Effect of L-carnitine on cerebral and hepatic energy metabolites in congenitally hyperammonemic sparse-fur mice and its role during benzoate therapy

REVIEW: Practical strategies to maintain anabolism by intravenous nutritional management in children with inborn metabolic diseases

One of the most vital elements of management for patients with inborn errors of intermediary metabolism is the promotion of anabolism, the state in which the body builds new components, and avoidance of catabolism, the state in which the body breaks down its own stores for energy. Anabolism is maintained through the provision of a sufficient supply of substrates for energy, as well as critical building blocks of essential amino acids, essential fatty acids, and vitamins for synthetic function and growth. Patients with metabolic diseases are at risk for decompensation during prolonged fasting, which often occurs during illnesses in which enteral intake is compromised. During these times, intravenous nutrition must be supplied to fully meet the specific nutritional needs of the patient. We detail our approach to intravenous management for metabolic patients and its underlying rationale. This generally entails a combination of intravenous glucose and lipid as well as early introduction of protein and essential vitamins. We exemplify the utility of our approach in case studies, as well as scenarios and specific disorders which require a more careful administration of nutritional substrates or a modification of macronutrient ratios.

Pathophysiology of hepatic encephalopathy and future treatment options

Understanding of the pathophysiology of hepatic encephalopathy has conditioned new treatment options. Ammonia detoxification in hepatic encephalopathy is regulated by two enzymes: glutaminase or glutamine synthetase. The first produces ammonia and the second detoxifies the ammonia, which is why treatments are aimed at glutaminase inhibition or glutamine synthetase activation. At present, we know that both enzymes are found not only in the liver, but also in the muscle, intestine, kidney, and brain. Therefore, current treatments can be directed at each enzyme at different sites. Awareness of those potential treatment sites makes different options of approach possible in the patient with hepatic encephalopathy, and each approach should be personalized.

Carnitine dynamics and their effects on hyperammonemia in cirrhotic Japanese patients

AIM

Supplementation with levocarnitine preparations has been reported to improve hepatic encephalopathy, but no detailed investigations have addressed the dynamics of carnitine or its supplementation indication in cirrhosis patients. We studied carnitine dynamics in cirrhotic patients by measuring serum and liver tissue carnitine levels and tested the effects of levocarnitine supplementation on concurrent hyperammonemia.

METHODS

In a pilot cohort of seven patients with liver cirrhosis and five patients without cirrhosis, the serum and liver carnitine concentrations were measured. Then the serum carnitine fractions were analyzed in 70 liver cirrhosis patients. Among them, a levocarnitine preparation (1800 mg/day) was supplemented orally for 3 months in 27 patients with refractory hyperammonemia, and the effects were evaluated.

RESULTS

A significant correlation was observed between serum and liver tissue carnitine concentrations (r = 0.69, P < 0.05). The serum total carnitine concentration was 68.4 ± 4.7 μmol/L, the free carnitine concentration was 53.2 ± 2.6 μmol/L, and the acylcarnitine concentration was 13.2 ± 1.1 μmol/L in 70 cirrhotic patients (reference values are 45-91, 36-74, 6-23 μmol/L, respectively). There was no correlation between blood ammonia and serum carnitine concentrations. The serum carnitine concentration rose with levocarnitine supplementation, reaching steady state after 1 month and, in parallel, refractory hyperammonemia was significantly improved. The cut-off level for a 20% decrease in blood ammonia was identified as 62.0 μmol/L total carnitine concentration by receiver-operating characteristic curve analysis, with an area under the curve of 0.69.

CONCLUSION

Serum carnitine concentrations were within standard levels in the majority of liver cirrhosis patients. In patients with concurrent hyperammonemia, the levocarnitine supplementation reduced blood ammonia levels.

