Use of Tissue Metabolite Analysis and Enzyme Kinetics To Discriminate between Alternate Pathways for Hydrogen Sulfide...

Hydrogen sulfide (H₂S) is an endogenously synthesized signaling molecule that is enzymatically metabolized in mitochondria. The metabolism of H₂S maintains optimal concentrations of the gasotransmitter and produces sulfane sulfur (S⁰)-containing metabolites that may be functionally important in signaling. Sulfide:quinone oxidoreductase (SQOR) catalyzes the initial two-electron oxidation of H₂S to S⁰ using coenzyme Q as the electron acceptor in a reaction that requires a third substrate to act as the acceptor of S⁰. We discovered that sulfite is a highly efficient acceptor and proposed that sulfite is the physiological acceptor in a reaction that produces thiosulfate, a known metabolic intermediate. This model has been challenged by others who assume that the intracellular concentration of sulfite is very low, a scenario postulated to favor reaction of SQOR with a considerably poorer acceptor, glutathione. In this study, we measured the intracellular concentration of sulfite and other metabolites in mammalian tissues. The values observed for sulfite in rat liver (9.2 μM) and heart (38 μM) are orders of magnitude higher than previously assumed. We discovered that the apparent kinetics of oxidation of H₂S by SQOR with glutathione as the S⁰ acceptor reflect contributions from other SQOR-catalyzed reactions, including a novel glutathione:CoQ reductase reaction. We used observed metabolite levels and steady-state kinetic parameters to simulate rates of oxidation of H₂S by SQOR at physiological concentrations of different S⁰ acceptors. The results show that the reaction with sulfite as the S⁰ acceptor is a major pathway in liver and heart and provide insight into the potential dynamics of H₂S metabolism.