Formation of a protonated trihydrobiopterin radical cation in the first reaction cycle of neuronal and endothelial nitric oxide synthase detected by electron paramagnetic resonance spectroscopy

Nitric oxide synthase (EC 1.14.13.39; NOS) converts L-arginine into NO and L-citrulline in a two-step reaction with Nomega-hydroxy-L-arginine (NOHLA) as an intermediate. The active site iron in NOS has thiolate axial heme-iron ligation as found in the related monooxygenase cytochrome P450. In NOS, tetrahydrobiopterin (BH4) is an essential cofactor for both steps, but its function is controversial. Previous optical studies of the reaction between reduced NOS with O2 at -30 degrees C suggested that BH4 may serve as an one-electron donor in the first cycle, implying formation of a trihydrobiopterin radical. We investigated the same reaction under identical conditions with electron paramagnetic resonance spectroscopy. With BH4-containing full-length neuronal NOS we obtained an organic free radical (g-value 2.0042) in the presence of Arg, and a similar radical was observed with the endothelial NOS oxygenase domain in the presence of Arg and BH4. Without substrate the radical yield was greatly (10x) diminished. Without BH4, or with NOHLA instead of Arg, no radical was observed. With 6-methyltetrahydropterin or 5-methyl-BH4 instead of BH4, radicals with somewhat different spectra were formed. On the basis of simulations we assign the signals to trihydropterin radical cations protonated at N5. This is the first study that demonstrates the formation of a protonated trihydrobiopterin radical with the constitutive isoforms of NOS, and the first time the radical was obtained without exogenous BH4. These results offer strong support for redox cycling of BH4 in the first reaction cycle of NOS catalysis (BH4 <--> BH3.H+).