Sung-Eun Kong

Branched-chain amino acids

The branched‐chain amino acids (BCAA), isoleucine, leucine and valine, are unique in that they are principally metabolized extrahepatically in the skeletal muscle. This observation led to the investigation of these nutrients in a number of clinical scenarios. By far the most intensively studied applications for BCAA have been in patients with liver failure and/or patients in catabolic disease states. However, the resulting studies have not demonstrated a clear clinical benefit for BCAA nutritional supplements. In patients with liver failure, the BCAA did improve nitrogen retention and protein synthesis, but their effect on patient outcome was less clear. Similarly, in critically ill septic patients, BCAA did not improve either survival or morbidity. The BCAA are important nutrients, and it seems that any specific benefits associated with their use will be based upon a greater understanding of the underlying cellular biology. Potential areas of further research may include the combination of BCAA supplements with other anabolic factors (e.g. growth hormone) in managing patients with catabolic disease states.

Review: Glutamine and Nucleotide Metabolism Within Enterocytes

Glutamine has an important role as a source of energy for enterocytes. However, it may also have a key role as a source of nitrogen for the synthesis of nucleotides. The relative contribution of de novo synthesis and salvage pathways seems to be affected by the position of enterocytes within the crypt-villus axis as well as the dietary intake of nucleic acids and glutamine. Nucleotides are especially important to enterocytes during intestinal development, maturation, and repair. Hence an understanding of nucleotide metabolism within enterocytes has important implications regarding both the composition and route of administration of nutrient solutions. Many important questions remain unanswered, in particular: Does glutamine stimulate intestinal de novo pyrimidine synthesis via the action of carbamoyl phosphate synthetase I? Can de novo purine synthesis maintain intestinal purine pools in the absence of dietary nucleic acids? And, what are the specific effects of parenterally administered nucleotides on the metabolism and well-being of enterocytes? A greater understanding of these issues will lead to a more rational approach toward the nutritional modulation of gut dysfunction.

Influence of glycine on intestinal ischemia-reperfusion injury

BACKGROUND It has been reported that glycine may protect donor small intestine against hypothermic ischemia before transplantation. This is consistent with the documented role of glycine as a natural cytoprotectant. OBJECTIVE Using an in vivo rodent model, we sought to determine whether exposure to a 20% glycine solution reduces the extent of warm ischemia-reperfusion injury. METHODS Wistar rats (n = 50) underwent laparotomy. A baseline group did not receive any further intervention. The remaining animals had cannulation of the aorta before the initiation of intestinal ischemia (30 minutes) followed by reperfusion (30 minutes). Using a factorial design, rats were randomized to receive local tissue perfusion with either normal saline or a 20% glycine solution during either the preischemia or the prereperfusion phase. Standardized segments of small intestine were removed at the end of the study period to determine the extent of ischemia-reperfusion injury. RESULTS Perfusion with 20% glycine increased mucosal protein content (p < .05), increased mucosal DNA content (p < .05), reduced intestinal myeloperoxidase activity (p < .05), and maintained mucosal glutaminase activity. This was true regardless of whether glycine was administered during the preischemia phase or the prereperfusion phase. CONCLUSIONS Local perfusion with 20% glycine can diminish warm ischemia-reperfusion injury to the rat small intestine in an in vivo model. The role of glycine supplementation should be evaluated in situations where hemodynamic instability may be responsible for breakdown in the gut barrier.

The role of glutaminase in the small intestine.

Glutaminase is the enzyme which hydrolyses glutamine, the main respiratory fuel of the intestine, to yield glutamate and ammonia. Glutaminase has a central role in intestinal metabolism: the products of the reaction catalyzed by glutaminase can be transaminated, catabolized to yield energy or used for the biosynthesis of pyrimidine nucleotides. Experimental treatments which deprive the intestine of glutamine induce intestinal atrophy. In this review, attention is paid to the role of glutaminase in intestinal metabolism. Background information on the structure, kinetics and distribution of glutaminase precede a discussion of the metabolism of glutamine within the intestine. In closing, we review the factors known to regulate glutaminase activity and emphasise that the regulation of glutaminase within the intestine is poorly understood.

Pretreatment with glycine reduces the severity of warm intestinal ischemic-reperfusion injury in the rat.

Free jejunal flaps may experience adverse effects immediately after revascularization because of ischemic-reperfusion injury. In this study the authors evaluated the ability of glycine to protect the small intestine against the effects of a warm ischemic-reperfusion injury. Male Wistar rats (N = 30) were randomized to either a baseline group (no intervention), a control group (local arterial infusion with normal saline), or a glycine group (local arterial infusion with 20% glycine). Pretreatment with 20% glycine increased significantly (p < 0.05) mucosal protein and deoxyribonucleic acid content, reduced intestinal myeloperoxidase activity, and maintained mucosal glutaminase activity. These results indicate that some of the indicators of ischemic-reperfusion injury are improved by pretreatment with a 20% glycine solution.

