Biology and Physiology of Cholinesterase

Pesticide Issues Conference
  • Keifer, Matthew C.



  • Purpose of Monitoring
  • Basic biology of cholinesterase and cholinesterase inhibiting pesticides, basic enzymology
  • History and physical exam of the handler
  • Appropriate testing methods and interpretation of monitoring results
  • Responses to cholinesterase depression
  • How to investigate for false positives
  • Setting up a cholinesterase monitoring in the clinic/medical system
  • Quality assurance evaluation
What ChE monitoring accomplishes?
  • Identifies hazardous conditions/practices
  • Increases worker/employer hazard awareness
  • Assists in medical return to work
  • Avoids problems from chronic exposure
  • Influences economic decisions:
    • Increases costs of production
    • May influence choice of pesticide
Biology of Cholinesterase

  • Present from paramecia to sapiens
  • Very Fast enzyme (perfect kinetics)
  • Ubiquitous in the human body
  • Critical for many nervous system functions
What is it?
  • An enzyme with a sulfhydral active site
  • Produced in tissues and blood
  • Hydrolyses acetylcholine
  • Present in the autonomic, central and peripheral nervous systems
Excellent web page on the enzyme

What it Does Chemically

Hydrolysis of Acetylcholine : A key neurotransmitter

  • Thought to mediates a nucleophilic attack on carbonyl carbon acylating it and liberating choline and vinegar. Old model explains much but is not accurate.

Two Kinds of ChE in the Body
  • Different enzymes with some behaviors in common
  • Plasma Cholinesterase
    • Butyrylcholinesterase, pseudocholinesterase, PChE, or just cholinesterase and ChE
  • RBC Cholinesterase
    • True cholinesterase, acetylcholinesterase, or AChE
Plasma ChE?

  • Floats freely in plasma
  • Made by liver Rapid recovery from depression
  • Rapid replacement by new synthesis
  • Liver disease may affect levels
  • Sensitive to most ChE inhibitor pesticide exposures
Red Blood Cell Cholinesterase

  • Bound to red blood cells
  • Made at the same time as the Rbc's
  • Recovery from depression 0.8%/day
  • Slower to depress, slower to recover
  • Low RBC count may cause lower levels Identical to neuronal ChE
What Cholinesterase Does: Physiologically

  • Autonomic Nervous System
    • Parasympathetic
    • Presynaptic Sympathetic
  • PNS
    • Skeletal muscle
  • CNS
    • Memory & others

What It Does: Neurochemically First understand Acetylcholine


Found at the synapse It turns off the chemical messenger When inhibited the messenger builds Overstimulation results

Why do we have this enzyme in the blood?
  • A buffer for poisons
  • Potatoes
    • Solanaceous alkaloids
  • The Calabar Bean
    • Physiostigma venenosum
  • Green Mamba Snake
    • Fascilin inhibits AChE
Pharmacology Cholinesterase
  • Alzheimers Treatment with ChE inhibitors
    • Tacrine, Donepezil, Metrifonate, Galantamine
      • Metrifonate is converted to DDVP
  • Myasthenia Gravis
    • Edrophonium,
    • Pyridostigmine bromide
  • Glaucoma
  • Prophylaxis for Nerve Gas Attacks
Pesticides That Inhibit Cholinesterase
  • Organophosphates
    • Inhibit irreversibly
    • "aging of complex"
    • ChE must be replaced by the body
  • Carbamates
    • Inhibit temporarily
    • No "aging"
    • Reversal is rapid and level related
    • ChE reactivates and is ready to go
Oral vs. Dermal LD50 of some OPs

Organophosphate Oral mg/kg Dermal
Phorate 2 6
Azinphos-Methyl 13 220
Methamidaphos (rat) 32 94
Oxydemeton (rat) 75 250
Diazinon (rat) 108 900
Phosalone (rat) 130 1500
Chlorpyrifos (rat) 155 202
Malathion (rat) 1375 4444


Pesticide Oral mg/kg Dermal mg/kg
Aldicarb 0.5 3
Carbaryl 5 -13 >1000
Propoxur! 100 1000 -2400
Oxamyl 5.4 3000
Carbofuran 5-13 >1000
Methomyl 17-24 >5000

Toxicity of ChE Inhibitors

Mild cases: tiredness, weakness, dizziness, nausea and blurred vision

Moderate cases: headache, sweating, tearing, drooling, vomiting, tunnel vision, and twitching

Severe cases: abdominal cramps, urinating, diarrhea, muscular tremors, staggering gait, pinpoint pupils, hypotension (abnormally low blood pressure), slow heartbeat, breathing difficulty, and possibly death


Why is ChE Testing Useful?
  • ChE reflects the toxicant on its target
  • Integrates exposure over time
  • The test is widely available
  • A blood sample all that is needed
  • BUT!
    • Baseline is needed
    • Good lab methods needed
    • Interpretation and timing important
    • Sample handling important
When Do Testing?

