Transmembrane movement of cholesterol in human erythrocytes.

Abstract

We studied the exchange of cholesterol between radioactively labeled plasma and human erythrocytes.Results from experiments in which [3H]cholesterol and ['4CJ cholesterol were exchanged sequentially into the cells and back out into unlabeled plasma, showed that transmembrane movement of cholesterol occurred with a half-time that was either less than 50 min or greater than 10 days. To obtain further information about the transmembrane movement of cholesterol, we used a technique [Jaeobson, B. S. & Branton, D. (1977) Science 195, 302-304] for exposure of the cytoplasmic surface of erythrocyte membranes. This method involved the ionic attachment of erythrocytes to derivatized glass beads followed by disruption of the cells, leaving the beads covered by membrane with the cytoplasmic surface exposed. [3HlCholesterol was exchanged into intact erythrocytes which then were attached to beads. The beads with attached membrane were incubated with phospholipid-cholesterol vesicles and the exchange of cholesterol between the membrane cytoplasmic surface and vesicles was studied. We found that [3Hlcholesterol was present at the cytoplasmic surface, indicating that transmembrane movement of cholesterol had occurred within the 2.5 hr required to complete the experiment. This result suggests that the more rapid rate of transmembrane cholesterol movement, inferred from-the experiments described above, is the one that applies.Cholesterol from erythrocyte membranes exchanges rapidly with the unesterified cholesterol of plasma lipoproteins both in vivo (1) and in vitro (2). The mechanisms involved in this exchange process are not understood. Some studies have indicated that all of erythrocyte cholesterol is exchangeable (3, 4), whereas others have shown that a significant portion does not exchange (5,6). The extent to which red cell cholesterol is available for exchange has important implications with regard both to membrane structure and to mechanisms of cholesterol exchange.It is widely believed that the lipids of the erythrocyte membrane are arranged in a bilayer that has transmembrane compositional asymmetry in its phospholipid components. The location of cholesterol in the membrane is not known and it is possible that it too is asymmetrically distributed across the bilayer. A recent study of the cholesterol content of the inner and outer halves of the erythrocyte membrane has indicated that there is cholesterol in both membrane halves, with possibly more on the outer half of the membrane (7).If cholesterol located on the cytoplasmic side of the membrane is to exchange with plasma lipoproteins, it first must move across the bilayer. Translocation across the erythrocyte membrane occurs for phosphatidylcholine. Bloj and Zilversmit (8) found a half-time for the process of 2.3 hr in rat erythrocyte ghosts; Renooij et al. (9) measured a half-time of 4.5 hr in intact rat red cells.The transbilayer movement of cholesterol in vesicles occurs with a half-time in excess of 6 days (10). In [14C]cholesterol were sequential...

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