Plasma clearances of branched-chain amino acids in control subjects and in patients with cirrhosis

Serum metabolite profiles are associated with the presence of advanced liver fibrosis in Chinese patients with chronic hepatitis B viral infection

BACKGROUND

Accurate and noninvasive diagnosis and staging of liver fibrosis are essential for effective clinical management of chronic liver disease (CLD). We aimed to identify serum metabolite markers that reliably predict the stage of fibrosis in CLD patients.

METHODS

We quantitatively profiled serum metabolites of participants in 2 independent cohorts. Based on the metabolomics data from cohort 1 (504 HBV associated liver fibrosis patients and 502 normal controls, NC), we selected a panel of 4 predictive metabolite markers. Consequently, we constructed 3 machine learning models with the 4 metabolite markers using random forest (RF), to differentiate CLD patients from normal controls (NC), to differentiate cirrhosis patients from fibrosis patients, and to differentiate advanced fibrosis from early fibrosis, respectively.

RESULTS

The panel of 4 metabolite markers consisted of taurocholate, tyrosine, valine, and linoelaidic acid. The RF models of the metabolite panel demonstrated the strongest stratification ability in cohort 1 to diagnose CLD patients from NC (area under the receiver operating characteristic curve (AUROC) = 0.997 and the precision-recall curve (AUPR) = 0.994), to differentiate fibrosis from cirrhosis (0.941, 0.870), and to stage liver fibrosis (0.918, 0.892). The diagnostic accuracy of the models was further validated in an independent cohort 2 consisting of 300 CLD patients with chronic HBV infection and 90 NC. The AUCs of the models were consistently higher than APRI, FIB-4, and AST/ALT ratio, with both greater sensitivity and specificity.

CONCLUSIONS

Our study showed that this 4-metabolite panel has potential usefulness in clinical assessments of CLD progression in patients with chronic hepatitis B virus infection.

Glucagon Receptor Signaling and Glucagon Resistance

Hundred years after the discovery of glucagon, its biology remains enigmatic. Accurate measurement of glucagon has been essential for uncovering its pathological hypersecretion that underlies various metabolic diseases including not only diabetes and liver diseases but also cancers (glucagonomas). The suggested key role of glucagon in the development of diabetes has been termed the bihormonal hypothesis. However, studying tissue-specific knockout of the glucagon receptor has revealed that the physiological role of glucagon may extend beyond blood-glucose regulation. Decades ago, animal and human studies reported an important role of glucagon in amino acid metabolism through ureagenesis. Using modern technologies such as metabolomic profiling, knowledge about the effects of glucagon on amino acid metabolism has been expanded and the mechanisms involved further delineated. Glucagon receptor antagonists have indirectly put focus on glucagon's potential role in lipid metabolism, as individuals treated with these antagonists showed dyslipidemia and increased hepatic fat. One emerging field in glucagon biology now seems to include the concept of hepatic glucagon resistance. Here, we discuss the roles of glucagon in glucose homeostasis, amino acid metabolism, and lipid metabolism and present speculations on the molecular pathways causing and associating with postulated hepatic glucagon resistance.

Protein tolerance to standard and high protein meals in patients with liver cirrhosis

AIM

To investigate the plasma amino acid response and tolerance to normal or high protein meals in patients with cirrhosis.

METHODS

The plasma amino acid response to a 20 g mixed protein meal was compared in 8 biopsy-proven compensated cirrhotic patients and 6 healthy subjects. In addition the response to a high protein meal (1 g/kg body weight) was studied in 6 decompensated biopsy-proven cirrhotics in order to evaluate their protein tolerance and the likelihood of developing hepatic encephalopathy (HE) following a porto-caval shunt procedure. To test for covert HE, the “number connection test” (NCT) was done on all patients, and an electroencephalogram was recorded in patients considered to be at Child-Pugh C stage.

RESULTS

The changes in plasma amino acids after a 20 g protein meal were similar in healthy subjects and in cirrhotics except for a significantly greater increase (P < 0.05) in isoleucine, leucine and tyrosine concentrations in the cirrhotics. The baseline branched chain amino acids/aromatic amino acids (BCAA/AAA) ratio was higher in the healthy persons and remained stable-but it decreased significantly after the meal in the cirrhotic group. After the high protein meal there was a marked increase in the levels of most amino acids, but only small changes occurred in the levels of taurine, citrulline, cysteine and histidine.The BCAA/AAA ratio was significantly higher 180 and 240 min after the meal. Slightly elevated basal plasma ammonia levels showed no particular pattern. Overt HE was not observed in any patients.

CONCLUSION

Patients with stable liver disease tolerate natural mixed meals with a standard protein content. The response to a high protein meal in decompensated cirrhotics suggests accumulation of some amino acids but it did not precipitate HE. These results support current nutritional guidelines that recommend a protein intake of 1.2-1.5 g/kg body weight/day for patients with cirrhosis.

Branched-chain amino acids and muscle ammonia detoxification in cirrhosis

Branched-chain amino acids (BCAA) are used as a therapeutic nutritional supplement in patients with cirrhosis and hepatic encephalopathy (HE). During liver disease, the decreased capacity for urea synthesis and porto-systemic shunting reduce the hepatic clearance of ammonia and skeletal muscle may become the main alternative organ for ammonia detoxification. We here summarize current knowledge of muscle BCAA and ammonia metabolism with a focus on liver cirrhosis and HE. Plasma levels of BCAA are lower and muscle uptake of BCAA seems to be higher in patients with cirrhosis and hyperammonemia. BCAA metabolism may improve muscle net ammonia removal by supplying carbon skeletons for formation of alfa-ketoglutarate that combines with two ammonia molecules to become glutamine. An oral dose of BCAA enhances muscle ammonia metabolism but also transiently increases the arterial ammonia concentration, likely due to extramuscular metabolism of glutamine. We, therefore, speculate that the beneficial effect of long term intake of BCAA on HE demonstrated in clinical studies may be related to an improved muscle mass and nutritional status rather than to an ammonia lowering effect of BCAA themselves.

