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

Sialylation: fate decision of mammalian sperm development, fertilization, and male fertility†

Sperm development, maturation, and successful fertilization within the female reproductive tract are intricate and orderly processes that involve protein translation and post-translational modifications. Among these modifications, sialylation plays a crucial role. Any disruptions occurring throughout the sperm's life cycle can result in male infertility, yet our current understanding of this process remains limited. Conventional semen analysis often fails to diagnose some infertility cases associated with sperm sialylation, emphasizing the need to comprehend and investigate the characteristics of sperm sialylation. This review reanalyzes the significance of sialylation in sperm development and fertilization and evaluates the impact of sialylation damage on male fertility under pathological conditions. Sialylation serves a vital role in the life journey of sperm, providing a negatively charged glycocalyx and enriching the molecular structure of the sperm surface, which is beneficial to sperm reversible recognition and immune interaction. These characteristics are particularly crucial during sperm maturation and fertilization within the female reproductive tract. Moreover, enhancing the understanding of the mechanism underlying sperm sialylation can promote the development of relevant clinical indicators for infertility detection and treatment.

Paleo-immunology of human anti-carbohydrate antibodies preventing primate extinctions

Two human natural anti-carbohydrate antibodies appeared in critical evolutionary events that brought primates and hominins to brink of extinction. The first is the anti-Gal antibody, produced in Old-World monkeys (OWM), apes and humans. It binds the carbohydrate-antigen 'α-gal epitope' (Galα1-3Galβ1-4GlcNAc-R) on carbohydrate-chains (glycans) synthesized by non-primate mammals, lemurs and New-World monkeys (NWM). The second is anti-N-glycolylneuraminic-acid (anti-Neu5Gc) antibody binding Neu5Gc on glycans synthesized by OWM, apes and most non-primate mammals. Ancestral OWM and apes synthesized α-gal epitopes and were eliminated ~20-30 million-years-ago (mya). Only few accidentally mutated offspring lacking α-gal epitopes, produced anti-Gal and survived. Hominin-populations living ~3 mya synthesized Neu5Gc and were eliminated, but few mutated offspring that accidently lost their ability to synthesize Neu5Gc, produced natural anti-Neu5Gc antibody. These hominins survived and ultimately evolved into present-day humans. It is argued that these two near-extinction events were likely to be the result of epidemics caused by highly virulent and lethal enveloped viruses that killed parental-populations. These viruses presented α-gal epitopes or Neu5Gc synthesized in host-cells of the parental-populations. Mutated offspring survived the epidemics because they were protected from the lethal virus by the natural anti-Gal or anti-Neu5Gc antibodies they produced due to loss of immune-tolerance to α-gal epitopes or to Neu5Gc, respectively.

Strategic feeding of NS0 and CHO cell cultures to control glycan profiles and immunogenic epitopes of monoclonal antibodies

The control of glycosylation profiles is essential to the consistent manufacture of therapeutic monoclonal antibodies that may be produced from a variety of cell lines including CHO and NS0. Of particular concern is the potential for generating non-human epitopes such as N-glycolylneuraminic acid (Neu5Gc) and Galα1-3 Gal that may be immunogenic. We have looked at the effects of a commonly used media supplements of manganese, galactose and uridine (MGU) on Mab production from CHO and NS0 cells in enhancing galactosylation and sialylation as well as the generation of these non-human glycan epitopes. In the absence of the MGU supplement, the humanized IgG1 antibody (Hu1D10) produced from NS0 cells showed a low level of mono- and di-sialylated structures (SI:0.09) of which 75 % of sialic acid was Neu5Gc. The chimeric human-llama Mab (EG2-hFc) produced from CHO cells showed an equally low level of sialylation (SI: 0.12) but the Neu5Gc content of sialic acid was negligible (<3%). Combinations of the MGU supplements added to the production cultures resulted in a substantial increase in the galactosylation of both Mabs (up to GI:0.78 in Hu1D10 and 0.81 in EG2-hFc). However, the effects on sialylation differed between the two Mabs. We observed a slight increase in sialylation of the EG2-hFc Mab by a combination of MG but it appeared that one of the components (uridine) was inhibitory to sialylation. On the other hand, MG or MGU increased sialylation of Hu1D10 substantially (SI:0.72) with an increase that could be attributed predominantly to the formation of Neu5Ac rather than Neu5Gc. The increased level of galactosylation observed with MG or MGU was attributed to an activation of the galactosyl transferase enzymes through enhanced intracellular levels of UDP-Gal and the availability of Mn²⁺ as an enzymic co-factor. However, this effect not only increased the desirable beta 1-4 Gal linkage to GlcNAc but unfortunately in NS0 cells increased the formation of Galα1-3 Gal which was shown to increase x3 in the presence of combinations of the MGU supplements. Supplementation of media with fetal bovine serum (FBS) increased the availability of free Neu5Ac which resulted in a significant increase in the sialylation of Hu1D10 from NS0 cells. This also resulted in a significant decrease in the proportion of Neu5Gc in the measured sialic acid from the Mab.

