Comparison of cholesterol and sitosterol uptake in different brush border membrane models.

(I) There is little discrimination between cholesterol and the plant sterol 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 sterols by the small-intestinal epithelium. (III) This discrimination occurs during intracellular processing involving the esterification and incorporation into lipoprotein particles of the two sterols. This conclusion is based on a comparative study of sterol uptake by brush border membrane vesicles (BBMV) and sterol absorption by Caco-2 cells. (IV) The uptake of sitosterol by the BBM is energy-independent and facilitated in a manner analogous to cholesterol uptake [Thurnhofer, H., & Hauser, H. (1990a) Biochemistry 29, 2142-2148]. (V) The rate of cholesterol and sitosterol uptake by BBMV from both mixed bile salt micelles and small unilamellar vesicles (SUV) as the donor is directly proportional to the sterol content of the donor. (VI) The pseudo-first-order rate constants k1 for sterol uptake from SUV are independent of the sterol content up to 10-20 mol %. Above that, competition between the two sterols leads to a reduction of the k1 values.