Biotinylation of bacterial lipopolysaccharide and its applications to electron microscopy

We describe a procedure for lipopolysaccharide (LPS) biotinylation using N-biotinyl-L-lysine and application of the biotinylated LPS (Bi-LPS) to localization of LPS binding sites and subcellular distribution. Biotinylation of LPS was confirmed by enzyme-linked immunosorbent assay (ELISA), gel immunodiffusion, and immunodot techniques. The biological and toxicological activity of the Bi-LPS was tested by Limulus amoebocyte lysate (LAL) assays and histopathological examinations, respectively. Results showed that biotin was conjugated to LPS without disrupting the biological/toxicological activity of the molecule, which indicates that the biotin is directly linked to the polysaccharide portion of LPS. Localization of binding sites and subcellular distribution of Bi-LPS in human platelets and monocytes were studied by electron microscopy using an avidin-biotin-horseradish peroxidase (HRP) or streptavidin-gold method. Platelet surfaces were intensely stained by the reaction product of horseradish peroxidase (HPR) 5 min after incubation, and Bi-LPS was localized in small vesicles and vacuoles of platelets and in the phagocytic vacuoles of monocytes 60 min post incubation. Bi-LPS provides a reliable, stable, and sensitive tool for determination of LPS binding sites and subcellular distribution.