Three different oxygen-induced radical species in endothelial nitric-oxide synthase oxygenase domain under regulation by L-arginine and tetrahydrobiopterin

Abstract

Endothelial nitric-oxide synthase (eNOS) plays important roles in vascular physiology and homeostasis. Whether eNOS catalyzes nitric oxide biosynthesis or the synthesis of reactive oxygen species such as superoxide, hydrogen peroxide, and peroxynitrite is dictated by the bioavailability of tetrahydrobiopterin (BH4) and L-arginine during eNOS catalysis. The effect of BH 4 and L-arginine on oxygen-induced radical intermediates has been investigated by single turnover rapid-freeze quench and EPR spectroscopy using the isolated eNOS oxygenase domain (eNOSox). Three distinct radical intermediates corresponding to >50% of the heme were observed during the reaction between ferrous eNOSox and oxygen. BH4-free eNOSox produced the superoxide radical very efficiently in the absence of L-arginine. L-Arginine decreased the formation rate of superoxide by an order of magnitude but not its final level or EPR line shape. For BH 4-containing eNOSox, only a stoichiometric amount of BH4 radical was produced in the presence of L-arginine, but in its absence a new radical was obtained. This new radical could be either a peroxyl radical of BH4 or an amino acid radical was in the vicinity of the heme. Formation of this new radical is very rapid, >150 s-1, and it was subsequently converted to a BH4 radical. The trapping of the superoxide radical by cytochrome c in the reaction of BH4(-) eNOSox exhibited a limiting rate of ∼15 s-1, the time for the superoxide radical to leave the heme pocket and reach the protein surface; this reveals a general problem of the regular spin-trapping method in determining radical formation kinetics. Cytochrome c failed to trap the new radical species. Together with other EPR characteristics, our data strongly support the conclusion that this new radical is not a superoxide radical or a mixture of superoxide and biopterin radicals. Our study points out distinct roles of BH4 and L-arginine in regulating eNOS radical intermediates. BH4 prevented superoxide formation by chemical conversions of the Fe(II)O2 intermediate, and L-arginine delayed superoxide formation by electronic interaction with the heme-bound oxygen.