Taicatoxin

Taicatoxin (TCX) is a snake toxin that blocks voltage-dependent L-type calcium channels and small conductance Ca²⁺-activated K⁺ channels. The name taicatoxin (TAIpan + CAlcium + TOXIN) is derived from its natural source, the taipan snake, the site of its action, calcium channels, and from its function as a toxin. Taicatoxin was isolated from the venom of Australian taipan snake, Oxyuranus scutellatus scutellatus. TCX is a secreted protein, produced in the venom gland of the snake.^[1]

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Chemistry

Through SDS-PAGE analysis, TCX (112154-17-3 ) was determined to be a complex held together by non-covalent forces of the following three polypeptides in a stoichiometry of 1:1:4 respectively:^[2]

• a α-neurotoxin-like peptide (8 kDa),
• a neurotoxic phospholipase (16 kDa; UniProt Q7LZG2), and
• a serine protease inhibitor (7 kDa; UniProt Q7LZE4).

The active complex was isolated by ion exchange chromatography through DE-Cellulose and two steps of Cm-Cellulose chromatography at pH = 4.7 and pH = 6.0, respectively. It migrates in beta-alanine-acetate-urea gel electrophoresis as a single compound. The phospholipase activity can be separated by affinity chromatography, using a phospholipid analog (PC-Sepharose). The alpha-neurotoxin-like peptide can be separated from the protease inhibitor, Sephadex G-50 gel filtration chromatography can be used, in the presence of high salt (1M NaCl) and alkaline conditions (pH = 8.2). The amino sequence of the protease inhibitor was determined by using the automatic Edman degradation method.

Target

Taicatoxin acts on the voltage-dependent L-type calcium channels from the heart, and on the small conductance Ca²⁺-activated K⁺ channels in the chromaffin cells and in the brain.^[3] It has a high affinity for the ¹²⁵I-apamin acceptor-binding sites of the rat synaptosomal membranes (K_i = 1.45±0.22 nM) and blocks affinity-labeling of a 33-kDa ¹²⁵I-apamin-binding polypeptide. Other neurotoxins that act on the calcium channels are calcicludine, calciseptine, ω-conotoxin, ω-agatoxin.

Mode of action

It lowers the plateau of the action potential, decreasing the duration and the concentration parameters in the heart muscle cells. It has been seen that the 16-kDa subunit exhibits phospholipase activity, inducing a release of acyl CoA and acyl carnitine, fact which has a negative effect on cell’s integrity and function. TCX is involved in the outer hair cell motility too, by blocking the calcium traffic and preventing the cell shortening and elongation.^[4][5] Taicatoxin has an inhibitory effect by reducing the affinity of ¹²⁵I-apamin for its acceptor and not by alteration of the acceptor binding site density.

Toxicity

A dose of 1 to 2 μg of taicatoxin can kill a mouse of 20 g in 2 hours. Pretreatment with taicatoxin (0.19 μM) on the outer hair cells of guinea pig prevented the cell shortening induced by high K⁺ (50 mM) and the cell elongation induced by ionomycine (10 μM).^[5] This is due to the fact that taicatoxin blocks the calcium influx through the calcium channels in the cell’s membrane. 50 nM of taicatoxin blocks the apamin-sensitive after-hyperpolarizing slow tail K⁺ currents in rat chromaffin cells, but not immediately; instead, 5 μM of this toxin immediately blocks the I_SK(Ca) tail current. It has been shown that taicatoxin blocks the calcium currents in heart cells with IC₅₀ between 10-500 nM. Also was seen to evoke severe arrhythmias and prolonged changes in the intercellular electrical coupling.^[6]

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