Hydrogen sulfide (H2S) has historically been considered to be a toxic gas, an environmental and occupational hazard. However, with the discovery of its presence and enzymatic production through precursors of L-cysteine and homocysteine in mammalian tissues, H2S has recently received much interest as a physiological signaling molecule. H2S is a gaseous messenger molecule that has been implicated in various physiological and pathological processes in mammals, including vascular relaxation, angiogenesis, and the function of ion channels, ischemia/reperfusion (I/R), and heart injury. H2S is an endogenous neuromodulator and present studies show that physiological concentrations of H2S enhance NMDA receptor-mediated responses and aid in the induction of hippocampal long-term potentiation. Moreover, in the field of neuronal protection, physiological concentrations of H2S in mitochondria have many favorable effects on cytoprotection.
Hydrogen sulfide (H2S) is well known as a transparent, toxic gas with the characteristic strong smell of rotten eggs [
Recent evidence clearly indicates that mammalian tissues can also produce H2S through an endogenous synthetic system, that consists primarily of two enzymes, cystathionine
In mitochondria, H2S acts as a cytoprotective factor by inhibiting the activity of cytochrome oxidase following ischemia/reperfusion (I/R), upregulating the level of superoxide dismutase (SOD), and downregulating levels of reactive oxygen species (ROS). H2S also acts as both a neuroprotectant by increasing the production of glutathione (GSH) and by modulating CSE translocation to mitochondria and the supply of ATP during hypoxia. Mitochondria play a key role in cell death pathways [
The physiological regulation of H2S as a gasotransmitter and modulator in both central and peripheral systems will be discussed below, along with the unique role that H2S plays in mitochondria.
Three enzymes have been identified that produce endogenous H2S: cystathionine
Understanding the distinct expression patterns of the three enzymes is helpful for drug design. Each enzyme may be a possible target for modulating endogenous H2S, while a lack of cysteine may lead to a nonspecific decrease of H2S.
CBS is a pyridoxal-5′-phosphate- (PLP-)dependent enzyme. Using northern blot assays, CBS was shown to be expressed in the hippocampus, cerebellum, cerebrum, and brainstem [
Several endogenous and exogenous compounds such as epidermal growth factor (EGF), transforming growth factor-
CSE is also a pyridoxal-5′-phosphate- (PLP-)dependent enzyme. CSE is mainly localized in the liver and kidney and in both vascular and nonvascular smooth muscle. Low levels of CSE are also detectable in the small intestine and stomach of rodents [
3MST and cysteine aminotransferase (CAT) are recently identified enzymes that can produce H2S from cysteine in the brain [
The major cellular sources of H2S and the mechanism of H2S release remain unknown, although several possibilities have been proposed. Two forms of sulfur that can release H2S have been detected and methods have been developed to measure the levels of free H2S.
Basal levels of free H2S must be kept low because frequent exposure to relatively high concentrations of H2S leads to desensitization of the response to H2S. Some endogenous H2S is likely to be released immediately after it is produced, but the majority are stored and released following stimulation. Two forms in which endogenous H2S can be stored are acid-labile sulfur and bound sulfane sulfur. However, many unanswered questions regarding the stored form of endogenous H2S still remain [
Acid-labile sulfur is mainly localized in the iron-sulfur center of mitochondrial enzymes. However, it can only release H2S at approximately pH 5.4, which suggests that it is not a physiological source of H2S [
In contrast to acid-labile sulfur, bound sulfane sulfur releases H2S under reducing conditions. Bound sulfane sulfur consists of divalent sulfur bound only to other sulfur atoms, in the forms of polysulfide, elemental sulfur, and persulfide. Under reducing conditions, approximately pH 8.4 is needed for cells to release H2S under physiological concentrations of glutathione and cysteine [
Several methods have been developed to measure free H2S under various conditions with relatively high accuracy [
Some studies claim that H2S inhibits human recombinant Ca2+-activated K+ channels (BKCa) and native BKCa channels expressed in the carotid body in rats. In addition, these channels are widely distributed in the central nervous system and vasculature. The inhibition of BKCa channels by H2S is of fundamental physiological importance to carotid body function. However, another report has indicated that H2S increased the activity of BKCa channels expressed in a rat pituitary cell line, leading to hyperpolarization and relaxation of smooth muscle cells (SMCs) [
T-type Ca2+ channels are a unique class of voltage-gated Ca2+ channel. Regulation of T-type Ca2+ channels is an important feature of both acute and chronic pain sensations. H2S can activate or sensitize the channels in primary afferent and spinal sensory neurons. This may, in part, account for hyperalgesia and chronic pain, because hyperalgesia and allodynia can be prevented by CSE inhibitors as well as by a T-type channel inhibitor. Hyperalgesia can also be suppressed by blocking endogenous H2S production. However, no detailed electrophysiological investigation of the modulation of T-type Ca2+ channels by H2S has been performed [
