Interorgan ammonia metabolism in liver failure: the basis of current and future therapies

Authors

  • Gavin Wright,

    1. Liver Failure Group, the UCL Institute of Hepatology, Division of Medicine, University College London, London, UK
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  • Lorette Noiret,

    1. Liver Failure Group, the UCL Institute of Hepatology, Division of Medicine, University College London, London, UK
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  • Steven W. M. Olde Damink,

    1. Department of General Surgery, Division of Surgery and Interventional Science, University College London, London, UK
    2. The Department of Surgery, Academic Hospital Maastricht, University, Maastricht, the Netherlands
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  • Rajiv Jalan

    1. Liver Failure Group, the UCL Institute of Hepatology, Division of Medicine, University College London, London, UK
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Correspondence
Rajiv Jalan, The Institute of Hepatology, UCL, 69-75 Chenies Mews, London WC1E 6H, UK
Tel: +44207 679 6552
Fax: +44207 380 0405
e-mail: r.jalan@ucl.ac.uk

Abstract

Hepatic encephalopathy complicates the course of both acute and chronic liver disease and its treatment remains an unmet clinical need. Ammonia is thought to be central in its pathogenesis and remains an important target of current and future therapeutic approaches. In liver failure, the main detoxification pathway of ammonia metabolism is compromised leading to hyperammonaemia. In this situation, the other ammonia-regulating pathways in multiple organs assume important significance. The present review focuses upon interorgan ammonia metabolism in health and disease describing the role of the key enzymes, glutamine synthase and glutaminase. Better understanding of these alternative pathways are leading to the development of new therapeutic approaches.

Abbreviations
ALF,

acute liver failure;

GDH,

glutamate dehydrogenase;

GS,

glutamine synthetase;

HE,

hepatic encephalopathy;

LOLA,

l-ornithine l-aspartate;

OP,

ornithine phenylacetate;

PAG,

phosphate-activated glutaminase.

For over 100 years, ammonia has been thought to be central in the pathogenesis of hepatic encephalopathy (1). Many organs are involved in regulating whole body ammonia homeostasis (2), with the regulation of arterial levels dependent upon ‘interorgan ammonia metabolism’ in the context of urea cycle disruption and portal-systemic shunting. Previous meta-analysis suggest that current therapeutic strategies aimed at targeting ammonia fall short of the desired impact on clinical HE (3, 4). This review describes interorgan ammonia and amino acid metabolism in health and in patients with liver failure and how these concepts may lead to the development of more targeted therapies.

Ammonia metabolism in health

Nitrogen is necessary for cellular structure and energy. Humans must therefore assimilate reduced nitrogenous compounds as part of our diet, such as protein, free amino acids and ammonia (derived from the splitting of urea and other amino acids). In the body, ammonia (NH3) is co-existent with its charged form ammonium (NH4+). Their relative concentrations are dependent on pH (5). At physiological pH, 98% of total ammonia exists as NH4+, with gaseous NH3 the main diffusible form transported across biological membranes. In this review, we will refer to NH3/NH4+ as simply, ‘ammonia’. Ammonia is hydrophilic and easily transported in plasma. In health, ammonia transport and metabolism are tightly regulated to maintain low plasma concentrations (normal range 10–40 μmol/L), but exists in the mmol/L range in organs such as intestine or kidney. Transport of unionized ammonia into the cell is through diffusion despite low lipid solubility. In metabolic alkalosis, the conversion of NH4+ to NH3 is increased, thus increasing membrane permeability. Conversely within the cell, negatively charged mitochondria show very poor permeability to NH3, but high NH4+ permeability (6). Transmembrane transport of ammonia can be facilitated by constitutive ion channels and transporters such as the Rhesus proteins (7–9) and Aquaporin (10–12), however, their precise role in whole body ammonia metabolism remains unclear.

Interorgan ammonia and amino acid metabolism

Relationship between ammonia, glutamine and glutamate

Whole body ammonia metabolism is dependent on the activity and differential concentrations of certain key enzymes in different organs (13). Glutamine synthetase (GS) converts ammonia and glutamate to glutamine, expending one ATP for every molecule of ammonia consumed. Phosphate-activated glutaminase (PAG) carries out the reciprocal reaction – glutamine to glutamate and ammonia. Glutamine is a non-essential amino acid which is abundant in protein and constitutes 50% of the body's free amino acid pool (14). Glutamine and glutamate are therefore central to organ and whole body nitrogen balance serving as either a nitrogen donor or acceptor; with glutamine acting as a sink for excess ammonia (via GS), or its source (via PAG) (15).

