Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are heme enzymes that catalyze the O-2-dependent oxidation of L-tryptophan to N-formyl-kynurenine. Previous proposals for the mechanism of this reaction have suggested that deprotonation of the indole NH group, either by an active-site base or by oxygen bound to the heme iron, as the initial step. In this work, we have examined the activity of 1-Me-L-Trp with three different heme dioxygenases and their site-directed variants. We find, in contrast to previous work, that 1-Me-L-Trp is a substrate for the heme dioxygenase enzymes. These observations suggest that deprotonation of the indole N-1 is not essential for catalysis, and an alternative reaction mechanism, based on the known chemistry of indoles, is presented.