Which of the following statements is not associated with serum cholinesterase?

• A. Inhibited by organic insecticides
• B. Referred to as "true" cholinesterase
• C. Decreased level causes prolonged apnea after administration of succinyldicholine
• D. Acts on the substrate propionylthiocholine

Answer: (B)

Serum cholinesterase, also called pseudocholinesterase or butyrylcholinesterase, is a different enzyme from acetylcholinesterase, which is the enzyme at nerve endings and in blood cells often referred to as the “true” cholinesterase. The important distinction is that while both enzymes hydrolyze choline esters, they have different substrate preferences and inhibitor sensitivities. Serum cholinesterase is inhibited by organophosphorus compounds such as organic insecticides, which is why exposure can lower its activity. Clinically, a lowered serum cholinesterase level leads to prolonged apnea after giving succinylcholine because this drug is hydrolyzed by butyrylcholinesterase; when the enzyme is deficient, the drug remains active longer. It also acts on substrates used in assays, such as propionylthiocholine, enabling measurement of its activity. The statement labeling serum cholinesterase as the “true” cholinesterase is not accurate, since that designation is reserved for acetylcholinesterase.