Cholinesterase

Question 1

Cholinesterase catalyses the hydrolysis of ___ to ___ and ____

Answer 1

ACh, acetate and choline

Question 2

Two forms of ChE are

Answer 2

1. AChE

2. Butyrylcholinesterase (pseudocholinesterase)

Question 3

Location of AChE

Answer 3

Membrane-bound, at all cholinergic synapses + red cells

Question 4

Location of butyrylcholinesterase

Answer 4

Usually a soluble enzyme (sometimes membrane-bound) found in plasma, liver + other tissues

Question 5

How does substrate specificity differ for 2 ChE’s

Answer 5

AChE is specific for ACh and closely similar esters. BuChE is rather unspecific + faster hydrolysis for butyrylcholine than for ACh; can hydrolyse many other esters e.g. procaine, suxamethonium, cocaine

Question 6

Inhibitor specificity

Answer 6

mainly selective for AChE

Question 7

BuChE is controlled by an ____ gene with ____

Answer 7

Autosomal, several alleles

Question 8

Those with mutant - low/reduced BuChE show 2 things:

Answer 8

1. Greatly reduced rate of hydrolysis of some drugs - imp. in anaesthesia (suxamethonium)
2. Reduced sensitivity of enzyme to inhibiton by dibucaine

Question 9

Outline dibucaine number test

Answer 9

Normal range 70-90 (percentage enzyme inhibition by 10^-4 M dibucaine)

• Heterozygotes 50-70 without abnormal suxamethonium sensiticity
• Homozygous 10-20 - extremely sensitive to suxamethonium

Question 10

ChE has 2 sites:

Answer 10

1. Catalystic site (esteratic) containing a reactive serine OH group
2. Anionic site - binds cationic quaternary ammonium group of ACh

Question 11

Enzymatic hydrolysis involves:

Answer 11

1. binding of ACh at 2 sites
2. Transfer of acetyl group to serine OH
3. Dissociation of choline spontaneous hydrolysis of acetylated serine OH

Question 12

Number of ACh hydrolysed at 1 active site/second

Answer 12

10^4

Question 13

transmitter (ACh) released at NMJ is normally hydrolysed in less than

Answer 13

1 ms

Question 14

Short-acting anticholinesterases

Answer 14

e. g. Endrophonium - quaternary NH4 but no group complementary to esteratic site
- act competitively - bind anionic site by ionic bond

Question 15

Medium/ long acting reversible anticholineserases

Answer 15

e. g. neostigmine, eserine
- most clinically imp.
- both sites bound - forms ester bond with serine OH of esteratic site

Question 16

When is spontaneous hydrolysis much slower?

Answer 16

When serine OH is bound to neostigmine rather than acetyl group + enzyme molecule inactivated for several mins

Question 17

Excamples of long-acting (irreversible) anticholinesterases

Answer 17

e.g. dyflos, parathion, ecothiopate + organophosphate compounds

Question 18

Long-acting (irreversible) anticholinesterases mech

Answer 18

Phosphorylate serine OH. Hydrolysis of phosphorylated enzyme = slow + recovery requires synthesis of new enzyme 
If ecothipate (few hour recovery) 
-> most organophosphates have no quaternary group = inhibit many serine enzymes besides ChE

Question 19

Peripheral effects of inhibtion

Answer 19

Parasympathetic - fall in IC pressure
Nmj - restoration of transmission at junctions blocked by competitive blocking agens or in myasthenia gravis
-: high doses -: depolarising muscle block

Question 20

Central effects of inhibitions

Answer 20

Mainly excitatory, agitation + convulsions followed by respiratory depression

Question 21

Main uses of anticholinesterases

Answer 21

1. Glaucoma - eye drops (eserine/ organophosphate)
2. Myasthenia gravis - quaternary comp. (neostigme) don’t reach brain, edrophonium used as a test
3. Reversal of competitive neuromuscular block after anaesthesia
4. Insectisides - organophosphate
5. AD symptoms - tacrine/ doneprezil cross BBB -: small effect, inconsistent + temp relief

Question 22

Toxic effects of anticholinesterases

Answer 22

acute -> demyelination -> paraylsis + sensory loss -> enzyme loss in myelin synthesis

Question 23

ChE reactivators

Answer 23

Oxime compounds e.g. pralidoxime which binds reversibly to anionic site -> oxime OH is at right distance to reach phosphate group + substitutes for serine OH

Question 24

What are ChE reactivators used for

Answer 24

treatment of organophosphate poisoning -: early treatment essential