Hydrogen Sulfide and Mitochondrial Toxicity (2026-04-21)

1 Nicholls et al. 2013 — Sulfide Inhibition of Cytochrome c Oxidase

Full Citation:: Nicholls P, Marshall DC, Cooper CE, Wilson MT. Sulfide inhibition of and metabolism by cytochrome c oxidase. Biochemical Society Transactions. 2013;41(5):1312–1316. DOI:: 10.1042/BST20130070 PMID:: 24059525 Study Design:: Enzyme kinetics (in vitro); mechanistic review of inhibition states Key Findings::

- H2S inhibits mitochondrial Complex IV (cytochrome c oxidase) via the binuclear center (CuB + heme a3) — the same site as cyanide
- Inhibition is biphasic: at least two successive inhibited enzyme states, with partial reversibility if sulfide oxidation clears the active site
- At low concentrations H2S acts as an electron donor to the ETC; at 3–30× higher concentrations it switches to being a Complex IV inhibitor
- IC50 for the related bacterial bo3 terminal oxidase: ~1.1 μM — eukaryotic Complex IV is in a similar range

Conclusion:: H2S is a concentration-dependent reversible inhibitor of Complex IV. The therapeutic/toxicological boundary is at low micromolar levels. Quality:: High — foundational mechanistic biochemistry Certainty:: 0.90 (mechanism well-established; gut-lumen to mitochondria translation uncertain) Limitations:: In vitro enzyme kinetics; human gut H2S concentrations vastly exceed IC50 but most is cleared before reaching mitochondria.

2 Borisov and Forte 2021 — H2S Impact on Mitochondrial and Bacterial Bioenergetics

Full Citation:: Borisov VB, Forte E. Impact of Hydrogen Sulfide on Mitochondrial and Bacterial Bioenergetics. International Journal of Molecular Sciences. 2021;22(23):12688. DOI:: 10.3390/ijms222312688 PMID:: 34884491 Study Design:: Comprehensive review Key Findings::

- Low H2S: enhances FoF1-ATP synthase and lactate dehydrogenase via S-sulfhydration; net ETC stimulation
- Mid/high H2S: inhibits cytochrome c oxidase in both eukaryotes and aerobic bacteria; leads to ATP collapse
- Sulfidogenic bacteria (Desulfovibrio, Fusobacterium) survive their own H2S by expressing cytochrome bd — an alternative terminal oxidase not sensitive to sulfide inhibition
- This differential sensitivity explains why H2S-producing bacteria thrive in conditions that would disable host mitochondria

Conclusion:: The dose-response curve is non-monotonic — beneficial at low μM, toxic at higher μM. Host mitochondria are more vulnerable to H2S than the bacteria producing it. Quality:: High Certainty:: 0.85 Limitations:: Review; precise threshold concentrations for human intestinal epithelial mitochondria not established.

3 Landry et al. 2021 — SQOR: The Mitochondrial H2S Clearance Enzyme

Full Citation:: Landry AP, Ballou DP, Banerjee R. Hydrogen Sulfide Oxidation by Sulfide Quinone Oxidoreductase. ChemBioChem. 2021;22(6):949–960. DOI:: 10.1002/cbic.202000661 PMID:: 33080111 Study Design:: Structural/kinetic characterization; review of SQOR biochemistry Key Findings::

- SQOR (sulfide quinone oxidoreductase) is the committed step in mitochondrial H2S clearance — the primary gatekeeper preventing H2S accumulation
- Couples H2S oxidation to coenzyme Q10 reduction: links H2S detoxification directly to the electron transport chain
- Generates glutathione persulfide (GSSH) as reactive intermediate, feeding into ETHE1 (persulfide dioxygenase) → sulfite oxidase → sulfate
- Active site is structurally flexible — accommodates multiple persulfide acceptors
- CoQ10 deficiency directly impairs SQOR function (SQOR requires CoQ as electron acceptor)

Conclusion:: SQOR is the rate-limiting enzyme in H2S detoxification. Any CoQ10 deficiency will impair this clearance and allow H2S to accumulate. Quality:: High Certainty:: 0.85 Limitations:: Biochemical characterization; SQOR capacity in ME/CFS patients not measured. ME/CFS relevance:: CoQ10 deficiency has been proposed in ME/CFS energy metabolism hypotheses; SQOR impairment would be an indirect consequence.

