On the Mechanism of the Pepsin-Catalyzed Exchange of Carboxylic Acids with Water-18O

The pD dependence of the pepsin-catalyzed exchange of DL-acetylphenylalanine with 18OD₂has been determined. The data at pD ≥ 4.0 are quantitatively accounted for by a kinetic scheme that postulates that the kinetic equivalent of the half-dissociated form of pepsin, ECOOHCOO-, and the anionic form (RCOO-) of the substrate undergoes reaction. Data at low pD require that an exchange path kinetically equivalent to reaction of ECOOHCOO- with the neutral form of the substrate (RCOOH) also be available. Possible mechanisms consistent with these data are discussed and evaluated for compatibility with other observations on pepsin's mechanism of action. The preferred mechanism for α-chymotrypsin-catalyzed exchange, a nucleophilic attack by enzyme upon undissociated substrate, is probably not operative with pepsin.