The cytotoxic action of BPI is limited to many species of Gram-negative bacteria; this specificity may be explained by a strong affinity of the very basic N-terminal half for the negatively charged lipopolysaccharides that are unique to the Gram-negative bacterial outer envelope. Has antibacterial activity against the Gram-negative bacterium P.aeruginosa, this activity is inhibited by LPS from P.aeruginosa. Monomer. Homodimer; disulfide-linked. Membrane-associated in polymorphonuclear Leukocytes (PMN) granules. Restricted to cells of the myeloid series. The N-terminal region may be exposed to the interior of the granule, whereas the C-terminal portion may be embedded in the membrane. During phagocytosis and degranulation, proteases may be released and activated and cleave BPI at the junction of the N- and C-terminal portions of the molecule, providing controlled release of the N-terminal antibacterial fragment when bacteria are ingested. The N- and C-terminal barrels adopt an identical fold despite having only 13% of conserved residues. Belongs to the BPI/LBP/Plunc superfamily. BPI/LBP family.