Biochemical mechanisms leading to tryptophan 2,3-dioxygenase activation

Tryptophan 2,3-dioxygenase (TDO) is the first enzyme in the tryptophan oxidation pathway. It is a hemoprotein and its heme prosthetic group is present as a heme-ferric (heme-Fe(3+)) form that is not active. To be able to oxidize tryptophan, the heme-Fe(3+) form of the enzyme must be reduced to a heme-ferrous (heme-Fe(2+)) form and this study describes conditions that promote TDO activation. TDO is progressively activated upon mixing with tryptophan in a neutral buffer, which leads to an impression that tryptophan is responsible for TDO activation. Through extensive analysis of factors resulting in TDO activation during incubation with tryptophan, we conclude that tryptophan indirectly activates TDO through promoting the production of reactive oxygen species. This consideration is supported by the virtual elimination of the initial lag phase when either pre-incubated tryptophan solution was used as the substrate or a low concentration of superoxide or hydrogen peroxide was incorporated into the freshly tryptophan and TDO mixture. However, accumulation of these reactive oxygen species also leads to the inactivation of TDO, so that both TDO activation and inactivation proceed with the specific outcome depending greatly on the concentrations of superoxide and hydrogen peroxide. As a consequence, the rate of TDO catalysis varies depending upon the proportion of the active to inactive forms of the enzyme, which is in a dynamic relationship in the reaction mixture. These data provide some insight towards elucidating the molecular regulation of TDO in vivo.