Acetyl-L-carnitine in hepatic encephalopathy

Hepatic encephalopathy is a common complication of hepatic cirrhosis. The clinical diagnosis is based on two concurrent types of symptoms: impaired mental status and impaired neuromotor function. Impaired mental status is characterized by deterioration in mental status with psychomotor dysfunction, impaired memory, and increased reaction time, sensory abnormalities, poor concentration, disorientation and coma. Impaired neuromotor function include hyperreflexia, rigidity, myoclonus and asterixis. The pathogenesis of hepatic encephalopathy has not been clearly defined. The general consensus is that elevated levels of ammonia and an inflammatory response work in synergy to cause astrocyte to swell and fluid to accumulate in the brain which is thought to explain the symptoms of hepatic encephalopathy. Acetyl-L-carnitine, the short-chain ester of carnitine is endogenously produced within mitochondria and peroxisomes and is involved in the transport of acetyl-moieties across the membranes of these organelles. Acetyl-L-carnitine administration has shown the recovery of neuropsychological activities related to attention/concentration, visual scanning and tracking, psychomotor speed and mental flexibility, language short-term memory, attention, and computing ability. In fact, Acetyl-L-carnitine induces ureagenesis leading to decreased blood and brain ammonia levels. Acetyl-L-carnitine treatment decreases the severity of mental and physical fatigue, depression cognitive impairment and improves health-related quality of life. The aim of this review was to provide an explanation on the possible toxic effects of ammonia in HE and evaluate the potential clinical benefits of ALC.

Acetyl-L-carnitine improves cognitive functions in severe hepatic encephalopathy: a randomized and controlled clinical trial

The aim of this study was to investigate the effects of ALC treatment on cognitive functions in patients with severe hepatic encephalopathy. This was a randomized, double-blind, placebo-controlled study. 61 patients with severe hepatic encephalopathy were recruited to the study. The 2 groups received either 2 g ALC twice a day (n = 30) or placebo (n = 30) for 90 days. Clinical and laboratory assessment, psychometric tests and automated electroencephalogram (EEG) analysis were performed for all patients. At the end of the study period, between the 2 groups we observed a significant difference in Everyday Memory Questionnaire -23.9 vs 4.4 (p < 0.001), Logical Memory (Paragraph recall) test 22.3 vs 0.7 (p < 0.001), Trail Making Test A -7.5 vs -2.6 (p < 0.001), Trail Making Test B -10.5 vs -3.1 (p < 0.001), Controlled Oral Word Association Test 4.2 vs 0.5 (p < 0.001), Hooper test 2.6 vs 0.1 (p < 0.05), Judgement of line orientation 2.8 vs 0.3 (p < 0.001), Digit Cancellation time -24.5 vs -2.4 (p < 0.001), NH₄⁺ 30.5 vs 13.5 (p < 0.001), prothrombin time 2 vs 2.4 (p < 0.05), alanine transaminase -10.7 vs -13.6 (p < 0.001). 88% of patients treated with ALC vs 72% of patients treated with placebo showed a significant improvement in EEG. The improvement of cognitive deficits, the reduction of ammonia, and the modification of EEG in patients treated with ALC suggest that ALC could represent a new tool in the treatment of severe hepatic encephalopathy.

Brain energy metabolism and mitochondrial dysfunction in acute and chronic hepatic encephalopathy

One proposed mechanism for acute and chronic hepatic encephalopathy (HE) is a disturbance in cerebral energy metabolism. It also reviews the current status of this mechanism in both acute and chronic HE, as well as in other hyperammonemic disorders. It also reviews abnormalities in glycolysis, lactate metabolism, citric acid cycle, and oxidative phosphorylation as well as associated energy impairment. Additionally, the role of mitochondrial permeability transition (mPT), a recently established factor in the pathogenesis of HE and hyperammonemia, is emphasized. Energy failure appears to be an important pathogenetic component of both acute and chronic HE and a potential target for therapy.

The mitochondrial permeability transition in neurologic disease

Mitochondria, being the principal source of cellular energy, are vital for cell life. Yet, ironically, they are also major mediators of cell death, either by necrosis or apoptosis. One means by which these adverse effects occur is through the mitochondrial permeability transition (mPT) whereby the inner mitochondrial membrane suddenly becomes excessively permeable to ions and other solutes, resulting in a collapse of the inner membrane potential, ultimately leading to energy failure and cell necrosis. The mPT may also bring about the release of various factors known to cause apoptotic cell death. The principal factors leading to the mPT are elevated levels of intracellular Ca2+ and oxidative stress. Characteristically, the mPT is inhibited by cyclosporin A. This article will briefly discuss the concept of the mPT, its molecular composition, its inducers and regulators, agents that influence its activity and describe the consequences of its induction. Lastly, we will review its potential contribution to acute neurological disorders, including ischemia, trauma, and toxic-metabolic conditions, as well as its role in chronic neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis.

