5-lipoxygenase (5-LO, also called 5-LOX or ALOX5) is a calcium-dependent, non-heme iron-containing enzyme that catalyzes the conversion of arachidonic acid into leukotrienes, a family of potent pro-inflammatory lipid mediators. Highly expressed in neutrophils, eosinophils, and M1 macrophages, 5-LO drives the acute inflammatory response and, when chronically activated, contributes to pathological inflammation in asthma, atherosclerosis, and inflammatory bowel disease.
Think of 5-LO as a specialized factory on a neutrophil production line that converts raw membrane fat (arachidonic acid) into chemical flares called leukotrienes. When tissue gets damaged—say a splinter in your finger—these flares shoot into the bloodstream, broadcasting a message: "ALL NEUTROPHILS, CONVERGE HERE." The factory runs hot for the first 3-4 days of injury, churning out LTB4 (the brightest flare, visible to neutrophils miles away) and cysteinyl-leukotrienes (which squeeze airways like a fist around a garden hose in asthma). Around day 4-5, as the wound shifts from war zone to reconstruction site, the factory undergoes a management change: M1 macrophages (high 5-LO, aggressive foremen) clock out, and M2 macrophages (low 5-LO, gentle renovators) clock in. The flares dim. Neutrophils stop arriving. Resolution begins. But if the factory never shuts down—chronic stress, Western diet, persistent infection—the flares keep burning, neutrophils keep flooding in, and you get chronic inflammation: the fire station that never stops blaring its alarm.
5-LO catalyzes the committed step in leukotriene biosynthesis, a two-step enzymatic process requiring calcium, ATP, and the scaffolding protein FLAP (5-lipoxygenase-activating protein):
Step 1: Arachidonic Acid Liberation
Step 2: 5-LO Activation and Translocation
- Intracellular Ca²⁺ rise (>1 μM) activates 5-LO in the cytosol
- ATP binding stabilizes active enzyme conformation
- 5-LO translocates to nuclear membrane, where it binds FLAP
- FLAP presents arachidonic acid to 5-LO active site
Step 3: Leukotriene Synthesis Cascade
graph TD
AA[Arachidonic Acid] -->|"5-LO + O₂"| HPETE[5-HPETE]
HPETE -->|5-LO dehydrase| LTA4[LTA4 - unstable epoxide]
LTA4 -->|LTA4 hydrolase| LTB4[LTB4 - neutrophil chemoattractant]
LTA4 -->|"LTC4 synthase + glutathione"| LTC4[LTC4 - cysteinyl-LT]
LTC4 -->|"γ-glutamyl transferase"| LTD4[LTD4]
LTD4 -->|dipeptidase| LTE4[LTE4]
LTB4 -.->|binds| BLT1[BLT1 receptor on neutrophils]
LTC4 -.->|binds| CysLT1[CysLT1 receptor - bronchoconstriction]
LTD4 -.->|binds| CysLT1
LTE4 -.->|binds| CysLT2[CysLT2 receptor]
Cellular Expression Dynamics
- Neutrophils: Highest 5-LO activity (peak days 1-4 post-injury), produce massive LTB4 for autocrine/paracrine recruitment
- M1 macrophages: High 5-LO, sustain inflammatory phase
- M2 macrophages: Low 5-LO (transcriptional downregulation via IL-4/IL-10 → STAT6/STAT3 → suppression of ALOX5 gene)
- Eosinophils: High 5-LO, major producers of cysteinyl-leukotrienes in allergic inflammation
Inhibition Mechanisms
- EPA and DHA compete as alternative substrates, producing less inflammatory 5-series leukotrienes
- Zileuton directly inhibits 5-LO active site
- Corticosteroids reduce PLA2 activity (upstream substrate limitation)
- Specialized pro-resolving mediators (lipoxins) paradoxically require 15-LO interaction with 5-LO for synthesis
High 5-LO Activity States
- Asthma: Cysteinyl-leukotrienes (LTC4, LTD4, LTE4) bind CysLT1 receptors on bronchial smooth muscle → bronchoconstriction 1000× more potent than histamine, mucus hypersecretion, eosinophil recruitment. Aspirin-exacerbated respiratory disease (AERD) shows extreme 5-LO upregulation.
- Atherosclerosis: LTB4 recruits neutrophils and monocytes into arterial plaques, drives foam cell formation, promotes plaque instability. 5-LO knockout mice show 90% reduction in atherosclerotic lesions.
- Inflammatory bowel disease: Elevated mucosal 5-LO activity in Crohn's disease and ulcerative colitis; LTB4 perpetuates neutrophil infiltration and epithelial damage.
- Chronic pain: LTB4 sensitizes TRPV1 channels on nociceptors, lowers pain threshold (peripheral sensitization mechanism).
