Amino acids with anabolic properties

Integrated multi-omic data reveal the potential molecular mechanisms of the nutrition and flavor in Liancheng white duck meat

The Liancheng white (LW) duck is one of the most valued Chinese indigenous poultry breeds. Its meat is rich in nutrients and has distinct flavors, but the molecular mechanisms behind them are unknown. To address this issue, we measured and compared multi-omic data (genome, transcriptome, and metabolome) of breast meat from LW ducks and the Mianyang Shelduck (MS) ducks. We found that the LW duck has distinct breed-specific genetic features, including numerous mutant genes with differential expressions associated with amino acid metabolism and transport activities. The metabolome driven by genetic materials was also seen to differ between the two breeds. For example, several amino acids that are beneficial for human health, such as L-Arginine, L-Ornithine, and L-lysine, were found in considerably higher concentrations in LW muscle than in MS duck muscle (p < 0.05). SLC7A6, a mutant gene, was substantially upregulated in the LW group (p < 0.05), which may lead to excessive L-arginine and L-ornithine accumulation in LW duck meat through transport regulation. Further, guanosine monophosphate (GMP), an umami-tasting molecule, was considerably higher in LW muscle (p < 0.05), while L-Aspartic acid was significantly abundant in MS duck meat (p < 0.05), showing that the LW duck has a different umami formation. Overall, this study contributed to our understanding of the molecular mechanisms driving the enriched nutrients and distinct umami of LW duck meat, which will provide a useful reference for duck breeding.

Malnutrition and Fracture Healing: Are Specific Deficiencies in Amino Acids Important in Nonunion Development?

With the increasing incidence of fractures now, and in the future, the absolute number of bone-healing complications such as nonunion development will also increase. Next to fracture-dependent factors such as large bone loss volumes and inadequate stabilization, the nutritional state of these patients is a major influential factor for the fracture repair process. In this review, we will focus on the influence of protein/amino acid malnutrition and its influence on fracture healing. Mainly, the arginine-citrulline-nitric oxide metabolism is of importance since it can affect fracture healing via several precursors of collagen formation, and through nitric oxide synthases it has influences on the bio-molecular inflammatory responses and the local capillary growth and circulation.

Simultaneous Infusion of Glutamine and Branched-Chain Amino Acids (BCAA) to Septic Rats Does Not Have More Favorable Effect on Protein Synthesis in Muscle, Liver, and Small Intestine Than Separate Infusions

BACKGROUND

Glutamine and branched-chain amino acids (BCAA; valine, leucine, and isoleucine) are used as nutrition supplements in the treatment of proteocatabolic illness. We hypothesized that simultaneous administration of BCAA and glutamine affects protein metabolism more significantly than separate administration. In the present study, we evaluated their effect on protein synthesis in skeletal muscle, liver, and jejunum of septic rats.

METHODS

Twenty-four hours after induction of sepsis by subcutaneous injection of turpentine, the rats were infused for 6 hours with 5 mL of 1.75% glutamine, 1.75% BCAA, 1.75% glutamine+BCAA, or saline solution. The control group consisted of intact rats infused with saline. Protein synthesis was measured at the end of infusion by a "flooding method" with [3,4,5-(3)H]phenylalanine.

RESULTS

In turpentine-treated animals, we observed a decrease in glutamine concentration in blood plasma and skeletal muscle, a decrease in BCAA concentration in liver and jejunum, and a decrease in protein synthesis in all tissues. Glutamine or glutamine+BCAA infusion increased glutamine concentration in plasma and muscle and stimulated protein synthesis in the liver. The BCAA infusion enhanced concentrations of BCAA in plasma and tissues, but the effect of BCAA on protein synthesis was insignificant. Synergistic effect of simultaneous infusion of glutamine and BCAA on protein synthesis was not observed.

CONCLUSIONS

We conclude that glutamine infusion to rats with septic injury may significantly improve impaired protein synthesis in the liver and that there is no synergistic effect of glutamine and BCAA infusion on protein synthesis in skeletal muscle, liver, and jejunum.

Indications and contraindications for infusing specific amino acids (leucine, glutamine, arginine, citrulline, and taurine) in critical illness

PURPOSE OF REVIEW

The review assesses the utility of supplementing parenteral or enteral nutrition of ICU patients with each of five specific amino acids that display pharmacological properties. Specifying indications implies also stating contraindications.Combined supplementation of amino acids with ω3-fatty acids and/or trace elements (immune-enhancing diets) will not be considered in this review because these mixtures do not allow the role of amino acids in the effect (positive or negative) of the mixture to be isolated, and so cannot show whether or not supplementation of a given amino acid is indicated.

RECENT FINDINGS

After decades of unbridled use of glutamine (GLN) supplementation in critically ill patients, recent large trials have brought a note of caution, indicating for example that GLN should not be used in patients with multiple organ failure. Yet these large trials do not change the conclusions of recent meta-analyses. Arginine (ARG), as a single dietary supplement, is probably not harmful in critical illness, in particular in a situation of ARG deficiency syndrome with low nitric oxide production. Citrulline supplementation strongly improves microcirculation in animal models with gut injury, but clinical studies are lacking. Taurine has a potent protective effect against ischemic reperfusion injury.

SUMMARY

Amino acid-based pharmaconutrition has displayed familiar 'big project' stages: enthusiasm (citrulline and taurine), doubt (GLN), hunt for the guilty (ARG), and backpedalling (leucine). Progress in this field is very slow, and sometimes gives way to retreat, as demonstrated by recent large trials on GLN supplementation.

