Lipopolysaccharide (LPS) and Protein-LPS complexes: Detection and Characterization by Gel Electrophoresis, Mass Spectrometry and Bioassays

A prerequisite to the discovery and characterization of lipopolysaccharide (LPS) interaction with specific receptors and the resulting pathophysiological effects is the comprehensive structural analysis of LPS species. This brief review is aimed to summarize the use of gel electrophoresis linked to other biochemical technologies for detecting and characterizing LPSs. Lipopolysaccharide aggregates alone or mixtures containing LPSs and proteins/peptides can be separated by native agarose gel electrophoresis (NAGE), after which LPSs are detected with imidazole and zinc salts. A double-staining process with Coomassie brilliant blue R-250 enables the use of NAGE for detecting and studying protein-LPS interactions. For compositional analysis, the LPS aggregates are separated, with high resolution, by surfactant-polyacrylamide gel electrophoresis. After reverse staining with zinc-imidazole and elution from gel microparticles, glycoform-specific LPSs are ready for structural and biological analysis. For sequence analysis based on tandem electrospray ionization mass spectrometry (ESI-MS/MS) of oligosaccharides, LPSs are subjected to mild acid hydrolysis, dephosphorylation and permethylation. Also, O-deacylated LPS forms can be analyzed by matrix-assisted laser desorption/ionization-time of flight-MS. By comparison to spectra of unpurified LPSs, mass spectra of the micropurified LPSs show reduce heterogeneity and increased signal-to-noise ratios. Furthermore, the micropurification of LPSs prior to MS allows a higher sensitivity of detection for less abundant LPS glycoforms. The micropurified LPS fractions can be used to form self-assembled nanoaggregates which may be detected by dynamic light scattering. The effect of the O-side chain length on the Z-potential of LPS aggregates may be estimated by measurements based on laser Doppler electrophoresis. The thus obtained glycoform-specific LPSs are not only intact chemically but also biologically active as tested by e.g. Limulus amoebocyte lysate test, TNF-α assay and agonistic effect on human Toll-like receptor 4.