Glucose phosphorylated on carbon 6 suppresses lipopolysaccharide binding to lipopolysaccharide-binding protein and inhibits its bioactivities
Lipid A comprises the active region of lipopolysaccharide (LPS), and its phosphate group is required for LPS activities. Additionally, it is essential for effects of inhibitors of LPS-induced coagulation activity in limulus amebocyte lysate (LAL) tests. Lipid A has phosphorylated glucosamine residues, which are structurally similar to glucose 1-phosphate (G1P) and glucose 6-phosphate (G6P). This study focused on the antagonistic effects of glucose phosphates on the action of protein or non-protein inhibitors against LAL coagulation, LPS–LPS-binding protein (LBP) interaction, and LPS bioactivities. These effects of glucose phosphates were evaluated and compared with those of other charged sugars such as fructose 6-phosphate and glucuronic acid by LAL tests, ELISA-based LPS–LBP binding assay, cell-based assay, and using a mouse endotoxin shock model. G6P neutralized the interfering actions of drug substances and plasma proteins on LPS coagulation activity in LAL tests. Compared to other sugars, G6P more strongly inhibited LPS binding to LBP, leading to significant inhibition of LPS-induced cellular responses in human umbilical vein endothelial cells and in the THP-1 human leukemic line. Consistent herewith, G6P inhibited inflammatory cytokine release and decreased serum alanine aminotransferase and hepatic caspase-3/7 activities and mortality in LPS-stimulated D-galactosamine-sensitized mice. These data indicated that the structural properties of G6P, such as its glucose moiety and phosphorylation on carbon 6, are important for suppressing the interaction of proteins with LPS. Therefore, G6P is useful to improve sensitivity and accuracy of plasma and drug LPS assays, and such structural property is more suitable to antagonize LPS activities.