Murein hydrolase (N-acetyl-muramyl-L-alanine amidase) in human serum

An enzyme was identified in human serum which unlike lysozyme cleaved the amide bond between N-acetyl-muramic acid and L-alanine of the peptide side chain of the rigid layer (murein) of Escherichia coli. The N-acetyl-muramyl-L-alanine amidase released all of the peptide side chains including those to which the lipoprotein is bound. A portion of the peptide side chains of the Micrococcus lysodeikticus murein was also hydrolysed from the polysaccharide chains. E. coli, M. lysodeikticus, Bacillus subtilis and Staphylococcus aureus were not killed by the amidase. Treatment of E. coli with EDTA or osmotic shock rendered the cells sensitive to the amidase and they were killed. Possible biological functions of the amidase are discussed. The enzyme was separated from lysozyme in human serum. Gel permeation chromatography indicated a molecular weight of the active enzyme of 82,000 while gel electrophoresis in the presence of sodium dodecyl sulfate revealed a molecular weight of 75,000. Thus, the enzyme probably consists of a single polypeptide chain. Incubation with neuraminidase rendered the amidase more basic suggesting the release of sialic acid residues. The modified glycoprotein disclosed an increased activity to murein. Enzyme activity was inhibited by p-chloromercuribenzene sulfonate and ethyleneglycol-bis(2-aminomethyl) tetraacetate (EGTA) at 1 and 0.2 mM concentration, respectively, whereas EDTA up to 5 mM was without effect. The amidase was also inactivated by agents that reduce disulfide bridges.