STRUCTURAL RELATION BETWEEN HDL-BINDING PROTEINS IN PORCINE LIVER

We have found strong evidence for a relation between three high-density lipoprotein (HDL)-binding proteins of 90, 110, and 180 kDa in porcine liver that were detected by ligand blotting. Because HDL-binding proteins with identical molecular masses were detected in human liver, all subsequent experiments were performed with porcine liver proteins. An antiserum raised against a highly purified preparation of the 90-kDa HDL-binding protein, designated 90-PC, showed cross-immunoreactivity with the 110- and 180-kDa HDL-binding proteins. Purified protein preparations of the 90-, 110-, and 180-kDa HDL-binding proteins were obtained and analyzed by polyacrylamide gel electrophoresis with sodium dodecyl sulfate. Under nonreducing conditions these preparations showed protein bands with the expected molecular masses. Reduction of these preparations resulted in protein bands of 90 kDa. Ligand blotting experiments with I-125-HDL showed protein bands of 90, 110, and 180 kDa under nonreducing conditions and a 90-kDa protein band in all three preparations under reducing conditions. Immunoblotting experiments with 90-PC antiserum resulted in a similar pattern. The three protein preparations were then subjected to cyanogen bromide cleavage and the resulting peptides separated on gel. Immunoblotting with the 90-PC antibody revealed a pattern of protein bands that was remarkably similar in all three protein preparations. Immunohistochemical localization studies with the 90-PC antibody showed that the HDL-binding proteins were present both at the borders of the sinusoids as well as within the hepatocellular plates. We conclude that the 180-kDa form is a homodimer of a monomeric HDL-binding protein present in two conformation variants of 90 and 110 kDa.