Mechanisms of myeloperoxidase catalyzed oxidation of H2S by H2O2 or O2 to produce potent protein Cys-polysulfide-inducing species.

The interaction of heme proteins with hydrogen sulfide is gaining attention as an important element in sulfide-mediated protection against oxidative stress and in regulation of redox signaling. In our previous study we reported the efficient reversible inhibition of myeloperoxidase (MPO) activity by sulfide and the kinetics of the reactions of sulfide with ferric MPO, Compound I and Compound II. Here we provide several lines of evidence that a central intermediate species in the turnover of MPO by sulfide is the Compound III state. Compound III is formed in the reactions of sulfide with ferric or ferrous MPO in the presence of oxygen or via the reductions of Compound I or Compound II by sulfide. The regeneration of active ferric MPO from Compound III is slow - representing the rate-limiting step during turnover - but facilitated by ascorbate or superoxide dismutase. These catalytic cycles produce inorganic sulfane sulfur species, which were shown to promote protein Cys persulfidation. Based on compiling experimental data we propose that in contrast to hemoglobin, myoglobin, catalase or lactoperoxidase the formation of a sulfheme derivative in the oxidative interactions of sulfide with MPO is not a major pathway. Using the Met243Val mutant we demonstrated that the sulfonium ion linkage of the Met243 sulfur to the heme pyrrole ring A, which is a unique feature of MPO, is pivotal in the catalytic oxidation of sulfide via Compound III. The proposed novel MPO-catalyzed sulfide oxidation model does not require the initial presence of hydrogen peroxide, only oxygen to provide a slow flux of sulfane sulfur species generation, which could be important in sulfide-mediated endogenous signaling. Furthermore, peroxide-induced formation of sulfane sulfur species by MPO may have a role in protection of regulatory or functional Cys residues during (for example neutrophil induced) inflammatory oxidative stress.