Immune-modulating enteral formulations: optimum components, appropriate patients, and controversial use of arginine in sepsis

Nutrients have traditionally been viewed as a means to provide basic calories to sustain homeostasis. However, critically ill, surgical, and trauma patients are in a constant dynamic state between systemic inflammatory response (SIRS) and compensatory anti-inflammatory response (CARS). Results from ongoing research strongly support the use of specific nutrients to modulate the immune and/or metabolic response. These agents can now be considered therapeutic tools in the management of complex hypermetabolic diseases. The principle of using nutrients as a therapeutic strategy rather than just as "nutritional support" requires a shift in the current dogma. The most common nutrients found in currently available enteral immune-modulating formulas are omega-3 fatty acids (eicosapentaenoic acid and docosahexaenoic acid), antioxidants, nucleotides, glutamine, and arginine. Multiple individual reports and at least five meta-analyses using combinations of immune-modulating nutrients have reported almost uniform beneficial results. However, certain conflicting hypotheses continue to revolve around the use of arginine in septic patients.

Effects of ornithine alpha-ketoglutarate on protein metabolism in Yoshida sarcoma-bearing rats

BACKGROUND & AIMS

Ornithine alpha-ketoglutarate (OKG) is recognized to improve nutritional status in various catabolic states, such as burn injury, trauma, and sepsis. However, in wasting diseases, such as induced by cancer, the data are scarce and the precise mechanisms by which OKG acts on protein metabolism are still unclear. The aim of this study was to evaluate the ability of OKG to affect protein metabolism in an aggressive model of cancer and to modulate the ubiquitin-proteasome-dependent pathway, which in skeletal muscle is the critical degradative pathway implicated in many catabolic states, including cancer-associated cachexia.

METHODS

Experiments were carried out in Yoshida sarcoma-bearing and healthy pair-fed rats. Three groups of 16 young male rats were studied during 9 days following tumor implantation: two groups of tumor-bearing rats fed a balanced regimen enriched with either OKG (5 g/kg body weight/day, OKG-K) or an isonitrogenous mixture of non-essential amino acids (C-K), and one group of healthy pair-fed rats (PF).

RESULTS

As expected, Yoshida sarcoma induced muscle atrophy, decreased nitrogen balance, enhanced 3-methylhistidine (3-MH) excretion and increased mRNA levels for ubiquitin and 14-kDa ubiquitin-conjugating enzyme E2. OKG supplementation did not improve muscle mass or protein balance and did not reduce enhanced 3-MH excretion in Yoshida sarcoma-bearing rats. Furthermore, OKG did not suppress in the cancer rats the enhanced expression of ubiquitin and 14-kDa E2, despite OKG decreased by 23% the ubiquitination rate in cancer rats (OKG-K vs. C-K, P<0.05).

CONCLUSIONS

These data suggest that OKG action is not universal; i.e. depending upon the model under study. In the circumstances, OKG did not counteract the increase in ubiquitin-proteasome-dependent proteolysis observed in Yoshida sarcoma-bearing rats.

Dose dependency of the effect of ornithine α-ketoglutarate on tissue glutamine concentrations and hypercatabolic response in endotoxaemic rats

The optimal dosage of ornithine α-ketoglutarate (OKG) for repleting tissue glutamine (Gln) concentrations and maintaining N homeostasis after injury is unknown. We set out to perform ‘dose-ranging’ of OKG supplementation after an endotoxaemic challenge. Sixty-one male Wistar rats were injected with 3 mg lipopolysaccharide (LPS) from Escherichia coli/kg (n 50) or saline vehicle (9 g NaCl/l; controls n 11). After a 24 h fast, survivors were fed by gavage for 48 h with a polymeric standard diet (879 kJ/kg per d and 1·18 g N/kg per d) supplemented with non-essential amino acids (control, n 11; LPS-OKG-0·0, n 9), or with 0·5 g OKG/kg per d (LPS-OKG-0·5, n 12), 1·5 OKG/kg per d (LPS-OKG-1.5, n 11) or 4·5 g OKG/kg per d (LPS-OKG-4·5, n 10). The diets for all groups were made isonitrogenous with the LPS-OKG-4·5 diet by adding an appropriate amount of non-essential amino acids. Rats were killed on day 3 for blood and tissue sampling (muscle, jejunum mucosa, liver). Urine was collected daily for 3-methylhistidine and total N assays. The OKG dose was correlated with Gln concentrations in every tissue and with cumulative N balance (Spearman test, P<0·01). 3-Methylhistidine excretion was increased in endotoxaemic groups compared with controls (ANOVA, P<0·05) except in the LPS-OKG-4·5 group. Only the LPS-OKG-4·5 group achieved a positive post-injury N balance (t test, P<0·05). In conclusion, OKG exerted a dose-dependent effect on tissue Gln concentration and N balance, but only the highest dosage counteracted myofibrillar hypercatabolism and caused a positive N balance.

Enhanced fracture and soft-tissue healing by means of anabolic dietary supplementation

BACKGROUND

Malnutrition is common in hospitalized injured patients. It contributes to delayed fracture-healing and increased morbidity. However, relatively little attention has been directed toward nutritional strategies for augmenting musculoskeletal recovery after a fracture. This animal study was designed to examine the effects of dietary protein intake and the role of conditionally essential amino acids in muscle and bone-healing after a fracture.

METHODS

One hundred adult male rats were used. Ten rats served as controls and received a 15% protein diet throughout the study. The remaining ninety rats received a 6% protein diet for five weeks to induce protein malnutrition. The rats underwent intramedullary nailing and closed midshaft fracture of one femur. After the fracture, they were separated into three isocaloric dietary groups. Group P6 received a diet with 6% protein; Group P15, a diet with 15% protein; and group P30, a diet with 30% protein with conditionally essential amino acids. At two, four, and six weeks after surgery, ten animals from each group were killed and the femora were evaluated with dual x-ray absorptiometry, histomorphometric assessment of callus, and torsional testing. The quadriceps muscles were analyzed for total mass, total protein content, and for mRNA expression of insulin-like growth factor-1 (IGF-1), IGF-2, IGF receptors, actin, myosin, and vascular endothelial growth factor (VEGF).

