5-lipoxygenase (5-LOX, ALOX5) is a dual-function enzyme in the arachidonic acid cascade that produces both pro-inflammatory leukotrienes (LTB4, LTC4/D4/E4) during acute inflammation and, via transcellular biosynthesis with 12/15-LOX, anti-inflammatory Lipoxins during resolution. It represents a critical molecular switch point in Lipid mediator class switching, making it a key regulator of the inflammation-to-Resolution of inflammation transition.
Imagine 5-LOX as a factory foreman who changes production lines depending on who's on shift. Early in a crisis (acute inflammation), the foreman runs the "emergency response" assembly line: he takes raw arachidonic acid and cranks out Leukotriene B4 — the most potent "call all workers NOW" signal, flooding the site with neutrophils. This is the pro-inflammatory side: maximum recruitment, maximum alarm.
But hours later, when the foreman notices a different team has clocked in (macrophages expressing 12-LOX or 15-LOX), he switches production. Now, the same foreman (5-LOX) partners with the new team in a transcellular handshake: he passes a half-finished product (15-HETE) to them, and together they assemble Lipoxins — the "stand down, clean up, go home" signal. Same enzyme, same raw material, completely opposite output. The foreman didn't change; the context and partners changed. This is why blocking 5-LOX entirely (e.g., zileuton in asthma) can paradoxically impair both inflammation AND resolution — you've fired the foreman mid-shift.
graph TD
AA[Arachidonic Acid] -->|"5-LOX + FLAP"| LTA4[LTA4 - Leukotriene A4]
LTA4 -->|LTA4 hydrolase| LTB4[LTB4 - potent neutrophil chemoattractant]
LTA4 -->|LTC4 synthase| LTC4["LTC4 → LTD4 → LTE4<br/>Cysteinyl leukotrienes<br/>bronchoconstriction"]
AA -->|15-LOX in macrophages| 15HETE[15-HETE intermediate]
15HETE -->|"transferred to neutrophils<br/>5-LOX converts"| LXA4[Lipoxin A4]
15HETE -->|or 5-LOX first| 5HETE[5-HETE]
5HETE -->|15-LOX| LXB4[Lipoxin B4]
ASA[Aspirin acetylates COX-2] -->|COX-2 produces| 15REPI["15R-HETE<br/>epi-intermediate"]
15REPI -->|5-LOX converts| ATLXA4["Aspirin-Triggered Lipoxin A4<br/>15-epi-LXA4"]
LXA4 --> ALX["ALX/FPR2 receptor<br/>on neutrophils/macrophages"]
ATLXA4 --> ALX
ALX --> RES["Resolution signals:<br/>stop neutrophil migration<br/>enhance efferocytosis<br/>M2 polarization"]
Inflammatory Phase (0-24h):
- 5-LOX requires FLAP (5-LOX activating protein) for membrane translocation and activation
- Calcium (Ca²⁺) binding causes 5-LOX translocation from cytosol to nuclear envelope
- ATP-dependent conformational change enables substrate binding
- 5-LOX catalyzes two sequential reactions:
- Oxygenation: arachidonic acid → 5-HpETE (5-hydroperoxyeicosatetraenoic acid)
- Dehydration: 5-HpETE → LTA4 (leukotriene A4, unstable epoxide)
- LTA4 hydrolase → Leukotriene B4 (LTB4), binds BLT1 receptor on neutrophils, causes chemotaxis at picomolar concentrations (EC₅₀ ~1 nM)
- LTC4 synthase → LTC4 → LTD4 → LTE4 (cysteinyl leukotrienes), bind CysLT1/CysLT2 receptors, cause bronchoconstriction in asthma, vascular permeability
Resolution Phase (24-72h):
- Transcellular biosynthesis: 5-LOX in neutrophils receives 15-HETE intermediate from macrophages expressing 15-LOX
- 5-LOX converts 15-HETE → Lipoxin A4 (LXA4) or starts with AA → 5-HETE, which 15-LOX converts → Lipoxin B4 (LXB4)
- LXA4/LXB4 bind ALX-FPR2 receptor → G-protein coupled signaling → stops neutrophil recruitment, enhances efferocytosis, promotes M2 macrophages polarization
- Aspirin-triggered pathway: acetylated COX-2 produces 15R-HETE (epimer), which 5-LOX converts → 15-epi-LXA4 (aspirin-triggered lipoxin, AT-LXA4), same pro-resolving function, more resistant to enzymatic degradation
Genetic Regulation:
- Sp1 and Egr-1 transcription factors upregulate 5-LOX expression during inflammation
- single nucleotide polymorphisms in ALOX5 promoter region (e.g., deletion variants) associated with reduced leukotriene production, altered asthma risk, cardiovascular disease susceptibility
Conditions Where 5-LOX Dysregulation Is Central:
- Asthma: Cysteinyl leukotrienes (LTC4/D4/E4) cause bronchoconstriction, mucus hypersecretion, airway remodelling. Zileuton (5-LOX inhibitor) reduces exacerbations but does NOT cure asthma — because it blocks both pro-inflammatory leukotrienes AND pro-resolving lipoxins.
- inflammatory bowel disease (IBD): LTB4 drives neutrophil infiltration in gut mucosa. Elevated LTB4:LXA4 ratio predicts flares in Crohn's disease and Ulcerative Colitis.
- Cardiovascular disease: Leukotrienes promote atherosclerotic plaque instability via macrophages activation, foam cell formation. 5-LOX inhibition shown to reduce atherosclerosis in animal models but impairs plaque resolution.
- Psoriasis, rheumatoid arthritis: LTB4 maintains chronic inflammatory infiltrates.
cPNI Metamodel Integration:
- Metamodel 3 (Movement-Nutrition-Recovery): Omega-3 fatty acids (EPA/DHA) compete with arachidonic acid for 5-LOX, shifting output from 4-series leukotrienes to less inflammatory 5-series leukotrienes and enabling Resolvins via 15-LOX.