Two patients with hepatic mtDNA depletion syndromes and marked elevations of S-adenosylmethionine and methionine

This paper reports studies of two patients proven by a variety of studies to have mitochondrial depletion syndromes due to mutations in either their MPV17 or DGUOK genes. Each was initially investigated metabolically because of plasma methionine concentrations as high as 15-21-fold above the upper limit of the reference range, then found also to have plasma levels of S-adenosylmethionine (AdoMet) 4.4-8.6-fold above the upper limit of the reference range. Assays of S-adenosylhomocysteine, total homocysteine, cystathionine, sarcosine, and other relevant metabolites and studies of their gene encoding glycine N-methyltransferase produced evidence suggesting they had none of the known causes of elevated methionine with or without elevated AdoMet. Patient 1 grew slowly and intermittently, but was cognitively normal. At age 7 years he was found to have hepatocellular carcinoma, underwent a liver transplant and died of progressive liver and renal failure at age almost 9 years. Patient 2 had a clinical course typical of DGUOK deficiency and died at age 8 ½ months. Although each patient had liver abnormalities, evidence is presented that such abnormalities are very unlikely to explain their elevations of AdoMet or the extent of their hypermethioninemias. A working hypothesis is presented suggesting that with mitochondrial depletion the normal usage of AdoMet by mitochondria is impaired, AdoMet accumulates in the cytoplasm of affected cells poor in glycine N-methyltransferase activity, the accumulated AdoMet causes methionine to accumulate by inhibiting activity of methionine adenosyltransferase II, and that both AdoMet and methionine consequently leak abnormally into the plasma.

Insulin resistance and the metabolism of branched-chain amino acids in humans

Peripheral resistance to insulin action is the major mechanism causing the metabolic syndrome and eventually type 2 diabetes mellitus. The metabolic derangement associated with insulin resistance is extensive and not restricted to carbohydrates. The branched-chain amino acids (BCAAs) are particularly responsive to the inhibitory insulin action on amino acid release by skeletal muscle and their metabolism is profoundly altered in conditions featuring insulin resistance, insulin deficiency, or both. Obesity, the metabolic syndrome and diabetes mellitus display a gradual increase in the plasma concentration of BCAAs, from the obesity-related low-grade insulin-resistant state to the severe deficiency of insulin action in diabetes ketoacidosis. Obesity-associated hyperinsulinemia succeeds in maintaining near-normal or slightly elevated plasma concentration of BCAAs, despite the insulin-resistant state. The low circulating levels of insulin and/or the deeper insulin resistance occurring in diabetes mellitus are associated with more marked elevation in the plasma concentration of BCAAs. In diabetes ketoacidosis, the increase in plasma BCAAs is striking, returning to normal when adequate metabolic control is achieved. The metabolism of BCAAs is also disturbed in other situations typically featuring insulin resistance, including kidney and liver dysfunction. However, notwithstanding the insulin-resistant state, the plasma level of BCAAs in these conditions is lower than in healthy subjects, suggesting that these organs are involved in maintaining BCAAs blood concentration. The pathogenesis of the decreased BCAAs plasma level in kidney and liver dysfunction is unclear, but a decreased afflux of these amino acids into the blood stream has been observed.

Elimination of infused branched-chain amino-acids from plasma of patients with non-obese type 2 diabetes mellitus

Increased plasma levels of branched-chain amino-acids (BCAA) have been demonstrated in poorly controlled diabetes mellitus, and related to absolute or relative insulin deficiency. To study the pathogenesis of this alteration, the elimination of BCAA from plasma was measured in 8 patients with non-obese type 2 diabetes mellitus and in 8 age-matched control subjects during steady-state BCAA concentrations induced by a primed-continuous infusion. Fasting BCAA levels were increased by 40-50% in patients with diabetes. The plasma clearances of valine, isoleucine, and leucine, calculated as infusion rate divided by steady-state concentration, were reduced by 20% in diabetics, despite 50% hyperinsulinemia (P < 0.01). Basal BCAA levels and BCAA clearance were negatively correlated (r(2) = 0.46 - 0.56). The endogenous basal appearance rates of BCAA, estimated by the basal concentrations multiplied by the plasma clearances, were normal in diabetics, and there was no difference in the apparent volumes of distribution of BCAA. The increased basal concentration of BCAA in poorly controlled type 2 diabetics (693 [SD 114; n = 8] mumol/l vs 479 [88; n = 8] in controls (P < 0.005) is attributable to changes in plasma clearances, without any change in the efflux of BCAA into plasma. This may be due to insulin resistance.

Acute dopamine depletion with branched chain amino acids decreases auditory top-down event-related potentials in healthy subjects

Cerebral dopamine homeostasis has been implicated in a wide range of cognitive processes and is of great pathophysiological importance in schizophrenia. A novel approach to study cognitive effects of dopamine is to deplete its cerebral levels with branched chain amino acids (BCAAs) that acutely lower dopamine precursor amino acid availability. Here, we studied the effects of acute dopamine depletion on early and late attentive cortical processing. Auditory event-related potential (ERP) components N2 and P3 were investigated using high-density electroencephalography in 22 healthy male subjects after receiving BCAAs or placebo in a randomized, double-blind, placebo-controlled crossover design. Total free serum prolactin was also determined as a surrogate marker of cerebral dopamine depletion. Acute dopamine depletion increased free plasma prolactin and significantly reduced prefrontal ERP components N2 and P3. Subcomponent analysis of N2 revealed a significant attenuation of early attentive N2b over prefrontal scalp sites. As a proof of concept, these results strongly suggest that BCAAs are acting on basic information processing. Dopaminergic neurotransmission seems to be involved in auditory top-down processing as indexed by prefrontal N2 and P3 reductions during dopamine depletion. In healthy subjects, intact early cortical top-down processing can be acutely dysregulated by ingestion of BCAAs. We discuss the potential impact of these findings on schizophrenia research.

