Survival after treatment with phenylacetate and benzoate for urea-cycle disorders

BACKGROUND

The combination of intravenous sodium phenylacetate and sodium benzoate has been shown to lower plasma ammonium levels and improve survival in small cohorts of patients with historically lethal urea-cycle enzyme defects.

METHODS

We report the results of a 25-year, open-label, uncontrolled study of sodium phenylacetate and sodium benzoate therapy (Ammonul, Ucyclyd Pharma) in 299 patients with urea-cycle disorders in whom there were 1181 episodes of acute hyperammonemia.

RESULTS

Overall survival was 84% (250 of 299 patients). Ninety-six percent of the patients survived episodes of hyperammonemia (1132 of 1181 episodes). Patients over 30 days of age were more likely than neonates to survive an episode (98% vs. 73%, P<0.001). Patients 12 or more years of age (93 patients), who had 437 episodes, were more likely than all younger patients to survive (99%, P<0.001). Eighty-one percent of patients who were comatose at admission survived. Patients less than 30 days of age with a peak ammonium level above 1000 micromol per liter (1804 microg per deciliter) were least likely to survive a hyperammonemic episode (38%, P<0.001). Dialysis was also used in 56 neonates during 60% of episodes and in 80 patients 30 days of age or older during 7% of episodes.

CONCLUSIONS

Prompt recognition of a urea-cycle disorder and treatment with both sodium phenylacetate and sodium benzoate, in conjunction with other therapies, such as intravenous arginine hydrochloride and the provision of adequate calories to prevent catabolism, effectively lower plasma ammonium levels and result in survival in the majority of patients. Hemodialysis may also be needed to control hyperammonemia, especially in neonates and older patients who do not have a response to intravenous sodium phenylacetate and sodium benzoate.

Alternative pathway therapy for urea cycle disorders: twenty years later

Alternative pathway therapy is currently an accepted treatment approach for inborn errors of the urea cycle. This involves the long-term use of oral sodium phenylbutyrate, arginine supplements, or both, depending on the specific enzyme deficiency, and treatment of acute hyperammonemic crises with intravenous sodium benzoate/sodium phenylacetate plus arginine. A review of 20 years of experience with this approach illustrates the strengths and limitations of this treatment. It has clearly decreased the mortality and morbidity from these disorders, but they remain unacceptably high. The medications are generally well tolerated, but severe accidental overdosage has been reported because of the infrequent use of the medication. There is also a difference in their metabolism between newborns and older children that must be addressed in determining dosage. To avoid these complications it is recommended that drug levels in blood be monitored routinely and that very specific treatment protocols and oversight be followed to avoid overdoses. Finally, it must be acknowledged that alternative pathway therapy has limited effectiveness in preventing hyperammonemia and must be combined with effective dietary management. Therefore in children with neonatal-onset disease or in those with very poor metabolic control, liver transplantation should be considered. There should also be the continued search for innovative therapies that may offer a more permanent and complete correction, such as gene therapy.

Persistent arterial hyperammonemia increases the concentration of glutamine and alanine in the brain and correlates with intracranial pressure in patients with fulminant hepatic failure

In this prospective study of patients with fulminant hepatic failure (FHF), we tested the hypothesis that arterial hyperammonemia results in cerebral accumulation of the osmotic active amino acids glutamine and alanine, processes that were expected to correlate with intracranial pressure (ICP). By using in vivo brain microdialysis technique together with ICP monitoring in 17 FHF patients (10 females/7 males; median age 49 (range 18 to 66) years), we found that arterial ammonia concentration correlated to brain content of glutamine (r=0.47; P<0.05) but not to alanine. A persisting high arterial ammonia concentration (above 200 micromol/L) characterized patients who developed high ICP (n=8) while patients who did not experience surges of increased ICP (n=9) had a decline in the ammonia level (P<0.05). Moreover, brain glutamine and alanine concentrations were higher at baseline and increased further in patients who developed intracranial hypertension compared with patients who experienced no surges of high ICP. Brain glutamine concentration increased 32% from baseline to 6536 (697 to 9712) micromol/L (P<0.05), and alanine 44% from baseline to 104 (81 to 381) micromol/L (P<0.05). Brain concentration of glutamine (r=0.59, P<0.05), but not alanine, correlated to ICP. Also arterial ammonia concentration correlated to ICP (r=0.73, P<0.01). To conclude, this study shows that persistence of arterial hyperammonemia is associated with profound changes in the cerebral concentration of glutamine and alanine. The elevation of brain glutamine concentration correlated to ICP in patients with FHF.