Starvation alters the activity and mRNA level of glutaminase and glutamine synthetase in the rat intestine

The metabolism of glutamine, the main respiratory fuel of enterocytes, is governed by the activity of glutaminase and glutamine synthetase. Because starvation induces intestinal atrophy, it might alter the rate of intestinal glutamine utilization. This study examined the effect of starvation on the activity, level of mRNA, and distribution of mRNA of glutaminase and glutamine synthetase in the rat intestine. Rats were randomized into groups and were either: (1) fed for 2 days with rat food ad libitum or (2) starved for 2 days. Standardized segments of jejunum and ileum were removed for the estimation of enzyme activity, level of mRNA, and in situ hybridization analysis. The jejunum of the fed rats had a greater activity of both enzymes per centimeter of intestine (P < 0.01), a greater glutaminase specific activity (1.97 +/- 0.45 vs. 1.09 +/- 0.34 micromol/hr/mg protein, P < 0.01), and a lower level of glutaminase and glutamine synthetase mRNA. The ileum of the fed rats had a greater activity of glutamine synthetase per centimeter of intestine (162.9 +/- 50.6 vs. 91.0 +/- 23.1 nmol/hr/cm bowel, P < 0.01), a lower level of glutaminase mRNA, and a greater level of glutamine synthetase mRNA. In situ hybridization analysis showed that starvation does not alter the distribution of glutaminase and glutamine synthetase mRNA in the intestinal mucosa. This study confirms that starvation decreases the total intestinal activity per centimeter of both glutaminase and glutamine synthetase. More importantly, the results indicate that the intestine adapts to starvation by accumulating glutaminase mRNA. This process prepares the intestine for a restoration of intake.

Glutamine-enriched parenteral nutrition regulates the activity and expression of intestinal glutaminase

The aim of this study was to examine the effect of glutamine-enriched parenteral nutrition on the activity, expression and distribution of glutaminase mRNA within the small intestine of rats. Central venous lines were inserted into 30 male Wistar rats before they were fed for 6 days with either: (a) conventional parenteral nutrition, (b) 2.5% glutamine-enriched parenteral nutrition, or (c) rat food ad libitum. Jejunal glutaminase activity per milligram of dry matter was greatest in the animals fed rat food (0.94+/-0.29), intermediate in the glutamine supplemented rats (0.69+/-0.19) and least in the rats nourished with conventional parenteral nutrition (0.55+/-0.24) (P<0.05). The data for glutaminase expression exhibited a similar trend (P<0.05). In situ hybridisation analysis confirmed that glutaminase is expressed in the mucosa along the whole length of the small intestine. It was concluded that provision of glutamine alters the activity and expression of glutaminase in intestinal enterocytes. The results suggest that glutamine increases glutaminase activity by promoting the accumulation of intestinal glutaminase mRNA.

The influence of ischemia/reperfusion injury on the jejunum.

Ischemia/reperfusion injury (IRI) after free tissue transfer of the small intestine results in transmural tissue damage. This study examined the effects of IRI on the jejunum. Wistar rats served either as controls (N = 10) or underwent clamping of the infrarenal aorta for 1 hour followed by 1 hour of reperfusion (N = 10). Both ischemia and reperfusion reduced the protein and deoxyribonucleic acid content of the jejunal mucosa (p < 0.05). Myeloperoxidase activity in the jejunal mucosa remained relatively low. The expression of leukocyte function-associated antigen 1 and intercellular adhesion molecule 1 (ICAM-1) on the surface of mucosal cells was not altered significantly by the ischemic insult, but was reduced after the period of reperfusion (p < 0.05). This coincided with an increase in messenger ribonucleic acid (mRNA) for ICAM-1 within isolated mucosal cells (p < 0.05). The specific activity of glutaminase in isolated jejunal mucosal cells was diminished after ischemia and reperfusion (p < 0.05), and this was not associated with an appreciable change in glutaminase mRNA expression. These results have identified some molecular mechanisms underlying IRI of the small intestine that are possible candidates for therapeutic intervention.

The Role of Enterocytes in Gut Dysfunction

Stem cells in the intestinal epithelium give rise to enterocytes, goblet cells, enteroendocrine cells, and Paneth cells. Each of these cell lines plays a role in cytoprotection of the intestinal mucosa. In particular, it has been demonstrated that mature enterocytes can act as antigen presenting cells. Parenteral and enteral nutrition are used to nourish critically ill patients. However, these regimens are unfortunately associated with gut atrophy. Glutamine, the preferred intestinal nutrient, reverses this gut atrophy and plays a key role in maintaining the barrier function of the gut. Specific nutrients (putrescine, spermidine, spermine) have been used to modulate intestinal adaption. In addition, ornithine has been shown to act as a regulator of intestinal adaption. In this review, we discuss the relationship between the biology of enterocytes and failure of the gut barrier.