Mode of action of Poisons

From a more extensive perspective, mode of action of poisons can be divided as physical and chemical disruption of the living procedure.

• Physical Action: Because of their physical properties, for example, lipid solubility, a few substances exert a non-specific inhibitory effect on enzymes. Their physical nature leads to accumulation in vital parts of cells where they depress cellular functions. Hypnotics and sedatives, such as hydrocarbons, chlorinated hydrocarbons, alcohols, ethers, ketones are its examples.
• Compound Action: Majority of poisons produce their impact because of their chemical associations with cell segments. It is the enzymes concerned in cell oxidation and oxidative phosphorylation which seem to be most vulnerable to the action of poisons. Enzymes have active sites and the toxic compounds may occupy those active sites thus preventing normal substrates to combine. The enzyme inhibition caused by a poison may be irreversible (as in the case of certain organophosphates) or reversible (as in the case of carbamates).

 Furthermore, mode of action can also be classified into non-specific and specific types:

• Non-specific action: It results narcosis; therefore, narcosis is a mode of toxic action. The target site and mechanism of toxic action through which narcosis affects organisms are still unclear, but there are hypotheses that support that it occurs through alterations in the cell membranes. Even though continuous exposure to a narcotic toxicant can produce death, if the exposure to the toxicant is stopped, narcosis can be reversible. Heavy metals causing developmental defects, warfarin causing injury to blood vascular system, cobalt involved with impairment of oxygen transport, DDT causing carcinogenesis etc are its examples.

• Specific action: Poisons at low concentrations modify some biological process by binding at a specific site having a specific acting mode of toxic action. However, at high enough concentrations, poisons with specific acting modes can produce narcosis that may or may not be reversible. However, the specific action of the poison is always shown first because it requires lower concentrations. Some of the specific modes of toxic actions are:

1. Uncoupling of oxidative phosphorylation: It involves poisons that uncouple the two processes that occur in oxidative phosphorylation: electron transfer and ATP production.
2. AChE inhibitors: When poisons bind to AChE, they inhibit the breakdown of Ach that results in continued nerve impulses across the synapses, and cause nerve system damage. Examples- organophosphates and carbamates.
3. Irritants: Cause an inflammatory effect on living tissue resulting hypertrophy, or hyperplasia. Examples – benzaldehyde, acrolein, zinc sulfate, and chlorine.
4. CNS seizure agents: CNS seizure agents inhibit cellular signaling by acting as receptor antagonists. Example- organochlorine pesticides.
5. Respiratory blockers: These are toxicants that affect respiration by interfering with the electron transport chain in the mitochondria. Examples- rotenone and cyanide.