Leukotrienes are a family of inflammatory lipid mediators biosynthesized from arachidonic acid via the 5-LOX enzyme pathway. They are named for their discovery in leukocytes and their conjugated triene structure (three alternating double bonds). The family divides into two functional classes: Leukotriene B4 (LTB4), a potent chemoattractant for neutrophils and other immune cells, and the cysteinyl leukotrienes (LTC4, LTD4, LTE4), which cause bronchoconstriction, increase vascular permeability 100-fold more than Histamine, and drive mucus hypersecretion.
Picture leukotrienes as a factory that manufactures two distinct emergency response teams from a single raw material stockpile. The warehouse holds omega-6 arachidonic acid (stored in every cell membrane like ammunition in a bunker). When the alarm sounds—infection, allergen, injury—the 5-LOX assembly line opens.
Out of this line come two specialized teams: The B4 squad (LTB4) acts like emergency flares fired into the sky, calling in battalions of neutrophils from the bloodstream to the battle site. These flares burn 1000 times brighter than ordinary signals, pulling immune troops through blood vessel walls with irresistible chemical magnetism.
Meanwhile, the cysteinyl crew (LTC4→LTD4→LTE4) behaves like overzealous security guards at the lung gates. They slam shut the airways (bronchoconstriction), open the vascular doors wide causing fluid to leak into tissues (permeability), and activate the mucus sprinkler system at maximum output. In asthma, these guards never stand down—they keep the airways locked tight, fluid flooding in, and mucus pouring out, long after the threat has passed.
The omega-3 fatty acids (EPA and DHA) act like replacing the ammunition stockpile with blanks. The factory still runs, but it produces far less potent emergency teams—the response stays proportional instead of catastrophic.
graph TD
A[Phospholipid Membrane] -->|PLA2 activation| B[Arachidonic Acid Release]
B -->|"5-LOX + FLAP"| C[5-HPETE]
C -->|5-LOX| D[LTA4]
D -->|LTA4 Hydrolase| E[LTB4]
D -->|LTC4 Synthase| F[LTC4]
E -->|BLT1/BLT2 Receptor| G[Neutrophil Chemotaxis]
G --> H["Ca²⁺ Mobilization"]
H --> I[Phagocyte Activation]
F -->|"γ-Glutamyl Transferase"| J[LTD4]
J -->|Dipeptidase| K[LTE4]
F --> L[CysLT1/CysLT2 Receptor]
J --> L
K --> L
L --> M[Gq Protein Activation]
M --> N[PLC Activation]
N --> O["IP3 + DAG"]
O --> P["Ca²⁺ Release"]
P --> Q[Smooth Muscle Contraction]
P --> R[Vascular Permeability]
P --> S[Mucus Secretion]
Initiation:
Phospholipase A2 (PLA2) cleaves arachidonic acid from membrane phospholipids in response to inflammatory stimuli (TNF-α, IL-1, Calcium flux, mechanical stress). Free arachidonic acid is the substrate pool for multiple eicosanoid pathways.
5-Lipoxygenase Pathway:
5-LOX requires the helper protein FLAP (5-lipoxygenase-activating protein) for membrane localization and catalytic activity. This enzyme catalyzes two sequential reactions:
- 5-HPETE (5-hydroperoxyeicosatetraenoic acid) formation via addition of molecular oxygen to carbon 5
- LTA4 (leukotriene A4) formation via dehydration of 5-HPETE, creating an unstable epoxide intermediate
Bifurcation Point — Two Enzymatic Routes:
Route 1: LTB4 Synthesis
- LTA4 hydrolase (a zinc metalloenzyme) converts LTA4 → LTB4
- LTB4 binds to BLT1 (high-affinity) and BLT2 (low-affinity) receptors on neutrophils, mast cells, and macrophages
- Receptor activation triggers Gi protein → PLC-β → IP3/DAG → Calcium release → chemotaxis, degranulation, superoxide production
- Peak LTB4 concentration: 10-100 nM at inflammatory sites (1000x more potent than C5a as chemoattractant)
Route 2: Cysteinyl Leukotriene Synthesis
- LTC4 synthase (a MAPEG family enzyme) conjugates glutathione to LTA4 → LTC4
- Extracellular γ-glutamyl transferase removes glutamate → LTD4
- Dipeptidase removes glycine → LTE4 (most stable, longest half-life)
- CysLT1 receptors (predominant in airways) and CysLT2 receptors (vascular endothelium) activate Gq proteins → PLC → IP3 → Calcium release → smooth muscle contraction, endothelial gap formation, goblet cell degranulation
Omega-3 Competitive Inhibition:
EPA and DHA compete with arachidonic acid for 5-LOX binding. When EPA is the substrate, the pathway produces 5-series leukotrienes (LTB5, LTC5) which bind receptors 10-100x more weakly, effectively dampening the inflammatory cascade without eliminating the defense response entirely.
