The small intestine is not only responsible for the terminal digestion and absorption of nutrients, it also plays an important role in amino acid metabolism. Both arterial and intestinal luminal glutamine are actively taken up by the small intestine of all the mammals studied (except for the guinea pig). Most of the dietary glutamine is degraded by the mammalian small intestine during the first pass. Intestinal glutamine degradation releases CO2 ammonia, alanine, citrulline, proline and, in the pig, arginine, into the venous circulation. Arterial glutamate and aspartate are not extracted by the gut. However, almost all of the dietary glutamate and aspartate are catabolized by the small intestine during the first pass and thus do not enter the peripheral circulation. The small intestine synthesizes proline and citrulline from enterally-delivered dietary glutamate in animals and humans. Intestinal luminal glutamine plus glutamate plus aspartate and arterial glutamine serve as major fuels for the mammalian small intestine, respectively, during enteral amino acid feeding and in the postabsorptive state. Enterocytes extensively degrade proline to form ornithine and citrulline, thus challenging the current belief that proline is not catabolized by the gut. Because of the synthesis of citrulline from glutamine, glutamate and proline in enterocytes, the small intestine is the major source of circulating citrulline for endogenous synthesis of arginine in all mammals studied (except for the cat and ferret) but not in birds. This intestinal synthesis of citrulline plays an important role in maintaining arginine homeostasis in neonates and growing animals. In the fed state, the small intestine plays a quantitatively important role in branched-chain amino acid degradation in monogastric animals (e.g., pig and dog) and in humans but not in ruminants. Recent findings that approximately 50% of dietary essential amino acids (including methionine, lysine, phenylalanine and threonine), glycine and serine are sequestered by the portal-drained viscera during the first pass suggest substantial utilization and possibly catabolism of these amino acids by the small intestine. Intestinal activities of most arginine-synthesizing enzymes, and rates of citrulline synthesis from glutamine, and proline, are greatest at birth and decline during the suckling period. Weaning results in increased synthesis of citrulline from glutamine, and of proline, nitric oxide and polyamines from arginine in enterocytes, which is likely mediated by increased plasma concentrations of glucocorticoids. Other factors that are known to affect intestinal glutamine metabolism include glucagon, cytokines, ketone bodies, glucose, diabetes and prolonged fasting. Intestinal amino acid metabolism plays a critical role in intestinal integrity and function, endogenous synthesis of amino acids (e.g., proline, citrulline and arginine), and selective modification of the pattern of absorbed dietary amino acids that enter the portal circulation. Knowledge of intestinal amino acid metabolism provides a biochemical basis for explaining 1) why arginine is an essential nutrient for the cat, ferret and chick, and for young mammals, 2) why proline is an essential amino acid for neonates and for the chick, 3) why enteral amino acid nutrition is required to maintain the intestinal mucosa, and 4) why deficiency of intestinal citrulline synthesis results in hypocitrullinemia, hypoargininemia, hyperammonemia, retarded growth, and even death in animals and humans. Much remains to be learned about comparative aspects of intestinal amino acid metabolism at molecular, cellular and whole body levels. This knowledge is essential to improving the efficiency of utilization of dietary amino acids, making recommendation on dietary amino acid requirements, and designing new enteral or parenteral amino acid nutrition to optimize health and growth of animals and humans.
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