Hemoglobin and red blood cells as tools for studying peroxynitrite biochemistry

Oxyhemoglobin represents a relevant intravascular sink of peroxynitrite. Indeed, peroxynitrite undergoes a fast isomerization (k = 1.7 x 10(4)M(-1)s(-1)) to nitrate in the presence of oxyhemoglobin; the reaction mechanism is complex and leads to methemoglobin and superoxide radical as additional products and a small amount (approximately 10%) of transient species, including ferrylhemoglobin, nitrogen dioxide, and globin-derived radicals. The mechanism of the reaction could be solved only after extensive quantitative analysis of reactants, intermediates, and products and setting up experimental conditions that favor direct reactions of peroxynitrite with hemoglobin versus peroxynitrite decay through proton- or carbon dioxide-catalyzed homolysis. Additionally, oxyhemoglobin has been used as a "reporter" molecule of peroxynitrite diffusion from extracellular to intracellular compartments, using red blood cells (RBCs) as a model system. In RBCs, peroxynitrite diffusion across the membrane is favored by the large abundance of anion channels, and average transit distances can vary as a function of cell density. Indeed, we have developed a mathematical model that incorporates competition between the extracellular consumption of peroxynitrite and the permeation to the erythrocytes as a function of the average diffusion distances. The RBC model presented herein serves to estimate biological diffusion distances of peroxynitrite in the presence of relevant molecular targets, and the theoretical approach can be successfully applied to study the diffusion of peroxynitrite in other cellular/tissue systems.