Effects of L-carnitine in patients with hepatic encephalopathy

AIM

To evaluate the influence of L-carnitine on mental conditions and ammonia effects on patients with hepatic encephalopathy (HE).

METHODS

One hundred and fifty patients (10 patients with alcoholism, 41 patients with hepatitis virus B infection, 78 patients with hepatitis C virus infection, 21 patients with cryptogenetic cirrhosis) meeting the inclusion criteria were randomized into group A receiving a 90-d treatment with L-carnitine (2 g twice a day) or into group B receiving placebo in double blind.

RESULTS

At the end of the study period, a significant decrease in NH4 fasting serum levels was found in patients with hepatic encephalopathy (P<0.05) after the treatment with levocarnitine (LC). Significant differences were also found between symbol digit modalities test and block design in patients with hepatic encephalopathy (P<0.05).

CONCLUSION

Results of our study suggest an important protective effect of L-carnitine against ammonia-precipitated encephalopathy in cirrhotic patients.

High glucose upregulates pantothenate kinase 4 (PanK4) and thus affects M2-type pyruvate kinase (Pkm2)

A new Rattus norvegicus PanK gene was isolated by mRNA differential display from high concentration glucose-stimulated rat, which encodes a human PanK4-like protein with KOG2201 and KOG4584 domain. Proteins that interact with rat PanK4 were identified by the application of the yeast two-hybrid system. One of the components, Pkm2, was found to be associated with rat PanK4 and its two domains under both in vitro and in vivo conditions. Immunofluorescence staining and confocal scanning experiments showed that PanK4 could transiently co-express with Pkm2 in the cytoplasm of HeLa cell and HEK293T cell. These findings suggest that PanK4 interacts with Pkm2 and thereby may modulate the glucose metabolism through regulating the activity of Pkm2.

Prevention of ammonia and glutamate neurotoxicity by carnitine: molecular mechanisms

Carnitine has beneficial effects in different pathologies and prevents acute ammonia toxicity (ammonia-induced death of animals). Acute ammonia toxicity is mediated by excessive activation of the NMDA-type of glutamate receptors, which mediates glutamate neurotoxicity. We showed that carnitine prevents glutamate neurotoxicity in primary cultures of cerebellar neurons. This supports the idea that the protective effect of carnitine against ammonia toxicity is due to the protective effect against glutamate neurotoxicity. We are studying the mechanism by which carnitine protects against glutamate neurotoxicity. Carnitine increases the binding affinity of glutamate for metabotropic glutamate receptors. The protective effect of carnitine is lost if metabotropic glutamate receptors are blocked with specific antagonists. Moreover, activation of metabotropic glutamate receptors by specific agonists also prevents glutamate neurotoxicity. This indicates that the protective effect of carnitine against glutamate neurotoxicity is mediated by activation of metabotropic glutamate receptors. The molecule of carnitine has a trimethylamine group. Different compounds containing a trimethylamine group (carbachol, betaine, etc.) also prevent ammonia-induced animal death and glutamate-induced neuronal death. Moreover, metabotropic glutamate receptor antagonists also prevent the protective effect of most of these compounds. We summarize here some studies aimed to identify the mechanism and the molecular target that are responsible for the protective effect of carnitine against ammonia and glutamate neurotoxicity. Finally it is also shown that carnitine inhibits the hydrolysis of inositol phospholipids induced by activation of different types of metabotropic receptors, but this effect seems not responsible for its protective effects.

