Identification of the sialic acids from the egg jelly coat of the sea urchin Pseudocentrotus depressus (Okayama)

Dietary sialic acids: distribution, structure, and functions

Sialic acids (Sias), a group of over 50 structurally distinct acidic saccharides on the surface of all vertebrate cells, are neuraminic acid derivatives. They serve as glycan chain terminators in extracellular glycolipids and glycoproteins. In particular, Sias have significant implications in cell-to-cell as well as host-to-pathogen interactions and participate in various biological processes, including neurodevelopment, neurodegeneration, fertilization, and tumor migration. However, Sia is also present in some of our daily diets, particularly in conjugated form (sialoglycans), such as those in edible bird's nest, red meats, breast milk, bovine milk, and eggs. Among them, breast milk, especially colostrum, contains a high concentration of sialylated oligosaccharides. Numerous reviews have concentrated on the physiological function of Sia as a cellular component of the body and its relationship with the occurrence of diseases. However, the consumption of Sias through dietary sources exerts significant influence on human health, possibly by modulating the gut microbiota's composition and metabolism. In this review, we summarize the distribution, structure, and biological function of particular Sia-rich diets, including human milk, bovine milk, red meat, and egg.

The polyfunctional polysialic acid: A structural view

Polysialic acid (polySia), a homopolymer of α2,8-linked sialic acid residues, modifies a small number of proteins and has central functions in vertebrate signalling. Here, we review the regulatory functions of polySia in signalling processes and the immune system of adult humans, as well as functions based on their chemical properties. The main focus will be on the structure-function relationship of polySia with its interaction partners in humans. Recent studies have indicated that the degree of polymerisation is an important parameter that can guide the regulatory effect of polySia in addition to its binding to target proteins. Therefore, the structures of polySia in solution and bound to interaction partners are compared in order to identify the key factors that define binding specificity.

Cloning and expression of a membrane-bound CMP-N-acetylneuraminic acid hydroxylase from the starfish Asterias rubens

The sialic acid N-glycolylneuraminic acid (Neu5Gc) is synthesized by the action of CMP-Neu5Ac hydroxylase. The enzyme from various mammals has been purified, characterized and sequenced by cDNA cloning. Although functional sequence motifs can be postulated from comparisons with several enzymes, no global homologies to any other proteins have been found. The unusual characteristics of this hydroxylase raise questions about its evolution. As echinoderms are phylogenetically the oldest organisms possessing Neu5Gc, they represent a starting point for investigations on the origin of this enzyme. Despite many similarities with its mammalian counterpart, CMP-Neu5Ac hydroxylase from the starfish A. rubens exhibits fundamental differences, most notably its association with a membrane and a requirement for high ionic strength. In order to shed light on the structural basis for these differences, the primary structure of CMP-Neu5Ac hydroxylase from A. rubens has been determined by PCR and cDNA-cloning techniques, using initial sequence information from the mouse enzyme. The complete assembled cDNA contained an ORF coding for a protein of 653 amino acids with a molecular mass of 75 kDa. The deduced amino-acid sequence exhibited a high degree of homology with the mammalian enzyme, although the C-terminus was some 60 residues longer. This extension consists of a terminal hydrophobic region, which may mediate membrane-binding, and a preceding hydrophilic sequence which probably serves as a hinge or linker. The identity of the ORF was confirmed by expression of active CMP-Neu5Ac hydroxylase in E. coli at low temperatures.

Cytidine monophosphate-N-acetylneuraminate hydroxylase in the starfish Asterias rubens and other echinoderms

The sialic acid N-glycolylneuraminic acid (Neu5Gc) is synthesised by an NADH-dependent hydroxylase which acts on CMP-N-acetylneuraminic acid (CMP-Neu5Ac). Although Neu5Gc is the predominant sialic acid in many echinoderms, little is known about the hydroxylase from organisms of this phylum. We show here that in contrast to the mammalian enzyme, the hydroxylase from various echinoderms is predominantly membrane-bound and exhibits optimal activity in the presence of 100 mM NaCl. A detailed characterisation of the hydroxylase from echinoderms was performed using fractionated gonads of the starfish Asterias rubens. Solubilisation using detergents led to an inactivation of the hydroxylase. However, the solubilised enzyme was reactivated by the addition of cytochrome b5 reductase together with the amphiphilic or soluble form of cytochrome b5. Although these latter proteins were only available from a mammalian source, the high affinity of the hydroxylase for cytochrome b5 suggests that, as with the mammalian enzyme, these electron carriers participate in the catalytic cycle of the hydroxylase from A. rubens in vivo. The relevance of these results to the interaction between cytochrome b5 and the hydroxylase is discussed.

