A New Method for the Exchange of Lipid Classes of Human Serum High Density Lipoprotein

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Abstract
Human serum high density lipoproteins (density = 1.063-1.21 g .times. cm-3) exchange their phospholipid, cholesterol and cholesterol ester components when the same lipid classes are brought into micellar solution with sodium cholate. The stoichiometry of the lipid and apoprotein components remains constant when the lipid concentration in the micellar solution is that of the native HDL. With the corresponding radioactive lipid classes an equilibrium at about 50% exchange is reached. Higher lipid and lipid cholate concentrations lead to an accumulation of lipid in the HDL particles. The procedure for lipid exchange described here yields the HDL preparation free of cholate. Phosphatidylcholine and sphingomyelin are incorpoarated at up to 50% of the starting concentration in a single lipid exchange cycle at the expense of phosphatidylcholine of the native HDL. Phosphatidylethanolamine is completely incorporated in the HDL particles. The method also allows the cholesterol and cholesterol ester exchange of HDL particles. Fluorescence spectroscopy and near UV circular dichroism measurements carried out before and during the lipid exchange process and with the final exchanged HDL preparation indicate that no conformational changes of the apoprotein components occur. The EM appearance of the HDL particles before and after the lipid exchange is identical. The use of this lipid exchange for lipoprotein structural and metabolic studies is discussed.