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. Author manuscript; available in PMC: 2016 Aug 1.
Published in final edited form as: Exp Gerontol. 2014 Dec 16;68:26–32. doi: 10.1016/j.exger.2014.12.010

Calorie restriction and methionine restriction in control of endogenous hydrogen sulfide production by the transsulfuration pathway

Christopher Hine 1, James R Mitchell 1,*
PMCID: PMC4464900  NIHMSID: NIHMS652096  PMID: 25523462

Abstract

H2S is a gas easily identified by its distinctive odor. Although environmental exposure to H2S has been viewed alternately as therapeutic or toxic through the centuries, H2S has recently regained recognition for its numerous beneficial biological effects. Most experiments documenting such benefits, including improved glucose tolerance, increased stress resistance, and even lifespan extension, are based on exposure of experimental organisms to exogenous sources of H2S. However, appreciation is growing for the importance of H2S produced endogenously by the evolutionary conserved transsulfuration pathway (TSP) in health and longevity. Recent data implicate H2S produced by the TSP in pleiotropic benefits of dietary restriction (DR), or reduced nutrient/energy intake without malnutrition. DR, best known as the most reliable way to extend lifespan in a wide range of experimental organisms, includes various regimens aimed at either reducing overall calorie intake (calorie restriction, intermittent/ every-other-day fasting) or reducing particular nutrients such as protein or the essential amino acid, methionine (methionine restriction), with overlapping functional benefits on stress resistance, metabolic fitness and lifespan. Here we will review the small but growing body of literature linking the TSP to the functional benefits of DR in part through the production of endogenous H2S, with an emphasis on regulation of the TSP and H2S production by diet and mechanisms of beneficial H2S action.

Keywords: Dietary restriction, Methionine, Transsulfuration, Hydrogen sulfide, Aging, Stress, Metabolism

1. Introduction

1.1. Dietary restriction

Dietary restriction (DR) encompasses a variety of regimens characterized by nutrient and/or energy restriction leading to generally bene-ficial, but reversible, adaptive changes on the organismal level. Because DR-related nomenclature is poorly defined, we will refer to regimens involving restriction of total food intake as calorie restriction (CR) in order to contrast them with regimens in which individual macronutrients such as protein, or specific amino acids such as methionine, are restricted (MetR) without enforced food restriction. CR and MetR result in overlapping phenotypes and associated benefits in multiple organisms, and as described below, may also share similar underlying mechanisms of benefits.

1.1.1. Calorie restriction

CR was originally identified as a lifespan extending regimen in rodents, and has been used as an experimental tool for nearly a century with which to study underlying mechanisms of the aging process. Functionally, CR regimens are diverse and organism specific; for example, in yeast reducing glucose in the media from 2% to 0.5% extends chronological and replicative lifespan, while in rodents daily restriction of food by 40% or alternating days of fasting and ad libitum feeding (every-other-day fasting) represent two extremes of CR regimens leading to longevity extension, improved metabolic fitness and multiple stress resistance (Anson et al., 2005). In recent decades, evolutionarily conserved pathways involved in nutrient and energy sensing, including insulin/IGF signaling, mTOR, AMPK, sirtuins, and GCN2 have been implicated in regulation of aging by CR, and associated benefits including stress resistance (Fontana et al., 2010). Because CR benefits are gained and lost rapidly upon the change from ad libitum to restricted feeding and vice versa, underlying mechanisms most likely involve adaptive changes linked to nutrient/energy restriction signal transduction pathways and downstream transcription factors including FOXO (Greer et al., 2007), NRF2 (Bishop and Guarente, 2007; Pearson et al., 2008), CREB (Mair et al., 2011) and ATF4 (Li et al., 2014). Nonetheless, evolutionarily conserved molecular requirements downstream of such transcriptional changes remain largely unresolved.

