Lactoferrin uptake by the rat liver. Characterization of the recognition site and effect of selective modification of arginine residues.
Recently it was found that lactoferrin, an iron-binding glycoprotein with a molecular weight of 76,500, inhibits the remnant receptor-mediated uptake of apolipoprotein E (apoE)-bearing lipoproteins by the liver. In the present study we characterized the hepatic recognition of lactoferrin. Intravenously injected 125I-lactoferrin was cleared rapidly from the circulation by the liver (92.8 +/- 9.5% of the dose at 5 min after injection). Parenchymal cells contained 97.1 +/- 1.5% of the hepatic radioactivity. Internalization, monitored by measuring the release of liver-associated radioactivity by the polysaccharide fucoidin, occurred slowly. Only about 40% of the liver-associated lactoferrin was internalized at 10 min after injection, and it took 180 min to internalize 90%. Subcellular fractionation indicated that internalized lactoferrin is transported to the lysosomes. Binding of lactoferrin to isolated parenchymal liver cells was saturable with a dissociation constant of 10 microM (20 x 10(6) binding sites/cell). The role of arginine residues on lactoferrin was studied by modifying these residues with 1,2-cyclohexanedione. The modification resulted in a strongly reduced liver association (15.9 +/- 1.6% of the dose at 5 min after injection). Furthermore, unlabeled 1,2-cyclohexanedione-modified lactoferrin did not inhibit the binding of 125I-lactoferrin to isolated parenchymal cells. Arginine residues on lactoferrin thus appear to be essential for its specific recognition by parenchymal liver cells. In particular the clustered N-terminal arginine residues, which resemble the arginine-rich receptor binding sequence in apoE, may be responsible for both the interaction of lactoferrin with its recognition site and the inhibition of the hepatic uptake of apoE-bearing lipoproteins.
Ziere
G J
GJ
Division of Biopharmaceutics, University of Leiden, Sylvius Laboratory, The Netherlands.
van Dijk
M C
MC
Bijsterbosch
M K
MK
van Berkel
T J
TJ
D000818:Animals; D001120:Arginine / Q000378:metabolism*; D001665:Binding Sites; D001692:Biological Transport; D001769:Blood; D003512:Cyclohexanones / Q000737:chemistry; D006801:Humans; D066298:In Vitro Techniques; D007457:Iodine Radioisotopes; D007781:Lactoferrin / Q000378:metabolism*; D008099:Liver / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D008247:Lysosomes / Q000378:metabolism; D008297:Male; D011134:Polysaccharides / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D014018:Tissue Distribution
D003512:Cyclohexanones; D007457:Iodine Radioisotopes; D011134:Polysaccharides; C018031:1,2-cyclohexanedione; C007789:fucoidan; D001120:Arginine; D007781:Lactoferrin