RESULTS

The P30 group demonstrated elevations in albumin, body mass, muscle mass, total protein content of muscle, and bone mineral density in the fracture callus compared with the P6 diet group at six weeks (p < 0.05). Molecular analysis of muscle revealed that IGF-1, IGF-2, IGF receptors, myosin, actin, and VEGF gene expression were significantly (p < 0.001) higher in the P6 group compared with the P30 group. Biomechanical testing of the femora, however, showed no significant differences.

CONCLUSIONS

Dietary supplementation with conditionally essential amino acids in malnourished animals had anabolic effects on bone mineralization, body mass, and muscle mass.

Aspects of Protein and Amino Acid Metabolism in a Model of Severe Glutamine Deficiency in Sepsis

BACKGROUND/AIMS

Growth hormone (GH) could have the potential to improve protein metabolism in sepsis but glutamine deficiency has been reported after GH treatment. The aim was to investigate the effects of glutamine deficiency in sepsis with and without GH treatment on protein and amino acid metabolism.

METHODS

Cecal ligation and puncture (CLP) was used as a model of sepsis. Serious glutamine deficiency was induced by administration of glutamine synthetase inhibitor, methionine sulfoximine (MSO). Young Wistar rats were divided into 5 groups: control; CLP; CLP+MSO; CLP+GH, and CLP+MSO+GH. Parameters of protein metabolism were measured on incubated soleus and extensor digitorum longus muscles: [1-14C]leucine was used to estimate protein synthesis and leucine oxidation, tyrosine release was used to evaluate protein breakdown. Amino acid concentrations in plasma, skeletal muscle and incubation media were measured by HPLC.

RESULTS/CONCLUSIONS

A reduced muscle glutamine concentration after MSO treatment is not associated with changes in the rates of protein synthesis or breakdown. MSO treatment decreased glutamine release from skeletal muscle and plasma glutamine concentration. Severe glutamine deficiency in GH-treated septic rats resulted in increased release of branched-chain amino acids from skeletal muscle.

L-alanyl-L-glutamine dipeptide-supplemented total parenteral nutrition reduces infectious complications and glucose intolerance in critically ill patients: the French controlled, randomized, double-blind, multicenter study

OBJECTIVE

Glutamine (Gln)-supplemented total parenteral nutrition (TPN) improves clinical outcome after planned surgery, but the benefits of Gln-TPN for critically ill (intensive care unit; ICU) patients are still debated.

DESIGN

Prospective, double-blind, controlled, randomized trial.

SETTING

ICUs in 16 hospitals in France.

PATIENTS

One-hundred fourteen ICU patients admitted for multiple trauma (38), complicated surgery (65), or pancreatitis (11).

INTERVENTIONS

Patients were randomized to receive isocaloric isonitrogenous TPN via a central venous catheter providing 37.5 kcal and 1.5 g amino acids.kg-1.day-1 supplemented with either L-alanyl-L-glutamine dipeptide (0.5 g.kg-1.day-1; Ala-Gln group, n=58) or L-alanine+L-proline (control group, n=56) over at least 5 days.

MEASUREMENTS AND MAIN RESULTS

Complicated clinical outcome was defined a priori by the occurrence of infectious complications (according to the criteria of the Centers for Disease Control and Prevention), wound complication, or death. The two groups were compared by chi-square test on an intention-to-treat basis. The two groups did not differ at inclusion for type and severity of injury (mean simplified acute physiology score II, 30 vs. 30.5; mean injury severity score, 44.9 vs. 42.3). Similar volumes of TPN were administered in both groups. Ala-Gln-supplemented TPN was associated with a lower incidence of complicated outcome (41% vs. 61%; p<.05), which was mainly due to a reduced infection rate per patient (mean, 0.45 vs. 0.71; p<.05) and incidence of pneumonia (10 vs. 19; p<.05). Early death rate during treatment and 6-month survival were not different. Hyperglycemia was less frequent (20 vs. 30 patients; p<.05) and there were fewer insulin-requiring patients (14 vs. 22; p<.05) in the Ala-Gln group.

CONCLUSIONS

TPN supplemented with Ala-Gln dipeptide in ICU patients is associated with a reduced rate of infectious complications and better metabolic tolerance.

Therapeutic use of branched-chain amino acids in burn, trauma, and sepsis

Various experimental studies conducted in the 1970s demonstrated, at least in the physiological situation, the anabolic and/or anticatabolic properties of branched-chain amino acids (leucine, valine, isoleucine) or their ketoacid derivatives. This led to several clinical studies in the late 1970s and early 1980s that aimed to evaluate the potential benefits of BCAA supplementation in nutritional support of the critically ill. The data on burn, trauma, and sepsis are, however, far from convincing. Besides significant discrepancies in their results and the fact that most of these studies involved very small populations of patients, few of them meet the current standards of therapeutic evaluation. However, some positive results in specific studies suggest that the underlying concept may be correct but that interpretation has been faulty. Indeed, we know now that while the BCAAs possess regulatory properties on protein metabolism, leucine is by far the most potent, while isoleucine and valine are inefficient. However, in the above-mentioned studies, BCAA-supplemented nutrition very frequently supplied almost equivalent amounts of all 3 BCAAs. Moreover, several studies were performed without adequate basal nutritional support, which most probably hampered the correct metabolic utilization of these amino acids. Taken together, these factors mean that the demonstrations of BCAA efficacy were fortunate in the least. In contrast, more recently, leucine was demonstrated to positively affect protein synthesis in an experimental model of sepsis or burn. In parallel, 2 prospective controlled trials of BCAA supplementation in septic patients also demonstrated an improvement in patients' nutritional status and outcome. Thus, we should abandon the concept of BCAA-supplemented nutrition for a more promising leucine-supplemented nutrition that requires further evaluation.

