in a number of affected individuals. Symptoms mimic those of CPSI deficiency; since CPSI is rendered inactive in the absence of NAG
The incidence of UCDs(Urea cycle disorders)is estimated to be at least 1:25,000 births; partial defects may make the number much higher.
Infants with a urea cycle disorder often appear normal initially but rapidly develop cerebral edema and the related signs of lethargy, anorexia, hyperventilation or hypo ventilation, hypothermia, Slurring of the speech, Blurring of vision, seizures, neurological posturing, and coma. In milder (or partial) urea cycle enzyme deficiencies, ammonia accumulation may be triggered by illness or stress at almost any time of life, resulting in multiple mild elevations of plasma ammonia concentration; the hyperammonemia is less severe and the symptoms more subtle. In individuals with partial enzyme deficiencies, the first recognized clinical episode may be delayed for months or years.
The diagnosis of a urea cycle disorder is based on evaluation of clinical, biochemical, and molecular genetic data.
· A plasma ammonia concentration of 150 mmol/L or higher is a strong indication for the presence of a UCD.
· Plasma quantitative amino acid analysis can be used to diagnose a specific urea cycle disorder: plasma concentration of arginine may be reduced in all urea cycle disorders, except ARG(Arginase) deficiency, in which it is elevated five- to sevenfold;
· Plasma concentration of Citrulline helps discriminate between the proximal and distal urea cycle defects, as Citrulline is the product of the proximal enzymes (OTC and CPSI) and a substrate for the distal enzymes (ASS, ASL, ARG).
· Urinary Orotic acid is measured to distinguish CPSI deficiency and NAGS (N-Acetyl Glutamate Synthase) deficiency from OTC deficiency.
· A definitive diagnosis of CPS-I deficiency, OTC deficiency, or NAGS deficiency depends on determination of enzyme activity from a liver biopsy specimen.
· However, the combination of family history, clinical presentation, amino acid and Orotic acid testing, and, in some cases, molecular genetic testing is often sufficient for diagnostic confirmation, eliminating the risks of liver biopsy.
The mainstays of treatment for urea cycle disorders include-
· Dialysis to reduce plasma ammonia concentration,
· Intravenous administration of arginine chloride and nitrogen scavenger drugs to allow alternative pathway excretion of excess nitrogen, Excess nitrogen is removed by intravenous phenyl acetate and that conjugate with glutamine and glycine, respectively, to form phenylacetylglutamine and Hippuric acid, water-soluble molecules efficiently excreted in urine.
· Arginine becomes an essential amino acid (except in arginase deficiency) and should be provided intravenously to resume protein synthesis. If these measures fail to reduce ammonia, hemodialysis should be initiated promptly.
· Restriction of protein for 24-48 hours to reduce the amount of nitrogen in the diet, providing calories as carbohydrates (intravenously as glucose) and fat (intralipid or as protein-free formula) to reduce catabolism,
· Physiologic stabilization with intravenous fluids
· Chronic therapy consists of a protein-restricted diet, phenyl butyrate (a more palatable precursor of phenyl acetate), arginine, or Citrulline supplements, depending on the specific diagnosis.
· Liver transplantation should be considered in patients with severe urea cycle defects that are difficult to control medically.
Deficiencies of CPSI, ASS, ASL, NAGS, and ARG are inherited in an autosomal recessive manner. OTC deficiency is inherited in an X-linked manner. Prenatal testing using molecular genetic testing is available for five of the six urea cycle disorders
A number of other disorders that perturb the liver can result in hyperammonemia and mimic the effects of a urea cycle disorder. The most common/significant ones are viral infection of the liver and vascular bypass of the liver.
Figure-4 showing the mechanism of Nitrogen removal by Glycine, phenyl acetate and Arginine
Figure- 5 showing an overview of Urea cycle. Glutamate is the ultimate precursor for both nitrogen atoms of urea. One through ammonia by oxidative deamination of Glutamate by glutamate dehydrogenase and the second through the activity of transaminase whereby aspartate is formed from Glutamate, hence both nitrogen actually come from Glutamate only.Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!