The ability of lactoferrin (Lf), an iron-binding glycoprotein that is also called lactotransferrin, to bind lipopolysaccharide (LPS) may be relevant to some of its biological properties. A knowledge of the LPS-binding site on Lf may help to explain the mechanism of its involvement in host defence. Our report reveals the presence of two Escherichia coli 055B5 LPS-binding sites on human Lf (hLf): a high-affinity binding site (Kd 3.6 +/- 1 nM) and a low-affinity binding site (Kd 390 +/- 20 nM). Bovine Lf (bLf), which shares about 70% amino acid sequence identity with hLf, exhibits the same behaviour towards LPS. Like hLf, bLf also contains a low- and a high-affinity LPS-binding site. The Kd value (4.5 +/- 2 nM) corresponding to the high-affinity binding site is similar to that obtained for hLf. Different LPS-binding sites for human serum transferrin have been suggested, as this protein, which is known to bind bacterial endotoxin, produced only 12% inhibition of hLf-LPS interaction. Binding and competitive binding experiments performed with the N-tryptic fragment (residues 4-283), the C-tryptic fragment (residues 284-692) and the N2-glycopeptide (residues 91-255) isolated from hLf have demonstrated that the high-affinity binding site is located in the N-terminal domain I of hLf, and the low-affinity binding site is present in the C-terminal lobe. The inhibition of hLf-LPS interaction by a synthetic octadecapeptide corresponding to residues 20-37 of hLf and lactoferricin B (residues 17-41), a proteolytic fragment from bLf, revealed the importance of the 28-34 loop region of hLf and the homologous region of bLf for LPS binding. Direct evidence that this amino acid sequence is involved in the high-affinity binding to LPS was demonstrated by assays carried out with EGS-loop hLf, a recombinant hLf mutated at residues 28-34.

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Human lactoferrin (hLf), a glycoprotein released from neutrophil granules during inflammation, and the lipopolysaccharide (LPS)-binding protein (LBP), an acute-phase serum protein, are known to bind to the lipid A of LPS. The LPS-binding sites are located in the N-terminal regions of both proteins, at amino acid residues 28 to 34 of hLf and 91 to 108 of LBP. Both of these proteins modulate endotoxin activities, but they possess biologically antagonistic properties. In this study, we have investigated the competition between hLf and recombinant human LBP (rhLBP) for the binding of Escherichia coli 055:B5 LPS to the differentiated monocytic THP-1 cell line. Our studies revealed that hLf prevented the rhLBP-mediated binding of LPS to the CD14 receptor on cells. Maximal inhibition of LPS-cell interactions by hLf was raised when both hLf and rhLBP were simultaneously added to LPS or when hLf and LPS were mixed with cells 30 min prior to the incubation with rhLBP. However, when hLf was added 30 min after the interaction of rhLBP with LPS, the binding of the rhLPS-LBP complex to CD14 could not be reversed. These observations indicate that hLf competes with rhLBP for the LPS binding and therefore interferes with the interaction of LPS with CD14. Furthermore, experiments involving competitive binding of the rhLBP-LPS complex to cells with two recombinant mutated hLfs show that in addition to residues 28 to 34, another basic cluster which contains residues 1 to 5 of hLf competes for the binding to LPS. Basic sequences homologous to residues 28 to 34 of hLf were evidenced on LPS-binding proteins such as LBP, bactericidal/permeability-increasing protein, and Limulus anti-LPS factor.