Apamin

Chemical Identity

Property	Value
Chemical Formula	C79H131N31O24S4
Molecular Weight	2027.34 g/mol
CAS Number	24345-16-2
IUPAC Name	Apamin (Apis mellifera)
Peptide Class	Bee Venom Neurotoxin
Origin	Apis mellifera (honeybee)
Disulfide Bonds	2

Structure

Apamin is an 18-amino acid neurotoxin from honeybee venom and the smallest known peptide neurotoxin. It contains two intramolecular disulfide bonds (Cys1-Cys11, Cys3-Cys15) that create a rigid compact structure with a characteristic bee venom toxin fold. The peptide has a net positive charge (+4) from two arginine residues and an N-terminal amino group. This rigid scaffold positions key residues for high-affinity SK channel binding.

Mechanism of Action

Apamin selectively blocks small-conductance calcium-activated potassium channels (SK channels, KCa2.1-KCa2.3):

• SK2 (KCa2.2): Highest sensitivity (IC50 ~ 60-140 pM)
• SK3 (KCa2.3): Moderate sensitivity (IC50 ~ 1-2 nM)
• SK1 (KCa2.1): Lower sensitivity (IC50 ~ 10 nM)

The toxin binds to the external face of the channel pore, physically blocking potassium efflux. SK channels mediate the afterhyperpolarization (AHP) following action potentials in neurons. Apamin blockade reduces the AHP, increasing neuronal excitability and firing rate.

Biological Functions

Apamin is a minor component (~2-3%) of honeybee venom:

• Neurotoxicity: Causes hyperexcitability, convulsions, and death at high doses in insects and mammals
• Complementary to melittin: Apamin targets neurons while melittin causes direct tissue damage
• Predatory function: Part of the defensive venom cocktail

Research Applications

Apamin is an invaluable research tool in neuroscience:

• SK channel identification: Defining the role of SK channels in neuronal excitability
• Learning and memory: SK channel modulation affects hippocampal synaptic plasticity
• Parkinson’s disease: SK channel blockade improves dopaminergic neuron function in models
• Atrial fibrillation: SK channels in cardiac atria; apamin-sensitive currents modulate atrial repolarization

No approved therapeutic uses, but apamin analogs are in development for neurological and cardiac conditions.

Safety and Side Effects

Apamin is neurotoxic, causing hyperexcitability, tremors, and convulsions at toxic doses. Bee stings typically deliver sub-toxic apamin doses (0.05-0.1 micrograms per sting). Allergic reactions to bee venom (apitoxin) may involve apamin as one of several allergenic components. The peptide is not absorbed through intact skin.

References

• Adelman, J.P., et al. (2012). Small-conductance Ca2+-activated K+ channels: form and function. Annual Review of Physiology, 74, 245-269.
• Castle, N.A., et al. (1989). Apamin block of small-conductance calcium-activated potassium channels. Journal of General Physiology, 93, 479-498.