Osmolality as a lever to modulate the N-glycolylneuraminicacid (Neu5Gc) level of a recombinant glycoprotein produced in Chinese hamster ovary cells

Glycoproteins could be highly sialylated, and controlling the sialic acid levels for some therapeutic proteins is critical to ensure product consistency and efficacy. N-acetylneuraminic acid (Neu5Ac, or NANA) and N-glycolylneuraminic acid (Neu5Gc, or NGNA) are the two most common forms of sialic acids produced in mammalian cells. As Neu5Gc is not produced in humans and can elicit immune responses, minimizing Neu5Gc formation is important in controlling this quality attribute for complex glycoproteins. In this study, a sialylated glycoprotein was used as the model molecule to study the effect of culture osmolality on Neu5Gc. A 14-day fed-batch process with osmolality maintained at physiological levels produced high levels of Neu5Gc. Increase of culture osmolality reduced the Neu5Gc level up to 70-80%, and the effect was proportional to the osmolality level. Through evaluating different osmolality conditions (300-450 mOsm/kg) under low or high pCO₂ , we demonstrated that osmolality could be an effective process lever to modulate the Neu5Gc level. Potential mechanism of osmolality impact on Neu5Gc is discussed and is hypothesized to be cytosol NADH availability related. Compared with cell line engineering efforts, this simple process lever provides the opportunity to readily modulate the Neu5Gc level in a cell culture environment.

Intestine specific regulation of pig cytidine-5'-monophospho-N-acetylneuraminic acid hydroxylase gene for N-glycolylneuraminic acid biosynthesis

N-glycolylneuraminic acid (Neu5Gc), a generic form of sialic acid, is enzymatically synthesized by cytidine-5′-monophospho-N-acetylneuraminic acid hydroxylase (CMAH). Although expression of pig CMAH gene pcmah encoding CMAH has been reported to be regulated by pathogenic infection and developmental processes, little is known about the mechanisms underlying the regulation of pcmah gene expression. The objective of this study was to determine mechanism(s) involved in intestine specific regulation of pcmah gene by identifying several cis-acting elements and nuclear transcription factors that could directly interact with these cis-acting elements. We identified intestine specific promoter region (Pi) of pcmah gene located at upstream regions of the 5′flanking region of exon 1a and found that the promoter region is responsible for the transcriptional regulation of 5′pcmah-1. Based on reporter assays using serially constructed luciferase genes with each deleted promoter, we demonstrated that the Pi promoter activity was more active in intestinal IPI-2I cells than that in kidney PK15 cells, corresponding to both mRNA expression patterns in the two cell lines. In addition, we found that Sp1 transcription factor was necessary for basal activity of Pi promoter and that Ets-1 contributed to intestine-specific activity of Pi promoter. This study helps us understand transcriptional regulation of pcmah in the intestine of pig tissues. It also allows us to consider potential roles of Neu5Gc in interaction with environmental factors present in the intestinal tissue during pathogenic infection and developmental process.

