EFFECTS OF APOLIPOPROTEINS ON THE KINETICS OF CHOLESTEROL EXCHANGE

The effects of apolipoproteins on the kinetics of cholesterol exchange have been investigated by monitoring the transfer of [C-14]cholesterol from donor phospholipid/cholesterol complexes containing human apolipoproteins A, B, or C. Negatively charged discoidal and vesicular particles containing purified apolipoproteins complexed with lipid (75 mol % egg PC, 15 mol % dicetyl phosphate, and 10 mol % cholesterol) and a trace of [C-14]cholesterol were incubated with a 10-fold excess of neutral, acceptor, small unilamellar vesicles (SUV; 90 mol % egg PC and 10 mol % cholesterol). The donor and acceptor particles were separated by chromatography on DEAE-Sepharose, and the rate of movement of labeled cholesterol was analyzed as a first-order exchange process. The kinetics of exchange of cholesterol from both vesicular and discoidal complexes that contain apoproteins are consistent with an aqueous diffusion mechanism, as has been established previously for PC/cholesterol SUV. The addition of 2-3 molecules of apo A-I to a donor SUV does not significantly alter the half-time (t1/2), which is 80 +/- 9 min at 37-degrees-C. However, addition of 5-12 apo A-I molecules progressively decreases t1/2 from 65 +/- 2 to 45 +/- 4 min. This enhancement in the rate of desorption of cholesterol molecules is presumed to arise from the creation of packing defects at boundaries around the apoprotein molecules, which are intercalated among the phospholipid and cholesterol molecules in the surface of the donor SUV. Apolipoproteins A-I, A-II, reduced and carboxymethylated A-II, and B-100 present in SUV at the same lipid/protein (w/w) ratio all enhance the rate of cholesterol exchange to about the same degree. Cholesterol molecules exchange more rapidly from discoidal complexes; t1/2 from a 150:1 (mol/mol) apo A-I/lipid disc is 14 +/- 1 min compared to 45 +/- 4 min for a SUV of the same composition. Generally, as the diameter of apoprotein/phospholipid/cholesterol discs decreases, t1/2 for cholesterol exchange decreases. The apoprotein molecules at the edges of the discoidal complexes perturb the phospholipid/cholesterol interactions and facilitate desorption of cholesterol molecules from these areas. Since small bilayer discs have a relatively high ratio of boundary to face surface area, cholesterol molecules desorb more rapidly than from larger discs. The modulation of lipid packing by the apoprotein molecules present at the surface of lipoprotein particles affects the rate of cholesterol exchange from such particles.