Abstract: | The dissociation constant (Kd) for CO from neuronal nitric oxide synthase heme in the absence of the substrate and cofactor was less than 10−3 μM. In the presence of -Arg, it dramatically increased up to 1 μM. In the presence of inhibitors such as NG-nitro--arginine methyl ester and 7-nitroindazole (NI), the Kd value further increased up to more than 100 μM. Addition of the cofactor, 5,6,7,8-tetrahydrobiopterin (H4B), increased the Kd value by 10-fold in the presence of -Arg, whereas it decreased the value to less than one 250th in the presence of NI. Addition of H4B increased the recombination rate constant (kon) for CO by more than two-fold in the presence of -Arg or N6-(1-iminoethyl)--lysine, whereas it decreased the kon value by three-fold in the presence of -thiocitrulline. Thus, the binding fashion of some of inhibitors, such as NI, may be different from that of -Arg with respect to the H4B effect. |