Isolation and characterization of lipid-protein complexes present in commercial albumin preparations

Most commercially available albumin preparations examined by us contain phospholipids, cholesterol and apolipoprotein impurities. As these albumin preparations are frequently used in large amounts in systems involving lipoprotein metabolism these impurities may reach remarkable levels to introduce exogenous effects in these studies. We have studied in detail tow bovine albumin preparations differing in their content of these contaminants. Using preparative ultracentrifugation, we have isolated from both albumins a lipid protein complex at a buoyant density of d = 1.063-1.21 g/ml with a chemical composition resembling plasma high density lipoproteins. This complex when further characterized proved also to have a similar apoprotein composition to bovine plasma high density lipoproteins. Electron microscopic study of this complex revealed discoidal particles closely resembling nascent high density lipoproteins recovered from rat liver or lymph. The similarity of these lipid-protein complexes to high density lipoproteins, accounts for some reported effects caused by commercially available albumin preparations on cholesterol excretion from cells in tissue culture and their ability to act as acceptors for surface remnants released upon VLDL catabolism in vitro.