Glutamine in the central nervous system: function and dysfunction

Glutamine (Gln) abounds in the central nervous system (CNS), and its interstitial and cerebrospinal fluid (CSF) concentrations are at least one order of magnitude higher than of any other amino acid. Gln transport from blood to the brain is insufficient to meet the demand of the brain tissues for this amino acid. This demand is met by intracerebral Gln synthesis from glutamate (Glu), a reaction carried out by glutamine synthetase (GS), an enzyme residing in astrocytes. A major proportion of astroglia-derived Gln is shuttled to neurons where it is degraded by phosphate-activated glutaminase (PAG) giving rise to the excitatory neurotransmitter amino acid Glu, which is also a precursor of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Glu released from neurons is taken up by astrocytes, and reconverted to Gln, closing the so called "glutamate-glutamine" cycle. A portion of Gln serves as an energy metabolite, and part of it leaves the brain to blood. Gln efflux from astrocytes, its neuronal uptake and egress to the blood via the cerebral capillary endothelial cells is mediated by different amino acid carriers showing i) considerable preference for Gln, ii) distribution between astrocytes and neurons that favors astrocyte-to-neuron fluxes of the amino acid. The Gln-specific carriers also largely contribute to Gln efflux from the brain to the vascular bed. Excessive accumulation of Gln in brain cells may be deleterious to brain function. In hyperammonemia associated with acute liver failure, excess Gln leads to cerebral edema, which largely results from its interference with mitochondrial function and partly from its osmotic action. Future analyses of the roles of Gln in both normal and abnormal cerebral metabolism and function will have to account for its newly recognized direct involvement in the regulation of gene transcription and/or translation.

Mechanisms of hyperammonemia

Hyperammonemia is mainly found in hepatic encephalopathy and in genetic defects of the urea cycle or other pathways of the intermediary metabolism. Clinically a difference has to be made between chronic moderate hyperammonemia and acutely increased concentrations. Pathogenetic mechanisms of ammonia toxicity to the brain are partly unraveled. In some animal models confounding variables, such as the reduced intake of food and amino acid imbalance due to liver insufficiency, do not allow to establish unequivocal causal relationships between the ammonia concentration and measured effects. In chronic moderate hyperammonemia an increased flux through the serotonin pathway is a key factor. It is caused by an increased transport of large neutral amino acids (including tryptophan) through the blood-brain barrier, accentuated by the imbalance of plasma amino acids in hepatic insufficiency. It is stimulated by D- or L-glutamine. Evidence is presented showing that a functioning gamma-glutamyl cycle (glutathione formation) is a prerequisite. In acute hyperammonemia involvement of NMDA receptors, glutamate, NO and cGMP plays an additional role. In hyperammonemic crises the increased cerebral blood flow leads to brain edema; factors discussed here are increased osmolytes in astrocytes and serotoninergic activity. Recent data indicate that axonal development is affected by ammonia and can be normalized in vitro by creatine supplementation in developing mixed brain cell aggregate cultures, thus reviving the old hypothesis of the impact of hyperammonemia on energy metabolism in the developing brain that could cause mental retardation.

Ammonia toxicity and its prevention in inherited defects of the urea cycle

The urea cycle is the final pathway for removal of surplus nitrogen from the body, and the major route in humans for detoxification of ammonia. The full complement of enzymes is expressed only in liver. Inherited deficiencies of urea cycle enzymes lead to hyperammonaemia, which causes brain damage. Severe defects present with hyperammonaemic crises in neonates. Equally devastating episodes may occur in previously asymptomatic adults with mild defects, most often X-linked ornithine transcarbamylase (OTC) deficiency. Several mechanisms probably contribute to pathogenesis. Treatment aims to reduce plasma ammonia quickly, reduce production of waste nitrogen, dispose of waste nitrogen using alternative pathways to the urea cycle and replace arginine. These therapies have increased survival and probably improve the neurological outcome. Arginine, sodium benzoate, sodium phenylbutyrate and, less often, sodium phenylacetate are used. Long-term correction is achieved by liver transplantation. Gene therapy for OTC deficiency is effective in animals, and work is ongoing to improve persistence and safety.

