Biosynthesis of N-glycolylneuraminic acid in porcine submandibular glands. Subcellular site of hydroxylation of N-acetylneuraminic acid in the course of glycoprotein biosynthesis

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Gene expression and pathway analysis of effects of the CMAH deactivation on mouse lung, kidney and heart

Background

N-glycolylneuraminic acid (Neu5Gc) is generated by hydroxylation of CMP-Neu5Ac to CMP-Neu5Gc, catalyzed by CMP-Neu5Ac hydroxylase (CMAH). However, humans lack this common mammalian cell surface molecule, Neu5Gc, due to inactivation of the CMAH gene during evolution. CMAH is one of several human-specific genes whose function has been lost by disruption or deletion of the coding frame. It has been suggested that CMAH inactivation has resulted in biochemical or physiological characteristics that have resulted in human-specific diseases.

Methodology/Principal Findings

To identify differential gene expression profiles associated with the loss of Neu5Gc expression, we performed microarray analysis using Illumina MouseRef-8 v2 Expression BeadChip, using the main tissues (lung, kidney, and heart) from control mice and CMP-Neu5Ac hydroxylase (Cmah) gene knock-out mice, respectively. Out of a total of 25,697 genes, 204, 162, and 147 genes were found to be significantly modulated in the lung, kidney, and heart tissues of the Cmah null mouse, respectively. In this study, we examined the gene expression profiles, using three commercial pathway analysis software packages: Ingenuity Pathways Analysis, Kyoto Encyclopedia of Genes and Genomes analysis, and Pathway Studio. The gene ontology analysis revealed that the top 6 biological processes of these genes included protein metabolism and modification, signal transduction, lipid, fatty acid, and steroid metabolism, nucleoside, nucleotide and nucleic acid metabolism, immunity and defense, and carbohydrate metabolism. Gene interaction network analysis showed a common network that was common to the different tissues of the Cmah null mouse. However, the expression of most sialytransferase mRNAs of Hanganutziu-Deicher antigen, sialy-Tn antigen, Forssman antigen, and Tn antigen was significantly down-regulated in the liver tissue of Cmah null mice.

Conclusions/Significance

Mice bearing a human-like deletion of the Cmah gene serve as an important model for the study of abnormal pathogenesis and/or metabolism caused by the evolutionary loss of Neu5Gc synthesis in humans.

Involvement of a non-human sialic Acid in human cancer

Sialic acids are common monosaccharides that are widely expressed as outer terminal units on all vertebrate cell surfaces, and play fundamental roles in cell–cell and cell–microenvironment interactions. The predominant sialic acids on most mammalian cells are N-glycolylneuraminic acid (Neu5Gc) and N-acetylneuraminic acid (Neu5Ac). Neu5Gc is notable for its deficiency in humans due to a species-specific and species-universal inactivating deletion in the CMAH gene encoding the hydroxylase that converts CMP-Neu5Ac to CMP-Neu5Gc. However, Neu5Gc is metabolically incorporated into human tissues from dietary sources (particularly red meat), and detected at even higher levels in some human cancers. Early life exposure to Neu5Gc-containing foods in the presence of certain commensal bacteria that incorporate dietary Neu5Gc into lipooligosaccharides can lead to generation of antibodies that are also cross-reactive against Neu5Gc-containing glycans in human tissues (“xeno-autoantigens”). Such anti-Neu5Gc “xeno-autoantibodies” are found in all humans, although ranging widely in levels among individuals, and displaying diverse and variable specificities for the underlying glycan. Experimental evidence in a human-like Neu5Gc-deficient Cmah^−/−mouse model shows that inflammation due to “xenosialitis” caused by this antigen–antibody interaction can promote tumor progression, suggesting a likely mechanism for the well-known epidemiological link between red meat consumption and carcinoma risk. In this review, we discuss the history of this field, mechanisms of Neu5Gc incorporation into tissues, the origin and specificities of human anti-Neu5Gc antibodies, their use as possible cancer biomarkers, implications of xenosialitis in cancer initiation and progression, and current and future approaches toward immunotherapy that could take advantage of this unusual human-specific phenomenon.