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.

Siglec expression on the surface of human, bull and ram sperm

Sialic acid (Sia) is a major constituent of both the sperm glycocalyx and female reproductive mucosal surface and is involved in regulating sperm migration, uterotubal reservoir formation and oocyte binding. Siglecs (sialic acid-binding immunoglobulin - like lectins) commonly found on immune cells, bind to Sia in a linkage- and sugar-specific manner and often mediate cell-to-cell interactions and signalling. Proteomic and transcriptomic analysis of human and bovine sperm have listed Siglecs, but to date, their presence and/or localisation on sperm has not been studied. Therefore, the aim of this study was to characterise the presence of Siglecs on the surface of bovine, human and ovine sperm using both immunostaining and Western blotting. Siglec 1, 2, 5, 6, 10 and 14 were identified and displayed both species- and regional-specific expression on sperm. Almost universal expression across Siglecs and species was evident in the sperm neck and midpiece region while variable expression among Siglecs, similar among species, was detected in the head and tail regions of the sperm. The possible role for these proteins on sperm is discussed.

Poor Patient and Graft Outcome After Induction Treatment by Antithymocyte Globulin in Recipients of a Kidney Graft After Nonrenal Organ Transplantation

Background

End-stage renal failure occurs in a substantial number of patients having received a nonrenal transplantation (NRT), for whom a kidney transplantation is needed. The medical strategy regarding the use of immunosuppression (IS) for a kidney graft in patients after an NRT is not well established. The prekidney grafts long-term IS advocates for a mild induction, such as using anti-IL-2R antibodies, whereas addition of new incompatibilities and anti-HLA preimmunization may suggest using stronger IS such as induction by polyclonal antithymocyte globulins (ATG).

Methods

We performed Cox multivariate and propensity score analysis of our validated transplant database to study the impact of the type of induction therapy on kidney graft survival of recipients of a kidney graft after NRT.

Results

We report here that kidney transplantation after NRT treated with an ATG induction has a poorer outcome (kidney and recipient survival) than that with an anti-IL-2R induction. After accounting for potential baseline differences with a multivariate Cox model, or by adjusting on a propensity score, we found that despite patients having received ATG cumulate more risk factors, ATG appears independently involved. As animal-derived biotherapeutics induce antiglycan antibodies and particularly anti-N-glycolylneuraminic acid (Neu5Gc) IgGs which may activate endothelial cells in patients and grafts, we also investigated the magnitude and the nature of the anti-Neu5Gc elicited by the induction and showed that induction was associated with a shift in anti-Neu5Gc IgG repertoire. Possible reasons and mechanisms of a deleterious ATG usage in these patients are discussed.

Conclusions

Our study suggests that ATG induction after a kidney transplantation in recipients already under maintenance IS for a NRT should be used cautiously.

N-glycolyl groups of nonhuman chondroitin sulfates survive in ancient fossils

Biosynthesis of the common mammalian sialic acid N-glycolylneuraminic acid (Neu5Gc) was lost during human evolution due to inactivation of the CMAH gene, possibly expediting divergence of the Homo lineage, due to a partial fertility barrier. Neu5Gc catabolism generates N-glycolylhexosamines, which are potential precursors for glycoconjugate biosynthesis. We carried out metabolic labeling experiments and studies of mice with human-like Neu5Gc deficiency to show that Neu5Gc degradation is the metabolic source of UDP-GlcNGc and UDP-GalNGc and the latter allows an unexpectedly selective incorporation of N-glycolyl groups into chondroitin sulfate (CS) over other potential glycoconjugate products. Partially N-glycolylated-CS was chemically synthesized as a standard for mass spectrometry to confirm its natural occurrence. Much lower amounts of GalNGc in human CS can apparently be derived from Neu5Gc-containing foods, a finding confirmed by feeding Neu5Gc-rich chow to human-like Neu5Gc-deficient mice. Unlike the case with Neu5Gc, N-glycolyl-CS was also stable enough to be detectable in animal fossils as old as 4 My. This work opens the door for investigating the biological and immunological significance of this glycosaminoglycan modification and for an "ancient glycans" approach to dating of Neu5Gc loss during the evolution of Homo.

