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Siegel GJ, Agranoff BW, Albers RW, et al., editors. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Philadelphia: Lippincott-Raven; 1999.

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Cover of Basic Neurochemistry

Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition.

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Urea Cycle

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Correspondence to Marc Yudkoff, Children's Hospital of Philadelphia, 1 Children's Center, Philadelphia, Pennsylvania 19104.

The urea cycle (Fig. 44-6) mediates the removal of ammonia as urea in the amount of 10 to 20 g per day in the healthy adult. The absence of a fully functional urea cycle may result in hyperammonemic encephalopathy and irreversible brain injury in severe cases. A failure of ureagenesis occurs because of acquired disease, such as cirrhosis secondary to alcoholism, or secondary to an inherited defect, usually a congenital enzymopathy.

The initial two steps of the urea cycle are mitochondrial. Carbamyl phosphate synthetase (CPS), which has been mapped to human chromosome 2, mediates the formation of carbamyl phosphate from NH3, HCO3 and ATP (Fig. 44-6, reaction 1). N-acetylglutamate (NAG), formed from glutamate and acetyl-CoA via NAG synthetase (Fig. 44-6, reaction 9), is an obligatory effector of CPS and an important regulator of ureagenesis. A variety of influences, including dietary protein, arginine and corticosteroids, augment the concentration of NAG in mitochondria.

Following condensation with ornithine, carbamyl phosphate is converted to citrulline in the ornithine transcarbamylase (OTC) reaction. OTC is coded on band p21.1 of the X chromosome, where the gene contains 8 exons and spans 85 kb of DNA. The activity of this enzyme is directly related to dietary protein. There may be “tunneling” of ornithine transported from the cytosol to OTC, with the availability of intramitochondrial ornithine serving to regulate the reaction.

Citrulline is released to the cytosol, where it condenses with aspartate to form argininosuccinate via argininosuccinate synthetase (AS) (Fig. 44-6, reaction 3). This enzyme is coded on human chromosome 9, where a 63-kb gene comprising 14 exons is located. The mRNA is markedly increased by starvation, treatment with corticosteroids or cAMP. Citrulline itself is a potent inducer of the mRNA.

Argininosuccinate is cleaved in the cytosol by argininosuccinate lyase (AL), which is coded on human chromosome 7 (Fig. 44-6, reaction 4). The products of the reaction are fumarate, which is oxidized in the TCA cycle, and arginine, which is rapidly cleaved to urea and ornithine via hepatic arginase. Both AL and arginase are induced by starvation, dibutyryl cAMP and corticosteroids. Several isozymes of arginase have been described.

Urea cycle defects cause hyperammonemia and may result in coma, convulsions and vomiting during the first few days of life

Clinical confusion with septicemia is common, and many infants are treated futilely with antibiotics. Hyperammonemia usually is severe, even in excess of 1 mM; normal in term infants is up to 100 μM.

Diagnosis usually is made from the blood aminogram. Plasma concentrations of glutamine and alanine, the major nitrogen-carrying amino acids, usually are high and that of arginine is low. Patients with citrullinemia, caused by a deficiency of AS, or argininosuccinic aciduria, caused by a deficiency of AL, will manifest marked increases of blood citrulline and argininosuccinate, respectively.

Urinary orotic acid generally is very elevated in babies with OTC deficiency and normal or even low in infants with CPS deficiency. Patients with OTC deficiency have increased excretion of orotic acid because carbamyl phosphate spills into the cytoplasm, where it enters the pathway of pyrimidine synthesis.

Diagnosis of the infant with either CPS or OTC deficiency may not always be apparent from the blood aminogram. Ornithine concentrations typically are normal in the latter disorder. The presence of hyperammonemia, hyperglutaminemia, hyperalaninemia and orotic aciduria in a critically ill infant affords strong presumptive evidence for OTC deficiency. Conversely, the presence of this pattern on the aminogram in the absence of an untoward orotic aciduria is suggestive of CPS deficiency.

Diagnosis of an urea cycle defect in the older child can be more problematic. Patients may present with psychomotor retardation, growth failure, vomiting, behavioral abnormalities, perceptual difficulties, recurrent cerebellar ataxia and headache. Thus, it is essential to monitor the blood ammonia in any patient with unexplained neurological symptoms. Measurement of blood NH3 alone may not be sufficient for diagnosis since hyperammonemia can be an inconstant finding with partial enzymatic defects. In the latter group, quantitation of blood amino acids and of urinary orotic acid is indicated.

Hyperammonemia also occurs in some organic acidurias, particularly those that affect neonates. Thus, the urine organic acids should be quantitated in all patients with significant hyperammonemia.

A variety of biochemical changes in brain metabolism have been described in experimental models of hyperammonemia. High ammonia concentrations impair the malate—aspartate shuttle, which mediates the transport of NADH from the cytosol to mitochondria. Changes also occur in the rate of oxidation of glucose and/or pyruvate. The intracellular ATP pool may be depleted, especially in the reticular activating system.

Hyperammonemia also may affect brain volume control; cell swelling is sometimes observed, perhaps because of the marked increase of brain glutamine. This change probably is most prominent in the astrocytes, where it would be expected to have an osmotic effect. Glial swelling is a common pathological finding in hyperammonemic patients.

