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Phenylmethanesulfonyl fluoride reacts stoichiometrically with a-chymotrypsin, producing an inactive monosulfonyl enzyme and one equivalent of free acid. Titration of the enzyme with the inhibitor results in nearly complete inactivation when the equivalent amount of inhibitor is present, and treatment of enzyme with a large excess of [7-14C]phenylmethanesulfonyl fluoride leads to incorporation of the expected amount of l4C. Irreversibly inhibited chymotrypsin and chymotrypsinogen do not react significantly with the radioactive inhibitor. The release of acid in the reaction was measured over a range of pH; in the p H region 7-8 nearly one equivalent of acid is released. The reaction rate is a function of pH, apparently being dependent upon a group of pK, 7.0 in the nonprotonated form. A Michaelis constant of 5.6 x M and kz of 3.1 min-l were determined a t p H 6.0, 10". The reaction is slowed in the presence of @-phenylpropionate ion. It is concluded that the reaction is highly specific for the active site and that the mechanism is analogous to that for acylation. The stability of the sulfonyl group in [7-'4C]-

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