Intracellular redox imbalance is mainly caused by overproduction of reactive oxygen species (ROS) or weakness of the natural antioxidant defense system. It is involved in the pathophysiology of a wide array of human diseases. Hydrogen sulfide (H2S) is now recognized as the third “gasotransmitters” and proved to exert a wide range of physiological and cytoprotective functions in the biological systems. Among these functions, the role of H2S in oxidative stress has been one of the main focuses over years. However, the underlying mechanisms for the antioxidant effect of H2S are still poorly comprehended. This review presents an overview of the current understanding of H2S specially focusing on the new understanding and mechanisms of the antioxidant effects of H2S based on recent reports. Both inhibition of ROS generation and stimulation of antioxidants are discussed. H2S-induced S-sulfhydration of key proteins (e.g., p66Shc and Keap1) is also one of the focuses of this review.
In 1777, a young Swedish apothecary, Carl Wilhelm Scheele, treated ferrous sulfide with a mineral acid and noted a colorless gas with a characteristic odor of rotten eggs. He described it as “sulfuretted hydrogen.” The notoriety of hydrogen sulfide (H2S) had been considered as a toxic gas for several hundreds of years. The Permissible Exposure Limit (PEL) of H2S is 10 ppm and sudden exposure to >400 ppm can cause rapid death. The biological effects of H2S in physiological condition began around the turn of the 20th century. H2S is now recognized as the third “gasotransmitter” along with nitric oxide (NO) and carbon monoxide (CO) [
A free radical is an unstable chemical species that contains one or more unpaired electrons in its outer orbital. In organisms, the highly reactive free radicals formed from metabolism might donate their unpaired electron to another molecule or pull an electron off a neighboring molecule. The term oxidative stress has been proposed indicating a disturbance in the equilibrium status of oxidant/antioxidant systems with a progressive accumulation of ROS in intact cells. ROS are short-lived and highly chemically reactive. At low concentrations, ROS serve as cellular signaling molecules [
In human body, more than 95% free radicals belong to oxygen free radicals. Recent studies suggest that oxygen-free radicals play an essential role in the control of cell functions and signal transmission [
The types of common oxygen-free radicals.
Radicals | Chemical formulas | Electron structures |
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Superoxide anion |
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Hydroxyl radical |
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Perhydroxyl radical |
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Alkoxyl radical |
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Alkyl peroxide radical |
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Hydrogen peroxide | H2O2 |
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Singlet oxygen | 1O2 |
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ROS are widespread in living organisms. Actually, they are being continuously produced in vivo and many of them are necessary to carry out certain cellular and biological reactions [
Exogenous sources are mainly generated by some stimulating factors. These include smoking, alcohol, certain drugs, air pollution, ionizing radiation, and hyperbaric oxygen poisoning. Compared with exogenous sources, endogenous sources play more important roles in the form of oxygen free radicals. Endogenous activities are the main sources of oxygen-free radicals in living organisms. The main endogenous sources are listed below (Figure
The main ROS generation and elimination pathways.
Endogenous superoxide anion (
In addition, the metabolism of arachidonic acid by cyclooxygenase [
In living organisms, ROS are continuously produced because of the reduction of molecular oxygen. Although free radicals play an important role in some physiological reactions, such as cell signal transduction and regulation of muscle tone [
Superoxide dismutase (SOD) is a common antioxidant enzyme which contains copper, zinc, and manganese as cofactors [
Catalase (CAT) is another antioxidant enzyme that is widely distributed in tissues [
There are also some nonenzymatic chemical antioxidants that play an important role in antioxidant, included glutathione (GSH),
At 37°C and pH 7.4, more than 80% of H2S molecules dissolve in surface waters and dissociate into H+,
As we mentioned before, ROS is counterbalanced in the body by a net of antioxidants, including enzymatic and nonenzymatic antioxidants. GSH and thioredoxin (Trx-1) are two biologically important nonenzymatic antioxidants in animal cells and attracted increasing attention as cellular protectants against oxidative stress in vivo.
GSH, a tripeptide consisting of cysteine, glutamate, and glycine, is a major antioxidant in the cellular defense against oxidative stress and a decreased GSH/GSSG ratio is usually taken as indicating oxidative stress. In cells, GSH is synthesized from cysteine. There are 2 cysteine forms, oxidized form cystine and reductive form cysteine. Because of its redox instability, extracellular cysteine is mostly present in cystine, which can be transported into cells through cystine/glutamate antiport system
H2S increases intracellular GSH synthesis. Cellular GSH is mainly synthesized from cysteine.
