Apamin

Apamin is a neurotoxin which selectively blocks SK channels, a type of Ca²⁺-activated K⁺ channels expressed in the central nervous system. The final 18 amino acid polypeptide is a component of apitoxin (bee venom).^[1] It is used primarily in biomedical research to study the electrical properties of SK channels and their role in the afterhyperpolarizations occurring immediately following an action potential.^[2]

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Origin

Apamin is a neurotoxin that was originally isolated from Apis mellifera, the Western honey bee. The venom of the honeybee consists of many more products, like melittin, the MCD peptide and phospholipase A2.

Chemistry

Apamin is a polypeptide possessing an amino acid sequence of C00H-Cys-Asn-Cys-Lys-Ala-Pro-Glu-Thr-Ala-Leu-Cys-Ala-Arg-Arg-Cys-Gln-Gln-His-NH2 (with disulfide bonds between Cys1-Cys11 and Cys3-Cys15). Because honeybee venom is a complex mixture of short peptides and proteins, it is difficult to isolate apamin. The isolation can be done by electrophoresis,^[3] or by chromatography.^[4][5]

Pharmacology

Apamin binds to the SK channels (small conductance Ca²⁺-activated K⁺ channels) in the brain and spinal cord and inhibits them.^[6] It inhibits the three cloned SK channel subtypes (SK1, SK2, and SK3) with different affinity, highest affinity for SK2, lowest for SK1, and intermediate for SK3 channels. Heteromers show intermediate sensitivity. Most likely, apamin acts as a pore blocker, although residues both inside and outside of the pore region of the SK channels participate in apamin binding.^[7] The SK channels are present in a wide range of excitable and non-excitable cells, including cells in the central nervous system, intestinal myocytes, endothelial cells, and hepatocytes. SK channels, when activated, contribute to afterhyperpolarizations in neurons, which control neuronal excitability. Intracellular Ca²⁺ binding to calmodulin can activate these channels. Channel deactivation can take place through dissociation of Ca²⁺ from calmodulin.^[8] Inhibition of SK channels by apamin will increase the neuronal excitability and lower the threshold for generating an action potential. Other toxins that block SK channels are tamapin and scyllatoxin.

Toxicity

Symptoms following bee sting or apamin poisoning may include:

• local effects: burning or stinging pain, swelling, redness.
• severe systemic reactions: swelling of the tongue and throat, difficulty breathing, and shock.
• development of optic neuritis and atrophy.
• atrial fibrillation, cerebral infarction, acute myocardial infarction, Fisher's syndrome, acute inflammatory polyradiculopathy (Guillain-Barre syndrome), claw hand (through a central action of apamin on the spinal cord and a peripheral action in the form of median and ulnar neuritis, causing spasms of the long flexors in the forearm).^[9]
• dramatic haemorrhagic effect in the lungs.^[10]

Patients poisoned with bee venom can be treated with anti-inflammatory medication, antihistamines and oral prednisolone.^[9]

Therapeutic Use

SK channel blockers such as apamin can have therapeutic applications, for example on the peripheral cells (e.g. the insulin releasing cells of the pancreas) and on the central nervous system where there is evidence for a role of SK channels in memory processes, both general and specifically hippocampal.^[8]

SK channels have been proposed as targets for the treatment of ataxia, epilepsy, memory disorders, and possibly schizophrenia and Parkinson's disease.

References