Cholinesterase inhibitors act indirectly by preventing the
enzyme from hydrolyzing (inactivating) acetylcholine at the receptor
site. This inhibition permits the buildup of acetylcholine and
results in more intensive and prolonged activation of the receptor
site. The effects of cholinergic stimulation include: vasodilattion
of blood vessels; slower heart rate; constriction of bronchioles
and reduced secretion of mucus in the respiratory tract; intestinal
cramps; secretion of salvia; sweat and tears; and constriction
of eye pupils.
Acetylcholine Inhibitors - Toxic Poisons
The main agents in this class are poisons such as organophosphate
insecticides and nerve gases.
Organophosphorus pesticides and Carbamate pesticides
Organophosphorus warfare nerve agents: Sarin, Soman, Tabun
Organophosphates account for about half (by amount sold) of
all insecticides used in the U.S. In addition to major crops
such as cotton, corn, and wheat, they are used on many important
minor crops. Some also are used for mosquito control to protect
public health against diseases such as malaria, dengue fever,
and encephalitis. These insecticides are developed to be highly
toxic to the target species, while being much less toxic to non-target
species, such as domestic animals and humans.
These compounds, as irreversible cholinesterase inhibitors,
are effective in very low concentrations and are capable of causing
death within minutes of exposure. The toxicity of organophosphates
and carbamates in humans is characterized by a variety of symptoms,
including tension, anxiety, headaches, slurred speech, tremor,
convulsions, and even death. If death occurs, it is caused by
asphyxia resulting from respiratory failure.
The structural similarities to acetylcholine should be examined
in the graphic on the left. Well known organophosphates include,
malathion and parathion. A carbamate, carbaryl - Sevin, is also
Although organophosphates are relatively toxic to both insects
and man, they do not persist in the environment since the phosphate
ester is relatively easily hydrolyzed by water.