Studies from other laboratories have also proven that H2S preserves the cellular GSH status and provides protection against oxidative damage in brain [
Classic thioredoxin (Trx-1) is a small (12 kDa) ubiquitous molecule containing a characteristic Cys-Gly-Pro-Cys motif and the oxidation-reduction of Trx-1 occurs at its two cysteine residues. It was reported that Trx-1 exerts extracellular and intracellular multifunctions in cell proliferation [
In 2008, Jha et al. reported that H2S protected murine liver against ischemia-reperfusion (I/R) injury through upregulation of intracellular Trx-1 along with an increase in hepatic tissue GSH/GSSG ratio [
Despite the potential role of H2S in the cellular antioxidant defense, studies on its antioxidant mechanism have been exceptionally limited. Recently, increasing evidence revealed that Nrf2 participated in the antioxidant effect of H2S by promoting cellular antioxidant gene expression.
Nuclear factor (erythroid-derived 2)-like 2, also known as nuclear factor-erythroid 2 (NF-E2) related factor 2 (Nrf2), is a transcription factor that regulates a wide variety gene expression. Nrf2 is found mostly in the cytoplasm as an inactive complex with Kelch-like ECH-associated protein 1 (Keap1) [
Effect of H2S on intracellular enzymatic and nonenzymatic antioxidant production.
Another major mechanism for cells to maintain redox equilibrium is based on the clearance ability processed by cellular antioxidant enzymes. Superoxide dismutase (SOD), CAT, and GPx are three main antioxidant enzymes that defend against oxidative damage in vivo. There are three isoforms of mammalian SOD: the cytosolic copper/zinc-containing SOD (Cu/ZnSOD, SOD-1), the mitochondrial manganese-containing SOD (MnSOD, SOD-2), and the extracellular SOD (ecSOD, SOD-3). SOD catalyzes the dismutation of
The signal transduction pathways for H2S to promote endogenous enzymatic antioxidant defense are much less understood. NF-
In addition to the activation of NF-
Besides the capacity of cellular antioxidant defense, sequential overproduction of ROS is another vital factor in response to oxidative stress. Mitochondria is the major source of intracellular ROS and leak from the electron transfer chain is thought to be the main route [
p66Shc is a 66 kD Src homologous-collagen homologue (Shc) adaptor protein, which is encoded by the
Proposed model for the effect of H2S on p66Shc mediated mitochondrial ROS generation. (a) Showing the effect of H2S. p66Shc is activated by
Recently, our group demonstrated for the first time that H2S may inhibit mitochondrial ROS production via a p66Shc-dependent signal transduction. Protein S-sulfhydration had been proposed to emerge as a major functional alteration of proteins, such as the potassium channels (like KATP, IKca, and SKca) [
The antioxidant activity of H2S discussed in this review illuminated the biochemical mechanisms of H2S on cellular redox homeostasis. However, the effects of H2S on redox status are highly divergent. H2S was also reported as a powerful prooxidant, which kills cancer cells in a ROS-dependent manner [
It should also be noted that the concentration- and time-dependent effects of H2S are very complicated. H2S was reported to display opposite effects at different concentrations/periods. GYY4137, a slow-releasing H2S donor, yielded very low concentrations of H2S and was proved to kill cancer cells. NaHS, which releases higher concentrations of H2S in short period, however, only exhibited weaker anticanner effect [
In summary, we discussed the current understanding of the antioxidant effect of H2S in this paper. Obviously, H2S does not produce antioxidant effect via a single/simple mechanism. Multiple targets and signaling pathways are involved. H2S can stimulate cellular enzymatic or nonenzymatic antioxidants to scavenge free radicals. This may be secondary to a direct effect on antioxidants or an indirect action through activation of various signaling proteins. H2S may also inhibit mitochondria ROS production through sulfhydration of p66Shc or membrane/cytosol ROS generation via inhibition of NADPH. To a weak extent, H2S also quenches free radicals directly due to its chemical reducing property. Future studies to explore more action sites of H2S in different signaling proteins and mechanisms underlying concentration- and time-dependent effects of H2S are still warranted.
3-Mercaptopyruvate sulfurtransferase
Adenosine diphosphate
Adenosine monophosphate
Antioxidant response element
Adenosine triphosphate
Catalase
Cystathionine
collagen homology domain 1
carbon monoxide
Cystathionine
Diallyl sulfide
Glutamate-cysteine ligase catalytic subunit
Glutamate-cysteine ligase modifier subunit
Glutathione peroxidase
Glutathione reductase
Reduced glutathione
Oxidized glutathione
Glutathione-S-transferase
Ischemia-reperfusion
Kelch-like ECH-associated protein 1
Nitric oxide
Nuclear factor-erythroid 2 (NF-E2) related factor 2
Permissible Exposure Limit
Prolyl isomerase
Dephosphorylated by phosphatase A2
Peroxiredoxin
The phosphor-tyrosine-binding domain
Quinone reductase
Reactive oxygen species
Src homologous-collagen
Superoxide dismutase
Thioredoxin 1
Xanthine dehydrogenase
Xanthine oxidase.
The authors declare no conflict of interests to this work.
This work was supported by the Construct Program of the Key Discipline in Hunan Province and by the NUHS B2B Research Grant (NUHSRO/2011/012/STB/B2B-08).