Role of meal carbohydrate content for the imbalance of plasma amino acids in patients with liver cirrhosis

BACKGROUND AND AIM

Imbalance of circulating branched chain amino acids (BCAA) versus aromatic amino acids (AAA) and hyperinsulinemia are common metabolic alterations in patients with liver cirrhosis. The aim of this study was to characterize the effect of the carbohydrate component of a protein-rich mixed meal on postprandial plasma concentrations of 21 amino acids, insulin and C-peptide in patients with compensated liver cirrhosis. Furthermore, the effect of a dietary intervention on the metabolic alterations in cirrhotic patients was examined.

METHODS

Eighteen patients with cirrhosis and 12 healthy volunteers received a protein-rich meal (pork filet 200 g) with or without carbohydrates (bread 50 g, glucose 20 g). A subgroup of four cirrhotic patients received an isoenergetic (117 kJ/kg bw) carbohydrate-enriched (60%) and -restricted (20%) diet for 7 days each.

RESULTS

In the cirrhotic patients, basal plasma insulin and C-peptide concentrations were significantly elevated. The ingestion of a protein-rich meal without additional carbohydrates led to a significantly greater increase of insulin and C-peptide in the cirrhotic patients compared to controls. Postprandial increases of leucine and isoleucine were reduced, whereas those of phenylalanine were higher in cirrhotic patients. The addition of carbohydrates led to higher insulin and C-peptide plasma concentrations in cirrhotic patients. Postprandial BCAA increases were more impaired in the cirrhotic group after additional carbohydrate ingestion (46%vs 82%). After the carbohydrate-restricted diet for 7 days BCAA plasma levels increased but the BCAA/AAA ratio remained unaltered.

CONCLUSIONS

The carbohydrate content of a meal enhances reduction of BCAA plasma concentrations in clinically stable cirrhotic patients. An imbalanced BCAA/AAA ratio cannot be avoided by a carbohydrate-reduced diet alone, supporting mandatory BCAA supplementation.

Emotion-based decision-making in healthy subjects: short-term effects of reducing dopamine levels

INTRODUCTION

Converging evidences from animal and human studies suggest that addiction is associated with dopaminergic dysfunction in brain reward circuits. So far, it is unclear what aspects of addictive behaviors are related to a dopaminergic dysfunction.

DISCUSSION

We hypothesize that a decrease in dopaminergic activity impairs emotion-based decision-making. To demonstrate this hypothesis, we investigated the effects of a decrease in dopaminergic activity on the performance of an emotion-based decision-making task, the Iowa gambling task (IGT), in 11 healthy human subjects.

MATERIALS AND METHODS

We used a double-blind, placebo-controlled, within-subject design to examine the effect of a mixture containing the branched-chain amino acids (BCAA) valine, isoleucine and leucine on prolactin, IGT performance, perceptual competency and visual aspects of visuospatial working memory, visual attention and working memory, and verbal memory. The expectancy-valence model was used to determine the relative contributions of distinct IGT components (attention to past outcomes, relative weight of wins and losses, and choice strategies) in the decision-making process.

OBSERVATIONS AND RESULTS

Compared to placebo, the BCAA mixture increased prolactin levels and impaired IGT performance. BCAA administration interfered with a particular component process of decision-making related to attention to more recent events as compared to more distant events. There were no differences between placebo and BCAA conditions for other aspects of cognition. Our results suggest a direct link between a reduced dopaminergic activity and poor emotion-based decision-making characterized by shortsightedness, and thus difficulties resisting short-term reward, despite long-term negative consequences. These findings have implications for behavioral and pharmacological interventions targeting impaired emotion-based decision-making in addictive disorders.

Assessment of branched-chain amino Acid status and potential for biomarkers

BCAAs are not synthesized in the body in humans, but they are crucial in protein and neurotransmitter synthesis. The protein anabolic role of BCAAs seems to be mediated not only by their important role as a promoter of the translation process (and possibly acting at the transcription level) but also by inhibition of protein degradation. Leucine may play a critical role in these signaling pathways. Supplementation with BCAAs spares lean body mass during weight loss, promotes wound healing, may decrease muscle wasting with aging, and may have beneficial effects in renal and liver disease. BCAA supplementation is extensively used in the athletic field with the assumption of improved performance and muscle mass. Measuring serum BCAAs has limited clinical utility beyond the controlled setting because levels are affected by a variety of clinical states, and optimal levels in these scenarios have not been completely elucidated. We discuss the effects diet, hormones, stress, aging, and renal or liver dysfunction have on BCAA levels and how understanding the biological effects of BCAAs may help to develop biomarkers of BCAA status. We also discuss potential biomarkers of BCAA status.

Plasma amino acid concentrations in patients with amnestic mild cognitive impairment or Alzheimer disease

BACKGROUND

Plasma concentrations of several amino acids may affect the availability of important neurotransmitter precursors in the brain. Abnormalities in the plasma amino acid profile have been reported in elderly persons with cognitive impairment, but no data exist for the prodromal phase of Alzheimer disease (AD), which is characterized by amnestic mild cognitive impairment (aMCI).

OBJECTIVE

The objective was to investigate whether the plasma amino acid profiles of elderly patients with aMCI or AD are abnormal.

DESIGN

The plasma amino acid profile was assessed in 29 cognitively normal control subjects (age: 86.7 +/- 5.9 y), 21 patients with aMCI (age: 84.9 +/- 7.0 y), and 51 patients with AD (age: 86.7 +/- 5.4 y). The participants were from the University of Bologna Research Center for Physiopathology of Aging, Italy.

RESULTS

Higher plasma concentrations of the aromatic amino acid phenylalanine were found in the aMCI (68 micromol/L; 95% CI: 63, 73) and AD (62 micromol/L; 95% CI: 59, 65) patients than in the control subjects (54 micromol/L; 95% CI: 48, 61; P < 0.05). The ratio of arginine to other basic amino acids was also higher in the aMCI (0.31 +/- 0.04) and AD (0.27 +/- 0.08) patients than in the control subjects (0.21 +/- 0.05; P < 0.05). Adjustment for differences in body composition, serum vitamin B-12 concentrations, and serum folate concentrations did not significantly affect the results.