Cellular Sources:
- Neutrophils: Primary LTB4 producers (100-200 ng per 10⁶ cells)
- Eosinophils: Major cysteinyl LT producers in allergic inflammation
- Mast cells: Produce both classes during IgE-mediated degranulation
- Macrophages, basophils (lesser but significant contributors)
Leukotrienes represent a critical therapeutic target in respiratory and allergic disease, and understanding their synthesis is foundational to evolutionary dietary interventions in cPNI practice.
Asthma and Allergic Airway Disease:
Cysteinyl leukotrienes are 1000x more potent bronchoconstrictors than Histamine and are the primary mediators in aspirin-exacerbated respiratory disease (AERD), exercise-induced bronchoconstriction, and nocturnal asthma. LTC4/LTD4 concentrations in bronchoalveolar lavage fluid correlate directly with asthma severity. Leukotriene receptor antagonists (montelukast, zafirlukast) block CysLT1 receptors and are effective in ~60% of asthma patients, particularly those with aspirin sensitivity or allergic rhinitis overlap.
Evolutionary Mismatch — The Omega-6/Omega-3 Substrate Problem:
Modern Western diets show omega-6:Omega-3 ratios of 15-20:1 (ancestral estimate: 1-4:1). This floods membrane phospholipids with arachidonic acid, creating massive substrate availability for leukotriene synthesis. The selfish immune system prioritizes inflammatory readiness when substrate is abundant—chronic low-grade LTB4 production drives persistent neutrophil activation, contributing to chronic inflammation, atherosclerosis, and inflammatory bowel disease. In cPNI practice, correcting omega-6:omega-3 ratios to <4:1 (ideally 2:1) is a foundational intervention for conditions involving leukotriene dysregulation.
Clinical Thresholds:
- Urinary LTE4 >100 pg/mg creatinine: Elevated leukotriene production (reference <40 pg/mg)
- Exhaled LTC4 in asthma exacerbation: >20 pg/mL (normal <5 pg/mL)
- Omega-3 index <4%: High substrate availability for arachidonic acid pathways
- Omega-3 index >8%: Competitive inhibition of leukotriene synthesis
Aspirin Sensitivity:
In ~10% of asthma patients, aspirin or NSAIDs paradoxically trigger severe bronchoconstriction. Mechanism: COX-1 inhibition shunts arachidonic acid toward the 5-LOX pathway, causing a leukotriene surge (urinary LTE4 can increase 10-fold). This reflects the evolutionary trade-off between prostaglandins (via COX enzymes) and leukotrienes—blocking one amplifies the other, revealing substrate competition between parallel pathways.
Neutrophil-Driven Pathology:
Leukotriene B4 is central to neutrophil recruitment in Crohn's disease (creeping fat contains LTB4-secreting adipocytes), rheumatoid arthritis (synovial fluid LTB4 >500 pg/mL), psoriasis, and acute gout. 5-LOX inhibitors (zileuton) reduce LTB4 production but show limited efficacy due to narrow therapeutic window and hepatotoxicity.
Resolution Phase and SPM Transition:
The shift from leukotriene production to Specialized pro-resolving mediators (SPMs) synthesis marks active resolution of inflammation. Lipid mediator class switching—triggered by declining prostaglandin E2 and rising cortisol—redirects 5-LOX and 12-LOX/15-LOX toward resolvins, protectins, and maresins. In chronic disease, this switch fails—leukotrienes persist, resolution stalls. Omega-3 supplementation (2-4g EPA+DHA daily) provides substrate for SPM synthesis, enabling metabolic switching even when inflammatory signals remain.