- Metamodel 5 (Evolutionary Mismatch): High omega-6:omega-3 ratios in modern diets oversupply 5-LOX with arachidonic acid, biasing toward leukotriene overproduction without corresponding resolution capacity. Hunter-gatherer ratios (~1:1) supported balanced 5-LOX function.
- Selfish Immune System: 5-LOX exemplifies immune prioritization — immediate threat response (leukotrienes) takes precedence over tissue repair (lipoxins) unless the system receives pro-resolving signals (efferocytosis, apoptotic cell clearance, omega-3 availability).
Clinical Thresholds:
- Urinary LTE4 >104 pg/mg creatinine: elevated leukotriene production, seen in aspirin-exacerbated respiratory disease (AERD)
- Plasma LTB4 >100 pg/mL: suggests active neutrophilic inflammation
- LTB4:LXA4 ratio >10:1: impaired resolution capacity, predicts chronic inflammation
Intervention Implications:
- 5-LOX is nuclear membrane-associated during activation, not cytoplasmic like COX enzymes
- FLAP co-localizes with 5-LOX at nuclear envelope; mutations in FLAP reduce leukotriene synthesis
- Leukotriene B4 is the most potent neutrophil chemoattractant known (10-100x more potent than IL-8)
- Cysteinyl leukotrienes (LTC4/D4/E4) cause 10,000-fold greater bronchoconstriction than histamine
- Zileuton (Zyflo) is the only FDA-approved 5-LOX inhibitor for asthma (requires liver function monitoring due to hepatotoxicity risk in 1-3% of patients)
- Aspirin-exacerbated respiratory disease (AERD): patients have upregulated 5-LOX and overproduction of cysteinyl leukotrienes, respond paradoxically to COX inhibitors
- Transcellular biosynthesis requires cell-cell contact or exosomal transfer of lipid intermediates between neutrophils and macrophages
- 5-LOX activity peaks 4-6 hours after acute inflammatory stimulus, then declines as 15-LOX expression rises in infiltrating monocytes (class switch timing)
- Redox-sensitive enzyme: oxidative stress inactivates 5-LOX via iron center oxidation; glutathione peroxidase protects activity
- Genetic polymorphisms: ALOX5 promoter Sp1 binding site deletions associated with reduced atherosclerosis risk in some populations, increased asthma severity in others (context-dependent effects)
- arachidonic acid — substrate for 5-LOX; availability determines leukotriene production capacity
- Leukotrienes — primary enzymatic pathway producing all leukotrienes (B4, C4, D4, E4)
- Leukotriene B4 — most potent product in inflammatory phase; drives neutrophil chemotaxis via BLT1 receptor
- Lipoxins — produces lipoxins via transcellular biosynthesis with 12/15-LOX during resolution
- 12-LOX — partner enzyme for lipoxin biosynthesis; provides 12-HETE intermediate
- 15-LOX — critical partner for lipoxin production; converts 5-HETE or provides 15-HETE to 5-LOX
- Lipid mediator class switching — 5-LOX is THE pivotal enzyme mediating the inflammatory-to-resolution lipid switch
- Specialized pro-resolving mediators (SPMs) — essential for biosynthesis of lipoxin class of SPMs
- aspirin — acetylates COX-2 to redirect 5-LOX toward aspirin-triggered lipoxin production
- Resolvins — indirectly supports resolvin production by enabling inflammation termination via lipoxins
- neutrophils — highly expressed in neutrophils; produces LTB4 that recruits more neutrophils (positive feedback)
- eosinophils — produces cysteinyl leukotrienes in eosinophils; drives allergic inflammation
- macrophages — provides lipid intermediates to macrophages for lipoxin production; macrophage 15-LOX completes biosynthesis
- M2 macrophages — lipoxins produced by 5-LOX promote M2 polarization during resolution
- asthma — cysteinyl leukotrienes cause bronchoconstriction, mucus hypersecretion; zileuton targets 5-LOX
- inflammation — initiates acute inflammation via leukotriene production
- Resolution of inflammation — terminates inflammation via lipoxin production when partnered with 12/15-LOX
- chronic inflammation — dysregulated 5-LOX activity (high leukotriene:lipoxin ratio) perpetuates chronic inflammation
- omega-6 — metabolizes omega-6 arachidonic acid; high omega-6 intake oversupplies 5-LOX
- EPA — omega-3 EPA competes with arachidonic acid for 5-LOX, producing less inflammatory 5-series leukotrienes
- DHA — omega-3 DHA shifts lipoxygenase pathways toward resolvin production
- atherosclerosis — leukotrienes promote foam cell formation, plaque instability; lipoxins stabilize plaques
- inflammatory bowel disease — elevated LTB4 drives neutrophilic gut inflammation in Crohn's and UC
- ALX-FPR2 — receptor for lipoxins produced by 5-LOX; mediates resolution signals
- efferocytosis — lipoxins produced by 5-LOX enhance macrophage efferocytosis of apoptotic neutrophils
- FLAP — 5-LOX activating protein; required for enzyme translocation and activity
- Calcium — calcium binding triggers 5-LOX translocation to nuclear membrane
- ATP — ATP required for conformational change enabling substrate binding
- COX-2 — aspirin-acetylated COX-2 produces 15R-HETE intermediate for aspirin-triggered lipoxins
- Curcumin — polyphenol that modulates 5-LOX activity, may favor lipoxin production
- Vitamin D — vitamin D deficiency impairs 15-LOX expression, disrupting lipoxin biosynthesis
- Zinc — zinc cofactor for lipoxygenase function; deficiency impairs class switching