Lecture 28/29 Flashcards
Anticholinesterase - Masserano
Cholinesterase Characterisitcs
- Two main forms, 54% identical
1. ACh-terase - membrane bound, ACh specific, responsible for rapid ACh hydrolysis at cholinergic synapses
2. Butyrylcholinesterase (Pseudocholinesterase) - non-selective, occurs in plasma and many tissues - Both are serine hydrolases (esterases)
- Many prodrugs are designed to be hydrolyzed via this pathway
Pseudocholinesterases
- AKA plasma cholinesterases, false cholinesterases, serum cholinesterases, and butyrylcholinesterases
- Widely distributed, especially in liver and plasma with exception of RBCs
- Broader spectrum of chemical substances
- Synthesized in liver
Liver Disease + Pseudocholinesterases
- Acute/chronic hepatitus - shows a 30-50% decrease in the Pseudoch.
- Advanced cirrhosis/carcinomas - shows a 50-70% decrease in the Pseudoch.
- This could lead to increased half lives of drugs metabolized by this enzyme
Enzyme Function - Pseudochol.
-Genetic Alterations
-1:3000 people have homogeneous atypical genes for Pseudoch., autosomal recessive inheritance, tested to find
-Decreased levels lead to decreased hydrolysis of drugs that are metabolized this way which increases their therapeutic effect
EX: Succinylcholine (muscle relaxation) apnea for 60-120 minutes, Pilocarpine causing prolonged muscarinic activation, Procacine and Cocaine (30-50%) increased effect and toxicity
Drug Toxicities
- 30-50% of injected cocaine is rapidly metabolized by Pseudochol. in liver
- Decreased hepatic perfusion (hypotension, CHF) increases cocaine levels
- Decreased pseudochol. activity increases cocaine levels (babies, pregnant, geriatric)
- Genetic deficiency of pseudochol. increases cocaine levels
- Puts them at an increased risk of toxicity and sudden death due to sensitivity of small doses
- *Also explains use of cholinesterase inhibitors (carbamates or organophosphates) with cocaine to increase its effects and abuse it**
AChE
- “True” cholinesterase or eryhtrocyte cholinesterase
- Maintains homeostasis and metabolizes potent toxins like venom, insecticides, and chemical weapons
- Found in RBC, entire chol. NS, and other tissues
- Main function is hydrolysis of ACh released from nerves
- Located mainly in neurons on post-synaptic membrane and some in axonal presynaptic membranes
- High concentration in skeletal muscle synapse (fast reactions)
Binding Sites
- Hydrophobic products on anionic binding regions, N+ reacts with aspartate
- Ester binding region includes tyrosine, histadine, and serine
- Induced fit strains ACh and weakens bonds
- Positioned for reaction with serine
Mechanism of ACh Hydrolysis
- AChE anionic pocket binds to amine of ACh by electrostatic attraction
- Acetyl (ester) carbon of ACh goes through nucleophilic attack by serine’s hydroxyl
- Intermediate formed and decomposed to acetylated enzyme conjugate and choline
- Deacetylation step of hydrolysis is rate limiting
AChE Inhibitors
- One of the fastest neuronal enzyme reactions
- When inhibited, ACh builds up in synapse and produces an enhanced, prolonged response
- Enzyme inhibitor = same effect as cholinergic agonist (ACh can’t leave, so it returns to receptor and activates it)
AChE Drug Types
- 3 types
- Based on duration of action with enzyme
1. Short-acting Anti-ChE
2. Medium-druation Anti-ChE
3. Irreversible Anti-ChE
Short-Acting Anti-ChE
- Enrophonium (Enlon) is an example
- Quaternary ammonium compound
- Binds anionic pocket only and is reversible, brief action (2-10) minutes, used to diagnose myasthenia gravis
Medium-Duration Anti-ChE
- Neostigmine (Prostigmine) and Pyrodostigmine (Mestinon, Regonol) are examples
- All quarternary ammonium compounds , carbamyl ester binds to anionic site and is transferred to serine, but this is slower to hydrolyze than acetate (30 mins - 6 hours)
- Physostigmine (Eserine) - tertiary amine, used to treat anticholinergic drug toxicity, may precipitate seizures (small potential benefit v.s. risk)
Irreversible Anti-ChE
- Echothiophate
- Liable group like fluoride or organic group
- Released, leaving same -OH group phosphorylated with no spontaneous hydrolysis on serine
- Need to synthesize new enzyme to continue action, can take days
Non-Selective Cholinesterase Inhibitors
- Stimulation of muscarinic receptor - in autonomic effector organs
- Stimulation followed by depression or paralysis of all autonomic ganglia and skeletal muscle (nicotinic)
- Stimulation of cholinergic sites in CNS
Therapeutic Doses = More Selective
- Quaternary Ammonium can’t penetrate all membranes readily like CNS and has a preferential effect on NMJ of skeletal muscle (some direct action)
- Lipid soluble molecules affect CNS and PNS, which can be absorbed by the lungs and skin easily
Anti-ChE Drug Action Sites
- Eye
- Intestine
- NMJ
- CNS
Eye +Anti-ChE Drug
- Echothiophate (Phospholine)
- Contract ciliary muscle and pupil sphincter to produce miosis, rarely used since irreversible
- Used to treat glaucoma
- Onset ranges from hours to days
- IOP decreases by facilitating outflow of aqueous humor
- Reserved for those intolerant or not responding to other treatments due to systemic effects
GI Tract
- Neostigmine used for this site
- Increases secretion of gastric acid
- Enhances gastric contractions
- Lower portions of esophagus is stimulated and increased sphincter tone and peristalsis in patients with achalasia and dilation of esophagus
- Augments motor activity of large and small intestine, can increase propulsive waves in those with ileus
NMJ
- Quarternary Ammonium - direct effect on skeletal muscle, produces contraction (Ex: Neostigmine), each nerve impulse to the muscle gives rise to a single wave of depolarization since ACh is rapidly metabolized
- Cholinesterase Inhibitor prolong ACh action and availability and potentiates muscle contraction
NMJ Uses
- Removes competitive neuromuscarinic blocking agents
- Therapy in myasthenia gravis
* *Potential side effects = DUMBELS**
Pesticides
- Inhibit cholinesterases
- Organophosphates - inhibit irreversibly, “aging of complex” (treat early if possible), AChE must be resynthezides
- Carbamates - inhibit temporarily, no”aging”, reversible (rapid and dose related), AChE reactivates