Petra G. Boelens

Intestinal and hepatic metabolism of glutamine and citrulline in humans

Glutamine plays an important role in nitrogen homeostasis and intestinal substrate supply. It has been suggested that glutamine is a precursor for arginine through an intestinal–renal pathway involving inter‐organ transport of citrulline. The importance of intestinal glutamine metabolism for endogenous arginine synthesis in humans, however, has remained unaddressed. The aim of this study was to investigate the intestinal conversion of glutamine to citrulline and the effect of the liver on splanchnic citrulline metabolism in humans. Eight patients undergoing upper gastrointestinal surgery received a primed continuous intravenous infusion of [2‐15N]glutamine and [ureido‐13C–2H2]citrulline. Arterial, portal venous and hepatic venous blood were sampled and portal and hepatic blood flows were measured. Organ specific amino acid uptake (disposal), production and net balance, as well as whole body rates of plasma appearance were calculated according to established methods. The intestines consumed glutamine at a rate that was dependent on glutamine supply. Approximately 13% of glutamine taken up by the intestines was converted to citrulline. Quantitatively glutamine was the only important precursor for intestinal citrulline release. Both glutamine and citrulline were consumed and produced by the liver, but net hepatic flux of both amino acids was not significantly different from zero. Plasma glutamine was the precursor of 80% of plasma citrulline and plasma citrulline in turn was the precursor of 10% of plasma arginine. In conclusion, glutamine is an important precursor for the synthesis of arginine after intestinal conversion to citrulline in humans.

Specific amino acids in the critically ill patient--exogenous glutamine/arginine: a common denominator?

Objective:Glutamine and arginine are both used as nutritional supplements in critically ill patients. Although glutamine has been shown to be beneficial for the metabolically stressed patient, considerations about arginine supplementation are not unanimously determined. Our aim is to review the current knowledge on the possible interplay between glutamine and arginine generation in the stressed patient and to elaborate on whether these amino acids may function as a common denominator. Because glutamine can be given by the parenteral and enteral routes, possible different actions on the metabolic fate (e.g., generation of citrulline) with both routes are analyzed. Data Source:A summary of data on the clinical effect of glutamine and arginine metabolism is given, incorporating data on glutamine and arginine supplementation. Differences between the route of administration, parenteral or enteral, and the molecular form of supplied glutamine, free or as dipeptide, on citrulline generation by the gut and production of arginine are discussed. Results:Glutamine and arginine influence similar organ systems; however, they differ in their targets. For example, glutamine serves as fuel for the immune cells, increases human leukocyte antigen-DR expression on monocytes, enhances neutrophil phagocytosis, and increases heat shock protein expression. Arginine affects the immune system by stimulating direct or indirect proliferation of immune cells. This indirect effect is possibly mediated by nitric oxide, which also enhances macrophage cytotoxicity. Furthermore, glutamine serves as a precursor for the de novo production of arginine through the citrulline-arginine pathway. Glutamine has shown to be beneficial in the surgical and critically ill patient, whereas arginine supplementation is still under debate. The route of glutamine administration (parenteral or enteral) determines the effect on citrulline and on the de novo arginine generation. There is a marked difference between the administration of free glutamine and dipeptide enterally or parenterally. Splanchnic extraction of the hydrolyzed glutamine in mice when administering the dipeptide enterally is higher compared with administering free glutamine from the enteral site. In patients, splanchnic extraction of the dipeptide given enterally is 100% when comparing supplementation of the dipeptide intravenously. Conclusions:The beneficial effects of free glutamine or dipeptide may depend on the route of administration but also on the metabolic fate of amino acids generated (e.g., citrulline, arginine). Glutamine serves as a substrate for de novo citrulline and arginine synthesis. More research needs to be done to establish the direct clinical relevance of the different metabolic pathways. Future perspectives might include combining enteral and parenteral routes of administrating free glutamine or dipeptide.

