Developmental sialic acid modifications in rat organs

Why Is N-Glycolylneuraminic Acid Rare in the Vertebrate Brain?

The sialic acids N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) differ by a single oxygen atom and are widely found at the terminal position of glycans on vertebrate cell surfaces. In animals capable of synthesizing Neu5Gc, most tissues and cell types express both sialic acids, in proportions that vary between species. However, it has long been noted that Neu5Gc is consistently expressed at trace to absent levels in the brains of all vertebrates studied to date. Although several reports have claimed to find low levels of Neu5Gc-containing glycans in neural tissue, no study definitively excludes the possibility of contamination with glycans from non-neural cell types. This distribution of a molecule - prominently but variably expressed in extraneural tissues but very low or absent in the brain - is, to our knowledge, unique. The evolutionarily conserved brain-specific suppression of Neu5Gc may indicate that its presence is toxic to this organ; however, no studies to date have directly addressed this very interesting question. Here we provide a historical background to this issue and discuss potential mechanisms causing the suppression of Neu5Gc expression in brain tissue, as well as mechanisms by which Neu5Gc may exert the presumed toxicity. Finally, we discuss future approaches towards understanding the mechanisms and implications of this unusual finding.

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.

Extensive enrichment of N-glycolylneuraminic acid in extracellular sialoglycoproteins abundantly synthesized and secreted by human cancer cells

N-Glycolylneuraminic acid (Neu5Gc) is the second most populous sialic acid (Sia). The only known biosynthetic pathway of Neu5Gc is the hydroxylation of cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac), catalyzed by CMP-Neu5Ac hydroxylase (CMAH). Neu5Gc is abundantly found in mammals except for human, in which CMAH is inactivated due to mutation in the CMAH gene. Evidence has accumulated to show occurrence of Neu5Gc-containing glycoconjugates in sera of cancer patients, human cancerous tissues and cultured human cell lines. Recently, occurrence of natural antibodies against Neu5Gc was shown in healthy humans and is a serious problem for clinical xenotransplantation and stem cell therapies. Studying human occurrence of Neu5Gc is of importance and interest in a broad area of medical sciences. In this study, using a fluorometric high performance liquid chromatography method, we performed quantitative analyses of Sias both inside and in the external environment of the cell and found that (i) incorporation of Neu5Gc was most prominent in soluble glycoproteins found both in the extracellular space and inside the cell as the major Sia compounds. (ii) Of the total Neu5Gc in the Sia compounds that the cells synthesized, 90% was found in the secreted sialoglycoproteins, whereas for Neu5Ac, 70% was found in the secreted sialoglycoproteins. (iii) The Neu5Gc ratio was higher in the secreted sialoglycoproteins (as high as 40% of total Sias) than in intracellular sialoglycoproteins. (iv) The majority of the secreted sialoglycoproteins was anchored on the culture dishes and solubilized by brief trypsin treatment. Based on these findings, a new idea on the mechanism of accumulation of Neu5Gc in cancer cells was proposed.

Sialic acid utilisation and synthesis in the neonatal rat revisited

Background

Milk is the sole source of nutrients for neonatal mammals and is generally considered to have co-evolved with the developmental needs of the suckling newborn. One evolutionary conserved constituent of milk and present on many glycoconjugates is sialic acid. The brain and colon are major sites of sialic acid display and together with the liver also of synthesis.

Methodology/Principal Findings

In this study we examined in rats the relationship between the sialic acid content of milk and the uptake, utilization and synthesis of sialic acid in suckling pups. In rat milk sialic acid was found primarily as 3′sialyllactose and at highest levels between 3 and 10 days postpartum and that decreased towards weaning. In the liver of suckling pups sialic acid synthesis paralleled the increase in milk sialic acid reaching and keeping maximum activity from postnatal day 5 onwards. In the colon, gene expression profiles suggested that a switch from sialic acid uptake and catabolism towards sialic acid synthesis and utilization occurred that mirrored the change of sialic acid in milk from high to low expression. In brain sialic acid related gene expression profiles did not change to any great extent during the suckling period.

