Which enzyme is required to break down the muscle relaxants succinylcholine and mivacurium?

• A. Pseudocholinesterase (serum)
• B. Acetylcholinesterase (nerve)
• C. Butyrylcholinesterase (plasma)
• D. Lipase

Answer: (A)

Succinylcholine and mivacurium are broken down by a plasma enzyme known as pseudocholinesterase, also called butyrylcholinesterase. This enzyme hydrolyzes the ester bonds in these drugs while they’re in the bloodstream, which rapidly terminates their neuromuscular blocking effect. Acetylcholinesterase, located at the neuromuscular junction, breaks down acetylcholine, not these two drugs, so it isn’t responsible for their clearance. Lipase is unrelated to their metabolism. Clinically, reduced or atypical pseudocholinesterase activity can lead to prolonged paralysis after these agents because the drug isn’t cleared as quickly.