Highly‐Time‐Resolved Measurements of Tryptophan's Distribution and Homeostatic Regulation in Live, Intact Animals

Current theories regarding tryptophan's homeostatic regulation, which are derived from in vitro, post‐mortem, and cell‐culture studies, lack any direct evidence supporting their occurrence in living subjects. In response, here we have used 20 s resolution, multi‐hour plasma measurements to confirm, for the first time, that the cooperativity and tryptophan‐induced stabilization of tryptophan 2,3‐dioxygenase (TDO) seen in vitro and in cell culture hold in live, intact animals. For example, a model coupling these regulatory mechanisms with a constant tryptophan production rate from tissue reservoirs describes our in vivo measurements quantitatively, and provides high‐precision estimates of tryptophan flux and tryptophan‐induced TDO up‐regulation in individual subjects. In bridging a longstanding gap between ex vivo, post‐mortem, and cell culture investigations and the physiology of live, intact animals, an advance that had previously only been reported for glucose, our results highlight the transformative power of applying only‐recently‐available, seconds‐resolved metabolite measurements to the study of metabolic regulation.