THE Joumar, OF BIOLOGICAL CHEMISTRY

The purification of a lipoprotein fraction from rat liver has been reported from this laboratory (I, 2). This lipoprotein fraction was precipitated from a sodium chloride extract by the low temperature, ethanol fractionation procedure of Cohn et al. (3), and exhibited flotation as observed in the analytical ultracentrifuge at a density of 1.21, analogous to the high density lipoproteins of serum (4). The high lipid content of mitochondria and microsomes (5) suggested that the cellular organelles were the source of these lipoprotein fractions, rather than the cytoplasm which is relatively free of bound lipid. The presumed importance of lipids in electron transport particles derived from mitochondria (6, 7), and the possible role of cellular lipoproteins as precursors of the plasma lipoproteins has stimulat,ed interest in a fuller characterization of cellular lipoproteins as chemical entities. The present communication describes the separation of lipoprotein fractions from cellular organelles by means of sonic oscillation, subsequent flotation of the solubilized lipoproteins, and chemical analysis of the resulting fractions.