Pesticides

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Pesticides
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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.

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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.

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History (Continued)

• The 1930s ushered in the era of modern synthetic
  chemistry.
• By the beginning of WWII there were a number of
  synthetic pesticides.

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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.

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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.

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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.

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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 .

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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

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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

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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.

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Organochlorines (Continued)

• Properties which made them good insecticides
• low volatility
• chemical stability
• lipid solubility
• slow rate of biotransformation and degradation

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Organochlorines (Continued)

• These properties also led to their demise because
  of their persistence in the environment,
  bioaccumulation, and biomagnification through the
  food chains.

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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.

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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.

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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.

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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.

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(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.

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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.

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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.

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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.

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(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.

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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.

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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.

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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.

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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.

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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

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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.

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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.

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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.

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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.