Differential uptake of cholesterol and plant sterols by rat erythrocytes in vitro

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Abstract
The in vitro uptake of radioactively labeled cholesterol and the plant sterol .beta.-sitosterol was examined in rat erythrocytes. From mixed micellar solutions containing egg yolk phospholipid and sodium taurocholate, the erythrocytes showed a nonlinear uptake of the 2 sterols. The uptake leveled off after .apprx. 45 min with the attainment of a 1:1 total sterol-to-phospholipid ratio within the cell membrane, as determined on a mass basis. From solutions containing egg yolk phospholipid, or purified egg yolk phosphatidylcholine, a preference for cholesterol over the plant sterol was observed, increasing with time from a cholesterol/.beta.-sitosterol uptake ratio of unity (the media ratio) to a maximum of 2 after a 60-min incubation. Correction of the data for nonspecifically bound sterol increased the ratio to a maximum of 5 at the 30-min time point. The increase in the cholesterol/.beta.-sitosterol uptake ratio with time, following an initial nonspecific association, showed that penetration of the plasma membrane by the sterol was required for the selectivity to be expressed. The presence of lysophosphatidylcholine or bovine serum albumin did not exert any noticeable influence over the extent or selectivity of absorption. Replacement of the egg yolk phospholipid with synthetic dipalmitoyl-phosphatidylcholine led to a loss of the sterol selectivity. No evidence was found to support a selective extraction of sterol from the erythrocyte membrane to account for the observed effects, nor was there any sign of a mass accumulation of phospholipid during the incubation. The media phospholipid may influence the membrane permeability toward cholesterol and .beta.-sitosterol.