Sialic acid content and surface hydrophobicity of group B streptococci

The sialic acid content and the cell-surface hydrophobicity index of 40 group B streptococci (GBS) strains were assessed. GBS isolated from invasive infections (virulent strains) presented an increased level of sialic acid content (1.4%) when compared with GBS isolated from asymptomatic patients (0.53%). Treatment of GBS strain 85634 with neuraminidase resulted in a decrease (about 25%) in the net negative surface charge as assessed by cell electrophoresis. This finding suggests that sialic acid residues are important anionogenic groups exposed on GBS cell surface. N-acetylneuraminic acid was the only sialic acid derivative characterized in the strain 85634 as evaluated by gas-liquid chromatography. GBS from different serotypes presented a hydrophobic index mean value of 0.9. Even though the sialic acid contributed effectively to the negative charge on GBS cell surface, no difference was observed in the hydrophobic index when virulent and avirulent strains were compared.

Nature and biosynthesis of sialic acids in the starfish Asterias rubens. Identification of sialo-oligomers and detection of S-adenosyl-L-methionine: N-acylneuraminate 8-O-methyltransferase and CMP-N-acetylneuraminate monooxygenase activities

Mass spectrometric and NMR spectroscopic analyses of bound sialic acids from the starfish Asterias rubens revealed the presence of N-acetylneuraminic acid (4%), N-acetyl-8-O-methylneuraminic acid (12%), N-acetyl-9-O-acetyl-8-O-methylneuraminic acid (less than 1%), N-glycoloylneuraminic acid (19%), N-glycoloyl-8-O-methylneuraminic acid (47%), and N-glycoloyl-9-O-acetyl-8-O-methylneuraminic acid (18%). Analysis of sialo-oligomeric material, obtained after mild acid hydrolysis, demonstrated that N-glycoloyl-8-O-methylneuraminic acid can occur as di- and tri-oligomers, linked through the anomeric center and the N-glycoloyl moiety, Neu5Gc8Me-alpha(2----O5)-Neu5Gc8Me and Neu5Gc8Me-alpha(2----O5)-Neu5Gc8Me-alpha (2----O5)-Neu5Gc8Me. Studies on the biosynthesis of N-acyl-8-O-methylneuraminic acid in A rubens, using the tracer S-adenosyl-L-[methyl-14C]methionine, showed that N-acylneuraminate 8-O-methyltransferase activity was present predominantly in the membrane fraction. CMP-N-acetylneuraminic acid monooxygenase activity was found in the soluble protein fraction, in agreement with investigations on the corresponding vertebrate enzyme.

Changes in cell surface anionogenic groups induced by propranolol in Herpetomonas muscarum muscarum

The effects of propranolol (10(-3) mM) on the surface anionic groups of Herpetomonas muscarum muscarum were analysed by cell electrophoresis, by ultrastructural cytochemistry and by identification of sialic acids using paper chromatography. Differentiation of H. muscarum muscarum induced by propranolol treatment caused a significant increase in the net negative surface charge. Binding of cationized ferritin (CF) and colloidal iron hydroxide particles was observed at the cell surface of both untreated and propranolol-treated cells. In cells incubated in the presence of the drug the CF particles were distributed in all membrane regions. However, there were small areas where the particles were absent. In H. muscarum muscarum exposed to propranolol the density of residues of sialic acid per cell was higher, and the agglutinating activity with Sendai virus was more intense. However, the pattern of sialic acid, characterized by the presence of N-acetylneuraminic acid derivative, was not modified upon cell interaction with the drug. Treatment of both control and propranolol-treated protozoa with neuraminidase significantly reduced the surface charge. These findings suggest that sialic acid residues are the major anionogenic groups exposed on the surface of H. muscarum muscarum.

