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Course: MCAT > Unit 2

Lesson 3: Foundation 3: Organ Systems

Neuromuscular junction: Acetylcholinesterase inhibitors


Acetylcholinesterase (AChE) is a serine hydrolase whose primary function is to degrade acetylcholine (ACh) into acetate and choline molecules, terminating neurotransmission. AChE inhibitors, or anticholinesterases, inhibit cholinesterase enzymes from breaking down ACh. According to the mode of action, AChE inhibitors can be divided into two groups based on their mode of action: reversible and irreversible. Reversible inhibitors, which can be competitive or noncompetitive, have mostly therapeutic applications, whereas irreversible inhibitors are mostly associated with toxic effects.
Sarin is a gaseous organophosphate compound that acts as an irreversible AChE inhibitor. Because of its extreme toxicity, it is traditionally used in chemical warfare. Exposure to sarin can result in tremors, seizures, hypothermia, paralysis, and ultimately peripherally-mediated respiratory arrest. The standard treatment for sarin-like nerve agent exposure includes post-exposure injection of atropine – an ACh receptor antagonist – accompanied by an oxime. Oximes such as pralidoxime reactivate organophosphate-inhibited AChE by hydrolyzing the phosphorylated enzyme and separating the nerve agent phosphate from the AChE active site.
Mechanisms of inactivation of AChE by sarin and reactivation by pralidoxime are shown in Figure 1.
Figure 1 Mechanism of AChE inactivation (a) and reactivation (b)
Reversible acetylcholinesterase inhibitors are used to treat various neurological disorders, including Alzheimer's disease and myasthenia gravis (an autoimmune disease caused by auto-antibodies that inhibit muscarinic ACh receptors). Pyridostigmine is a competitive inhibitor of acetylcholinesterase that helps relieve symptoms experienced by myasthenia gravis patients.
Where does acetylcholine NOT function as the primary neurotransmitter?
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