Reduction of Postoperative Ileus by Early Enteral Nutrition in Patients Undergoing Major Rectal Surgery Prospective, Randomized, Controlled Trial

Background:The current trend in postoperative nutrition is to promote a normal oral diet as early as possible. However, postoperative ileus is a frequent and common problem after major abdominal surgery. This study was designed to investigate whether early enteral nutrition (EEN), as a bridge to a normal diet, can reduce postoperative ileus. Methods:Patients undergoing major rectal surgery for locally advanced primary or recurrent rectal carcinoma (after neoadjuvant (chemo)-radiation, with or without intraoperative radiotherapy) were randomly assigned to EEN (n = 61) or early parenteral nutrition (EPN, n = 62) in addition to an oral diet. Early nutrition was started 8 hours after surgery. Early parenteral nutrition was given as control nutrition to obtain caloric equivalence and minimize confounding. The primary endpoint was time to first defecation; secondary outcomes were morbidity, other ileus symptoms, and length of hospital stay. Results:Baseline characteristics were similar for both groups. In intention-to-treat analysis, the time to first defecation was significantly shorter in the enteral nutrition arm than in the control arm (P = 0.04). Moreover, anastomotic leakage occurred significantly less frequently in the enteral group (1 patient) compared with parenteral supplementation (9 patients, P = 0.009). Mean length of stay in the enteral group was 13.4 ± 2.2 days versus 16.7 ± 2.3 days in the parenteral group (P = 0.007). Conclusions:Early enteral nutrition is safe and associated with significantly less ileus. Early enteral nutrition is associated with less anastomotic leakage in patients undergoing extensive rectal surgery.

The Route of Administration (Enteral or Parenteral) Affects the Conversion of Isotopically Labeled L-[2-15N]Glutamine Into Citrulline and Arginine in Humans.

BACKGROUND Glutamine exhibits numerous beneficial effects in experimental and clinical studies. It has been suggested that these effects may be partly mediated by the conversion of glutamine into citrulline and arginine. The intestinal metabolism of glutamine appears to be crucial in this pathway. The present study was designed to establish the effect of the feeding route, enteral or parenteral, on the conversion of exogenously administered glutamine into citrulline and arginine at an organ level in humans, with a focus on gut metabolism. METHODS Sixteen patients undergoing upper gastrointestinal surgery received an IV or enteral (EN) infusion of L-[2-(15)N]glutamine. Blood was sampled from a radial artery and from the portal and right renal vein. Amino acid concentrations and enrichments were measured, and net fluxes of [(15)N]-labeled substrates across the portal drained viscera (PDV) and kidneys were calculated from arteriovenous differences and plasma flow. RESULTS Arterial [(15)N]glutamine enrichments were significantly lower during enteral tracer infusion (tracer-to-tracee ratio [labeled vs unlabeled substrate, TTR%] IV: 6.66 +/- 0.35 vs EN: 3.04 +/- 0.45; p < .01), reflecting first-pass intestinal metabolism of glutamine during absorption. Compared with IV administration, enteral administration of the glutamine tracer resulted in a significantly higher intestinal fractional extraction of [(15)N]glutamine (IV: 0.15 +/- 0.03 vs EN: 0.44 +/- 0.08 micromol/kg/h; p < .01). Furthermore, enteral administration of the glutamine tracer resulted in higher arterial enrichments of [(15)N]citrulline (TTR% IV: 5.52 +/- 0.44 vs EN: 8.81 +/- 1.1; p = .02), and both routes of administration generated a significant enrichment of [(15)N]arginine (TTR% IV: 1.43 +/- 0.12 vs EN: 1.68 +/- 0.18). This was accompanied by intestinal release of [(15)N]citrulline across the PDV, which was higher with enteral glutamine (IV: 0.38 +/- 0.07 vs EN: 0.72 +/- 0.11 micromol/kg/h; p = .02), and subsequent [(15)N]arginine release in both groups. CONCLUSIONS In humans, the gut preferably takes up enterally administered glutamine compared with intravenously provided glutamine. The route of administration, enteral or IV, affects the quantitative conversion of glutamine into citrulline and subsequent renal arginine synthesis in humans.

Coagulopathy following major liver resection: the effect of rBPI21 and the role of decreased synthesis of regulating proteins by the liver.

