Pesticides
Definition of a Pesticide
US EPA - any substance or mixture of
substances intended for preventing,
destroying, repelling, or mitigating any
pest. The term pest means any
harmful, destructive, or troublesome
animals, plants, or microorganisms.
History of Pesticides
Sulfur used as a fumigant by the Chinese
before 1000 B.C. and as a fungicide in the
1800s in Europe against powdery mildew on
fruit; Sulfur containing compounds are still
used in CA today.
The Chinese applied moderate amounts of
arsenic-containing compounds as insecticides
in the 16th century. In the 1800s arsenic
trioxide was used as a weed killer.
History (Continued)
The 1930s ushered in the era of modern
synthetic chemistry.
By the beginning of WWII there were a
number of synthetic pesticides.
Pesticides
It is important to appreciate
despite the modern day development of
second- and third- generation derivatives
of the early chemical pesticides, all
pesticides are inherently toxic to some
living organism, otherwise they would be
of no practical use.
There is no such thing as a completely safe
pesticide.
Human Problems
Target species and nontarget species can be
affected.
On a world wide basis, intoxications due to
pesticides are estimated to be 3 million cases
of acute, severe poisoning annually with an
equal number of unreported cases, and some
220,000 deaths.
In CA, ~ 25,000 cases of pesticide-related
illness occur annually among agricultural
workers. National ~ 80,000 cases per year.
Pesticide Regulation
Federal Insecticide, Fungicide, and
Rodenticide Act of 1947.
Taken over by EPA in 1972. Each pesticide
has a registration requirement that must
include appropriate chemical, toxicological,
and environmental impact studies. There are
label specifications, use restrictions and
tolerances for pesticide residues on raw
agricultural products. The EPA is responsible
for monitoring pesticide residue levels in
foods.
Pesticide Regulation
Studies related to environmental impact on
birds, mammals, aquatic organisms, plants,
soils, environmental persistence, and
bioaccumulation are required.
The average development cost for a new
pesticide is on the order of 30 - 50 million $.
Insecticides
All of the chemical insecticides in use
today are neurotoxicants and they act
by poisoning the nervous system of
target (also non-target) organisms.
Classes of Insecticides:
Organochlorine Insecticides
Organophosphate/Carbamate Insecticides
Pyrethroid Insecticides
Organochlorine Insecticides
The organochlorine (chlorinated hydrocarbon)
insecticides are a diverse group of agents
belonging to the following chemical classes:
dichlorodiphenylethanes
chlorinated cyclodienes
chlorinated benzenes
chlorinated cyclohexanes
Organochlorines (Continued)
These agents were used extensively from the
mid 1940s to the mid 1960s in all aspects of
agriculture and forestry, building and
structural protection, and in human situations
to control a wide variety of insect pests.
Organochlorines (Continued)
Properties which made them good
insecticides:
low volatility
chemical stability
lipid solubility
slow rate of biotransformation and
degradation
Organochlorines (Continued)
These properties also led to their demise
because of their persistence in the
environment, bioaccumulation, and
biomagnification through the food chains.
Organochlorines (Continued)
Studies in wildlife species and laboratory
animals have demonstrated potent estrogenic
and enzyme-inducing properties, which
interfere with fertility and reproduction.
In birds, the interference is related to steroid
metabolism and the inability of the bird to mobilize
sufficient calcium to produce a strong eggshell.
Eggshell cracking allows bacteria into the egg,
which kills the embryo.
Organochlorines (Continued)
Reproduction in fish is affected by the
bioconcentrations of these agents in the
yolk-sac of fry.
The o,p'-isomer of DDT has been shown to
compete with estradiol for binding with the
estrogen receptor in rat uterine cells.
Typical Mechanism of Action
For DDT Type Pesticides:
periodic sequences of persistent tremoring and/or
convulsive seizures suggestive of repetitive
discharges in neurons.
these tremors, seizures can be initiated by tactile
and auditory stimuli, indicating that the sensory
nervous system appears to be much more
responsive to stimuli.
Chlorinated Cyclodiene-, Benzene-
and Cyclohexane- type Insecticides
These insecticides act in the CNS.
(1) Mimic the action of the chemical
picrotoxin, a nerve excitant and antagonist
of the neurotransmitter GABA found in the
CNS.
GABA induces the uptake of chloride ions by
neurons.
The blockage of this uptake results in only
partial repolarization of the neuron and a
state of uncontrolled excitation.