Human CMP-N-acetylneuraminic acid hydroxylase is a novel stem cell marker linked to stem cell-specific mechanisms

Human stem cells contain substantial amounts of the xenoantigen N-glycolylneuraminic acid (Neu5Gc), although the levels of Neu5Gc are low or undetectable in human body fluids and most other human tissues. The lack of Neu5Gc in human tissues has been previously explained by the loss of hydroxylase activity of the human CMP-N-acetylneuraminic acid hydroxylase (CMAH) protein caused by a genetic error in the human Cmah gene. We thus wanted to investigate whether the human redundant Cmah gene could still function in stem cell-specific processes. In this study, we show that CMAH gene expression is significantly upregulated in the adult stem cell populations studied, both of hematopoietic and mesenchymal origin, and identify CMAH as a novel stem cell marker. The CMAH content co-occurs with higher levels of Neu5Gc within stem cells as measured by mass spectrometric profiling. It seems that despite being enzymatically inactive, human CMAH may upregulate the Neu5Gc content of cells by enhancing Neu5Gc uptake from exogenous sources. Furthermore, exposure to exogenous Neu5Gc caused rapid phosphorylation of beta-catenin in both CMAH overexpressing cells and bone marrow-derived mesenchymal stem cells, thereby inactivating Wnt/beta-catenin signaling. The data demonstrate the first molecular evidence for xenoantigen Neu5Gc-induced alteration of crucial stem cell-specific signaling systems for the maintenance of self renewal. These results add further emphasis to the crucial need for completely xenofree culturing conditions for human stem cells.

Cloning and functional characterization of pig CMP-N-acetylneuraminic acid hydroxylase for the synthesis of N-glycolylneuraminic acid as the xenoantigenic determinant in pig–human xenotransplantation

In the present study, the pig CMP-N-acetylneuraminic acid hydroxylase gene (pcmah), a key enzyme for the synthesis of NeuGc (N-glycolylneuraminic acid), was cloned from pig small intestine and characterized. The ORF (open reading frame) of pcmah was 1734 bp, encoding 577 amino acids and consisting of 14 exons. Organ expression pattern analysis reveals that pcmah mRNA is mainly expressed in pig rectum, tongue, spleen and colon tissues, being the most highly expressed in small intestine. In the ectopic expression of pcmah, when pig kidney PK15 cells and human vascular endothelial ECV304 cells were transfected with the cloned pcmah, the NeuGc contents of these transfectants were greater in comparison with vector transfectants used as controls. In addition, in the functional analysis of NeuGc, HSMC (human-serum-mediated cytotoxicity) was elevated in the ectopic NeuGc-expressing pcmah-transfected cells compared with controls. Moreover, binding of human IgM to the pcmah-transfected cells was significantly increased, whereas binding of IgG was slightly increased, indicating that the human IgM type was a major anti-NeuGc antibody. Furthermore, pcmah silencing by shRNA (short hairpin RNA) resulted in a decrease in NeuGc content and xenoantigenicity in PK15. From the results, it was concluded that the pcmah gene was capable of synthesizing the NeuGc acting as a xenoantigen in humans, confirming the NeuGc-mediated rejection response in pig–human xenotransplantation.

Analysis of N-acetyl and N-glycolylneuraminic acid in rat serum and tissues with Walker 256 carcinoma by high-performance liquid chromatography

Serum and tissue specimens from healthy Wistar rats and from rats with Walker 256 carcinoma were analysed for N-acetyl and N-glycolylneuraminic acid by high performance liquid chromatography (HPLC) as per-O-benzoylated derivatives. Both neuraminic acids were identified, while N-acetylneuraminic acid was the predominant sialic acid. Samples from rats with generalized metastasis showed a significant increase (45-80%) of total sialic acids. This phenomenon in serum is caused by the overproduction of sialic acids, as a result of synthesis of both types of neuraminic acids to a similar molar ratio. The increase of sialic acids in rat bones with metastatic cancer is mainly because of increased N-acetylneuraminic acid synthesis. These results suggest that the molecular mechanisms responsible for cancer metastasis in different tissues may be closely associated with increased synthesis of dominating neuraminic acid.

Screening of N-acylneuraminic acids in serum and tissue specimens of mouse C57BI with Lewis' lung cancer by high-performance liquid chromatography

Serum and tissue specimens from healthy C57BI mice and from mice with Lewis' lung cancer after metastasis were analyzed for N-acetyl- and N-glycolylneuraminic acid by high-performance liquid chromatography. Both neuraminic acids were present, while N-glycolylneuraminic acid was the predominant sialic acid in all tissues. Samples from mice with metastatic cancer showed a significant increase (67-200%) of total sialic acids mainly as a result of increased N-glycolylneuraminic acid synthesis. These results suggest that cancer metastasis in various tissues is closely associated with increased synthesis of the predominant neuraminic acid and may help to understand the underlying mechanisms of tumor development.