Ammonia and Alzheimer's disease

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder. Behavioural, cognitive and memory dysfunctions are characteristic symptoms of AD. The formation of amyloid plaques is currently considered as the key event of AD. Other histological hallmarks of the disease are the formation of fibrillary tangles, astrocytosis, and loss of certain neuronal systems in cortical areas of the brain. A great number of possible aetiologic and pathogenetic factors of AD have been published in the course of the last two decades. Among the toxic factors, which have been considered to contribute to the symptoms and progression of AD, ammonia deserves special interest for the following reasons: (a) Ammonia is formed in nearly all tissues and organs of the vertebrate organism; it is the most common endogenous neurotoxic compounds. Its effects on glutamatergic and GABAergic neuronal systems, the two prevailing neuronal systems of the cortical structures, are known for many years. (b) The impairment of ammonia detoxification invariably leads to severe pathology. Several symptoms and histologic aberrations of hepatic encephalopathy (HE), of which ammonia has been recognised as a pathogenetic factor, resemble those of AD. (c) The excessive formation of ammonia in the brains of AD patients has been demonstrated, and it has been shown that some AD patients exhibit elevated blood ammonia concentrations. (d) There is evidence for the involvement of aberrant lysosomal processing of beta-amyloid precursor protein (beta-APP) in the formation of amyloid deposits. Ammonia is the most important natural modulator of lysosomal protein processing. (e) Inflammatory processes and activation of microglia are widely believed to be implicated in the pathology of AD. Ammonia is able to affect the characteristic functions of microglia, such as endocytosis, and cytokine production. Based on these facts, an ammonia hypothesis of AD has first been suggested in 1993. In the present review old and new observations are discussed, which are in support of the notion that ammonia is a factor able to produce symptoms of AD and to affect the progression of the disease.

Cloning and characterization of a novel human pantothenate kinase gene

Pantothenate kinase (PanK) is a key regulatory enzyme in the CoA biosynthetic pathway in bacteria and mammalian cells. It catalyzes the first committed step in the universal biosynthetic pathway leading to CoA. Here we report the molecular cloning and characterization of a new human PanK gene. The gene encoded a protein of 314 amino acid residues, which share high homology to mouse pantothenate kinase (mPanK) 1 beta. Northern blot analysis revealed a transcript of the gene of 2.6 kb in human liver and kidney. The human PanK gene was located to human chromosome between 10q22.3 and 10q23.2 by bioinformatics analysis.

Rumen undegradable protein, rumen-protected choline and mRNA expression for enzymes in gluconeogenesis and ureagenesis in periparturient dairy cows

The objective of this study was to profile mRNA expression of argininosuccinate synthetase (AS) and ornithine transcarbamylase (OTC), two enzymes that participate in the formation of urea in liver and compare these with changes in mRNA for pyruvate carboxylase (PC) and phosphoenolpyruvate carboxykinase (PEPCK) during the periparturient period in dairy cows. Forty-eight multiparous Holstein cows were fed isoenergetic prepartum diets that contained 10% RDP and either 4.0% RUP or 6.2% RUP and either 0, 6, or 12 g/d of rumen-protected choline (RPC) as CapShure (Balchem Corp., Slate Hill, NY). After calving cows received a common diet and continued RPC as per their prepartum assignments. Liver biopsies were obtained on d -28, -14, 1, 28, and 56 relative to calving, and the abundances of AS, OTC, PC, PEPCK, and 18S mRNA were determined by Northern blot analysis of total RNA. The abundance of OTC mRNA was lowest at calving and was decreased by RPC and 6.2% RUP feeding. Feeding 6.2% RUP did not alter AS, PC, or PEPCK mRNA. The expression of AS mRNA increased and PEPCK mRNA tended to increase from calving to 56 DIM. Pyruvate carboxylase mRNA increased more than twofold at calving. The data indicated adaptation to lactation for gluconeogenic enzymes that is not matched in direction and magnitude by changes in mRNA for urea cycle enzymes. Feeding additional protein, as RUP, failed to induce mRNA for key enzymes in gluconeogenesis or ureagenesis.

The nutritional management of urea cycle disorders

Diet is one of the mainstays of the treatment of patients with urea cycle disorders. The protein intake should be adjusted to take account of the inborn error and its severity and the patient's age, growth rate, and individual preferences. Currently, the widely used standards for protein intake are probably more generous than necessary, particularly for those with the more severe variants. Most patients, except those with arginase deficiency, will need supplements of arginine, but the value of other supplements including citrate and carnitine is unclear. Any patient on a low-protein diet should be monitored clinically and with appropriate laboratory tests. All should have an emergency (crisis) regimen to prevent decompensation during periods of metabolic stress.