Intestines

Feeding increases intestinal ammonia generation (16) (meats > dairy (17) > vegetable protein (18)), possibly influenced by carbohydrate (19). In addition to dietary protein and intestinal bacterial ammonia production, studies on post-absorptive healthy animals show that 50% of intestinal ammonia is generated from amino acids derived from its blood supply (Fig. 1) (15, 20, 21). The main energy source for enterocytes is circulating glutamine, which is converted by PAG to ammonia and glutamate, for later release into the mesenteric vein (22). In both humans and rats, about 80% of intestinal PAG is found in the small bowel and 20% in the large bowel (23, 24). The intestines are a major glutamine-consuming and ammonia-producing organ (25, 26), with the important role of intestinal PAG in ammonia production reflected by demonstration of substantial intestinal ammonia production in germ free rats (27). These observations serve to highlight that intestinal ‘bacterial’ ammonia production is not an important contributor to systemic ammonia as has been thought historically. In a study of patients with gastrointestinal malignancy who underwent elective abdominal surgery, differences in arterial-venous ammonia concentrations support animal data, with glutamine extraction three-fold higher in the Jejunum than Ileum (and also colon), correlating with jejunal and ileal ammonia release (28), and distribution of intestinal PAG (23, 24). The colon provides the remaining intestinal ammonia, which in dogs has been estimated to be derived from only 9% glutamine, 42% arterial urea and 49% from luminal bacterial breakdown products, glucose, short-chain fatty acids and ketones (20). Therefore in the post-absorptive state, only about 50% of intestinal ammonia arises directly from dietary nitrogen and 50% from circulating amino acids; with equal contributions from the small bowel (with predominant conversion from circulating amino acid) and large bowel (with predominant bacterial amino acid and urea breakdown). Although as discussed later, the small bowel may become increasingly important with progressive liver injury (29).

Figure 1.

 Intestinal ammonia and amino acid production.

Liver

A typical diet contains ∼100 grams protein/day most of which is metabolized by the liver. Excessive dietary nitrogen is either excreted or converted to a non-toxic form. This is achieved in the peri-portal and peri-venous hepatocytes of the liver acinus where specific enzymes reactions are compartmentalized.

Peri-portal hepatocytes

Peri-portal hepatocytes are highly abundant and a prominent site for the ‘hepatic urea cycle’ (and other important enzymes) (Fig. 2). The ‘urea cycle’ converts ammonia to urea, the major end-product of nitrogen metabolism; with 1 mole of urea removing 2 mol of waste nitrogen. This process also involves bicarbonate, such that ammonia detoxification may be affected by alterations in pH. Peri-portal hepatocytes also contain PAG that has a low affinity and high capacity for ammonia (2), which also regulates its activity. PAG is also critical to the urea cycle by providing intramitochondrial glutamate for N-acetylglutamate synthesis, activating carbamoyl-phosphate synthetase (CPS), the first and rate-limiting step in the urea cycle. Therefore, the greater the supply of intestinal ammonia (and glutamine, transported in to the cells by the ‘sodium-coupled neutral amino acid transporter’ (30)), the greater its turnover to urea (31, 32). Alanine is another important source for urea through the actions of the periportal aminotransferases. Collectively, the main substrates for urea synthesis are – portal ammonia (33%), portal glutamine (6–13%), mitochondrial glutamine (20%) derived from e.g. glutamate, and others such as portal and hepatic alanine and glutamine (33–40%) (33).

Figure 2.

 Schematic of the interaction between the urea cycle and Krebs cycle.

Perivenous hepatocytes

These cells surround terminal venules and constitute ∼7% of hepatocytes. They convert ammonia to glutamine (34) via abundant GS, which has high affinity but low capacity for ammonia (2). Therefore, if any ammonia escapes periportal hepatocytes, it can be scavenged and detoxified by perivenous hepatocytes. GS and PAG act in concert to accommodate rapid changes in systemic ammonia levels from one of glutamine uptake to glutamine release (35–37). Hepatic glutamine metabolism, in concert with urea synthesis, is therefore important in systemic ammonia detoxification (32). As such, in the post-absorptive state with a normal healthy liver, hyperammonaemia should never occur in the environment of adequate liver blood flow (Fig. 3).