Conclusions/Significance

Our results support the views that (i) when milk sialic acid levels are high, in the colon this sialic acid is catabolized to GlcNAc that in turn may be used as such or used as substrate for sialic acid synthesis and (ii) when milk sialic acid levels are low the endogenous sialic acid synthetic machinery in colon is activated.

Expression of DeltaF508 Cystic Fibrosis Transmembrane Regulator (CFTR) Decreases Membrane Sialylation

Chronic colonization and infection of the lung with Pseudomonas aeruginosa is a major cause of morbidity and mortality in cystic fibrosis (CF) patients. Imundo, et al. determined that CF cells had a higher concentration of an asialoganglioside (asialo-G(M1)), to which both P. aeruginosa and S. aureus bound preferentially. We sought to determine if the expression of mutant CFTR is associated with altered sialylation. Our study of epithelial cells transfected with normal and mutant DeltaF508 CFTR, the defect in the majority of CF patients in the United States, were analyzed by ELISA and FACS analysis of cell membranes labeled with lectins which bind to Neu5Ac. We determined that DeltaF508 CFTR is associated with decreased membrane sialic acid residues in the alpha2, 3 position and increased concentrations of asialo- G(M1). Quantitation of sialic acids released from the cellular membranes demonstrated that the presence of the DeltaF508 CFTR is associated with markedly decreased membrane sialylation, but similar cytoplasmic sialylation. Thus, DeltaF508 defect is correlated with decreased expression of G(M1) and with decreased sialylation of all cell surface structures, and this change occurs during post-translational modification of glycoproteins and glycolipids. This may be one factor involved in the chronic bacterial colonization seen in these patients.

Sialic acids: fascinating sugars in higher animals and man

Sialic acids are acidic monosaccharides, which are among the most important molecules of higher animals, and occur in some microorganisms. They are bound to complex carbohydrates and occupy prominent positions, especially in cell membranes. Their structural diversity is high and, correspondingly, the mechanisms for their biosynthesis are complex. Sialic acid substituents strongly influence the activity of catabolic enzymes, in particular the sialidases, and thus the turnover rate of glycoconjugates. These sugars are involved in manifold cell functions. Due to the surface location of the acidic molecules they shield macromolecules and cells from enzymatic and immunological attacks. But they also represent recognition sites for various physiological receptors as well as for toxins and microorganisms, and thus allow their colonization. Many viruses use sialic acids for the infection of cells. As sialic acids also play a decisive role in tumor biology they prove to be rather versatile molecules that modulate cell biological events in a sensitive way. It is discussed that their evolvement may have stimulated evolution and rendered organisms less vulnerable to environmental attacks. However, disturbance of their metabolism may cause diseases.

Regulation of the intestinal glycoprotein glycosylation during postnatal development: role of hormonal and nutritional factors

This review focuses on the regulation of the glycoprotein glycosylation process in small intestine and colon during postnatal development. Glycoproteins play a prominent part in intestine as mucins secreted by the goblet cells and as molecules of biological interest largely present in the microvillus membrane of the enterocytes (digestive enzymes, transporters). The age-related changes in the intestinal glycosylation control the quality of glycan chains of glycoproteins. Postnatal maturation is observed at all stages of the glycoprotein glycosylation. But it is essentially characterised in the external glycosylation by a shift from sialylation to fucosylation depending on the transcriptional regulation of the corresponding glycosyltransferases, but also on coordinate changes in the activities of glycosyltransferases and of their regulatory proteins, in nucleotide-sugar bioavailability and in product degradation by oxidases. Many factors have been evoked to trigger these changes, among which are hormonal (glucocorticoids, insulin) and dietary factors. Changes in the structure of the glycoprotein glycans might be important for the transport, the barrier function, the implantation of the immune defences and of the microflora and even probably for the biological activity of some digestive enzymes.