H2S produced via CSE has been shown to relax vascular smooth muscle via the opening of ATP-sensitive K+ (
The effects on cell hyperpolarization in intact and endothelium-denuded mesenteric arteries are not mediated by KATP, but by the combination of intermediate- and small-conductance Ca2+ activated K+ channels (IKCa/SKCa channels), as hyperpolarization is completely blocked by selective IKCa and SKCa channel inhibitors such as charybdotoxin and apamin [
The cardiovascular effects of H2S include relaxing vascular smooth muscle
H2S has an inhibitory effect on L-type Ca2+ currents in normotensive and spontaneously hypertensive rat strains, and it is speculated that this important modulatory effect of H2S may contribute not only to a reduction in blood pressure, but also to longer term protective effects [
Kubo et al. investigated the inhibitory role of H2S on endothelial NO synthase, using sodium hydrogen sulfide (NaHS) as a H2S donor and glibenclamide as a KATP channel inhibitor [
H2S is also an important endogenous vasorelaxant factor [
NO donors upregulate expression and activity of CSE in vascular tissues and cultured aortic SMCs. NO inhibition and subsequent vascular tension are magnified by endogenous H2S, which may contribute to circulatory regulation under physiological conditions [
Current evidence suggests that H2S plays an important role in brain functions. It plays a neuromodulatory role in maintaining the balance of excitation and inhibition by a series of ion channel and receptor-mediated effects, which results in upregulation the
Moreover, possible physiological functions of H2S include long-term potentiation through activation of NMDA receptors, regulating the redox status, and inhibiting oxidative damage through scavenging free radicals and reactive species. Together, this indicates that H2S has a positive impact on protecting neurons from oxidative stress in both extracellular and intracellular microenvironments. It can also fine-tune the inhibitory impact on hyperpolarizing neurons by increasing K+ efflux via KATP channels or through stimulation of the postsynaptic receptors that generate long-lasting inhibitory postsynaptic potentials [
Endogenous H2S is a novel neuromodulator and transmitter in the brain. H2S is also involved in pathologies of the central nervous system such as stroke and Alzheimer’s disease. In stroke, H2S appears to act as a mediator of ischemic injuries and thus inhibition of its production has been suggested to be a potential therapeutic approach.
Nonsteroidal anti-inflammatory drugs (NSAIDs) are among the most commonly used anti-inflammatory drugs but they have significant side effects, such as gastrointestinal ulceration and bleeding, allergy, and coagulation disorder. NSAIDs are, therefore, limited in their application.
There is an emerging evidence that physiological concentrations of H2S can modulate inflammatory processes or even exert a range of anti-inflammatory effects and accelerate healing by downregulating inflammatory responses [
Mitochondrial dysfunction plays a vital role in many human disease because of the important roles of mitochondria in cellular metabolism. DNA mutation, hypoperfusion, and generation of ROS may be key factors in the induction of mitochondrial damage and dysfunction [
In many diseases such as Friedreich’s ataxia, hereditary spastic paraplegia, and Wilson’s disease, genetic defects lead to dysfunction of mitochondrial proteins [
Many seemingly unrelated diseases such as Alzheimer’s disease, Parkinson’s disease, stroke, cardiovascular disease, and diabetes mellitus may be caused by a common factor: ROS [
Comparing with exogenous antioxidants, endogenous antioxidants like
ATP, which contains high-energy phosphate bonds, is produced in mitochondria and the cytosol via glycolysis, substrate-level phosphorylation, and oxidative phosphorylation. With hydrolysis of the phosphate bond, energy is released. Many photoautotrophic and chemoautotrophic bacteria and certain animals use sulfide as an energy substrate. H2S can improve mitochondrial ATP production in SMCs with impaired ATP production, especially following hypoxia [
Under resting conditions, CSE is localized only in the cytosol, but not in the mitochondria of SMCs. Cysteine levels inside mitochondria are approximately three times higher than in the cytosol. However, in response to hypoxia CSE can translocate from the cytosol to mitochondria to confer resistance by increasing ATP synthesis. The promotion of CSE translocation is promoted by increased intracellular calcium levels via the calcium ionophore. Tissue metabolism relying on oxygen supply and oxidative phosphorylation or H2S production is greatly dependent on CSE, such as in vascular SMCs. Therefore, the stimuli for CSE translocation to mitochondria to sustain ATP production under stress conditions may be diverse. Translocation of CSE to mitochondria metabolizes cysteine, produces H2S inside mitochondria, and increases ATP production.
