Biochemical Effects of Arsenic:
The insecticides, fungicides and herbiacides contain arsenic. Among the compounds of arsenic, those of As (III) are most toxic. As (III) exerts its toxic action by attacking SH groups of an enzyme (as in case of Hg” Figure 1) thereby inhibiting enzyme action Figure 2
As (III) deactivates the enzyme pyruvate dehydrogenase by complexion with SH groups and in this way the generation of ATP is prevented. Being similar to P, arsenic interferes with the generation of ATP (adenosine triphosphate Normally ATP is obtained from glecereldehyde -3 -phosphate as shown in figure 3. However, in presence of arsenite, 1-arseno-3-phosphoglycerate, is formed . which prevents ATP formation.
In biological systems, As (III) compounds at high concentration coagulates proteins by attacking the sulphur bonds maintaining the remaining part of the structure. The general antidote for arsenic poisoning are chemicals having .SH groups which can bind to As (III). One such examples 2, 3-dimercaptopropanol
Biochemical Effects of Cadmium:
Cadmium is found to occur in nature along with zinc ores. When the plants acquire zinc, they also take up cadmium. When animals eat the plants or when the human eat the animals which have consumed plants containing cadmium, the cadmium gets accumulated in human bodies. The first case of cadmium poisoning was reported in Japan in the form of ‘itai itai’ or ‘ouch ouch’ disease. A large number of people suffered from the disease in which their bones became fragile. Other effects of cadmium in the human system are Renal Dysfunction, anemia, hypertension bone marrow disorder, and cancer.
The cadmium ingested in our body is trapped in the kidneys and gets eliminated. However, a small fraction is bound by the body proteins, metallothionein, present in the kidney. With age the accumulation of cadmium in the body increases. When excessive amounts of Cd2+ are present in the body, it replaces Zn 2+ at key enzyme sites.
Biochemical Effects of Lead:
The important sources of lead for biological systems are mining, smelting, winning of metals, automobiles, etc. In urban areas, citizens are exposed to it through emission of vehicles and food. About 200-300 mg/day of lead intake through food and about 10-15 mg/day through air and water is quite common in urban settlements. Lead is present in tap water to some extent as a result of dissolution from natural sources but mainly from household plumbing systems containing lead pipes. It is satisfying to note that emissional lead from vehicular emission has ceased since leaded petrol is no more used in cities like Delhi.
When present in < 40 mg/100 mL of blood, lead is considered as normal. However >100 mg/100 mL of blood is unacceptable.
Lead gets deposited in bones and is considered as a cumulative poison. Lead is a cumulative poison. It enters the body through inhalations, ingestion and absorption through the skin. On an average natural water contains about 20ppb of lead. It enters water bodies from mining, smelter discharges lead plumbing and weathering of rocks. After entering the human body, it gets concentrated in the bones, from where it enters the blood stream.
Lead interferes with hence synthesis leading to hematological damage. It interferes with the production of red blood cells and can also damage the brain, kidney and liver. Lead is particulars affects children and pregnant women; in the later case it may be responsible for miscarriage and birth defects.
Biochemical Effects of Mercury:
Mercury is one of the most well known toxic metal. The toxicity or the toxic effects of mercury came to lime light in 1953-1960 in Japan by the incidence of Minamata Disease. At Minamata Bay in Japan, more than 100 people died and thousands were permanently paralysed by consuming fish contaminated with mercury. Even genetic defects were observed in some new born babies whose mothers had consumed contaminated fish from the Bay. The cause of the disease was contamination of water with mercury rich effluents discharged into the Bay by Chisso Chemical Company. It was discovered (1963) that the illness was due to methyl mercury poisoning, caused by eating contaminated fish from the bay.
The Chisso plant used mercuric chloride catalyst, a non-toxic inorganic mercury compound in the production of acetaldehyde which was released in the industrial effluents. Subsequently, the sediments form the Minamata Bay was found to be rich in methyl mercury chloride. It was, therefore, clear that inorganic mercury underwent bio-methylation in aquatic system. It was also evident from the data obtained which showed the presence of methyl mercury chloride in concentrated form in the body of the fish.
It has been found that methylation of mercury in water bodies is brought about by a number of microorganisms called methanogenic bacteria. This process is called bio-methylation. The bio-methylation takes place under anaerobic conditions. The process is facilitated by the enrichment of water with organic impurities which permit growth of methanogenic bacteria, the bio-methylation proceeds smoothly in the pH range 5.5-6.5.
The Minamata incident was followed by a more tragic incident of mercury poisoning form Iraq (1972) where about 500 people died after consuming wheat which had been sprayed with mercury containing pesticide.