Physicochemical characterization of ten fractions of bovine alpha lipoproteins.

With the onset of milk production, serum concentrations of alpha lipoproteins in the dairy cow steadily increase, frequently attaining values greater than 1.5 g/dl. Since these lipoproteins comprise a highly polydisperse system, we have carried out studies to explore differences among bovine alpha lipoproteins in the density interval between 1.05 to 1.21 g/ml. Separation into ten fractions was achieved ultracentrifugally in an isopycnic gradient. Agarose gel electrophoresis showed that all but the bottom fraction contained alpha lipoproteins as either the major or sole lipoprotein class. Compositional analyses revealed an increasing percentage of both protein and phospholipid and a decreasing percentage of cholesterol with increasing fraction density. The esterified to unesterified cholesterol ratio ranged from 3 to 8 from the top to the bottom of the gradient. The densities of the particles obtained from the various fractions were calculated both from sedimentation velocity measurements and from compositional analyses. The resulting density values agreed well with the solution densities of these isopycnic gradient fractions. The major apoprotein of each fraction was apoA-I. Combining diffusion coefficient data obtained by intensity fluctuation spectroscopy with sedimentation velocity data, we were able to calculate molecular weights, frictional ratios, and Einstein-Stokes radii for three of the fractions. Results are discussed in terms of previously published data on bovine lipoproteins as well as other mammalian data.