Reduction in the MK-801 binding sites of the NMDA sub-type of glutamate receptor in a mouse model of congenital hyperammonemia: prevention by acetyl-L-carnitine

Our earlier studies on the pharmacotherapeutic effects of acetyl-L-carnitine (ALCAR), in sparse-fur (spf) mutant mice with X linked ornithine transcarbamylase deficiency, have shown a restoration of cerebral ATP, depleted by congenital hyperammonemia and hyperglutaminemia. The reduced cortical glutamate and increased quinolinate may cause a down-regulation of the N-methyl-D-aspartate (NMDA) receptors, observed by us in adult spf mice. We have now studied the kinetics of [3H]-MK-801 binding to NMDA receptors in spf mice of different ages to see the effect of chronic hyperammonemia on the glutamate neurotransmission. We have also studied the Ca2+-dependent and independent (4-aminopyridine (AP) and veratridine-mediated) release of glutamate and the uptake of [3H]-glutamate in synaptosomes isolated from mutant spf mice and normal CD-1 controls. All these studies were done with and without ALCAR treatment (4 mmol/kg wt i.p. daily for 2 weeks), to see if its effect on ATP repletion could correct the glutamate neurotransmitter abnormalities. Our results indicate a normal MK-801 binding in 12-day-old spf mice but a significant reduction immediately after weaning (21 day), continuing into the adult stage. The Ca2+-independent release of endogenous glutamate from synaptosomes was significantly elevated at 35 days, while the uptake of glutamate into synaptosomes was significantly reduced in spf mice. ALCAR treatment significantly enhanced the MK-801 binding, neutralized the increased glutamate release and restored the glutamate uptake into synaptosomes of spf mice. These studies point out that: (a) the developmental abnormalities of the NMDA sub-type of glutamate receptor in spf mice could be due to the effect of sustained hyperammonemia, causing a persistent release of excess glutamate and inhibition of the ATP-dependent glutamate transport, (b) the modulatory effects of ALCAR on the NMDA binding sites could be through a repletion of ATP, required by the transporters to efficiently remove extracellular glutamate.

Restoration of hepatic cytochrome c oxidase activity and expression with acetyl-L-carnitine treatment in spf mice with an ornithine transcarbamylase deficiency

The sparse fur (spf) mutant mouse, with an X-linked ornithine transcarbamylase deficiency, is a model of congenital hyperammonemia in children. Our earlier studies indicated a deficiency of hepatic carnitine, CoA-SH, acetyl CoA, and ATP in spf mice. We have now studied the effects of a 7-day treatment with acetyl-L-carnitine (ALCAR) in the spf/Y mice on the activity and expression of the respiratory chain enzyme cytochrome c oxidase (COX; EC 1.9.3.1). We found decreased hepatic activity and expression of COX in the untreated hyperammonemic spf/Y mice, which was restored upon ALCAR treatment. Because COX is a mitochondrial membrane protein, we also carried out studies to explain the mechanism of ALCAR through its effect on membrane stability. Our results indicate a decrease of the mitochondrial membrane cholesterol/phospholipid molar ratio (CHOL/PL ratio) with the activity and expression of COX in untreated spf/Y mice. While ALCAR treatment normalized the ratios, it also restored the hepatic ATP production to normal. To study further if there was any effect of ALCAR on the mitochondrial matrix urea cycle enzymes, we measured the activity and expression of mutant ornithine transcarbamylase (OTC; EC 2.1.3.3) and normal carbamyl phosphate synthase-I (CPS-I; EC 6.3.4.16) in spf/Y mice. There was no general effect on the specific activities of the matrix enzymes upon ALCAR treatment, although their mRNA levels were enhanced. Our studies point towards the feasibility of an ALCAR treatment in conjunction with other treatment modalities, e.g. sodium benzoate and/or arginine, to improve the availability of cellular ATP and to counteract the effects of hereditary hyperammonemic syndromes in children.

Alternative pathway therapy for urea cycle disorders

In man the major pathway for the disposal of waste nitrogen is the urea cycle; in inborn errors of this pathway, nitrogen flux is reduced. As a result there is accumulation of ammonia and glutamine with disordered metabolism of other amino acids. Nitrogen homeostasis can be restored in these patients with a low-protein diet combined with compounds that create alternative pathways for nitrogen excretion. The introduction of these compounds has been a major advance in the management of these inborn errors and as a result the outcome, particularly for those treated early, has improved.

