The peptidoglycan-degrading property of lysozyme is not required for bactericidal activity in vivo.
Lysozyme is an abundant, cationic antimicrobial protein that plays an important role in pulmonary host defense. Increased concentration of lysozyme in the airspaces of transgenic mice enhanced bacterial killing whereas lysozyme deficiency resulted in increased bacterial burden and morbidity. Lysozyme degrades peptidoglycan in the bacterial cell wall leading to rapid killing of Gram-positive organisms; however, this mechanism cannot account for the protective effect of lysozyme against Gram-negative bacteria. The current study was therefore designed to test the hypothesis that the catalytic activity (muramidase activity) of lysozyme is not required for bacterial killing in vivo. Substitution of serine for aspartic acid at position 53 (D53S) in mouse lysozyme M completely ablated muramidase activity. Muramidase-deficient recombinant lysozyme (LysM(D53S)) killed both Gram-positive and Gram-negative bacteria in vitro. Targeted expression of LysM(D53S) in the respiratory epithelium of wild-type (LysM(+/+)/LysM(D53S)) or lysozyme M(null) mice (LysM(-/-)/LysM(D53S)) resulted in significantly elevated lysozyme protein in the airspaces without any increase in muramidase activity. Intratracheal challenge of transgenic mice with Gram-positive or Gram-negative bacteria resulted in a significant increase in bacterial burden in LysM(-/-) mice that was completely reversed by targeted expression of LysM(D53S). These results indicate that the muramidase activity of lysozyme is not required for bacterial killing in vitro or in vivo.
Nash
James A
JA
Division of Pulmonary Biology, Department of Pediatrics, Cincinnati Children's Hospital Medical Center and University of Cincinnati College of Medicine, Cincinnati, OH 45229, USA.
Ballard
Tiffany Nicole S
TN
Weaver
Timothy E
TE
Akinbi
Henry T
HT
D000818:Animals; D001224:Aspartic Acid / Q000235:genetics; D001770:Blood Bactericidal Activity / Q000235:genetics / Q000276:immunology*; D001992:Bronchoalveolar Lavage Fluid / Q000276:immunology / Q000382:microbiology; D007710:Klebsiella Infections / Q000201:enzymology / Q000235:genetics / Q000276:immunology; D007711:Klebsiella pneumoniae / Q000254:growth & development* / Q000276:immunology; D051379:Mice; D018345:Mice, Knockout; D008822:Mice, Transgenic; D009113:Muramidase / Q000097:blood* / Q000172:deficiency / Q000235:genetics; D016297:Mutagenesis, Site-Directed; D010457:Peptidoglycan / Q000097:blood*; D011552:Pseudomonas Infections / Q000201:enzymology / Q000235:genetics / Q000276:immunology; D011550:Pseudomonas aeruginosa / Q000254:growth & development* / Q000276:immunology; D011994:Recombinant Proteins / Q000097:blood / Q000235:genetics; D020545:Respiratory Mucosa / Q000201:enzymology / Q000276:immunology / Q000382:microbiology; D012694:Serine / Q000235:genetics; D013203:Staphylococcal Infections / Q000201:enzymology / Q000235:genetics / Q000276:immunology; D013211:Staphylococcus aureus / Q000254:growth & development* / Q000276:immunology
D010457:Peptidoglycan; D011994:Recombinant Proteins; D001224:Aspartic Acid; D012694:Serine; D009113:Muramidase; C472450:lysozyme M, mouse