Tyrosine nitration by superoxide and nitric oxide fluxes in biological systems: Modeling the impact of superoxide dismutase and nitric oxide diffusion

Tyrosine nitration is a posttranslational modification observed in many pathologic states that can be associated with peroxynitrite (ONOO-) formation. However, in vitro, peroxynitrite-dependent tyrosine nitration is inhibited when its precursors, superoxide (O-2(.-)) and nitric oxide ((NO)-N-.), are formed at ratios (O-2(.-)/(NO)-N-.) different from one, severely questioning the use of 3-nitrotyrosine as a biomarker of peroxynitrite-mediated oxidations. We herein hypothesize that in biological systems the presence of superoxide dismutase (SOD) and the facile transmembrane diffusion of (NO)-N-. preclude accumulation of O-2(.-) and (NO)-N-. radicals under flux ratios different from one, preventing the secondary reactions that result in the inhibition of 3-nitrotyrosine formation. Using an array of reactions and kinetic constants, computer-assisted simulations were performed in order to assess the flux of 3-nitrotyrosine formation (J(NO2-Y)) during exposure to simultaneous fluxes of superoxide (J(O2)(.-)) and nitric oxide (J(NO)(.)), varying the radical flux ratios (J(O)(.-)/J(NO)(.)), in the presence of carbon dioxide. With a basic set of reactions, J(NO2-Y) as a function of radical flux ratios rendered a bell-shape profile, in complete agreement with previous reports. However, when superoxide dismutation by SOD and (NO)-N-. decay due to diffusion out of the compartment were incorporated in the model, a quite different profile of J(NO2-Y) as a function of the radical flux ratio was obtained: despite the fact that nitration yields were much lower, the bell-shape profile was lost and the extent of tyrosine nitration was responsive to increases in either O-2(.-) or (NO)-N-., in agreement with in vivo observations. Thus, the model presented herein serves to reconcile the in vitro and in vivo evidence on the role of peroxynitrite in promoting tyrosine nitration. (c) 2005 Elsevier Inc. All rights reserved.