Spirulina and Arachidonic Acid / Eicosanoid Metabolism.

Arachidonate Release: cPLA2 and Substrate Pool

Arachidonic acid (AA; 20:4 n-6) is the primary precursor of pro-inflammatory eicosanoids (PGE&sub2;, TXA&sub2;, LTB4, LTC4/D4/E4). AA is predominantly stored in the sn-2 position of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) in the plasma membrane. Cytoplasmic phospholipase A2α (cPLA2α; PLA2G4A; C2 domain; Ser228 active site; Ca²&sup+;-dependent membrane translocation at [Ca²&sup+;] >100 nM; Ser505 phosphorylation by MAPK ERK/p38 → activity ↑ ~2–5-fold) hydrolyses the sn-2 ester bond → AA release. NF-κB transcriptionally induces cPLA2α (two κB sites in PLA2G4A promoter). Secreted PLA2 (sPLA2; group IIA/V/X; pancreatic; synovial) also generates AA extracellularly. The AA pool is reduced when DGLA (dihomo-gamma-linolenic acid; 20:3 n-6) competes for the sn-2 membrane position, which it does when dietary GLA (18:3 n-6) is supplemented→Δ6-desaturase→DGLA ↑.

COX-1/2 Prostaglandin/Thromboxane Pathway

Cyclooxygenase (COX-1/PTGS1; constitutive; COX-2/PTGS2; inducible by NF-κB κB sites at −447/−651 bp; CREB; AP-1) catalyse AA → PGG&sub2; (bis-dioxygenase domain; Tyr385 radical initiates; haem peroxidase reduces PGG&sub2;→PGH&sub2;). PGH&sub2; is diverged by: PGIS (prostacyclin synthase; CYP8A1) → PGI&sub2;/prostacyclin (IP receptor; cAMP ↑; platelet inhibition; vasodilation); TXS (thromboxane synthase; CYP5A1) → TXA&sub2; (TP receptor; platelet aggregation; vasoconstriction); mPGES-1 (microsomal PGES-1; GSTM; induced by NF-κB/IL-1β) → PGE&sub2; (EP1–4 receptors; pyrexia; pain; immunomodulation; EP4→cAMP anti-inflammatory; EP2→cAMP); H-PGDS → PGD&sub2; (DP1/2; mast cells; CRTH2; anti-inflammatory in late phase). DGLA at COX-1/2 generates PGE&sub1; (anti-inflammatory; IP receptor partial agonist) and TXA&sub1; (weak TP agonist), with ~1/10 the inflammatory potency of AA-derived PGE&sub2;/TXA&sub2;.

5-LOX Leukotriene Pathway and Lipoxins

5-lipoxygenase (5-LOX; ALOX5; non-haem Fe²&sup+;; C-terminal catalytic domain; N-terminal β-barrel PLAT domain; nuclear translocation upon Ca²&sup+;/arachidonate stimulation; 5-LOX-activating protein FLAP/ALOX5AP essential membrane co-factor) converts AA → 5-HPETE → LTA&sub4; (unstable epoxide). LTA&sub4; is processed by: LTA4H (leukotriene A4 hydrolase; zinc aminopeptidase) → LTB4 (BLT1/2 receptors; neutrophil chemotaxis; PMN activation; potent at ng/mL) or LTC4S (LTC4 synthase; glutathione conjugation) → LTC4 → LTD4 → LTE4 (cysteinyl LTs; CysLT1/2 receptors; bronchoconstriction; vascular permeability; mast cell/eosinophil products). 15-LOX-1 (ALOX15; Nrf2-induced; IL-4-induced) converts AA → 15-HPETE → 15-HETE; 15-HETE + 5-LOX → lipoxin A4 (LXA4) and B4 → FPR2/ALX receptor → resolution of inflammation (PMN apoptosis; macrophage phagocytosis of apoptotic PMNs; anti-inflammatory gene expression). EPA (20:5 n-3; from GLA→Δ5-desaturation or marine) feeds 5-LOX to generate LTB5 (1/10–1/30 potency of LTB4) and E-series resolvins (18-HEPE precursor).

Spirulina’s Mechanistic Actions

• GLA → DGLA → AA dilution: Spirulina GLA (~0.5–1.5% DW)→Δ6-desaturase→DGLA; DGLA incorporated at sn-2 of membrane PL competing with AA insertion→AA pool ↓ 10–20% in cell membranes after 4–8 weeks supplementation; PGE&sub1;:PGE&sub2; ratio ↑; TXA&sub1;:TXA&sub2; ratio ↑→net pro-inflammatory eicosanoid output ↓.
• NF-κB ↓ → COX-2 ↓: PCB→NF-κB↓→COX-2 mRNA ↓ 30–50%; mPGES-1 ↓ 25–40% in LPS models→PGE&sub2; ↓ 25–45%; TXB&sub2; ↓ 20–35%.
• PCB → 5-LOX ↓: PCB inhibits 5-LOX non-haem Fe²&sup+; redox cycling (Kd ~5–20 μM estimated from inhibition kinetics)→5-HPETE ↓ 20–35%→LTB4 ↓ 20–35%; LTC4 ↓ 20–35%; urinary LTE4 ↓ 15–25% in spirulina-treated allergic subjects (two trials).
• GLA → EPA (partial Δ5-desaturation) → LTB5: DGLA →Δ5-desaturase → EPA (trace; competes with AA at 5-LOX) → LTB5 (weak BLT1) + RvE series precursors; LTB4:LTB5 ratio ↓→net neutrophil chemotaxis signal ↓.
• Nrf2 → 15-LOX-1 → LXA4 (resolution): Nrf2→ALOX15 ARE element→15-LOX-1 ↑ 15–25%; 15-HETE ↑→LXA4 ↑ 15–25% (anti-inflammatory resolution mediator)→FPR2/ALX→PMN apoptosis ↑; M2 macrophage efferocytosis ↑.
• GLA → PGI&sub2; ↑: COX-1/PGIS converts DGLA/AA → PGI&sub2;/PGI&sub1; mix; PGIS is Nrf2-responsive (haem enzyme protected from oxidative inactivation); PGI&sub2; ↑ 10–20%→IP receptor→cAMP→tPA exocytosis ↑; platelet TXA&sub2; signalling ↓ (prostacyclin-thromboxane balance shifted anti-thrombotic).

Clinical Correlates and Dosing

Human RCTs: 4–8 g/day spirulina for 8–12 weeks reduces: urinary LTE4 ↓ 15–25% in allergic rhinitis (two RCTs; n=150+); TXB&sub2; ↓ 20–35% in T2DM; PGE&sub2; ↓ 20–35% in arthritis models; platelet aggregation ↓ 15–25% (TXA&sub2;/PGI&sub2; balance). Histamine ↓ 30–45% (mast cell 5-LOX + FcεRI suppression). Animal: COX-2 protein ↓ 30–50%; LTB4 ↓ 20–35% in colitis, asthma, and peritonitis models. Interactions: NSAIDs (COX-1/2 inhibitors) + spirulina: additive PGE&sub2; suppression; monitor GI/renal effects at high doses. 5-LOX inhibitors (zileuton) + spirulina: additive LTB4 suppression; monitor liver function.