Enzymic Reactions of Nitric Oxide

In 1987 it was announced by two groups that the endothelium-derived relaxing factor, EDRF, is nitric oxide. Previous work had shown that this factor is produced in the endothelium and diffuses into the underlying muscle, where it causes smooth muscle relaxation by 

With the benefit of hindsight, it is not surprising that NO is involved in smooth muscle relaxation. Angina pain has been treated for many years by vasodilators, such as glyceryl trinitrate, amyl nitrite and isosorbide dinitrate, all of which are potential sources of NO. There is still some question as to whether NO itself or some derivative is the actual EDRF, and nitrosothiols, RS— NO, are most often invoked as possibilities.  

In the biological system, NO is produced by nitric oxide synthase, NOS, which occurs in at least three forms the endothelial form, eNOS, found in endothelial cells the neuronal form, nNOS, found in neural tissues the inducible form, iNOS, found in macrophages. Formation of the latter is triggered by the calcium-binding protein, calmodulin. The NOS enzymes contain iron in a coordination environment like that in cytochrome P-4S0-CAM. The substrate is L-arginine, and the products are NO and citrulline  

The mechanistic details are described in several s ific reviews as well as in more general reviews on P-450 enzymes. The first step is thought to be insertion of O from Fe =0 into an NH bond of the C=NH group to yield C=NOH. Then, Fe(III) is reduced and reacts with another Oj to form the superoxo Fe fCj ) species and possibly the Fe (02H ) hydroperoxo species. There is some debate about which of these species is responsible for the transfer of the second O that ultimately leads to formation of NO. 

The NOS enzymes are known to be inhibited by NO. This effect is at least in part due to binding of NO to Fe(III) and Fe(II) centers in the enzymes. There have been many studies of the binding of NO to various Fe-heme proteins and the work has been reviewed. Ford and co-workers studied the following equilibrium reaction 

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