Biological roles of tryptophan and its metabolism: Potential implications for pig feeding

Tryptophan is an essential amino acid. Apart from its incorporation into body proteins, tryptophan is known to play important biological roles, most of them being associated to metabolic pathways involved in tryptophan catabolism. Tryptophan is the precursor for the synthesis of serotonin, an important neuromediator associated to mood, stress response, sleep and appetite regulation. From a quantitative point of view, the proportion of tryptophan used for the production of serotonin is very low: less than 10% of tryptophan that is degraded would be into serotonin. Tryptophan transport and availability in the brain seems to be one of the limiting steps for the synthesis of brain serotonin. Low tryptophan diets are known to depress feed intake. The relationship between tryptophan and serotonin is usually considered as the mechanism involved in the depressive effect of low tryptophan diet on appetite. Other mechanisms such as modulation of insulin sensitivity and ghrelin secretion are presented and discussed. Most of tryptophan degradation occurs through the complex kynurenine pathway. Two enzymes are involved in the first step of tryptophan catabolism into kynurenine: TDO located in the liver degrades tryptophan in excess; in this review, we focus on the second enzyme, IDO expressed by immune cells and tissues targets by inflammation. Increased tryptophan catabolism through the IDO pathway occurs during inflammatory states and immune stress leading to reduced tryptophan availability when this amino acid is not provided at a sufficient amount by the diet. Pigs fed with a low tryptophan diet developed a more important inflammatory response. Tryptophan catabolism through IDO pathway is probably involved in the regulation of T cells proliferation and the production of antioxidant molecules. Finally, we will question the utilisation of growth rate as a single criterion for the evaluation of tryptophan requirement. (C) 2007 Elsevier B.V All rights reserved.