Chicken IDO2 and TDO2: Biochemical Characteristics and Regulatory Roles in Avian Tryptophan Metabolism and Inflammatory Response

Abstract

Simple Summary Indoleamine 2,3-dioxygenase 2 (IDO2) and tryptophan 2,3-dioxygenase (TDO2) serve as rate-limiting enzymes for the kynurenine pathway. Distinct from mammals, chickens lack the IDO1 gene and rely exclusively on IDO2 and TDO2 for tryptophan catabolism. Supplemental dietary tryptophan increases the combined hepatic total catalytic activity of IDO2/TDO2, accompanied by only modest transcriptional changes and no major changes at the protein level, and is associated with reduced PPAR signaling and altered lipid–energy balance. Recombinant enzyme assays demonstrated that chicken IDO2 and TDO2 achieve optimal catalytic performance at avian physiological pH of 7.2 and body temperature of 42 °C; chicken IDO2 possesses measurable catalytic efficiency, which is lower than that of chicken TDO2, whereas its human ortholog showed no detectable activity under the same conditions. Inflammatory stimuli including LPS and APEC infection significantly suppressed the combined hepatic IDO2/TDO2 dioxygenase activity, showing an avian-specific metabolic–immune regulatory feature. This study characterizes avian IDO2 and TDO2 biochemically and functionally, and provides theoretical basis for poultry nutritional regulation and disease prevention.