Streptococcus pneumoniae Senses a Human-like Sialic Acid Profile via the Response Regulator CiaR

Streptococcus pneumoniae is a human-adapted pathogen that encounters terminally sialylated glycoconjugates and free sialic acid (Sia) in the airways. Upon scavenging by the bacterial sialidase NanA, Sias serve as carbon sources for the bacteria. Unlike most animals in which cytidine-monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) converts Sia N-acetylneuraminic acid (Neu5Ac) into N-glycolylneuraminic acid (Neu5Gc), humans have an inactive CMAH, causing an absence of Neu5Gc and excess Neu5Ac. We find that pneumococcal challenge in Cmah(-/-) mice leads to heightened bacterial loads, virulence, and NanA expression. In vitro, NanA is upregulated in response to Neu5Ac compared with Neu5Gc, a process controlled by the two-component response regulator CiaR and requiring Sia uptake by the transporter SatABC. Additionally, compared with Neu5Gc, Neu5Ac increases pneumococcal resistance to antimicrobial reactive oxygen species in a CiaR-dependent manner. Thus, S. pneumoniae senses and responds to Neu5Ac, leading to CiaR activation and increased virulence and potentially explaining the greater susceptibility in humans.

Natural anti-carbohydrate antibodies contributing to evolutionary survival of primates in viral epidemics?

Humans produce multiple natural antibodies against carbohydrate antigens on gastrointestinal bacteria. Two such antibodies appeared in primates in recent geological times. Anti-Gal, abundant in humans, apes and Old-World monkeys, appeared 20-30 million years ago (mya) following inactivation of the α1,3GT gene (GGTA1). This gene encodes in other mammals the enzyme α1,3galactosyltransferase (α1,3GT) that synthesizes α-gal epitopes (Galα1-3Galβ1-4GlcNAc-R) which bind anti-Gal. Anti-Neu5Gc, found only in humans, appeared in hominins <6 mya, following elimination of N-glycolylneuraminic-acid (Neu5Gc) because of inactivation of CMAH, the gene encoding hydroxylase that converts N-acetylneuraminic-acid (Neu5Ac) into Neu5Gc. These antibodies, were initially produced in few individuals that acquired random mutations inactivating the corresponding genes and eliminating α-gal epitopes or Neu5Gc, which became nonself antigens. It is suggested that these evolutionary selection events were induced by epidemics of enveloped viruses, lethal to ancestral Old World primates or hominins. Such viruses presented α-gal epitopes or Neu5Gc, synthesized in primates that conserved active GGTA1 or CMAH, respectively, and were lethal to their hosts. The natural anti-Gal or anti-Neu5Gc antibodies, produced in offspring lacking the corresponding carbohydrate antigens, neutralized and destroyed viruses presenting α-gal epitopes or Neu5Gc. These antibodies further induced rapid, effective immune responses against virus antigens, thus preventing infections from reaching lethal stages. These epidemics ultimately resulted in extinction of primate populations synthesizing these carbohydrate antigens and their replacement with offspring populations lacking the antigens and producing protective antibodies against them. Similar events could mediate the elimination of various carbohydrate antigens, thus preventing the complete extinction of other vertebrate species.

Housekeeping promoter 5'pcmah-2 of pig CMP-N-acetylneuraminic acid hydroxylase gene for NeuGc expression

In the present study, we isolated pCMAH house-keeping promoter regions (Ph), which are responsible for transcriptional regulation and which are located upstream of the alternative transcript pcmah-2. Luciferase reporter assays using serial construction of each deleted promoter demonstrated that the Ph promoter was highly active in pig-derived kidney PK15. Ph promoter of pcmah lacked a TATA box, but contained three putative Sp1 binding sites. Mutations of these Sp1 binding sites always resulted in the reduction of luciferase activities in Ph-334. In addition, treatment with mithramycin A (25-100 nM) decreased the luciferase activities of the Ph promoters and NeuGc expression in a dose-dependent manner. Electrophoretic mobility shift assay analysis revealed that the probes containing each Sp1 binding site bound to Sp1. Taken together, the results indicate that Sp1 bind to their putative binding sites on the Ph promoter regions of pcmah and positively regulate the promoter activity in pig kidney cells. Interspecies comparison of 5'UTRs and 5'flanking regions shows high homology between pig and cattle, and Sp1 binding sites existing in genomic regions corresponding Ph region are evolutionally conserved.