Regulation of N-glycolylneuraminic acid biosynthesis in developing pig small intestine

N -Glycolylneuraminic acid (Neu5Gc), an abundant sialic acid in animal glycoconjugates, is formed by the enzyme CMP-N-acetylneuraminic acid (CMP-Neu5Ac) hydroxylase. The amount of Neu5Gc relative to other sialic acids is highly dependent on the species, tissue and developmental stage. Although the activity of the hydroxylase is a key factor in controlling Neu5Gc incorporation in adult animals, little is known about the regulation of hydroxylase expression and the role of this enzyme in determining changes in Neu5Gc during development. Using pig small intestine as a model system, the appearance of total sialic acid and the regulation of Neu5Gc biosynthesis during development were studied in various regions of this tissue. The amount of total sialic acid and Neu5Gc declined markedly in 2 weeks after birth. Although in subsequent developmental phases there were no positional differences in total sialic acid, a significant proximal-to-distal increase in Neu5Gc was detected. In all cases, a good correlation between the amount of Neu5Gc, the activity of the hydroxylase and the level of hydroxylase mRNA was observed. However, Western-blot analysis revealed considerable accumulation of less active enzyme in the post partum period, which persisted until adulthood. No evidence for cytosolic factors influencing the hydroxylase activity or for the formation of truncated enzyme was found, raising the possibility that other regulatory mechanisms are involved. The relevance of these results in the formation of Neu5Gc as a receptor for certain pig enteric pathogens is also discussed.

Isolation and characterization of cytidine-5'-monophosphate-N-acetylneuraminate hydroxylase from the starfish Asterias rubens

The sialic acid N-glycolylneuraminic acid (Neu5Gc) is formed by cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac) hydroxylase (EC 1.14.13.45). The enzyme from mammals exhibits several unusual characteristics, raising questions about its evolution. Since echinoderms are the most primitive organisms possessing glycoconjugate-bound Neu5Gc, studies on the hydroxylase from members of this phylum may yield insights into the origin and development of the hydroxylase. Investigations on crude CMP-Neu5Ac hydroxylase in gonads from the starfish Asterias rubens revealed that it shares many properties with its mammalian counterpart. However, the echinoderm hydroxylase also exhibits fundamental differences, particularly its association with a membrane and a requirement for high ionic strength for optimal activity. Here, we describe the isolation of the CMP-Neu5Ac hydroxylase from A. rubens gonads using anion exchange chromatography and chromatography on immobilized cytochrome b(5). The enzyme was enriched 137-fold with a yield of 13%. The preparation exhibited a main polypeptide of 76 kDa, consistent with a cDNA sequence published earlier, and a minor protein of 64 kDa. A kinetic characterization showed that salt activation of this enzyme results from an increase in affinity for CMP-Neu5Ac. Evidence for the formation of a ternary complex of hydroxylase, CMP-Neu5Ac and cytochrome b(5) is also presented. The mechanistic and physiological significance of these results is discussed.

Inactivation of CMP-N-acetylneuraminic acid hydroxylase occurred prior to brain expansion during human evolution

Humans are genetically deficient in the common mammalian sialic acid N-glycolylneuraminic acid (Neu5Gc) because of an Alu-mediated inactivating mutation of the gene encoding the enzyme CMP-N-acetylneuraminic acid (CMP-Neu5Ac) hydroxylase (CMAH). This mutation occurred after our last common ancestor with bonobos and chimpanzees, and before the origin of present-day humans. Here, we take multiple approaches to estimate the timing of this mutation in relationship to human evolutionary history. First, we have developed a method to extract and identify sialic acids from bones and bony fossils. Two Neanderthal fossils studied had clearly detectable Neu5Ac but no Neu5Gc, indicating that the CMAH mutation predated the common ancestor of humans and the Neanderthal, approximately 0.5-0.6 million years ago (mya). Second, we date the insertion event of the inactivating human-specific sahAluY element that replaced the ancestral AluSq element found adjacent to exon 6 of the CMAH gene in the chimpanzee genome. Assuming Alu source genes based on a phylogenetic tree of human-specific Alu elements, we estimate the sahAluY insertion time at approximately 2.7 mya. Third, we apply molecular clock analysis to chimpanzee and other great ape CMAH genes and the corresponding human pseudogene to estimate an inactivation time of approximately 2.8 mya. Taken together, these studies indicate that the CMAH gene was inactivated shortly before the time when brain expansion began in humankind's ancestry, approximately 2.1-2.2 mya. In this regard, it is of interest that although Neu5Gc is the major sialic acid in most organs of the chimpanzee, its expression is selectively down-regulated in the brain, for as yet unknown reasons.

