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

Abstract

Hydrogen sulfide (HS) is an endogenously synthesized signaling molecule that is enzymatically metabolized in mitochondria. The metabolism of HS maintains optimal concentrations of the gasotransmitter and produces sulfane sulfur (S)-containing metabolites that may be functionally important in signaling. Sulfide:quinone oxidoreductase (SQOR) catalyzes the initial two-electron oxidation of HS to S using coenzyme Q as the electron acceptor in a reaction that requires a third substrate to act as the acceptor of S. 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 HS by SQOR with glutathione as the S 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 HS by SQOR at physiological concentrations of different S acceptors. The results show that the reaction with sulfite as the S acceptor is a major pathway in liver and heart and provide insight into the potential dynamics of HS metabolism.

Metabolism of sulfur compounds in homocystinurias

“…The major finding in the catabolic pathway of H 2 S is the significant elevation of thiosulfate in urine indicating a possible higher rate of H 2 S detoxification using sulfite in the TST reaction, which is indirectly supported also by non‐significantly decreased sulfite. Moreover, the slightly lower sulfite may be also due to low production of Cys sulfinate via CDO as evidenced by low Hyp and Tau and/or due to decreased formation of GSH persulfide (Augustyn et al ., ) resulting from GSH depletion, which was shown in the liver of a CBS‐deficient mouse model (Maclean et al ., ). To sum up, it is conceivable that patients with CBSD may produce slightly less H 2 S from Cys, which is compensated by production of H 2 S from Hcy and that this slightly higher H 2 S is further metabolized to thiosulfate (Figure A).…”

“…Model of mitochondrial oxidation of H 2 S. Several models of mitochondrial H 2 S oxidation have been proposed (Augustyn et al ., ; Carter and Morton, ; Libiad et al ., ; Melideo et al ., ; Olson, ). This scheme summarizes the alternative pathways of H 2 S oxidation and interconversions of selected sulfur compounds within mitochondria.…”

“…Degradation of H 2 S is located mostly in the mitochondria (see Figure ) and starts by persulfidation of sulfide : quinone oxidoreductase (SQOR). Two alternative pathways for further oxidation of persulfidated SQR were proposed involving sulfur dioxygenase (SDO), thiosulfate transferase (TST) and sulfite oxidase (SO) with formation of intermediate metabolites sulfite, thiosulfate and thiocyanate, and of the final H 2 S oxidation product sulfate (Augustyn et al ., ; Olson, ; Kohl et al ., ).…”

Hydrogen Sulfide in Pharmacotherapy, Beyond the Hydrogen Sulfide-Donors

“…The sulfane can be further transferred to glutathione to form glutathione persulfide or to sulfite and form thiosulfate. The glutathione persulfide may be oxidized by ETHE1 and thiosulfate by TST to regenerate sulfite, which is oxidized by SO to sulfate [36,40,41]. Additionally, a part of H 2 S is exhaled or scavenged in the blood by methemoglobin to form sulfhemoglobin [42][43][44].…”

The dichotomous role of H2S in cancer cell biology? Déjà vu all over again

“…Mitochondrial oxidation via three consecutive reactions is how much of the H 2 S is metabolized [116]. In fact, H 2 S is the only gasotransmitter that is enzymatically metabolized and the only inorganic compound that can be used by mammalian mitochondria to generate ATP [117, 118]. This oxidation requires one mole of oxygen for every mole of H 2 S oxidized along the electron transport chain.…”