Neuroinflammation increases GABAergic tone and impairs cognitive and motor function in hyperammonemia by increasing GAT-3 membrane expression. Reversal by sulforaphane by promoting M2 polarization of microglia

BACKGROUND

Hyperammonemia induces neuroinflammation and increases GABAergic tone in the cerebellum which contributes to cognitive and motor impairment in hepatic encephalopathy (HE). The link between neuroinflammation and GABAergic tone remains unknown. New treatments reducing neuroinflammation and GABAergic tone could improve neurological impairment. The aims were, in hyperammonemic rats, to assess whether: (a) Enhancing endogenous anti-inflammatory mechanisms by sulforaphane treatment reduces neuroinflammation and restores learning and motor coordination. (b) Reduction of neuroinflammation by sulforaphane normalizes extracellular GABA and glutamate-NO-cGMP pathway and identify underlying mechanisms. (c) Identify steps by which hyperammonemia-induced microglial activation impairs cognitive and motor function and how sulforaphane restores them.

METHODS

We analyzed in control and hyperammonemic rats, treated or not with sulforaphane, (a) learning in the Y maze; (b) motor coordination in the beam walking; (c) glutamate-NO-cGMP pathway and extracellular GABA by microdialysis; (d) microglial activation, by analyzing by immunohistochemistry or Western blot markers of pro-inflammatory (M1) (IL-1b, Iba-1) and anti-inflammatory (M2) microglia (Iba1, IL-4, IL-10, Arg1, YM-1); and (e) membrane expression of the GABA transporter GAT-3.

RESULTS

Hyperammonemia induces activation of astrocytes and microglia in the cerebellum as assessed by immunohistochemistry. Hyperammonemia-induced neuroinflammation is associated with increased membrane expression of the GABA transporter GAT-3, mainly in activated astrocytes. This is also associated with increased extracellular GABA in the cerebellum and with motor in-coordination and impaired learning ability in the Y maze. Sulforaphane promotes polarization of microglia from the M1 to the M2 phenotype, reducing IL-1b and increasing IL-4, IL-10, Arg1, and YM-1 in the cerebellum. This is associated with astrocytes deactivation and normalization of GAT-3 membrane expression, extracellular GABA, glutamate-nitric oxide-cGMP pathway, and learning and motor coordination.

CONCLUSIONS

Neuroinflammation increases GABAergic tone in the cerebellum by increasing GAT-3 membrane expression. This impairs motor coordination and learning in the Y maze. Sulforaphane could be a new therapeutic approach to improve cognitive and motor function in hyperammonemia, hepatic encephalopathy, and other pathologies associated with neuroinflammation by promoting microglia differentiation from M1 to M2.

Unmasked adult-onset urea cycle disorders in the critical care setting

Most often, urea cycle disorders have been described as acute onset hyperammonemia in the newborn period; however, there is a growing awareness that urea cycle disorders can present at almost any age, frequently in the critical care setting. This article presents three cases of adult-onset hyperammonemia caused by inherited defects in nitrogen processing in the urea cycle, and reviews the diagnosis, management, and pathophysiology of adult-onset urea cycle disorders. Individuals who have milder molecular urea cycle defects can lead a relatively normal life until a severe environmental stress triggers a hyperammonemic crisis. Comorbid conditions such as physical trauma often delay the diagnosis of the urea cycle defect. Prompt recognition and treatment are essential in determining the outcome of these patients.

The kidney plays a major role in the hyperammonemia seen after simulated or actual GI bleeding in patients with cirrhosis

Upper gastrointestinal (UGI) bleeding in cirrhosis is associated with enhanced ammoniagenesis, the site of which is thought to be the colon. The aims of this study were to evaluate interorgan metabolism of ammonia following an UGI bleed in patients with cirrhosis. Study 1: UGI bleed was simulated in 8 patients with cirrhosis and a transjugular intrahepatic portasystemic stent-shunt (TIPSS) by intragastric infusion of an amino acid solution that mimics the hemoglobin molecule. We sampled blood from the femoral artery and a femoral, renal, portal, and hepatic vein for 4 hours during the simulated bleed and measured plasma flows across these organs. Study 2: In 9 cirrhotic patients with an acute UGI bleed that underwent TIPSS insertion, blood was sampled from an artery and a hepatic, renal, and portal vein, and plasma flows were measured. Study 1: During the simulated bleed, arterial concentrations of ammonia increased significantly (P =.002). There was no change in ammonia production from the portal drained viscera, but renal ammonia production increased 6-fold (P =.008). In contrast to an unchanged ammonia removal by the liver, a significant increase in muscle ammonia removal was observed. Study 2: In patients with an acute UGI bleed, ammonia was only produced by the kidneys (572 [184] nmol/kg bw/min) and not by the splanchnic area (-121 [87] nmol/kg bw/min). In conclusion, enhanced renal ammonia release has an important role in the hyperammonemia that follows an UGI bleed in patients with cirrhosis. During this hyperammonemic state, muscle is the major site of ammonia removal.

