Bactericidalpermeability increasing proteins BPI

BPl is a 55-60 kDa, lysine-rich azurophil granule protein that accounts for less than 1% of total human PMN protein. BPI binds to bacterial lipo-polysaccharide (LPS) with high affinity, and can mitigate several potentially toxic effects of LPS in vitro. Although BPI is bactericidal for rough gram-negative bacteria, including E. coli and S. typhimurium, it lacks activity against gram-positive bacteria and fungi. BPI exerts maximal bactericidal activity at neutral pH, is relatively insensitive to ionic strength, but is inhibited by millimolar concentrations of calcium or magnesium ions.

The molecular organization of BPI includes an antibacterial N-terminal domain that is both highly cationic and amphiphilic in nature, and a slightly acidic C-terminal domain that contains three hydrophobic regions which might anchor BPI to the azurophil granule membrane and prevent its external secretion. These domains are separated by a hydro-philic and proline-rich region which may serve as a hinge. BPI was reported to bind rapidly to the outer membrane of susceptible gram-negative bacteria and to cause increased outer membrane permeability to hydrophobic molecules and selective activation of enzymes that degrade envelope phospholipids and peptidoglycans. More recently, BPI was also shown to affect the integrity of the inner bacterial membrane.

BPI shows substantial homology to LBP, a lipo-polysaccharide-binding protein that is synthesized by hepatocytes and circulates as an acute phase reactant in the plasma of humans and animals. BPI and LBP share the ability to bind the lipid A region of LPS and are immunologically cross-reactive; however, LBP lacks bactericidal activity. The genes for BPI and LBP are located in the same region of the long arm of chromosome 20.

Autoantibodies to BPI are commonly present in the serum of patients with inflammatory bowel disease or primary sclerosing cholangitis, a disorder of small bile ducts. What role, if any, these antibodies play in the pathogenesis of these disorders is unknown.

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