(Continued)
(2) Inhibitors of Na/K ATPase and Ca/Mg
ATPase that is essential for the transport of
Ca++ across membranes.
The inhibition of Ca/Mg ATPase located in
synaptic terminals of neurons results in an
accumulation of intracellular free Ca++ which
stimulates neurotransmitter release,
depolarization of adjacent neurons, and
propagation of stimuli throughout the CNS.
Metabolism of Compounds
Very slow rate for DDT due to complex
aromatic ring structure and chlorination.
Half-life is 335 days in cattle.
DDT broken down to DDE both
nonenzymatically and by cytochrome P450
reductive dechlorination. All metabolites are
highly lipophilic.
Metabolism (Continued)
Biotransformation is also slow for cyclodiene-
type insecticides.
Aldrin and Heptachlor are converted by
cytochrome P450 oxidation reactions to
dieldrin and heptachlor epoxide without a
change in lipid solubility or toxicity.
Organophosphate /
Carbamate Pesticides
~ 200 different organophosphate insecticides
~ 25 different carbamate insecticides
These two types of insecticides are
considered together because they have a
similar mechanism of action.
(Continued)
Organophosphorus insecticides were first
synthesized in 1937 by German chemists as
potential chemical warfare agents.
Sarin is an organophosphate compound that was
used against Kurdish people in northern Iraq in
1988.
The first carbamates were synthesized in the
1930s as fungicides.
Mechanism of Action
Both organophosphate and carbamate
insecticides act by inhibiting the action of
acetylcholinesterase (AchEase) at the
synapse between two neurons or between a
motor neuron and a muscle.
AChEase is the enzyme responsible for the
breakdown of the neurotransmitter acetylcholine
(ACh).
Organophosphate compounds are irreversible
inhibitors of AChEase while carbamate compounds
are reversible inhibitors of AChEase.
Mechanism (Continued)
The reaction between an organophosphate
and the active site on AChEase (a serine
hydroxyl group) results in the formation of an
intermediate that undergoes partial hydroylsis
with the loss of the "Z" group, leaving a
stable, phosphorylated, permanently inhibited
enzyme.
Signs and symptoms are prolonged and
persistent. Without intervention, the toxicity
will persist until sufficient amounts of "new"
AChEase are synthesized in 20 to 30 days.
Mechanism (Continued)
Carbamate compounds attach to the
serine hydroxyl group, the "X" group is
removed by hydrolysis forming a
carbamylated enzyme, and then
decarbamylation occurs regenerating
free, active AChEase.
Carbamate pesticides are really just poor
substrates for AChEase.
Metabolism
Both types of insecticides undergo extensive
metabolism in all forms of life. The routes
and rates are highly species specific. Both
phase I and phase II mechanisms are
involved.
One reaction, oxidative desulphuration, results in
a significant increase in toxicity. In parathion,
methyl parathion and malathion, the presence of
the P = S reduces the AChEase inhibiting
properties of the compound.
Mechanism (Continued)
There is a characteristic prolongation of the
falling phase of the action potential. The
nerve membrane remains in a partially
depolarized state and is extremely sensitive
to complete depolarization again by very
small stimuli.
See handout
Why Prolonged Repolarization
(1) DDT affects the permeability of the nerve
cell membrane to K+ ions reducing K+
transport across the membrane.
(2) DDT alters the Na+ channels, they open
normally but are closed (inactivated) slowly.
(3) DDT inhibits neuronal ATPase activity
particularly Na/K ATPase and Ca ATPase
which play a role in repolarization of the
neuron.
Why (Continued)
All of these factors reduce the rate at
which repolarization occurs and
increase the sensitivity of the neurons
to small stimuli that would not elicit a
response in a fully repolarized neuron.
Pyrethroid Insecticides
This is the newest class of insecticides, a
group of chemicals that entered the
marketplace in 1980 but by 1982 accounted
for approx. 30% of worldwide insecticide use.
These synthetic chemicals are similar to the
insecticides extracted from pyrethrum or
chrysanthemum flowers.
Pyrethroids (Continued)
Type I pyrethroids affect Na+ channels in
nerve membranes (both sensory and motor)
and gives long afterpotential similar to DDT.
Type II pyrethroids cause persistent
depolarization and prolonged repetitive firing
of sensory receptors and muscle fibers.
Both types also inhibit Ca/Mg ATPase
resulting in increased intracellular Ca++
levels and increased neurotransmitter
release.