Current strategies for the management of neonatal urea cycle disorders

The treatment of newborns with urea cycle disorders has evolved over the years into a complex multidisciplinary effort. The complexity derives from the number of issues that must be addressed simultaneously. At the Urea Cycle Disorders Consensus Meeting held in Washington, D.C., a panel of physicians and other professionals with extensive experience in this field was assembled to bring some systematization to this task. This manuscript is a condensation of the collective opinion and experience of that group. The outcome of untreated or poorly treated patients with urea cycle disorders is universally bad. Although a favorable outcome is not always feasible, even with the best therapy, the methods outlined here should help treat such a patient by drawing on the experience of others who have treated patients with urea cycle disorders. This article does not purport to be the final word in treating children with these disorders. However, by establishing some common ground, new methods can be tried and compared with existing ones. In a future that holds the prospect of gene therapy "cures" for these diseases, striving for the best possible outcome in the critical newborn period is a worthy goal.

Disorders of carnitine biosynthesis and transport

Carnitine is a hydrophilic quaternary amine that plays a number of essential roles in metabolism with the main function being the transport of long-chain fatty acids from the cytosol to the mitochondrial matrix for β-oxidation. Carnitine can be endogenously synthesized. However, only a small fraction of carnitine is obtained endogenously while the majority is obtained from diet, mainly animal products. Carnitine is not metabolized and is excreted in urine. Carnitine homeostasis is regulated by efficient renal reabsorption that maintains carnitine levels within the normal range despite variabilities in dietary intake. Diseases occurring due to primary defects in carnitine metabolism and homeostasis are comprised in two groups: disorders of carnitine biosynthesis and carnitine transport defect. While the hallmark of carnitine transport defect is profound carnitine depletion, disorders of carnitine biosynthesis do not cause carnitine deficiency due to the fact that both carnitine obtained from diet and efficient renal carnitine reabsorption can maintain normal carnitine levels with the absence of endogenously synthesized carnitine. Carnitine transport defect phenotype encompasses a broad clinical spectrum including metabolic decompensation in infancy, cardiomyopathy in childhood, fatigability in adulthood, or absence of symptoms. The phenotypes associated with the carnitine transport defect result from the unavailability of enough carnitine to perform its functions particularly in fatty acid β-oxidation. Carnitine biosynthetic defects have been recently described and the phenotypic consequences of these defects are still emerging. Although these defects do not result in carnitine deficiency, they still could be associated with pathological phenotypes due to excess or deficiency of intermediate metabolites in the carnitine biosynthetic pathway and potential carnitine deficiency in early stages of life when brain and other organs develop. In addition to these two groups of primary carnitine defects, several metabolic diseases and medical conditions can result in excessive carnitine loss leading to a secondary carnitine deficiency.

Cerebral metabolic disturbances in the brain during acute liver failure: from hyperammonemia to energy failure and proteolysis

Several observations suggest that patients with fulminant hepatic failure may suffer from disturbances in cerebral metabolism that can be related to elevated levels of arterial ammonia. One effect of ammonia is the inhibition of the rate limiting TCA cycle enzyme alpha-ketoglutarate dehydrogenase (alphaKGDH) and possibly also pyruvate dehydrogenase, but this has been regarded to be of no quantitative importance. However, recent studies justify a revision of this point of view. Based on published data, the following sequence of events is proposed. Inhibition of alphaKGDH both enhances the detoxification of ammonia by formation of glutamine from alpha-ketoglutarate and reduces the rate of NADH and oxidative ATP production in astrocytic mitochondria. In the astrocytic cytosol this will lead to formation of lactate even in the presence of sufficient oxygen supply. Since the aspartate-malate shuttle is compromised, there is a risk of depletion of mitochondrial NADH and ATP unless compensatory mechanisms are recruited. One likely compensatory mechanism is the use of amino acids for energy production. Branched chain amino acids, like isoleucine and valine can supply carbon skeletons that bypass the alphaKGDH inhibition and maintain TCA cycle activity. Large-scale consumption of certain amino acids can only be maintained by cerebral proteolysis, as has been observed in these patients. This hypothesis provides a link between hyperammonemia, ammonia detoxification by glutamine production, cerebral lactate production, and cerebral catabolic proteolysis in patients with FHF.

