Differences in LDL subspecies involve alterations in lipid composition and conformational changes in apolipoprotein B

In order to investigate causes of variability in low density lipoprotein (LDL) particle size, we have assessed LDL composition in plasma from 66 subjects, each with a single LDL band, by 2-16% gradient gel electrophoresis, with a total of eight discrete sizes (LDL-1 to LDL-8). Lipoprotein concentrations were analyzed by standard methods; specific proteins were assessed by immunoassay and electrophoresis. Results showed decreased anhydrous molecular weight with size (2.67 +/- 0.07 x 10(6) to 1.78 +/- 0.19 x 10(6)), along with decreased relative content for cholesteryl ester (41.5% to 24.3%), free cholesterol (10.1% to 4.6%), and phospholipid (23.7% to 18.9%), and increased triglyceride (4.1% to 21.0%) and protein (20.5% to 31.2%) content. As LDL size decreased, the ratio of surface cholesterol to phospholipid decreased from 0.53 to 0.29, and the fraction of surface area covered by lipid decreased from 0.74 to 0.47. Moreover, core volume decreased with size from 24.2 A3 x 10(5) to 15.9 A3 x 10(5), and the ratio of surface-to-core lipids fell from 0.59 to 0.46. Based on surface pressures of 30 mN/m, the area covered by surface lipid was calculated to range from 6.45 A2 x 10(4) in the largest LDL, to 3.10 A2 x 10(4) in the smallest. Computer modeling indicates that alterations in the tertiary structure of apoB-100 are required to account for surface changes. The estimated core surface area requiring coverage by apoB increased with decreasing particle size from 2.26 A2 x 10(4) to 3.46 A2 x 10(4). To accommodate coverage of increasing relative surface area associated with decreasing size, apoB thickness at the interface was calculated to decrease from approximately 25 A to 16 A. Such conformational changes in apoB may alter exposed epitopes, possibly causing changes in LDL receptor binding affinity and resistance to oxidation.