A COMPARATIVE-STUDY OF STEROL ABSORPTION IN DIFFERENT SMALL-INTESTINAL BRUSH-BORDER MEMBRANE MODELS

We reported previously that the absorption of cholesterol and long-cha in cholesteryl esters by rabbit small-intestinal brush border membrane s (BBMV) is protein-mediated (Thurnhofer, H., and H. Hauser. 1990. Bio chemistry. 29: 2142-2148; Compassi, S., M. Werder, D. Boffelli, F. E. Weber, H. Hauser, and G. Schulthess. 1995. Biochemistry. 34: 16473-164 82). Evidence is presented for similar cholesterol transport activitie s in rabbit, pig, and human BBMV. As BBMV are subject to a number of l imitations and the influence of these on sterol absorption is unknown, it is desirable to verify results obtained with this model system in other brush border membrane models more closely related to tile in viv o situation. Sterol absorption in intact enterocytes parallels the abs orption measured in BBMV, provided that both model systems are normali zed to equal sucrase activity. The parallel behavior of the two brush border membrane models lends support to our previous conclusion that t he brush border membrane takes up free and esterified cholesterol in a facilitated and energy-independent process. The absorption of sterols in small-intestinal segments mounted in the Ussing chamber is shown t o be a complex process in which the diffusion of the bile salt micelle s to the brush border membrane is rate-limiting. All brush border memb rane models share die disadvantage of being unstable and subject to de gradation. The seriousness of the problem increases apparently with th e complexity of the model, i.e., in the order BBMV --> enterocytes --> intestinal segments. One main conclusion of this study is that no bru sh bolder membrane model is sufficient and satisfactory, therefore con clusive work in lipid absorption can never be based on a single brush border membrane model.