Interaction of Cu,Zn Superoxide Dismutase with Hydrogen Sulfide

Addition of HS− enhanced the O−2scavenging activity of bovine erythrocyte Cu,Zn superoxide dismutase (EC 1.15.1.1) by about twofold. The positive effect was measured using a diverse selection of SOD activity assays, and cannot be an artifact restricted to any single technique. Km values for HS− varied in different assay techniques, but we estimate Km ≍ 80 μM HS−. In contrast to HS−, other small molecules tested with SOD either had little effect or were inhibitory. Consumption of HS− and O−2 occurred in nearly 1:1 mole ratio. The products were H2O2 and sulfane sulfur, such as either elemental sulfur or polysulfide. Binding of HS− to the enzyme was rapid, with k > 107 M−1 s−1. The resulting complex exhibited a Cu-to-S charge-transfer absorbance band at 345 nm and an altered Cu(II) EPR spectrum. Taken together, these observations suggest that HS− binds at the catalytic Cu center of SOD and can be a genuine substrate of the enzyme.