Particle size and protein content of six fractions of the Sf 20 plasma lipoproteins isolated by density gradient centrifugation.

A procedure is described for the separation of plasma S(f) > 400 and S(f) 20-400 lipoproteins each into three fractions. Serum samples are overlayered with a sodium chloride density gradient in a preparative ultracentrifuge tube and thin layers are removed at the top of the tube after successive centrifugations at different speeds in a swinging bucket rotor. The procedure was evaluated by electron microscopy of the S(f) > 400 lipoprotein fractions and schlieren analysis of the S(f) 20-400 lipoprotein fractions. Protein content of each fraction was measured by elemental N, C, H, and lipid-P analysis. Protein coverage was calculated for all fractions on the assumption that there is a surface layer 20 A thick. For the entire S(f) > 400 lipoprotein spectrum and for a part of the S(f) 20-400 lipoprotein distribution the proportion of surface covered by protein was constant (approximately 20% coverage). Therefore, for these portions of the lipoprotein spectrum, the increase in surface: volume ratio as particle size decreases is approximately compensated for by an increase in the weight percentage of protein.