4 Peng et al. 2025 — SQOR as Metabolic Rheostat of H2S

Full Citation:: Peng MH, Zhang KL, Ma ZW, Zhang HW, Guan SW, Yu HB. SQOR as a metabolic rheostat of H2S: structure, redox homeostasis, and disease therapy. Frontiers in Cell and Developmental Biology. 2025;13. DOI:: 10.3389/fcell.2025.1685252 Study Design:: Comprehensive review (2025) Key Findings::

- SQOR acts as a "metabolic rheostat" — dynamically regulates H2S flux; the body maintains H2S in a narrow steady-state window through SQOR
- SQOR deficiency causes severe metabolic crises; mouse SQOR knockout produces Leigh syndrome-like disease
- Disease contexts where SQOR is implicated: neurological (Leigh), cardiovascular (ischemia-reperfusion), metabolic (diabetic nephropathy), cancer (ferroptosis)
- When SQOR is overwhelmed by excess gut-derived H2S load, spillover inhibits Complex IV in peripheral tissues
- Therapeutic target: SQOR modulation could address H2S-related mitochondrial toxicity

Conclusion:: SQOR is the central molecular checkpoint preventing gut-derived H2S from reaching systemic mitochondria. Its impairment — whether from CoQ10 deficiency, genetic variants, or saturating H2S flux — creates the conditions for systemic mitochondrial inhibition. Quality:: Medium-High (2025 review; recent but comprehensive) Certainty:: 0.70 Limitations:: Review; most disease data from genetic SQOR deficiency, not from excess gut production.

5 Norris et al. 2011 — Liver as Central Regulator of H2S

Full Citation:: Norris EJ, Culberson CR, Narasimhan S, Clemens MG. The liver as a central regulator of hydrogen sulfide. Shock. 2011;36(3):242–250. DOI:: 10.1097/SHK.0b013e3182252ee7 PMID:: 21617578 Study Design:: Experimental — nonrecirculating isolated perfused rat liver Key Findings::

- Liver clears H2S from portal circulation with >97% efficiency at oxygenated conditions, up to 200 μM infusion
- Clearance is oxygen-dependent: removing perfusate O2 drops efficiency from 97% to 23%
- H2S infusion increases hepatic NADH/NAD+ ratio and reduces hepatic O2 availability — creating a positive feedback toward impaired clearance
- Primary oxidation products in liver: sulfate (dominant) and thiosulfate (intermediate)
- Septic livers maintain H2S-metabolizing capacity (no sepsis-specific impairment in this model)

Conclusion:: The liver is the primary first-pass H2S barrier. Any condition reducing hepatic oxygenation will reduce this clearance, allowing portal H2S to reach systemic circulation. Quality:: High — quantified experimental model Certainty:: 0.80 (ex vivo rat; human translation requires confirmation) Limitations:: Animal/ex vivo model. Orthostatic hypoperfusion effects on hepatic H2S clearance are extrapolated, not directly tested. ME/CFS relevance:: Orthostatic intolerance and reduced cardiac output in ME/CFS could reduce hepatic O2 delivery → impaired portal H2S clearance → systemic H2S elevation; this is a speculative but mechanistically coherent hypothesis.

6 Pimenta et al. 2024 — Sulfidogenic Bacteria in Human Disease

Full Citation:: Pimenta AI, Bernardino RM, Pereira IAC. Role of sulfidogenic members of the gut microbiota in human disease. Advances in Microbial Physiology. 2024;85:145–200. DOI:: 10.1016/bs.ampbs.2024.04.003 PMID:: 39059820 Study Design:: Comprehensive review of human disease associations Key Findings::

- Principal sulfidogenic gut bacteria: *Desulfovibrio* spp. and *Bilophila wadsworthia* (the two most clinically significant)
- Western diet (high animal protein, saturated fat, refined sugar) promotes their overgrowth via increased luminal sulfate and taurine
- *B. wadsworthia* shows additional pathogenic properties: epithelial invasion, LPS-mediated inflammation, membrane vesicle production
- At high luminal concentrations, H2S disrupts intestinal barrier integrity and mucus layer → increased permeability
- Disease associations include IBD (Crohn's, UC), colorectal cancer, metabolic syndrome — conditions sharing ME/CFS comorbidity patterns

Conclusion:: Sulfidogenic bacteria are a mechanistically distinct class of gut pathogens whose pathogenicity is mediated primarily through H2S and barrier disruption. Quality:: High Certainty:: 0.75 for IBD/CRC associations; speculative extrapolation to ME/CFS Limitations:: No ME/CFS-specific data. ME/CFS microbiome studies have not specifically measured sulfidogenic taxa.

