Enteral administration of alanyl-[2-(15)N]glutamine contributes more to the de novo synthesis of arginine than does intravenous infusion of the dipeptide in humans

The Preventive Role of Glutamine Supplementation in Cardiac Surgery-Associated Kidney Injury from Experimental Research to Clinical Practice: A Narrative Review

Acute kidney injury represents a significant threat in cardiac surgery regarding complications and costs. Novel preventive approaches are needed, as the therapeutic modalities are still limited. As experimental studies have demonstrated, glutamine, a conditionally essential amino acid, might have a protective role in this setting. Moreover, the levels of glutamine after the cardiopulmonary bypass are significantly lower. In clinical practice, various trials have investigated the effects of glutamine supplementation on cardiac surgery with encouraging results. However, these studies are heterogeneous regarding the selection criteria, timing, dose, outcomes studied, and way of glutamine administration. This narrative review aims to present the potential role of glutamine in cardiac surgery-associated acute kidney injury prevention, starting from the experimental studies and guidelines to the clinical practice and future directions.

Perioperative glutamine supplementation restores disturbed renal arginine synthesis after open aortic surgery: a randomized controlled clinical trial

Postoperative renal failure is a common complication after open repair of an abdominal aortic aneurysm. The amino acid arginine is formed in the kidneys from its precursor citrulline, and citrulline is formed from glutamine in the intestines. Arginine enhances the function of the immune and cardiovascular systems, which is important for recovery after surgery. We hypothesized that renal arginine production is diminished after ischemia-reperfusion injury caused by clamping of the aorta during open abdominal aortic surgery and that parenteral glutamine supplementation might compensate for this impaired arginine synthesis. This open-label clinical trial randomized patients who underwent clamping of the aorta during open abdominal aortic surgery to receive a perioperative supplement of intravenous alanyl-glutamine (0.5 g·kg(-1)·day(-1); group A, n = 5) or no supplement (group B, n = 5). One day after surgery, stable isotopes and tracer methods were used to analyze the metabolism and conversion of glutamine, citrulline, and arginine. Whole body plasma flux of glutamine, citrulline, and arginine was significantly higher in group A than in group B (glutamine: 391 ± 34 vs. 258 ± 19 μmol·kg(-1)·h(-1), citrulline: 5.7 ± 0.4 vs. 2.8 ± 0.4 μmol·kg(-1)·h(-1), and arginine: 50 ± 4 vs. 26 ± 2 μmol·kg(-1)·h(-1), P < 0.01), as was the synthesis of citrulline from glutamine (4.8 ± 0.7 vs. 1.6 ± 0.3 μmol·kg(-1)·h(-1)), citrulline from arginine (2.3 ± 0.3 vs. 0.96 ± 0.1 μmol·kg(-1)·h(-1)), and arginine from glutamine (7.7 ± 0.4 vs. 2.8 ± 0.2 μmol·kg(-1)·h(-1)), respectively (P < 0.001 for all). In conclusion, the production of citrulline and arginine is severely reduced after clamping during aortic surgery. This study shows that an intravenous supplement of glutamine increases the production of citrulline and arginine and compensates for the inhibitory effect of ischemia-reperfusion injury.

Enteral Glutamine Administration in Critically Ill Nonseptic Patients Does Not Trigger Arginine Synthesis

Glutamine supplementation in specific groups of critically ill patients results in favourable clinical outcome. Enhancement of citrulline and arginine synthesis by glutamine could serve as a potential mechanism. However, while receiving optimal enteral nutrition, uptake and enteral metabolism of glutamine in critically ill patients remain unknown. Therefore we investigated the effect of a therapeutically relevant dose of L-glutamine on synthesis of L-citrulline and subsequent L-arginine in this group. Ten versus ten critically ill patients receiving full enteral nutrition, or isocaloric isonitrogenous enteral nutrition including 0.5 g/kg L-alanyl-L-glutamine, were studied using stable isotopes. A cross-over design using intravenous and enteral tracers enabled splanchnic extraction (SE) calculations. Endogenous rate of appearance and SE of glutamine citrulline and arginine was not different (SE controls versus alanyl-glutamine: glutamine 48 and 48%, citrulline 33 versus 45%, and arginine 45 versus 42%). Turnover from glutamine to citrulline and arginine was not higher in glutamine-administered patients. In critically ill nonseptic patients receiving adequate nutrition and a relevant dose of glutamine there was no extra citrulline or arginine synthesis and glutamine SE was not increased. This suggests that for arginine synthesis enhancement there is no need for an additional dose of glutamine when this population is adequately fed. This trial is registered with NTR2285.

