Processing of Bacillus subtilis peptidoglycan by a mouse macrophage cell line

It has previously been established that muramyl dipeptide (N-acetylmuramyl-L-alanyl-D-isoglutamine) is an effective immunostimulant whose primary target cell type is the macrophage. Muramyl dipeptide is known to be structurally identical to a portion of the monomer unit of peptidoglycan, a nearly ubiquitous component of bacterial cell walls. To establish whether muramyl dipeptide or glycopeptides structurally related to it are formed as a result of macrophage processing of peptidoglycan, Bacillus subtilis cell walls radiolabeled in the muramic acid, glucosamine, and alanine residues of the constituent peptidoglycan were incubated in the presence of the cultured macrophage-like cell line RAW264, and the glycopeptides which released into the medium were fractionated and analyzed. Although muramyl dipeptide was not found in the culture medium, at least three glycopeptides structurally related to it were found, namely GlcNAc-MurNAc-Ala-isoGln-Dap-Ala, GlcNAc-MurNAc-Ala-isoGln-Dap, and GlcNAc-MurNAc-Ala-isoGln.