Nitroarginine and tetrahydrobiopterin binding to the haem domain of neuronal nitric oxide synthase using a scintillation proximity assay

Nitric oxide synthases (NOS) have a bidomain structure comprised of an N-terminal oxygenase domain and a C-terminal reductase domain. The oxygenase domain binds haem, (6R)-5,6,7,8-tetrahydro-l-biopterin (tetrahydrobiopterin) and arginine, is the site where nitric oxide synthesis takes place and contains determinants for dimeric interactions. A novel scintillation proximity assay has been established for equilibrium and kinetic measurements of substrate, inhibitor and cofactor binding to a recombinant N-terminal haem-binding domain of rat neuronal NOS (nNOS). Apparent Kd values for nNOS haem-domain-binding of arginine and Nomega-nitro-L-arginine (nitroarginine) were measured as 1.6 microM and 25 nM respectively. The kinetics of [3H]nitroarginine binding and dissociation yielded an association rate constant of 1.3x10(4) s-1.M-1 and a dissociation rate constant of 1.2x10(-4) s-1. These values are comparable to literature values obtained for full-length nNOS, suggesting that many characteristics of the arginine binding site of NOS are conserved in the haem-binding domain. Additionally, apparent Kd values were compared and were found to be similar for the inhibitors, L-NG-monomethylarginine, S-ethylisothiourea, N-iminoethyl-L-ornithine, imidazole, 7-nitroindazole and 1400W (N-[3-(aminomethyl) benzyl] acetamidine). [3H]Tetrahydrobiopterin bound to the nNOS haem domain with an apparent Kd of 20 nM. Binding was inhibited by 7-nitroindazole and stimulated by S-ethylisothiourea. The kinetics of interaction with tetrahydrobiopterin were complex, showing a triphasic binding process and a single off rate. An alternating catalytic site mechanism for NOS is proposed.