Effects of membrane lipid composition on the kinetics of cholesterol exchange between lipoproteins and different species of red blood cells

Abstract

To better understand the effects of plasma membrane structure on the kinetics of cellular cholesterol efflux to extracellular lipoprotein particles, the influence of plasma membrane sphingomyelin (SM) on the kinetics of cholesterol exchange was examined in both a model membrane system comprised of egg SM/egg phosphatidylcholine (PC) unilamellar vesicles and in various types of mammalian red blood cells (RBC) containing differing levels of SM. The kinetics and mechanism of the bidirectional flux of unesterified cholesterol (FC) between RBC and lipoproteins were established by using human RBC (labeled with [ 14C]FC) incubated with varying concentrations of human [ 3H]FC high-density lipoprotein (HDL 3) or [ 3H]FC low-density lipoprotein (LDL). A maximal rate constant for FC efflux was obtained when the lipoprotein FC was in excess (6-fold and 15-fold, for HDL 3 and LDL, respectively) of RBC FC; under this condition, the rate-limiting step is desorption of cholesterol molecules from the RBC membrane into the extracellular aqueous phase. At 37°C, the halftime ( t 1 2 ) for efflux was 4.6 ± 0.6 h for HDL 3 and 6.2 ± 0.2 h for LDL; FC efflux exhibited first-order kinetics and the RBC FC comprised a single kinetic pool. To investigate the effect of different membrane SM/PC ratios on the rate of FC desorption from the plasma membrane, the kinetics of cholesterol efflux from bovine RBC (5:1, w/w ratio of SM/PC), human RBC (1:1 ratio), rabbit RBC (0.6:1 ratio) and rat RBC (0.3:1 ratio) were compared. With excess HDL 3 present, bovine, rabbit, and rat RBC exhibited t 1 2 of 5.5 ± 0.8, 4.0 ± 0.2, and 3.7 ± 0.6 h, respectively, for cholesterol efflux. Changing the ratio from 0.3:1 to 3:1 in egg SM/egg PC small unilamellar vesicles increased the t 1 2 for cholesterol efflux at 45°C from 1.1 to 6.9 h. The results described in this paper suggest that increasing membrane SM content raises the t 1 2 for cholesterol exchange in both the RBC plasma membrane and in simple mixed SM/PC bilayers. However, the influence of SM is less in the natural plasma membrane, perhaps because of modulating factors such as membrane proteins and the presence of a complex phospholipid mixture.