CMP-N-Acetylneuraminic acid hydroxylase is exclusively inactive in humans

We cloned cDNAs for mouse and human CMP-N-acetylneuraminic acid (CMP-NeuAc) hydroxylases and showed that the human CMP-NeuAc hydroxylase protein is inactive because of a partial deletion in the hydroxylase gene. We report here that no other active CMP-NeuAc hydroxylases are present in humans. Southern blot analysis showed that the human homologue of the mouse CMP-NeuAc hydroxylase is one gene in the human genome and no other homologues of the mouse hydroxylase exist in human genome. The mouse and the human CMP-NeuAc hydroxylases were mapped to chromosome 13A3 and chromosome 6p22, respectively, by fluorescence in situ hybridization. The chromosomal location of the human hydroxylase is syntenic to that of the mouse hydroxylase. These results demonstrate that the human CMP-NeuAc hydroxylase is the only homologue of the mouse hydroxylase, and CMP-NeuAc hydroxylase is exclusively inactive in humans.

The molecular basis for the absence of N-glycolylneuraminic acid in humans

N-Glycolylneuraminic acid (NeuGc) is abundantly expressed in most mammals, but it is not detectable in humans. The expression of NeuGc is controlled by cytidine monophospho-N-acetylneuraminic acid (CMP-NeuAc) hydroxylase activity. We previously cloned a cDNA for mouse CMP-NeuAc hydroxylase and found that the human genome contains a homologue. We report here the molecular basis for the absence of NeuGc in humans. We cloned a cDNA for human CMP-NeuAc hydroxylase from a HeLa cell cDNA library. The cDNA encodes a 486-amino acid protein, and its deduced amino acid sequence lacks a domain corresponding to the N-terminal 104 amino acids of the mouse CMP-NeuAc hydroxylase protein, although the human protein is highly identical (93%) to the rest of the mouse hydroxylase protein. The N-terminal truncation of the human hydroxylase is caused by deletion of a 92-base pair-long exon in human genomic DNA. The human hydroxylase expressed in COS-7 cells exhibited no enzymatic activity, and a mouse hydroxylase mutant, which lacks the N-terminal domain, was also inactive. A chimera composed of the human hydroxylase and the N-terminal domain of the mouse hydroxylase displayed the enzyme activity. These results indicate that the human homologue of CMP-NeuAc hydroxylase is inactive because it lacks an N-terminal domain that is essential for enzyme activity. The absence of NeuGc in human glycoconjugates is due to a partial deletion in the gene that encodes CMP-NeuAc hydroxylase.

Molecular cloning of cytidine monophospho-N-acetylneuraminic acid hydroxylase. Regulation of species- and tissue-specific expression of N-glycolylneuraminic acid

Cytidine monophospho-N-acetylneuraminic acid (CMP-NeuAc) hydroxylase, which is the key enzyme for the synthesis of N-glycolylneuraminic acid (NeuGc), has been purified from the cytosolic fraction of mouse liver, as described in our previous paper. The amino acid sequences of the purified CMP-NeuAc hydroxylase, and peptides obtained by lysylendopeptidase digestion, were used to synthesize specific oligonucleotide primers. A mouse cDNA clone of the enzyme was obtained by a combination of the polymerase chain reaction and rapid amplification of cDNA ends. The sequence of the clone contained an open reading frame coding for a protein of 577 amino acids with a predicted molecular mass of 66 kDa. The deduced sequence included the amino acid sequences obtained for the purified enzyme and peptides, and a complete match was obtained for 159 residues. The enzyme has neither a signal peptide sequence nor a membrane spanning domain, which is consistent with localization of the enzyme in the cytosol. Transfection of a cDNA construct to COS-1 cells increased the enzyme activity and the amount of NeuGc. Comparison of the sequence with GenBank data indicated that no similar sequence has been reported so far. Northern blot analysis of various mouse tissues with the enzyme cDNA as a probe indicated that expression of NeuGc is related to the level of CMP-NeuAc hydroxylase mRNA. On Southern blot analysis with the same probe, cross-hybridizing bands were detected in the human and fish genomes.

Donor substrate specificities of Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase and Gal beta 1,3GalNAc alpha 2,3-sialyltransferase: comparison of N-acetyl and N-glycolylneuraminic acids

Using cloned sialyltransferases, Gal beta 1,3GalNAc alpha 2,3-sialyltransferase (ST3Gal I) and Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase (ST6Gal I) from both chicken and mouse, CMP-NeuAc and CMP-NeuGc were compared as donor substrates with pyridylamino-oligo-saccharides as acceptors. ST6Gal I showed 4-7-times higher activity toward CMP-NeuGc than CMP-NeuAc, while for ST3Gal I there was no significant difference between them, irrespective of the origin of the enzymes. Also, the difference in donor substrate (i.e., NeuAc and NeuGc) had little effect on the preference to acceptor substrates of these enzymes. Thus, the results showed that the cloned sialyltransferases can utilize both CMP-NeuAc and CMP-NeuGc as donor substrates, and that the preference difference between the sialyltransferases to CMP-NeuGc and CMP-NeuAc could, at least partly, explain the discrepancy in the ratio of NeuAc and NeuGc in glycolipids and glycoproteins in individual tissues.

