Metabolism of hydrogen sulfide (H 2 S) and Production of Reactive Sulfur Species (RSS) by superoxide dismutase.

Reactive sulfur species (RSS) such as H2 S, HS• , H2 Sn , (n = 2-7) and HS2 •- are chemically similar to H2 O and the reactive oxygen species (ROS) HO• , H2 O2 , O2 •- and act on common biological effectors. RSS were present in evolution long before ROS, and because both are metabolized by catalase it has been suggested that "antioxidant" enzymes originally evolved to regulate RSS and may continue to do so today. Here we examined RSS metabolism by Cu/Zn superoxide dismutase (SOD) using amperometric electrodes for dissolved H2 S, a polysulfide-specific fluorescent probe (SSP4), and mass spectrometry to identify specific polysulfides (H2 S2 -H2 S5 ). H2 S was concentration- and oxygen-dependently oxidized by 1μM SOD to polysulfides (mainly H2 S2 , and to a lesser extent H2 S3 and H2 S5 ) with an EC50 of approximately 380μM H2 S. H2 S concentrations > 750μM inhibited SOD oxidation (IC50 = 1.25mM) with complete inhibition when H2 S > 1.75mM. Polysulfides were not metabolized by SOD. SOD oxidation preferred dissolved H2 S over hydrosulfide anion (HS- ), whereas HS- inhibited polysulfide production. In hypoxia, other possible electron donors such as nitrate, nitrite, sulfite, sulfate, thiosulfate and metabisulfite were ineffective. Manganese SOD also catalyzed H2 S oxidation to form polysulfides, but did not metabolize polysulfides indicating common attributes of these SODs. These experiments suggest that, unlike the well-known SOD-mediated dismutation of two O2 •- to form H2 O2 and O2 , SOD catalyzes a reaction using H2 S and O2 to form persulfide. These can then combine in various ways to form polysulfides and sulfur oxides. It is also possible that H2 S (or polysulfides) interact/react with SOD cysteines to affect catalytic activity or to directly contribute to sulfide metabolism. Our studies suggest that H2 S metabolism by SOD may have been an ancient mechanism to detoxify sulfide or to regulate RSS and along with catalase may continue to do so in contemporary organisms.