The First Cleavage Site in Pepsinogen Activation

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Abstract
The carboxyl proteases which are responsible for initiating gastric digestion of proteins in most mammals are synthesized in precursor form. The pepsinogens from pig, cow and calf prochymosin, in particular, are well-characterized proteins (see Foltmann and Pedersen, Chapter 1 in this volume). These zymogens convert themselves into the more active forms of their respective enzymes by undergoing limited proteolysis in which an NH2-terminal ‘activation’ segment is cleaved off (Fig.l). In porcine pepsinogen, the activation follows an intramolecular mechanism at pH 3 and below, whereas above this pH value the reaction involves the intermolecular action of newly formed pepsin interacting with pepsinogen to form even more pepsin (1–3).