Two forms of prolyl endopeptidase with different activities

Prolyl endopeptidase, an enzyme exhibiting high specificity towards the Pro-Xaa bond, is thought to play an important role in the metabolism of biologically active peptides. We have purified the enzyme from pig muscle and observed significant differences in its kinetic behavior as compared to the extensively studied serine proteases, such as chymotrypsin. Thus, pH-dependence of, and kinetic deuterium isotope effects on, the rate constants indicated that the enzyme has two forms, which exhibit different activities and interconvert with changing pH. It can be concluded that a general base/acid-catalyzed acylation step is rate-limiting in the lower pH range, and an isotopically silent step, probably a conformational change preceding the acylation dominates the reaction in the physiological pH range.