Differential expression of three sialidase genes in rat development

Mammalian sialidases have been reported to give a great influence on a number of cellular functions including cell differentiation and cell growth by removal of sialic acids from glycoproteins and gangliosides. To understand the roles of the sialidases during development, we investigated expression pattern of three types of sialidase in developing rat brain and liver. For this purpose we cloned a new membrane-associated sialidase cDNA from rat brain. The cDNA encodes 418 amino acids containing three ASP-boxes characteristic of sialidases and the major transcript of 3.5 kb is highly expressed in brain and cardiac muscle but low in liver. Competitive polymerase chain reaction methods were developed to evaluate the mRNA level together with activity assays in comparison with cytosolic and lysosomal sialidases previously obtained. The results indicate that the expression of individual sialidase genes is spatiotemporally controlled with distinct roles in determining the concentration and components of sialo-glycoconjugates during development.

Paracrine effects of a uterine agglutinin are mediated via the sialic acids present in the rat uterine endometrium

A 32 kDa estrogen-induced, sialic acid-specific agglutinin (P-SAS) was isolated from rat endometrium in its proestrus stage. To investigate the functional importance of P-SAS in the uterine milieu, specific binding assays were carried out with 125I-labeled P-SAS and different cellular components of the uterus (epithelial, stromal and myometrial cells), that were isolated from different stages of the estrus cycle. The results indicate that although the protein is secreted from the epithelial cells in the estrogenic phase, it binds specifically to the stromal cells, especially to those isolated from the diestrus stage of the estrus cycle. The specific binding, however, is seen to decrease with the progression of pregnancy. Scatchard analysis performed with varying amounts of 125I-P-SAS in the presence of excess cold P-SAS revealed that the binding occurs with a Ka = 1.69 x 10(8) M(-1). As P-SAS binds specifically to sialic acids on the stromal cell surface, further characterization of the sialic acid molecule to which P-SAS binds was carried out by gas liquid chromatography (GLC). The studies revealed that P-SAS preferentially binds to N-glycolylneuraminic acid, which is attached to the penultimate sugar of the stromal cell surface glycoprotein chain via alpha2,6 linkage. As P-SAS is further known to be mitogenic, the effect of P-SAS on cultured stromal cells was studied in vitro. The growth regulatory assays revealed that P-SAS induced 3H-thymidine uptake by stromal cells in culture. Thus, from the above observations, paracrine effects of P-SAS on the stromal cells and on the subsequent growth and development of the uterus can be assumed.

Achievements and challenges of sialic acid research

Slalic acids are one of the most important molecules of life, since they occupy the terminal position on macromolecules and cell membranes and are involved in many biological and pathological phenomena. The structures of sialic acids, comprising a family of over 40 neuraminic acid derivatives, have been elucidated. However, many aspects of the regulation of their metabolism at the enzyme and gene levels, as well as of their functions remain mysterious. Sialic acids play a dual role, not only are they indispensable for the protection to and adaptation of life, but are also utilised by life-threatening infectious microorganisms. In this article the present state of knowledge in sialobiology, with an emphasis on my personal experience in this research area, is outlined including a discussion of necessary future work in this fascinating field of cell biology.

