Selective Fe3 Research Articles

Iron Catalyzes both Decomposition and Synthesis ofS-Nitrosothiols: Optical and Electron Paramagnetic Resonance Studies

Formation ofS-nitrosothiols was demonstrated in 1–50 mmaqueous solutions of cysteine or glutathione (cys-NO or GS-NO, respectively) upon contact of thiols with gaseous nitric oxide under a pressure of 50–600 mm Hg and anaerobic conditions. The yield ofS-nitrosothiols was increased by mixing with NO plus air at a molar ratio [NO]/[O2from air] of no less than 40. In this instance, theS-nitrosothiol formation was optimum at a NO pressure of 100–150 mm Hg. The addition of 0.25 mmo-phenanthroline, a selective Fe2+chelator, to thiol solutions prior to the treatment with NO or NO + air completely blocked the formation ofS-nitrosothiols. On the other hand, this process was potentiated by the addition of Fe2+but not Cu2+ions. These data indicated a crucial influence of Fe2+on the process. The contact ofo-phenanthroline withS-nitrosothiols synthesized by a routine method (treatment of thiol solutions with the NO + NO2mixture at pH <1) did not induce their degradation at pH 3–10. Moreover,o-phenanthroline strikingly enhanced the cys-NO stability at neutral pH. Cysteine, glutathione, and desferal, a selective Fe3+chelator, exerted a similar effect on cys-NO. The stabilizing effect of thiols on cys-NO was accompanied by the formation of dinitrosyl–iron complexes with thiol-containing ligands containing admixed (intrinsic) iron (1–2 μm). The addition of Fe2+at a concentration higher than 10 μmabolished the stabilizing effect of thiols on cys-NO. Therefore iron can induce both degradation and synthesis ofS-nitrosothiols. According to the proposed mechanisms such opposite effects of iron onS-nitrosothiols are determined by the ratio betweenS-nitrosothiols, thiols, iron, and NO in the reaction system.