Differential effects of mutations in human endothelial nitric oxide synthase at residues Tyr-357 and Arg-365 on l-arginine hydroxylation and GN-hydroxy-l-arginine oxidation

Biosynthesis of nitric oxide (NO) is catalyzed by NO synthase (NOS) through a two-step oxidation of l-arginine (Arg) with formation of an intermediate, GN-hydroxy-l-Arg (NHA). In this study we have employed mutagenesis to investigate how residues Y357 and R365 which interact primarily with the substrate Arg and (6R)-5,6,7,8-tetrahydro-l-biopterin (H4B) modulate these two steps of the NOS reaction. Mutant Y357F preserved most wild-type heme characteristics and NADPH oxidation ability. However, mutation of this residue markedly increased the dissociation constants for both Arg and NHA by 20-fold and decreased the NO synthesis from Arg by 85% compared to that of wild type. Mutation of Y357 had less effect on the rate of NO generated from NHA. Mutant R365L purified in the presence of Arg had a normal heme environment and retained 9 and 55% of the wild-type NO formation rate from Arg and NHA, respectively. When Arg was removed from buffer, R365L instantly became a low-spin state (Soret peak at 418 nm) with the resultant loss of H4B and instability of the heme–CO complex. The low-spin R365L exhibited an NADPH oxidation rate higher than that of wild type. Its Arg-driven NO formation was decreased to near the limit of detection, whereas the rate of NHA-driven NO synthesis was one third that of wild type. This NHA-driven NO formation completely relied on H4B and was not sensitive to superoxide dismutase or catalase but was inhibited by imidazole. The wild-type eNOS required 14 μM NHA and 0.39 μM H4B to reach the half-maximal NHA-driven NO formation rate (EC50), while R365L needed 59 μM NHA and 0.73 μM H4B to achieve EC50. The differential effect of mutation on Arg and NHA oxidation suggests that distinct heme-based active oxidants are responsible for each step of NO synthesis.