Isolation of a porcine liver plasma membrane fraction that binds low density lipoproteins.

Abstract

Large amounts of injected radiolabeled low density lipoproteins have been found by others to accumulate primarily in the liver and studies in various types of isolated cells, including hepatocytes, have indicated the presence of specific cell membrane recognition sites for lipoproteins. In the present studies, the high affinity binding of radiolabeled low density lipoproteins ([125I]LDL, d 1.020--1.063 g/mL) was measured in the major subcellular fractions of porcine liver homogenates. The nuclear and mitochondrial fractions were 1.9- and 1.4-fold enriched in binding activity with respect to unfractionated homogenates and contained 15% and 12% of the total binding activity, respectively. The microsomes, which contained most of the plasma membranes and endoplasmic reticulum, were approximately 4-fold enriched in binding and contained 73% of the binding activity. Microsomal subfractions obtained by differential homogenization and centrifugation procedures were 5.6--7.0-fold enriched in LDL binding and contained 54--58% of the homogenate binding activity. They were separated by discontinuous sucrose density gradient centrifugation into fractions which contained "light" and "heavy" plasma membranes and endoplasmic reticulum. The heavy membrane fraction was 2--4 fold in binding with respect to the parent microsomes (16--22 fold with respect to the homogenate). There was no enrichment of binding activity in the other two fractions. Two plasma membrane "marker" enzymes, nucleotide pyrophosphatase and 5'-nucleotidase, were also followed. Of the two, binding in the sucrose density gradient subfractions most closely followed nucleotide pyrophosphatase, which was also most highly enriched (3.2--3.3-fold) in the heavy membrane fraction, but did not follow it exactly. The enzyme was 2-fold richer in the light membranes than in the parent microsomes, though the light membrane binding activity was only 0.4--1.4 times that of the parent microsomes. High affinity binding was time and temperature dependent, saturable, and inhibited by unlabeled low density lipoproteins but not by unrelated proteins. Binding was stimulated 2--3 fold Ca2+, was not affected by treatment with Pronase or trypsin and was inhibited by low concentrations of phospholipids and high density lipoproteins (HDL). Heparin-Mn2+ treatment of HDL did not affect its ability to inhibit [125I] LDL binding. The LDL recognition site was distinct from the liver membrane asialoglycoprotein receptor; LDL binding was not inhibited by desialidated fetuin. We conclude that porcine liver contains a high affinity binding site that recognizes features common to both pig low density and high density lipoproteins. Further studies may elucidate the significance of this binding site in lipoprotein metabolism.