Poisons act in a variety of ways, but the most deadly of them is by inhibiting enzymes. They may do this by tying up an enzyme in the form of a stable complex, by denaturing it, or by blocking its formation from its apoenzyme and cofactor.
Cyanide acts almost instantly and only a small amount is needed for a lethal dose. The average fatal dose is only 50 or 60 milligram. Cyanide is used as gaseous hydrogen cyanide (H-C=N) and as solid salts, which contain the cyanide ions (H=N). The gas is used for extermination of insects and rodents in ships, warehouses, and railway cars and on certain fruit trees.
Cyanide has more affinity with iron atoms. So it gets tied immediately with the iron atom, which forms haeme part of the Haemoglobin. This makes the iron atom unavailable to carry oxygen atom to the tissues through haemoglobin. So oxygen deficiency at the tissue level occurs. This is called Hypoxia.
Since the poisonous substance causes it, it is also called histotoxic hypoxia. In histotoxic hypoxia the brain is affected first. It results in loss of consciousness in 10-20 seconds and death in 4-5 minutes.
Cyanide blocks the oxidation of glucose inside a cell by forming a stable complex with the oxidation enzymes. Certain enzymes of our body cells, like cytochrome oxidases, contain iron and copper atoms. They normally act by providing electrons for the reduction of oxygen in the cell. Cyanide ties up those mobile electrons, rendering them unavailable for the reduction process.
Thus, cyanide brings an abrupt end to cellular respiration. When this process, which is holding the life of an individual is stopped abruptly, it causes death in a matter of minutes, since all the cells in the body die immediately.
Administration of antidote for cyanide poison is not possible, since the fatal end comes immediately within minutes to a person who has consumed the poison. But if the quantity consumed is below the lethal dose sodium nitrate and sodium thiosulphate may be used to treat cyanide poisoning.
Source : The Hindu