Alexandra Lindsay

Cyanide Poisoning

An increasing problem

 

Murder, terrorism and suicide...

 

These are the scenarios generally associated with cyanide poisoning. One would hope these are rare occurrences but incidences of cyanide poisoning are all too common and increasing. The reason for this is that when man-made polymers such as polyacrylonitrile, nylon and melamine are burned they produce hydrogen cyanide (HCN) gas. These substances are used in clothes and furnishings and so HCN gas can be produced during a fire and anyone near the fire can is exposed to it.

Hydrogen cyanide was first isolated in 1782 by

Scheele. He later went on to provide a practical

demonstration of its toxic effects by accidentally

becoming its victim. It is often associated with

the smell of bitter almonds but in fact only about

40% of people can smell it.

Cyanide is naturally present in everyone's blood

in very small amounts, and people who smoke tend

to have more in their blood than people who don't.

This would be a good moment to define 'cyanide'.

Cyanide is the negatively charged ion, CN- but at

physiological pH 7.4, when unbound, this is in the

form of hydrogen cyanide, HCN. Once cyanide is taken

into the blood stream the majority (92-99%) is found

bound to hemoglobin (Hb) in red blood cells. From

there it is taken to the body's tissues where it

binds to an enzyme called cytochrome oxidase and

stops cells from being able to use oxygen. The signs

and symptoms of cyanide poisoning range from headache,

difficulty in breathing and vomiting to unconsciousness

and death. People who have recovered from cyanide

poisoning do not usually suffer any long-term effects.

Cyanide can be metabolized rapidly and is generally

converted to thiocyanate by an enzyme called rhodanese.

Thiocyanate is much less toxic than cyanide and

the body can then get rid of this. But (there had

to be a but!) the enzyme needs another chemical,

thiosulfate, to be able to do this and this can

be used up quite quickly.

Cyanide and cyanide containing compounds are used

in lots of industrial processes such as electroplating,

chemical synthesis and fumigation. Some food types

contain compounds called cyanogenic glycosides which

can be converted to cyanide in the body; these include

cassava roots, lima beans and bamboo shoots. In

addition drugs such as sodium nitroprusside, sometimes

used for the treatment of hypertension and Laetrile,

an anti-cancer agent, also release cyanide into

the circulation.

Cyanide salts are the forms which historically

have been used for suicide and murder. They can't

be bought as easily now by the public and so they

aren't used much for this now. Nevertheless there

are recent cases, for example in 1982 seven people

in Chicago were killed by Tylenol tablets (painkillers)

which were spiked with cyanide salts.

Figure 1: The effects of cyanide within the body. Hydrogen cyanide gas (HCN) is inhaled and locks onto haemoglobin, the oxygen-carrying molecule in red blood cells (bottom right). It is then distributed via the bloodstream to cells throughout the body where it binds to an important metabolic enzyme called cytochrome oxidase (bottom left), preventing cells from using oxygen to produce energy. In this way cyanide effectively chemically asphyxiates the body.

 

There are antidotes available for cyanide poisoning

but they can have bad side effects or be toxic themselves.

For example some drugs act to change normal hemoglobin

to another form called methemoglobin, as cyanide

has a high affinity for this type of hemoglobin,

which binds the cyanide and stops it from reaching

the tissues. Unfortunately oxidized hemoglobin does

not bind oxygen and so obviously it is not desirable

to have too much of this form present. Extra thiosulfate

can also be given so that more cyanide can be metabolized,

this is often used with methemoglobin formers. Cobalt

(II) compounds form cyanide complexes and these

are used but they are highly toxic. Hydroxycobalamin

(Vitamin B12a) which forms cyanocobalamin (Vitamin

B12) has lower toxicity but it is not widely available.

Due to the toxicity of these drugs it is important

they are given in doses related to the amount of

cyanide in the body.

So, yet another reason to make sure you've checked

the batteries work in your smoke alarm!

 

- April 2006

About the Author

Alexandra Lindsay is a post-docoral scientist in the Department of Bioengineering at Imperial College London.