Identification of imidazole as L-arginine-competitive inhibitor of porcine brain nitric oxide synthase.

Imidazole acts as a heme-site inhibitor of nitric oxide synthase (NOS). We used this compound to investigate whether the substrate L-arginine binds directly to the heme or to a separate domain of brain NOS. Enzyme kinetic experiments showed that imidazole enhanced the apparent Km for L-arginine without affecting maximal enzyme activity, and binding studies revealed that the inhibitor displaced the radioligand NG-nitro-L-[3H]arginine in a concentration-dependent fashion. These results demonstrate that imidazole exerts its effects on NOS in an L-arginine-competitive manner and that the substrate site of the enzyme may be identical with the prosthetic heme group.