Identification of free deaminated sialic acid (2-keto-3-deoxy-D-glycero-D-galacto-nononic acid) in human red blood cells and its elevated expression in fetal cord red blood cells and ovarian cancer cells

Chemical studies have shown the occurrence of the deaminated sialic acid 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN) in paired samples of blood obtained from mothers and newborns of healthy human individuals. Most of the KDN was found in red blood cells, although low levels were detected in mononuclear cells. No N-glycolylneuraminic acid was detected. Unexpectedly, nearly all of the KDN in fetal cord and matched maternal red blood cells was present as the free sugar and comparatively little occurred conjugated or as cytidine 5'-KDN phosphate. The amount of free KDN in fetal newborn red blood cells was 2.4-fold higher than in red blood cells from the mothers or from healthy nonpregnant women. Free KDN was also identified in normal human ovaries, in ovarian tumors, and in ascites cells obtained from ovarian cancer patients. Importantly, as in fetal cord red blood cells, a distinguishing feature of KDN expression in ovarian tumor cells was an elevated level of free KDN compared with normal controls. A positive correlation was found between an increase in the ratio of free KDN/N-acetylneuraminic acid in ovarian adenocarcinomas and the stage of malignancy. This was particularly evident in tumor cells isolated from the ascites fluid. The central importance of these new findings is 2-fold. First, they show that free KDN is a minor but ubiquitous sialic acid in human red blood cells and that its elevated expression in red blood cells from fetal cord blood compared with maternal red blood cells may be developmentally related to blood cell formation during embryogenesis. Second, the enhanced expression of KDN in ovarian cancer cells suggests that this sialic acid, like the alpha2,8-linked polysialic acid glycotope, may be an oncofetal antigen in these tumors and thus could be an "early warning" signal for onset of disease and/or a marker for detection of recurrence of disease. These new findings highlight the importance of elucidating the role that KDN and KDN-containing glycoconjugates may play in normal development and malignancy.

A structural difference between the cell surfaces of humans and the great apes

The sialic acids are major components of the cell surfaces of animals of the deuterostome lineage. Earlier studies suggested that humans may not express N-glycolyl-neuraminic acid (Neu5Gc), a hydroxylated form of the common sialic acid N-acetyl-neuraminic acid (Neu5Ac). We find that while Neu5Gc is essentially undetectable on human plasma proteins and erythrocytes, it is a major component in all the four extant great apes (chimpanzee, bonobo, gorilla and orangutan) as well as in many other mammals. This marked difference is also seen amongst cultured lymphoblastoid cells from humans and great apes, as well as in a variety of other tissues compared between humans and chimpanzees, including the cerebral cortex and the cerebrospinal fluid. Biosynthetically, Neu5Gc arises from the action of a hydroxylase that converts the nucleotide donor CMP-Neu5Ac to CMP-Neu5Gc. This enzymatic activity is present in chimpanzee cells, but not in human cells. However, traces of Neu5Gc occur in some human tissues, and others have reported expression of Neu5Gc in human cancers and fetal tissues. Thus, the enzymatic capacity to express Neu5Gc appears to have been suppressed sometime after the great ape-hominid divergence. As terminal structures on cell surfaces, sialic acids are involved in intercellular cross-talk involving specific vertebrate lectins, as well as in microbe-host recognition involving a wide variety of pathogens. The level of sialic acid hydroxylation (level of Neu5Ac versus Neu5Gc) is known to positively or negatively affect several of these endogenous and exogenous interactions. Thus, there are potential functional consequences of this widespread structural change in humans affecting the surfaces of cells throughout the body.

