Handle With Care The Worlds Five Deadliest Poisons

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When asked to name a poison, people may well think of cyanide, arsenic or strychnine. But these are not the most toxic substances known. More poisonous than these, but still not near the upper part of the tree, is tetrodotoxin, the pufferfish toxin that poisons around 50 Japanese people every year. The fish is a delicacy in Japan, but can be lethal if prepared incorrectly. Incidentally, this was the poison favoured by evil assassin Rosa Klebb in James Bond film From Russia With Love. It also crops up in the blue-ringed octopus and was more recently discovered in tiny frogs in Brazil.

The LD5 0( Lethal Dose, 50%) the amount needed to kill 50% of the test population is how toxicity is most often assessed, and is usually quoted per kilogram of body weight. On this scale, for example, sodium cyanide comes out at around 6 milligrams per kg. The LD50 of tetrodoxotin, by comparison, is around 300 micrograms per kg if orally ingested, and as little as 10 micrograms per kg if injected.

Assessing toxicity is not easy. The chemical state of a substance is important, as is how we ingest it. If we swallowed liquid mercury metal( as distinct from inhaling the vapour ), it would very likely pass straight through us harmlessly. And yet when in 1996 an American professor got merely a drop-off or two of the compound dimethyl mercury on her rubber gloves, it penetrated the gloves and her scalp, sending her into a fatal coma some months later.

Nevertheless, here is a representative selection, in ascending order, of five genuinely deadly poisons, all at least a hundred times more toxic than cyanide, arsenic or strychnine.

5. Ricin

This extremely toxic plant poison was famously used to kill the Bulgarian dissident Georgi Markov, exiled in London. On September 7 1978, he was waiting for a bus near Waterloo Bridge, when he felt an impact on the back of his right thigh. Appearing round he saw a man bending down to pick up an umbrella. Markov was soon taken to hospital with a high fever and died 3 days later. An autopsy exposed a tiny sphere made of a platinum-iridium alloy in Markovs thigh. The realm had been drilled to take a small amount of ricin and may have been fired from an air rifle hidden in the umbrella.

Ricinus communis: find ricin here. Musum de Toulouse/ Flickr, CC BY-SA

Ricin is obtained from the beans of the castor petroleum plant( Ricinus communis ), which is cultivated to extract the petroleum the ricin remains in the solid fiber. It is a glycoprotein that interferes with protein synthesis in the cell, causing cell death. It has an LD50 of 1-20 milligrams per kg if orally ingested, but much less is required to kill if inhaled or injected( as in Markovs occurrence ).

4. VX

The only synthetic compound in our top five, VX is a nerve agent with the coherence of engine petroleum. It emerged from ICIs research into new insecticides in the early 1950 s but proved too toxic to use in agriculture. VX kills by interfering with the transmission of nerve messages between cells; this requires a molecule called acetylcholine. After acetylcholine has passed on its message, it needs to be broken down( otherwise it will maintain sending the message) by an enzyme catalyst called acetylcholinesterase. VX and other nerve agents stop this enzyme from running, so muscle contractions go out of control and you die of asphyxiation.

Nerve agents explained.

Nerve agents were made by both sides during the Cold War, but VX became particularly well-known after featuring in Hollywood blockbuster film The Rock. Merely one person is known to have been killed by VX, a former member of the Aum Shinrikyo cult, though some 4,000 sheep were killed by it in an accident in Skull Valley, Utah in 1968. It has an LD50 of as little as 3 micrograms per kg( although some reports suggest the above figures is a little higher ).

3. Batrachotoxin

Weve all heard of South American Indians utilizing venom-tipped blowpipes to hunt their prey. Curare is the best known, and comes from a plant. The more toxic, however, come from the scalps of tiny frogs and the deadliest of all is Batrachotoxin.

Native Indians in Western Colombia collect these frogs golden Phyllobates terribilis and multicoloured Phyllobates bicolor and sweat out the poison over a fire before putting it on their darts. The LD50 is around 2 micrograms per kg, means that an amount the size of two grains of table salt will kill you.

Batrachotoxin kills by interfering with sodium ion channels in the cells of muscles and nerves, jamming them open so that they are able to do not close. The continued migration of Na+ ions outcomes ultimately in heart failure.

A very poisonous bird.

Interestingly, captive-born frogs of these species are not poisonous, is recommended that the poison is derived from their diet. Indeed, virtually 30 years ago, Jack Dumbacher, an American ornithologist, was working in Papua New Guinea when he was scratched on the hand by one of the local Pitohui birds. He instinctively set his hand to his mouth, which started to go numb. Eventually, it was found that these birds on the opposite side of the world to Colombia have plumage containing the same poisonous molecule as the frogs. It is thought that both the birds and frogs procure the toxin from the beetles they eat although the poison is far less potent in the birds

2. Maitotoxin

There are a number of potent marine toxins, such as Saxitoxin, which are often the cause of poisoning after eating polluted shellfish. These are often links with harmful algal buds in the sea.

Dinoflagellate. fickleandfreckled/ Flickr, CC BY

Maitotoxin is the most lethal of these substances, reckoned to have a LD50 about an order of magnitude less than batrachotoxin. Formed by a dinoflagellate, a kind of marine plankton, it has a very complicated structure, which presents a massive challenge to synthetic chemists. Maitotoxin is a cardiotoxin, it exerts its effects by increasing the flow of calcium ions through the cardiac muscle membrane, causing heart failure.

1. Botulinum toxin

Scientists differ about the relative toxicities of substances, but they seem to agree that botulinum toxin, produced by anaerobic bacteria, is the most toxic substance known. Its LD50 is tiny at most 1 nanogram per kilogram can kill a human. Extrapolating from its effect on mice, an intravenous dose of merely 10 -7 g would be fatal to a 70 kg person.

It was first identified as a cause of food poisoning due to incorrectly prepared sausage( Latin, botulus ) in late-1 8th century Germany. There are several botulinum toxins, with kind A being the most potent. These are polypeptides, consisting of over 1,000 amino acid molecules joined together. They cause muscle paralysis by preventing the release of the signalling molecule( neurotransmitter) acetylcholine.

This same paralysing property is fundamental to the clinical use of the botulinum toxin in cosmetic Botox. Targeted injections of tiny amounts of the toxin stop particular muscles from running, relaxing muscles that would otherwise cause wrinkly scalp. But it has also been applied to a range of clinical conditions, such as paralysing muscles that, if untreated, would cause crossed eyes( strabismus ).

There is increasing interest in using the properties of toxic substances medicinally. The venom of the lethal Brazilian cavity viper, Bothrops jararaca , for example, contains blood-pressure reducing molecules that have led to pioneering therapies for high blood pressure.

As Paracelsus is reported to have said 500 years ago: All things are poison, and nothing is without poison: the dose alone makes a thing not poison. And he had a point. Ultimately, we are surrounded by potentially dangerous substances its the dose that makes it deadly.

Simon Cotton, Senior Lecturer in Chemistry, University of Birmingham

Read more: www.iflscience.com

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