Figure 3.

 Ammonia-glutamine metabolism in the liver.

Kidney

Urinary ammonia excretion is tightly regulated by mechanisms such as tubular urine flow, apical/basolateral ion exchangers (e.g. Na+–K+–NH4+–ATPase), acid–base balance and the ammonia counter-current system. In the post-absorptive state, glutamine is the main substrate for renal ammoniagenesis (38), but unlike other organs kidney-PAG is strongly inhibited by its product glutamate. Ammonia synthesized by proximal tubular cells is excreted into tubular fluid and quickly reabsorbed by the medullary thick ascending limb (MTAL) (Fig. 4), where it accumulates in the interstitium before being excreted as NH4+ in the medullary collecting ducts. In the post-absorptive state in humans, renal ammonia metabolism is in a steady state (38, 39). However, acute changes in acid–base balance alters renal metabolism of ammonia. The percentage of total renal ammonia excreted in urine can increase from 50 to 70% with acute acidosis and be as low as 18% in alkalosis (reviewed in (40)). In the post-absorptive state in certain animal species (e.g. dogs (41), pigs (42) and rats (43)) 70% of renal ammonia is released into the circulation (renal vein) with only 30% excreted in urine. The kidney can become a net producer or excretor of ammonia (44) under different conditions. In the kidney of healthy volunteers, mild metabolic acidosis induced by prolonged oral ammonium chloride challenge, lead to net urinary ammonia excretion (38); the reverse being with alkalosis. Hormonal control also has a role, with angiotensin II dose-dependently increasing renal ammoniagenesis in the PCT, especially with acidosis (45) by directly increasing MTAL absorption via its effect on the Na+–K+–NH4+–Cl ion-exchanger. Similarly renal ammonia transport is sensitive to the action of specific diuretics such as Frusemide on these ion-exchangers. This could be a potential factor in patients with cirrhosis, as patients may commonly be on drugs that block angiotensin II (e.g. losartan), or diuretics that interact with aldosterone, leading to a reversal of the compensatory increase in urinary ammonia excretion with hyperammonaemia, acidosis or both. This is truer in respect of acid–base balance as the PCT is the major site of control of acidosis, increasing its rates of NH3 production and secretion while generating bicarbonate. However, one should also remember that frusemide and other diuretics may induce encephalopathy, likely consequent on reduced renal perfusion for over diuresis, and potential electrolyte imbalance (worsened by attempts to influence acid–base balance).

Figure 4.

 Renal ammonia and amino acid homeostasis.

Muscle

Although the GS activity of skeletal muscle is low (46), by virtue of mass it can impact greatly on interorgan ammonia metabolism. In healthy volunteers, one study suggested an estimated 50% arterial 13N-ammonia extraction from skeletal thigh muscle (47). However, other studies demonstrate a zero arterial–venous difference across forearm and leg suggesting an absence of muscle ammonia uptake (46, 48–50). Skeletal muscle ammonia consumption also provides an additional route for nitrogen (and carbon) transport from muscle to liver (Cori cycle), with muscle pyruvate transaminated to alanine and then transported to the liver for conversion to ammonia (by ALT). Transfer of the nitrogen chain to α-ketoglutarate regenerates pyruvate, a further substrate for gluconeogenesis (via the Krebs cycle product – acetyl CoA); called the ‘glucose–alanine cycle’ (Fig. 5). The ‘glucose-alanine cycle’ therefore allows skeletal muscle to eliminate nitrogen while replenishing its energy supply. Given the influence of the muscle on whole body ammonia and amino acid metabolism, this highlights the significance of alanine in whole body nitrogen metabolism.

Figure 5.

 Skeletal muscle ammonia and amino acid metabolism.

