An electron paramagnetic resonance study of the affinity of nitrite for methemoglobin

Recent data suggests that reactions of nitrite with ferric hemoglobin are potentially important in heme-protein dependent NO signaling. Our group and others are evaluating the role of reductive nitrosylation reactions in the generation of N2O3 as a signaling molecule. The latter reaction is hypothesized to involve reactions on NO, nitrite and methemoglobin to form N2O3 in an anhydrase reaction. Of potential importance to these reactions is the affinity of methemoglobin for nitrite and the reactivity of nitrite-bound methemoglobin with nitric oxide. In this paper, we review work related to the electronic structure of nitrite-bound methemoglobin and its dissociation constant. We present new data using electron paramagnetic resonance spectroscopy which confirm that methemoglobin has a much higher affinity for nitrite, under certain conditions, than reported in classical observations. Interestingly the affinity is greatest at lower pH and low nitrite:methemoglobin ratios. These data suggest additional interesting chemistry in the reaction of nitrite with ferric and ferrous heme species. Moreover, this reaction could serve as a paradigm for ferric heme reactions with nitrite.