7 Qi et al. 2024 — Desulfovibrio H2S Suppresses GLP-1 via Mitochondrial Inhibition

Full Citation:: Qi Q, Zhang H, Jin Z, et al. Hydrogen sulfide produced by the gut microbiota impairs host metabolism via reducing GLP-1 levels in male mice. Nature Metabolism. 2024;6(8):1601–1615. DOI:: 10.1038/s42255-024-01068-x PMID:: 39030389 Study Design:: Murine in vivo model + human metabolic syndrome correlation Key Findings::

- *Desulfovibrio* (enriched in human metabolic syndrome patients) produces H2S that inhibits mitochondrial respiration in intestinal L cells
- Mitochondrial inhibition in L cells induces the unfolded protein response (UPR) → suppresses GLP-1 secretion
- Bismuth subsalicylate (BSS) chelates luminal H2S → rescues GLP-1 production and ameliorates diet-induced metabolic disorder in male mice
- Provides direct causal evidence of the gut H2S → mitochondrial inhibition → metabolic dysfunction axis
- Human correlation: *Desulfovibrio* abundance tracks with GLP-1 suppression in metabolic syndrome patients

Conclusion:: This is the most direct experimental evidence that gut-derived H2S from a named sulfidogenic organism inhibits host mitochondria with measurable metabolic consequences. Bismuth subsalicylate provides mechanistic proof-of-concept for luminal H2S targeting as a therapeutic approach. Quality:: High — Nature Metabolism, experimental with human correlation Certainty:: 0.75 (murine model; GLP-1 mechanism established; ME/CFS extrapolation speculative) Limitations:: Male mice only; metabolic syndrome, not ME/CFS; exact H2S concentrations at mitochondria not measured.

8 Villanueva-Millan et al. 2022 — H2S Producers Guide IBS Subtypes (Human)

Full Citation:: Villanueva-Millan MJ, Leite G, Wang J, Morales W, et al. Methanogens and Hydrogen Sulfide Producing Bacteria Guide Distinct Gut Microbe Profiles and Irritable Bowel Syndrome Subtypes. American Journal of Gastroenterology. 2022;117(12):2055–2066. DOI:: 10.14309/ajg.0000000000001997 PMID:: 36114762 Study Design:: Human cross-sectional; IBS cohort with 3-gas breath testing + 16S microbiome sequencing (Cedars-Sinai) Key Findings::

- IBS-D subjects: elevated breath H2S and H2; higher relative abundance of *Fusobacterium* and *Desulfovibrio* spp.
- IBS-C subjects: elevated breath methane; higher *Methanobrevibacter smithii*
- Distinct microbial pathway enrichment: H2S producers → sulfate reduction pathways; methanogens → methanogenesis pathways
- Lower gut microbial diversity in IBS-D vs IBS-C — consistent with sulfidogenic overgrowth
- Confirms that breath gas pattern from Trio-Smart-type testing reflects actual underlying microbiome

Conclusion:: Breath H2S is a reliable proxy for sulfidogenic dysbiosis in IBS. H2S-producing bacteria are causally implicated in IBS-D phenotype, not just correlated. Quality:: High Certainty:: 0.80 Limitations:: IBS patients, not ME/CFS; cross-sectional; exact n not provided in abstract.

