Simple-sugar meals target GLUT2 at enterocyte apical membranes to improve sugar absorption: a study in GLUT2-null mice.

The physiological significance of the presence of GLUT2 at the food-facing pole of intestinal cells is addressed by a study of fructose absorption in GLUT2-null and control mice submitted to different sugar diets. Confocal microscopy localization, protein and mRNA abundance, as well as tissue and membrane vesicle uptakes of fructose were assayed. GLUT2 was located in the basolateral membrane of mice fed a meal devoid of sugar or containing complex carbohydrates. In addition, the ingestion of a simple sugar meal promoted the massive recruitment of GLUT2 to the food-facing membrane. Fructose uptake in brush-border membrane vesicles from GLUT2-null mice was half that of wild-type mice and was similar to the cytochalasin B-insensitive component, i.e. GLUT5-mediated uptake. A 5 day consumption of sugar-rich diets increased fructose uptake fivefold in wild-type tissue rings when it only doubled in GLUT2-null tissue. GLUT5 was estimated to contribute to 100 % of total uptake in wild-type mice fed low-sugar diets, falling to 60 and 40 % with glucose and fructose diets respectively; the complement was ensured by GLUT2 activity. The results indicate that basal sugar uptake is mediated by the resident food-facing SGLT1 and GLUT5 transporters, whose mRNA abundances double in long-term dietary adaptation. We also observe that a large improvement of intestinal absorption is promoted by the transient recruitment of food-facing GLUT2, induced by the ingestion of a simple-sugar meal. Thus, GLUT2 and GLUT5 could exert complementary roles in adapting the absorption capacity of the intestine to occasional or repeated loads of dietary sugars.

Inhibition of the cathepsin B like proteinase by a low molecular weight cysteine-proteinase inhibitor from ascitic fluid and plasma alpha 2 macroglobulin.

The cathepsin B like proteinase present in ascitic fluid of a patient with neoplasia has been purified and characterized after pepsin activation. From this fluid we have prepared the low molecular weight (LMW) cysteine-proteinase inhibitors. Three major inhibitor forms were found with isoelectric points of 7.4, 5.4, and 5.1, respectively. The interaction of the enzyme with the former inhibitor was studied because this inhibitor was the most abundant. The Ki value was found to be 4.3 X 10(-8) M. Two molecules of this proteinase were bound by one molecule of plasma alpha 2 macroglobulin (alpha 2M). The LMW inhibitor was able to bind to the enzyme entrapped in alpha 2M and reduced its endopeptidase activity on benzyloxycarbonyl-L-phenylalanyl-L-arginine-4-methyl-7-coumarylamide. These results may have a physiological significance in the regulation of the enzyme which, among other extracellular hydrolases, probably plays an important role in tumor invasion.