Evidence for a free N-acetylneuraminic acid-hydroxylating enzyme in pig mandibular gland soluble fraction

The activity of a free N-acetylneuraminic acid (Neu5Ac)-hydroxylating enzyme which converted Neu5Ac into N-glycolyl-neuraminic acid (Neu5Gc) was demonstrated in the soluble fraction of pig mandibular gland. The hydroxylation was possible only with NADPH as the electron donor. The apparent Km was 4.5 mM Neu5Ac. At 0.5 mM monovalent cations had no effect on the hydroxylation of Neu5Ac whereas bivalent cations gave varied inhibition capacities ranging from 14 to 75%. EDTA gave a time-dependent enhancement of activity. It was concluded that the enzyme does not require an exogenously added inorganic cofactor. Results from salt fractionation of the soluble fraction and the use of inhibitors such as mercurials suggested that the hydroxylation of Neu5Ac to Neu5Gc may involve other, as yet unknown, component(s) and the possibility of electrons donated by NADPH being transferred to activated molecular oxygen (second substrate). We propose to name this enzyme N-acetyl-neuraminic acid hydroxylase.

Biosynthesis and shedding of murine lymphoma gangliosides

Ganglioside biosynthesis and subsequent shedding are a potential mechanism contributing to tumor cell escape from the host immune response. As a first step in identifying active molecular species, structural characterization and quantification of the purified individual cellular and shed gangliosides of YAC-1 murine lymphoma cells were undertaken. These studies uncovered three striking changes in ganglioside metabolism in cells passaged in vivo, compared with cells cultured in vitro. (i) Marked inhibition of GalNAcGM1b synthesis: GM1b was present in an equal proportion to its biosynthetic product GalNAcGM1b in vitro, but was present in a 6-fold higher concentration in vivo. (ii) Marked inhibition of NeuGc synthesis: NeuGc, present in vitro in an up to 7-fold higher concentration than its biosynthetic precursor NeuAc, was decreased in relative concentration in vivo (1:1). (iii) Selectivity of shedding: ganglioside shedding in vitro was generalized with respect to both carbohydrate structure and ceramide structure (mainly d18:1-C24:1 and d18:1-C16:0), while in vivo, there was selective shedding of gangliosides containing NeuGc and the shorter chain fatty acid. The reduced synthesis of NeuGc and of GalNAcGM1b in vivo, and the selective shedding of more polar ganglioside species, also in vivo, show that the extracellular environment can markedly affect cellular ganglioside metabolism.

A study on the regulation of N-glycoloylneuraminic acid biosynthesis and utilization in rat and mouse liver

The relative contribution of N-glycoloyl-beta-D-neuraminic acid (Neu5Gc) to total sialic acids expressed in mouse and rat liver glycoconjugates was found to be 95% and 11%, respectively. This considerable difference in sialic acid composition made these two tissues suitable models for a comparative investigation into the regulation of Neu5Gc biosynthesis and utilization. An examination of the CMP-glycoside specificity of Golgi-associated sialyltransferases using CMP-N-acetyl-beta-D-neuraminic acid (CMP-Neu5Ac) and CMP-Neu5Gc revealed no significant tissue-dependent differences. The Golgi membrane CMP-sialic acid transport system from rat liver did, however, exhibit a slightly higher internalisation rate for CMP-Neu5Ac, though no preferential affinity for this sugar nucleotide over CMP-Neu5Gc was observed. In experiments, where Golgi membrane preparations were incubated with an equimolar mixture of labelled CMP-Neu5Ac and CMP-Neu5Gc, no significant tissue-dependent differences in [14C]sialic acid composition were observed, either in the luminal soluble sialic acid fraction or in the precipitable sialic acid fraction, results which are consistent with the above observations. From this experiment, evidence was also obtained for the presence of a Golgi-lumen-associated CMP--sialic acid hydrolase which exhibited no apparent specificity for either CMP-Neu5Ac or CMP-Neu5Gc. The specific activity of the CMP-Neu5Ac hydroxylase, the enzyme responsible for the biosynthesis of Neu5Gc, was found to be 28-fold greater in high-speed supernatants of mouse liver than of rat liver. No hydroxylase activity was detected in the Golgi membrane preparations. It is therefore proposed that the cytoplasmic ratio of CMP-Neu5Ac and CMP-Neu5Gc produced by the hydroxylase, remains largely unmodified after CMP-glycoside uptake into the Golgi apparatus and transfer on to growing glycoconjugate glycan chains. The close relationship between the total sialic acid composition and the sialic acid pattern in the CMP-glycoside pools of the tissues lends considerable weight to this hypothesis.