A mutation in human CMP-sialic acid hydroxylase occurred after the Homo-Pan divergence

Sialic acids are important cell-surface molecules of animals in the deuterostome lineage. Although humans do not express easily detectable amounts of N-glycolylneuraminic acid (Neu5Gc, a hydroxylated form of the common sialic acid N-acetylneuraminic acid, Neu5Ac), it is a major component in great ape tissues, except in the brain. This difference correlates with lack of the hydroxylase activity that converts CMP-Neu5Ac to CMP-Neu5Gc. Here we report cloning of human and chimpanzee hydroxylase cDNAs. Although this chimpanzee cDNA is similar to the murine homologue, the human cDNA contains a 92-bp deletion resulting in a frameshift mutation. The isolated human gene also shows evidence for this deletion. Genomic PCR analysis indicates that this deletion does not occur in any of the African great apes. The gene is localized to 6p22-p23 in both humans and great apes, which does not correspond to known chromosomal rearrangements that occurred during hominoid evolution. Thus, the lineage leading to modern humans suffered a mutation sometime after the common ancestor with the chimpanzee and bonobo, potentially affecting recognition by a variety of endogenous and exogenous sialic acid-binding lectins. Also, the expression of Neu5Gc previously reported in human fetuses and tumors as well as the traces detected in some normal adult humans must be mediated by an alternate pathway.

The role of CMP-N-acetylneuraminic acid hydroxylase in determining the level of N-glycolylneuraminic acid in porcine tissues

The biosynthesis of the sialic acid N-glycolylneuraminic acid (Neu5Gc) occurs by the action of cytidine monophosphate-N-acetylneuraminate (CMP-Neu5Ac) hydroxylase. Previous investigations on a limited number of tissues suggest that the activity of this enzyme governs the extent of glycoconjugate sialylation with Neu5Gc. Using improved analytical procedures and a panel of nine porcine tissues, each expressing different amounts of Neu5Gc, we have readdressed the issue of the regulation of Neu5Gc incorporation into glycoconjugates. The following parameters were measured for each tissue: the molar ratio Neu5Gc/Neu5Ac, the activity of the hydroxylase, and the relative amount of hydroxylase protein, as determined by enzyme-linked immunosorbent assay (ELISA). A positive correlation between the activity of the hydroxylase and the molar ratio Neu5Gc/Neu5Ac was observed for each tissue. In addition, the hydroxylase activity correlated with the amount of enzyme protein, though in heart and lung disproportionately large amounts of immunoreactive protein were detected. Taken together, the results suggest that the incorporation of Neu5Gc into glycoconjugates is generally controlled by the amount of hydroxylase protein expressed in a tissue.

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.

Developmental changes in UDP-N-acetylglucosamine 2-epimerase activity of rat and guinea-pig liver

The activity of UDP-N-acetylglucosamine 2-epimerase was determined in the liver of rats and guinea-pigs of different ages. The activity of this enzyme in rats was low at birth, increased to a maximum value on day 15, and fell gradually until day 30. Thereafter, it increased up to the 60th day. The activity profile of the enzyme from guinea-pig liver was very similar. However, guinea-pig activity was 2-5 times lower than in rats. Both rats and guinea-pigs displayed similar liver sialic acid contents which increased from birth to 2 months of age. Rats also showed a N-glycolylneuraminic acid content that decreased from birth to 2 months. From these results we can inferred that postnatal UDP-N-acetylglucosamine 2-epimerase activity seems to be correlated with age and the developmental states of rats and guinea-pigs.

A critical analysis of total sialic acid and sialoglycoconjugate contents of bovine milk-based infant formulas

BACKGROUND

Several infant formulas were bovine milk-based products. Mature bovine milk has a very low sialoglycoconjugate content compared with human milk from the first phases of lactation.

METHODS

The present study was undertaken to determine total sialic acid and oligosaccharide, glycoprotein, and ganglioside sialic acid contents of bovine milk-based formulas.

RESULTS

Starter formulas, designed for the first days/weeks after birth, have very similar sialic acid contents (233-266 mg/L fresh milk). We found more oligosaccharide-bound sialic acids (167-174 mg/L fresh milk) than those bound to proteins (53-84 mg/L fresh milk) in these formulas. The ganglioside sialic acid contents of starter formulas (952-1135 micrograms/L fresh milk) vary slightly from formula to formula. However, all the above-mentioned contents are lower than in human colostrum or transitional milk.