Glutamine and antioxidants: status of their use in critical illness

PURPOSE OF REVIEW

Many studies in critically ill patients have addressed enteral or parenteral supplementation of glutamine and antioxidants to counteract assumed deficiencies and induce immune-modulating effects to reduce infections and improve outcome. Older studies showed marked reductions in mortality, infectious morbidity and length of stay. Recent studies no longer show beneficial effects and in contrast even demonstrated increased mortality. This opiniating review focuses on the latest information and the consequences for the use of glutamine and antioxidants in critically ill patients.

RECENT FINDINGS

Positive effects in systematic reviews and meta-analyses are based on results from older, smaller and mainly single-centre studies. New information has challenged the conditional deficiency hypothesis concerning glutamine in critically ill patients. The recent REDOXS and MetaPlus trials studying the effects of glutamine, selenium and other antioxidants have shown no benefits and increased mortality.

SUMMARY

Given that the first dictum in medicine is to do no harm, we cannot be confident that immune-modulating nutrient supplementation with glutamine and antioxidants is effective and well tolerated for critically ill patients. Until more data are available, it is probably better not to routinely administer glutamine and antioxidants in nonphysiological doses to mechanically ventilated critically ill patients.

l-Arginine depletion blunts antitumor T-cell responses by inducing myeloid-derived suppressor cells

Enzymatic depletion of the nonessential amino acid l-Arginine (l-Arg) in patients with cancer by the administration of a pegylated form of the catabolic enzyme arginase I (peg-Arg I) has shown some promise as a therapeutic approach. However, l-Arg deprivation also suppresses T-cell responses in tumors. In this study, we sought to reconcile these observations by conducting a detailed analysis of the effects of peg-Arg I on normal T cells. Strikingly, we found that peg-Arg I blocked proliferation and cell-cycle progression in normal activated T cells without triggering apoptosis or blunting T-cell activation. These effects were associated with an inhibition of aerobic glycolysis in activated T cells, but not with significant alterations in mitochondrial oxidative respiration, which thereby regulated survival of T cells exposed to peg-Arg I. Further mechanistic investigations showed that the addition of citrulline, a metabolic precursor for l-Arg, rescued the antiproliferative effects of peg-Arg I on T cells in vitro. Moreover, serum levels of citrulline increased after in vivo administration of peg-Arg I. In support of the hypothesis that peg-Arg I acted indirectly to block T-cell responses in vivo, peg-Arg I inhibited T-cell proliferation in mice by inducing accumulation of myeloid-derived suppressor cells (MDSC). MDSC induction by peg-Arg I occurred through the general control nonrepressed-2 eIF2α kinase. Moreover, we found that peg-Arg I enhanced the growth of tumors in mice in a manner that correlated with higher MDSC numbers. Taken together, our results highlight the risks of the l-Arg-depleting therapy for cancer treatment and suggest a need for cotargeting MDSC in such therapeutic settings.

l-Arginine depletion blunts antitumor T-cell responses by inducing myeloid-derived suppressor cells

