- The dual roles of H2S as an endogenously synthesized respiratory substrate and as a toxin raise questions as to how it is cleared when the electron transport chain is inhibited. Sulfide quinone oxidoreductase (SQOR) catalyzes the first step in the mitochondrial H2S oxidation pathway, using CoQ as an electron acceptor, and connects to the electron transport chain at the level of complex III. We have discovered that at high H2S concentrations, which are known to inhibit complex IV, a new redox cycle is established between SQOR and complex II, operating in reverse.
- Mammalian cells synthesize H2S from sulfur-containing amino acids and are also exposed to exogenous sources of this signaling molecule, notably from gut microbes. As an inhibitor of complex IV in the electron transport chain, H2S can have a profound impact on metabolism, suggesting the hypothesis that metabolic reprogramming is a primary mechanism by which H2S signals. In this study, we report that H2S increases lipogenesis in many cell types, using carbon derived from glutamine rather than from glucose.
- Hydrogen sulfide is synthesized by enzymes involved in sulfur metabolism and oxidized via a dedicated mitochondrial pathway that intersects with the electron transport chain at the level of complex III. Studies with H2S are challenging since it is volatile and also reacts with oxidized thiols in the culture medium, forming sulfane sulfur species. The half-life of exogenously added H2S to cultured cells is unknown. In this study, we first examined the half-life of exogenously added H2S to human colonic epithelial cells.
- Unlike most other tissues, the colon epithelium is exposed to high levels of H2S derived from gut microbial metabolism. H2S is a signaling molecule that modulates various physiological effects. It is also a respiratory toxin that inhibits complex IV in the electron transfer chain (ETC). Colon epithelial cells are adapted to high environmental H2S exposure as they harbor an efficient mitochondrial H2S oxidation pathway, which is dedicated to its disposal. Herein, we report that the sulfide oxidation pathway enzymes are apically localized in human colonic crypts at the host–microbiome interface, but that the normal apical-to-crypt gradient is lost in colorectal cancer epithelium.
- In this sequel to the thematic collection of Minireviews on redox metabolism and signaling published last year, five articles plumb the redox metabolic pathways relevant to cell proliferation, stress response, and survival post-detachment from the extracellular matrix. The sixth article provides unexpected insights into the hepatic NAD(P)ome, revealing that more than half of these proteins reside outside the cytoplasmic and mitochondrial compartments, pointing to the paucity of knowledge on their functions.
- Life on oxygen predisposes cells to reactive oxygen species (ROS) generation by electron slippage in the electron transfer chain. Aerobic metabolism also generates superoxide (O2̇̄) and hydrogen peroxide (H2O2) as bona fide products in reactions involving 1- or 2-electron reduction of O2. Although often viewed as dangerous, ROS are now recognized as important messengers in redox signaling pathways. A delicate balance between needed versus excessive ROS production distinguishes health from an array of disease states.
- Long before the recent thrust of scientific research on the microbiome, the importance of its interface with the host was being acknowledged by practices such as probiotic supplementation, e.g. after a course of antibiotics, which has the unwanted side effect of depleting commensal bacteria. The shared metabolite capital between the host and the microbiome is extensive and tightly controlled. However, despite the influence of microbe-derived metabolites on many aspects of host physiology, behavior, and pathology, our understanding of this metabolic interface is still in its infancy and its therapeutic targeting is largely untapped.
- Substrate ambiguity and relaxed reaction specificity underlie the diversity of reactions catalyzed by the transsulfuration pathway enzymes, cystathionine β-synthase (CBS) and γ-cystathionase (CSE). These enzymes either commit sulfur metabolism to cysteine synthesis from homocysteine or utilize cysteine and/or homocysteine for synthesis of H2S, a signaling molecule. We demonstrate that a kinetically controlled heme-dependent metabolite switch in CBS regulates these competing reactions where by cystathionine, the product of CBS, inhibits H2S synthesis by the second enzyme, CSE.
- Background: RBCs produce H2S but, lacking mitochondria, are devoid of the canonical sulfide oxidation pathway.Results: RBCs utilize methemoglobin to catalyze H2S oxidation producing thiosulfate and polysulfide.Conclusion: In the presence of NADPH and a reductase, ferric sulfide hemoglobin is converted to oxyhemoglobin, completing the sulfide oxidation cycle.Significance: We describe a novel mechanism for H2S oxidation that may be pertinent to other hemeproteins.