Immunohistochemical detection of heterophile Hanganutziu-Deicher antigen in human malignant melanoma

The Hanganutziu-Deicher (HD) antigen is a heterophile antigen whose immunodominant molecule is N-glycolylneuraminic acid, a sialic acid that cannot be found in normal tissues of either humans or chickens. Using biotinylated chicken anti-HD antibody purified with affinity chromatography, expression of HD antigen was immunohistochemically investigated in formalin-fixed tissues of human malignant melanoma. HD antigen-positive melanoma cells were clearly demonstrated in 7 of 11 lesions of malignant melanoma. No HD antigen-positive cells were found in 8 lesions of melanocytic nevus, and no components of normal human skin including epidermal melanocytes were stained with the antibody. This study is the first that immunohistochemically demonstrates HD antigen in tissue sections of human malignant melanoma. The expression of the HD antigen in transformed human melanocytes may have great immunological significance because the antigen is absent from normal human tissues and is immunogenic in humans.

Detection of CMP-N-acetylneuraminic acid hydroxylase activity in fractionated mouse liver

The finding that N-glycoloylneuraminic acid (Neu5Gc) in pig submandibular gland is synthesized by hydroxylation of the sugar nucleotide CMP-Neu5Ac [Shaw & Schauer (1988) Biol. Chem. Hoppe-Seyler 369, 477-486] prompted us to investigate further the biosynthesis of this sialic acid in mouse liver. Free [14C]Neu5Ac, CMP-[14C]Neu5Ac and [14C]Neu5Ac glycosidically bound by Gal alpha 2-3- and Gal alpha 2-6-GlcNAc beta 1-4 linkages to fetuin were employed as potential substrates in experiments with fractionated mouse liver homogenates. The only substrate to be hydroxylated was the CMP-Neu5Ac glycoside. The product of the reaction was identified by chemical and enzymic methods as CMP-Neu5Gc. All of the CMP-Neu5Ac hydroxylase activity was detected in the high-speed supernatant fraction. The hydroxylase required a reduced nicotinamide nucleotide [NAD(P)H] coenzyme and molecular oxygen for activity. Furthermore, the activity of this enzyme was enhanced by exogenously added Fe2+ or Fe3+ ions, all other metal salts tested having a negligible or inhibitory influence. This hydroxylase is therefore tentatively classified as a monooxygenase. The cofactor requirement and CMP-Neu5Ac substrate specificity are identical to those of the enzyme in high-speed supernatants of pig submandibular gland, suggesting that this is a common route of Neu5Gc biosynthesis. The relevance of these results to the regulation of Neu5Gc expression in sialoglycoconjugates is discussed.

Natural occurrence and preparation of O-acylated 2,3-unsaturated sialic acids

Three O-acylated, unsaturated sialic acids, N-acetyl-9-O-acetyl-, N-acetyl-9-O-lactoyl-, and 2-deoxy-N-glycoloyl-9-O-lactoyl-2,3-didehydroneuraminic acid (5-acetamido-9-O-acetyl-, 5-acetamido-9-O-lactoyl-, and 2,6-anhydro-3,5-dideoxy-5-glycoloylamido-9-O-lactoyl-D-glycero-D-g alacto-non-2- enonic acid) were isolated from urine or submandibular glands of rat, pig, and cow. Mass spectrometric evidence for the existence of 2,3-unsaturated 9-O-acetyl-N-glycoloylneuraminic acid in porcine urine was also obtained. The sialic acids were purified by dialysis, gel- and ion-exchange chromatography, and preparative thin-layer chromatography. They were analyzed by thin-layer chromatography, high-pressure liquid chromatography, and capillary gas-liquid chromatography-mass spectrometry. For comparison, O-acetylated unsaturated sialic acids were synthesized.

Porcine submaxillary mucin contains alpha 2----3- and alpha 2----6-linked N-acetyl- and N-glycolylneuraminic acid

Four acidic trisaccharides have been obtained by alkaline borohydride reductive cleavage (beta-elimination) of a fraction of porcine submaxillary mucin precipitating at 60-75% ethanol. Their structures have been investigated using the techniques of methylation analysis involving gas-liquid chromatography/mass spectrometry along with high-resolution 1H-NMR analysis. Two of the four oligosaccharides, B1-B4, contain N-acetylneuraminic acid (NeuAc) while two contain N-glycolylneuraminic acid (NeuGc). The following structures are proposed for the acidic trisaccharide fraction: (B1) NeuAc alpha(2----3)Gal beta(1----3)GalNAcol, (B2) NeuGc alpha(2----3)Gal beta(1----3)GalNAcol, (B3) Gal beta(1----3)[NeuAc alpha(2----6)]GalNAcol and (B4) Gal beta(1----3)[NeuGc alpha(2----6)]GalNAcol, (GalNAcol = reduced N-acetylgalactosamine. These oligosaccharides were present in a molar ratio of 69:22:4:5. Although oligosaccharide B4 has previously been found in porcine submaxillary mucin B1, B2 and B3 have not. Furthermore, oligosaccharide B2 is a novel structure.