1.1.2. Methionine restriction

MetR also extends lifespan and stress resistance in yeast (Johnson and Johnson, 2014; Ruckenstuhl et al., 2014; Wu et al., 2013), flies (Troen et al., 2007), worms (Cabreiro et al., 2013) and rodents (Miller et al., 2005; Orentreich et al., 1993). In humans, it has been used to compliment cancer treatment (Thivat et al., 2007) and to improve metabolic fitness (Lees et al., 2014; Plaisance et al., 2011). Furthermore, MetR in yeast can be phenocopied by genetic manipulation of methionine bio-synthetic pathways Met15/17/25 or Met2 and shMTR in human and mouse cells (Fig. 1), imparting multiple stress resistance phenotypes (Johnson and Johnson, 2014). In mammals, MetR benefits actually require combined methionine and cysteine restriction (Elshorbagy et al., 2013), and thus could be more accurately referred to as sulfur amino acid (SAA) restriction. Because MetR regimens in rodents are given on an ad libitum basis without enforced restriction of calorie intake, it is currently unclear to what degree SAA restriction and CR share underlying molecular mechanisms of protection despite clear phenotypic overlap (Lopez-Torres and Barja, 2008). Evidence in favor of mechanistic overlap comes from flies, in which CR-mediated lifespan extension can be specifically abrogated by essential amino acids (EAA) including Met, but not EAA lacking Met (Grandison et al., 2009); and in mice, where SAA abrogate benefits of stress resistance (Hine et al., in press) as discussed further below.

Fig. 1.

Fig. 1

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Pathways of H2S production. Model of the transmethylation and transsulfuration pathway (TSP) in metazoans (left) and in yeast (right). Solid arrows trace canonical sulfur transfer from Met to Cys (in metazoans), or from inorganic sulfate and/or Met to Cys (in yeast), through various metabolites and downstream cellular processes via the enzymes Cystathionine Beta-Synthase (CBS;STR4) and Cystathionine Gamma-Lyase (CGL;STR1). Dotted arrows trace alternative pathways/usage of transmethylation products or TSP enzymes for production of H2S. MAT: methionine adenosyl transferase, SAM: S-adenosylmethionine, SAH: S-adenosylhomocysteine, SAHH: S-adenosylhomocysteine hydrolase, and MS: methionine synthase.

1.2. Hydrogen sulfide

H2S gas is released into the environment from inorganic sources or produced by sulfate-reducing bacteria, and is thus found in varying concentrations from different sources including well-water, thermal baths and volcanoes. When present in high concentrations, H2S blocks respiration by inhibiting cytochrome c oxidase and interfering with iron-dependent biochemical reactions. However, as is typical of hormetic compounds that are toxic at high doses, at lower doses H2S has a number of beneficial effects, probably via a variety of different mechanisms (discussed below). Interestingly, although H2S was in vogue for centuries past as a cure-all (Forster, 1994), it is currently viewed by environmental/regulatory bodies as hazardous with little to no acceptable level of exposure (WHO, 2003). Nonetheless, in biology and medicine, interest in H2S has entered a renaissance since the recognition that it acts as a vasodilator, similar to nitric oxide (NO) and carbon monoxide (CO), and has a number of other benefits in health and medicine (Zhang et al., 2013).

1.2.1. Exogenous hydrogen sulfide benefits

Exposure to exogenous H2S can induce a state of suspended animation in rats, allowing them to survive hypoxia over six hours without irreversible effects (Blackstone et al., 2005; Blackstone and Roth, 2007). H2S can also protect against global ischemia associated with severe blood loss (Morrison et al., 2008). In yeast and worms, H2S significantly extends median lifespan (Hine et al., in press; Miller and Roth, 2007). Furthermore, H2S is beneficial against metabolic syndrome (Xue et al., 2013), cancer (Lee et al., 2011, 2014), neurodegeneration (Kida et al., 2011), and multiple stress resistance, including heat shock in worms (Miller et al., 2011) and oxidative stress and hypoxia in mammalian cells (Hine et al., in press; Wen et al., 2013).

1.2.2. Endogenous hydrogen sulfide benefits

H2S is also produced endogenously by organisms including mammals via the transsulfuration pathway (TSP) via the tandem enzymatic activity of CBS and CGL (Kimura, 2011). The importance of endogenous H2S production was first demonstrated in mice lacking CGL, the final enzyme in the Cys biosynthesis pathway (Fig. 1). Under dietary conditions with adequate levels of Cys, KO mice still suffer from high blood pressure due to the lack of endogenous H2S (Yang et al., 2008). H2S production by CGL is also implicated in protection from a growing list of maladies including neurodegeneration associated with Huntington's disease (Paul et al., 2014), atherosclerosis (Mani et al., 2013) and type 1 diabetes (Manna et al., 2014). Because defects in TSP enzymes preventing H2S production also affect other metabolites in the pathway, leading in some cases to cystathionuria or homocysteinuria (Zhu et al., 2008), or changes in downstream metabolites GSH and taurine, the causative role of H2S depletion in these disorders is strongly suspected but remains to be proven.

