Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are heme enzymes that catalyze the O2-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 N1 is not essential for catalysis, and an alternative reaction mechanism, based on the known chemistry of indoles, is presented.