Colorimetric estimation and histochemical demonstration of serum cholinesterase.

The presence of enzymes in mammalian blood which hydrolyze acetylcholine was first suspected by Dale (1)) and later demonstrated by Plattner (2) and by Engelhart and Loewi (3). A serum cholinesterase isolated by Stedman et al. (4) from horse blood was first thought to be the same as the cholinesterase in the central nervous system.’ Distinct differences between the cholinesterase of erythrocytes and of serum and species differences in serum cholinesterase’ and in the cell-serum partition of total cholinesterase activity have been demonstrated (5, 6). Two fundamentally different types of cholinesterase are now recognized and are differentiated primarily with respect to their action on various substrates. Acetylcholinesterase of the central nervous system (and of erythrocytes) hydrolyzes acetylcholine faster than any other ester. It also attacks propionylcholine and acetyl+methylcholine, but not benzoylcholine or butyrylcholine. It exhibits optimal activity with a substrate concentration of 2 to 3 X 10e3 M and is moderately inhibited at higher concentrations of the substrate. Serum cholinesterase also attacks acetylcholine faster than any other ester. It also attacks propionylcholine, benzoylcholine, and butyrylcholine, but not acetyl-/%methylcholine. Its activity is not significantly influenced by substrate concentration. Central nervous system cholinesterase is said not to attack non-choline esters,2 such as tributyrin, while