1.3. Linking DR and H2S

Recently, we reported a link between endogenous H2S by the TSP and multiple DR benefits, including stress resistance in rodents and longevity extension in yeast, flies and worms (Hine et al., in press). While the requirement for a functional TSP in DR-mediated longevity was previously shown in flies (Kabil et al., 2011), this was the first attempt to elucidate the specific role of the TSP metabolite H2S in this process. In this study, we made use of the fact that 50% CR offers robust protection against the multifactorial stress of hepatic ischemia reperfusion injury to elucidate first dietary, and then molecular, mechanisms of protection. We found that protein restriction (PR) lent equal protection to 50% CR independent of calorie intake, and that adjusting the concentration of SAA, or just Cys, back to ad libitum levels significantly reduced CR- or PR-mediated protection. Together, these data implicate reduced Cys as the major trigger of protection in this model of DR-mediated stress resistance, prompting us to consider alterations in TSP activity and associated metabolites.

To uncover relevant molecular mechanisms underlying protection, we used a genetic model that fails to gain DR-mediated protection against hepatic ischemic injury due to hepatocyte-specific ablation of the mTORC1 repressor TSC1, and hence constitutively active mTORC1 in the liver (Harputlugil et al., 2014). Comparative analysis of TSP gene and protein expression and metabolomics profiling were strongly suggestive of increased H2S production upon DR as the key diet/genotype interaction leading to stress resistance. To test this, we demonstrated the sufficiency of exogenous H2S for protection, and that genetic or pharmacological inhibition of CGL abrogated DR-mediated protection, while CGL overexpression induced H2S production capacity and stress resistance independent of diet (Hine et al., in press). We also found that species-specific DR regimens associated with extended longevity and stress resistance in mice (MetR, every-other-day fasting), flies (optimized protein/methionine levels), worms (eat-2 mutant) and yeast (glucose restriction) resulted in increased H2S production capacity. Finally, overexpression of one particular TSP gene in worms, CBS-1, increased longevity independent of diet, while knocking down the same gene abrogated DR-mediated longevity extension.

Based on these data, we proposed that increased TSP activity is an evolutionarily conserved response to multiple DR regimens, including CR, PR and MetR, and that increased H2S production under these conditions represents a common molecular mechanism underlying multiple DR benefits. In the following sections, we review recent findings regarding the regulation of TSP activity and H2S production, and potential molecular mechanisms underlying H2S benefits.

2. Regulation of transsulfuration pathway gene expression and activity

2.1. Dietary SAA requirements

Because most metazoans cannot reduce inorganic sulfur, sulfur-containing amino acids (SAA) methionine (Met) and cysteine (Cys) must be consumed in the diet. Met is of critical importance not only as the first amino acid incorporated into the growing polypeptide chain during protein synthesis, but with its X-CH3 moiety is also the source of all one-carbon transfer reactions. In addition to its essential role in protein synthesis, Cys is also critical for glutathione production and thus redox homeostasis. Because Cys can be synthesized de novo from Met by the TSP (Fig. 1), Cys is nonessential unless Met is limiting. If dietary intake of both Met and Cys is low, then Cys also becomes an essential amino acid. Currently, the recommended daily intake of SAAs for humans independent of age or sex is approximately 14–20 mg/kg of body weight a day, with Met recommendations accounting for 10–19 mg/kg of body weight a day. These numbers are not universally agreed upon, especially in light of the lack of age and/or health dependent requirements (Fukagawa, 2013; Nimni et al., 2007). Nonetheless, long-term, severe deficiencies of SAAs can lead to infertility, embryonic developmental defects, reduced growth and bone strength, liver damage and death, while overconsumption of SAAs is conversely associated with metabolic syndrome.

2.2. Cys biosynthesis via the transsulfuration pathway

Cys can be produced from Met via a step-wise enzymatic process involving the transmethylation pathway (TMP) and TSP (Fig. 1) when dietary Cys intake is low. Two key enzymes of the TSP are cystathionine β-synthase (CBS) and cystathionine γ-lyase (CGL). Under normal physiological conditions, CBS converts serine and homocysteine, a product of Met methyl transfer, into cystathionine. Cystathionine is then converted into α-ketobutyrate and Cys by CGL. In mice lacking CGL, Cys is an essential amino acid, and mice die within 3 weeks of 50% cysteine restriction (Ishii et al., 2010). Mice lacking CBS, on the other hand, have a severe phenotype even when fed a normal diet (Gupta et al., 2009).