Regulation of SNARK activity in response to cellular stresses

SNARK is a member of the AMPK subfamily of serine/threonine protein kinases. In this study, we examined the regulation of SNARK activity in kidney (BHK, HEK293), pancreatic beta-cell insulinoma (INS-1), hepatocarcinoma (H4IIE) and keratinocyte (NRKC)-derived cell lines in response to diverse cellular stresses. We show that SNARK activity is regulated by glucose- or glutamine-deprivation, induction of endoplasmic reticulum stress by homocysteine or DTT, elevation of cellular AMP and/or depletion of ATP, hyperosmotic stress, salt stress, ultraviolet B radiation and oxidative stress caused by hydrogen peroxide. Moreover, the regulation of SNARK activity in response to cellular stresses depends greatly upon cell type. Furthermore, SNARK activity is downregulated by metformin in a dose- and time-dependent manner in H4IIE cells. These observations support a role for SNARK as a molecular component of the cellular stress response.

Effects of an immune-enhancing diet in endotoxemic rats

OBJECTIVE

This work compared the nutritional efficiency of a recently available enteral formula enriched with arginine, omega-3 fatty acids, and antioxidants and supplied nitrogen as peptides (Crucial, Nestle Clinical Nutrition) with that of a standard polymeric formula (Sondalis HP, Nestle Clinical Nutrition) in endotoxemic rats.

METHODS

Male Wistar rats (209 +/- 2 g) underwent catheter gastrostomy and received Sondalis HP until they recovered their preoperative weight. At that time (day 0), an endotoxemic shock was induced by an intraperitoneal injection of lipopolysaccharide (Escherichia coli, 8 mg/kg) and rats then received 290 kcal x kg(-1) x d(-1) and 3.29 g of nitrogen x kg(-1) x d(-1) in the form of Crucial (IED group, n = 7) or Sondalis HP (S group, n = 6) for 3 d. Another group underwent no treatment and was fed ad libitum (AL group). Rats were killed on day 3. Results are presented as mean +/- standard error of the mean (analysis of variance and Newman-Keuls test).

RESULTS

The endotoxemic shock induced a weight loss in group S on days 1 and 2 and a weight gain in group IED (-3.5 +/- 1.3 g in group S versus +6.0 +/- 2.2 g in group IED, P < 0.05). In the same way, atrophy of extensor digitorum longus muscle was observed in group S, whereas wasting was limited in group IED (102 +/- 4 mg in group IED versus 90 +/- 3 mg in group S versus 119 +/- 3 mg in group AL, P < 0.05). Muscular atrophy was associated with muscular glutamine depletion and correlated with hyperphenylalaninemia (R = 0.60), with the latter being blunted in group IED (57 +/- 1 microM/L in group AL versus 77 +/- 4 microM/L in group S versus 66 +/- 2 microM/L in group IED, P < 0.05). No difference was observed between the experimental groups of endotoxemic rats with respect to nitrogen balance, urinary excretion of 3-methyl histidine, or total tissue protein content.

CONCLUSION

Crucial counteracts injury-mediated weight loss, extensor digitorum longus muscle atrophy, and hyperphenylalaninemia in endotoxemic rats.

Arginine and glutamine availability and macrophage functions in the obese insulin-resistant Zucker rat

Increased susceptibility to infections in obese patients may be related to decreased availability of arginine and glutamine, which may affect immune cell functions. Our aim was to evaluate the in vitro effects of these amino acids on the function of macrophages from obese insulin-resistant Zucker rats. Macrophages, isolated from male Zucker obese or lean rats by peritoneal lavage, were incubated in Dulbecco's modified Eagle medium (DMEM) without arginine or glutamine. Arginine or glutamine was added to the medium at increasing final concentrations (0, 0.25, 0.5, 1 or 2 mM). After stimulation by lipopolysaccharide (LPS) from E. coli (40 microg/ml), productions of tumour necrosis factor alpha (TNFalpha) and of nitric oxide (NO) were measured after 3 or 48 h incubation, respectively. NO production, lower in macrophages from obese rats, decreased in macrophages from lean rats (0 mM: 2,423 +/- 1,174 vs. 2 mM: 198 +/- 31 microM/mg protein/24 h; P < 0.05), but not in those from obese rats, when glutamine was added. TNFalpha production, lower in macrophages from obese rats, was inversely correlated with glutamine concentration. In the presence of arginine, NO production was constantly higher in macrophages from obese rats. It peaked at 0.5 mM arginine and decreased thereafter in both groups. TNFalpha production in macrophages from lean rats was unaffected by arginine, but decreased in macrophages from obese rats (0 mM: 1920 +/- 450 vs. 2 mM: 810 +/- 90 microM/mg protein/3 h; P < 0.05). These results suggest that abnormalities in cell signalling or in arginine and glutamine metabolism in macrophages of obese rats, resulting in decreased TNFalpha production and increased NO release, may contribute to increased susceptibility to infection in insulin-resistant states.