Cerebral effects of ammonia in liver disease: current hypotheses

Hyperammonemia is necessary for development of the cerebral complications to liver disease including hepatic encephalopathy and cerebral edema but the mechanisms are unclear. Ammonia is taken up by the brain in proportion to its arterial concentration. The flux into the brain is most likely by both diffusion of NH3 and mediated transport of NH4 (+) . Astrocytic detoxification of ammonia involves formation of glutamine at concentrations high enough to produce cellular edema, but compensatory mechanisms reduce this effect. Glutamine can be taken up by astrocytic mitochondria and initiate the mitochondrial permeability transition but the clinical relevance is uncertain. Elevated astrocytic glutamine interferes with neurotransmission. Thus, animal studies show enhanced glutamatergic neurotransmission via the NMDA receptor which may be related to the acute cerebral complications to liver failure, while impairment of the NMDA activated glutamate-NO-cGMP pathway could relate to the behavioural changes seen in hepatic encephalopathy. Elevated glutamine also increases GABA-ergic tone, an effect which is aggravated by mitochondrial production of neurosteroids; this may relate to decreased neurotransmission and precipitation of encephalopathy by GABA targeting drugs. Hyperammonemia may compromise cerebral energy metabolism as elevated cerebral lactate is generally reported. Hypoxia is unlikely since cerebral oxygen:glucose utilisation and lactate:pyruvate ratio are both normal in clinical studies. Ammonia inhibits α-ketoglutaratedehydrogenase in isolated mitochondria, but the clinical relevance is dubious due to the observed normal cerebral oxygen:glucose utilization. Recent studies suggest that ammonia stimulates glycolysis in excess of TCA cycle activity, a hypothesis that may warrant further testing, in being in accordance with the limited clinical observations.

Gut microbiota drive the development of neuroinflammatory response in cirrhosis in mice

The mechanisms behind the development of hepatic encephalopathy (HE) are unclear, although hyperammonemia and systemic inflammation through gut dysbiosis have been proposed. The aim of this work was to define the individual contribution of hyperammonemia and systemic inflammation on neuroinflammation in cirrhosis using germ-free (GF) and conventional mice. GF and conventional C57BL/6 mice were made cirrhotic using CCl4 gavage. These were compared to their noncirrhotic counterparts. Intestinal microbiota, systemic and neuroinflammation (including microglial and glial activation), serum ammonia, intestinal glutaminase activity, and cecal glutamine content were compared between groups. GF cirrhotic mice developed similar cirrhotic changes to conventional mice after 4 extra weeks (16 vs. 12 weeks) of CCl4 gavage. GF cirrhotic mice exhibited higher ammonia, compared to GF controls, but this was not associated with systemic or neuroinflammation. Ammonia was generated through increased small intestinal glutaminase activity with concomitantly reduced intestinal glutamine levels. However, conventional cirrhotic mice had intestinal dysbiosis as well as systemic inflammation, associated with increased serum ammonia, compared to conventional controls. This was associated with neuroinflammation and glial/microglial activation. Correlation network analysis in conventional mice showed significant linkages between systemic/neuroinflammation, intestinal microbiota, and ammonia. Specifically beneficial, autochthonous taxa were negatively linked with brain and systemic inflammation, ammonia, and with Staphylococcaceae, Lactobacillaceae, and Streptococcaceae. Enterobacteriaceae were positively linked with serum inflammatory cytokines.

CONCLUSION

Gut microbiota changes drive development of neuroinflammatory and systemic inflammatory responses in cirrhotic animals. (Hepatology 2016;64:1232-1248).

Hyperammonemia and proteostasis in cirrhosis

PURPOSE OF REVIEW

Skeletal muscle loss or sarcopenia is a frequent complication of cirrhosis that adversely affects clinical outcomes. As skeletal muscle is the largest store of proteins in the body, proteostasis or protein homeostasis is required for maintenance of muscle mass. This review will focus on disordered skeletal muscle proteostasis in liver disease.