CONCLUSIONS

The plasma amino acid profiles of elderly patients with aMCI or AD show abnormalities in aromatic and basic amino acids that potentially affect neurotransmitter biosynthesis.

Wasting and the substrate-to-energy controlled pathway: a role for insulin resistance and amino acids

Amino acids contained in proteins can be transformed either in glucose precursors or in acetate, the end product of free fatty acid (FFA) oxidation. The dynamics of glucose, FFA, and amino acid competition for entry into the citric acid cycle (tricarboxylic acid [TCA] cycle) are very complex and not fully understood. Conditions where glucose is insufficiently driven to full oxidation are characterized by lowest efficiency in energy production per mole of oxygen consumed. Moreover, acetate provided by oxidation of FFA increases consumption of amino acids as precursors of the oxaloacetate required for condensation with acetate and for maintenance of citrate synthesis. Increased consumption of amino acids in the TCA cycle, if not matched by adequate intake, leads to muscular wasting and cachexia. Therefore, amino acid needs are very complex, and their intake must provide a balanced ratio of glucogenic and ketogenic precursors suitable to trigger entry of glucose to full oxidation and blunt the level of FFA utilization. Optimization of substrate entry into energy production must also be coupled with sufficient availability of amino acids in ratios suitable for maintaining protein synthesis, inhibiting the catabolic drive, and promoting integrity of cellular proteic structures. Alimentary proteins have a content of amino acids that is far from the stoichiometric ratios of essential amino acids required by humans. An amino acid formulation suitable to match energy needs, control carbohydrate and lipid flow into the TCA cycle, and promote protein synthesis in contracting cells is detailed in this article.

Branched-chain amino acids for hepatic encephalopathy

BACKGROUND

Hepatic encephalopathy may be caused by a decreased plasma ratio of branched-chain amino acids (BCAA) to aromatic amino acids. Treatment with BCAA may therefore have a beneficial effect on patients with hepatic encephalopathy.

OBJECTIVES

To evaluate the beneficial and harmful effects of BCAA for patients with hepatic encephalopathy.

SEARCH STRATEGY

We identified trials through The Cochrane Hepato-Biliary Group Controlled Trials Register (September 2002), (Issue 3, 2002), MEDLINE (1966-2002/09) and EMBASE (1980-2002/05), manual searches of bibliographies and journals, authors of trials, and pharmaceutical companies.

SELECTION CRITERIA

Randomised trials comparing BCAA with any kind of control therapy for hepatic encephalopathy were included, regardless of blinding, language, or publication status.

DATA COLLECTION AND ANALYSIS

Trial inclusion and data extraction were made independently by two reviewers. Our primary outcome was improvement of hepatic encephalopathy. Statistical heterogeneity was tested using random effects and fixed effect models. Binary outcomes are reported as risk ratios (RR) based on a random effects model.

MAIN RESULTS

Eleven randomised trials (556 patients) assessing BCAA versus carbohydrates, neomycin/lactulose, or isonitrogenous control were included. The median number of patients in each trial was 55 (range 22 to 75). Follow-up after treatment was reported in four trials (median 17 days (range 6 to 30 days)). Compared to the control regimens, BCAA significantly increased the number of patients improving from hepatic encephalopathy at the end of treatment (risk ratio (RR) 1.31, 95% confidence interval (CI) 1.04 to 1.66, nine trials). We found no evidence of an effect of BCAA on survival (RR 1.06, 95% CI 0.98 to 1.14, eight trials) or adverse events (RR 0.97, 95% CI 0.41 to 2.31, three trials). Sensitivity analyses indicated that methodological quality had significant impact on the results. We found no evidence of an effect of BCAA on improvement of hepatic encephalopathy in trials with adequate generation of the allocation sequence (RR 1.01, 95% CI 0.84 to 1.23, three trials), adequate allocation concealment (RR 1.09, 95% CI 0.89 to 1.33, five trials), or adequate double-blinding (RR 1.20, 95% CI 0.83 to 1.73, three trials).

REVIEWER'S CONCLUSIONS

We did not find convincing evidence that BCAA had a significant beneficial effect on patients with hepatic encephalopathy. The trials performed in this field were small with short follow-up and most had low methodological quality.

Effects of hyperglycaemia and hyperinsulinaemia on plasma amino acid levels in obese subjects with normal glucose tolerance

OBJECTIVE

To investigate the effects of hyperglycaemia and hyperinsulinaemia on amino acid disposal in human obesity.

DESIGN

Four sequential experimental conditions: (1) overnight fasting; (2) hyperglycaemia with hyperinsulinaemia (2 h hyperglycaemic clamp at 11 mmol/l); (3) hyperglycaemia with basal insulin (1 h hyperglycaemic clamp during somatostatin infusion), (4) hyperglycaemia with resuming hyperinsulinaemia (1 h hyperglycaemic clamp after somatostatin discontinuation).

SUBJECTS

Seven non-obese and seven obese non-diabetic, normo-insulinaemic subjects.

MEASUREMENTS

Glucose infused to maintain steady-state hyperglycaemia. Plasma insulin, glucagon, free fatty acid and amino acid concentrations in the last 20 min of the four experimental conditions. Net rates of plasma amino acid disappearance and appearance (micromol/l per hour), calculated as the slopes of the regression of amino acid concentration on time.

RESULTS

The amount of glucose infused to maintain hyperglycaemia was reduced by nearly 50% in obese subjects. During hyperinsulinaemia, FFA suppression was lower in obese subjects. In all experimental conditions plasma amino acid levels were slightly, non-significantly higher in obese than in non-obese subjects. In both groups plasma amino acids decreased slightly with ongoing fasting, decreased remarkably during hyperglycaemia-hyperinsulinaemia, rose promptly when insulin concentration was suppressed by somatostatin infusion, and declined again after somatostatin discontinuation. Also the time-course of plasma branched-chain amino acids, which paralleled that of total amino acids, was similar in the two groups. The net rates of amino acid disappearance from plasma did not differ in obese and non-obese subjects both at fasting and during hyperglycaemia-hyperinsulinaemia. Also plasma amino acid appearance during hyperglycaemia with basal insulin was not different in the two groups.