Brain

Ammonia is important for the regulation of glial cell metabolism. In health, ammonia readily traverses the blood–brain barrier, with positive arterial–venous differences suggesting net brain ammonia uptake; with the rate of brain uptake directly correlating with arterial concentration. Though some studies suggest little/no net ammonia uptake (49, 51), others show that arterial extraction maybe as high as 47% within grey matter (47). Astrocytes, which constitute 20–50% of cortical brain cells, contain 80% of brain GS compared with only 20% in neurons (52, 53). Astrocyte GS preferentially takes up ammonia to form glutamine, which is then transferred to neurons for deamination to re-form GABA and glutamate (important neurotransmitters). Therefore in health, the brain is a significant organ for ammonia utilization and detoxification (47).

Lung

As with other organs the lung possesses an appreciable amount of PAG and GS (13), but little data exists regarding its role in human ammonia metabolism as studies of lung arterial–venous differences are difficult to interpret because of very high and variable blood flow altering the ‘metabolic flux’, which is blood flow dependent. It is not surprising therefore that although there is the potential of the lung to play a role in whole body ammonia metabolism, so far there is little to suggest that in man the lung has a significant net effect. However, in a recent study of 13N-ammonia metabolism in rat lung, there was rapid conversion of ammonia to glutamine (first-pass extraction ∼30%), likely because of GS activity, with little escaping in exhaled air suggesting that the lungs may indeed have an important role in whole body ammonia metabolism (54).

Heart

As the heart is muscle and contains a significant amount of GS (37) and to a lesser extent PAG (55) it may have a potential role in ammonia metabolism, but evidence is limited.

Adipose tissue

There is also limited data on the impact of adipose tissue in whole body ammonia metabolism. Although adipose tissue is a net producer of glutamine (derived from intracellular proteolysis), glycerol and lactate, this appears to be without any net change in uptake or release of ammonia whether in the starved or fed state (56).

Immune cells

Glutamine (and thus ammonia metabolism) is thought to be essential to immune function. Lower glutamine availability from either reduced production or increased metabolism, may impact on immune cell (lymphocyte and macrophage) proliferation, cytokine production and macrophage-mediated phagocytosis (57), though their role in regulating ammonia metabolism is not clear.

Ammonia metabolism in liver failure

The importance of ammonia in patients with liver disease was first identified by Nencki & Pavlov in 1896' who demonstrated that hyperammonaemia induced neurobehavioral change in portacaval shunted (PCS) dogs (58). Because innumerable studies provide the consensus view that ammonia is central to the pathogenesis of HE. The presence of HE defines transition of patients from acute liver injury to acute liver failure (ALF). In ALF, arterial ammonia levels of >150 umol/L predict a poor outcome (59) and correlate with increased ICP and cerebral oedema (59–61); likely because of increased brain ammonia delivery and uptake related to hyperammonemia.

Interorgan ammonia and amino acid metabolism in liver failure (Fig. 6)

Figure 6.

 Interorgan ammonia metabolism in health and cirrhosis.

In liver failure, widespread disturbance in ammonia and glutamine metabolism has been observed in both human (62–66) and animal models (67, 68) which are summarized below.

Intestines

Cirrhosis is associated with a four-fold increase in intestinal PAG activity in the small bowel (29), pointing to the importance of PAG activity in small bowel ammoniagenesis in patients with liver disease (29); though in terms of ‘whole body’ ammoniagenesis in liver failure the small bowels contribution appears relatively small. Hence, PAG might be a new therapeutic target in the management of hepatic encephalopathy. Further studies using PAG inhibitors or PAG knockout mice help clarify the role of increased PAG expression in the pathophysiology of hepatic encephalopathy. In stable cirrhotic patients with a transjugular intrahepatic portosystemic shunt (TIPS), there is net intestinal ammonia production, which directly correlates with glutamine uptake (69). In cirrhotic TIPS patients made hyperammonaemic by either a simulated bleed (amino acid solution mimicking haemoglobin) or acute variceal GI bleeding, there was no sign of increased net intestinal ammonia production, no efflux from the splanchnic circulation or muscle uptake, but instead a significant increase in renal production (six-fold) (64). In pigs, induction of ALF did not provoke net intestinal ammonia production (68). In PCS rats, induction of ALF with hepatic artery ligation (HAL) caused an early, but only slight increase in intestinal ammonia production and glutamine uptake (70). These data suggest that during liver failure the contribution of the intestines to the hyperammonaemic state is mainly due to PCS and not to changes in the ability of the intestines to produce ammonia (68).

