Studies on the Specificity of Choline Esterase

The present investigation was undertaken to determine the relation between the structure of various substrates and the action of serum choline esterase upon them. The comparison of activities with different esters should take into account two factors : the hydrolytic velocities compared should be linear functions of time, and these velocities should be independent of substrate concentration. The first factor requires observations of the amount of ester split after certain intervals to establish the slope of the activity-time curve, and the second necessitates the determination of the initial velocities of hydrolysis with more than one substrate concentration to insure that further addition of substrate will not increase the speed of the splitting. In the past these points have been neglected in certain instances. Previous work upon the specificity of choline esterase was carried out chiefly by Stedman and coworkers (l-4), Kahane and Levy (5), Roepke (6, 7), and Vahlquist (8). Stedman’s group collected evidence indicating that choline esterase is an enzyme distinct from lipase or simple esterase, and they rejected the implication of Vahlquist that this distinction was doubtful. Kahane and Levy studied the hydrolysis of a variety of choline esters in connection with the effect of eserine in sensitizing tissues to the pharmacological action of these esters, and Roepke and associates investigated the affinity of the enzyme for a few substrates. In the present work a systematic study was made of the effect on the enzyme action of variations in the structure of the acid component and the hydrocarbon portion of the alcohol component of the substrate. In several cases the effect of changing the associated anion was also observed. In addition the stereochemical specificity of choline esterase was investigated for the first time.