CONCLUSION

The net traffic of amino acids to and from plasma in relation to insulin drive and prevailing glucose is not impaired in obese subjects with normal glucose tolerance, in spite of a decreased insulin sensitivity of glucose and lipid metabolism.

The kidney is an important site for in vivo phenylalanine-to-tyrosine conversion in adult humans: A metabolic role of the kidney

Synthesis of Tyr in the human body occurs by hydroxylation of the indispensable amino acid Phe. Until now, it was believed that in humans, this process was restricted to the liver, but we provide compelling evidence of production of Tyr from Phe in the kidney. To determine whether the human kidney produces Tyr, we measured Tyr balance, the Tyr appearance rate, and the Phe-to-Tyr conversion in 12 healthy human subjects by using [(15)N]Phe and [(2)H(4)]Tyr as tracers. Renal plasma flow was measured by using paraaminohippurate, and sampling from the femoral artery and renal veins was performed. The results were compared with those obtained in 12 control subjects undergoing hepatic vein catheterization and infusion of identical tracers. In all 12 subjects, there was a net uptake of Phe by the kidney (2.2 +/- 1.2 micromol/min), whereas Tyr was released (5.3 +/- 1.5 micromol/min). In contrast, there was a net uptake of both Phe (9.5 +/- 1.2 micromol/min) and Tyr (14.3 +/- 1.3 micromol/min) by the splanchnic bed. Phe conversion to Tyr occurred at a rate of 5.2 +/- 1.2 micromol/min in kidney and 3.0 +/- 0.7 micromol/min in the splanchnic bed. The kidney contributed a substantial amount of Tyr to the systemic circulation where the splanchnic bed was a net remover of Tyr. Our results demonstrate that the kidney is the major donor of Tyr to the systemic circulation by its conversion of Phe to Tyr. This observation may have important clinical implications for patients with both renal and hepatic disease, who may be at risk of Phe overloading and Tyr deficiency, and it should be considered when parenteral or enteral nutrients are administered rich in Phe and low in Tyr.

Abnormalities in branched-chain amino acid metabolism in cirrhosis: influence of hormonal and nutritional factors and directions for future research

Plasma branched-chain amino acid (BCAA) levels are decreased in patients with liver cirrhosis, owing to an increase in BCAA tissue uptake and/or catabolism and a decrease in BCAA production from proteins. Non-specific factors such as malnutrition worsen this picture. Studies of BCAA fluxes and protein turnover in cirrhotic patients have given conflicting results due to patient heterogeneity, differences in method and bias in the expression of results. In well compensated cirrhosis, muscle wasting is moderate and probably due more to decreased protein synthesis than to increased protein catabolism. Hyperinsulinemia has been suggested as the main cause of decreased BCAA levels, by increasing BCAA uptake in muscle and additionally in adipose tissue. However, as depletion of fat stores is frequent in cirrhosis, this effect is certainly quantitatively weak. Also, there is no correlation between state of hyperinsulinemia and decrease in BCAA levels. An effect of cytokines (IL1 and TNF) on muscle BCAA catabolism is a possibility. Until recently, the contribution of the liver to abnormal BCAA metabolism has been underestimated. In cirrhotic liver an increase in liver transamination of branched-chain keto acids (BCKAs) has been suggested and may result from inhibition of liver BCKA dehydrogenase. A modification of protein turnover in cirrhotic liver must be also considered. Lastly, the contribution of non-hepatocyte liver cells, which are activated in cirrhosis, remains to be assessed.

Effects of systemic prostaglandin E1 on hepatic amino acid-nitrogen metabolism in patients with cirrhosis

Prostaglandins of the E (PGE) series have long been considered "catabolic" hormones, but recent data suggest that they may be secreted in critically ill patients to counteract stress hormones, stimulating protein synthesis. Their use is under scrutiny to improve hepatic microcirculation and as cytoprotective agents. We tested the effects of PGE1 on hepatic and whole-body nitrogen metabolism in eight patients with cirrhosis. Urea-nitrogen synthesis rate, alpha-amino-nitrogen levels, and nitrogen exchange were measured in the basal, postabsorptive state and in response to continuous alanine infusion, in paired experiments, during superinfusion of PGE1 or saline. Splanchnic and systemic hemodynamics were assessed by echo-Doppler at the beginning and at the end of each experiment. PGE1 produced a rapid fall in plasma amino acids and in urea-nitrogen synthesis rate, as well as a positive nitrogen exchange. The slope of the regression of alpha-amino-nitrogen levels on urea-nitrogen synthesis rate, a measure of liver cell metabolic activity, was not affected, but the regression line was shifted rightward, suggesting a nitrogen-sparing effect of PGE1. Mesenteric artery and portal flow were unchanged, whereas femoral artery flow increased by 30%. Insulin and glucagon levels were not systematically different. We conclude that PGE1 reduces hepatic urea synthesis rate, independent of hormones and/or hepatic flow, possibly acting at the peripheral level on amino acid transport, thus reducing amino acid supply to the liver. The resulting net nitrogen sparing might be the basis for the beneficial effect of PGE1 in clinical hepatology.

Leucine metabolism in rats with cirrhosis

BACKGROUND/AIMS

This study aimed to investigate the pathogenesis of reduced plasma levels of branched-chain amino acids leucine, isoleucine and valine in cirrhosis.

METHODS

Cirrhosis was induced by intragastric administration of 36 doses of carbon tetrachloride in olive oil over a period of 12 weeks. Rats treated with oil alone served as controls. The rates of leucine turnover, clearance, oxidation and incorporation into proteins were evaluated using [1-14C]leucine, [4,5-3H]leucine and alpha-keto[1-14C]isocaproate 3 days after the last intragastric treatment in vivo and in the isolated perfused liver.

