Acetylcholine synthesis + degradation

Category: biosynthesis

Overview

ACh synthesis: choline + acetyl-CoA → acetylcholine (CHOLINE ACETYLTRANSFERASE, ChAT — cholinergic neuron marker). Released into synapse, binds nicotinic (NMJ + ganglia + CNS) and muscarinic (M1-M5 — autonomic + CNS) receptors. Rapid hydrolysis: ACh → choline + acetate (ACETYLCHOLINESTERASE, AChE — postsynaptic NMJ + RBCs). Plasma butyrylcholinesterase (BChE / pseudocholinesterase) is a related enzyme — succinylcholine + mivacurium hydrolyzed here. Cholinesterase inhibitors: reversible (DONEPEZIL / rivastigmine / galantamine for Alzheimer; PYRIDOSTIGMINE / neostigmine for myasthenia gravis + NMJ reversal); irreversible (organophosphates — sarin, VX, malathion; pralidoxime + atropine for poisoning). BChE-deficient phenotypes have prolonged paralysis with succinylcholine (the classic "succinylcholine apnea" complication).

Organ Systems

• nervous
• musculoskeletal

Pathway Steps

1. choline → acetylcholine — via ChAT (choline acetyltransferase) — cholinergic neuron marker. Acetylcholine is synthesized from choline and acetyl-CoA by choline acetyltransferase (ChAT), the marker enzyme of cholinergic neurons. High-affinity choline uptake (CHT) is rate-limiting, so choline availability can influence cholinergic transmission.
2. acetylcholine → choline-acetate — via AChE (acetylcholinesterase) — postsynaptic; sarin / donepezil / pyridostigmine target. Acetylcholine is hydrolyzed in the synaptic cleft by acetylcholinesterase (AChE) into choline and acetate within milliseconds, terminating the signal. AChE inhibitors (donepezil, neostigmine, organophosphates) prolong ACh action — the basis of Alzheimer’s and myasthenia therapy, and of nerve-agent toxicity.

Known Modulators

• donepezil (inhibitor) — AChE — Alzheimer disease
• pyridostigmine (inhibitor) — AChE — myasthenia gravis
• alpha gpc (substrate) — choline donor (upstream)
• citicoline (substrate) — choline donor (upstream)
• scopolamine (inhibitor) — muscarinic M1 (broad). tropane alkaloid; transdermal motion-sickness; CNS-penetrant — useful in chemo-induced + post-op N/V
• oxybutynin (inhibitor) — muscarinic M1/M3 (urinary). overactive bladder; CNS side effects (cognitive dulling in elderly) limit chronic use — transdermal reduces them
• tolterodine (inhibitor) — muscarinic M2/M3 (urinary). overactive bladder; less CNS penetration than oxybutynin; CYP2D6 substrate
• dicyclomine (inhibitor) — muscarinic M1/M3 (GI). antispasmodic for IBS-D + functional bowel; centrally active
• orphenadrine (inhibitor) — muscarinic M1 + NMDA antagonist. skeletal muscle relaxant; anticholinergic side effect tail
• ipratropium (inhibitor) — muscarinic M3 (bronchial). short-acting muscarinic antagonist (SAMA); inhaled — minimal systemic anticholinergic load; COPD
• tiotropium (inhibitor) — muscarinic M3 (bronchial, long-acting). LAMA; once-daily COPD maintenance; kinetic selectivity (slower M3 dissociation vs M2)
• aclidinium (inhibitor) — muscarinic M3 (bronchial). LAMA; twice-daily COPD; faster plasma hydrolysis → lower systemic exposure
• umeclidinium (inhibitor) — muscarinic M3. once-daily LAMA; component of Trelegy (ICS/LABA/LAMA triple)
• galantamine (inhibitor) — AChE + nAChR positive allosteric modulator. Alzheimer's dual-action AChE-i + nicotinic enhancer; CYP2D6 + CYP3A4 substrate
• rivastigmine (inhibitor) — AChE + BuChE (dual). Alzheimer's + Parkinson's dementia; pseudo-irreversible (carbamate); transdermal patch reduces GI side effects
• huperzine a (inhibitor) — AChE (selective reversible). Lycopodium alkaloid; nootropic + Alzheimer's (off-label) supplement; sold OTC despite Rx-grade pharmacology
• tacrine (inhibitor) — AChE. first Alzheimer's AChE-i (1993); withdrawn 2013 for hepatotoxicity; CYP1A2 substrate (cross-link: caffeine_demethylation, fluvoxamine DDI Wave 2a v1.1)
• varenicline (activator) — α4β2 nicotinic (partial agonist). smoking cessation; reduces nicotine reward + craving; neuropsychiatric tail historically (FDA boxed warning removed 2016)
• nicotine (activator) — nicotinic acetylcholine receptors (full agonist). tobacco alkaloid + smoking cessation gum/patch; CYP1A2 substrate (smoking induces own clearance)
• dexmedetomidine (activator) — α2-adrenergic (NOT muscarinic — sedation via locus coeruleus disinhibition). ICU + procedural sedation; preserves respiration; rebound HTN on abrupt discontinuation; cross-link: adrenergic_receptor_signaling
• piperine (inhibitor) — CYP3A4 + UGT inhibitor (black pepper). black-pepper alkaloid; turmeric absorption enhancer (12x); broad CYP modulation
• vinpocetine (inhibitor) — PDE1 + Na channel (vinca alkaloid). periwinkle alkaloid; cerebral vasodilator + neuroprotection claim; EU Rx
• 9 me bc (activator) — TAAR1 + MAO modulation. 9-methyl-β-carboline; research compound; mood + cognition
• colchicine low dose cv (inhibitor) — tubulin (low-dose cardiovascular). low-dose colchicine; COLCOT/LoDoCo2 trials — CV-event reduction post-MI + chronic CAD
• acetylcholine (substrate) — endogenous ligand; ChAT product, AChE substrate

References

• PMID 23040810
• PMID 11283752