Nitrile hydratase (NHase) from Rhodococcus sp. N771 is a non-heme iron enzyme catalyzing the hydration of various nitriles to the corresponding amides. We report a novel catalytic activity of NHase. When NHase was incubated with an large excess of commercially available isovaleronitrile, the charge transfer band from the sulfur ligand to the Fe atom shifted from 710 nm to 820 nm, but recovered within 4 min. Similar UV-Vis absorption changes were observed after the addition of isobutylisonitrile (iBuNC), a major impurity in commercially available isovaleronitrile, suggesting that NHase catalyzes the conversion of iBuNC to other compounds. The reaction product was identified as isobutylamine (iBuNH(2)) by liquid chromatography tandem mass spectrometry. NHase also converts t-butylisonitrile and 1,1,3,3,-tetramethylbutylisonitrile to the corresponding amines. Kinetic analysis of the conversion of iBuNC to iBuNH(2) showed a K(m) value comparable to that for nitriles, while the V(max) value was more than 10(5) times smaller than that for methacrylonitrile. This is the first report suggesting that NHase is a bifunctional enzyme catalyzing a reaction other than the hydration of nitriles.