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.[1]References
- Lactoferrin uptake by the rat liver. Characterization of the recognition site and effect of selective modification of arginine residues. Ziere, G.J., van Dijk, M.C., Bijsterbosch, M.K., van Berkel, T.J. J. Biol. Chem. (1992) [Pubmed]
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