Monday, 22 October 2012

Lecture : Nature's Chemical Warfare Agents

A recent talk in the Nottingham University Public Science series, entitled “Chemical Warfare to Chemotherapy”, looked at the way in which chemical originally used in warfare have found surprising uses in the medicine.

Presented by Dr Rob Stockman, Associate Professor and Reader of Organic Chemistry at the University of Nottingham, the talk was fascinating and forms the basis of this post, together with various additions from the Internet, which are largely here to dumb things down sufficiently for Notts Science to understand them.

The talk looked at a number of specific examples, the first of which occurred hundreds of years ago..

Poison Plants
Spanish soldiers colonising South America noted that the indigenous peoples used poison tipped darts to kill their prey when hunting. The darts were tipped with a plant extract known as “curare” which acted as a muscle relaxant, causing the animal to die from asphyxiation as it could no longer contract its respiratory muscles.

Curare attracted the interest of a number of medical men, including Sir Benjamin Collins Brody, whoc demonstrated that, if an animals respiration is maintained artificially, the effects of curare wear off after a few hours.

Further investigations and research over the years, together with wider advances in medical science, resulted in the active ingredient of curare (d-tubocurarine) being introduced into clinical practice as an anaesthetic in 1942, having only been isolated from curare in 1935.

Prepare to feel a little numb. . . .

Here comes the science part
It is worth taking a moment to describe the structure of a neuron, which is shown below. The signals travel from left to right.

A Neuron, yesterday

The electrical nerve signals from the dendrites travel down the axon (which has a protective Mylin sheath) to the Axon terminal, where there is a junction with a muscle cell. Some of the key elements of the junction are shown in the schematic below. When a signals travelling along the axon reaches the terminal they cause a vesicle to release neurotransmitters across the synaptic gap where they activate the receptors and thus the muscle fibre. The neurotransmitters are then broken down by enzymes and recycled to produce fresh neurotransmitters.

A Synapse

d-tubocurarine works by blocking the receptors on the post synaptic terminal temporarily. Whilst it is not longer used due to side-effects, a number of derivative drugs are still widely administered, largely to allow the intubation (a procedure in which a flexible plastic tube is inserted into the windpipe to maintain an open airway), examples being Suxamethonium, which has the twin benefits of being very fast acting and also only lasting 5-10minutes.

The neurotransmitters mentioned in the diagram earlier are a chemical called Acetylcholine, or ACh. The most important part of this molecule is the Nitrogen atom and its surrounding CH3 groups.

Chemicals that replicate the effect of a biological signal are called AGONISTS, while those that block the signal are called ANTAGONISTS.

ACH in a the naked and clothed forms.

Toxic Mushrooms
A number of fungi found around the world, such as the Ivory Funnel toadstool or the Wood Pinkgill contain potentially fatal amounts of Muscarine (first isolated in 1869), which acts on the receptors in the synapse, causing them to fire continually. Muscarine is not easily metabolised (broken down) by the body, which partly explains why it is so toxic. The receptors that are susceptible to activation by Muscarine are known as Muscarinic ACH receptors.

Research on Muscarine has resulted in a number of useful drugs, such as Carbachol (used in the treatment of Glaucoma and for other ophthalmic purposes); Methacholine (used to diagnose asthma) and Bethanechol (which has many uses, including reversal of the effects of Atrophine which is given preoperatively to prevent voiding of the bowel or bladder during surgery)

Pretty. As in Pretty Deadly

Toxic Frogs
And if you can avoid the deadly mushrooms, you aren’t home free, as there are also deadly frogs out there whose skins contain called Histrionicotoxins, which bind to other types of ACh receptors, known as Nicotinic ACh receptors.

So, if you see the Harlequin Poison Frog, or the be sure to give it a respectfully wide berth. (Interestingly, the frogs do not manufacture the toxins themselves, but instead absorb them from the insects they eat.)

The Harlequin Poison Frog, so called because
of their support for the London Rugby Union team

Another type of Histrionicotoxin is carried in the skin of the operatically named “Phantasmal Poison Frog” which is also used by native South Americans to produce poison darts. In this case the chemical found in the frogs skin, Epibatidine, was researched in the 1970s and found to be an extremely powerful painkiller.

