Ricin
Ricin is a highly toxic heterodimeric ribosome-inactivating protein from castor beans (Ricinus communis) that inhibits protein synthesis through N-glycosidase activity.
Chemical Identity
| Property | Value |
|---|---|
| Chemical Formula | Approximately C1498H2348N416O472S18 |
| Molecular Weight | 62,932 g/mol (holotoxin) |
| CAS Number | 9009-86-3 |
| IUPAC Name | Ricin (Ricinus communis agglutinin) |
| Peptide Class | Ribosome-Inactivating Protein (Type II) |
| Origin | Ricinus communis (castor bean) |
| Subunits | A chain (267 aa) + B chain (262 aa) |
Structure
Ricin is a heterodimeric glycoprotein consisting of two subunits linked by a single disulfide bond:
- A chain (RTA): 27 kDa enzymatically active subunit with N-glycosidase activity
- B chain (RTB): 32 kDa lectin subunit with two galactose-binding sites for cell attachment
The B chain binds to galactose-containing glycoproteins and glycolipids on cell surfaces, facilitating receptor-mediated endocytosis. The A chain is translocated to the cytosol where it irreversibly inactivates ribosomes.
Mechanism of Action
Ricin toxicity occurs through a multi-step pathway:
- Cell binding: RTB binds to cell surface galactose residues
- Endocytosis: The toxin is internalized via clathrin-coated pits and macropinocytosis
- Retrograde transport: Ricin travels through the Golgi to the endoplasmic reticulum
- Translocation: RTA unfolds and crosses the ER membrane into the cytosol
- Ribosome inactivation: RTA depurinates a specific adenine (A4324) in the 28S rRNA of the 60S ribosomal subunit, permanently inhibiting protein synthesis
One molecule of RTA can inactivate ~1500 ribosomes per minute.
Biological Functions
Ricin is a plant defense protein:
- Seed protection: Concentrated in castor bean meal (5-10% of seed protein)
- Anti-herbivory: Toxic to insects and mammals that consume the seeds
- Commercial production: Castor oil is extracted from seeds; ricin remains in the waste meal
Clinical Significance
Ricin exposure can occur through inhalation, ingestion, or injection:
- Ingestion: Abdominal pain, vomiting, bloody diarrhea, multi-organ failure (lethal dose ~1-20 mg/kg)
- Inhalation: Respiratory distress, pulmonary edema, ARDS (lethal dose ~3-5 micrograms/kg)
- Injection: Local necrosis, multi-organ failure
No antidote exists; treatment is supportive. Ricin has been weaponized and is classified as a Schedule 1 chemical weapon agent. Vaccine candidates (RiVax) are in clinical development.
Research Applications
- Immunotoxins: RTA conjugated to antibodies for targeted cancer therapy
- Vaccine development: Ricin toxoid vaccines for biodefense
- Cell biology: Tool for studying retrograde transport and ER stress responses
- Structural biology: Model for understanding protein toxin translocation
Safety and Side Effects
Ricin is extremely toxic. Castor bean ingestion typically causes delayed toxicity (12-48 hours). Occupational exposure during castor oil processing is a concern. Decontamination requires bleach or sodium hydroxide. Medical countermeasures and public health surveillance are critical for biodefense preparedness.
References
- Audi, J., et al. (2005). Ricin poisoning: a comprehensive review. JAMA, 294, 2342-2351.
- Lord, J.M., et al. (2003). Ricin. Applied Microbiology and Biotechnology, 62, 58-64.
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