Cholinesterase Flashcards
Cholinesterase catalyses the hydrolysis of ___ to ___ and ____
ACh, acetate and choline
Two forms of ChE are
- AChE
2. Butyrylcholinesterase (pseudocholinesterase)
Location of AChE
Membrane-bound, at all cholinergic synapses + red cells
Location of butyrylcholinesterase
Usually a soluble enzyme (sometimes membrane-bound) found in plasma, liver + other tissues
How does substrate specificity differ for 2 ChE’s
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
Inhibitor specificity
mainly selective for AChE
BuChE is controlled by an ____ gene with ____
Autosomal, several alleles
Those with mutant - low/reduced BuChE show 2 things:
- Greatly reduced rate of hydrolysis of some drugs - imp. in anaesthesia (suxamethonium)
- Reduced sensitivity of enzyme to inhibiton by dibucaine
Outline dibucaine number test
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
ChE has 2 sites:
- Catalystic site (esteratic) containing a reactive serine OH group
- Anionic site - binds cationic quaternary ammonium group of ACh
Enzymatic hydrolysis involves:
- binding of ACh at 2 sites
- Transfer of acetyl group to serine OH
- Dissociation of choline spontaneous hydrolysis of acetylated serine OH
Number of ACh hydrolysed at 1 active site/second
10^4
transmitter (ACh) released at NMJ is normally hydrolysed in less than
1 ms
Short-acting anticholinesterases
e. g. Endrophonium - quaternary NH4 but no group complementary to esteratic site
- act competitively - bind anionic site by ionic bond
Medium/ long acting reversible anticholineserases
e. g. neostigmine, eserine
- most clinically imp.
- both sites bound - forms ester bond with serine OH of esteratic site
When is spontaneous hydrolysis much slower?
When serine OH is bound to neostigmine rather than acetyl group + enzyme molecule inactivated for several mins
Excamples of long-acting (irreversible) anticholinesterases
e.g. dyflos, parathion, ecothiopate + organophosphate compounds
Long-acting (irreversible) anticholinesterases mech
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
Peripheral effects of inhibtion
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
Central effects of inhibitions
Mainly excitatory, agitation + convulsions followed by respiratory depression
Main uses of anticholinesterases
- Glaucoma - eye drops (eserine/ organophosphate)
- Myasthenia gravis - quaternary comp. (neostigme) don’t reach brain, edrophonium used as a test
- Reversal of competitive neuromuscular block after anaesthesia
- Insectisides - organophosphate
- AD symptoms - tacrine/ doneprezil cross BBB -: small effect, inconsistent + temp relief
Toxic effects of anticholinesterases
acute -> demyelination -> paraylsis + sensory loss -> enzyme loss in myelin synthesis
ChE reactivators
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
What are ChE reactivators used for
treatment of organophosphate poisoning -: early treatment essential
