Serum Cholinesterase Variants: Examination of Several Differential Inhibitors, Salts, and Buffers Used to Measure Enzyme Activity

Abstract

Dibucaine, used as a differential inhibitor with acetyl-, propionyl-, and butyrylthiocholine as substrate, clearly identified the "usual" and "atypical" serum cholinesterases. Succinylcholine was also used successfully as a differential inhibitor with butyrylthiocholine as substrate. Sodium fluoride, used as a differential inhibitor, gave conflicting results, depending on whether Tris or phosphate buffer was used in the assay. Mono- and divalent cations (NaCl, KCl, MgCl₂, CaCl₂, and BaCl₂) activated the "usual" and inhibited the "atypical" enzyme at low concentrations. The "usual" enzyme had the same activity in 0.05 mol of Tris or phosphate buffer per liter, while the heterozygous and "atypical" enzymes showed 12 and 42% inhibition, respectively, when assayed in the phosphate buffer. Kinetic studies showed the phosphate acted as a competitive inhibitor of "atypical" enzyme. K_m values, determined for "usual" and "atypical" enzymes, were 0.057 and 0.226 mmol/liter, respectively, with butyrylthiocholine as substrate.