Protein metabolism in guanethidine-treated rats

The aim of the present study was to evaluate the effect of short-term adrenergic blockade on the rate of whole-body protein turnover and leucine oxidation, and on protein synthesis in specific tissues in male rats. Adrenergic blockade was induced by guanethidine (100 mg/kg body weight subcutaneously). The control group was treated with saline. On the second day, the parameters of whole-body protein and leucine metabolism were evaluated using a primed constant intravenous infusion of L-[1-(14)C]leucine. Protein synthesis in tissues was determined on the basis of L-[1-(14)C]leucine incorporation. Guanethidine treatment caused a decrease in norepinephrine in skeletal muscle. Whole-body leucine oxidation and leucine oxidized fraction were higher in guanethidine-treated rats. There was an insignificant effect of guanethidine on whole-body proteolysis, protein synthesis and leucine clearance. However, protein balance was negative due to the larger difference between protein synthesis and proteolysis in guanethidine-treated animals compared to controls. In guanethidine-treated rats, protein synthesis was higher in the gastrocnemius muscle and in the kidneys and lower in liver and spleen. Changes in the small intestine and colon were insignificant. In addition, a marked decrease in concentration of several amino acids has been observed in the liver, the kidneys and the spleen. It is concluded that adrenergic blockade induced by guanethidine is associated with significant changes in protein metabolism, leucine oxidation and amino acid concentrations in several tissues. The most important consequences of treatment are considered to be a negative effect on protein balance, increased protein turnover in skeletal muscle and kidneys and decreased protein synthesis in the liver and spleen. These changes may also be induced by administration of other sympathetic blocking agents, e.g. in treatment of hypertension.

The gastrointestinal tract in critical illness: nutritional implications

PURPOSE OF REVIEW

Recognition that the gastrointestinal tract is a key element of the immune system has led to a greater interest in understanding its role as a central figure in host defenses. Biologic systems that are perturbed by any destabilizing stimulus are known to respond by adaptive strategies in an attempt to maintain or return to global homeostasis. In critically ill patients, the gut has previously been described as a promoter of progression to sepsis and multi-organ failure. However, with better understanding of gastrointestinal tract mucosal immunity, we are now provided with a new arsenal to combat nosocomial infection and significantly impact return to health.

RECENT FINDINGS

In this review we focus on five key topics in the rapidly expanding landscape of knowledge on the gastrointestinal tract in the critical care setting. These include a discussion of probiotic therapy, now the new frontier of immuno-nutrition, the concept of ischemia/reperfusion injury and changes in gut permeability, anti-oxidant and micronutrient therapy, blood glucose regulation, and enhancement of gut motility, all in the intensive care setting.

SUMMARY

Ongoing research in nutritional support in both normal and pathologic gastrointestinal function and response to injury has opened the door to several new opportunities for enhancing rapid recovery in critical care.

Acute effects of decreased glutamine supply on protein and amino acid metabolism in hepatic tissue: a study using isolated perfused rat liver

Glutamine deficiency, a common finding in severe illness, has a negative influence on immune status, protein metabolism, and disease outcome. In several studies, a close relationship between glutamine, branched-chain amino acid (BCAA), and protein metabolism was demonstrated. The aim of the present study was to investigate the effect of glutamine deficiency on amino acid and protein metabolism in hepatic tissue using a model of isolated perfused rat liver (IPRL). Parameters of protein metabolism and amino acid metabolism were measured using both recirculation and single pass technique with L-[1-(14)C]leucine and [1-(14)C]ketoisocaproate (KIC) as a tracer. Glutamine concentration in perfusion solution was 0.5 mmol/L in control and 0 mmol/L in the glutamine-deficient group. The net release of glutamine (about 11 micromol/g/h) and higher net uptake of most of the amino acids was observed in the glutamine-deficient group. There was an insignificant effect of lack of glutamine on hepatic protein synthesis, proteolysis, and the release of urea. However, significantly lower release of proteins by the liver perfused with glutamine-deficient solution was observed. The lack of glutamine in perfusion solution caused a significant decrease in leucine oxidation (6.66 +/- 1.04 v 13.67 +/- 2.38, micromol/g dry liver/h, P <.05) and an increase in KIC oxidation (163.7 +/- 16.5 v 92.0 +/- 12.9 micromL/g dry liver/h, P <.05). We conclude that decreased delivery of glutamine to hepatic tissue activates glutamine synthesis, decreases resynthesis of essential BCAA from branched-chain keto acids (BCKA), increases catabolism of BCKA, and has an insignificant effect on protein turnover in hepatic tissue.

In vivo induction of insulin secretion by ornithine alpha-ketoglutarate: involvement of nitric oxide and glutamine

We previously demonstrated that ornithine alpha-ketoglutarate (OKG), known for its anabolic properties, induces insulin secretion in vitro. The present study was undertaken to further characterize this effect in vivo and investigate a possible interaction with glucose both in vivo and in vitro. Male Wistar rats received an intravenous bolus of OKG (25 mg/kg) and/or glucose (0.8 g/kg) or saline, and their plasma insulin and glucose levels were monitored for 30 minutes. OKG alone increased plasma insulin to a similar extent to glucose. In combination with glucose, OKG significantly increased glucose-induced insulin secretion in vivo and in vitro, and led to a significant increase in glucose utilization in vivo. The absence of significant variations in plasma arginine and glutamine suggests a direct effect of OKG on the pancreas. To assess the involvement of the synthesis of nitric oxide and glutamine in OKG-induced insulin secretion, the experiments were repeated in the presence of inhibitors of these 2 pathways, respectively L-nitroarginine-methylester (L-NAME) and methionine sulfoximine (MSO). Both inhibitors were able significantly to reduce OKG-induced insulin secretion without affecting either basal or glucose-induced insulin release. Thus OKG acts directly with glucose on islets to induce insulin secretion via mechanisms involving NO and glutamine synthesis. In addition, our results suggest that OKG and glucose act via separate pathways.