Enzymatic depletion of the nonessential amino acid l-Arginine (l-Arg) in patients with cancer by the administration of a pegylated form of the catabolic enzyme arginase I (peg-Arg I) has shown some promise as a therapeutic approach. However, l-Arg deprivation also suppresses T-cell responses in tumors. In this study, we sought to reconcile these observations by conducting a detailed analysis of the effects of peg-Arg I on normal T cells. Strikingly, we found that peg-Arg I blocked proliferation and cell-cycle progression in normal activated T cells without triggering apoptosis or blunting T-cell activation. These effects were associated with an inhibition of aerobic glycolysis in activated T cells, but not with significant alterations in mitochondrial oxidative respiration, which thereby regulated survival of T cells exposed to peg-Arg I. Further mechanistic investigations showed that the addition of citrulline, a metabolic precursor for l-Arg, rescued the antiproliferative effects of peg-Arg I on T cells in vitro. Moreover, serum levels of citrulline increased after in vivo administration of peg-Arg I. In support of the hypothesis that peg-Arg I acted indirectly to block T-cell responses in vivo, peg-Arg I inhibited T-cell proliferation in mice by inducing accumulation of myeloid-derived suppressor cells (MDSC). MDSC induction by peg-Arg I occurred through the general control nonrepressed-2 eIF2α kinase. Moreover, we found that peg-Arg I enhanced the growth of tumors in mice in a manner that correlated with higher MDSC numbers. Taken together, our results highlight the risks of the l-Arg-depleting therapy for cancer treatment and suggest a need for cotargeting MDSC in such therapeutic settings.

Intravenous glutamine supplementation enhances renal de novo arginine synthesis in humans: a stable isotope study

BACKGROUND

Arginine plays a role in many different pathways in multiple cell types. Consequently, a shortage of arginine, caused by pathologic conditions such as cancer or injury, has the potential to disturb many cellular and organ functions. Glutamine is the ultimate source for de novo synthesis of arginine in humans via the intestinal-renal axis. Therefore, we hypothesized that parenteral glutamine supplementation may stimulate the interorgan pathway of arginine production.

OBJECTIVES

The objectives were to quantify arginine production from its precursor glutamine and to establish the contribution of the kidneys to de novo synthesis of arginine in patients receiving intravenous supplementation of glutamine dipeptide during major abdominal surgery.

DESIGN

Whole-body and renal metabolism of glutamine, citrulline, and arginine was assessed by stable isotope techniques in 7 patients receiving a perioperative supplement of intravenous alanyl-glutamine (0.5 g · kg(-1) · d(-1)).

RESULTS

Plasma glutamine, citrulline, and arginine concentrations increased significantly in patients receiving intravenous glutamine dipeptide. At whole-body level, 91% of total citrulline turnover was derived from glutamine, whereas 49% of whole-body citrulline turnover was used for de novo synthesis of arginine. The kidneys were responsible for 75% of whole-body arginine production from citrulline.

CONCLUSIONS

Glutamine and citrulline are important sources for de novo arginine synthesis. The kidneys are the main production site for endogenous arginine. After comparison of these results with previous similar studies, our data suggest that an intravenous glutamine supplement doubles renal arginine production from citrulline. This trial was registered at www.trialregister.nl as NTR2914.

Amino acids in human and animal nutrition

Amino acids are key components of human and animal nutrition, both as part of a protein-containing diet, and as supplemented individual products. In the last 10 years there has been a marked move away from the extraction of amino acids from natural products, which has been replaced by efficient fermentation processes using nonanimal carbon sources. Today several amino acids are produced in fermentation plants with capacities of more than 100,000 tonnes to serve the requirements of animal feed and human nutrition. The main fermentative amino acids for animal nutrition are L-lysine, L-threonine, and L-tryptophan. DL-Methionine continues to be manufactured for animal feed use principally by chemical synthesis, and a pharmaceutical grade is manufactured by enzymatic resolution. Amino acids play an important role in medical nutrition, particularly in parenteral nutrition, where there are high purity requirements for infusion grade products. Amino acids are also appearing more often in dietary supplements, initially for performance athletes, but increasingly for the general population. As the understanding of the effects of the individual amino acids on the human metabolism is deepened, more specialized product mixtures are being offered to improve athletic performance and for body-building.

