Demonstration of receptor binding of two apo-B containing lipoproteins by differential labelling with colloidal gold.

The interaction of two types of apo-B containing lipoproteins, human low density lipoprotein (LDL) and lipoprotein-a (Lp(a], with cultured human skin fibroblasts was studied by differential colloidal gold labelling in conjunction with thin sectioning and surface replication techniques. After separate exposure of the fibroblasts to either gold labelled LDL or Lp(a) for 15 to 30 min at 37 degrees C, labelled lipoproteins were predominantly found in coated pit areas. Excess of unlabelled LDL or Lp(a) completely displaced the gold labelled lipoproteins, indicating specific binding by the LDL-receptor. Simultaneous exposure of fibroblasts to LDL-16 nm gold and Lp(a)-40 nm gold conjugates revealed that both LDL and Lp(a) are bound in the same coated pit and internalized into the same endosome. In contrast to native lipoproteins, gold labelled acetylated lipoproteins were found diffusely distributed on membrane surface areas predominantly representing fibronectin-containing fibrils.