Liver

Hepatocyte loss reduces ammonia detoxification by reducing the quantity of periportal urea and perivenous glutamine synthesis (36, 71), but there is usually enough ammonia detoxifying capacity left until advanced liver failure occurs (59). Portal–systemic shunting further reduces ammonia detoxification and may account for 50% of portal flow in patients with cirrhosis (72, 73), or as much as 93% following a TIPS (72, 73). With progressive liver injury, despite increases in periportal glutaminase activity (∼six-fold) and ureagenesis (36), increasing amounts of ammonia pass through to the terminal venules (74). As peri-venous GS capacity also becomes swamped and fails to scavenge ammonia, it spills into the hepatic vein with post-absorptive arterial levels of ammonia approximating 40–60 μM in cirrhosis, 70–90 μM in acute-on-chronic liver failure and 200–240 μM in ALF (59). The lower ammonia levels with stable cirrhosis suggest that the liver is still able to remove the majority of ammonia from the portal vein and hepatic artery until quite late in the disease. It is therefore likely that demonstrable ammonia removal may be occurring despite advanced disease, but somewhat masked by even more significant portal–systemic shunting (59). However, an increase in nitrogen load invariably results in hyperammonaemia.

Muscle

Skeletal muscle ammonia uptake is correlated to arterial levels at various stages of ALF (49, 50) and cirrhosis (46, 48). In ALF patients with advanced HE, skeletal muscle consumes ammonia (100 nmol/100 g/min) with the stoichiometric release of glutamine (62). However, such increased glutamine release may in-part reflect muscle catabolism and resultant amino acid production (62), rather than ammonia conversion from increased GS expression and activity (22, 75). In hyperammonaemic cirrhotics who underwent TIPS for gastrointestinal bleeding, skeletal muscle was also the main site of ammonia removal (64, 76). The severity of hyperammonaemia can be limited in stable cirrhotics who have a normal muscle mass (49), but cirrhotics with significant muscle wasting (46, 48–51), are more likely to exhibit hyperammonaemia. Skeletal muscle therefore provides an alternative therapeutic target for ammonia detoxification (77).

Early clinical studies reported increased circulating glutamate with liver failure (78–80), but recent evidence tends to confirm the converse – a circulating glutamate deficiency (62, 65, 81); due to a reduction in hepatic synthesis and/or conversion of stores to glutamine. However, in ALF (PCS+HAL) devascularized pigs, despite an initially raised arterial glutamine at 2 h, levels eventually fell (eight-fold), possibly due to a fall in glutamate provision (68). In contrast to clinical studies, in these ALF devascularized pigs there was no net skeletal muscle ammonia uptake 6 h post-ALF, despite initially increased at 2 h (68). Absent muscle ammonia uptake was also reported in PCS rats with acute liver ischaemia with no net glutamine efflux despite increased muscle glutamine levels (82). Contrasting results between animal experiments and clinical studies may only reflect differences in species physiology and/or type and extent of liver injury. Also, skeletal muscle ammonia uptake and glutamine release does not necessarily lead to net whole body ammonia detoxification as muscle-derived glutamine can be taken up by the splanchnic region or kidneys and converted to ammonia for release into the circulation.

Kidney

Ammonia excretion is not directly correlated to plasma levels (48). With advancing liver disease the kidney becomes an important source of ammonia (64, 68, 83), especially in patients with declining renal function (e.g. diuretic therapy (84) and hepatorenal syndrome). Animal models allow study of the influence of renal metabolism on hyperammonaemia (85–87). In PCS rats, in response to moderate hyperammonaemia (arterial levels ∼250 μmol) (86), the kidneys adapt by increasing glutamine production and/or decreasing plasma release (64) (reversing the normal 30/70 urinary excretion/renal venous release ratio), resulting in a shift to ammonia excretion. However, in PCS rats 6 h post-induction of ALF, severe hyperammonaemia (arterial levels ∼950 μmol) was associated with reduced ammonia excretion and net renal ammonia production (86). Elevated renal ammonia release in this model was likely triggered by increased muscle glutamine production and renal uptake. Furthermore, in ALF pigs there is a significant reduction in renal glutamine uptake and ammonia excretion in urine with time (68), indicating that in more advanced disease the ability of the kidney to compensate for hyperammonaemia by excreting ammonia is overwhelmed. Splanchnic vasodilatation, associated with over-activated renin–aldosterone–angiotensin system (RAAS), is a characteristic feature of cirrhosis (63, 88). In cirrhotic patients, correcting hypovolaemia therefore increases renal ammonia excretion and reduces plasma ammonia (89). This is supported by the observation of improved renal ammonia excretion following TIPS (66), with a fall in renal ammonia circulatory release to very low values. Collectively these observations support the presence of renal ammonia adaptation with early hyperammonaemia, which with worsening hyperammonaemia and renal dysfunction (e.g. diuretics, dehydration, hepatorenal syndrome etc.) switches to net renal ammonia production and circulatory release.

