Catalytic generation of N₂O₃ by the concerted nitrite reductase and anhydrase activity of hemoglobin

Nitrite reacts with deoxyhemoglobin to form nitric oxide (NO) and methemoglobin. Though this reaction is experimentally associated with NO generation and vasodilation, kinetic analysis suggests that NO should not be able to escape inactivation in the erythrocyte. We have discovered that products of the nitrite-hemoglobin reaction generate dinitrogen trioxide (N ₂O₃) via a novel reaction of NO and nitrite-bound methemoglobin. The oxygen-bound form of nitrite-methemoglobin shows a degree of ferrous nitrogen dioxide (Fe(II)-NO₂^•) character, so it may rapidly react with NO to form N₂O₃. N ₂O₃ partitions in lipid, homolyzes to NO and readily nitrosates thiols, all of which are common pathways for NO escape from the erythrocyte. These results reveal a fundamental heme globin- and nitrite-catalyzed chemical reaction pathway to N₂O₃, NO and S-nitrosothiol that could form the basis of in vivo nitrite-dependent signaling. Because the reaction redox-cycles (that is, regenerates ferrous heme) and the nitrite-methemoglobin intermediate is not observable by electron paramagnetic resonance spectroscopy, this reaction has been 'invisible' to experimentalists over the last 100 years.