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 (O2 −) and nitric oxide ( NO), are formed at ratios (O2 −/ NO) 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 preclude accumulation of O2 − and NO 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 (JNO2−Y) during exposure to simultaneous fluxes of superoxide (JO2 −) and nitric oxide (J NO), varying the radical flux ratios (JO2 −/ J NO), in the presence of carbon dioxide. With a basic set of reactions, JNO2−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 decay due to diffusion out of the compartment were incorporated in the model, a quite different profile of JNO2−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 O2 − or NO, 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.