In presence of oleate and taurocholate, differentiated CaCo-2 cell monolayers on membranes were able to assemble and secrete chylomicrons. Under these conditions, both cellular uptake and secretion into chylomicrons of ␤ -carotene ( ␤ -C) were curvilinear, time-dependent (2-16 h), saturable, and concentration-dependent (apparent K m of 7-10 M) processes. Under linear concentration conditions at 16 h incubation, the extent of absorption of all-trans ␤ -C was 11% (80% in chylomicrons), while

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... of 9-cis-and 13cis-␤ -C were significantly lower (2-3%). The preferential uptake of the all-trans isomer was also shown in hepatic stellate HSC-T6 cells and in a cell-free system from rat liver (microsomes), but not in endothelial EAHY cells or U937 monocyte-macrophages. Moreover, extents of absorption of ␣ -carotene ( ␣ -C), lutein (LUT), and lycopene (LYC) in CaCo-2 cells were 10%, 7%, and 2.5%, respectively. Marked carotenoid interactions were observed between LYC/ ␤ -C and ␤ -C/ ␣ -C. The present results indicate that ␤ -C conformation plays a major role in its intestinal absorption and that cis isomer discrimination is at the levels of cellular uptake and incorporation into chylomicrons. Moreover, the kinetics of cellular uptake and secretion of ␤ -C, the inhibition of the intestinal absorption of one carotenoid by another, and the cellular specificity of isomer discrimination all suggest that carotenoid uptake by intestinal cells is a facilitated process. -During, A., M. M. Hussain, D. W. Morel, and E. H. Harrison. Carotenoid uptake and secretion by CaCo-2 cells: ␤ -carotene isomer selectivity and carotenoid interactions. J.