Incomplete intracellular forms of intestinal surface membrane sucrase-isomaltase.

Sucrase-isomaltase (S-I) is an intestinal membrane enzyme consisting of two active moieties each with its hydrolytic site available for nutrient digestion at the luminal-cell interface. At least 90% of hydrolytic activity can be localized to the brush border membrane and the remainder in the cytoplasm has been considered to originate from brush border contamination. The intracellular cytosol from rat jejunum was examined for the presence of a precursor of the fully formed brush border membrane enzyme. The cytosol contained 1 to 4% of the total cellular sucrase activity. Whereas brush border sucrase is essentially all (99%) in the form of S-I hybrid, the predominant form in the cytosol was free sucrase (60 to 85%). Smaller amounts of free isomaltase were also identified. As estimated from gel filtration chromatography and density gradient centrifugation, sucrase-isomaltase, free sucrase, and free isomaltase in the cytosol were distinctly smaller in molecular weight (-20,00O/enzyme moiety) than the corresponding membrane enzymes. Although devoid of some antigenic domains, the cytosol sucrases reacted in a specific solid phase radioimmunoassay designed for the brush border sucrases. Approximately 10 to 50% of the cytosol sucrases were enzymatically inactive as estimated by the specific radioimmunoassay. Like their brush border counterparts, the cytosol sucrases bound nearly completely to concanavalin ASepharose (95 to 99%) but were more readily uncoupled by cY-methylmannoside (brush border, 10 to 30%; cytosol, 45 to 65%). Pulse-labeling studies in uiuo revealed that [3H]leucine was maximally incorporated into cytosol sucrase in only 15 min; maximal incorporation did not occur in the brush border sucrase-isomaltase until at least 3 h. The intracellular cytosol sucrases appear to be closely related to brush border membrane sucrase-isomaltase and have the appropriate characteristics to serve as intracellular precursors for the fully formed brush border membrane enzyme at some stage of the intracellular assembly process.