Studies on Cholinesterase Vi. Kinetics of the Inhibition of Acetylcholine Esterase*

Enzyme inhibitors have long been used in the analysis of cellular function for separating different steps in complex chemical reactions and for studying the effect of blocking these reactions in the intact cell. Inhibitors of choline ester-splitting enzymes, especially the alkaloids prostigmine and eserine, have attracted the interest of investigators in view of the physiological rale of acetylcholine. In contrast to the inhibitors previously known, some powerful agents recently developed inactivate the enzymes irreversibly. This property has made it possible to approach many new problems for which the compounds previously available were not appropriate. Particularly, the diisopropyl fluorophosphate (DFP), in the presence of which the irreversible enzyme inactivation is a relatively slow process, has been an excellent tool for testing the essentiality of acetylcholine-hydrolyzing enzymes in nerve conduction (l-3). In addition, the new compounds offer an opportunity for studying whether the signs of toxicity must be attributed exclusively to interaction with the enzymes. As such an interaction depends on many factors, in vitro and still more in viva, it is imperative to study the kinetics of the inhibition of specific esterases, the inactivation of which appears to be primarily responsible for the toxic symptoms (4, 5). The properties of the esterases of conductive tissue and erythrocytes are distinctly different from other choline ester-splitting enzymes (6, 7). The term acetylcholine esterase (ACh-esterase) has been proposed (8) for this type of esterases. In recent studies on the kinetics of the inhibition of ACh-esterase some differences between DFP and the alkaloids have been described (9, 10). As before, the ACh-esterase used was exclusively the highly purified preparation obtained from the electric tissue of Electrophorus electricus.