CO binding studies of nitric oxide synthase: effects of the substrate, inhibitors and tetrahydrobiopterin

The dissociation constant (K-d) for CO from neuronal nitric oxide synthase heme in the absence of the substrate and cofactor was less than 10(-3) mu M. In the presence of L-Arg, it dramatically increased up to 1 mu M. In the presence of inhibitors such as N-G-nitro-L-arginine methyl ester and 7-nitroindazole (NI), the K-d value further increased up to more than 100 mu M. addition of the cofactor, 5,6,7,8-tetrahydrobiopterin (H4B), increased the K-d value by 10-fold in the presence of L-Arg, whereas it decreased the value to less than one 250th in the presence of NI, Addition of H4B increased the recombination rate constant (k(on)) for CO by more than two-fold in the presence of L-Arg or N-6-(1-iminoethyl)-L-lysine, whereas it decreased the k(on) value by three-fold in the presence of L-thiocitrulline. Thus, the binding fashion of some of inhibitors, such as NI, mag be different from that of L-Arg with respect to the H4B effect, (C) 1998 Federation of European Biochemical Societies.