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Mechanism of S-nitrosothiol decomposition

Aside from the reaction of iron(II) complexes with peroxides that yield reactive radicals, the Fenton reaction, iron also catalyzes the decompositions of nitrosothiols that play a role in the homostasis of vascular tension.

The mechanisms of S-nitrosothiol transformation into paramagnetic dinitrosyl iron complexes with thiol- or non-thiol ligands, or into mononitrosyl iron complex with N-methyl-D-glucamine dithiocarbamate, catalyzed by Fe2+ ions under anaerobic conditions, were studied by monitoring EPR and optical features of the complexes and S-nitrosothiols. Kinetic investigations demonstrated the appearance of a short-living paramagnetic mononitrosyl iron complex with L-cysteine prior to formation of a stable dinitrosyl iron complex with cysteine in a solution, micromolar in Fe2+-citrate and S-nitrosocysteine and millimolar in L-cysteine at pH 7.4.

The addition of deoxyhemoglobin did not influence the process, which points to a direct interaction between S-nitrosocysteine and Fe2+ ions to yield dinitrosyl iron compounds. The reaction to form dinitrosyliron cysteine is first- and second-order in iron and S-nitrosocysteine, respectively. The third-order rate constant is (1.0+/-0.2) x 10'5 M-2s-1 (estimated from EPR results) or (2.0+/-0.1) x 10'4 M-2s-1, as estimated by optical methods. The formation of di- and mononitrosyl iron complexes from S-nitrosothiols and Fe2+ ions takes place in a concerted mechanism.

Reported by

Hoenggerberg, HCI
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