Reductive nitrosylation and S-nitrosation of hemoglobin in inhomogeneous nitric oxide solutions

Elucidating the reaction of nitric oxide (NO) with oxyhemoglobin [HbFe(II)O2] is critical to understanding the metabolic fate of NO in the vasculature. At low concentrations of NO, methemoglobin [HbFe(III)] is the only detectable product from this reaction; however, locally high concentrations of NO have been demonstrated to result in some iron-nitrosylhemoglobin [HbFe(II)NO] and S-nitrosohemoglobin (SNO-Hb) formation. Reductive nitrosylation through a HbFe(III) intermediate was proposed as a viable pathway under such conditions. Here, we explore another potential mechanism based on mixed valenced Hb tetramers. The oxidation of one or two heme Fe(II) in the R-state HbFe(II)O2 has been observed to lower the oxygen affinity of the remaining heme groups, thus creating the possibility of oxygen release and NO binding at the heme Fe(II) sites. This mixed valenced hypothesis requires an allosteric transition of the Hb tetramer. Hence, this hypothesis can account for HbFe(II)NO formation, but not SNO-Hb formation. Here, we demonstrate that cyanide attenuated the formation of SNO-Hb by 30-40% when a saturated NO bolus was added to 0.1-1.0 mM HbFe(II)O2 solutions. In addition, HbFe(II)NO formation under such inhomogeneous conditions does not require allostericity. Therefore, we concluded that the mixed valenced theory does not play a major role under these conditions, and reductive nitrosylation accounts for a significant fraction of the HbFe(II)NO formed and approximately 30-40% of SNO-Hb. The remaining SNO-Hb is likely formed from NO oxidation products.