Endothelial cells metabolize NG-monomethyl-L-arginine to L-citrulline and subsequently to L-arginine.

NG-monomethyl-L-arginine (MeArg) inhibits the release of endothelium-derived relaxing factor (EDRF) from endothelial cells (EC) and the formation of nitric oxide (NO) from L-arginine (Arg) in EC and activated macrophages. We have compared the inhibitory potency of MeArg to that of N omega-nitro-L-arginine (NO2Arg), a more potent inhibitor of EDRF synthesis in vitro. NO2Arg (100 microM) was significantly more potent than MeArg in inhibiting the endothelium-dependent relaxation of rabbit aorta induced by acetylcholine. MeArg and NO2Arg (10 and 30 microM) also inhibited the release of EDRF from bovine aortic cultured EC. In the anaesthetized rat in vivo, the pressor effect of NO2Arg (3 and 10 mg kg-1) was significantly larger and longer lasting than that of MeArg. These differences in potency could be due to the extensive metabolism of MeArg but not NO2Arg to L-citrulline (Cit) and subsequently to Arg by EC. The enzyme responsible for the conversion of MeArg to Cit had the characteristics of a novel deiminase, NG,NG-dimethylarginine dimethylaminohydrolase, recently isolated from rat kidney.