Hyperammonemia also affects neurotransmitter metabolism. Major effects on the handling of GABA and serotonin have been observed. In the latter instance, a possible mechanism may be increased passage of tryptophan across the blood—brain barrier and consequent increased synthesis of serotonin. Treatment of patients with blockers of serotonergic receptors may alleviate the anorexia that is common in this population. Extracellular glutamic acid tends to increase, and recent experimental evidence suggests excitotoxic injury. Ammonia also has been shown to affect ion flux, in particular that of C1. This might cause hyperpolarization of membranes.

Surprisingly little is known about the changes in the brain of patients dying with hyperammonemia. Abnormal myelination with cystic degeneration of neurons has been described. Cell swelling, particularly of the astrocytes, is common. Cortical atrophy may occur in youngsters with long-standing disease.

Except for patients with argininosuccinic aciduria, who may demonstrate varying degrees of hepatic fibrosis, there is very little evidence of pathological changes outside of the CNS.

Carbamyl phosphate synthetase deficiency prevents the formation of carbamyl phosphate from ammonia

CPS deficiency is relatively rare. Neonates quickly develop lethargy, hypothermia, vomiting and irritability. The hyperammonemia typically is severe, even exceeding 1 mM. Occasional patients with partial enzyme deficiency have had a relapsing syndrome of lethargy and irritability upon exposure to protein. Brain damage can occur in both neonatal and late-onset groups.

N-Acetylglutamate synthetase deficiency leads secondarily to carbamyl phosphate synthetase deficiency

A deficiency of CPS activity also can arise because of the congenital absence of NAG synthetase, which catalyzes the formation of NAG from glutamate and acetyl-CoA. NAG is an obligatory effector of CPS. The few patients reported have had a malignant course of neonatal onset.

Ornithine transcarbamylase deficiency prevents the conversion of carbamyl phosphate to citrulline and is the most common of the urea cycle defects

Presentation is variable, ranging from a fulminant, fatal disorder of neonates to a schizophrenia-like illness in an otherwise healthy adult. Affected men characteristically fare more poorly than women with this X-linked disorder. This difference reflects the random inactivation of the X chromosome, termed Lyon's hypothesis. If the inactivation affects primarily the X chromosome bearing the mutant OTC gene, then a more favorable outcome can be anticipated. Conversely, if the wild-type X chromosome is inactivated, the woman is expected to have a much more active disease.

The human OTC gene spans 73 kb, comprising 10 exons and nine introns. In the mouse, a 750-bp promoter 5′ to the transcription initiation codon confers tissue specificity.

The diagnosis has been aided by the use of genetic markers based on intragenic restriction fragment length polymorphisms (RFLPs) (see also Chap. 40). More than 80% of carriers can be detected in this manner, and antenatal diagnosis is possible in many cases. Approximately one-third of the mothers of boys and two-thirds of the mothers of girls have been found to be noncarriers, reflecting the greater propensity for mutation in the male gamete.

Diagnosis of carriers can be made with protein-loading tests, in which the excretion of urinary orotic acid has been used as a marker. This approach detects 85 to 90% of carriers. A recent elaboration of these tests involves the administration of allopurinol to favor orotic acid excretion. Loading studies with 15NH4C1 as metabolic tracer indicate that symptomatic female carriers for OTC produce less 15N urea compared with a control population. Asymptomatic heterozygotes form urea at a normal rate, but they produce excessive [5-15N]glutamine. Thus, whole-body nitrogen metabolism is abnormal even in this group [33].

Animal models for OTC deficiency have been developed. These include the sparse fur (spf) mouse and the sparse fur—abnormal skin and hair (spf-ash) mouse. In the former model, a histidine residue replaces an asparagine at position 117 of the gene, resulting in an enzymatic activity that is 15% of control. The spf-ash mutation entails a base change in exon 4, resulting in a splicing mutation and a reduction of OTC activity to 5% of normal. Both kinds of mutant mice manifest hyperammonemia, orotic aciduria, growth failure and sparse fur.

OTC deficiency must be suspected in any patient, male or female, with unexplained neurological symptoms. The absence of hyperammonemia in a casual sample should not rule out the diagnosis, especially if the history is positive for protein intolerance or an untoward reaction to infections. Family history also may be suggestive. Blood amino acids and urinary orotic acid should be quantitated in such individuals.

Deficiencies in arginosuccinate synthetase cause citrullinemia

Neonates with AS deficiency (Fig. 44-6, reaction 3) usually die, and most survivors suffer major brain injury. Patients with a partial deficiency may have a milder course, and a few individuals with citrullinemia have been phenotypically normal.

The diagnosis usually is apparent from the hyperammonemia and the extreme hypercitrullinemia. The activity of AS can be determined in both fibroblasts and chorionic villus samples, thus simplifying the problem of antenatal diagnosis.

