Identification of imidazole as l‐arginine‐competitive inhibitor of porcine brain nitric oxide synthase

Abstract

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 K _m for l-arginine without affecting maximal enzyme activity, and binding studies revealed that the inhibitor displaced the radioligand N ^G-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.