Formation of S-nitrosothiols was demonstrated in 1-50 mM aqueous 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 of S-nitrosothiols was increased by mixing with NO plus air at a molar ratio [NO]/[O2 from air] of no less than 40. In this instance, the S-nitrosothiol formation was optimum at a NO pressure of 100-150 mm Hg. The addition of 0.25 mM o-phenanthroline, a selective Fe2+ chelator, to thiol solutions prior to the treatment with NO or NO + air completely blocked the formation of S-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 of o-phenanthroline with S-nitrosothiols synthesized by a routine method (treatment of thiol solutions with the NO + NO2 mixture 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 microM). The addition of Fe2+ at a concentration higher than 10 microM abolished the stabilizing effect of thiols on cys-NO. Therefore iron can induce both degradation and synthesis of S-nitrosothiols. According to the proposed mechanisms such opposite effects of iron on S-nitrosothiols are determined by the ratio between S-nitrosothiols, thiols, iron, and NO in the reaction system.