Unfortunately, the dose required to achieve a painkilling effect was very close to the dose required to kill you. So Epibatimine is something that is unlikely to ever appear , even in prescription only form, at your local Boots.

The Phantasmal Poison Frog, with a rather dodgy Henna tattoo

Toxic Bacteria
Neurotoxins are not restricted to the animal world. For example, some bacterial algae blooms are composed of bacteria that contain a neurotoxin that bears the technical name Anatoxin-A, but is also known as the “Very Fast Death Factor” - a name that does not leave much room for misinterpretation.

This toxin binds permanently to Nicotinic receptors, causing muscles to contract permanently so that the victim cannot breathe and quickly dies.

Don't touch, this really is not good for you

And some ACh Antagonists
In contrast to the above examples, which artificially activate neuron receptors, there are other chemicals in the natural world that bind with receptors but do not actually activate them. These Antagonists essentially cause some degree of paralysis.

Perhaps the most famous example of this is the chemical Atrophine, which can be extracted from plants such as Deadly Nightshade or Mandrake. Indeed, the Italian name for Deadly Nightshade is “Belladona” (Beautiful woman) due to its use as a means of dilating women’s pupils to make them more beautiful. But high doses of Atrophine can cause hallucinations, confusion and dilated pupils. It has been used as a poison since Roman times.

Another source of Atrophine and other neuron receptor antagonists is the plant Datura, which has long been used as a hallucinogen and poison. In fact, the wide variety of toxicity from plant to plant has resulted in many youngsters looking for the former effect to end up falling victim to the latter property

But medicinally, Atrophine is very widely used to dilate the pupils and to increase heart rate.

Datura also contains the chemical Scopolamine, which can be used as a “truth drug” due to its ability to cross the blood-brain barrier. And is also widely used to treat sea-sickness.

WARNING : Datura contains high levels of Atrohpine

AChE inhibitors
After Acetylcholine has activated the neuron receptor, it needs to be removed from the receptor and broken down. This is done by the fiendishly complicated looking enzyme Acetyl Choline Esterase.

Enzymes and Proteins are just the coolest things. This is AChE

But the action of AChE can be blocked, for example by nerve agents such as Sarin (developed during WW2. These agents are particularly dangerous because they can be absorbed through the skin and eyes. With AChE blocked, the neurotransmitters cannot be broken down and remain in the “on” position, constantly activating the muscles - causing paralysis, respiratory failure and eventually death.

However, research has also shown that some AChE inhibitors can be medically useful, for example Neostigmine is used to treat Myasthernia, which is a progressive weakness of the muscles, possibly because ACh is metabolised too quickly in these patients.

Mustard Gas
This infamous poison gas used in WW1 was responsible for some 4,000 deaths and 20,000 casualties by causing internal and external bleeding, blistering and stripping of the mucous membranes in the lungs - something so painful that many affected soldiers had to be strapped to be their beds - until they died some 4-5 weeks later.
In 1943, an Allied ship carrying a secret load of Mustard gas was berthed at the Italian port of Bari when it was hit in an air raid. The resulting escape of mustard gas injured hundreds - but medics noted that the victims had lowered counts of white blood cells, suggesting that the poison might be of use in treating the cancer Hodgkins Lymphona.
Incredibly, this was the birth of the science of chemotherapy, which aims to kill cancer by interfering with cell division. Drugs related to mustard gas, such as the so-called “Nitrogen Mustards” Chlorambucil and Melphalam were developed. These still have unpleasant side-effects and affect realtivley fast dividing cells such as hair and the stomach lining.

Explosives as a Heart Drug
Even explosives such as Nitroglycerin and PETN can find medicinal uses. In this case as vascularodilators for the treatment of heart conditions. The drugs (e.g. Lentonitrat) work by releasing the signalling gas nitric oxide.

The Opiates
No discussion of drugs that have originated in the natural world would be complete without mentioning Opiates (such as Morphine, Codeine and Diamorphine(Heroin) )

They work as agonists on the central nervous system and all have serious side effects such as respiratory depression, nausea and dependency.

Good explanation of neurotransmitters here
Nice item on Histrionicotoxin

Tubocurarine,Algal Bloom, Neuron, Amanita_muscaria, Datura Oophaga histrionica, Phantasm Frog, AChE

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