Cancer intelligence acquired (CIA): tumor glycosylation and sialylation codes dismantling antitumor defense

Aberrant glycosylation is a key feature of malignant transformation and reflects epigenetic and genetic anomalies among the multitude of molecules involved in glycan biosynthesis. Although glycan biosynthesis is not template bound, altered tumor glycosylation is not random, but associated with common glycosylation patterns. Evidence suggests that acquisition of distinct glycosylation patterns evolves from a 'microevolutionary' process conferring advantages in terms of tumor growth, tumor dissemination, and immune escape. Such glycosylation modifications also involve xeno- and hypersialylation. Xeno-autoantigens such as Neu5Gc-gangliosides provide potential targets for immunotherapy. Hypersialylation may display 'enhanced self' to escape immunosurveillance and involves several not mutually exclusive inhibitory pathways that all rely on protein-glycan interactions. A better understanding of tumor 'glycan codes' as deciphered by lectins, such as siglecs, selectins, C-type lectins and galectins, may lead to novel treatment strategies, not only in cancer, but also in autoimmune disease or transplantation.

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.

Colloquium paper: uniquely human evolution of sialic acid genetics and biology

Darwinian evolution of humans from our common ancestors with nonhuman primates involved many gene-environment interactions at the population level, and the resulting human-specific genetic changes must contribute to the "Human Condition." Recent data indicate that the biology of sialic acids (which directly involves less than 60 genes) shows more than 10 uniquely human genetic changes in comparison with our closest evolutionary relatives. Known outcomes are tissue-specific changes in abundant cell-surface glycans, changes in specificity and/or expression of multiple proteins that recognize these glycans, and novel pathogen regimes. Specific events include Alu-mediated inactivation of the CMAH gene, resulting in loss of synthesis of the Sia N-glycolylneuraminic acid (Neu5Gc) and increase in expression of the precursor N-acetylneuraminic acid (Neu5Ac); increased expression of alpha2-6-linked Sias (likely because of changed expression of ST6GALI); and multiple changes in SIGLEC genes encoding Sia-recognizing Ig-like lectins (Siglecs). The last includes binding specificity changes (in Siglecs -5, -7, -9, -11, and -12); expression pattern changes (in Siglecs -1, -5, -6, and -11); gene conversion (SIGLEC11); and deletion or pseudogenization (SIGLEC13, SIGLEC14, and SIGLEC16). A nongenetic outcome of the CMAH mutation is human metabolic incorporation of foreign dietary Neu5Gc, in the face of circulating anti-Neu5Gc antibodies, generating a novel "xeno-auto-antigen" situation. Taken together, these data suggest that both the genes associated with Sia biology and the related impacts of the environment comprise a relative "hot spot" of genetic and physiological changes in human evolution, with implications for uniquely human features both in health and disease.

Germinal center marker GL7 probes activation-dependent repression of N-glycolylneuraminic acid, a sialic acid species involved in the negative modulation of B-cell activation

