Inhibitors of brain nitric oxide synthase. Binding kinetics, metabolism, and enzyme inactivation

Nitric oxide (NO) is synthesized from L-arginine by different NO synthase isozymes, which are inhibited by the substrate analogs NG-methyl- and NG-nitro-L-arginine. We studied binding of 3H-labeled NG-nitro-L-arginine to purified brain NO synthase and compared the data with results obtained in enzyme kinetic experiments. Binding data revealed a single binding site for NG-nitro-L-[3H]arginine (KD = 0.17 microM). Binding was competitively antagonized by L-arginine (KI = 2.9 microM). The half-time of dissociation was remarkably slow (9.4 min) and closely correlated with the time necessary for surmounting NO synthase inhibition by dilution. Although an apparently less potent inhibitor, NG-methyl-L-arginine exhibited the same affinity for brain NO synthase as the nitro derivative (KI = 0.17 microM), and in initial rate experiments, almost equal KI values were obtained for NG-methyl-L-arginine (0.61 microM) and NG-nitro-L-arginine (0.53 microM). However, after prolonged incubation periods, NG-nitro-L-arginine induced a rapid inactivation of the enzyme, whereas the methyl derivative turned out to be a substrate of NO synthase, which was slowly converted into stoichiometric amounts of NO and L-citrulline.