Spirulina and Arginine–Nitric Oxide Signalling.

NOS Isoforms: Structure, Cofactors, and Regulation

Three nitric oxide synthase (NOS) isoforms catalyse L-arginine + O&sub2; + NADPH → L-citrulline + NO + NADP&sup+;. All share a reductase domain (FMN/FAD/NADPH-binding) and an oxygenase domain (haem Fe, BH4, L-Arg, Zn²&sup+;-thiolate) connected by a calmodulin (CaM) recognition loop. Endothelial NOS (eNOS; NOS3) is myristoylated and palmitoylated at Cys15/Cys26, targeting it to caveolae where CAV1 CSD (residues 82–101) occludes the CaM-binding site, maintaining tonic inhibition. Ca²&sup+;/CaM displaces CAV1 → eNOS activation; AMPK phosphorylates eNOS Ser1177 independently of Ca²&sup+;, providing a Ca²&sup+;-independent flux. eNOS Thr495 (constitutively phosphorylated by PKC/CaMKII) reduces CaM affinity; Ser1177 phosphorylation outweighs Thr495 suppression when both occur. Inducible NOS (iNOS; NOS2) is Ca²&sup+;-independent (CaM constitutively bound) and requires de novo transcription driven by NF-κB (κB sites at −76 and −115 bp of NOS2 promoter) and IRF1/STAT1 (IFN-γ); iNOS produces >1,000-fold more NO than eNOS. Neuronal NOS (nNOS; NOS1) interacts with PSD-95 via PDZ domain at excitatory synapses; nNOS Ser1416 phosphorylation by CaMKII reduces activity.

BH4 Coupling, Uncoupling, and Peroxynitrite

Tetrahydrobiopterin (BH4) occupies the oxygenase domain dimer interface near the haem propionate, stabilises the eNOS homodimer, and participates in electron transfer from haem Fe to O&sub2; → NO. When BH4 is limiting or oxidised to BH2 (dihydrobiopterin), eNOS “uncouples”: electrons flow to O&sub2; producing superoxide (O&sub2;•−) rather than NO. O&sub2;•− + NO → peroxynitrite (ONOO−; k ~1.9×10¹° M−s−¹) — which nitrates eNOS Tyr657, reducing activity, and oxidises BH4 → BH2, amplifying uncoupling. GTP cyclohydrolase I (GCH1; EC 3.5.4.16) is the rate-limiting BH4 biosynthetic enzyme, induced by Nrf2 via an ARE at −3.4 kb. DHFR recycles BH2 → BH4 using NADPH; GSH/NADPH status is thus a key coupling determinant.

Arginase Competition and ADMA

Arginase I (ARG1; cytoplasmic; liver) and arginase II (ARG2; mitochondrial; kidney, macrophages) compete with NOS for L-Arg (Km ARG1 ~1–5 mM vs. NOS ~2–20 μM). M2 macrophage polarisation (IL-4/IL-13→STAT6) strongly induces ARG1, diverting L-Arg toward urea/ornithine and reducing NO. ARG2 in endothelium is induced by oxidised LDL via LOX-1/Akt, causing eNOS substrate limitation and endothelial dysfunction. Asymmetric dimethylarginine (ADMA), an endogenous NOS inhibitor, is degraded by DDAH1/2 (Cys273 active site); oxidative stress inactivates DDAH1 Cys273 → ADMA accumulates → eNOS further inhibited.

Spirulina’s Mechanistic Actions

• AMPK → eNOS Ser1177 ↑: AMPK Thr172 ↑ → eNOS Ser1177 +25–40%; Ca²&sup+;-independent NO ↑ 20–35% in endothelial cells; AMPK→HSP90 Ser226 → HSP90-eNOS chaperone complex stabilised → eNOS membrane retention improved.
• NF-κB ↓ → iNOS ↓: PCB → IKKβ↓ 30–50% → NF-κB nuclear ↓ → NOS2 mRNA ↓ 40–65% in LPS/IFN-γ models, shifting NO from inflammatory burst to constitutive vascular tone.
• Nrf2 → BH4 coupling: Nrf2→GCH1 +15–25% → BH4 synthesis ↑; Nrf2→GCLC/GSH/NADPH → DHFR-driven BH2→BH4 recycling ↑; net BH4:BH2 +20–35% → eNOS coupling improved → O&sub2;•− ↓, NO ↑.
• Arginase suppression: NF-κB↓→ARG2 ↓ 20–35% in macrophages; AMPK→PPAR-α reduces IL-4/STAT6-driven ARG1 indirectly; net L-Arg bioavailability for eNOS ↑.
• DDAH Cys273 preservation: PCB→Nox2↓→ROS↓ → DDAH1 Cys273 oxidation ↓ → ADMA degradation ↑ → eNOS inhibition by ADMA ↓.
• Peroxynitrite defence: Nrf2→TXNRD1/TRX1→Prx II/V → ONOO− detoxification ↑; eNOS Tyr657 nitration ↓.
• Direct L-Arg provision: Spirulina protein ~9–10% arginine by weight; 5 g dose ≈ 450–500 mg L-Arg, supporting eNOS substrate without pharmacological loading.

Clinical Correlates and Dosing

Human RCTs: 4.5–8 g/day spirulina for 8–12 weeks reduced systolic BP 4–8 mmHg in hypertensive adults; flow-mediated dilation (FMD) improved ~2–4% (absolute). In T2DM cohorts, ADMA ↓ 15–20% correlated with CRP ↓, suggesting DDAH restoration. Animal data: iNOS protein ↓ 40–65%; eNOS protein +20–30%; vascular O&sub2;•− ↓ 35–50%. Interactions: PDE5 inhibitors (sildenafil) + spirulina — additive hypotension possible; monitor. High-dose L-Arg supplements (>6 g/day) + spirulina: redundant substrate loading; no known harm but unnecessary.