RECENT FINDINGS

Increased skeletal muscle uptake of ammonia initiates responses that result in disordered proteostasis including impaired protein synthesis and increased autophagy. The cellular response to the stress of hyperammonemia (hyperammonemic stress response, HASR) involves the coordinated action of diverse signaling pathways targeting the molecular mechanisms of regulation of protein synthesis. Transcriptional upregulation of myostatin, a TGFβ superfamily member, results in impaired mTORC1 signaling. Phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α) also relates to decreased global protein synthesis rates and mTORC1 signaling. Ammonia also causes mitochondrial and bioenergetic dysfunction because of cataplerosis of α-ketoglutarate. Lowering ammonia, targeting components of HASR and regulating cellular amino acid levels can potentially restore proteostasis.

SUMMARY

Signaling via myostatin and eIF2α phosphorylation causes decreases in protein synthesis and mTORC1 activity with a parallel mitochondrial dysfunction and increased autophagy contributing to proteostasis perturbations during skeletal muscle hyperammonemia of liver disease.

Systemic oxidative stress is implicated in the pathogenesis of brain edema in rats with chronic liver failure

Chronic liver failure leads to hyperammonemia, a central component in the pathogenesis of hepatic encephalopathy (HE); however, a correlation between blood ammonia levels and HE severity remains controversial. It is believed oxidative stress plays a role in modulating the effects of hyperammonemia. This study aimed to determine the relationship between chronic hyperammonemia, oxidative stress, and brain edema (BE) in two rat models of HE: portacaval anastomosis (PCA) and bile-duct ligation (BDL). Ammonia and reactive oxygen species (ROS) levels, BE, oxidant and antioxidant enzyme activities, as well as lipid peroxidation were assessed both systemically and centrally in these two different animal models. Then, the effects of allopurinol (xanthine oxidase inhibitor, 100mg/kg for 10days) on ROS and BE and the temporal resolution of ammonia, ROS, and BE were evaluated only in BDL rats. Similar arterial and cerebrospinal fluid ammonia levels were found in PCA and BDL rats, both significantly higher compared to their respective sham-operated controls (p<0.05). BE was detected in BDL rats (p < 0.05) but not in PCA rats. Evidence of oxidative stress was found systemically but not centrally in BDL rats: increased levels of ROS, increased activity of xanthine oxidase (oxidant enzyme), enhanced oxidative modifications on lipids, as well as decreased antioxidant defense. In PCA rats, a preserved oxidant/antioxidant balance was demonstrated. Treatment with allopurinol in BDL rats attenuated both ROS and BE, suggesting systemic oxidative stress is implicated in the pathogenesis of BE. Analysis of ROS and ammonia temporal resolution in the plasma of BDL rats suggests systemic oxidative stress might be an important "first hit", which, followed by increases in ammonia, leads to BE in chronic liver failure. In conclusion, chronic hyperammonemia and oxidative stress in combination lead to the onset of BE in rats with chronic liver failure.

Inborn Errors of Metabolism with Hyperammonemia: Urea Cycle Defects and Related Disorders

The urea cycle disorders are a group of inherited biochemical diseases caused by a complete or partial deficiency of any one of the enzymes or transport proteins required to convert toxic ammonia into urea and to produce arginine and citrulline. The clinical manifestations of these disorders are mostly the result of acute or chronic hyperammonemia, which affects the central nervous system. Affected individuals can also develop hepatic dysfunction. These disorders can present at any age from the immediate newborn to later in life. Early diagnosis and treatment are key to improving outcomes.

N-carbamylglutamate protects patients with decompensated propionic aciduria from hyperammonaemia

In patients with propionic aciduria, the accumulating metabolite propionyl-CoA causes a disturbance of the urea cycle via the inhibition of N-acetylglutamate synthesis. Lack of this allosteric activator results in an inhibition of carbamoylphosphate synthase (CPS). This finally leads to hyperammonaemia. In two patients with decompensated propionic aciduria the CPS activator carbamylglutamate was tested for its ability to antagonize the propionyl-CoA associated hyperammonaemia. Oral carbamyl glutamate administration resulted in a significant increase in ammonia detoxification and could avoid further dialysis therapy. Safe, fast and easy to administer, carbamyl glutamate improves the acute therapy of decompensated propionic aciduria by increasing ammonia detoxification and avoiding hyperammonaemia.

