Heme oxygenase-1 (HO-1), also known as heat shock protein 32 (HSP32), is the inducible isoform of heme oxygenase that catalyzes the rate-limiting step in heme degradation. HO-1 cleaves heme into biliverdin (later converted to bilirubin), carbon monoxide (CO), and free iron (Fe²⁺). As a stress-responsive enzyme activated by oxidative stress, heavy metals, and inflammation, HO-1 plays a dual role in neurodegeneration—protective through antioxidant bilirubin production but potentially harmful through iron release.
Structure and Domains
HO-1 is anchored to the endoplasmic reticulum membrane:
Transmembrane domain (C-terminal): ER membrane anchoring
Heme-binding pocket: Contains catalytic histidine (His25) that coordinates heme iron
α-helical structure: Multiple α-helices form the catalytic core
N-terminal regulatory region: Contains signal sequences and cleavage sites
Heme oxygenase-1 (HO-1), also known as heat shock protein 32 (HSP32), is the inducible isoform of heme oxygenase that catalyzes the rate-limiting step in heme degradation. HO-1 cleaves heme into biliverdin (later converted to bilirubin), carbon monoxide (CO), and free iron (Fe²⁺). As a stress-responsive enzyme activated by oxidative stress, heavy metals, and inflammation, HO-1 plays a dual role in neurodegeneration—protective through antioxidant bilirubin production but potentially harmful through iron release.
Structure and Domains
HO-1 is anchored to the endoplasmic reticulum membrane:
Transmembrane domain (C-terminal): ER membrane anchoring
Heme-binding pocket: Contains catalytic histidine (His25) that coordinates heme iron
α-helical structure: Multiple α-helices form the catalytic core
N-terminal regulatory region: Contains signal sequences and cleavage sites
The enzyme requires NADPH-cytochrome P450 reductase as an electron donor for catalytic activity.
Normal Function
Heme Degradation
HO-1 catalyzes the oxidative degradation of heme:
First oxygenation: Heme + O₂ → α-mesohydroxyheme (Fe³⁺)
Second oxygenation: Verdoheme formation with CO release
Biliverdin formation: Final cleavage produces biliverdin and free iron
Cytoprotective Products
HO-1 products have distinct biological activities:
| Product | Function | Neuroprotective Effect | |---------|----------|------------------------| | Bilirubin | Antioxidant | Scavenges peroxyl radicals, more potent than vitamin E | | Carbon monoxide | Signaling gas | Anti-inflammatory, vasodilatory, anti-apoptotic | | Free iron | Pro-oxidant | Activates ferritin transcription (protective feedback) |
Role in Neurodegeneration
Alzheimer's Disease
HO-1 expression is markedly increased in AD brain tissue:
Location: Enriched in neurofibrillary tangles and senile plaques[@smith1994]
Iron accumulation: HO-1-mediated iron release may contribute to AD iron overload[@ham2015]
Oxidative stress: Chronic induction reflects persistent oxidative stress in AD
Mitochondrial dysfunction: HO-1 translocation to mitochondria impairs respiration[@suliman2016]
The role of HO-1 in AD is complex—acute induction is protective, but chronic upregulation may exacerbate iron-mediated oxidative damage.
Parkinson's Disease
HO-1 is upregulated in dopaminergic [neurons](/entities/neurons) of the substantia nigra:
Protective induction: Early HO-1 upregulation may protect against oxidative stress[@schipper1998]
Iron release: HO-1 contributes to the characteristic iron accumulation in substantia nigra
Lewy bodies: HO-1 colocalizes with [α-synuclein](/proteins/alpha-synuclein) in Lewy bodies[@schipper2015]
Glial response: Astrocytic and microglial HO-1 induction reflects neuroinflammation
Ferroptosis Connection
HO-1 is intimately linked to [ferroptosis](/entities/ferroptosis), an iron-dependent cell death pathway:
Iron source: HO-1 provides free iron for lipid peroxidation
Ferroptosis trigger: Excessive HO-1 activity can initiate ferroptosis[@adedoyin2021]
Labile iron pool: HO-1 increases the labile iron pool available for Fenton chemistry
Therapeutic target: HO-1 inhibition may protect against ferroptosis
Multiple Sclerosis
HO-1 expression is elevated in MS lesions:
Anti-inflammatory: CO suppresses pro-inflammatory cytokine production
Myelin protection: HO-1 may protect oligodendrocytes from oxidative damage
Hemin treatment: HO-1 induction ameliorates EAE in animal models
Therapeutic Targeting
HO-1 Inhibitors
Pharmacological inhibition may be beneficial in iron-overload conditions:
| Compound | Mechanism | Status | |----------|-----------|--------| | Tin protoporphyrin (SnPP) | Competitive inhibitor | Research tool | | Zinc protoporphyrin (ZnPP) | Competitive inhibitor | Research tool | | OB-24 | Novel inhibitor | Preclinical |
HO-1 Inducers
Induction may be beneficial in acute oxidative stress:
[Smith et al, Heme oxygenase-1 is associated with the neurofibrillary pathology of Alzheimer's disease (1994)](https://pubmed.ncbi.nlm.nih.gov/8081566/)
[Ham et al, Iron and lipid peroxidation in the Alzheimer's disease brain: the role of heme oxygenase-1 (2015)](https://doi.org/10.1007/s00726-014-1846-1)
[Suliman et al, Mitochondrial quality control via mitophagy in heme oxygenase-1 deficient cells (2016)](https://doi.org/10.1016/j.freeradbiomed.2016.10.498)
[Schipper et al, Heme oxygenase-1 expression in dopaminergic neurons of the substantia nigra in Parkinson's disease (1998)](https://pubmed.ncbi.nlm.nih.gov/9614231/)
[Schipper et al, Heme oxygenase-1 and the dopamine system in Parkinson's disease (2015)](https://doi.org/10.1007/s00726-014-1866-x)
[Adedoyin et al, Heme oxygenase-1 promotes ferroptosis by increasing the labile iron pool (2021)](https://doi.org/10.1038/s41419-021-03590-2)
[Ayuso et al, Association between the (GT)n polymorphism of the HMOX1 gene promoter and Alzheimer's disease (2019)](https://pubmed.ncbi.nlm.nih.gov/30951520/)