This prospective study investigated the role of reduced hepatic synthesis of regulating proteins in coagulopathy after partial hepatectomy (PH) compared with major abdominal surgery (MAS) without involvement of the liver. Furthermore, we studied the effect of rBPI21, an endotoxin-neutralizing agent, on coagulopathy after PH was studied. Compared with MAS, PH resulted in significantly elevated levels of thrombin-antithrombin-III and plasmin-alpha2-antiplasmin complexes. Levels of antithrombin-3, alpha2-antiplasmin, fibrinogen, plasminogen, alpha2-macroglobulin (alpha2-M), and C1-inhibitor remained lower following PH. Treatment with rBPI21 led to significantly lower levels of tissue-type plasminogen activator (t-PA). Post-operative disseminated intravascular coagulation (DIC) was associated with significantly higher bilirubin and t-PA plasma levels and significantly lower levels of alpha2-M. This study indicates that PH induced hepatic failure results in decreased synthesis of hepatic regulating plasma proteins and subsequent activation of coagulation and fibrinolysis. Prevention of t-PA release by rBPI21 may have important clinical implications. Decreased availability of alpha2-M may be a factor in post-operative DIC.

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

BACKGROUND We previously confirmed in humans the existence of a pathway of glutamine into citrulline and arginine, which is preferentially stimulated by luminally provided glutamine. However, because glutamine is unstable, we tested this pathway with a stable dipeptide of glutamine. OBJECTIVES The objectives were to explore whether alanyl-glutamine contributes to the synthesis of arginine in humans and whether this depends on the route of administration. DESIGN The study was conducted under postabsorptive conditions during surgery. Sixteen patients received alanyl-[2-(15)N]glutamine enterally or intravenously together with intravenously administered stable-isotope tracers of citrulline and arginine. Blood was collected from an artery, the portal vein, a hepatic vein, and the right renal vein. Arterial and venous enrichments and (tracer) net balances of alanyl-glutamine and glutamine, citrulline, and arginine across the portal-drained viscera, liver, and kidneys were determined. Parametric tests were used to test results (mean +/- SEM). P < 0.05 was considered significant. RESULTS Twice as much exogenous glutamine was used for the synthesis of citrulline when alanyl-glutamine was provided enterally (5.9 +/- 0.6%) than when provided intravenously (2.8 +/- 0.3%) (P < 0.01). Consequently, twice as much exogenous glutamine was used for the synthesis of arginine when alanyl-glutamine was provided enterally (5 +/- 0.7%) than when provided intravenously (2.4 +/- 0.2%) (P < 0.01). However, results at the organ level did not explain the differences due to route of administration. CONCLUSIONS Alanyl-glutamine contributes to the de novo synthesis of arginine, especially when provided enterally. A stable-isotope study using a therapeutic dose of alanyl-glutamine is needed to investigate the clinical implications of this finding.

Accurate perioperative flow measurement of the portal vein and hepatic and renal artery: a role for preoperative MRI?

BACKGROUND Quantification of abdominal blood flow is essential for a variety of gastrointestinal and hepatic topics such as liver transplantation or metabolic flux measurement, but those need to be performed during surgery. It is not clear whether Duplex Doppler Ultrasound during surgery or MRI before surgery is the tool to choose. OBJECTIVE To examine whether preoperative evaluation of abdominal blood flow using MRI could prove to be a useful and reliable alternative for the perioperative sonographic approach. METHODS In this study portal and renal venous flow and hepatic arterial flow were sequentially quantified by preoperative MRI, preoperative and perioperative Duplex Doppler Ultrasound (DDUS). 55 Patients scheduled for major abdominal surgery were studied and methods and settings were compared. Additionally, average patient population values were compared. RESULTS Mean (±SD) plasmaflow measured by perioperative DDUS, preoperative DDUS and MRI, respectively was 433±200/423±162/507±96 ml/min (portal vein); 96±70/74±41/108±91 ml/min (hepatic artery); 248±139/201±118/219±69 ml/min (renal vein). No differences between the different settings of DDUS measurement were detected. Equality of mean was observed for all measurements. Bland Altman Plots showed widespread margins. Hepatic arterial flow measurements correlated with each other, but portal and renal venous flow correlations were absent. CONCLUSIONS Surgery and method (DDUS vs. MRI) do not affect mean flow values. Individual comparison is restricted due to wide range in measurements. Since MRI proves to be more reliable with respect to inter-observer variability, we recommend using mean MRI results in experimental setups.