Alterations in glutamine metabolism and its conversion to citrulline in sepsis

In enterocytes, glutamine serves as the major source of energy; another metabolic fate of glutamine is conversion to citrulline. Because sepsis can affect gut function and integrity, alterations in glutamine metabolism may exist and lead to decreased citrulline production. This study aimed to investigate how sepsis affects glutamine metabolism, including its conversion to citrulline, by measuring glutamine and citrulline flux, fractional splanchnic extraction of glutamine and leucine, and the contribution of glutamine nitrogen to citrulline in septic patients and healthy controls. Eight patients with severe sepsis and 10 healthy controls were given primed, constant intravenous infusion of [(2)H2]citrulline and sequential administration of intravenous and enteral [α-(15)N]glutamine and [(13)C]leucine in the postabsorptive state. The results showed that, compared with healthy controls, septic patients had a significantly lower whole body citrulline flux and plasma concentration, higher endogenous leucine flux, and higher glutamine clearance. Fractional splanchnic extraction of leucine was higher in septic patients than in controls, but fractional extraction of glutamine was not different. The majority of the (15)N label transferred from glutamine to citrulline was found at the α-position. These results demonstrate that lower glutamine plasma concentrations in sepsis were a result of increased glutamine clearance. Despite adequate splanchnic uptake of glutamine, there is decreased production of citrulline, suggesting a defect in the metabolic conversion of glutamine to citrulline, decreased uptake of glutamine by the enterocyte but increased uptake by the liver, and/or shunting of glutamine to other metabolic pathways.

Applications of stable isotopes in clinical pharmacology

This review aims to present an overview of the application of stable isotope technology in clinical pharmacology. Three main categories of stable isotope technology can be distinguished in clinical pharmacology. Firstly, it is applied in the assessment of drug pharmacology to determine the pharmacokinetic profile or mode of action of a drug substance. Secondly, stable isotopes may be used for the assessment of drug products or drug delivery systems by determination of parameters such as the bioavailability or the release profile. Thirdly, patients may be assessed in relation to patient-specific drug treatment; this concept is often called personalized medicine. In this article, the application of stable isotope technology in the aforementioned three areas is reviewed, with emphasis on developments over the past 25 years. The applications are illustrated with examples from clinical studies in humans.

Arginine synthesis from enteral glutamine in healthy adults in the fed state

Recent studies have documented transfer of labeled nitrogen from [2-(15)N]glutamine to citrulline and arginine in fasting human adults. Conversely, in neonates and piglets we have shown no synthesis of arginine from [2-(15)N]glutamate, and others have shown in mice that glutamine is a nitrogen, but not a carbon donor, for arginine synthesis. Therefore, we performed a multitracer study to determine whether glutamine is a nitrogen and/or carbon donor for arginine in healthy adult men. Two glutamine tracers, 2-(15)N and 1-(13)C, were given enterally to five healthy men fed a standardized milkshake diet. There was no difference in plasma enrichments between the two glutamine tracers. 1-(13)C isotopomers of citrulline and arginine were synthesized from [1-(13)C]glutamine. Three isotopomers each of citrulline and arginine were synthesized from the [2-(15)N]glutamine tracer: 2-(15)N, 5-(15)N, and 2,5-(15)N(2). Significantly greater enrichment was found of both [5-(15)N]arginine (0.75%) and citrulline (3.98%) compared with [2-(15)N]arginine (0.44%) and [2-(15)N]citrulline (2.62%), indicating the amino NH(2) from glutamine is mostly transferred to arginine and citrulline by transamination. Similarly, the enrichment of the 1-(13)C isotopomers was significantly less than the 2-(15)N isotopomers, suggesting rapid formation of α-ketoglutarate and recycling of the nitrogen label. Our results show that the carbon for 50% of newly synthesized arginine comes from dietary glutamine but that glutamine acts primarily as a nitrogen donor for arginine synthesis. Hence, studies using [2-(15)N]glutamine will overestimate arginine synthesis rates.