It was not possible to use higher substrate concentrations in the kinetics, because at substrate concentrations above 20 mM small amounts of the over-reduced product 4a could be detected. These over-reduction products would distort the indazole formation rate. Nevertheless, the results show that the KM,app value of NfsA towards 1a is more than three orders of magnitude higher than the KM,app value for nitrofurazone (5.5 µM 31 ). The unusually high KM,app values of NfsA and BaNTR1 for the nitro substrates 1a in combination with the finding that 4a formation could only be observed at higher substrate concentrations and thus higher formation of nitroso intermediates suggests that the abinity for the nitroso compounds is also low. This explains the complete selectivity towards indazole formation via the spontaneous cyclization/aromatization reaction.