cPNI Intervention Priorities:
- Dietary omega-6 reduction: Eliminate seed oils (soybean, corn, sunflower), reduce grain-fed meat
- Omega-3 supplementation: Target omega-3 index >8% (2-4g EPA+DHA daily from fish oil)
- 5-LOX pathway modulation: Curcumin (inhibits 5-LOX at 10-20 μM), boswellia (AKBA inhibits FLAP)
- Gut barrier repair: Leaky gut increases LPS exposure → NF-κB → PLA2 activation → increased substrate release
- Stress management: Cortisol resistance prevents lipid mediator class switching
- Leukotrienes are 1000x more potent than Histamine in causing bronchoconstriction (cysteinyl LTs)
- Leukotriene B4 is 100x more potent than C5a as a neutrophil chemoattractant
- Cysteinyl leukotrienes increase vascular permeability 100-fold more than histamine
- LTB4 production capacity: 100-200 ng per 10⁶ neutrophils within 5 minutes of stimulation
- Half-life: LTB4 ~1 minute in circulation, LTE4 ~4 hours (stable urinary marker)
- 5-LOX requires FLAP (5-lipoxygenase-activating protein) for membrane localization and activity
- ~60% of asthma patients respond to leukotriene receptor antagonists (montelukast)
- Aspirin-sensitive asthma patients show 10-fold increase in urinary LTE4 after aspirin challenge
- Omega-3 index >8% reduces LTB4 production by 40-60% compared to index <4%
- Western diet omega-6:omega-3 ratio (15-20:1) vs. ancestral estimate (1-4:1)
- Urinary LTE4 reference: <40 pg/mg creatinine (elevated: >100 pg/mg)
- CysLT1 receptor antagonists reduce asthma exacerbations by 30-40% in responders
- 5-LOX gene polymorphisms (repeat variants in promoter) alter asthma susceptibility and NSAID sensitivity
- arachidonic acid — The omega-6 precursor fatty acid from which all leukotrienes are biosynthesized via enzymatic oxygenation
- 5-LOX — The rate-limiting enzyme initiating leukotriene synthesis; requires FLAP for activity and is inhibited by Omega-3 fatty acids
- Leukotriene B4 — The primary chemoattractant leukotriene recruiting neutrophils, eosinophils, and monocytes to inflammatory sites
- prostaglandins — Parallel eicosanoid pathway from arachidonic acid via COX enzymes; aspirin shunts substrate toward leukotrienes
- Omega-3 fatty acids — EPA and DHA competitively inhibit 5-LOX, producing less inflammatory 5-series leukotrienes and providing substrate for Specialized pro-resolving mediators (SPMs)
- asthma — Cysteinyl leukotrienes are primary mediators of bronchoconstriction, mucus hypersecretion, and airway remodeling in asthmatic inflammation
- Allergy — Leukotrienes drive IgE-mediated allergic responses including urticaria, angioedema, and anaphylaxis alongside Histamine
- inflammation — Leukotrienes amplify acute inflammatory responses through immune cell recruitment, vascular permeability, and pain sensitization
- chronic inflammation — Persistent leukotriene production characterizes failed resolution in inflammatory bowel disease, atherosclerosis, and rheumatoid arthritis
- neutrophils — Primary cellular source of Leukotriene B4; recruited to inflammatory sites by LTB4 gradients in positive feedback loops
- mast cells — Produce massive cysteinyl leukotriene bursts during IgE-mediated degranulation in allergic reactions
- eosinophils — Major producers of cysteinyl leukotrienes in allergic inflammation, asthma, and parasitic infections
- Specialized pro-resolving mediators (SPMs) — Resolvins, protectins, and maresins counter-regulate leukotriene pro-inflammatory effects during active resolution
- Lipid mediator class switching — Critical transition from leukotriene/prostaglandin production to SPM synthesis marking shift from inflammation to resolution
- aspirin — Aspirin-sensitive individuals show excessive cysteinyl leukotriene production when COX-1 is blocked, shunting substrate to 5-LOX
- phospholipase A2 — The enzyme releasing arachidonic acid from membrane phospholipids, providing substrate for leukotriene synthesis
- NF-κB — Master transcription factor upregulating 5-LOX, LTC4 synthase, and BLT1 receptor expression during inflammatory activation
- Calcium — Intracellular calcium mobilization is both upstream trigger (activating phospholipase A2) and downstream effect (cysteinyl LT-induced contraction)
- diet — Dietary omega-6:Omega-3 ratio determines membrane arachidonic acid content and substrate availability for leukotriene synthesis
- gut permeability — Increased intestinal permeability allows LPS translocation → TLR4 activation → NF-κB → 5-LOX upregulation
- COX-2 — Parallel eicosanoid enzyme; COX-2 inhibition can shunt arachidonic acid toward 5-LOX pathway in susceptible individuals
- Crohn's disease — Elevated intestinal and adipose tissue LTB4 drives neutrophil infiltration, ulceration, and creeping fat formation
- atherosclerosis — Leukotriene B4 recruits monocytes into arterial walls, promoting foam cell formation and plaque instability
- resolution — Active resolution requires termination of leukotriene synthesis and metabolic switch to Specialized pro-resolving mediators (SPMs) production
- selfish immune system — Leukotrienes exemplify immune system prioritization of defense readiness over metabolic economy when substrate is abundant