Interaction of bovine erythrocyte N-glycolylneuraminic acid-containing gangliosides and glycoproteins with a human Hanganutziu-Deicher serum

A human Hanganutziu-Deicher (H-D) serum reacted with N-glycolylneuraminic acid (NeuGc)-containing gangliosides and glycoproteins isolated from bovine erythrocyte membranes. Three populations of H-D antibodies were identified in the human H-D serum. One population very likely recognized the NeuGc-Gal sequence; a second population appears to recognize additional sugars in the oligosaccharide sequence, e.g. NeuGc-Gal-GlcNAc; while a third population may also recognize polypeptide determinants in addition to the NeuGc-Gal-GlcNAc sequence. The H-D serum distinguished two high mol. wt glycoproteins (HMGP I and II) present in crude extracts of bovine erythrocyte membranes. These glycoproteins were separated by repetitive fractionation on Sephacryl S-1000 in the presence of urea and their composition determined.

Structural parameters and natural occurrence of 2-deoxy-2,3-didehydro-N-glycoloylneuraminic acid

2-Deoxy-2,3-didehydro-N-glycoloylneuraminic acid has been found to occur in porcine, bovine and equine submandibular glands as well as in the urine of pig, horse and rat. This novel, unsaturated sialic acid was isolated by gel filtration and ion-exchange chromatography. Final purification was achieved by column chromatography or by preparative thin-layer chromatography on cellulose. The structural analysis was performed by combined capillary gas-liquid chromatography/mass spectrometry. The various data were compared with those from synthetic 2-deoxy-2,3-didehydro-N-glycoloylneuraminic acid. Besides of the unsaturated N-glycoloylated sialic acid, also the corresponding N-acetylated derivative was present in the materials analyzed. The inhibitory effect of 2-deoxy-2,3-didehydro-N-glycoloylneuraminic acid on Vibrio cholerae sialidase using N-acetylneuraminyl-(alpha 2----3)-lactose as substrate is slightly higher (50% inhibition at 10 microM) when compared with 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (50% inhibition at 15 microM).

Metabolic labeling of sialic acids in tissue culture cell lines: methods to identify substituted and modified radioactive neuraminic acids

The parent sialic acid N-acetylneuraminic acid can be modified or substituted in various ways, giving rise to a family of more than 25 compounds. The definitive identification of these compounds has previously required isolation of nanomole amounts for mass spectrometry or NMR. We have explored the possibility of using the known metabolic precursors of the sialic acids, particularly N-acetyl-[6-3H]mannosamine, to label and identify various forms of sialic acids in tissue culture cells. Firstly, we defined several variables that affect the labeling of sialic acids with N-acetyl-[6-3H]mannosamine. Secondly, we have devised a simple screening method to identify cell lines that synthesize substituted or modified sialic acids. We next demonstrate that it is possible to definitively identify the natures of the various labeled sialic acids without the use of mass spectrometry, even though they are present only in tracer amounts. The methods used include paper chromatography, analytical de-O-acetylation, periodate release of the 9-3H as [3H]formaldehyde (which is subsequently converted to a specific 3H-labeled chromophore), acylneuraminate pyruvate lyase treatment with identification of [3H]acylmannosamines, gas-liquid chromatography with radioactive detection, and two new high-pressure liquid chromatography methods utilizing the amine-adsorption:ion suppression and ion-pair principles. The use of an internal N-acetyl-[4-14C]neuraminic acid standard in each of these methods assures precision and accuracy. The combined use of these methods now allows the identification of radioactive tracer amounts of the various types of sialic acids in well-defined populations of tissue culture cells; it may also allow the identification of hitherto unknown forms of sialic acids.

Antiglycolipid antibodies in normal and pathologic human sera and synovial fluids

Antiglycolipid antibodies were measured in normal and pathologic sera and synovial fluids by means of a modified microplate method of complement-mediated immune lysis of fluorescent dye-trapped liposomes. All sera of normal subjects had antibodies against globopentaosylceramide (IV3 GalNAcGbOse4Cer), ganglioside GM1, gangliotriaosylceramide, gangliotetraosylceramide, and galactosylneolactotetraosylceramide antigens. Most sera of normal subjects had antibodies against lactotriaosylceramide, N-glycolylneuraminosyl-neolactotetraosylceramide (NeuGcnLcOse4Cer), GM3 ganglioside with N-glycolylneuraminic acid (NeuGcGM3) and GD1a antigens. Differences of titers against IV3GalNAcGbOse4Cer, neolactotetraosylceramide, NeuGcGM3 and NeuGcnLcOse4Cer antigens were observed between sera of normal subjects and pathologic sera from cases of leukemias, lymphomas, several autoimmune diseases and liver diseases.