Effects of ornithine alpha-ketoglutarate on insulin secretion in rat pancreatic islets: implication of nitric oxide synthase and glutamine synthetase pathways

Ornithine alpha-ketoglutarate (OKG) administration in human subjects elicits insulin secretion. We investigated whether this action was related to an effect of OKG on islets of Langerhans, and addressed the underlying mechanisms of action. For this purpose the influence of OKG on insulin secretion was measured in isolated rat islets of Langerhans under two different conditions. In incubated islets, OKG (0.25 to 2.5 mmol/l) significantly and dose-relatedly increased insulin secretion (1.7- to 4.2-fold; P<0.05 v. basal). To study the kinetics of OKG-stimulated insulin secretion, perifusion experiments were performed, which showed that OKG affected insulin secretion in both initial and later phases. Experiments using alpha-ketoglutarate (alpha-KG) (1 mmol/l) or ornithine (Orn) (2 mmol/l) alone, in concentrations equal to that of OKG, showed that the OKG-induced insulin secretion could not be obtained by either component alone, suggesting that an alpha-KG-Orn interaction is mandatory for the insulin-secreting effect to occur. Since data obtained in vivo suggest that effects of OKG may depend on the synthesis of NO, glutamine and/or polyamines, three metabolic pathways potentially involved in insulin secretion, we then evaluated their contribution by means of their respective inhibitors: l-NG-nitroarginine methyl ester (l-NAME), methionine sulfoximine (MSO) and difluoromethylornithine (DFMO). Both l-NAME and MSO were able significantly to reduce OKG-induced insulin secretion (30 and 40 % respectively; P<0.05), while DFMO was ineffective. Thus OKG is an effective stimulator of insulin secretion, requiring the joint presence of both Orn and alpha-KG, and acting mainly via the synthesis of NO and glutamine. A better understanding of OKG insulino-secretory properties and its mechanisms of action are a prerequisite for its use in insulin-compromised situations.

Alpha-ketoglutarate-supplemented enteral nutrition: effects on postoperative nitrogen balance and muscle catabolism

Enteral feeding in the early postoperative phase may improve gut integrity and reduce infectious complications after trauma and surgery. The aim of the current study was to evaluate the feasibility of alpha-ketoglutarate enrichment of enteral feeding and the effect on protein metabolism after major surgery. Patients undergoing elective abdominal surgery were randomly allocated to receive a standard whole-protein-based enteral nutrition solution (n = 9) or an isonitrogenous, isocaloric solution enriched with alpha-ketoglutarate (n = 11) for 5 d postoperatively. The nutritional goals by day 4 were 25 kcal and 0.17 g of nitrogen, respectively, per kilogram of body weight every 24 h. Standard blood analysis, including prealbumin and C-peptide, was performed preoperatively and on days 1, 3, and 6. Urine was collected daily for nitrogen and 3-methylhistidine analyses. Due to restricted tolerance to enteral feeding, the nitrogen delivery reached only 0.10 g of nitrogen per kilogram of body weight. Transthyretin decreased by 25% in both groups, and albumin decreased significantly in the enriched group compared with the standard nutrition. There were no significant differences in nitrogen balance, excretion of 3-methylhistidine, or clinical outcome between groups. Enrichment of a whole-protein-based formula with alpha-ketoglutarate did not improve protein metabolism or decrease muscle catabolism after major abdominal surgery.

Evidence that glutamine modulates respiratory burst in stressed rat polymorphonuclear cells through its metabolism into arginine

Glutamine (GLN) and arginine (ARG) are recognized for their ability to modulate immune cell function. However, the metabolic pathways involved in their action remain unclear. It was recently shown that GLN- or ARG-enriched diets increased radical oxygen species (ROS) production by neutrophils from stressed rats. Since these two amino acids have a tied metabolism, we hypothesized that conversion between GLN and ARG (and its active metabolites NO* and polyamines) might be involved. To test this hypothesis male Sprague-Dawley rats (n 117) were randomized into thirteen groups: rats in eleven groups were rendered catabolic by dexamethasone injection (1.5 mg/kg per d for 5 d) and 6.8 mmol either GLN, ARG or non-essential amino acids (NEAA; glycine, alanine and histidine)/kg per d were given by the enteral route; one group was pair-fed to the treated groups. The regimens of all the groups were rendered isonitrogenous by the addition of NEAA. The last group was not treated and was fed ad libitum. For each supplementation three subgroups were formed, each of which received a specific inhibitor: methionine sulfoximine (inhibitor of GLN synthase; 100 mg/kg per d), S-methylthiourea (inhibitor of inducible NO* synthase (iNOS); 50 mg/kg per d) and difluoromethylornithine (inhibitor of ornithine decarboxylase (ODC); 50 mg/kg per d). Oxidative metabolism, intracellular H2O2, and extracellular O2*- production were measured in unstimulated and phorbol myristate acetate-stimulated polymorphonuclear neutrophils. GLN- and ARG-enriched diets increased respiratory burst by neutrophils (oxidative metabolism of 152 (sem 24) and 138 (sem 45) v. 57 (sem 18) mV for GLN-, ARG- and NEAA-enriched diets respectively, P<0.05). In vivo inhibition of iNOS or ODC decreased ROS production induced by GLN and ARG. In vivo inhibition of GLN synthase did not modify the effect of ARG on ROS production. In conclusion, GLN and ARG modulate ROS production in neutrophils from stressed rats by the same pathway involving polyamine and NO* synthesis.

