COMPARISON OF CHOLESTEROL AND SITOSTEROL UPTAKE IN DIFFERENT BRUSH-BORDER MEMBRANE MODELS

(I) There is little discrimination between cholesterol and the plant s terol sitosterol in the uptake at the brush border membrane (BBM). (II ) This difference cannot account for the marked discrimination between cholesterol and sitosterol observed in the absorption of these two st erols by the small-intestinal epithelium. (III) This discrimination oc curs during intracellular processing involving the esterification and incorporation into lipoprotein particles of the two sterols. This conc lusion is based on a comparative study of sterol uptake by brush borde r membrane vesicles (BBMV) and sterol absorption by Caco-2 cells. (IV) The uptake of sitosterol by the BBM is energy-independent and facilit ated in a manner analogous to cholesterol uptake [Thurnhofer, H., & Ha user, H. (1990a) Biochemistry 29, 2142-2148]. (V) The rate of choleste rol and sitosterol uptake by BBMV from both mixed bile salt micelles a nd small unilamellar vesicles (SUV) as the donor is directly proportio nal to the sterol content of the donor, (VI) The pseudo-first-order ra te constants k(1) for sterol uptake from SUV are independent of the st erol content up to 10-20 mol %, Above that, competition between the tw o sterols leads to a reduction of the k(1) values.