Wound Healing Context (Module 5 focus)
- Days 0-4: High 5-LO in neutrophils establishes inflammatory gradient, recruits immune cells
- Day 4+: M1→M2 transition reduces 5-LO → decreased neutrophil recruitment, shift to resolution
- Failure to downregulate 5-LO = chronic wound (diabetic ulcers, pressure sores)
Intervention Implications
- Omega-3 supplementation (EPA 2-4g/day): Competitive substrate inhibition, produces weaker 5-series leukotrienes
- 5-LO inhibitors (zileuton): FDA-approved for asthma, reduces LTB4 and cysteinyl-LTs
- Anti-inflammatory diet: Reduce arachidonic acid intake (decrease red meat, increase fish), provide polyphenols that inhibit 5-LO (quercetin, EGCG)
- Metamodel 5 application: In chronic inflammatory states, support M1→M2 transition via cold exposure, exercise, circadian alignment
Evolutionary Mismatch
Modern Western diet provides excessive arachidonic acid substrate (grain-fed meat, vegetable oils high in linoleic acid → arachidonic acid conversion) while lacking omega-3 competitors. Ancestral omega-6:omega-3 ratio ~1:1; modern ratio often 20:1, creating 5-LO substrate surplus and chronic leukotriene overproduction.
- Catalyzes arachidonic acid → 5-HPETE → LTA4 (first committed step in leukotriene synthesis)
- LTB4 is one of the most potent neutrophil chemoattractants known (effective at picomolar concentrations)
- Cysteinyl-leukotrienes cause bronchoconstriction 1000× more potent than histamine
- Requires calcium (>1 μM), ATP, and FLAP (5-LO activating protein) for full enzymatic activity
- Peak expression in neutrophils days 1-4 of wound healing, decreases sharply day 4+ during resolution
- M1 macrophages produce 10-fold more 5-LO products than M2 macrophages
- Omega-3 fatty acids (EPA/DHA) compete as substrates, producing 5-series leukotrienes with 10-100× less inflammatory potency
- Genetic variants in ALOX5 promoter (tandem SP1-binding repeats) affect asthma risk and leukotriene response
- 5-LO knockout mice are protected from atherosclerosis, asthma models, and colitis
- Zileuton inhibits 5-LO with IC50 ~0.5 μM; reduces urinary LTE4 by 70% in asthma patients
- arachidonic acid — direct substrate that 5-LO converts to 5-HPETE, then leukotrienes
- leukotrienes — family of pro-inflammatory lipid mediators produced by 5-LO enzymatic activity
- LTB4 — major 5-LO product, potent neutrophil and monocyte chemoattractant binding BLT1 receptor
- neutrophils — primary cell type expressing highest 5-LO levels, especially during acute inflammation
- PLA2 — liberates arachidonic acid substrate from membrane phospholipids for 5-LO
- inflammation — 5-LO drives acute inflammatory response via leukotriene production
- wound healing — 5-LO activity high in inflammatory phase (days 1-4), decreases during resolution (day 4+)
- M1 macrophages — express high 5-LO, sustain inflammatory phase with leukotriene production
- M2 macrophages — downregulate 5-LO during resolution phase, shift to anti-inflammatory phenotype
- COX-2 — parallel enzyme in arachidonic acid cascade producing prostaglandins while 5-LO produces leukotrienes
- asthma — cysteinyl-leukotrienes (LTC4, LTD4, LTE4) cause bronchoconstriction and mucus hypersecretion
- EPA — omega-3 fatty acid that competes with arachidonic acid for 5-LO, producing less inflammatory products
- DHA — omega-3 that reduces arachidonic acid availability and competes as 5-LO substrate
- eicosanoids — lipid mediator superfamily including leukotrienes (5-LO products) and prostaglandins (COX products)
- specialized pro-resolving mediators — lipoxins and resolvins involve 5-LO (plus 12-LOX or 15-LOX) in biosynthesis
- chronic inflammation — sustained 5-LO activity drives chronic inflammatory diseases (IBD, atherosclerosis, chronic pain)
- eosinophils — highly express 5-LO, major producers of cysteinyl-leukotrienes in allergic inflammation
- atherosclerosis — LTB4 recruits leukocytes into arterial plaques, drives foam cell formation
- inflammatory bowel disease — elevated mucosal 5-LO perpetuates neutrophil infiltration and epithelial damage
- TRPV1 — nociceptor channel sensitized by LTB4, contributing to inflammatory pain and hyperalgesia
- hypoxia — activates 5-LO via HIF-1α pathway, increases leukotriene production in ischemic tissues
- omega-3 fatty acids — competitive inhibitors of 5-LO, reduce inflammatory leukotriene production
- aspirin — triggers formation of aspirin-triggered lipoxins via acetylated COX-2 interacting with 5-LO pathway
- Module 4: Eicosanoid metabolism, arachidonic acid cascade
- Module 5: Wound healing phases, neutrophil function, macrophage polarization (M1→M2 transition)