A short history of heme dioxygenases: rise, fall and rise again.

It is well established that there are two different classes of enzymes-tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO)-that catalyse the O2 -dependent oxidation of L-tryptophan to N-formylkynurenine. But it was not always so. This perspective presents a short history of the early TDO and IDO literature, the people that were involved in creating it, and the legacy that this left for the future.