How does potassium cyanide cause sudden death?

A: Ingestion of potassium cyanide causes death by gradually cutting off the oxygen supply to the body’s cells through the formation of complexes with hemoglobin and cytochrome (a protein that helps cells breathe), thus depriving them of their ability to transport or exchange oxygen.

Normally, oxygen is transported through the blood from the lungs to different parts of the body with the help of hemoglobin, the iron-containing, oxygen-carrying molecule in red blood cells.

Hemoglobin is composed of a globular protein and four heme groups. The iron (in the ferrous state) present in these heme complexes can easily bind to either an oxygen molecule or a water molecule, or exchange one for the other. This ability allows hemoglobin to absorb oxygen from the lungs, transport it to the cells, and bring water in return.

Cells breathe oxygen with the help of myoglobin (hemoglobin-like proteins in cells) and cytochrome, which transports electrons. Certain forms of cytochrome and hemoglobin also cause sudden death in cyanide poisoning.

When potassium cyanide is consumed, it splits into a potassium ion and a cyanide ion. The cyanide ion has a greater affinity for the iron ion than the cyanide ion. As a result, it occupies the space in the hemoglobin intended for oxygen. This process is irreversible and prevents oxygen transport.

A form of cytochrome, the so-called cytochrome-a, also binds to the cyanide ion and stabilizes the iron to such an extent that it does not participate in the transport of electrons to the cell. This prevents the cell from absorbing oxygen.

The symptoms of cyanide poisoning are dizziness, headaches and a bluish discoloration of the skin. If the poisoning is not treated immediately, unconsciousness and death can result.

A. Karthikeyani, Udumalpet; C. Ravi Chandra, Secunderabad; and R. Ramaa, Chennai