Changes in cell surface anionogenic groups during differentiation of Herpetomonas samuelpessoai mediated by dimethylsulfoxide

The surface anionic groups of untreated or dimethyl sulfoxide (DMSO)-treated Herpetomonas samuelpessoai cells were analyzed by cell electrophoresis, ultrastructural cytochemistry, and identification of sialic acids using thin-layer chromatography. Differentiation of H. samuelpessoai induced by DMSO treatment caused a significant increase in the net negative surface charge. In flagellates exposed to DMSO, more cationized ferritin, colloidal iron hydroxide, and sendai virus particles bound to the cell surface. Treatment of both untreated and DMSO-treated flagellates with neuraminidase decreased markedly the EPM of cells to the cathodic pole. These findings suggest that sialic acid residues are the major anionogenic groups exposed on the surface of H. samuelpessoai. Thin-layer chromatography showed that N-acetyl and N,O-diacylneuraminic acids, in equal proportions, were present in H. samuelpessoai. However, N-acetylneuraminic acid predominates in DMSO-treated cells.

Polysaccharide and glycolipid composition in Tritrichomonas foetus

1. The polysaccharide and glycolipid composition in Tritrichomonas foetus was studied by paper, thin-layer and gas-liquid chromatographic analysis. 2. The carbohydrate components of the polysaccharide were glucose (47%), galactose (34%) and mannose (19%). N-acetylneuraminic acid was the sialic acid derivative characterized in the flagellate whole cells. 3. The sialic acid density was estimated as 2.7 x 10(7) residues/cell. 4. The long-chain base dihydrosphingosine, the carbohydrates galactose (67%), glucose (21%) and mannose (12%) as well as the fatty acids myristic (48%) and palmitic (52%) acids were characterized as components of the total glycolipids of T. foetus. 5. Total glycolipids were fractionated: a galactocerebroside and a ganglioside were identified.

Identification of sialic acids on the cell surface of hyphae and conidia of the human pathogen Fonsecaea pedrosoi

Sialic acids were characterized on the cell surface of conidia and hyphae of Fonsecaea pedrosoi, one of the agents of chromoblastomycosis. Neuraminidase-treated conidia had a reduced negative electrophoretic mobility and, in comparison with untreated cells, bound fewer particles of colloidal iron hydroxide and of cationized ferritin. Sialic acid residues in conidia are linked to galactopyranosyl units as indicated by the increased reactivity of neuraminidase-treated cells with peanut agglutinin. N-acetylneuraminic acid was the only derivative found in the mycelium whereas conidia contained both N-glycolyl- and N-acetylneuraminic acids.

Structural studies of 4-O-acetyl-alpha-N-acetylneuraminyl-(2 goes to 3)-lactose, the main oligosaccharide in echidna milk

The main oligosaccharide (50%) in the milk of the Australian echidna (Tachyglossus aculeatus) has been identified unequivocally as 4-O-acetyl-alpha-N-acetylneuraminyl-(2 goes to 3)-lactose. The 4-O-acetyl substituent of the sialic acid residue was characterized by g.l.c.-m.s. of the isolated (after mild, acid hydrolysis) and trimethyl-silylated/esterified sialic acid, and by m.s. (after derivatisation) and 500-MHz, 1H-n.m.r. spectroscopy of the intact oligosaccharide. Information about the glycosidic bonds was obtained by methylation analysis and 500-MHz, 1H-n.m.r. spectroscopy. This animals species is the third one known to produce 4-O-acetylated sialic acid.

The nature of sialic acids in human lymphocytes

Analysis of the sialic acids obtained by mild acid hydrolysis of B lymphocytes reveals the presence of N-acetylneuraminic acid and 9-O-acetyl-N-acetylneuraminic acid. For T lymphocytes only N-acetylneuraminic acid has been demonstrated to occur. The applied methods include quantitative colorimetry, thin-layer chromatography and combined gas-liquid chromatography-mass spectrometry.