Argininosuccinic aciduria results from a deficiency in arginosuccinic lyase, preventing the formation of arginine

Patients with arginosuccinic aciduria excrete an enormous amount of argininosuccinate in their urine. The CSF also contains this polar molecule in high concentration. Neonates have a stormy clinical course, and almost all die or sustain severe brain injury. A peculiar finding in many cases is trichorrhexis nodosa, or dry brittle hair with nodular protrusions, which are best visible with light microscopy. The precise cause is unknown.

Arginase deficiency blocks the conversion of arginine to urea and ornithine and causes a progressive, spastic tetraplegia, especially in the lower extremities

Most patients are thought to have psychomotor retardation during the first year of life. Seizures and growth failure may occur, although some patients are of normal size. The motor dysfunction usually comes to clinical attention by age 2 to 3 years. Leukodystrophic changes are seen. Blood NH3 is elevated less than in neonatal-onset disorders. The plasma arginine concentration usually is two to five times normal. Urine orotic acid excretion is extremely high, perhaps because arginine stimulates flux through the CPS reaction by favoring the synthesis of NAG.

Hyperornithinemia, hyperammonemia and hypercitrullinemia may also be caused by a failure of mitochondrial ornithine uptake

Electron microscopy of the liver has shown irregularities of mitochondrial shape. This results in a failure of citrulline synthesis and a consequent hyperammonemia. Urinary orotic acid is high, presumably because of underutilization of carbamyl phosphate. In contrast, excretion of creatine is low, reflecting the inhibition of glycine transamidinase by excessive concentrations of ornithine.

These conditions may result in growth failure and varying degrees of mental retardation. Sometimes symptoms are deferred until adulthood. Vomiting, lethargy and hypotonia are noted after protein ingestion. Recurrent hospitalizations for hyperammonemia are the rule. Some patients have manifested a bleeding diathesis and hepatomegaly.

Lysinuric protein intolerance is caused by defects in the transport of lysine, ornithine and arginine

The clinical course in neonates usually is not severe. After weaning or upon exposure to foods high in protein, the infants manifest growth failure, hepatomegaly, splenomegaly, vomiting, hypotonia, recurrent lethargy, coma, abdominal pain and, in rare instances, psychosis. Rarefaction of the bones is common, and both fractures and vertebral compression have been reported. Most patients are not mentally retarded, although this may occur. Some patients have died with interstitial pneumonia, which may respond to corticosteroid therapy.

The dibasic aminoaciduria reflects a failure of reabsorption of lysine, ornithine and arginine by the proximal tubule. There also is a failure to absorb these compounds by the intestinal mucosa. The transport defect occurs at the basolateral, rather than the luminal, membrane. Hyperammonemia reflects a deficiency of intramitochondrial ornithine. An effective treatment is oral citrulline supplementation, which corrects the hyperammonemia by allowing replenishment of the mitochondrial pool of ornithine.

Protein restriction is the mainstay of therapy for the management of urea cycle defects

In patients with very severe disease, tolerance for dietary protein may be so limited that it is not possible to support normal growth.

Treatment with sodium benzoate and sodium phenylacetate represents an important advance in the management of urea cycle defects. Benzoyl-CoA reacts rapidly in the liver with glycine to form hippurate, and phenylacetyl-CoA reacts with glutamine to yield phenylacetylglutamine. Thus, waste nitrogen is eliminated from the body not as urea but as amino acid conjugates of benzoate and phenylacetate [3436]. Excretion of ammonia as phenylacetylglutamine is more efficient than excretion as hippurate because 2 mol of ammonia are excreted with each mole of phenylacetylglutamine. The clinical utility of phenylacetate is limited by its objectionable odor. Sodium phenylbutyrate, which is less malodorous and is converted in the liver to phenylacetate, has been used with success in place of phenylacetate. Acylation therapy has greatly improved the survival and morbidity for selected patients. Thus, the outlook is favorable for heterozygote girls with OTC deficiency treated from an early age [36].

Most patients who survive the neonatal period can be successfully maintained with a diet low in protein and treatment with sodium benzoate. A useful adjunct to treatment in cases of citrullinemia and argininosuccinic aciduria is supplementation of the diet with arginine, which enhances the ability to eliminate ammonia as either citrulline or argininosuccinate. In addition, maintenance of arginine concentrations in the normal range facilitates protein synthesis.

Liver transplantation has been utilized in children with urea cycle defects. The long-term utility is still uncertain. It appears to afford good metabolic correction, although some abnormalities of amino acid metabolism persist even after transplantation. The high morbidity of organ transplantation restricts the utility of this approach.

Dialysis, including hemodialysis and peritoneal dialysis, relieves acute toxicity during fulminant hyperammonemia. Exchange transfusions also have been performed, but this technique has not been equally useful in removing ammonia.

The possibility of gene therapy for these disorders has been a subject of intense scrutiny [37]. An adenoviral vector containing a cDNA for the OTC gene has been given to mice with a congenital deficiency of OTC. The result was complete correction of hepatic OTC activity over a 2-month period. Transient correction of serum glutamine and urine orotic acid was reported. This experimental approach holds enormous promise for the management of this enzymopathy and other inborn errors of intermediary metabolism.

Image ch44f6

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 1999, American Society for Neurochemistry.
Bookshelf ID: NBK27982

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