Mechanism of action of toxicants varies from chemical to chemical. Some chemicals cause cytotoxicity, i.e., they act by causing cell death.
A number of chemicals are able to cause genotoxicity, i.e. they are able to produce their effect by altering gene expression.
Generally, toxicants produce their effects by interference with physiological or biochemical homeostatic (regulatory) mechanisms. Some of them can modify the regulation of cell division with harmful effects. Many toxic chemicals cause proliferative lesions on the skin of the organism.
A dinofiagellate toxin causes wounds (ulcers) on the body of fish (ichthyotoxin). When the toxic algal bloom dies, they are decomposed by oxygen-requiring microorganisms, thereby causing hypoxic condition in water, which may lead to massive fish kills. Humans who eat shellfish that have fed on red tide organisms (blooms of dinoflagallates) may suffer from paralytic shellfish poisoning caused by the toxin that they produce (Hill, 1997). Certain environmental agents called carcinogens cause mutations that can lead to cancer.
These mutations activate oncogenes (or inactivate tumor-suppressor genes). An oncogene (Gk. onco = txmior) is a cancer- causing gene. A tumor-suppressor gene is so-called because it prevents cancer from occurring. Certain pathogens produce toxins. The most dangerous food poisoning, called botulism, is attributed to Clostridium botulinum. This is a widely distributed soil bacterium. It is developed in incompletely sterilised meat products. It derives its name from its occurrence in sausages (Latin, botulus = sausage).
It produces a toxin that on injection of the food, causes paralysis of the nervous system (neurotoxin) and is lethal by virtue of respiratory paralysis. Botulinus toxin A, which is excreted by C. botulinum, is a heat-labile protein. It has LD 50 value of one nanogram per kg body weight. Thus, this neurotoxin is the most effective of all lethal poisons for humans (Schlegel, 1995). Table 8.2 shows some examples of how toxicants exert their adverse effects.
