Autoxidation rates of neuronal nitric oxide synthase: Effects of the substrates, inhibitors, and modulators

Autoxidation rates of the full-length neuronal nitric oxide synthase (nNOS) were analyzed and found to be composed of three phases, 60 s(-1) (28%), 5. 5 s(-1) (11%) and 0.048 s(-1) (61%), Addition of L-Arg, N-G-hydroxy-L-Arg ( NHA), and N-G-monomethyl-L-Arg markedly decreased the rate constants for th e first and second phases down to 12-20 s(-1) and 0.32-2.6 s(-1), respectiv ely. Addition of (6R)-5,6,7,8-tetrahydro-L-biopterin (H4B) increased the am plitude of the second phase up to 29% of the total. Addition of NHA decreas ed the rate of the first phase by 4.4-fold in the presence of H4B, whereas addition of L-Arg and other modulators did not significantly affect the rat es under the same conditions. Thus, we deduce that (1) L-Arg stabilizes the O-2-bound ferrous complex for efficient O-O bond cleavage to occur; (2) H4 B influences the O-2-bound ferrous complex in a fashion different from L-Ar g; and (3) NHA induces a characteristic distal-site structure in the presen ce of H4B, reflecting a difference in the mechanism of activation of O-2 in the first step (monooxygenation of L-Arg) and the second step (monooxygena tion of NHA). (C) 1998 Academic Press.