Toxicity of Mercury and Cyanide

Sources and Toxicity of Mercury (Hg)

Sources: Industrial mining, waste, pesticides, coal etc.

The Toxicity of Mercury depends on its chemical species as shown below

1) Properties of Mercury (Hg)
o Elemental Hg is fairly inert and non-toxic.
o Vapour highly toxic when inhailed
o If swallowed, it excreted without serious damage.

2) Mercuous ion(Hg2+)
o Insoluble as chloride
o Low toxicity


3) Mercuric ion(Hg3+)
o Toxic but not across biological membrane
o It attacks the sulphur containing amino acid of protein.


4) Organomercurials
o Highly toxic
o Partially causes irreversible nerve and brain damage.
o Easily transported to the biological membrane.
o Stored in flat tissue.
5) Diorganomercurials
o Low toxic
o Can be converted into Organomercurials.

6) Mercuric Sulphide (HgS): highly insoluble and non-toxic.




Biochemical effects of Cyanides

Cyanides occurs in seeds of fruits such as apples, apricots, cherries, peaches and plums. Cyanide in the plant is bonded to glycoside(sugar) called amygdalin and is released by enzymatic or acidic hydrolysis. Cyanide enters the environment from many sources. HCN is employed as a fumigating agent to destroy rodents in grain bins, building in electroplating and metal cleaning industry.

Cyanide exerts its toxic action by inhibiting oxidative enzymes from mediating the process by which oxygen is utilized to complete the production of ATP n the mitochondria

In the first step cyanide bind to ferricoytochrome oxydase. The important products are the ATP

Step 1: Fe (3)-oxide + glucose >> Fe (2)-oxide
Step 2: Fe (2)-oxide + 2H+ + 1/2O2 >> Fe(3) + H2O + ADP/ATP

Cyanide interferes with step 1 above by forming a bond with Fe(3)-oxide, which is thereby inactivated so that the reaction in step 2, the energy producing process is prevented.