Sialic acid (Sia) is a family of acidic nine-carbon sugars that occupies the nonreducing terminus of glycan chains. Diversity of Sia is achieved by variation in the linkage to the underlying sugar and modification of the Sia molecule. Here we identified Sia-dependent epitope specificity for GL7, a rat monoclonal antibody, to probe germinal centers upon T cell-dependent immunity. GL7 recognizes sialylated glycan(s), the alpha2,6-linked N-acetylneuraminic acid (Neu5Ac) on a lactosamine glycan chain(s), in both Sia modification- and Sia linkage-dependent manners. In mouse germinal center B cells, the expression of the GL7 epitope was upregulated due to the in situ repression of CMP-Neu5Ac hydroxylase (Cmah), the enzyme responsible for Sia modification of Neu5Ac to Neu5Gc. Such Cmah repression caused activation-dependent dynamic reduction of CD22 ligand expression without losing alpha2,6-linked sialylation in germinal centers. The in vivo function of Cmah was analyzed using gene-disrupted mice. Phenotypic analyses showed that Neu5Gc glycan functions as a negative regulator for B-cell activation in assays of T-cell-independent immunization response and splenic B-cell proliferation. Thus, Neu5Gc is required for optimal negative regulation, and the reaction is specifically suppressed in activated B cells, i.e., germinal center B cells.

Genetic basis for the lack of N-glycolylneuraminic acid expression in human tissues and its implication to human evolution

Sialic acid is a family of acidic monosaccharides and consists of over 30 derivatives. Two major derivatives are N-acetylneuraminic acid (NeuAc) and N-glycolylneuraminic acid (NeuGc), and the hydroxylation of CMP-NeuAc is the rate limiting reaction for the production of NeuGc. The hydroxylation was carried out by a complex formed with hydroxylase, cytochrome b5, and NADH-cytochrome b5 reductase. Mouse hydroxylase was purified from the cytosolic fraction of the liver and its cDNA was cloned. Normal human tissues do not contain NeuGc. Human hydroxylase cDNA was also cloned and the sequence revealed that human hydroxylase has 92 bp deletion. The deletion is the cause of defective expression of NeuGc in human. Chimpanzee has intact hydroxylase gene and the 92 bp deletion occurred after the divergence of human ancestor from chimpanzee ancestor. Biochemical and molecular biological studies on the biosynthesis of NeuGc and biological functions of NeuGc are reviewed.

Reduction of CMP-N-acetylneuraminic acid hydroxylase activity in engineered Chinese hamster ovary cells using an antisense-RNA strategy

Rodent cells, widely used for the industrial production of recombinant human glycoproteins, possess CMP-N-acetylneuraminic acid hydroxylase (CMP-Neu5Ac hydroxylase; EC 1.14.13.45) which is the key enzyme in the formation of the sialic acid, N-glycolylneuraminic acid (Neu5Gc). This enzyme is not expressed in an active form in man and evidence suggests that the presence of Neu5Gc in recombinant therapeutic glycoproteins may elicit an immune response. The aim of this work was, therefore, to reduce CMP-Neu5Ac hydroxylase activity in a Chinese Hamster Ovary (CHO) cell line, and thus the Neu5Gc content of the resulting glycoconjugates, using a rational antisense RNA approach. For this purpose, the cDNA of the hamster hydroxylase was partially cloned and sequenced. Based on the sequence of the mouse and hamster cDNAs, optimal antisense RNA fragments were selected from preliminary in vitro translation tests. Compared to the parental cell line, the new strain (CHO-AsUH2), which was transfected with a 199-bp antisense fragment derived from the mouse CMP-Neu5Ac hydroxylase cDNA, showed an 80% reduction in hydroxylase activity. An analysis of the sialic acids present in the cells' own glycoconjugates revealed a decrease in the percentage of Neu5Gc residues from 4% in the parental cells to less than 1% in the CHO-AsUH2 cell line.

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.

Anti-N-glycolylneuraminic acid antibodies identified in healthy human serum

The first and major clinical obstacle in xenotransplantation is antibody-mediated hyperacute rejection. Although human natural antibodies against Galalpha1,3Gal (Gal) antigens, which are common on porcine cells and organs, have been identified to play a major role in hyperacute rejection, other natural antibodies against non-Gal epitopes may be also involved in the process. Here, we present evidence suggesting that the majority of human anti-non-Gal antibodies are specific for carbohydrate structures carrying terminally linked N-glycolylneuraminic acid (NeuGc), a xenoantigen existing in almost all animals except humans. Furthermore, this anti-NeuGc activity is detectable in 85% of healthy humans, implicating the involvement of NeuGc in hyperacute rejection and the importance of developing strategies for removing NeuGc for clinical xenotransplantation.

