Heme-dependent dioxygenase that catalyzes the oxidative cleavage of the L-tryptophan (L-Trp) pyrrole ring and converts L-tryptophan to N-formyl-L-kynurenine. Catalyzes the oxidative cleavage of the indole moiety. L-tryptophan + O2 = N-formyl-L-kynurenine Binds 1 heme group per subunit. Weakly inhibited by D-tryptophan. Amino-acid degradation; L-tryptophan degradation via kynurenine pathway; L-kynurenine from L-tryptophan: step 1/2. Homotetramer. Belongs to the tryptophan 2,3-dioxygenase family.