Association between glutamine extraction and release of citrulline and glycine by the human small intestine

Although glutamine is an important fuel for the intestinal epithelium, the metabolic fate of glutamine extracted by the human intestine remains unclear. The aim of this study was to investigate the relationship between glutamine extraction and the release of other amino acids by the human intestine. In 21 patients undergoing major abdominal cancer surgery, differences in the plasma concentrations of 22 amino acids including glutamine across the superior or inferior mesenteric vein draining viscera were measured using a high-performance liquid chromatography. Arterial minus venous (A − V) or venous minus arterial (V − A) balances of the amino acids were calculated, and then the correlations between A − V differences of glutamine and V − A differences of amino acids released from the intestine were analyzed. Mean extraction rate of glutamine by the small intestine was 28.45%, approximately 3 times higher than 9.41% in the distal colon. Citrulline, proline, alanine, glycine, and arginine were released by the small intestine into the portal circulation. Positive correlations were found between glutamine uptake and the production of citrulline (r = 0.814, P = 0.0013) and glycine (r = 0.734, P = 0.0080). In conclusion, the synthesis of citrulline from glutamine by the small intestine is highly suspected, and the contribution of gut glutamine extraction to the release of glycine into the portal circulation is also supposed.