On the kinetics of inhibition of rat-brain acetylcholine esterase by Sevin.

The inhibition of rat-brain acetylcholinesterase with Sevin was shown to be partly reversible. The rate of progression of the irreversible inhibition is extremely slow and follows the kinetics of a unimolecular reaction, with K = 3.65 • 1 0 3 m i n 1 and t 0.5 190 minutes. The reversible inhibition of the enzyme was found to be of competitive nature. The dissociation constant of E —I (K\) was determined —by two different methods —to be 5.7 and 7 . 6 1 0 _ 6 M . The presence of acetylcholine protects the enzyme against the irreversible attack of Sevin. It is believed that acetylcholine esterase does not participate in the biological degradation of Sevin in the nervous tissue.