Apolipoprotein E-binding proteins isolated from dog and human liver.
Chylomicron remnant catabolism appears to be mediated by apolipoprotein (apo) E binding to hepatic lipoprotein receptors. Previously, the apo B,E(LDL) receptor and a unique apo E-binding protein (referred to as the apo E receptor) were isolated from solubilized canine and human livers. In the present study, the apo E-binding fraction was further characterized and found to contain at least three proteins, all of which bind apo E-containing lipoproteins with high affinity. The 56-kDa band was found to contain the alpha- and beta-subunits of F1-ATPase, presumably derived from mitochondrial membranes. In addition, an apo E-binding protein with an apparent Mr approximately equal to 59,000 was identified. The 59-kDa protein displays calcium-independent binding on ligand blots, but displays both calcium-dependent and -independent binding in assays performed with detergent-solubilized protein. The 59-kDa protein recognized lipid-free as well as lipid-bound apo E in ligand blots, and also bound apo E-2, apo E-3, and apo E-4 in a comparable way. Monoclonal antibodies produced against the 59-kDa protein did not react with the 56-kDa proteins. Normal human liver, as well as the liver of a patient lacking the apo B,E(LDL) receptor, possessed the 56-kDa and 59-kDa proteins. These data indicate that liver cells possess at least three proteins, in addition to the apo B,E(LDL) receptor, that bind apo E-containing lipoproteins with high affinity. The physiological role of these proteins in apo E metabolism remains to be determined.
Beisiegel
U
U
Medizinische Kernklinik and Poliklinik, Universitätskrankenhaus Eppendorf, Hamburg, Federal Republic of Germany.
Weber
W
W
Havinga
J R
JR
Ihrke
G
G
Hui
D Y
DY
Wernette-Hammond
M E
ME
Turck
C W
CW
Innerarity
T L
TL
Mahley
R W
RW
D000818:Animals; D001057:Apolipoproteins E / Q000378:metabolism*; D002846:Chromatography, Affinity; D002847:Chromatography, Agarose; D002848:Chromatography, DEAE-Cellulose; D002914:Chylomicrons / Q000378:metabolism; D004285:Dogs; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D007118:Immunoassay; D008099:Liver / Q000378:metabolism*; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D008565:Membrane Proteins / Q000032:analysis / Q000302:isolation & purification; D008970:Molecular Weight; D011485:Protein Binding; D011956:Receptors, Cell Surface / Q000032:analysis / Q000302:isolation & purification*; D012995:Solubility
D001057:Apolipoproteins E; D002914:Chylomicrons; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D008565:Membrane Proteins; D011956:Receptors, Cell Surface