New techniques for the investigation of structure and metabolism of sialic acids

Sialic acid analysis in biological material including gangliosides is often confronted with the necessity to determine trace amounts of various N,O-substituted species. Therefore, techniques of high sensitivity and resolution are required, such as capillary gas-liquid chromatography (GLC) and high performance liquid chromatography (HPLC). Both methods in combination with mass spectrometry allow structural analysis of the different neuraminic acid derivatives. Thus, the number of natural sialic acids known so far has increased to more than 30, including non only saturated, but also 2,3-unsaturated and 2,7-anhydro-sialic acids. Furthermore, HPLC has proved to be especially useful for the study of enzyme reactions, as the sialic acids of enzyme assay mixtures in most cases can be analyzed without prior extensive purification or derivatization.

Synthesis of N-acetylneuraminic acid and of CMP-N-acetylneuraminic acid in the rat liver cell

Adult male rats, under starving and normal conditions, were injected intravenously with N-acetyl[3H]mannosamine and after various time intervals the specific radioactivities of free N-acetylneuraminic acid (NeuAc) and CMP-N-acetylneuraminic acid were determined in the liver. The specific radioactivity of free NeuAc was high even within 20s after injection; the maximum was reached between 7 and 10 min. The specific radioactivity of CMP-NeuAc showed a lag phase of approx. 1 min. Thereafter it increased quickly and rose above the specific radioactivity of free NeuAc, reaching a maximum about 20 min after injection. These results point to a channelling of the newly synthesized NeuAc molecules into a special compartment, from which they are preferentially used by the enzyme CMP-sialic acid synthetase. It is suggested that the cytosolic enzyme N-acetylneuraminic acid 9-phosphate phosphatase is working in concert with the nuclear localized enzyme CMP-N-acetylneuraminic acid synthetase. Incorporation of radioactive sialic acid into sialoglycoproteins in liver occurred 2 min after injection, and after 10 min bound radioactivity began to appear in the circulation, indicating a transport time of 8 min of sialoglycoproteins from the point of attachment of sialic acid to the point of excretion.

Radioenzymatic determination of CMP-N-acetylsialic acid and free N-acetylsialic acid in biological material

Free N-acetylsialic acid (NeuNAc) and CMP-N-acetylsialic acids (CMP-NeuNAc) are extracted from freeze-clamped or liquid nitrogen-frozen biological material by sequential extraction with cold acetone and acetone/water. [14C]NeuNAc and [14C]CMP-NeuNAc (20,000 dpm each) are added to the frozen material to correct for small losses occurring during the subsequent steps. NeuNAc and CMP-NeuNAc are separated by anion-exchange chromatography. CMP-NeuNAc is hydrolyzed with formic acid and again chromatographed on an ion-exchange column. The NeuNAc-containing fractions (representing free NeuNAc and CMP-NeuNAc) are converted to [14C]CMP-NeuNAc in the presence of [14C]CTP and CMP-NeuNAc synthetase. [14C]CMP-NeuNAc is separated by paper chromatography and the radioactivity measured by liquid scintillation counting. The amount of NeuNAc is calculated from a calibration curve obtained with NeuNAc standards. The small amounts of [14C]NeuNAc and [14C]CMP-NeuNAc added initially do not interfere with the final assay. The method gives reliable values down to 50 pmol/assay, but the sensitivity can be easily increased by a factor of 10. Recoveries, with NeuNAc and CMP-NeuNAc added to biological extracts, were 98.3 and 98.5% for NeuNAc and CMP-NeuNAc, respectively. With this method values of 61.2 +/- 12.8 and 24.4 +/- 5.2 nmol/g wet wt were found in rat liver for free NeuNAc and CMP-NeuNAc, respectively. Values for free NeuNAc found in human blood plasma were 600 +/- 476 and 373 +/- 180 pmol/g plasma for healthy persons and patients with breast cancer, respectively. Free CMP-NeuNAc could not be found in plasma.

Free N-acetylneuraminic acid in tissues in Salla disease and the enzymes involved in its metabolism

Salla disease is a lysosomal storage disorder of unknown etiology, characterized biochemically by increased urinary excretion of N-acetylneuraminic acid. This compound has now been shown to occur in abnormally large amounts in liver and cultured skin fibroblasts from these patients. Quantification of N-acetylneuraminic acid was performed using a new gas-chromatography/mass spectrometric single-ion method which is sensitive and specific. No abnormalities in the activity of several enzymes involved in sialic acid metabolism (N-acetylneuraminate:pyruvate lyase, neuraminidase, CMP-N-acetylneuraminate N-acylneuraminohydrolase and CTP:N-acyl-neuraminate cytidylyltransferase) were demonstrable. A possible explanation for the defect is a malfunctioning active transport of N-acetylneuraminic acid across the lysosomal membrane.