2.3. Dietary regulation of TSP expression

In mammals, TSP genes are upregulated on the transcriptional level in response to a lack of dietary Cys, consistent with a primary role in Cys biosynthesis (Lee et al., 2008). Deprivation of even a single amino acid can activate the amino acid starvation response kinase GCN2, resulting in phosphorylation of its only known target, eIF2α, and stabilization of transcription factors such as ATF4. The amino acid starvation response is activated in mouse liver upon PR and SAA restriction (Sikalidis and Stipanuk, 2010). ATF4 has been implicated in the regulation of CGL, as cells lacking this factor must be supplemented with Cys (Harding et al., 2003). Interestingly, ATF4 is increased in mouse liver upon CR and MetR, but also in other models of slow aging, including hypopituitary dwarfism, independent of diet (Li et al., 2014).

In addition to GCN2, eIF2α can be phosphorylated on the same site by additional kinases such as PERK in response to oxidative or endoplasmic reticulum stress as part of the integrated stress response. Thus, whether ATF4 stabilization and CGL transcription upon Cys deprivation is due to the amino acid starvation response and GCN2, or redox stress and PERK, remains unclear.

The general transcription factor SP1 has also been implicated in promoting CGL expression (Paul et al., 2014; Yin et al., 2012). CGL is under negative regulation by glucocorticoids (Zhu et al., 2010) and anterior pituitary hormones lacking in long-lived Snell (Vitvitsky et al., 2013) and Ames (Uthus and Brown-Borg, 2006) dwarf mice. On the mRNA and protein level, CGL is increased upon 50% CR in mouse liver, and this increase in mRNA can be abrogated by specific add back of both SAAs or just Cys (Hine et al., in press). Additionally, the diet responsive non-coding micro RNA (miRNA) 21 and miRNA 30 family members epigenetically repress CGL expression and subsequent H2S production (Khanna et al., 2011; Shen et al., 2014; Yang et al., 2012).

2.4. Regulation of H2S production by TSP

The canonical product of the TSP is Cys, an amino acid used in protein synthesis. However, Cys is also integral for production of glutathione (GSH) (Lyons et al., 2000), taurine (Stipanuk, 2004) and H2S (Kimura, 2011). Of these, H2S is the most challenging to measure accurately because of its relatively short biological half-life (Wang, 2009). While the capacity of CGL to produce H2S can be ascertained fairly easily in organ extracts or live cells by providing them with an excess of substrate (Cys) and cofactor (vitamin B6), the measurement of endogenously produced levels of this short-lived gas species is much more challenging. Using techniques such as gas chromatography, sulfide probes, and differential extraction methods, a number of issues arise due to sample preparation and oxidation, the release of endogenous and labile sulfides, sensitivity and specificity for H2S, giving rise to differences of opinion on the actual levels in vivo (Olson, 2009).

Whatever the actual levels, the fact that H2S is produced by the same enzymes that synthesize its substrate, Cys, presents an apparent paradox: how can H2S levels be increased when its production depends on a substrate whose limitation increases the responsible enzymes in the first place? Although the answer isn't known, several non-mutually exclusive possibilities exist. First, Cys isn't the only substrate of H2S production. TSP enzymes CBS and CGL are rather promiscuous, and can catabolize cysteine, cystathionine, and homocysteine to produce H2S. A second possibility is based on localization of the enzyme CGL to the mitochondria, where local free Cys concentrations are high. A recent report indicates that CGL localizes to mitochondria upon conditions of ER stress via the translocase, Tom20 (Fu et al., 2012). A third possibility is that the source of free Cys for H2S production comes not from de novo Cys via the TSP, but from autophagy. Autophagy is expected to be upregulated under conditions of nutrient/energy limitation as in DR and PR/ MetR. In support of this latter possibility, extended chronological longevity upon MetR in yeast requires autophagy (Ruckenstuhl et al., 2014). Interestingly, in long-lived Ames dwarfs with upregulated TMP and TSP enzyme activity, the flux of Met through these pathways is higher (Uthus and Brown-Borg, 2006; Vitvitsky et al., 2013). Inevitably, measurements of metabolite flux combined with accurate quantitation of H2S will be required to determine the substrate of H2S in vivo under conditions of CR or PR/MetR.