Effects of ornithine alpha-ketoglutarate on circulatory antioxidants and lipid peroxidation products in ammonium acetate treated rats

The effects of ornithine alpha-ketoglutarate (OKG) on ammonium acetate induced hepatotoxicity were studied biochemically in rats. The levels of urea, nonprotein nitrogen, and thiobarbituric acid reactive substances were significantly increased in ammonium acetate treated rats; these levels were significantly decreased in rats treated with ammonium acetate and OKG. Similar patterns of alterations were observed in the levels of free fatty acids, triglycerides, and phospholipids. Furthermore, nonenzymatic antioxidants (vitamins C and E) were significantly decreased in ammonium acetate treated rats, when compared with control rats, and increased in OKG and ammonium acetate treated rats. The biochemical alterations during OKG treatment could be (1) by detoxifying excess ammonia; (2) by participating in nonenzymatic oxidative decarboxylation in the hydrogen peroxide decomposition process, and (3) by enhancing the proper metabolism of fats which could suppress oxygen radical generation and thus prevent the lipid peroxidative damages in rats.

L-arginine supplementation in pigs decreases liver protein turnover and increases hindquarter protein turnover both during and after endotoxemia

BACKGROUND

Accumulating evidence suggests that L-arginine, under conditions of septicemia, not only enhances immune function but also improves protein metabolism.

OBJECTIVE

Because the effect of L-arginine administration on the protein metabolism of different organs is unknown, the aim of the study was to elucidate the effects of exogenous supplementation of L-arginine during endotoxemia on the in vivo protein metabolism of individual organs and at the whole-body level.

DESIGN

Female pigs were cannulated with catheters in the aorta and the splenic, caval, portal, hepatic, and renal veins, enabling measurements across the hindquarter, portal-drained viscera, liver, and kidneys. Endotoxemia was induced by a 24-h continuous intravenous infusion of endotoxin (3 microg x kg body wt(-1) x h(-1)). At 8 h, an intravenous infusion of L-arginine was started (n = 8). Control pigs (n = 6) received L-alanine. At 24 h, blood was sampled. After cessation of the endotoxin infusion, L-arginine and L-alanine infusions were continued as a supplement in the enterally infused diet. At 48 h, blood samples were obtained during the postendotoxemic and nutritionally supported conditions. Stable isotopes were used to assess protein metabolism and phenylalanine hydroxylation.

RESULTS

Both during and after the endotoxin challenge, L-arginine administration enhanced protein synthesis and degradation across the hindquarter and simultaneously reduced protein synthesis and degradation in the liver at equal rates. Protein turnover across the kidneys and portal-drained viscera remained unaffected. After endotoxemia, L-arginine infusion decreased whole-body protein turnover without affecting the net protein balance.

CONCLUSION

L-Arginine administration affects protein turnover of the muscle area and the liver oppositely.

Do we have unrealistic expectations of the potential of immuno-nutrition?

Heavy sports training schedules and competition is often associated with immuno-suppression, and so there is a theoretical justification for providing athletes with nutrients that display immuno-regulatory properties. Among such immuno-nutrients, considerable attention has been paid in recent years to two amino acids, arginine (ARG) and glutamine (GLN). ARG and GLN availability regulate the function of T lymphocytes, macrophages and polymorphonuclear cells. ARG acts through nitric oxide and polyamine synthesis. The mechanism of action of GLN in immune cells remains unclear. Experience in clinical nutrition suggests that an ARG-enriched diet may limit infectious morbidity in critically ill patients. Data concerning oral/enteral GLN supplementation are more controversial. There have been few trials of supplementation in sports medicine, but results are promising, justifying further studies in which dosages and administration schedules should be taken into account.

Relation between glutamine, branched-chain amino acids, and protein metabolism

The branched-chain amino acids (BCAAs; valine, isoleucine, and leucine) are the major nitrogen source for glutamine and alanine synthesis in muscle. Synthesis of glutamine, alanine, and BCAA use is activated in critical illnesses such as in sepsis, cancer, and trauma. The use of glutamine often exceeds its synthesis, resulting in the lack of glutamine in plasma and tissues. In critical illness, resynthesis of BCAA from branched-chain keto acids is activated, particularly in hepatic tissue. The BCAA released to circulation may be used for protein synthesis or synthesis of alanine and glutamine. Glutamine and/or alanine infusion has an inhibitory effect on the breakdown of body proteins and decreases BCAA catabolism in postabsorptive control, endotoxemic, and irradiated rats. Decreased protein breakdown also was observed when glutamine synthesis was activated by ammonia infusion. In conclusion some favorable effects of BCAA supply can be explained by its role in the synthesis of glutamine and some positive effects of glutamine exogenous supply can be explained by its effect on metabolism of BCAA.

Regulation of amino acid transporters by amino acid availability

There is growing recognition that amino acid availability has profound effects on many aspects of cell function, including the control of membrane transport mechanisms, cell signalling, and gene expression. The precise mechanisms by which amino acids are able to elicit control over such diverse processes have become the focus of intense investigation recently. One particular area that has seen considerable advances is the molecular characterization of amino acid transporters, including members of the System A family, which are known to be regulated by amino acid supply. Recent developments concerning how cells sense and signal amino acid availability, and how this process influences the expression and function of amino acid transporters, are reviewed here. Elucidating the molecular mechanisms of these events will be important in clarifying how amino acid transporters might be regulated during altered nutritional states, and will be crucial for the design of new strategies aimed at improving nutritional support.

Total parenteral nutrition enriched with arginine and glutamate generates glutamine and limits protein catabolism in surgical patients hospitalized in intensive care units

OBJECTIVES

To study the effect of a parenteral nutrition solution enriched with potential precursors of glutamine, i.e., arginine and glutamate, on plasma glutamine concentrations and protein metabolism.

DESIGN

Prospective, randomized, single-blind, comparative study.

SETTING

Two intensive care units in two different hospitals.

PATIENTS

Fifteen surgical patients.

