Structural studies on the lipid A component of enterobacterial lipopolysaccharides by laser desorption mass spectrometry. Location of acyl groups at the lipid A backbone

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Abstract
In the present paper laser desorption mass spectrometry (LDMS) was applied to dephosphorylated free lipid A preparations obtained from lipopolysaccharides of Re mutants of Salmonella minnesota, Escherichia coli and Proteus mirabilis. The purpose of this study was to elucidate the location of (R)-3-hydroxytetradecanoic acid and 3-O-acylated (R)-3-hydroxytetradecanoic acid residues which are bound to amino and hydroxyl groups of the glucosamine disaccharide backbone of lipid A. Based on the previous finding from biochemical analyses that the amino group of the nonreducing glucosamine residue (GlcN II) of the backbone carries, in S. minnesota and E. coli, 3-dodecanoyloxytetradecanoic acid and, in P. mirabilis, 3-tetradecanoyloxytetradecanoic acid, a self-consistent interpretation of the LDMS was possible. It was found that: (a) in all three lipids A GlcN II is, besides the amide-linked 3-acyloxyacyl residue, substituted by ester-linked 3-tetradecanoyloxytetradecanoic acid; (b) the reducing glucosamine (GlcN I) is substituted by ester-linked 3-hydroxytetradecanoic acid; (c) the amino group of GlcN I carries a 3-hydroxytetradecanoic acid which is non-acylated in E. coli and which is partially acylated by hexadecanoic acid in S. minnesota and P. mirabilis. In lipids A which were obtained from the P. mirabilis Re mutant grown at low temperature (12°C) LDMS analysis revealed that specifically the one fatty acid bound to the 3-hydroxyl group of amide-linked 3-hydroxytetradecanoic acid at GlcN II is positionally replaced by Δ9-hexadecenoic acid (palmitoleic acid). It appears, therefore, that enterobacterial lipids A resemble each other in that the 3-hydroxyl groups of the two 3-hydroxytetradecanoic acid residues linked to GlcN II are fully acylated, while those of the two 3-hydroxytetradecanoic acid groups attached to GlcN I are free or only partially substituted.