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.