Murein lipoprotein, peptidoglycan-associated lipoprotein, and outer membrane protein A are present in purified rough and smooth lipopolysaccharides

Purified lipopolysaccharides (LPSs) have been used for many decades to gain insight into processes that occur during sepsis. Previous studies indicate that purified LPSs often contain trace protein contaminants. To identify protein contaminants of LPSs, we performed immunoblotting using, as antigen, purified LPS from various species of bacteria and, as primary antibodies, anti-murein lipoprotein (MLP), peptidoglycan-associated lipoprotein ( PAL), and outer membrane protein A ( OmpA). MLP, PAL, and/or OmpA were detected in 10 of the 13 LPS preparations and were present in LPS from rough and smooth bacteria. PAL and MLP have been reported to stimulate inflammation. The studies indicate that PAL and MLP are common contaminants of purified LPS and raise the possibility that these contaminants may influence results of studies performed using purified LPS.