Mechanism of Intramolecular Activation of Pepsinogen EVIDENCE FOR AN INTERMEDIATE 6 AND THE INVOLVEMENT OF THE ACTIVE SITE OF PEPSIN IN THE INTRAMOLECULAR ACTIVATION OF PEPSINOGEN*

Intramolecular pepsinogen activation is inhibited either by pepstatin, a potent pepsin inhibitor, or by purified globin from hemoglobin, a good pepsin substrate. Also, pepsinogen at pH 2 can be bound to a pepstatin-Sepharose column and recovered as native zymogen upon elution in pH 8 buffer. Kinetic studies of the globin inhibition of pepsinogen activation show that globin binds to a pepsinogen intermediate. This interaction gives rise to competitive inhibition of intramolecular pepsinogen activation. The evidence presented in this paper suggests that pepsinogen is converted rapidly upon acidification to the pepsinogen intermediate 6. In the absence of an inhibitor, the intermediate undergoes conformational change to bind the activation peptide portion of this same pepsinogen molecule in the active center to form an intramolecular enzyme.substrate complex (intermediate 0). This is followed by the intramolecular hydrolysis of the peptide bond between residues 44 and 45 of the pepsinogen molecule and the dissociation of the activation peptide from the pepsin. Intermediate d apparently does not activate another pepsinogen molecule via an intermolecular process. Neither does intermediate 6 hydrolyze globin substrate.