The Autoactivation of Trypsinogen

Abstract

The ability of trypsinogen to catalyze its own activation was studied at pH 8.1 in the presence of Ca++ ions. The lag phase portion of the classical S-shaped curve of activation was shortened by increasing the trypsinogen concentration but was extended by pretreatment of the zymogen with diisopropyl fluorophosphate or soybean trypsin inhibitor. Despite these pretreatments, however, full activation was achieved. In confirmation of the proposed catalytic activity of trypsinogen, acetyltrypsinogen, which cannot be activated by trypsin or acetyltrypsin, was able to activate chymotrypsinogen at a rate about 10-5 that of trypsin and to hydrolyze p-toluenesulfonyl arginine methyl ester at a very slow rate. Pretreatment of the acetyltrypsinogen with diisopropyl fluorophosphate and soybean trypsin inhibitor did not affect the rate at which it carried out these two reactions, thus excluding the presence of trypsin or acetyltrypsin. The conclusion is drawn that the activities observed were not due to contamination of the zymogen preparations with active enzyme but were due to an activity inherent in the zymogens. The S-shaped curve of trypsinogen activation is now interpreted as two separate phases: a slow initial formation of trypsin by self-activation of the zymogen, followed by a rapid acceleration of the reaction due to catalysis by the product, trypsin.