Decomposition And Synthesis Of Snitrosothiols By Iron

In principle, the same oxidative pathways of RS-NO synthesis and decomposition similar to shown in Schemes 2, 3 and 4 could also be catalyzed by iron. Just like Cu2+ ions, ferric iron could initiate the reactions. However, high redox potential of Fe3+/Fe2+ pair (Eo = +0.77 V)

makes improbable that the reductive mechanism of RS-NO decomposition as in Scheme 1 actually works for iron as well. This redoxpotential is the value for aqueous solution and quoted in many textbooks. However, the redoxcouple of iron is notoriously sensitive to the ligand field around the iron as caused by ligands like thiols, solvent molecules like H2O or OH-, or small solutes like phosphate or Cl-. The wide variation in redoxpotentials found in iron enzymes is a good illustration of this sensitivity. In addition, even when thermody-namically allowed by the redoxpotential, the actual transfer of electrons for certain redox reactions might be inhibited by geometrical constraints imposed by the ligands. Therefore, the presence of iron in an aqueous solution should be regarded as a superposition of iron pools spanning a considerable range of redoxpotentials.

Actual experiments demonstrate that ferrous iron catalyzes both decomposition and synthesis of RS-NO [10,15-19]. In both cases, ferrous iron does not change its redox state. It acts only as a complex-forming metal capable of binding NO, RS- or RS-NO ligands. The d-electrons of the iron provide electronic coupling between the reactants necessary for the S-nitrosation/denitrosation reactions to proceed. Invariably, the combination of iron, thiols and free NO leads to the formation of paramagnetic dinitrosyl iron complexes (DNIC) with thiol-containing ligands [thiol-DNIC with formula {(RS-)2Fe+(NO+)2}+ were considered in Chapter 2]. The pathways of DNIC formation were investigated mainly by our groups [10,19,24], other investigators have not devoted extensive attention to these reactions. Since endogenous NO levels usually remain far below the micromolar range, the formation of a significant pool of a dinitrosyl species was expected to be highly improbable. However, our investigations have shown that such dinitrosyl complexes play an important role in the iron-catalyzed reactions between the pools of thiols and S-nitrosothiols. Up to our knowledge, there are only a few investigations devoting to the role of ferrous iron in the processes of RS-NO decomposition or synthesis [9,14-16]. The main objective of this chapter is to review the evidence that not only copper but also iron, particularly ferrous iron, determine the status of S-nitrosothiols in cells and tissues.

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