RESULTS

In animals with cirrhosis we observed a profound fall in plasma branched-chain amino acid levels and significant decreases in leucine turnover, oxidation and incorporation into tissue proteins. A more pronounced fall in leucine incorporation in proteins resulted in a significant increase in the oxidized leucine fraction in rats with cirrhosis as compared to controls. Leucine clearance was higher in the cirrhosis group. Concomitant to the fall of whole body leucine turnover, decreases of leucine incorporation into protein and of ketoisocaproic acid decarboxylation were observed in the isolated perfused liver of rats with cirrhosis. However, leucine oxidation was increased compared with control rats.

CONCLUSIONS

Our results indicate that the predominant mechanism of the decrease in plasma leucine levels in rats with cirrhosis is an increase in the oxidized leucine fraction associated with a decrease in leucine turnover. An increase in leucine oxidation in the cirrhotic liver is one of the mechanisms involved.

Transamination of methionine after loading in patients with cirrhosis

BACKGROUND/AIMS

An impaired methionine degradation along the transsulfuration pathway has been widely described in cirrhosis. Evidence has been provided that methionine can also be degraded via a transamination pathway, leading to formation of methanethiol and its metabolites, protein-S-SCH3 (a mixed disulphide of blood proteins and methanethiol), alpha-ketomethylthiobutyrate and X-S-SCH3 (a mixed disulphide of a thiol with an unknown component X and methanethiol). This pathway seems to be of little importance in normal subjects, even after methionine loading, but its role in the presence of an acquired transsulfuration defect has never been tested.

METHODS

We measured the plasma concentration of methanethiol metabolites in six normal subjects and 11 patients with cirrhosis receiving a primed-continuous infusion of L-methionine, at rates able to increase plasma methionine to levels approximately 20 times basal concentrations.

RESULTS

Before methionine infusion, the sum of transamination metabolites was similar in the two groups (0.29 +/- SD 0.07 mumol/l in controls and 0.45 +/- SD 0.22 in patients with cirrhosis). During methionine infusion and after the end of infusion, there was a progressive increase of transamination metabolites, which reached values approximately 10 times basal concentrations, with no difference between groups.

CONCLUSIONS

We conclude that transamination cannot represent a quantitatively important exit for excess methionine in subjects with cirrhosis, in the presence of an acquired block along the transsulfuration pathway.

Effects of beta-blockade on hepatic conversion of amino acid nitrogen and on urea synthesis in cirrhosis

beta-Blockers are widely used to prevent gastrointestinal hemorrhage in cirrhosis. The metabolic effects of treatment are scarcely studied: hepatic function reportedly does not change significantly, but beta-adrenoceptors have been reported to regulate protein and amino acid metabolism. We studied hepatic nitrogen metabolism in response to constant alanine infusion in seven patients with cirrhosis before and 7 to 10 days after treatment with oral propranolol (60 to 100 mg/d). Beta-blockade was effective: it decreased heart rate by 25%, abolished orthostatic tachycardia, and reduced portal blood flow by 20%. Alanine-stimulated urea nitrogen synthesis rate (UNSR) was higher in patients with propranolol treatment, without any difference in aminonitrogen concentration. The kinetics of hepatic conversion of amino acid nitrogen into urea--ie, functional hepatic nitrogen clearance (FHNC)--increased by 30%, from (mean +/- SD) 17.0 +/- 4.1 to 22.0 +/- 6.6 L/h (P < .01). Increased urea production during alanine infusion resulted in negative nitrogen exchange even at the peak of alpha-aminonitrogen concentration. Basal insulin level was only slightly reduced during propranolol treatment, whereas the insulin response to alanine was significantly blunted. No differences in glucagon and cortisol were demonstrated. Epinephrine and norepinephrine levels were high-normal and did not vary after treatment. Increased urea production and stimulation of hepatic nitrogen clearance during beta-blockade may be mediated by relative hypoinsulinemia or by direct involvement of beta-adrenoceptors in the control of nitrogen metabolism, possibly by regulation of amino acid uptake and release in peripheral tissues.

Hepatic conversion of amino-nitrogen to urea in thyroid diseases. II. A study in hyperthyroid patients

Conflicting data have been reported on the influence of thyroid hormones on hepatic nitrogen metabolism and on liver metabolic activity. We studied the urea-nitrogen synthesis rate (UNSR) and the kinetics of the process of hepatic amino-nitrogen to urea-nitrogen conversion in response to constant alanine infusion (ie, the functional hepatic nitrogen clearance [FHNC]) in five hyperthyroid female patients before and after the achievement of a stable euthyroid status. In the same patients, galactose elimination capacity and antipyrine clearance were also measured as quantitative indices of hepatic function. The basal urea synthesis rate was nearly doubled in hyperthyroid patients (35.6 +/- 8.5 mmol.h-1 v 17.6 +/- 7.7 in euthyroid patients, P < .05) and increased linearly with increasing alpha-amino-nitrogen (alpha-AN) concentrations in both conditions. The urea synthesis rate during alanine infusion was still higher by approximately 30 mmol.h-1 in hyperthyroid subjects. The FHNC, calculated as the slope of the linear relation between the UNSR in each time interval and the corresponding average alpha-AN concentration, was not different (hyperthyroidism, 30.6 +/- 7.2 L.h-1; euthyroidism, 28.5 +/- 4.4; normal values > 25). The hepatic microsomal and cytosolic activities (antipyrine clearance and galactose elimination) were normal in hyperthyroid patients and did not change significantly after therapy. Our data show that the hepatic nitrogen metabolism of hyperthyroid patients is characterized by an upregulation of amino-nitrogen catabolism and loss of the sparing mechanism at low plasma amino acid levels, without any change in different metabolic activities.