Comparative study on ganglioside compositions of various rabbit tissues. Tissue-specificity in ganglioside molecular species of rabbit thymus

Ganglioside compositions of various organs of rabbit (NIBS strain, male, 10 months old) were studied. Organs examined contained lipid-bound sialic acid at various concentrations but the amounts in extraneural tissues were less than one-fifth of that in brain. The gangliosides of various tissues were analyzed by ganglioside-mapping and by isolating individual components and determining their structures chemically or enzymatically. According to their backbone asialocarbohydrate chain, the major gangliosides of various tissues were classified into three groups: (1) lactose and ganglio-N-triose (lung, stomach, liver, intestine, kidney, testis and muscle); (2) ganglio-N-tetraose (brain); (3) lacto-N-neotetraose (thymus). 70% of all thymus gangliosides had a lacto-N-neotetraose backbone, which was tissue-specific. In marked contrast to the case in other tissues, in thymus N-glycoloylneuraminic acid constituted 90% of the total lipid-bound sialic acid, and all molecular species of thymus gangliosides contained N-glycoloylneuraminic acid. Palmitic acid was a major fatty acid of thymus gangliosides. Distinct differences were found in the fatty acid compositions of gangliosides with longer carbohydrate chains in various tissues.

Lipid composition and molecular species of sialic acids of a mouse cell line (JLS-V9) and its ouabain-resistant mutant

Some properties of a mouse cell line (JLS-V9) and its ouabain-resistant mutant clone (JLS-V9OR) were compared. Specific activities of (Na+ + K+)-ATPase (EC 3.6.1.3) of the cell homogenate, particulate fraction and the plasma membrane fraction were 1.12, 1.72 and 8.75 mu mol/h per mg protein, respectively, in the parent cell and 0.97, 1.68 and 9.36 mu mol/h per mg protein in the mutant cell. The half-maximal concentration of ouabain for the inhibition of the (Na+ + K+)-ATPase was 3.4 . 10(-5) M in the parent cell and 4.0 . 10(-4) M in the resistant clone. The contents of phospholipid and cholesterol on a basis of protein in the mutant were 135 and 105% of those in the parent cell. The mutant's monohexosylceramide (HexCer), lactosylceramide (LacCer), trihexosylceramide (GbOse3Cer) and sialyllactosylceramide (GM3) were 111, 145, 274 and 114% of those of the parent cell. Sialic acids of GM3, analyzed by GC-MS, were 98% N-acetylneuraminic and 2% N-glycolylneuraminic acids in the parent cell, whilst 69% N-acetylneuraminic and 31% N-glycolylneuraminic acids in the mutant clone. The in vivo syntheses of these glycolipids were confirmed by the incorporation of radioactive galactose. No significant difference in fatty acid composition was observed between the two cell types. Neutral glycolipids contained mainly 24:0, 24:1, 22:0 and 16:0, whilst in GM3 18:0 and 18:1 predominated. These results show that the lipid content per mg protein is elevated in the ouabain-resistant cell compared to the parent cell.

Gangliosides of calf thymus and of normal and leukemic bovine lymphocytes

The gangliosides of calf thymus and of lymphocytes from blood, lymph nodes, lymph and spleen of normal and leukemic cows were investigated in an attempt to determine whether there exists a relation between the ganglioside composition and the maturity of the lymphocytes. With all normal peripheral lymphocytes studied the largely prevailing (up to 97%) ganglioside component was found to be N-glycoloylneuraminosyllactosylceramide. The ganglioside spectrum of calf thymus was much more complex and included at least six different components. By thin-layer chromatography, methylation analysis and neuraminidase treatment they were identified as N-glycoloylneuraminosyl and N-acetylneuraminosyl lactosylceramides, N-glycoloylneuraminosylneolactotetraosylceramide, di-(N-glycoloylneuraminosyl), di-(N-acetylneuraminosyl) lactosylceramides and N-glycoloylneuraminosyl-N-acetylneuraminosyllactosylceramide. The ganglioside spectra of leukemic peripheral lymphocytes were different from those of normal peripheral lymphocytes and resembled the ganglioside profile of the thymus. The data obtained indicate that, in the normal animal, the development of thymocytes into mature peripheral lymphocytes is accompanied by loss of disialosyl-gangliosides and sialosylneolactotetraosylceramide. It is concluded that both the structure and the composition of the lymphocyte ganglioside become more simple during maturation and ageing of the lymphocyte.