Antimicrobial Proteins and Peptides Bactericidal Permeability-increasing another bacteric Protein (BPI) lastase ed by Bactericidal permeability-increasing protein (BPD)is a highly cationic 55-60-kDa prote Cathepsin G gene is located in another cluster of serine prote. n chromosom dim/CAP37 ompa to th and like LBP,binds to lipopolysaccharide with high affinity It kills many Gram-negative bacteria by a process that activity (serine and histidine).In granulocytes.serpr initially involves the perme dins are located in the phagocytic (primary,azurophil) granule Their collective abundancein human neutrophil BPI is ve ag recombinant BPI has promise as an e rmental treatment in meningococcal and other forms of Gram-negative sepsis terial properties of serprocidins can often be dissociated Human BPI is a proteolytic activity,and d by i An N of BPr a prolin easing ionic bicidal,it also converts inactive procathelicidins into quired for binding ated bac PI,LBP th re in cl ntraper are suscepu cteri some 20 (g11.23 sting a common and origin.De pite the Secretory Phospholipase A2 logic LBP does not kill bacteria and greatly enhances the pro mn groupⅡphospholipase A3s.The」 DI A nte an target bac ia differ in their preferred substrates.Human group II inv the ns of BPI and the phospholipase eA preterentially acts on the ester linkages near the highly conserved lipid A region of LPS.In the pnos ane t bacteria,the insertion of BP by other phospholipases even though their catalytic outer me PS mechanisms are very y similar.Group II PLA2 is found in and PS- granulo Paneth cells,epith al secre ions,and infam n hydrophobic molecules.increased susce tory I PI pe otibility of nhos pholipids to phospholipase A attack,and bacterial growth inhibition n ivit e cell wall),penetra many Gram-negative bacteria.Some defensins.cathelici- entrations of complement potentiate the ependent antibacterial activity of mammalian group I Gram-negativ eria with PLA2 requires millimolar concentrations and is augmented elatively resistant to rei y complemen Moreover,other host detence substar y-mcrea of the PLA by disrupting the bacteral outer membrane Proteases Human and other mammalian granulocytes contain Lactoferrin highly cationie rot a -30-kDa senne protea (serprocidins) is by a gene in the ENCYCLOPEDIA OF LIFE SCIENCES/e 2001 Nature Publishing Group /www.els.net Bactericidal Permeability-increasing Protein (BPI) Bactericidal permeability-increasing protein (BPI) is a highly cationic 55–60-kDa protein, abundant in human and other mammalian neutrophils. It is structurally related to the lipopolysaccharide-binding protein (LBP) in plasma, and like LBP, binds to lipopolysaccharide with high affinity. It kills many Gram-negative bacteria by a process that initially involves the permeabilization of their outer membranes, but is inactive against Gram-positive bacteria. BPI is an effective scavenger of lipopolysaccharide and recombinant BPI has promise as an experimental treatment in meningococcal and other forms of Gram-negative sepsis. Human BPI is a highly elongated molecule formed by two domains of similar size connected by a proline-rich linker. An N-terminal domain of BPI expresses the bactericidal and endotoxin-neutralizing activities of BPI while the C-terminal domain may be required for binding events that facilitate phagocytosis of BPI-coated bacteria. The human genes encoding BPI, LBP and several other lipid-binding proteins are in close proximity on chromo￾some 20 (q11.23–q12), suggesting a common ancestral origin. Despite their similarity in overall structural and functional design, the biological activities of BPI and LBP are strikingly different: whereas BPI is bactericidal toward Gram-negative bacteria and inhibits endotoxin signalling, LBP does not kill bacteria and greatly enhances the pro￾inflammatory action of low doses of LPS. Initial interactions between BPI and target bacteria probably involve electrostatic attraction between the cationic regions of BPI and the anionic moieties clustered near the highly conserved lipid A region of LPS. In the outer membrane of intact bacteria, the insertion of BPI triggers the rearrangement of outer membrane lipids (LPS and phospholipids) and displacement of LPS-boundMg2+ and Ca2+, resulting in increased permeability to small hydrophobic molecules, increased susceptibility of phos￾pholipids to phospholipase A attack, and bacterial growth inhibition. In a rabbit model of peritoneal inflammation, extracellular BPI secreted by granulocytes into inflamma￾tory exudates confers potent bactericidal activity against many Gram-negative bacteria. Some defensins, cathelici￾dins and low concentrations of complement potentiate the activity of BPI. Gram-negative bacteria with long lipopo￾lysaccharide chains (‘Smooth’ colony type) tend to be relatively resistant to BPI. Proteases Human and other mammalian granulocytes contain elastase, cathepsin G and proteinase 3, all of which are highly cationic 25–30-kDa serine proteases with antibac￾terial properties (serprocidins). Azurocidin/CAP37 is another bactericidal, but enzymatically inactive, member of the same protein family. Azurocidin/CAP37, neutrophil elastase and proteinase 3 are encoded by a cluster of genes located at the tip of the short arm of chromosome 19. Cathepsin G gene is located in another cluster of serine protease genes on chromosome 14q11.2. Compared to the active serine proteases, azurocidin/CAP37 contains amino acid substitutions in two residues essential for catalytic activity (serine and histidine). In granulocytes, serproci￾dins are located in the phagocytic (primary, azurophil) granules. Their collective abundance in human neutrophils is similar to the other major granule proteins (lactoferrin, lysozyme, myeloperoxidase and defensins). The antibac￾terial properties of serprocidins can often be dissociated from their proteolytic activity, and are competitively inhibited by increasing ionic strength, indicating the importance of charge interactions in their mechanism of action. Although neutrophil elastase is modestly micro￾bicidal, it also converts inactive procathelicidins into highly microbicidal cathelicidins. Transgenic mice lacking neutrophil elastase are susceptible to sepsis and death from intraperitoneal infections with Gram-negative bacteria. Secretory Phospholipase A2 Human group II phospholipase A2 (PLA2) is a potent 14- kDa enzyme encoded by a gene on chromosome 1p35. The enzyme belongs to a large family of secretory PLA2 enzymes that share similar structure and catalytic machinery, but differ in their preferred substrates. Human group II phospholipase A2 preferentially acts on the ester linkages of phospholipids found in bacterial membranes. The antimicrobial activity of this phospholipase is not shared by other phospholipases even though their catalytic mechanisms are very similar. Group II PLA2 is found in granulocytes, Paneth cells, epithelial secretions, and inflam￾matory fluids. Against many strains and species of Gram￾positive bacteria, purified group II PLA2 is active at nanomolar concentrations. Bacterial killing by PLA2 depends on binding to the bacterial surface (Ca2+- independent, presumably to sites in the cell wall), penetra￾tion of the cell wall and Ca2+-dependent degradation of membrane phospholipids. Against Gram-negative bacteria, independent antibacterial activity of mammalian group II PLA2 requires millimolar concentrations and is augmented by complement. Moreover, other host defence substances (e.g. bactericidal permeability-increasing protein, BPI) potentiate the action of nanogram per millilitre amounts of the PLA2 by disrupting the bacterial outer membrane. Lactoferrin Similar to the iron-binding plasma protein, transferrin, and encoded by a gene in the same region of chromosome Antimicrobial Proteins and Peptides 4 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net