Conversion of pepsinogen to pepsin. Further evidence for intramolecular and pepsin catalyzed activation

Exposure of pepsinogen to acid for less than 2 min yields a product with proteolytic activity. This activity is due to intramolecular and intermolecular formation of pepsin from pepsinogen. No evidence was found for intermolecular proteolytic activity in the zymogen. These conclusions are based upon 2 sets of experiments. First, chemical cleavage of pepsinogen during short activation is demonstrated by quantitative analysis of the NH₂ terminal 2 residues of the pepsin and pepsinogen in an activation mixture. In addition, quantitative NH₂ terminal analyses after activation under different condition confirm a previous inference that the product of unimolecular pepsinogen activation is homogeneous whereas bimolecular activation produces a pepsin product with a variety of NH₂ termini. Second, spectral changes which occur upon acidification of a pepsinogen solution and are reversed by neutralization are shown to be consistent with the chemical cleavage of pepsinogen during acidification. The first order rate constant for pepsinogen activation, calculated from these spectral experiments, agrees well with the value determined previously.