On the mechanism of the nitric oxide synthase-catalyzed conversion of N omega-hydroxyl-L-arginine to citrulline and nitric oxide.

The mechanism of oxidation of N omega-hydroxyl-L-arginine (NHA) by the iron-dioxygen complex in nitric oxide synthase (NOS) is still uncertain. The uncertainty has not been helped by a lack of precision in the notation used to describe the oxidation states and electrical charges on the iron and oxygen in some of the suggested mechanisms. These problems of notation are addressed, and, in addition, a cyclic voltammetric measurement of the oxidation potential of NHA, namely +0.10 +/- 0.04 V versus normal hydrogen electrode, is used to argue that the sometimes postulated oxidation of NHA by the iron-dioxygen complex to form an intermediate radical cation, NHA.+, is very unlikely for thermodynamic reasons. Instead, it is suggested that this oxidation occurs by a thermodynamically favored abstraction of the hydrogen atom from the > C = NOH moiety of NHA to form an intermediate iminoxyl radical, > C = NO(.). A subsequent nucleophilic attack by the iron-hydroperoxide species formed by this H-atom abstraction on the carbon atom of the iminoxyl radical moiety leads to the production of nitric oxide (NO) and citrulline.