A Manual and Automated Procedure for Measuring Serum Cholinesterase Activity and Identifying Enzyme Variants

A simplified manual and automated method is described for measuring serum cholinesterase activity and identifying the homozygous "usual," heterozygous, and homozygous "atypical" forms of the enzyme. With butyrylthiocholine as substrate at pH 7.4, the activity is determined in two 0.05 mol/liter buffer systems, Tris and phosphate. The "usual" enzyme has the same activity in both buffer systems. The phosphate buffer inhibits the heterozygous enzyme about 13% and the "atypical" enzyme about 43% relative to its activity in Tris buffer. The range of normal serum cholinesterase values (measurements on plasma from 1842 preschool children) is 2.10 to 5.25 U (mean, 3.67 U). In this population, 3.28% of the Caucasians, but only 0.29% of the Negroes, were heterozygous for the trait.