Saxitoxin
Saxitoxin is a potent non-peptide neurotoxin produced by dinoflagellates that blocks voltage-gated sodium channels, responsible for paralytic shellfish poisoning.
Chemical Identity
| Property | Value |
|---|---|
| Chemical Formula | C10H17N7O4 |
| Molecular Weight | 299.29 g/mol |
| CAS Number | 35523-89-8 |
| IUPAC Name | Saxitoxin (paralytic shellfish toxin) |
| Peptide Class | Non-peptide neurotoxin (marine toxin) |
| Origin | Alexandrium, Gymnodinium dinoflagellates |
Structure
Saxitoxin (STX) is a non-peptide tetrahydropurine neurotoxin produced by marine dinoflagellates (Alexandrium, Gymnodinium, Pyrodinium species) and freshwater cyanobacteria (Anabaena, Cylindrospermopsis, Lyngbya). Like tetrodotoxin, it is not a peptide but is included in marine toxin classifications. The molecule contains a hydrated ketone, two guanidinium groups (one protonated at physiological pH), and a tricyclic skeleton. Over 50 saxitoxin analogs (neoSTX, GTX, C toxins) exist with varying potency.
Mechanism of Action
Saxitoxin blocks voltage-gated sodium channels (Nav) by binding to site 1:
- Binding: The two guanidinium groups interact with acidic residues in the channel outer pore
- Selectivity: Blocks Nav1.1-Nav1.7 with high affinity (IC50 ~ 1-10 nM)
- Effect: Prevents sodium influx and action potential propagation
- Difference from TTX: Slightly different binding orientation; STX interacts with more channel residues
The result is rapid neuromuscular paralysis through blockade of peripheral motor and sensory nerves.
Biological Functions
Saxitoxin functions as a chemical defense:
- Dinoflagellates: Produced during harmful algal blooms (red tides)
- Bioaccumulation: Filter-feeding shellfish concentrate STX from dinoflagellates
- Food web: Transfer through fish, crabs, and other marine organisms
- Ecological role: Protects producer organisms from predation
Clinical Significance
Paralytic shellfish poisoning (PSP) is caused by ingestion of saxitoxin-contaminated shellfish:
- Symptoms: Perioral tingling, numbness, ataxia, progressive paralysis, respiratory failure
- Onset: 15-60 minutes after ingestion
- Lethality: Fatal in 1-12% of cases; no antidote
- Treatment: Supportive care (mechanical ventilation)
- Monitoring: Shellfish harvesting closures during algal blooms
Saxitoxin has been classified as a potential chemical weapon (Schedule 1, Chemical Weapons Convention).
Research Applications
- Ion channel research: Mapping sodium channel subtypes and distributions
- Environmental monitoring: STX detection in shellfish using receptor binding assays
- Neuroscience: Tool for studying action potential propagation
- Military interest: Studied for protective equipment and detection methods
Safety and Side Effects
Saxitoxin is extremely toxic (lethal dose ~0.5-1 mg in humans). Cooking does not destroy the toxin. The FDA action level for saxitoxin in shellfish is 80 micrograms/100 g. Routine monitoring of shellfish harvesting areas prevents most human cases. Tetrodotoxin and saxitoxin have additive toxic effects when co-ingested.
References
- Llewellyn, L.E. (2006). Saxitoxin, a toxic marine natural product. Natural Product Reports, 23, 1-23.
- Etheridge, S.M. (2010). Paralytic shellfish poisoning: seafood safety and human health perspectives. Toxicon, 56, 108-122.
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