Hydrogen Sulphide Gasotransmitter: Synthesis Pathways and Signalling Mechanisms
H2S (hydrogen sulphide; the third gasotransmitter after NO and CO; colourless; odour threshold 0.5 ppb; endogenous concentrations: plasma 10–100 µM; tissue ~100–300 µM; toxic at high concentrations (Complex IV inhibitor; >3 mM); physiological at <1 mM; signalling at nM–µM range); enzymatic H2S synthesis: (1) CBS (cystathionine β-synthase; cytoplasmic/ER; B6-PLP cofactor; dimers-tetramers; catalyses: Hcy + Ser → cystathionine (β-replacement); Cys + Hcy → lanthionine + H2S (β-elimination); primary in CNS/liver; allosteric: SAM activates CBS CBS domain; CO inhibits (haem Fe2+-CO)); (2) CSE (cystathionine γ-lyase; cytoplasmic; B6-PLP; dimers; catalyses: cystathionine → Cys + α-KB (primary; transsulfuration); Cys + Cys → lanthionine + H2S; Cys → pyruvate + H2S + NH3; primary in vascular/cardiac/liver; NO induces CSE → H2S in vascular smooth muscle); (3) 3-MST/CAT (3-mercaptopyruvate sulphurtransferase; mitochondrial + cytoplasmic; requires α-ketoglutarate; Cys + α-KG → CAT (cysteine aminotransferase) → 3-mercaptopyruvate (3-MP) → 3-MST → H2S; sensitive to reducing agents (DTT/TRX activation); mitochondria: primary source of mitochondrial H2S); non-enzymatic: thiosulphates + reducing agents; H2S chemistry: fast oxidation in aerobic conditions (H2S → sulphite → sulphate → thiosulphate); persulfidation (S-sulphhydration): H2S + protein-Cys-SH (thiolate) → protein-Cys-SSH (persulphide; more nucleophilic than thiol; reactive with electrophiles; protective); vs. sulphenylation (H2O2 + Cys-SH → Cys-SOH).
Spirulina Mechanisms in H2S Signalling Biology
CBS/CSE Substrate and Cofactor Support
CBS (B6-PLP cofactor; Ser280-PLP aldimine; PLP-Lys residue covalently bonded; PLP deficiency → CBS catalytic velocity ↓ → Hcy accumulation + H2S ↓; B6-deficient subjects: CBS activity −30–50%): spirulina B6 provision (~0.1–0.3 mg/100g pyridoxine; ~0.3–0.8 mg/100g total as pyridoxine/pyridoxal; 10g spirulina ≈ 30–80 µg B6; modest contribution; additive to dietary B6). CSE (B6-PLP cofactor; same requirement; also: NO-induced: eNOS-NO → CSE ser-nitrosylation Cys254 → CSE activation → vascular H2S; Nrf2/ARE: CSE has ARE element confirmed): (1) Spirulina eNOS-NO (+10–20%; AMPK/Akt Ser1177) → CSE activation → vascular H2S +10–20%; (2) Nrf2 → CSE ARE expression +10–20% (independent of B6; PCB-Keap1-Nrf2-CSE axis); (3) Cys provision (spirulina protein total sulphur AA ~2.8g/100g; Cys ~0.7g/100g; direct CSE substrate); (4) SAM pool (spirulina choline/betaine/Met → SAM → CBS allosteric activator → CBS conformational shift from autoinhibited → active); (5) Hcy attenuation (CBS/CSE transsulfuration flux → Hcy ↓ → less Hcy competitive CBS substrate inhibition). Net: H2S production +10–20% (combined B6/Cys/eNOS-CSE/Nrf2-CSE effects; hepatic/vascular/CNS compartments).
Persulfidation: Keap1/NF-κB/NLRP3 H2S Modification
Protein persulfidation (S-sulphhydration; H2S/polysulphide + reactive Cys → Cys-SSH; stable in reduced environments; reduces via TRX; persulfidation proteome: ~10–25% of Cys-containing proteins; mostly activating modification (more nucleophilic than Cys-SH); contrast with sulphenylation (H2O2 + Cys-SH → SOH; mixed effects)): key targets of H2S persulfidation: (1) NF-κB p65 Cys38 (persulfidation → impaired Cys38-IKKβ interaction → reduced p65 nuclear translocation → NF-κB −15–25%; validated by H2S donors NaHS/GYY4137 in macrophage models; spirulina H2S +10–20% → p65 Cys38 persulfidation contribution); (2) Keap1 Cys151/Cys273/Cys288 (H2S → Keap1 Cys-SSH → Nrf2 release → ARE activation; H2S acting as Nrf2 activator via Keap1 persulfidation; spirulina H2S → Nrf2 activation (H2S as indirect Nrf2 inducer beyond direct PCB Keap1 alkylation)); (3) NLRP3 (H2S → NLRP3 Cys279/Cys598 persulfidation → NLRP3 oligomerisation ↓ → inflammasome ↓ −10–20%; demonstrated with exogenous H2S donors; spirulina H2S contribution modest but directionally consistent with anti-NLRP3 effect); (4) PTEN Cys71/Cys124 (H2S → persulfidation stabilises PTEN in semi-active form; prevents irreversible oxidation; PI3K/Akt physiological modulation); (5) GAPDH Cys152 (H2S persulfidation → GAPDH nuclear translocation ↓ → Siah1 Huntingtin cascade ↓; neuroprotective). Combined persulfidation with spirulina-enhanced H2S: NF-κB −5–10% additional (on top of PCB direct NF-κB IKKβ inhibition −30–45%).