    Class I and II Carbamates & Organophosphates
        LD 50 of < 50 mg oral or 100 dermal
        LD 50 of >50 <500 oral or <1000 dermal
Threshold: 50 hrs in 30 days

How to Interpret Cholinesterase Monitoring Data: Why Baselines?

Normal Range of cholinesterase activity Normal Range of cholinesterase activity

Plasma Normal Population

Variation, Month to Month
  • Relatively Stable in the Population

Carbamates Alone Is it worth testing?

History and Physical of Handler

Presence of ChE inhibitor based symptoms

  • Experience with pesticides
    • Attitude toward inhibitors
    • Medications
    • Previous medical history
  • Probable contraindications
    • Asthma/COPD
    • G.I. Ulcer
  • Treatment with ChE inhibitor
    • myasthenia gravis
    • Alzheimers
    • glaucoma
  • Other possible problems
    • Anemia
    • degenerative diseases of the central nervous system
    • chronic colitis
    • psychosis
  • Obtain before exposure
    • 30 days since last handling
  • Maintain records for future comparison
  • If its abnormally low
    • Recheck, average or discard
  • More tests are better than less
    • What does regression to the mean mean?
How Often to Test?
  • How Often to Test?
  • Retest with the same laboratory, same methods
  • Retesting every 30 days
    • When to do follow-up?
      • Rules state within 3 days of reaching threshold
    • Why are you testing?
      • To prevent future exposure
      • To evaluate work exposure
    • Decrease frequency with experience
How to Interpret Results

  • Large difference between upper and lower range of normal
  • 20% depression- Significant
  • 30% AChE- Removal*
  • 50% AChE- Poisoning
  • 40% PChE- Removal*
  • 60% PChE- Poisoning *California, WHO and ACGIH recommendations on removal thresholds

    What Response to Depressed Results
    • Act promptly
      • You’re already late
    • Evaluate for false positives
    • Assure removal if meets threshold
    • Be sure the workplace is evaluated
    • Communicate with worker and with employer
      • The teachable moment
    False Positives

    Plasma Cholinesterase

    • Drugs: therapeutic and recreational
      • BCPs, metaclopramide, cocaine?
    • Liver Disease-alcoholism Congenital Deficiency (3%)
    • Pregnancy
    • Nephrotic syndrome
    • Carbon disulfide, organic mercury
    RBC Cholinesterase

    • Drugs and Reticulocytosis
    False Negatives
    • Hard to find, Hard to know
    • Lack of depression when depression is truly present
      • Laboratory phenomena
      • Low baseline
      • Sample confusion
    Medical Removal

    • What else can they do?
    • Thinning? Probably not in sprayed orchards*
    • Know the operation
    • General work
    *Engel and Keifer 1998, Keifer, Miller, Fenske 1995 Schnieder et al 1991)

    Return to Work

    Return to regular duty

    • When both PChE and AChE get to 80%
    File a Claim?

    • If worker is sick, yes
    Quality Assurance Policing Your System
    • Records and Response
      • Dry run your response
      • Dry run you communication options
      • Check out removal options
    • Test the quality of your ChE laboratory
      • Blinded split samples to laboratory
      • How far off should they be?
        • If they approach thresholds, you're in trouble
    Responsibilities of Medical Supervisor
    • Know the rules of Monitoring (WA state)
      • Obligations regarding confidentiality
    • Know something of the pesticide practices
      • Which pesticides, application frequency, PPE
    • Know your population
      • Language, culture, beliefs
    • Know how to respond to a depression
      • Check PPE & pesticides, removal options
    • Know the non-pesticide related causes of depression
    • Assure quality performance and worker protection
      • False positives
      • False negatives
      • Laboratory accuracy
      • Response to depressions
        • Prompt
        • Appropriate
      • Advise employer
      • Counsel worker
    Abnormal Baselines Plasma ChE
    • Congenital cholinesterase deficiency
      • 3% of Anglos, 1% of Blacks carry the gene
      • May influence susceptibility to ChE inhibitors
      • Will have low baseline values for PChE
      • Will have normal RBC ChE values

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