In addition to sulfur-containing substrates, both CBS and CGL require the cofactor vitamin B6 for transsulfuration activities including H2S production. Vitamin B6 from the diet is thus also relevant, as mutations in CGL that result in cystathioninuria affect binding of the cofactor to the enzyme and decrease its ability to synthesize H2S (Zhu et al., 2008). Finally, in addition to causing relocalization of TSP enzymes, oxidative stress stimulates CGL mRNA, protein and enzyme activity during the fetal to neonatal transition in rats. It remains to be seen if this results in increased H2S production as well (Martin et al., 2007). H2S may serve as a redox sensor in blood vessels, as hypoxia and H2S have the same effects in multiple vessel types (Olson et al., 2006). In conclusion, H2S production by the TSP can be influenced by the availability of substrate and enzyme cofactors, both of which are influenced by diet, as well as environmental conditions such as oxidative stress or endoplasmic reticulum stress that can influence subcellular localization and TSP enzyme activity levels.

2.5. Other pathways of H2S production

In addition to the TSP, the enzyme 3MST in mammals can produce H2S. However, the relevance of DR, PR or MetR to 3MST is unclear. In yeast, H2S production is a major problem in wine-making, as it interferes with the smell and taste of the final product. Thus, there is considerable interest in strains that produce less H2S. Interestingly, yeast are thought to produce most of their H2S not by the TSP pathway, but by the assimilatory pathway that converts inorganic sulfate to sulfite, sulfide and then eventually into the organic homocysteine (Fig. 1). In the absence of this pathway, yeast cannot fix sulfur and are thus dependent on the addition of Met to the media for proper growth. They also produce considerably less H2S under normal growth conditions. However, we found that glucose restriction, which extends chronological longevity, also increases H2S production, and that this did not depend on the assimilatory pathway, strongly suggesting activity of the TSP as the source for H2S. Interestingly, genetic mutants in the sulfate assimila-tory pathway, including Met2 and/or Met15/17/25 (Fig. 1), produce an abundant amount of H2S and have increased chronological lifespans (Johnson and Johnson, 2014).

3. Mechanisms of H2S benefits

Like the other gasotransmitters NO and CO, H2S is thought to act predominantly as a signaling molecule. H2S can readily diffuse across membranes, and the production from one cell is thought to reach within a 200 cell radius of its origin (Cuevasanta et al., 2012). The major molecular mechanism of H2S action is thought to be sulfhydration, or the formation of –SSH moieties, on surface-exposed Cys and thiol residues of proteins. This posttranslational modification can have a number of different effects on target protein function. Because exposed Cys residues are subject to nitrosylation, sulfhydration can compete for these moieties and affect protein function. For example, sulfhydration of Cys 38 of NFkB subunit p65 competes for nitrosylation and regulates the antiapoptotic activity of this transcription factor (Sen et al., 2012). Another target of H2S with potential in health and longevity is the E3 ubiquitin ligase, parkin, whose dysfunction is implicated in aging-related neurodegeneration associated with Parkinson's disease. While nitrosylation of parkin inhibits protein activity, sulfhydration of specific Cys residues increases catalytic activity, thus potentially contributing to neuroprotection (Vandiver et al., 2013).

H2S also has the ability to activate and/or open transmembrane receptors and channels via sulfhydration. As part of its ability to stimulate vascular angiogenesis, H2S reduces a disulfide bond in the VEGFR2 cytoplasmic domain, changing the physical conformation of the receptor into the active kinase state (Tao et al., 2013). The role of H2S as a vasodilator is based on its ability to sulfhydrate the Kir6.2 regulatory subunit of the KATP channel in endothelial cells, leading to membrane hyperpolarization and vessel relaxation (Mustafa et al., 2011). Mutations in CGL or in KATP channel sulfhydration sites abolish the ability to relax, resulting in hypertension. Sulfhydration of KATP channels in insulin-secreting cells of the pancreas may also influence the ability of H2S to regulate insulin secretion (Yang et al., 2005). Additionally, H2S produced by the TSP in bone marrow mesenchymal stem cells (BMMSCs) regulates their self-renewal and osteogenic differentiation by maintaining proper calcium flux via sulfhydration of Cys residues on multiple calcium transient receptor potential channels (TRP) channels (Liu et al., 2014). Thus, either by allosteric interaction, prevention of intra-molecular or intermolecular disulfide bond formation, structural changes, or competition for other potential modifications such as nitrosylation at the same residue, sulfhydration can serve to alter protein and cellular functions.

