How does potassium cyanide cause sudden death?

Potassium cyanide when consumed causes death by gradually arresting the supply of oxygen to our body cells by forming stable complexes with hemoglobin (present in the blood) and cytochrome (a protein which helps in the respiration of the cells) and depriving them of their capacity to transport or exchange oxygen.

Normally, oxygen is carried to different parts of the body from the lungs by the blood using hemoglobin -the iron-containing, oxygen-carrying molecule of the red blood cells.

Hemoglobin is made up of a globular protein and four heme groups. The iron (in ferrous state) present in these heme complexes can bond to either an oxygen molecule or a water molecule or exchange them one for the other without much difficulty. It is because of this ability to exchange them, hemoglobin is able to pick up oxygen from the lungs, carry it to the body cells and bring back water in return.

The body cells ‘respire’ oxygen with the help of Myoglobin (hemoglobin like proteins present in the cells) and cytochrome (which function as electron carrier). One form of this cytochrome and hemoglobin are responsible for the sudden death due to cyanide poisoning.

When potassium cyanide is consumed, it splits into a potassium ion and a cyanide ion. The cyanide ion has a strong affinity to the ferrous ion than what oxygen has. As a result it occupies the site meant for oxygen in the hemoglobin. This process is irreversible and so it prevents transfer of oxygen.

Also, one form of cytochrome, designated as cytochrome-a, also binds with the cyanide ion and stabilizes the iron to such an extent that it does not take part in the electron transfer to the cell. This prevents oxygen in take by the cell. The symptoms of cyanide poisoning are giddiness, headache and bluish tinge of the skin. All these are indicators of lack of oxygen supply to various parts of the body. If not treated immediately, unconsciousness and death will follow.

Inhalation of amyl nitrate or injection of sodium nitrite to oxidize some of the hemoglobin to methymoglobin provides relief. Methymoglobin binds to cyanide ion more tightly than hemoglobin or cytochrome-a and helps in the removal of cyanide from the system. Carbon monoxide (CO) also has a similar effect when inhaled. It forms a stable compound called carboxy hemoglobin and deprives it of its oxygen carrying capacity.