STRUCTURAL RELATION BETWEEN HDL-BINDING PROTEINS IN PORCINE LIVER

We have found strong evidence for a relation between three high-densit y lipoprotein (HDL)-binding proteins of 90, 110, and 180 kDa in porcin e liver that were detected by ligand blotting. Because HDL-binding pro teins with identical molecular masses were detected in human liver, al l subsequent experiments were performed with porcine liver proteins. A n 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 prepa rations of the 90-, 110-, and 180-kDa HDL-binding proteins were obtain ed and analyzed by polyacrylamide gel electrophoresis with sodium dode cyl sulfate. Under nonreducing conditions these preparations showed pr otein bands with the expected molecular masses. Reduction of these pre parations resulted in protein bands of 90 kDa. Ligand blotting experim ents 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 prepara tions under reducing conditions. Immunoblotting experiments with 90-PC antiserum resulted in a similar pattern. The three protein preparatio ns were then subjected to cyanogen bromide cleavage and the resulting peptides separated on gel. Immunoblotting with the 90-PC antibody reve aled a pattern of protein bands that was remarkably similar in all thr ee 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 hepatocellu lar plates. We conclude that the 180-kDa form is a homodimer of a mono meric HDL-binding protein present in two conformation variants of 90 a nd 110 kDa.