CHOLINESTERASE INTRODUCTION CLINICAL UTILITY METHOD OF MEASUREMENT

By Dr. Kamal Modi Biochemistry Dept.

INTRODUCTION Cholinesterase (ChE) is the general term for two enzymes

in the human body: acetylcholinesterase (AChE) and Pseudocholinesterase (PChE).

Acetylcholinesterase Pseudocholinesterase

Found in erythrocytes, lungs, spleen, nerve endings & gray matter of brain

Found in liver, pancreas, heart, white matter of brain, serum

Responsible for hydrolysis of acetylcholine at nerve endings to mediate impulse transmission across synapse

Deactivation of octanoyl ghrelin, hormone that stimulates feeding & promotes weight gain.

In the nervous system AChE acts to “turn off” the signal delivered by cholinergic nerves by removing the neurotransmitter acytelcholine.

Thus if AChE is inhibited, acetylcholine builds up causing over-stimulation of cholinergic systems.

This over-stimulation of muscles, glands and other nerves is what causes most of the symptoms associated with overexposure to cholinesterase-inhibiting pesticides.

OP and N-methyl-carbamates inhibit cholinesterase by binding to the serine hydroxyl group on acetylcholinesterase Which is active site of the enzyme.

OPs may bind irreversibly with the enzyme while the N-methyl-carbamate bond is reversible.

Genetic basic • Most common allelic forms of genes that control synthesis

of cholinesterase are Eu, Ea, Ef, Es which describes as below:

Eu Normal phenotype

Ea Weakly active towards most substrate for CHEIncresed resistance to inhibition of enzyme activity by dibucaine.

Ef Increased resistance to flouride inhibition

Es Absence of enzyme or presence of protein with minimal or no catalytic activity

Clinical significanceAs liver function testAs an indicator of possible insecticide

poisoningFor detection of patients with atypical

forms of enzyme who are at risk for prolonged responses to certain muscle relaxants

Serum Cholinesterase

RBC Cholinesterase

Labile more Less

Inactivation faster Slow

Regeneration faster Slow

Accuracy less More

Indication For acute cases

For chronic cases

Blood test measurements of both AChE and PChE can be used as surrogate measures of nervous system AChE activity.

Because cholinesterase levels vary greatly between individuals (inter-individual variability) it is necessary to establish a pre-exposure baseline functioning level for each individual in order to determine meaningful change in cholinesterase levels.

If no baseline exists, and the initial test reading during the illness episode is within the laboratory’s normal range, the possibility of poisoning cannot be excluded.

Follow-up testing should be done at 1-2 week intervals until a stable value is evident.

If the values show an increasing trend, but eventually reach a stable value 30% or more above the first AChE level or 40% or more above the first PChE value, it is evidence that an overexposure did occur.

RBC Cholinesterase Serum Cholinesterase

Low levelAntimalarial drugs Oral contraceptives

Acute infectionsChronic debilitating disease CocaineCodeineDermatomyositisGenetic deficiencyHepatic parenchymal d's MalnutritionMorphinePregnancyOral contraceptivesOrganic mercurySuccinylcholineUse of collection tubes containing fluoride

High level Nephrotic syndromeNephrotic syndrome

PRINCIPLE OF S.CHOLINESTRASE ESTIMATION

The principle of the method is the measurement of the rate of production of thiocholine as acetylthiocholine is hydrolyzed.

This is accomplished by the continuous reaction of the thiol with dithiobisnitrobenzoate Ion to produce the yellow anion of 5-mercapto-2-nitro-benzoic acid.

The rate of color production is measured at 412 nm in a spectrophotometer.

(enzyme)Acetylthiocholine ----> thiocholine + acetate Thiocholine + dithiobisnitrobenzoate(DTNB)-----> yellow color

Cholinesterase Monitoring GuidelinesAdapted from guidelines adopted by the states of Washington and California

Utilize cholinesterase monitoring for individuals who apply OP pesticides occupationally on more than 3 consecutive days, or for 30 or more hours within any 30-day period.

Measure both acetylcholinesterase (red blood cell cholinesterase) and butyryl cholinesterase (plasma cholinesterase).

Use the same laboratory and the same methodology for all testing so that results may be accurately compared.

Obtain a baseline reading of both measures during the non-exposed period, at least 30 days since the last exposure to OP pesticides. Repeat testing every 3-4 weeks during intensive OP and carbamate application periods.

Test within 3 days of any 30-day period in which the individual has met or exceeded the handling hours threshold.

Compare each reading to the individual’s baseline. Take action as specified in the following table.

Thresholds for cholinesterase decision-making

Decrease from baseline

Action

20% decrease in AChE or PChE

Evaluate work practices

30% decrease in AchE or 40% decrease in PChE

Remove worker from exposure to organophosphates

and carbamates until levels return to within 80% of

baseline

SIGNING OUT……