Catalyzes the first step in tryptophan catabolism in order to supply de novo nicotinamide adenine dinucleotide (NAD(+)) via the kynurenine pathway. Plays a role in the cellular response to telomere uncapping. D-tryptophan + O2 = N-formyl-D-kynurenine L-tryptophan + O2 = N-formyl-L-kynurenine Binds 1 heme group per subunit. Cofactor biosynthesis; NAD(+) biosynthesis. Expression is mediated by the AFT2, HCM1, and SUM1 transcription factors. Up-regulated in absence of NPT1. Present with 149 molecules/cell in log phase SD medium. Belongs to the indoleamine 2,3-dioxygenase family.