Lipopolysaccharides with different structure, isolated from different mutant strains of Escherichia coli and Salmonella bacteria, and chemical degradation products of these lipopolysaccharides have been employed to investigate which part of the lipopolysaccharide molecule exerts mitogenic effects on bone marrow-derived mouse lymphocytes. Within the structure of lipopolysaccharide consisting of lipid A, a core polysaccharide, and the O-polysaccharide antigen, lipid A was found to be the mitogenic part. The mitogenic effect of lipid A, consisting of phosphorylated glucosamine disaccharide units with ester- and amide-linked fatty acids, was lost after alkali treatment, which removes ester-linked fatty acids. Insertion of the lipid A portion of lipopolysaccharides into the lipid bilayer of the plasma membranes of bone marrow-derived lymphocytes is discussed as the initial mitogenic action.

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