Evidence of degradation process of sucrase-isomaltase in jejunum of adult rats

Abstract

To evaluate degradation processes of sucrase-isomaltase in adult rat jejunum, enzymic activity of sucrase and isomaltase was determined and compared with the amount of immunoreactive sucrase-isomaltase. In rats fed or starved for 18 h, killed at 10:00 or 22:00 h, sucrase activity (expressed on the basis of total protein or of immunoreactive sucrase-isomaltase) was significantly (P < 0.02) lower in the lower jejunum than in the upper jejunum; isomaltase activity was similar in both segments. Crossed immunoelectrophoresis demonstrated the existence of a 2nd sucrase-isomaltase antigen reacting with anti-(sucrase-isomaltase) serum. This antigen was present in larger amounts in the lower jejunum than in the upper jejunum, exhibited immunological partial identity with the intact sucrase-isomaltase, and had isomaltase activity but no sucrase activity. Apparently, this antigen is a degradation product of sucrase-isomaltase in which the sucrase active site was broken down. To examine the role of pancreatic enzymes in degradation of sucrase-isomaltase, common pancreatico-biliary ducts were ligated. Within 18 h after the operation, the difference of sucrase activity between the upper and the lower jejunum disappeared and the amount of the 2nd sucrase-isomaltase antigen markedly decreased in the lower jejunum. Apparently, during the degradation of intestinal sucrase-isomaltase by the pancreatic proteinases, degradation of the sucrase active site precedes that of the isomaltase active site.