On the apparent inhibition of intramolecular activation of pepsinogen by pepsin substrates

Marciniszyn et al. (Marciniszyn, J., Huang, J. S. Hartsuch, J. A., Tang, J. (1976) J. Biol. Chem. 251, 7095-7102) have recently suggested an intermediate in the intramolecular activation of pepsinogen. As evidence, they showed apparent competitive inhibition of activation by globin, indication a pepsinogen-globin complex. Previous work had shown pepsinogen activation to occur very rapidly in the presence of high concentrations of hemoglobin, a very similar pepsin substrate (McPhie, P. (1974) Biochem. Biophys. Res. Commun. 56, 789-792). This contradiction has been resolved by a re-evaluation of the techniques used in the two investigations. The experimental conditions of Marciniszyn et al. Were inadequately defined to ensure denaturation of pepsin, a prerequisite of their method. A small decrease in pH, caused by the presence of extraneous protein, prevents this denaturation and leads to consistent underestimates of the rate of zymogen activation.