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High-affinity binding of the bactericidal/permeability-increasing protein and a recombinant amino-terminal fragment to the lipid A region of lipopolysaccharide

Bactericidal/permeability-increasing protein (BPI) is a 55-kDa cationic protein (nBPI55) elaborated by polymorphonuclear neutrophils (PMN). BPI has potent bactericidal activity against a wide variety of gram-negative organisms and neutralizes endotoxin activities. An N-terminal fragment of nBPI55 exhibits the bactericidal and antiendotoxin properties of the holoprotein. To further characterize the biological activities of the N-terminal fragment, a recombinant protein (rBPI23) corresponding to the first 199 amino acids of human BPI was produced and purified. rBPI23 had antibacterial activity equivalent to that of nBPI55 against Escherichia coli J5. Furthermore, both rBPI23 and nBPI55 bound identically to a broad range of R- and S-form lipopolysaccharides (LPS) and to natural and synthetic lipid A. Binding of radiolabeled nBPI55 to LPS was inhibited in an identical fashion by either nBPI55 or rBPI23. The binding of both proteins to immobilized E. coli J5 lipid A was inhibited in a comparable fashion by long- or short-chain LPS or lipid A. The binding of both rBPI23 and nBPI55 was specific, saturable, and of high affinity, with an apparent Kd of approximately 2 to 5 nM for all ligands tested. These results demonstrate that BPI recognizes the highly conserved lipid A region of bacterial LPS via residues contained within the amino-terminal portion of the BPI molecule.