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.

N-Glycolylneuraminic acid in human tumours

N-Glycolylneuraminic acid (Neu5Gc) is an abundant sialic acid, occurring in the glycoconjugates of most deuterostome animals. Homo sapiens is a notable exception, since Neu5Gc is effectively absent from normal human tissues. This is due to a deletion in the human gene coding for CMP-Neu5Ac hydroxylase, the enzyme usually responsible for Neu5Gc biosynthesis. Despite this mutation, persistent reports in the literature suggest that Neu5Gc occurs in the glycoconjugates of many human tumours, where it might be responsible for the formation of so-called Hanganutziu-Deicher antibodies. However, the variety of systems studied and the various experimental approaches adopted have yielded a complex picture of Neu5Gc occurrence in human neoplasias. The aim of this paper is therefore to provide a critical review of the evidence for Neu5Gc in human tumours, paying particular attention to the analytical methods employed. The possible clinical applications of Neu5Gc-containing glycoconjugates and Hanganutziu-Deicher antibodies in the diagnosis and treatment of breast cancer and melanoma are also discussed. In view of the lack of CMP-Neu5Ac hydroxylase in human cells, alternative metabolic pathways for the biosynthesis of glycoconjugate-bound Neu5Gc are considered.

Purification and identification of N-glycolylneuraminic acid (Neu5Gc) from the holothuroidea Gumi, Cucumaria echinata

N-Glycolylneuraminic acid (Neu5Gc), precious sialic acid which could not be synthesized by a chemical method, occurrs in the body of holothuroidea, Gumi Cucumaria echinata. Gumi contains 85% of total sialic acid, as Neu5Gc, in the body. Neu5Gc was purified from dry powder of the body using Dowex 1-x8 (HCOO* form) anion exchange chromatography after mild acid hydrolysis with 0.1 N trifluoroacetic acid. Using GC-MS and 1H-NMR spectroscopy, the purified Neu5Gc was correctly identified to be Neu5Gc. The purity of Neu5Gc was more than 99%. This is the first report of purification and identification of Neu5Gc from holothuroidea by using anion exchange chromatography, GC-MS, and 1H-NMR.

Distribution and localization of CMP-N-acetylneuraminic acid hydroxylase and N-glycolylneuraminic acid-containing glycoconjugates in porcine lymph node and peripheral blood lymphocytes

An immunohistochemical analysis was performed on paraplast-embedded sections of porcine lymph node with antibodies specific for CMP-N-acetylneuraminic acid hydroxylase (h-3 antibody) and glycoconjugate-bound N-glycolylneuraminic acid (Neu5Gc), which appears as a result of the hydroxylase reaction (a-Gc antibody). The observed localization of the enzyme in cells of the perifollicular zone, including lymphocytes, was reflected in a similar distribution of glycoconjugate-bound Neu5Gc. This result confirms previous biochemical investigations on the role of the hydroxylase in regulating Neu5Gc biosynthesis in vitro on a histological level. An analysis of lymphocytes isolated from porcine thymus, spleen, lymph node and peripheral blood revealed differences in the amount of Neu5Gc in the various lymphocytes that correlated well with the activity of the hydroxylase determined in these cells. The largest amount of Neu5Gc and highest activity of the enzyme were detected in the peripheral blood lymphocytes (PBL). Immunohistochemical studies with a-Gc and h-3 antibodies on sections of paraplast-embedded PBL showed that these antigens were located at the cell surface and in the cytosol, respectively. Ultrastructural immunocytochemistry with the h-3 antibody and immunogold labelling was used to investigate the subcellular localization of the hydroxylase. The enzyme was detected in the cytosol in the vicinity of the nuclear membrane and the outer membrane of mitochondria, in particular those close to the nucleus. The antigen was also detected on cytoplasmic tubular structures. In addition, a weak labelling of the Golgi apparatus was also observed occasionally. The possibility that this localization may be related to the availability of the substrate CMP-Neu5Ac and the redox partner cytochrome b5 is discussed.

