Potent inhibition of human neuronal nitric oxide synthase by NG-Nitro-L-Arginine methyl ester results from contaminating NG-NITRO-L-arginine

Abstract N G -Nitro-L-arginine methyl ester (L-NAME), inhibits the three isozymes of nitric oxide synthase (NOS) in vitro and in vivo .. The mechanism of NOS inhibition by L-NAME is uncertain. L-NAME was a time-dependent inhibitor of neuronal NOS (nNOS). Concommitantly, L-NAME was hydrolyzed, non-enzymatically, to N G -Nitro-L-arginine (L-NA) during the enzyme assay. The time-dependent inhibition of nNOS by L-NAME was the result of this time-dependent formation of L-NA. Furthermore, N G -Nitro-L-arginine methyl amide, which is an isosteric analogue of L-NAME that is much less susceptible to hydrolysis, was a rapidly reversible weak inhibitor of NOS. These data suggested that L-NAME itself was a weak and rapidly reversible inhibitor of nNOS. Most of the inhibition of nNOS by a solution of L-NAME is the result of the formation of L-NA. L-NAME was a substrate for porcine liver esterase.