In vivo evaluation of free chymotrypsin activity in the lumen using benzoyl- L-tyrosyl-p-aminobenzoic acid in portal cannulated rats

Abstract

In vivo chymotrypsin activity in lumen was estimated by portal absorption of p-aminobenzoic acid (PABA) in the portal cannulated rats after feeding of diets containing benzoyl- L-tyrosyl-PABA (BT-PABA), to predict protease activities regarding the negative feedback regulation of exocrine pancreatic secretion. The PABA concentrations of portal plasma depending on hydrolysis of BT-PABA in the lumen was maximum at 20 min after feeding of both an 8% casein and an 8% soybean protein isolate (SPI) diet containing 3% BT-PABA. The concentrations of the casein group were decreased more rapidly, and were significantly lower at 80 and 100 min than that of the SPI group. The concentrations were minimum at 120 min in both the groups, and then increased rapidly in the casein group. Casein, SPI, and their peptic hydrolysates inhibit in vitro chymotrypsin-catalyzed hydrolysis of BT-PABA more than 50%. Thus, the rapid decrease in the portal concentration of PABA may depend on the competition with the dietary proteins or their hydrolysates in the lumen. The inhibition of in vitro hydrolysis by SPI and its hydrolysate were a little stronger than that by casein and its hydrolysate, respectively. The discrepancy between in vivo and in vitro BT-PABA hydrolysis suggests that factors other than the competition also affect the in vivo hydrolysis of BT-PABA. High activities of chymotrypsin in the lumen at 20 min after feeding may be able to inhibit the exocrine pancreatic secretion, and low level of the protease activity in the casein group at 120 min may be able to enhance the secretion by removal of the negative feedback mechanism.