Participation of pancreatic enzymes in the degradation of intestinal sucrase-isomaltase.

The pancreatic ducts of the rats were bypassed with a catheter placed within the common bile duct to prevent the entry of pancreatic enzymes into the duodenum without interrupting bile flow. For 8 days, the animals were fed a diet (peptones, sucrose, coconut oil, vitamins, and minerals) that could be digested without pancreatic enzymes. Control animals were sham operated and pair-fed with the same diet. Relative rates of synthesis and degradation were estimated by pulse labeling and double labeling, respectively, for sucrase and for total protein, in intestinal mucosa and along the gradient of cells collected from the tip of the villus to the bottom of the crypt. The rate of degradation of sucrase was 1.7 times greater than that of total protein in controls, whereas in animals with the pancreatic bypass it was equal to that of total protein. This decrease in rate of degradation produced a proportional increase of activity of sucrase in experimental animals. The hydrolytic effect of pancreatic enzymes on sucrase was apparent along the entire length of the villus but not in the crypt. These data support the hypothesis that pancreatic proteases release sucrase-isomaltase from the brush border membrane, resulting in the observed increase of the rate of degradation. Electrophoretic separation of immunoprecipitated sucrase-isomaltase showed that the intact pro-sucrase-isomaltase observed in operated animals is split into two subunits (sucrase and isomaltase) by action of pancreatic proteases in control animals.