Sodium phenylbutyrate decreases plasma branched-chain amino acids in patients with urea cycle disorders

Sodium phenylbutyrate (NaPBA) is a commonly used medication for the treatment of patients with urea cycle disorders (UCDs). Previous reports involving small numbers of patients with UCDs have shown that NaPBA treatment can result in lower plasma levels of the branched-chain amino acids (BCAA) but this has not been studied systematically. From a large cohort of patients (n=553) with UCDs enrolled in the Longitudinal Study of Urea Cycle Disorders, a collaborative multicenter study of the Urea Cycle Disorders Consortium, we evaluated whether treatment with NaPBA leads to a decrease in plasma BCAA levels. Our analysis shows that NaPBA use independently affects the plasma BCAA levels even after accounting for multiple confounding covariates. Moreover, NaPBA use increases the risk for BCAA deficiency. This effect of NaPBA seems specific to plasma BCAA levels, as levels of other essential amino acids are not altered by its use. Our study, in an unselected population of UCD subjects, is the largest to analyze the effects of NaPBA on BCAA metabolism and potentially has significant clinical implications. Our results indicate that plasma BCAA levels should to be monitored in patients treated with NaPBA since patients taking the medication are at increased risk for BCAA deficiency. On a broader scale, these findings could open avenues to explore NaPBA as a therapy in maple syrup urine disease and other common complex disorders with dysregulation of BCAA metabolism.

Evidence of a vicious cycle in glutamine synthesis and breakdown in pathogenesis of hepatic encephalopathy-therapeutic perspectives

There is substantial clinical and experimental evidence that ammonia is a major factor in the pathogenesis of hepatic encephalopathy. In the article is demonstrated that in hepatocellular dysfunction, ammonia detoxification to glutamine (GLN) in skeletal muscle, brain, and likely the lungs, is activated. In addition to ammonia detoxification, enhanced GLN production may exert beneficial effects on the immune system and gut barrier function. However, enhanced GLN synthesis may exert adverse effects in the brain (swelling of astrocytes or altered neurotransmission) and stimulate catabolism of branched-chain amino acids (BCAA; valine, leucine, and isoleucine) in skeletal muscle. Furthermore, the majority of GLN produced is released to the blood and catabolized in enterocytes and the kidneys to ammonia, which due to liver injury escapes detoxification to urea and appears in peripheral blood. As only one molecule of ammonia is detoxified in GLN synthesis whereas two molecules may appear in GLN breakdown, these events can be seen as a vicious cycle in which enhanced ammonia concentration activates synthesis of GLN leading to its subsequent catabolism and increase in ammonia levels in the blood. These alterations may explain why therapies targeted to intestinal bacteria have only a limited effect on ammonia levels in patients with liver failure and indicate the needs of new therapeutic strategies focused on GLN metabolism. It is demonstrated that each of the various treatment options targeting only one the of the ammonia-lowering mechanisms that affect GLN metabolism, such as enhancing GLN synthesis (BCAA), suppressing ammonia production from GLN breakdown (glutaminase inhibitors and alpha-ketoglutarate), and promoting GLN elimination (phenylbutyrate) exerts substantial adverse effects that can be avoided if their combination is tailored to the specific needs of each patient.

Zinc and protein metabolism in chronic liver diseases

The capacity to metabolize proteins is closely related to the hepatic functional reserve in patients with chronic liver disease, and hypoalbuminemia and hyperammonemia develop along with hepatic disease progression. Zinc deficiency, which is frequently observed in patients with chronic liver disease, significantly affects protein metabolism. Ornithine transcarbamylase is a zinc enzyme involved in the urea cycle. Its activity decreases because of zinc deficiency, thereby reducing hepatic capacity to metabolize ammonia. Because the glutamine-synthesizing system in skeletal muscles compensates for the decrease in ammonia metabolism, hyperammonemia does not develop in the early stages of chronic liver disease. However, branched-chain amino acids (BCAAs) are consumed with the increase in glutamine-synthesizing system reactions, leading to a decreased capacity to synthesize proteins, including albumin, due to amino acid imbalance. Upon further disease progression, skeletal muscle mass decreases because of nutritional deficiency, as well as the further decreased capacity to metabolize ammonia in the liver, whereby the capacity to detoxify ammonia reduces as a whole, resulting in hyperammonemia. BCAA supplementation therapy for nutritional deficiency in liver cirrhosis improves survival by correcting amino acid imbalance via recovery of the capacity to synthesize albumin, while zinc supplementation therapy improves the capacity to metabolize ammonia in the liver. Here, the efficacy of a combination of BCAA and zinc preparation for nutritional deficiency in liver cirrhosis, as well as its theoretical background, was reviewed.

