Critical role of ring structure in the differential uptake of cholesterol and plant sterols by membrane preparations in vitro.

To determine the role of the ring structure in the differential absorption of sterols, we have used rat jejunal brush border vesicles and erythrocytes to examine the uptake of cholesterol, campesterol, and sitosterol following successive chemical degradations of rings A and B. The cell and membrane preparations were incubated with the sterols and sterol analogues (about 30 micromolar each) dissolved in 7 mM sodium taurocholate and 0.6 mM egg phospholipid. The uptake of the analogues was analyzed by high performance liquid chromatography and capillary gas--liquid chromatography. In both membrane preparations, the uptake of the 7-dehydroanalogues of cholesterol, campesterol, and sitosterol was linear with time. 7-Dehydrocholesterol was absorbed 4-5 times faster than 7-dehydrositosterol by both preparations. The uptake of the campesterol analogue was intermediate between that of the analogues of cholesterol and sitosterol at all time points. Following conversion of the 7-dehydrosterols to their calciferol derivatives, the 27-carbon sterols were absorbed only 1.9 and 1.4 times faster than those of the 29-carbon sterols by the erythrocyte and brush border membranes, respectively. A similar degree of selectivity was expressed in the erythrocytes during the uptake of a steroid series possessing keto-4-ene ring system. Complete oxidation of the calciferol derivatives to the des-AB-8-ones resulted in a total loss of discrimination among the various side-chain homologues during absorption from micellar solutions. It is concluded that the selective absorption of animal and plant sterols depends upon the presence of a ring system having the bulk of the cholestane nucleus, although not necessarily a rigid or planar one containing a hydroxyl group.