CRYOELECTRON MICROSCOPY OF LOW-DENSITY-LIPOPROTEIN AND RECONSTITUTED DISCOIDAL HIGH-DENSITY-LIPOPROTEIN - IMAGING OF THE APOLIPOPROTEIN MOIETY

Cryo-electron microscopy was used to analyze the structure of low dens ity lipoprotein from normolipidemic subjects (N-LDL), phospholipid-dep leted N-LDL (PD-LDL), small dense LDL from hypertriglyceridemic subjec ts (SD-LDL), and reconstituted discoidal high density lipoproteins (rH DL). In different projections of N-LDL, a high density component of th e particle was visible as two parallel bands or as a single ring. Proj ections of PD-LDL were very similar to those of N-LDL, indicating that the contribution of phospholipid headgroups to the observed high dens ity structure is minor. In preparations of SD-LDL, projections with tw o high density bands or a single high density ring were rare. Instead, triangular and diamond-shaped projections were recognized. In differe nt projections of discoidal rHDL, a high density component was visible as a single band or as a single ring. The present results indicate th at cryo-electron microscopy reveals the distribution of apolipoprotein s within lipoprotein particles. Thus, apolipoprotein B-100 (apoB) in N -LDL appears to be organized as a double ring around the particle, whi le apoB in SD-LDL is indicated to have a different conformation. Cryo- electron micrographs of rHDL are consistent with the presence of apoli poprotein AI on the periphery of the lipoprotein disc.