The mechanisms of S-nitrosothiol decomposition catalyzed by iron

Abstract

The mechanisms of S-nitrosothiol transformation into paramagnetic dinitrosyl iron complexes (DNICs) with thiol- or non-thiol ligands or mononitrosyl iron complex (MNICs) with N-methyl- d-glucamine dithiocarbamate catalyzed by iron(II) ions under anaerobic conditions were studied by monitoring EPR or optical features of the complexes and S-nitrosothiols. The kinetic investigations demonstrated the appearance of short-living paramagnetic mononitrosyl–iron complex with l-cysteine prior to the formation of stable dinitrosyl–iron complex with cysteine in the solution of iron(II)–citrate complex (50–100 μM), S-nitrosocysteine (400 μM), and l-cysteine (20 mM) in 100 mM Hepes buffer (pH 7.4). The addition of deoxyhemoglobin (100 μM) did not influence the process, which points to a direct interaction between S-nitrosocysteine and iron(II) ions to yield DNIC. The reaction of DNIC–cysteine formation is first- and second-order in iron and S-nitrosocysteine, respectively. The third-order rate constant is (1.0 ± 0.2) × 10 5 M −2 s −1 (estimated from EPR results) or (2.0 ± 0.1) × 10 4 M −2 s −1 (estimated by optical method). A similar process of DNIC–cysteine formation was observed in a solution of iron(II)–citrate complex, l-cysteine, and NO-proline (200 μM) as a NO donor. The appearance of a less stable dinitrosyl–iron complex with phosphate was detected when solutions of iron(II)–citrate containing 100 mM phosphate buffer (pH 7.4) were mixed with S-nitrosocysteine or NO-proline. The rapid formation of DNIC with phosphate was followed by its decay. When the concentration of l-cysteine in solutions was reduced from 20 to 1 mM, the life-time of the DNIC–cysteine diminished notably; this was caused by consumption of l-cysteine in the process of DNIC–cysteine formation from S-nitrosocysteine and iron. Thus, l-cysteine is consumed. Formation of DNIC with glutathione was also observed in a solution of glutathione (20 mM), S-nitrosoglutathione (400 μM), and iron(II) complex (800 μM) in 100 mM Hepes buffer (pH 7.4), but the rate of formation was about 10 times slower than the formation of the DNIC–cysteine. The rate of MNIC–MGD formation from iron(II)–MGD complexes and S-nitrosocysteine was first-order in both reactants. The second-order rate constant for this reaction, estimated from EPR measurements, was 30 ± 5 M −1 s −1. Rate constants of MNIC–MGD formation from iron(II)–MGD and the more stable S-nitrosoglutathione and S-nitroso- d, l-penicillamine were equal to 3.0 ± 0.3 and 0.3 ± 0.05 M −1 s −1, respectively. Thus, the concerted mechanism of DNIC and MNIC formation from S-nitrosothiols and iron(II) ions can be suggested to be predominant.