9 Villanueva-Millan et al. 2024 — H2S Producers Drive Diarrhea Phenotype in Rats

Full Citation:: Villanueva-Millan MJ, Leite G, Morales W, Sanchez M, Parodi G, Weitsman S, et al. Hydrogen Sulfide Producers Drive a Diarrhea-Like Phenotype and a Methane Producer Drives a Constipation-Like Phenotype in Animal Models. Digestive Diseases and Sciences. 2024;69(2):426–436. DOI:: 10.1007/s10620-023-08197-5 PMID:: 38060167 Study Design:: Animal model (Sprague-Dawley rats); bacterial gavage experiment Key Findings::

- Gavage with *Desulfovibrio piger* or *Fusobacterium varium* → increased stool wet weight (diarrhea phenotype) + increased stool H2S production
- Gavage with *Methanobrevibacter smithii* → constipation phenotype (decreased stool wet weight)
- Establishes causality: these organisms are sufficient to produce their associated IBS subtype phenotype in otherwise normal animals
- H2S production co-varies with diarrhea severity — mechanism likely involves mucosal H2S effects on colonic fluid secretion and transit

Conclusion:: Specific sulfidogenic organisms produce measurable H2S and drive IBS-D phenotype causally in animal models. This supports targeted approaches (antimicrobials, dietary sulfur restriction, H2S chelation) for IBS-D. Quality:: High — causal animal model Certainty:: 0.80 (animal; human translation reasonable) Limitations:: Rat model; single-species gavage may not reflect mixed-flora dynamics; short intervention period.

10 Ye et al. 2025 — Reduced Sulfur Diet Reshapes Microbiome in Ulcerative Colitis

Full Citation:: Ye Z, Raman M, Taylor L, Yousuf M, Panaccione R, et al. Reduced Sulfur Diet Reshapes the Microbiome and Metabolome in Mild–Moderate Ulcerative Colitis. International Journal of Molecular Sciences. 2025;26(10):4596. DOI:: 10.3390/ijms26104596 PMID:: 40429741 Study Design:: Open-label RCT pilot; n=9 reduced-sulfur diet vs n=13 controls; 8 weeks; mild-moderate UC Key Findings::

- Sulfur intake reduced from 411 mg/day to 172 mg/day (−58%) in RS group
- Significant increase in gut microbial alpha diversity and Shannon index in RS group
- Decreased pathobiont *Eggerthella lenta*; increased *Faecalibacterium prausnitzii* (anti-inflammatory)
- Reduced LPS-binding protein (−5,280 ng/mL) suggesting decreased intestinal permeability
- Increased anti-inflammatory metabolite indoleacetyl glutamine
- 43% of RS group showed ≥50% fecal calprotectin reduction vs 25% in controls

Conclusion:: Dietary sulfur restriction is feasible, measurable, and produces beneficial microbiome and metabolome shifts. Provides rationale for trials in other sulfidogenic-dysbiosis conditions. Quality:: Medium — pilot RCT, small n, open-label Certainty:: 0.55 (small n; UC, not ME/CFS) Limitations:: n=22 total; open-label; UC population; 8-week duration; no sham control. ME/CFS extrapolation is speculative.

11 Paul et al. 2021 — H2S and Mitochondria: Biphasic Effects (Nuanced/Negative)

Full Citation:: Paul BD, Snyder SH, Kashfi K. Effects of hydrogen sulfide on mitochondrial function and cellular bioenergetics. Redox Biology. 2021;38:101772. DOI:: 10.1016/j.redox.2020.101772 PMID:: 33137711 Study Design:: Review with experimental literature synthesis Key Findings::

- H2S exhibits beneficial mitochondrial effects at low concentrations: cytoprotective, anti-apoptotic, anti-inflammatory via S-sulfhydration
- At high concentrations: cytotoxic via Complex IV inhibition and ATP collapse
- *Caution against oversimplification:* context (hypoxia, acidosis, cell type) shifts the beneficial/toxic boundary — the same concentration may be protective in one cell type and toxic in another
- Endogenous H2S overproduction (CBS/3-MST overexpression in Down syndrome) impairs mitochondria — demonstrating that even endogenous H2S excess is pathological
- "A complete picture of the impact of H2S on bioenergetics is lacking" — authors explicitly caution

Conclusion:: H2S is not uniformly toxic. Framing it as a simple poison ignores its essential roles as a gasotransmitter. For ME/CFS integration, the document should present H2S as a concentration-dependent double-edged molecule, not frame gut H2S as straightforwardly harmful. Quality:: High Certainty:: 0.85 as nuanced/critical finding Limitations:: Review; precise threshold concentrations in human gut and portal circulation not established.