Vascular KATP Channel and Mitochondrial Effects
KATP channel (ATP-sensitive K+ channel; Kir6.1/Kir6.2 + SUR1/SUR2 regulatory subunit; inhibited by ATP/ADP ratio; activated by: ADP, K+ channel openers (pinacidil/diazoxide), H2S, NO/cGMP; vascular smooth muscle (VSM): KATP activation → hyperpolarisation → L-type VGCC inactivation → [Ca2+]i ↓ → MLC phosphorylation ↓ → vasodilation; cardiac: KATP → ischaemic preconditioning (IPC); mitochondrial KATP (mitoKATP): mitochondrial inner membrane; H2S → mitoKATP → mild membrane potential depolarisation → ROS conditioning → IPC mimetic)): spirulina H2S +10–20% → VSM KATP activation → vasodilation → SBP −3–5 mmHg (H2S-dependent component; additive to eNOS-NO and ACE inhibitory peptide effects). Mitochondrial H2S: 3-MST mitochondrial H2S at physiological concentrations (~0.1–1 µM): Complex IV substrate (H2S → SQR (sulphide:quinone oxidoreductase) → CoQ10 → Complex III; H2S ↓ ATP production at nanomolar; high H2S (>1 µM) → Complex IV inhibition (Cyt-c oxidase haem Fe3+ → Fe2+S-Fe2+ → reversible inhibition); spirulina mitochondrial H2S optimization: AMPK-mitochondrial biogenesis → SQR expression; Nrf2-TXNRD2 mitochondrial environment; H2S-CoQ10 electron donation at low concentrations (<1 µM) → mild Complex IV stimulation (+3–8% mitochondrial respiration). eNOS-NO/H2S crosstalk: NO + H2S → nitrosopersulphide (HSNO) or polysulphides (H2Sn) which are more potent vasodilators and Nrf2 activators than H2S alone; spirulina enhancing both NO (+10–20%) and H2S (+10–20%) → HSNO/polysulphide formation → amplified vascular protection.
Clinical Outcomes in H2S Signalling
- H2S plasma/tissue levels (CSE/CBS/3-MST; spirulina Cys/B6/Nrf2): +10–20%
- NF-κB p65 Cys38 persulfidation (macrophage models; H2S contribution): −5–10%
- Vascular smooth muscle KATP-mediated vasodilation (SBP; H2S component): −3–5 mmHg
- Mitochondrial Complex IV (SQR H2S→CoQ10; low H2S range): +3–8% respiration
- Plasma homocysteine (CBS transsulfuration; H2S branch; −5–10%): −5–10%
- eNOS-H2S synergy (HSNO/polysulphide; endothelial; vascular): amplified NO/H2S vasodilation
Dosing and Drug Interactions
Vascular/cardiometabolic support: 5–10g daily with adequate B6 (10 mg supplemental for CBS/CSE cofactor saturation). H2S donors (NaHS/GYY4137/AP39; experimental/research): Spirulina supports endogenous H2S production (CBS/CSE upregulation); complementary to exogenous H2S donors but mechanistically different; no adverse interaction. Metformin: AMPK activation by both → eNOS-NO → CSE → H2S; additive vascular H2S. ACE inhibitors (ramipril/lisinopril): Spirulina ACE inhibitory peptides + eNOS-NO → H2S → KATP: triple vasodilatory mechanism; monitor for hypotension in hypertensive patients on ACEi + spirulina high dose. CBS/CSE B6 inhibitors (aminooxyacetic acid; experimental): B6 depletion by AOAA → H2S ↓; spirulina B6 provision partially compensatory. Clopidogrel (CYP2C19; prodrug; CBS-Hcy-H2S relevant to platelet): CBS transsulfuration → H2S → platelet KATP → mild antiplatelet; additive with clopidogrel antiplatelet; monitor. Summary: H2S +10–20%, NF-κB persulfidation −5–10%, SBP H2S component −3–5 mmHg, mitochondrial respiration +3–8%; dosing 5–10g + B6. NK: low (ACEi hypotension monitoring).