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A centipede weighing about 3 grams subdued a mouse weighing about 45 grams within 30 seconds in an observation reported by researchers in 2018. They traced much of the effect to a peptide in the venom they named Ssm Spooky Toxin, or SsTx. The finding helps explain how the venom of one centipede species can rapidly affect prey—but it does not mean SsTx is the only active venom component or establish a treatment for human bites.
What is the deadly component?
SsTx is a peptide purified from the venom of the Chinese red-headed, or golden-head, centipede Scolopendra subspinipes mutilans. Lei Luo and colleagues reported identifying it in a study published in Proceedings of the National Academy of Sciences (PNAS) on January 22, 2018. The study linked the toxin to the striking prey observation and found that it blocks KCNQ-family potassium channels. Read the PNAS study.
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The name Ssm Spooky Toxin refers to the species abbreviation and the peptide, not to a substance shown to be present in every centipede’s venom. The experiments focused on this one species; centipede venoms are mixtures, and the results do not establish SsTx as the sole contributor to all effects.
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How does SsTx affect potassium channels?
KCNQ channels help regulate electrical activity in cells by allowing potassium ions to move across cell membranes. In laboratory channel assays, SsTx inhibited KCNQ1, KCNQ2, KCNQ4 and KCNQ5. The reported IC50 values were about 2.5–2.8 micromolar for the tested channel forms; an IC50 is the concentration that produces 50% inhibition under the assay conditions, not a dose that predicts an outcome in a person.
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The researchers’ experiments pointed to interaction with the channels’ outer pore region. Changing either of two positively charged toxin residues—arginine at position 12 (R12) or lysine at position 13 (K13)—substantially weakened inhibition. This supports a role for those residues in the toxin’s binding surface.
What do the structure and size tell us?
The mature SsTx peptide contains 53 amino acids and has a reported molecular weight of 6,017.5 daltons. The paper describes two disulfide bridges. The toxin is produced from a 76-amino-acid precursor; removal of a 23-amino-acid signal peptide leaves the mature peptide. The authors determined its structure by solution NMR; the structure is deposited as PDB 5X0S.
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What did the experiments establish about venom effects?
The study examined effects at several experimental levels. Purified-toxin channel assays showed KCNQ inhibition. Tissue and animal experiments reported cardiovascular effects, alongside nervous-system and respiratory effects. Removing SsTx from crude venom greatly reduced the venom’s activity in a vessel assay, evidence that it was a major cardiovascular-active component of this species’ venom in that test—not proof that other venom constituents are unimportant.
The mouse observation is similarly specific: the authors reported a roughly 3-gram S. subspinipes mutilans subduing a roughly 45-gram mouse within 30 seconds. It is an observation in the study, not a typical or guaranteed duration for centipede attacks generally.
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Does retigabine treat a centipede bite?
The researchers tested retigabine, a drug that opens KCNQ channels, and reported that it reversed effects in their experimental work. They proposed KCNQ opening as a possible therapeutic strategy. That result is not evidence that retigabine is a proven or safe treatment for a human centipede bite, and the study did not establish a clinical protocol. Do not take or administer retigabine for a bite on the basis of this research; seek medical advice for a bite or concerning symptoms.
How far can the finding be generalized?
The paper’s evidence concerns one centipede species and laboratory, tissue and animal models. It supports a role for SsTx in the studied venom’s effects, especially its cardiovascular activity, but does not show that all centipedes use the same toxin, that every bite causes the same symptoms, or that KCNQ blockade explains every effect of centipede venom. A Chemistry World report published January 25, 2018, quoted venom expert Ronald Jenner describing the work as evidence that a potassium-channel-blocking neurotoxin may be a major weapon against large vertebrate prey; neuroscientist Ian Mellor cautioned that multiple neurotoxic components may combine to cause rapid death. Chemistry World’s report provides that outside perspective.
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