Lysozyme is one of the most abundant antimicrobial proteins in the airspaces of the lung. Mice express two lysozyme genes, lysozyme M and P, but only the M enzyme is detected in abundance in lung tissues. Disruption of the lysozyme M locus significantly increased bacterial burden and mortality following intratracheal infection with a Gram-negative bacterium. Unexpectedly, significant lysozyme enzyme activity (muramidase activity) was detected in the airspaces of uninfected lysozyme M−/− mice, amounting to 25% of the activity in wild-type mice. Muramidase activity in lysozyme M−/− mice was associated with increased lysozyme P mRNA and protein in lung tissue and bronchoalveolar lavage fluid respectively. The muramidase activity of recombinant lysozyme P was less than that of recombinant M lysozyme. Recombinant P lysozyme was also less effective in killing selected Gram-negative bacteria, requiring higher concentrations than lysozyme M to achieve the same level of killing. The lower antimicrobial activity of P lysozyme, coupled with incomplete compensation by P lysozyme in lysozyme M−/− mice, probably accounts for the increased susceptibility of null mice to infection. Recombinant lysozyme M and P were equally effective in killing selected Gram-positive organisms. This outcome suggests that disruption of both M and P loci would significantly increase susceptibility to airway infections, particularly those associated with colonization by Gram-positive organisms.
Comparison of the microbicidal and muramidase activities of mouse lysozyme M and P
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Philipp MARKART, Nicole FAUST, Thomas GRAF, Cheng-Lun NA, Timothy E. WEAVER, Henry T. AKINBI; Comparison of the microbicidal and muramidase activities of mouse lysozyme M and P. Biochem J 1 June 2004; 380 (2): 385–392. doi: https://doi.org/10.1042/bj20031810
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