This enzyme, isolated from the bacterium Geobacillus thermodenitrificans, participates in the pathway of tryptophan degradation. The enzyme is part of a system that also includes a bifunctional riboflavin kinase/FMN adenylyltransferase and EC 22.214.171.124, anthranilate 3-monooxygenase (FAD). It can reduce either FAD or flavin mononucleotide (FMN) but prefers FAD. The enzyme has a slight preference for NADPH as acceptor. cf. EC 126.96.36.199, FAD reductase (NADH).