Renal replacement therapy in the treatment of confirmed or suspected inborn errors of metabolism

OBJECTIVE

Analysis of mortality and risk factors for mortality in the use of renal replacement therapy to correct metabolic disturbances associated with confirmed or suspected inborn errors of metabolism.

STUDY DESIGN

A retrospective review of an institutional review board-approved pediatric acute renal failure data base at the University of Michigan. Eighteen patients underwent 21 renal replacement therapy treatments for metabolic disturbances caused by urea cycle defects (n = 14), organic acidemias (n = 5), idiopathic hyperammonemia (n = 1), and Reye syndrome (n = 1).

RESULTS

There were 14 boys (74%) and 4 girls (26%), with a mean age and weight of 56.2 +/- 71.0 months and 18.5 +/- 19.2 kg, respectively, at the initiation of renal replacement therapy. Overall treatment mortality rate was 57.2% (12 of 21 treatments), with 11 of the 18 patients (61.1%) dying before hospital discharge. Two-year follow-up on those patients demonstrated that 5 patients (71.4%) remained alive. Initial therapy with hemodialysis was associated with improved survival. Ten treatments (47.6%) required transition to another form of renal replacement therapy to maintain ongoing metabolic control, with a mean duration of 6.1 +/- 9.8 days. Time to renal replacement therapy >24 hours was associated with an increased risk of mortality, whereas a blood pressure >5th percentile for age at the initiation of therapy and the use of anticoagulation were associated with a decreased risk of mortality.

CONCLUSIONS

Renal replacement therapy can correct the metabolic disturbances that accompany suspected or confirmed inborn errors of metabolism. Our experience demonstrates an approximately 60% mortality rate associated with renal replacement treatment, with more than 70% of survivors living longer than 2 years.

Ultrasonographic findings in dogs with hyperammonemia: 90 cases (2000–2002)

OBJECTIVE

To determine ultrasonographic abnormalities in dogs with hyperammonemia.

DESIGN

Retrospective study.

ANIMALS

90 client-owned dogs with hyperammonemia.

PROCEDURE

Ultrasonography of the abdominal vessels and organs was performed in a systematic way. Dogs in which the ultrasonographic diagnosis was a congenital portosystemic shunt were included only if they underwent laparotomy or necropsy. Dogs in which the abdominal vasculature appeared normal and dogs in which the ultrasonographic diagnosis was acquired portosystemic shunts and portal hypertension were included only if liver biopsy specimens were submitted for histologic examination.

RESULTS

Ultrasonography excluded portosystemic shunting in 11 dogs. Acquired portosystemic shunts were found in 17 dogs, of which 3 had arterioportal fistulae and 14 had other hepatic abnormalities. Congenital portosystemic shunts were found in 61 dogs, of which 19 had intrahepatic shunts and 42 had extrahepatic shunts. Intrahepatic shunts originated from the left portal branch in 14 dogs and the right portal branch in 5. Extrahepatic shunts originated from the splenic vein, the right gastric vein, or both and entered the caudal vena cava or the thorax. Ultrasonography revealed splenic-caval shunts in 24 dogs, right gastric-caval shunts in 9 dogs, splenic-azygos shunts in 8 dogs, and a right gastric-azygos shunt in 1 dog.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that ultrasonography is a reliable diagnostic method to noninvasively characterize the underlying disease in dogs with hyperammonemia. A dilated left testicular or ovarian vein was a reliable indicator of acquired portosystemic shunts.

Effect of ammonia on astrocytic glutamate uptake/release mechanisms

Hyperammonemic disorders such as acute liver failure (ALF) or urea cycle enzymopathies are associated with hyperexcitability, seizures, brain edema and increased extracellular brain glutamate. Mechanisms responsible for increased glutamate content in the extracellular space of the brain include decreased uptake by perineuronal astrocytes and/or increased release from neurons and/or astrocytes. Exposure of astrocytes to millimolar concentrations of ammonia results in cell swelling, loss of expression of the glutamate transporters excitatory amino acid transporter (EAAT-1) and EAAT-2 and increased release of glutamate. Three distinct mechanisms are theoretically possible to explain ammonia-induced glutamate release from astrocytes namely: release due to swelling; reversal of glutamate transporters and due to Ca2+-dependent vesicular release. Recent identification of vesicular docking and fusion proteins in astrocytes together with glutamate-release (due to intracellular alkanization and mobilization of intracellular Ca2+-stores) studies implies that vesicular release is a predominant mechanism responsible for ammonia-induced release of glutamate from astrocytes.