Hepatic conversion of amino nitrogen to urea nitrogen in hypothyroid patients and upon L-thyroxine therapy

Conflicting studies have been reported regarding the influence of thyroid hormones on hepatic nitrogen metabolism and liver metabolic activity. We studied urea N synthesis rate (UNSR), functional hepatic N clearance (FHNC), galactose elimination capacity, and antipyrine clearance in six hypothyroid female patients before and after achievement of a stable euthyroid status. In both conditions, UNSR measured at intervals in response to constant alanine infusion was linearly related to the average alpha-amino N concentrations. In the hypothyroid state, peak UNSR was decreased by 31% in comparison with values measured in euthyroidism, which were in the normal range. FHNC (ie, the slope of the linear relation between UNSR and blood alpha-amino N concentration) is a measure of the kinetics of the process of hepatic amino N to urea N conversion; it was 19.8 +/- 4.0 L.h-1 in hypothyroid patients and increased to normal values after L-thyroxine replacement (30.4 +/- 3.3 L.h-1, P < .01; normal values > 25 L.h-1). Hepatic microsomal and cytosolic activities (antipyrine clearance and galactose elimination) were normal in hypothyroid patients and did not change significantly after therapy. Our data show a specific defect in hepatic handling of amino acids in hypothyroid patients, leading to reduced alpha-amino N to urea N conversion, in the absence of any detectable impairment in different hepatic metabolic activities.

Effect of S-adenosyl-L-methionine administration on plasma levels of sulphur-containing amino acids in patients with liver cirrhosis

An impaired transsulphuration pathway has been described in patients with liver cirrhosis. The defective metabolic step is located at the site of S-adenosyl-L-methionine (SAMe) formation from methionine. In a placebo-controlled study, we measured the fasting plasma levels of sulphur-containing amino-acids in cirrhotic patients with hypermethioninemia and/or severe hepatocellular failure, during treatment with exogenous SAMe (1.2 g i.v. for 3 days, followed by an oral administration of 1.2 g for an additional 30 days; 8 cases) or saline and placebo tablets (8 cases). All subjects were initially treated during hospital admission, and completed the oral study as out-patients. In patients given SAMe, long-term treatment doubled the plasma concentration of the secondary sulphur-containing amino acid cystine (from 36 [SD 18] mumol.l(-1) to 67 [36]) and taurine (from 42 [13] mumol.l(-1) to 89 [33]), which were on average low-normal at baseline, without any change in the concentration of methionine, of neutral amino acids, and of polyamines. No changes in plasma amino acids were observed in the control group. Two-factor, repeated measures of analysis of variance revealed differences between SAMe- and placebo-treated patients, consistent with an effect of long-term SAMe administration on secondary sulphur-containing amino acids. The potential therapeutic advantage of such treatment remains to be determined in clinical studies.

Defective methionine metabolism in cirrhosis: relation to severity of liver disease

A block in the transsulfuration pathway has previously been suggested in cirrhosis on the basis of increased fasting methionine concentrations, decreased methionine elimination and low levels of methionine end products. To date, methionine elimination has never been studied under controlled steady-state conditions, and the relation of the severity of liver disease to impaired methionine metabolism has not been clarified. We measured methionine plasma clearance in 6 control subjects and in 12 patients with cirrhosis during steady-state conditions obtained by a primed, continuous methionine infusion. In the presence of high-normal fasting methionine concentrations (range = 14 to 69 mumol.L-1 in controls and 26 to 151 mumol.L-1 in cirrhotic patients), methionine plasma clearance was reduced in cirrhotic patients (2.25 +/- S.D. 0.43 ml.sec-1 vs. 2.86 +/- S.D. 0.43 ml.sec-1 in controls; p less than 0.05), whereas methionine half-life was increased (282 +/- 90 min vs. 187 +/- 25 min in controls; p less than 0.05). Fasting methionine significantly correlated with methionine clearance. The infused methionine was not degraded to urea to any significant extent in cirrhotic patients, whereas a threefold increase in urinary urea nitrogen excretion rate was observed in controls. Similarly, taurine concentrations significantly increased both in plasma and in the urine in controls but not in cirrhotic patients. In cirrhotic patients methionine plasma clearance significantly correlated with galactose elimination capacity (r = 0.818) and with the Child-Pugh score (rs = -0.795). The study supports a major role of impaired liver cell function in the reduced metabolism of methionine and decreased formation of methionine end products that occur in cirrhosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose disposal, beta-cell secretion, and hepatic insulin extraction in cirrhosis: a minimal model assessment

Factors controlling glucose metabolism after IV load were studied in nine patients with compensated cirrhosis and in six age-matched controls. The time courses of glucose, insulin, and C peptide were analyzed by means of the minimal model technique. In cirrhosis, insulin sensitivity was reduced by approximately 70% and glucose-dependent glucose uptake (glucose effectiveness) by 45%. Decreased glucose effectiveness explained 65% of the variance of glucose disappearance and correlated with the ratio of urinary creatinine to height, an independent measure of muscle mass (r = 0.839). beta-cell responsiveness to glucose, measured on C-peptide kinetics, was variable and increased on average by 170% and 107% (first-phase and second-phase, respectively). The total amount of insulin secreted by beta-cells in the course of the study was nearly doubled, whereas the basal insulin secretion rate was in the normal range. The time courses of hepatic extraction of insulin did not differ between groups, and basal extraction was on average 58% in controls and 56% in patients with cirrhosis. It was reduced to 30% in a single patient who had severe hepatocellular failure and large spontaneous portosystemic shunting. We conclude that the alterations in glucose metabolism of cirrhosis include a decreased insulin sensitivity, a reduced glucose effectiveness, and an increased pancreatic responsiveness to glucose, leading to hyperinsulinemia. The hepatic extraction of insulin is reduced only in the very advanced stages of the disease, possibly because of a large reserve capacity of the hepatic parenchyma.

Mathematical model to analyse urea synthesis following alanine infusion in control subjects and in patients with liver cirrhosis

A three-compartment model was used to analyse the urea response to an alanine infusion in control subjects and patients with liver cirrhosis. Discriminant analysis showed a good separation between model coefficients of the two groups. A single parameter was derived, able to quantify the liver functional capacity. The method provides a useful diagnostic tool in patients with liver disease.