Progressive decrease of cerebral cytochrome C oxidase activity in sparse-fur mice: role of acetyl-L-carnitine in restoring the ammonia-induced cerebral energy depletion

Sparse-fur (spf) mice with a deficiency of hepatic ornithine transcarbamylase (OTC) are congenitally hyperammonemic, showing elevated cerebral ammonia and glutamine and depleted levels of energy metabolites. This mouse disorder is akin to the human OTC deficiency, in which neuronal loss and Alzheimer's type II astrocytosis is reported. Reduced cytochrome C oxidase (COX) activity is characteristic of neurodegeneration in Alzheimer's type disorders. We have studied the causal relationship between cerebral COX activity and energy depletion in spf mice. Our results indicate a progressive decrease in the COX activity in various brain regions in spf mice, up to 40 weeks of age, which severely effected the cerebral levels of various energy metabolites. A quantitative estimation of cerebral COX subunit I mRNA also showed a tendency to decrease in spf mice. Short-term acetyl L-carnitine (ALCAR) treatment restored these abnormalities. Our study points out that: (a) ammonia-induced alterations in the cerebral reducing equivalents could cause a decrease in COX activity and its mRNA expression, and (b) ALCAR administration could normalize the cerebral energy metabolism and induce COX mRNA expression and activity.

Carnitine metabolism in chronic liver disease

The liver is a central organ for carnitine metabolism and for the distribution of carnitine to the body. It is therefore not surprising that carnitine metabolism is impaired in patients and experimental animals with certain types of chronic liver disease. In this review, the changes in carnitine metabolism associated with chronic liver disease and the role of carnitine as a therapeutic agent in some of these conditions are discussed.

Effect of carnitine administration on glycine metabolism in patients with isovaleric acidemia: significance of acetylcarnitine determination to estimate the proper carnitine dose

In isovaleric acidemia (IVA), accumulated isovaleryl-CoA in the mitochondrion induces variable metabolic disturbances. To remove intramitochondrial isovaleryl groups, glycine therapy has been advocated primarily. On the other hand, secondary carnitine deficiency has been documented in this disorder and carnitine supplementation alone has been reported to be effective. In the present study, we administered carnitine and glycine to patients with IVA, and investigated serum carnitine and urinary excretion of total and free carnitine, acylcarnitine profile (i.e., isovalerylcarnitine and acetylcarnitine), and isovalerylglycine. By adding carnitine to glycine supplementation, more isovalerylglycine, not only isovalerylcarnitine, was excreted in the urine. Acetylcarnitine was detected in the urine only when sufficient carnitine was supplemented. We concluded that combined therapy of glycine and carnitine is more effective and safer to eliminate isovaleryl-CoA in IVA than conventional therapy using either glycine or carnitine. Urinary acetylcarnitine concentration might be a good marker indicating the optimal dose of L-carnitine supplementation.

Molecular mechanism of acute ammonia toxicity and of its prevention by L-carnitine

In summary, we propose that acute ammonia intoxication leads to increased extracellular concentration of glutamate in brain and results in activation of the NMDA receptor. Activation of this receptor mediates ATP depletion and ammonia toxicity since blocking the NMDA receptor with MK-801 prevents both phenomena. Ammonia-induced metabolic alterations (in glycogen, glucose, pyruvate, lactate, glutamine, glutamate, etc) are not prevented by MK-801 and, therefore, it seems that they do not play a direct role in ammonia-induced ATP depletion nor in the molecular mechanism of acute ammonia toxicity. The above results suggest that ammonia-induced ATP depletion is due to activation of Na+/K(+)-ATPase, which, in turn, is a consequence of decreased phosphorylation by protein kinase C. This can be due to decreased activity of PKC or to increased activity of a protein phosphatase. We also show that L-carnitine prevents glutamate toxicity in primary neuronal cultures. The results shown indicate that carnitine increases the affinity of glutamate for the quisqualate type (including metabotropic) of glutamate receptors. Also, blocking the metabotropic receptor with AP-3 prevents the protective effect of L-carnitine, indicating that activation of this receptor mediates the protective effect of carnitine. We suggest that the protective effect of carnitine against acute ammonia toxicity in animals is due to the protection against glutamate neurotoxicity according to the above mechanisms.