The reaction between nitrite and oxyhemoglobin

The nitrite anion (NO2-) has recently received much attention as an endogenous nitric oxide source that has the potential to be supplemented for therapeutic benefit. One major mechanism of nitrite reduction is the direct reaction between this anion and the ferrous heme group of deoxygenated hemoglobin. However, the reaction of nitrite with oxyhemoglobin (oxyHb) is well established and generates nitrate and methemoglobin (metHb). Several mechanisms have been proposed that involve the intermediacy of protein-free radicals, ferryl heme, nitrogen dioxide (NO2), and hydrogen peroxide (H2O2) in an autocatalytic free radical chain reaction, which could potentially limit the usefulness of nitrite therapy. In this study we show that none of the previously published mechanisms is sufficient to fully explain the kinetics of the reaction of nitrite with oxyHb. Based on experimental data and kinetic simulation, we have modified previous models for this reaction mechanism and show that the new model proposed here is consistent with experimental data. The important feature of this model is that, whereas previously both H2O2 and NO2 were thought to be integral to both the initiation and propagation steps, H2O2 now only plays a role as an initiator species, and NO2 only plays a role as an autocatalytic propagatory species. The consequences of uncoupling the roles of H2O2 and NO2 in the reaction mechanism for the in vivo reactivity of nitrite are discussed.