Heavy metals are transition metals found in the middle of the periodic table that are of environmental or physiological concern. The most important heavy metals that can be and have been used as poisons are lead(Pb), mercury(Hg), arsenic(As), cadmium(Cd), and iron(Fe).
The toxic effects produced by heavy metals are accomplished by binding to one or more reactive groups essential for normal physiological functions.
Mechanism of action of heavy metal toxicity
Of the four, Hg is highly toxic in its elemental form while the others are more dangerous in their cation forms (Pb2+ or Pb4+; Cd2+; As3+). This is because in their ionic forms they become soluble in water and are more easily transported in the body.
The metal cations have a strong affinity for –SH (sulfhydryl) Groups which are found in proteins (muscles in the body) and enzymes. These metals bind to the enzymes preventing them from working properly, stopping or altering their metabolic process.
Mercury is a metal that is a liquid at room temperature. It has three highly toxic forms: elemental (Hg liquid or vapor), compounds of inorganic salts (e.g. Hg(NO3)2, and organometallics (e.g. CH3Hg).
Mercury can enter the body by a number of different methods:
Mercury ions produce toxic effects by protein precipitation, enzyme inhibition, and generalized corrosive action. Mercury not only binds to sulfhydryl groups but also to phosphoryl (-PO3-), carboxyl (-COOH), amide (R-N-R), and amine (-NH2) groups. Proteins (including enzymes) with such groups readily available are susceptible to reaction with mercury. Once bound to mercury, most proteins are rendered inactive.
In the simplest terms, mercury binds irreversibly to an enzyme (proteins that create molecules) changing its conformation and preventing its normal substrate (molecule it works on) from binding.
Lead is not dangerous in its elemental form but rather when it forms ions (Pb2+, Pb4+)making it soluble. Lead is normally ingested through contaminated water or food.
Lead can interfere with the normal operation of hemoglobin, adversely affect the Central Nervous System (CNS) and causes developmental issues in children.
Lead affects the production of Heme by binding to and blocking the enzyme ALA-D. The reduced amount of Heme (the oxygen carrying molecule in blood) leads to anemia. As the amount of lead increases in concentration further damage is done to the circulatory system eventually leading to CNS damage and death.
Cadmium (Cd) is a metal found in Coal and released into the environment as coal is burned. Cadmium is most toxic in its ionic form Cd2+. The normal pathway by which cadmium finds its way into the body is through consumption. As long as concentrations remain low, some intake of cadmium is normal and acceptable. Acute concentrations, however lead to severe joint pain, bone diseases and kidney problems.
Cadmium acts as a poison by a number of pathways:
Arsenic oxides were the common poisons used for murder and suicide from roman times through to the middle ages.
Arsenic, which often is described as tasting like bitter almonds, is generally taken into the body by consumption or inhalation from smoke.
Arsenic kills by disrupting the biochemical pathways in the body that produce ATP. As we saw earlier, ATP is an energy molecule that is necessary for the energy that runs the majority of the body's respiratory system. Arsenic both blocks and competes with the chemicals that form ATP, leaving the body depleted of what it needs to keep up even the most basic cellular processes.
Bangladesh Man with Arsenic Lesions
The lack of ATP will cause massive neurological and cardiovascular distress. In sufficient concentrations the arsenic will eventually cause multi-system organ failure, most likely from cell apoptosis and hemorrhaging, and then death.