The reaction of p-nitrophenyl esters with chymotrypsin and insulin

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Abstract
The rate of inhibition of the nitrophenyl esterase activity of chymotrypsin by diethyl p-nitrophenyl phosphate (E 600) does not correspond with the rate of reaction of enzyme with inhibitor. After complete reaction with E 600, chymotrypsin still reacts with p-nitrophenyl acetate (NPA) and p-nitrophenyl ethyl carbonate (NPC) to liberate nitrophenol. NPA and NPC react with amino and phenolic groups in derivatives and analogues of tyrosine. NPA and NPC also react with insulin and protamine to liberate nitrophenol. The reaction with insulin is a catalyzed hydrolysis of the esters for which a mechanism is proposed. A similar reaction of NPA or NPC in proteins would explain the observed reaction with inhibited chymotrypsin. Chymotrypsin catalyzes the hydrolysis of these esters at the same active center as is responsible for the other enzymic activities. The kinetics of this reaction are discussed, and the catalytic activities of insulin and chymotrypsin are compared.