Long-term oral branched-chain amino acid treatment in chronic hepatic encephalopathy. A randomized double-blind casein-controlled trial. The Italian Multicenter Study Group

In a double blind randomized study, branched-chain amino acids and placebo (casein) were compared as a treatment for chronic hepatic encephalopathy in cirrhosis. After a 15-day run-in period with controlled diet (45-65 g protein), the patients were administered, in addition to their diet, branched-chain amino acids (0.24 g/kg, 30 patients) or an equinitrogenous amount of casein (34 patients). One patient on branched-chain amino acids and two on casein were lost to the study. After 3 months, the index of portal-systemic encephalopathy significantly improved in patients on active treatment (from 40 [S.D. 14]% to 21 [17]), but was not in subjects receiving casein (from 37 [13]% to 36 [12]). Two or more parameters of the index improved in 24 patients treated with amino acids (80%; confidence limits, 61-92%), and only in 12 receiving casein (35%; confidence limits, 20-54%; p less than 0.001). Patients who did not improve were given an alternative treatment for 3 more months. Casein-treated patients given branched-chain amino acids rapidly improved. The changes in neuropsychologic function were associated with an improvement in semiquantitative nitrogen balance, which became consistently positive in amino acid-treated subjects; there was also a mild improvement in nutritional parameters and in liver function tests. The supplementation of oral branched-chain amino acids to the diet is superior to casein as a treatment for providing adequate nitrogen supply and improving the mental state of cirrhotic patients with chronic encephalopathy.

Glucagon increases hepatic efficacy for urea synthesis

The effect of glucagon on the relation between urea synthesis and blood amino acid concentration was studied in seven healthy volunteers. Alanine was given as prime-continuous infusions and, after 1 hr for equilibration, the urea nitrogen synthesis rate was measured in two periods of about 2 hrs as urinary excretion corrected for accumulation and intestinal hydrolysis. During one of the periods, glucagon was infused to obtain a constant concentration of 200-1200 ng/l. The spontaneous urea synthesis during the alanine infusion was 86-141 mmol/hr and linearly related to the alanine concentrations of 1.33-2.99 mmol/l. The hepatic clearance of alanine-nitrogen to urea-nitrogen, assessed by the ratio between the increase in the urea synthesis rate and alanine concentration, was 23 +/- 4 l/hr (mean +/- S.D.). Glucagon increased the rate of urea synthesis by 35 +/- 11 mmol/hr (p less than 0.02) and decreased the alanine concentration by 0.22 +/- 0.06 mmol/l (p less than 0.01). Glucagon increased the hepatic nitrogen clearance to an average of 42 +/- 13 l/hr (p less than 0.01). The difference between infusion of amino-nitrogen and appearance of urea-nitrogen was +15 +/- 10 mmol/hr during alanine infusion alone and -11 +/- 25 mmol/hr during exogenous glucagon. The loss of nitrogen could be accounted for by depletion of non-alanine amino acids from the blood. Glucagon increases the efficacy of urea synthesis, which may be of importance for catabolism by changing the hepatic contribution to nitrogen homeostasis.

Mechanisms and consequences of the impaired trans-sulphuration pathway in liver disease: Part II. Clinical consequences and potential for pharmacological intervention in cirrhosis

The liver is actively involved in the metabolism of the sulphur-containing essential amino acid, methionine. Methionine is transformed into S-adenosyl-L-methionine (SAMe) and then into sulphur-containing metabolites (cysteine, taurine and glutathione) via the trans-sulphuration pathway. Liver disease may affect the trans-sulphuration pathway and decrease the clearance of methionine, which leads to increased fasting methionine concentrations in blood and reduced formation of cysteine and glutathione. There is evidence that this defect, located at the level of SAMe-synthetase, may cause nutritional defects and contribute to negative nitrogen balance whenever non-essential sulphur-containing amino acids are not supplied in adequate amounts. In addition, cirrhotic patients may be at increased risk of hepatotoxicity after treatment with substances which are detoxified via glutathione. The SAMe-synthetase block may be overcome by administration of oral or intravenous SAMe, which improves the fasting amino acid profile and increases the hepatic glutathione concentration. Controlled studies on long term SAMe treatment in patients with cirrhosis are needed to confirm this possible beneficial effect.

Free amino acids in plasma and skeletal muscle of patients with liver cirrhosis

Free amino acids were measured under postabsorptive conditions in plasma and intracellular water of skeletal muscle obtained by needle biopsy in nine healthy controls and 14 subjects suffering from clinically stable liver cirrhosis. The aromatic amino acids phenylalanine and tyrosine in cirrhotics were elevated to the same extent in plasma and in muscle water. Branched-chain amino acids were uniformly reduced in plasma, but in muscle water only valine was significantly lower (222 +/- 92 mumoles per kg intracellular water vs. 368 +/- 82, p less than 0.001), while isoleucine (142 +/- 63 vs. 103 +/- 30), leucine (223 +/- 88 vs. 226 +/- 36) and branched-chain amino acids as a whole (589 +/- 186 vs. 681 +/- 88) were normal or elevated with an increased muscle:plasma ratio (3.12 +/- 2.03 vs. 1.41 +/- 0.37, p less than 0.05 for isoleucine; 3.00 +/- 1.28 vs. 1.85 +/- 0.27, p less than 0.025 for leucine; 2.24 +/- 0.64 vs. 1.69 +/- 0.13, p less than 0.05 for total branched-chain amino acids. Our data show that, in cirrhosis, plasma concentrations of branched-chain amino acids do not reflect their levels in muscle cellular water; only the intracellular pool of valine is severely depleted. This suggests that higher amounts of valine supplementation may be useful in nutritional treatment of liver cirrhosis. The elevated muscle:plasma gradients for branched-chain amino acids may result from abnormalities in their transport through muscle-plasma membrane.(ABSTRACT TRUNCATED AT 250 WORDS)