Which enzyme hydrolyzes ester local anesthetics in plasma?

• A. Pseudocholinesterase
• B. Cytochrome P450
• C. Monoamine oxidase
• D. Amylase

Answer: (A)

Esters are inactivated rapidly in the bloodstream by a specific plasma enzyme that cleaves the ester bond. The enzyme responsible is pseudocholinesterase, also known as butyrylcholinesterase, which is present in plasma and efficiently hydrolyzes ester local anesthetics such as procaine, benzocaine, and tetracaine. This swift hydrolysis explains why ester-type local anesthetics have a relatively short duration of action compared with amide-type anesthetics, which are mainly metabolized in the liver by amidases. The other enzymes listed do not perform this plasma hydrolysis: cytochrome P450 enzymes act in the liver to oxidize many drugs, monoamine oxidase targets monoamines, and amylase digests carbohydrates. If a person has a deficiency of pseudocholinesterase, the ester local anesthetics can persist longer in the plasma, increasing the potential for systemic effects.