Sulfhydration can also serve a different purpose in energy metabolism. The mitochondrial protein SQR, an evolutionarily conserved component of the mitochondrial electron transport chain (ETC) functionally overlapping with Complex 2, has two exposed Cys residues that become sulfhydrated sequentially by H2S. Because SQR can donate electrons to Coenzyme Q, each sulfhydration results in the donation of electrons to the ETC with the potential to generate ATP. Reoxidation of the two Cys residues occurs with the concerted action of a number of mitochondrial proteins and results in the formation of thiosulfate, which can be exported and secreted in the urine, or under some condition such as hypoxia can itself be converted to H2S (Olson et al., 2013). In this context, H2S can thus be thought of as an inorganic energy source. While this may be extremely useful during periods of hypoxia, whether or not this can contribute significantly during periods of nutrient/energy deprivation, as in DR, remains to be determined. Furthermore, whether mitochondrial H2S production requires CGL or the non-TSP component 3MST remains unclear (Modis et al., 2013). 3MST in the brain may be a major contributor to H2S in that organ (Shibuya et al., 2009).

Finally, H2S has the capacity to work directly as an antioxidant, protecting various cell types against hydrogen peroxide induced oxidative stress in HUVECs (Wen et al., 2013) and primary mouse endothelial cells (Hine et al., in press).

4. Conclusions

DR regimens, whether through restriction of overall calories or limitation of essential nutrient such as protein or SAAs, clearly have numerous health benefits. Similarly, H2S from either exogenous sources or produced endogenously imparts benefits that overlap with DR (Fig. 2). Recently, we proposed that upregulation of the TSP and increased H2S are causative of at least some of these benefits, including stress resistance and extended longevity (Hine et al., in press). As the biological effects of both DR and H2S are pleiotropic, much remains to be done in order to determine the extent to which DR benefits depend on H2S, as well as the molecular mechanisms underlying these effects. For example, H2S production is not mutually exclusive with other consequences of increased TSP activity, including biosynthesis of downstream metabolites GSH and taurine. As we learn more about the potential of nutrient/energy sensors and downstream transcription factors already implicated in evolutionarily conserved DR benefits to regulate the TSP and H2S production, the specific contributions of this gas as a downstream mediator of DR benefits should come into focus.

Fig. 2.

Fig. 2

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Overlapping benefits of DR and H2S. Shared benefits of reduced intake of SAA common to various DR regimens including CR, PR and SAA restriction, and H2S from endogenous or exogenous sources.

How could sulfhydration of target proteins such as NFkB, KATP channels or VEGFR2, or electron donation to the mitochondrial ETC, confer benefits of DR? Obviously much remains to be learned about the protein targets of sulfhydration, particularly by endogenously produced H2S, and how this affects individual protein function and physiology on the organism level. Nonetheless, changes in energy metabolism, mitochondrial biogenesis, ROS generation, insulin sensitivity, cardiovascular health and inflammatory capacity have all been implicated in DR benefits and can also be modulated by H2S.

Resolving the potential overlap between CR and PR/MetR to DR benefits, and the contribution of H2S to each, is of great interest and importance in both basic and clinical scientific realms. Identification of convergent points will undoubtedly lead to a better molecular understanding of this fascinating phenomenon. For the purposes of clinical translation, however, each beneficial treatment has different advantages and disadvantages. Relative to CR and the associated difficulties of reduced overall food intake, temporary or long lasting restriction of SAA intake, common in vegan diets already adopted by millions of people throughout the world (McCarty et al., 2009), may be more appealing to people wishing to obtain said benefits for various clinical applications or life-long improved health. For other purposes, H2S supplementation may be easier, but whether exogenously added H2S can safely recapitulate benefits of endogenously produced H2S remains to be fully explored. While improved overall health is a worthy long-term goal, immediate impact on planned stressful events such as elective surgery or chemotherapy requiring only a short intervention would appear to be the low-hanging fruit with the most immediate potential for clinical translation.

Acknowledgments

This work was supported by grants from NIH (DK090629, AG036712) and the Glenn Foundation for Medical Research to JRM; CH was supported by T32CA0093823.

Footnotes

The authors claim no conflicts of interest.

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