Chymotrypsinogen in the Intestine of Rats Fed Soybean Trypsin Inhibitor and Its Inability to Suppress Pancreatic Enzyme Secretions

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Abstract
Recent investigations have shown that intestinal trypsin and chymotrypsin exert a negative feedback control on pancreatic enzyme secretion in the rat. The present study was conducted to see if soybean trypsin inhibitor (SBTI) effected changes in the intestinal activities of these enzymes which could provide an explanation for the pancreatic enzyme response evoked when SBTI is fed to rats. Rats were intubated with either a control diet or diet containing SBTI. One hour after feeding, intestinal contents were removed and washed with either saline or saline containing chicken ovomucoid. Trypsin and amylase activities were determined and chymotrypsin activity was measured before and after incubation with trypsin. In control rats, intestinal chymotrypsin activity was not increased by incubation with trypsin. In animals fed SBTI, however, trypsin incubation increased intestinal chymotrypsin activity to 3 to 5 times the preincubation levels. Intestinal trypsin activity of rats fed SBTI was reduced to less than 10% of control values. Continuous infusion of purified bovine chymotrypsin or chymotrypsinogen into rats with a bile-pancreatic juice fistula demonstrated that while chymotrypsin suppressed pancreatic enzyme output, chymotrypsinogen did not. It was concluded, therefore, that SBTI acts to stimulate pancreatic secretion by binding intestinal trypsin so tightly it cannot fully activate chymotrypsinogen. Since chymotrypsinogen cannot suppress pancreatic secretion, SBTI effectively removes both enzymes from the intestine. An increased pancreatic enzyme secretion is initiated because of loss of the negative feedback regulation normally exerted by the active enzymes.