Reactivity of hydrogen sulfide with peroxynitrite and other oxidants of biological interest

Hydrogen sulfide (H2S) is an endogenously generated gas that can also be administered exogenously. It modulates physiological functions and has reported cytoprotective effects. To evaluate a possible antioxidant role, we investigated the reactivity of hydrogen sulfide with several one- and two-electron oxidants. The rate constant of the direct reaction with peroxynitrite was (4.8 ± 1.4) × 103 M− 1 s− 1 (pH 7.4, 37°C). At low hydrogen sulfide concentrations, oxidation by peroxynitrite led to oxygen consumption, consistent with a one-electron oxidation that initiated a radical chain reaction. Accordingly, pulse radiolysis studies indicated that hydrogen sulfide reacted with nitrogen dioxide at (3.0 ± 0.3) × 106 M− 1 s− 1 at pH 6 and (1.2 ± 0.1) × 107 M− 1 s− 1 at pH 7.5 (25 °C). The reactions of hydrogen sulfide with hydrogen peroxide, hypochlorite, and taurine chloramine had rate constants of 0.73 ± 0.03, (8 ± 3) × 107, and 303 ± 27 M− 1 s− 1, respectively (pH 7.4, 37 °C). The reactivity of hydrogen sulfide was compared to that of low-molecular-weight thiols such as cysteine and glutathione. Considering the low tissue concentrations of endogenous hydrogen sulfide, direct reactions with oxidants probably cannot completely account for its protective effects.