INTERVENTIONS

Patients were randomized to receive total parenteral nutrition for 5 days with the enriched glutamine precursor solution (GlnP+ group) or a conventional solution (control group), both total parenteral nutrition providing 0.25 gN/kg per day and 35 kcal/kg per day (glucose/lipids, 70%:30%).

MEASUREMENTS AND MAIN RESULTS

Plasma amino acid concentrations before (T0) and after 3 hrs (T3) of perfusion, nitrogen balance (daily and cumulated), and urinary excretion of 3-methylhistidine were measured daily from day 1 to day 5. The two groups were identical for age, weight, severity score, and nitrogen and energy intakes. After a 3-hr perfusion, plasma concentrations of arginine, ornithine, and glutamine increased, and the differences (T3 - T0) were significantly higher in the GlnP+ group: arginine, 107.6+/-7.0 vs. 51.9+/-3.3 (mean over 5 days; p < .001); ornithine, 78.9+/-7.1 vs. 43.6+/-3.1 (p < .001); and glutamine, 32.4+/-8.6 vs. 6.7+/-5.0 micromol/L (p < .05), respectively. A positive correlation was found between arginine and glutamine plasma increases only in the GlnP+ group: r = .45; p < .01 (Spearman's rank-correlation test). Daily and cumulated nitrogen balances were not significantly different between the two groups but were positive (difference from 0) only in the GlnP+ group. The urinary 3-methylhistidine/creatinine ratio decreased significantly from day 1 to day 5 only in the GlnP+ group: 24.5+/-2.7 vs. 18.8+/-2.7 micromol/mmol (p < .05).

CONCLUSIONS

Total parenteral nutrition enriched with arginine and glutamate promotes a better nitrogen balance, limits protein myofibrillar catabolism, and generates glutamine, with arginine (not glutamate) probably being the main contributor to the glutamine-generating effect of the solution through the formation of ornithine.

Glutamine and the gastrointestinal tract

The amino acid glutamine has become one of the most intensively studied nutrients in the field of nutrition and metabolic support. A variety of studies in cell culture systems, animal models of gut mucosal atrophy, injury/repair and adaptation and a limited number of clinical trials demonstrate trophic and cytoprotective effects of glutamine in small bowel and colonic mucosal cells. Although the routine clinical use of glutamine-enriched parenteral and enteral nutrient solutions remains controversial, available data demonstrate both the safety and metabolic and clinical efficacy of glutamine treatment in selected patient groups. Basic investigations are elucidating underlying mechanisms of glutamine action in intestinal cells. These will inform preclinical and clinical investigations designed to determine glutamine efficacy in selected gastrointestinal disorders. Emerging clinical trials will further define the utility of adjunctive glutamine supplementation as a component of specialized nutrition support in gastrointestinal disease.

N-acetyl-cysteine in the therapy of HIV-positive patients

Randomly selected asymptomatic HIV-positive persons reveal, on average, a massive daily loss of sulphur, which appears to represent in first approximation the mean loss throughout the asymptomatic stage, and may explain the widely observed decrease in cyst(e)ine and glutathione levels. This sulphur loss is reasonably expected to lead, within a few years, to a life-threatening condition and may, therefore, contribute decisively to disease progression. Importantly, the rate of sulphur loss is not ameliorated by highly active antiretroviral therapy and may contribute to antiretroviral treatment failure. Several clinical trials on N-acetyl-cysteine treatment of HIV-positive patients have revealed various therapeutic effects, but did not meet the rigorous standards for approval by the health authorities.

Cysteine and glutathione in catabolic conditions and immunological dysfunction

The conspicuous increase in the plasma cysteine disulphide/thiol ratio in elderly persons and cancer patients indicates a shift of the plasma redox state. The most important redox buffers in skeletal muscle tissue and blood plasma, i.e. glutathione and albumin, respectively, are significantly decreased in different models of cachexia. Treatment with N-acetyl cysteine, i.e. a thiol-containing antioxidant, was found to increase the plasma albumin level and to ameliorate the loss of body cell mass in cancer patients and healthy individuals. The treatment of HIV infection with N-acetyl cysteine, in contrast, serves mainly as a tool to ameliorate the physiological and immunological consequences of the virus-induced cysteine deficiency.

Potential intracellular targets for anabolic/anti-catabolic therapies

The intracellular signalling pathways controlling muscle protein synthesis and proteolysis are potential targets for anabolic/anti-catabolic therapy. In this review, we consider both the potentiation of the effect of anabolic hormones and suppression of the catabolic action of cytokines. Potential candidates, in particular isoforms of the protein kinase C family, and their role in the control of ribosomal action and the ubiquitin-proteasome proteolytic system are discussed.

The use of alpha-ketoglutarate salts in clinical nutrition and metabolic care

Theoretically, alpha-ketoglutarate is a precursor of glutamine, a fact that may be of importance given the key regulatory properties of this amino acid. Although the literature suggests that glutamine synthesis accounts only for a marginal part of the disposal of exogenously supplied alpha-ketoglutarate, administered alpha-ketoglutarate has a potent 'sparing' effect on endogenous glutamine pools. When alpha-ketoglutarate is supplied as an ornithine salt, a synergistic effect of the two parts of the molecule increases the synthesis of glutamine or the 'sparing' of endogenous glutamine pools. In addition, alpha-ketoglutarate in combination with ornithine dramatically increases the synthesis of arginine, proline and polyamines, which also play key roles in metabolic adaptation to trauma. The recent literature suggests that the administration of alpha-ketoglutarate in combination with ornithine improves gut morphology and functions, counteracts trauma-induced dysimmunity and exerts anabolic/anticatabolic actions on protein metabolism.