Abrin
Abrin is a highly toxic ribosome-inactivating protein from jequirity beans (Abrus precatorius) that inhibits protein synthesis, closely related to ricin in structure and mechanism.
Chemical Identity
| Property | Value |
|---|---|
| Chemical Formula | Approximately C1438H2260N402O458S16 |
| Molecular Weight | 60,000-65,000 g/mol (holotoxin) |
| CAS Number | 1393-62-0 |
| IUPAC Name | Abrin (Abrus precatorius) |
| Peptide Class | Ribosome-Inactivating Protein (Type II) |
| Origin | Abrus precatorius (jequirity bean) |
| Subunits | A chain (~270 aa) + B chain (~270 aa) |
Structure
Abrin is a heterodimeric glycoprotein closely related to ricin, consisting of two subunits linked by a disulfide bond:
- A chain: ~30 kDa enzymatically active subunit with RNA N-glycosidase activity
- B chain: ~35 kDa galactose-binding lectin subunit for cell attachment
The overall structure and domain organization is highly similar to ricin, with approximately 50% sequence identity between the A chains. Abrin exists in multiple isoforms (abrin-a, abrin-b, abrin-c, abrin-d) with varying potency and tissue distribution.
Mechanism of Action
Abrin follows the same intracellular trafficking pathway as ricin:
- Cell binding: B chain binds terminal galactose residues on cell surface glycoconjugates
- Endocytosis: Clathrin-dependent and -independent internalization
- Retrograde transport: Transport through Golgi to ER
- Translocation: A chain unfolds and crosses the ER membrane into cytosol
- Ribosome inactivation: N-glycosidase cleavage of adenine 4324 in 28S rRNA, halting protein synthesis
Abrin is approximately 75 times more toxic than ricin when injected intravenously, making it one of the most potent natural toxins known.
Biological Functions
Abrin is a seed storage protein in jequirity beans:
- Seed distribution: Concentrated in the seed coat and cotyledons
- Anti-herbivory: Deters seed predation by insects and mammals
- Ecological role: Allows seed dispersal through the intact passage through predator digestive systems
The bright red and black seeds are used in jewelry and traditional crafts, posing ingestion risk, particularly to children.
Clinical Significance
Abrin poisoning can occur through ingestion or inhalation:
- Ingestion: Gastrointestinal hemorrhage, liver and kidney failure, death within 3-5 days
- Inhalation: Respiratory distress, pulmonary edema
- Lethal dose: Estimated ~0.1-1 micrograms/kg (intravenous in humans)
- Treatment: Supportive care; no specific antidote
Decontamination requires bleach or sodium hydroxide. Abrin is a CDC Category B bioterrorism agent.
Research Applications
- Immunotoxins: Abrin A chain conjugates for targeted cancer therapy
- Anti-cancer research: Cytotoxicity studies against various tumor cell lines
- Structural studies: Comparative studies with ricin to understand toxin evolution
- Detection methods: Development of rapid field assays for biodefense
Safety and Side Effects
Abrin is extremely toxic. Even handling intact seeds can cause dermatitis. Seed ingestion is a medical emergency requiring immediate treatment. Laboratory work with purified abrin requires BSL-2 containment and careful decontamination protocols. All cases of abrin exposure should be reported to poison control and public health authorities.
References
- Hegde, R., et al. (2009). Abrin poisoning: a review. Toxicon, 54, 233-240.
- Bhatt, D.L., et al. (2013). Ribosome-inactivating proteins from Abrus precatorius. Journal of Biological Chemistry, 288, 1453-1461.
Test Your Knowledge
Reinforce what you learned about Abrin with interactive quizzes on Wikipept.
Take a Quiz on Wikipept