Taicatoxin (TCX) is a snake
toxin
A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849– ...
that blocks voltage-dependent
L-type calcium channel
The L-type calcium channel (also known as the dihydropyridine channel, or DHP channel) is part of the high-voltage activated family of voltage-dependent calcium channel.
"L" stands for long-lasting referring to the length of activation. This c ...
s and
small conductance Ca2+-activated K+ channels. The name taicatoxin (TAIpan + CAlcium + TOXIN) is derived from its natural source, the
taipan
Taipans are snakes of the genus ''Oxyuranus'' in the elapid family. They are large, fast-moving, highly venomous, and endemic to Australia and New Guinea. Three species are recognised, one of which, the coastal taipan, has two subspecies. Taipan ...
snake, the site of its action,
calcium channel
A calcium channel is an ion channel which shows selective permeability to calcium ions. It is sometimes synonymous with voltage-gated calcium channel, although there are also ligand-gated calcium channels.
Comparison tables
The following tables e ...
s, and from its function as a
toxin
A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849– ...
. Taicatoxin was isolated from the venom of Australian taipan snake, ''Oxyuranus scutellatus scutellatus''. TCX is a secreted
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
, produced in the
venom
Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a sti ...
gland of the snake.
Chemistry
Through
SDS-PAGE analysis, TCX () was determined to be a complex held together by
non-covalent forces of the following three polypeptides in a
stoichiometry
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
of 1:1:4 respectively:
* a α-
neurotoxin
Neurotoxins are toxins that are destructive to nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insultsSpencer 2000 that can adversely affect function in both developing and mature ner ...
-like
peptide
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A ...
(8 kDa),
* a neurotoxic
phospholipase (16 kDa;) and
* a
serine protease inhibitor
Serpins are a superfamily of proteins with similar structures that were first identified for their protease inhibition activity and are found in all kingdoms of life. The acronym serpin was originally coined because the first serpins to be i ...
(7 kDa;
).
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
Gel electrophoresis is a method for separation and analysis of biomacromolecules ( DNA, RNA, proteins, etc.) and their fragments, based on their size and charge. It is used in clinical chemistry to separate proteins by charge or size (IEF ...
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 Ca
2+-activated K
+ channels in the
chromaffin cells and in the brain.
It has a high affinity for the
125I-
apamin
Apamin is an 18 amino acid globular peptide neurotoxin found in apitoxin ( bee venom). Dry bee venom consists of 2–3% of apamin. Apamin selectively blocks SK channels, a type of Ca2+-activated K+ channel expressed in the central nervous syste ...
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
Calciseptine (CaS) is a natural neurotoxin isolated from the black mamba ''Dendroaspis p. polylepis'' venom. This toxin consists of 60 amino acids with four disulfide bonds. Calciseptine specifically blocks L-type calcium channels, but not other v ...
, ω-
conotoxin, ω-
agatoxin.
Mode of action
It lowers the plateau of the
action potential
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
, 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
Motility is the ability of an organism to move independently, using metabolic energy.
Definitions
Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
too, by blocking the calcium traffic and preventing the cell shortening and elongation.
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
The guinea pig or domestic guinea pig (''Cavia porcellus''), also known as the cavy or domestic cavy (), is a species of rodent belonging to the genus ''Cavia'' in the family Caviidae. Breeders tend to use the word ''cavy'' to describe the ani ...
prevented the cell shortening induced by high K
+ (50 mM) and the cell elongation induced by ionomycine (10 μM).
This is because 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 IC
50 between 10-500 nM. Also was seen to evoke severe
arrhythmias and prolonged changes in the intercellular electrical coupling.
References
External links
*
{{Toxins
Neurotoxins
Ion channel toxins
Vertebrate toxins