Antioxidant-enriched enteral nutrition and immuno-inflammatory response after major gastrointestinal tract surgery

Major surgery induces an immuno-inflammatory response accompanied by oxidative stress that may impair cellular function and delay recovery. The objective of the study was to investigate the effect of an enteral supplement, containing glutamine and antioxidants, on circulating levels of immuno-inflammatory markers after major gastrointestinal tract surgery. Patients (n 21) undergoing major gastrointestinal tract surgery were randomised in a single-centre, open-label study. The effects on circulating levels of immuno-inflammatory markers were determined on the day before surgery and on days 1, 3, 5 and 7 after surgery. Major gastrointestinal surgery increased IL-6, TNF receptor 55/60 (TNF-R55) and C-reactive protein (CRP). Surgery reduced human leucocyte antigen-DR (HLA-DR) expression on monocytes. CRP decrease was more pronounced in the first 7 d in the treatment group compared with the control group. In the treatment group, from the moment Module AOX was administered on day 1 after surgery, TNF receptor 75/80 (TNF-R75) level decreased until the third post-operative day and then stabilised, whereas in the control group the TNF-R75 level continued to increase. The results of the present pilot study suggest that enteral nutrition enriched with glutamine and antioxidants possibly moderates the immuno-inflammatory response (CRP, TNF-R75) after surgery.

Effects of preoperative flavonoid supplementation on different organ functions in rats

BACKGROUND Previously it has been reported that preoperative feeding preserves heart function in rats after intestinal ischemia-reperfusion. To further improve postoperative organ function, bioactive nutrition compounds were selected in vitro against the xanthine oxidase radical cascade, an enzyme suggested to play a key role in the induction of single- or multiple-organ dysfunction. METHODS Flavonoids were selected in vitro for their capacity to (1) inhibit xanthine oxidase, (2) scavenge superoxide, and (3) scavenge peroxylradicals. The most bioactive flavonoids were added to the preoperative nutrition to study their effect on postintestinal ischemia-reperfusion organ function. RESULTS A combination of flavonoids selected on basis of effective flavonoid xanthine oxidase inhibition and superoxide scavenging resulted in increased superoxide scavenging. In vivo, the selected flavonoid mixture significantly lowered postischemic intestinal apoptosis and intestinal oxidative stress indicated by malondialdehyde concentration when compared with ischemia-reperfusion fasted and sham-fasted animals. Moreover, this flavonoid mixture significantly lowered plasma creatinine and urea concentration, both indicating a better postoperative kidney function. Furthermore, oxidative stress measured as this flavonoid mixture when compared with control significantly lowered plasma malondialdehyde concentration in fed rats. CONCLUSIONS Coadministration of bioactive flavonoid mixture to preoperative nutrition, in contrast to fasting, attenuates ischemia-reperfusion injury by preserving kidney function in the rat and decreasing apoptosis in the intestine.

Taurine and the Relevance of Supplementation in Humans, in Health and Disease

Taurine, a beta-sulphur amino acid with a zwitterionic function, plays an important role in several biological processes. In humans taurine synthesis is limited. Therefore, intake by food is important. A normal diet, including animal products, contains a sufficient amount of taurine to provide the physiological necessary quantity of taurine. Taurine is a component of bile salts, which function as detergents for emulsification of dietary lipids and fat-soluble vitamins. Also, taurine is involved in the development of the brain and retina, which makes it an essential nutritional substrate in neonates and small children, who are limited in the synthesis of taurine. Moreover, taurine can act as an osmoregulator and antioxidant, which makes it an important amino acid during pathological conditions such as ischemic reperfusion injury, inflammation and brain oedema. Its osmolytic function helps to prevent alteration in the intra-cellular concentrations of all substances. In many models of oxidant-induced injury, taurine was shown to be tissue-protective as a non-toxic endogenous antioxidant. This review focuses on the biological actions of taurine, to illuminate possible clinical benefits of taurine.