title: 4-O-Acetyl-sialic acid (Neu4,5Ac2) in acidic milk oligosaccharides of the platypus (Ornithorhynchus anatinus) and its evolutionary significance.
abstract: Monotremes (echidnas and platypus) retain an ancestral form of reproduction: egg-laying followed by secretion of milk onto skin and hair in a mammary patch, in the absence of nipples. Offspring are highly immature at hatching and depend on oligosaccharide-rich milk for many months. The primary saccharide in long-beaked echidna milk is an acidic trisaccharide Neu4,5Ac2(α2-3)Gal(β1-4)Glc (4-O-acetyl 3'-sialyllactose), but acidic oligosaccharides have not been characterized in platypus milk. In this study, acidic oligosaccharides purified from the carbohydrate fraction of platypus milk were characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and (1)H-nuclear magnetic resonance spectroscopy. All identified structures, except Neu5Ac(α2-3)Gal(β1-4)Glc (3'-sialyllactose) contained Neu4,5Ac2 (4-O-acetyl-sialic acid). These include the trisaccharide 4-O-acetyl 3'-sialyllactose, the pentasaccharide Neu4,5Ac2(α2-3)Gal(β1-4)GlcNAc(β1-3)Gal(β1-4)Glc (4-O-acetyl-3'-sialyllacto-N-tetraose d) and the hexasaccharide Neu4,5Ac2(α2-3)Gal(β1-4)[Fuc(α1-3)]GlcNAc(β1-3)Gal(β1-4)Glc (4-O-acetyl-3'-sialyllacto-N-fucopentaose III). At least seven different octa- to deca-oligosaccharides each contained a lacto-N-neohexaose core (LNnH) and one or two Neu4,5Ac2 and one to three fucose residues. We conclude that platypus milk contains a diverse (≥ 20) array of neutral and acidic oligosaccharides based primarily on lactose, lacto-N-neotetraose (LNnT) and LNnH structural cores and shares with echidna milk the unique feature that all identified acidic oligosaccharides (other than 3'-sialyllactose) contain the 4-O-acetyl-sialic acid moiety. We propose that 4-O-acetylation of sialic acid moieties protects acidic milk oligosaccharides secreted onto integumental surfaces from bacterial hydrolysis via steric interference with bacterial sialidases. This may be of evolutionary significance since taxa ancestral to monotremes and other mammals are thought to have secreted milk, or a milk-like fluid containing oligosaccharides, onto skin surfaces.
authors.affiliation: Graduate School of Animal and Food Hygiene, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido 080-8555, Japan.
authors.forename: Tadasu
authors.initials: T
authors.lastname: Urashima
authors.affiliation: Graduate School of Animal and Food Hygiene, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido 080-8555, Japan.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Inamori
authors.affiliation: Graduate School of Animal and Food Hygiene, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido 080-8555, Japan.
authors.forename: Kenji
authors.initials: K
authors.lastname: Fukuda
authors.affiliation: Graduate School of Agriculture, Tohoku University, Sendai, Miyagi 981-8555, Japan.
authors.forename: Tadao
authors.initials: T
authors.lastname: Saito
authors.affiliation: School of Molecular Bioscience, University of Sydney, The University of Sydney, Sydney, NSW 2006, Australia.
authors.forename: Michael
authors.initials: M
authors.lastname: Messer
authors.affiliation: Smithsonian Environmental Research Center, Smithsonian Institution, Edgewater, MD 21037, USA oftedalo@si.edu.
authors.forename: Olav T
authors.initials: OT
authors.lastname: Oftedal
mesh_terms: D000818:Animals; D002851:Chromatography, High Pressure Liquid; D009682:Magnetic Resonance Spectroscopy; D008892:Milk / Q000737:chemistry*; D009844:Oligosaccharides / Q000737:chemistry* / Q000302:isolation & purification; D010987:Platypus; D012794:Sialic Acids / Q000737:chemistry*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
chemical_list: D009844:Oligosaccharides; D012794:Sialic Acids
keywords: evolution; milk oligosaccharides; monotreme; platypus; sialic acid