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Diphenylcarbamyl chloride (DPCC) was found to inactivate chymotrypsin and trypsin by means of a 1:1 stoichiometric reaction. The reaction was relatively specific for chymotrypsin, which was inactivated 80 times faster than trypsin. Am for the reaction with chymotrypsin was found to be 0.6 ± 0.2 X 10-4 m at pH 8.0. The inactivation of chymotrypsin could be competitively inhibited by indole. DPCC was unreactive toward chymotrypsinogen, diethylphosphorylchymotrypsin, and pepsin. The pH-rate profile of the reaction

Three photochromic reagents were synthesized and examined for their ability to inactivate acetylcholinesterase.

Phenothiazine-N-carbonyl chloride inactivated chymotrypsin and trypsin by means of a 1:1 stoicheiometric reaction. Its reaction with chymotrypsin was 29 times as fast as that with trypsin and was inhibited by indole. The reaction of phenothiazine-N-carbonyl chloride with chymotrypsin resembled an enzyme-substrate reaction in which the deacylation step is rate-limiting. Slow deacylation occurred, resulting in complete regeneration of active enzyme in 15h. The pH-rate profile of the inactivation process had a maximum at pH7.8. These data and other evidence indicate that the reaction of phenothiazine-N-carbonyl chloride with chymotrypsin exhibits ;kinetic specificity'. Therefore any hypothesis that attempts to describe the topography of the active site of chymotrypsin should take into account the reactivity of phenothiazine-N-carbonyl chloride. The above findings, as well as recent reports of others, are examined within the context of a hypothesis given in an earlier paper (Erlanger, 1967).