ON THE MECHANISM OF THE NITRIC-OXIDE SYNTHASE-CATALYZED CONVERSION OFN-OMEGA-HYDROXY-L-ARGININE TO CITRULLINE AND NITRIC-OXIDE

The mechanism of oxidation of N-omega-hydroxy-L-arginine (NHA) by the iron-dioxygen complex in nitric oxide synthase (NOS) is still uncertai n. The uncertainty has not been helped by a lack of precision in the n otation used to describe the oxidation states and electrical charges o n the iron and oxygen in some of the suggested mechanisms. These probl ems 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 sometime s postulated oxidation of NHA by the iron-dioxygen complex to form an intermediate radical cation, NHA(.+), is very unlikely for thermodynam ic reasons. Instead, it is suggested that this oxidation occurs by a t hermodynamically favored abstraction of the hydrogen atom from the >C= NOH moiety of NHA to form an intermediate iminoxyl radical, 2C=NO.. A subsequent nucleophilic attack by the iron hydroperoxide species forme d by this H-atom abstraction on the carbon atom of the iminoxyl radica l moiety leads to the production of nitric oxide (NO) and citrulline.