CONCLUSIONS

Infants fed starter formulas have total sialic acid and oligosaccharide, glycoprotein, and ganglioside sialic acid intakes of 36, 28, 50, and 20%, respectively, of those fed human colostrum or transitional milk. By contrast, follow-on formulas, used from 4 to 5 months of age, provide total sialic acid and oligosaccharide, glycoprotein, and ganglioside sialic acid contents similar to those furnished by mature human milk. Since the reference standard for optimal nutrition in the early months of infancy is human milk, a supplementation with sialic acid-containing glycoconjugates of infant formulas recommended for the first days after delivery could be advisable when breast-feeding is not possible.

Purification, characterization and reconstitution of CMP-N-acetylneuraminate hydroxylase from mouse liver

CMP-N-acetylneuraminate hydroxylase was isolated from mouse liver high speed supernatant with a yield of 0.4% and an apparent 1000-fold purification. The enzyme is a monomeric protein with a molecular weight of 66 kDa, as determined by gel filtration and SDS-PAGE. The hydroxylase system was reconstituted with Triton X-100-solubilized mouse liver microsomes and purified soluble or microsomal forms of cytochrome b5 reductase and cytochrome b5. The systems were characterized in detail and kinetic parameters for each system were determined.

CMP-N-acetylneuraminic acid hydroxylase from mouse liver and pig submandibular glands. Interaction with membrane-bound and soluble cytochrome b5-dependent electron transport chains

In this report, the nature of the protein components involved in the functioning of cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5 Ac) hydroxylase in high-speed supernatants of mouse liver has been investigated. Fractionation and reconstitution experiments showed that this enzyme system consists of NADH-cytochrome b5 reductase, cytochrome b5 and a 56-kDa terminal electron acceptor having the CMP-Neu5 Ac hydroxylase activity. This enzyme system is extracted in a soluble protein fraction; however, the amphipathic, usually membrane-associated, forms of cytochrome b5 and the reductase were found to predominate and are presumably the forms which support the turnover of the hydroxylase in vivo. Although the majority of cellular cytochrome b5 and cytochrome b5 reductase is membrane-bound, the addition of intact microsomes elicited no significant increase in the hydroxylase activity of supernatants. Detergent-solubilised microsomes, however, potently activated the hydroxylase, probably due to the greater accessibility of the cytochrome b5. Accordingly, in reconstitution experiments, pure hydrophilic cytochrome b5 interacts more effectively with the hydroxylase than isolated amphipathic cytochrome b5. Studies on the CMP-Neu5 Ac hydroxylase system in fractionated porcine submandibular glands and bovine liver suggest that the composition of this enzyme system is conserved in all mammals possessing sialoglycoconjugates containing N-glycolylneuraminic acid.

Regulation of biosynthesis of N-glycolylneuraminic acid-containing glycoconjugates: characterization of factors required for NADH-dependent cytidine 5'monophosphate-N-acetylneuraminic acid hydroxylation

The hydroxylation of CMP-NeuAc has been demonstrated to be carried out by several factors including the soluble form of cytochrome b5. In the present study, mouse liver cytosol was subjected to ammonium sulfate fractionation and cellulose phosphate column chromatography for the separation of two other essential fractions participating in the hydroxylation. One of the fractions, which bound to a cellulose phosphate column, was able to reduce the soluble cytochrome b5, using NADH as an electron donor. The other fraction, which flowed through the column, was assumed to contain the terminal enzyme which accepts electrons from cytochrome b5, activates oxygen, and catalyses the hydroxylation of CMP-NeuAc. Assay conditions for the quantitative determination of the terminal enzyme were established, and the activity of the enzyme in several tissues of mouse and rat was measured. The level of the terminal enzyme activity is associated with the expression of N-glycolylneuraminic acid in these tissues, indicating that the expression of the terminal enzyme possibly regulates the overall velocity of CMP-NeuAc hydroxylation.