Brain

Brain delivery, extraction and uptake of ammonia increases in ALF (47, 65, 90), and correlates with arterial levels (51). However, ammonia extraction data are variable, with some reporting extraction only in comatose patients, while others show 11–15% extraction in the non-comatose. Ammonia detoxification (via the amidation of glutamate by GS) produces glutamine accumulation and thus osmotic stress (91), – the ‘ammonia-glutamine-brain swelling hypothesis’. In ALF, brain glutamine correlates with arterial ammonia (92), with about 66% of brain ammonia is metabolized to glutamine (93). As intracellular glutamine increases, astrocytes expel myo-inositol and other weaker osmolytes to try and maintain osmotic equilibrium. However in ALF patients a rapid rise in ammonia may outstrip compensatory mechanisms leading to oedema. In cirrhosis, there is some protection from intracranial hypertension and brain oedema because of the more gradual increase in plasma ammonia concentration allowing time for a compensatory expulsion of weaker intracellular osmolytes (94), though oedema may still occur (95).

Lung

In ALF pigs, lung tissue ammonia levels are about 5–10 × circulating levels (68). However, the role of the lung in regulating ammonia homeostasis is unclear due to very high blood flow, which does not allow transorgan studies (68). With progression of liver disease, the increase in cardiac output and resultant increase in pulmonary perfusion leads to increased glutamine (and inversely ammonia) flux. This is likely related to increased GS activity (96, 97).

Heart

There is limited data on the contribution of the heart to ammonia and glutamine metabolism with liver failure, but given the significant presence of GS (37) and PAG (55) the heart muscle could potentially have an impact.

Adipose tissue

There is little data on the impact of adipose tissue in whole body ammonia tissue during liver failure.

Immune cells

Ammonia has recently been shown to impair neutrophil phagocytosis, potentially via p38-MAPKinase related intracellular signaling pathways and effect on cell morphology, as neutrophils incubated in pathological ammonia levels begin to swell (98). As immune cells are rich in Glutaminase, inflammation may result in increased ammonia production from inflammatory cells but no data in this population is available.

Current and future therapies targeting interorgan ammonia metabolism

Collectively, the evidence for interorgan ammonia and amino acid metabolism in the post-absorptive state in health and liver disease suggests that aside from the liver, systemic ammonia levels are chiefly determined by intestinal and renal ammonia metabolism, though with progressive liver disease the skeletal muscle may have increasingly significant roles. Hence, in liver failure patients, these peripheral organs are important potential targets. For example, purgatives target large intestinal ammonia production, and hypothermia primarily influences cerebral inflammation and energy utilization. But there are a number of interventions targeting multiple organs (Fig. 7). The concept of manipulating endogenous biosynthetic pathways to eliminate non-urea waste nitrogen as a substitute for defective urea synthesis is not new (99). Decreasing total body nitrogen by promoting the synthesis of non-urea nitrogen-containing metabolites that have high excretion rates appears to be of benefit.

Figure 7.

 Therapeutic interventions manipulating interorgan ammonia and amino acid metabolism.

