Inactivation of macrophage nitric oxide synthase activity by NG-Methyl-L-arginine

Abstract

[middle dot]N=O synthase catalyzes the oxidation of one of the two chemically equivalent guanido nitrogens of L-arginine to nitric oxide ([middle dot]N=O). NG-Methyl-L-arginine has been previously characterized as a potent competitive inhibitor of both major types of [middle dot]N=O synthases. Initial rate kinetics were performed with a spectrophotometric assay based on the oxidation of oxy- to methemoglobin by [middle dot]N=O. NG-Methyl-L-arginine was a competitive inhibitor of [middle dot]N=O synthase activity derived from activated murine macrophages with a Ki of 6.2 [mu]M. When the enzyme was pre-incubated in the presence of the required cofactors NADPH and tetrahydrobiopterin, time- and concentration-dependent irreversible inactivation of the activity was observed. At 37[deg] C the kinact was 0.050 min-1. This inactivation process exhibited substrate protection, saturation kinetics and required the cofactors necessary for enzymatic turnover. These data indicate that NG-methyl-L-arginine asts as a mechanism-based enzyme inactivator of murine macrophage [middle dot]N=O synthase.