Mitochondria are the major source of oxidative stress. Acute oxidative stress causes serious damage to tissues, and persistent oxidative stress is one of the causes of the aging process and of many common diseases, such as cancer [
Acute human exposure to relatively low concentrations of H2S results in ocular and respiratory mucous membrane irritation leading to nasal congestion, pulmonary edema, and a syndrome known as gas eye, which is characterized by corneal inflammation. Acute human exposure to high concentrations of H2S leads to rapid onset of respiratory paralysis and unconsciousness that can result in death within minutes. Persistent sequelae of H2S poisoning are often related to the olfactory system and may include hyposmia, dysosmia, and phantosmia. In animals, the olfactory system is especially sensitive to H2S inhalation. Acute exposure to moderately high concentrations of H2S in rats resulted in regeneration of the nasal respiratory mucosa and full thickness necrosis of the olfactory mucosa.
The release of cytochrome C into the cytosol is an apoptogenic factor that induces cell death. Dorman et al. evaluated the relationship between the sulfide concentrations and cytochrome oxidase activity in target tissues following acute exposure to sublethal concentrations of inhaled H2S and examined the toxicokinetics of H2S in rats following acute exposure to sublethal concentrations of the gas [
H2S protects neurons from oxidative stress by increasing the levels of GSH, a major intracellular antioxidant [
Since H2S is a reducing substance and cysteine is present in plasma and blood at certain concentrations, H2S may inhibit the reaction of reducing cystine into cysteine in the extracellular space and increase the transmembrane transport of cysteine into cells for GSH production. Increased cysteine transport contributes to a greater extent to the synthesis of GSH. Increased GSH production by H2S is prominent under conditions of oxidative stress caused by glutamate. H2S increases the production of GSH and its redistribution to mitochondria. Also, its production in mitochondria may result in suppressing oxidative stress.
To determine whether the protective effect of H2S is effective, one should not only examine for glutamate toxicity but also for other markers of oxidative stress. In cerebral tissues, glutamate is not solely responsible for producing neuronal damage. The effect of H2O2-induced oxidative stress should not be neglected. H2S recovers the levels of GSH suppressed by H2O2, indicating that H2S protects cells from a range of oxidative stress stimuli. H2S can also reinstate GSH levels in the embryonic brain that have been decreased by ischemia/reperfusion and cystine import suppressed by glutamate.
In summary, H2S increases intracellular GSH concentrations by increasing the transport of cysteine to a greater extent than that of cystine. In addition, H2S increases the redistribution of GSH into mitochondria. Moreover, H2S produced in mitochondria may also contribute to the protection of cells from oxidative stress [
Many studies have shown that the physiological actions of H2S make this gas ideally suited to protect the heart, brain, liver, kidney, and lungs against injury during ischemia/reperfusion (I/R) [
The life of a cell is partly dependent on the degree of mitochondrial functionality. During I/R, mitochondria are subjected to oxygen deprivation, ROS overproduction, and mitochondrial membrane potential depolarization. Mitochondria are central to oxidative phosphorylation and most metabolic processes and are also involved in many aspects of cell death. ROS is one of the major causes of acute and chronic diseases. H2S at high levels can induce a state of hypothermia in mice by inhibiting cytochrome oxidase, which decreases their metabolic rate and core body temperature. This effect of suspension can prevent ischemic damage to cells. During myocardial ischemia, the production of ROS is accelerated and all cellular antioxidants become depleted. H2S is a cytochrome oxidase inhibitor and therefore inhibits respiration. Inhibition of respiration has been shown to decrease the production of ROS. We are only just beginning to understand the role of H2S in I/R injury. In addition, H2S can decrease the production of ROS and preserve mitochondrial function at low concentrations. Therefore, H2S acts to preserve mitochondrial function, thereby imparting cytoprotection. Under physiological conditions, ROSs are generated in cells, and increased ROS levels induce I/R damage in cardiomyocytes. The regulation of ROS levels during I/R is associated with the cardioprotection of H2S by inhibiting oxidative stress [
The mitochondrial respiratory chain is the main source of ROS during energy metabolism. The production of ROS increases during pathological conditions, such as I/R injury to the heart. However, excessive ROSs have a pivotal role in the pathogenesis of myocardial I/R injury [
The activation of KATP channels, which are found in mitochondrial as well as plasmalemmal membranes, contributes to myocardial protection against I/R injury. H2S effects include control of respiratory chain ROS release, control of apoptosis, and promotion of GSH availability in mitochondria (Figure
H2S can improve mitochondrial ATP productions that have impaired ATP production. The enhancement of GSH production by H2S is prominent under conditions of oxidative stress caused by glutamate. H2S increases the production of GSH and its redistribution to mitochondria.
This study was supported by The National Basic Research Program (973 Program) (no. 2010CB 912603) and Fundamental Research Funds for the Central Universities (no. 10FX072).