Arginine supplementation

l-arginine is an important dietary substrate for the urea cycle, allowing for ammonia detoxification to urea via arginase (Fig. 7). As only a semi-essential amino acid, as only partially metabolically produced, in some disease states dietary l-arginine supplementation may be required. In childhood urea cycle disorders such CPT deficiency, as ureagenesis is impaired nitrogen accumulates as related amino acids, such that dietary L-arginine restriction over just 15–68 h triggers symptomatic hyperammonaemia (e.g. vomiting, lethargy or irritability) (100). Provision of additional dietary l-arginine is therefore beneficial as it promotes the preferential synthesis of nitrogenous products such as citrulline and argininosuccinate, which can be renally excreted. However, there have been no good clinical studies evaluating a role for l-arginine supplementation in hepatic encephalopathy.

Phenylbutyrate

Phenylbutyrate, which is converted to phenylacetate in vivo, is used for the hyperammonaemia associated with urea cycle enzyme deficiencies (101). Phenylacetate covalently combines with circulating glutamine to form phenylacetylglutamine, which is excreted by the kidneys. In metabolic disorders characterized by elevated glutamine levels (e.g. urea cycle defects), this excess can be mopped up by phenylacetate, thereby removing glutamine as a substrate for ammoniagenesis. Phenylbutyrate is being trialed in HE associated with liver failure.

Sodium benzoate

Similarly increases the renal excretion of ammonia but as hippuric acid. Benzoate has been shown to improve encephalopathy with inborn errors of metabolism (102), and in at least one double-blind randomized control trial was found to be as effective as lactulose in the treatment of acute portosystemic HE (103).

Combined intravenous sodium phenylbutyrate and benzoate in patients with urea cycle enzyme disorders (Ammonul)

Has proved beneficial for the treatment of urea-cycle disorders. In a recent 25-year open-labelled study, combination therapy lead to a 79% reduction in plasma ammonia and 84% improved survival (dependent on peak ammonia level and age – poor in neonates and 98% in late onset disease) in patients with urea cycle enzyme disorders (104). This compares favourably with 16% survival in neonates and 72% with late onset disease in patients without therapy.

l-ornithine l-aspartate

Provides l-ornithine and l-aspartate as substrates for glutamine production at the expense of ammonia. In a double-blind randomized control study of cirrhotics with mild HE, 1 week of LOLA reduced ammonia and improved mental function (105). A cross-over study showed that 20–40 g/day of LOLA infusions ameliorated post-prandial increases in ammonia following oral protein loading (106). Liver failure models further suggest that LOLA reduces brain oedema of advanced HE (107). However, in large randomized controlled clinical trial of patients with ALF, LOLA was not shown to be useful (106). Critically, there are concerns that the ammonia-lowering effects of LOLA may be only transient, as there are reports of rebound hyperammonaemia and HE recurrence on discontinuing LOLA (75), as a significant rise in glutamine levels eventually becomes a source for ammoniagenesis by the kidney and intestinal (through PAG) (76).

l-ornithine phenylacetate (OP)

Is a new novel ammonia-lowering agent, which may overcome the risk of rebound hyperammonaemia with LOLA. OP provides l-ornithine as a substrate muscle GS, thereby detoxifying ammonia into glutamine (Fig. 8). Phenylacetate combines with the glutamine that is generated, excreting it as phenylacetylglutamine (PAG). Studies in animal models of cirrhosis and in acute liver failure have shown convincingly that this molecule successfully reduces ammonia and brain water (108, 109). Importantly, in the ALF pigs, the increase in intracranial pressure was reduced to values that were not significantly different to sham animals (108). Early studies in patients with cirrhosis have been initiated.

Figure 8.

 Manipulation of interorgan ammonia metabolism with ornithine phenylacetate.

The relative contribution of either PAG of GS activity in certain organs has been central to this review. Focusing at particular enzymatic sites like intestinal PAG may therefore prove to be a worthwhile therapeutic target (e.g. with PAG inhibitors) in ‘whole body’ ammoniagenesis in the foreseeable future.

Conclusion

This review highlights the importance of interorgan ammonia and amino acid metabolism in health and diseased states, with systemic ammonia levels not only dictated by intestinal and renal ammonia efflux, but also the ability of skeletal muscle to detoxify ammonia. This greater understanding is already proving invaluable in the development of future therapies to ameliorate the hyperammonaemic state of liver disease.

Acknowledgements

Potential conflict of interest: UCL has filed patents surrounding the use of l-ornithine and phenylacetate for the treatment of hyperammonaemia and hepatic encephalopathy which has been licensed to Ocera Therapeutics.

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