Substitution of amino acid residue in influenza A virus hemagglutinin affects recognition of sialyl-oligosaccharides containing N-glycolylneuraminic acid

Sialic acids are essential components of cell surface receptors used by influenza viruses. To determine the molecular mechanisms of viral recognition of two major species of sialic acids, N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc), we tested the binding reactivity of nine human H3 influenza A viruses to sialylglycolipids containing type II sugar chain and different molecular species of terminal sialic acids. All human H3 viruses tested except A/Memphis/1/71 bound both Neu5Ac and Neu5Gc. Nucleotide sequence analysis suggests that amino acids at 143, 155, and 158 are linked to the viral recognition of Neu5Gc.

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.

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.

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.

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.

Structural analysis of the sialylated N- and O-linked carbohydrate chains of recombinant human erythropoietin expressed in Chinese hamster ovary cells. Sialylation patterns and branch location of dimeric N-acetyllactosamine units

The N-linked carbohydrate chains of recombinant human erythropoietin expressed in CHO cells were quantitatively released with peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F, separated from the remaining O-glycoprotein by gel-permeation chromatography, and subsequently fractionated via FPLC on Mono Q, HPLC on Lichrosorb-NH2 and high-pH anion-exchange chromatography on CarboPac PA1. The purified sialylated oligosaccharides were analyzed by one-dimensional and two-dimensional 500-MHz 1H-NMR spectroscopy. When necessary, oligosaccharides were treated with endo-beta-galactosidase (and N-acetyl-beta-glucosaminidase) followed by 1H-NMR analysis of the incubation products, to obtain additional structural information. Di-, tri-, tri'- and tetraantennary N-acetyllactosamine-type oligosaccharides occur which can be completely (major) or partially (minor) sialylated. Three different types of alpha 2-3-linked sialic acids are present, namely, N-acetylneuraminic acid (95%), N-glycolylneuraminic acid (2%) and N-acetyl-9-O-acetylneuraminic acid (3%). In the case of partial sialylation, a non-random distribution of the sialic acids over the branches is observed. One or two extra N-acetyllactosamine units, being exclusively located in the branches attached to the alpha 1-6-linked Man residue, can be present in completely or partially sialylated di-, tri'-, and tetraantennary oligosaccharides. Tetraantennary oligosaccharides with N-acetyllactosamine repeats could be digested quantitatively with endo-beta-galactosidase from Bacteroides fragilis, whereas under the same conditions tri' antennary oligosaccharides hardly reacted (< 15%). Using endo-beta-galactosidase from Escherichia freundii, these tri'antennary oligosaccharides could be digested more extensively (> 75%). The O-linked carbohydrate chains were released from the O-glycoprotein by alkaline borohydride treatment, and purified via FPLC on Mono Q and HPLC on Lichrosorb-NH2. Two O-glycans were found, namely, Neu5Ac alpha 2-3Gal beta 1-3GalNAc-ol and Neu5Ac alpha 2-3Gal beta 1-3(Neu5Ac alpha 2-6)GalNAc-ol.

Combined chemical and enzymatic synthesis of the sialylated non reducing terminal sequence of GM1b glycolylated ganglioside, a potential human tumor marker

N-Glycolylglucosamine 8 was synthesized in 4 steps from anisal glucosamine, via the new crystalline monochloracetyl derivatives 3, 4 and 7. N-Glycolylneuraminic acid 10 was prepared in 59% yield starting from pyruvate and a mixture of 8 and its manno epimer 9 in a 2:3 ratio, with immobilized sialic acid aldolase. Neu5Gc 10 was converted into CMP-NeuGc 11 in the presence of immobilized calf brain CMP-sialate synthetase. Finally 11 was used as a donor in the transfer to the acceptor beta-D-Gal-(1-3)-beta-D-GalNAc-OBn 12 catalyzed by a preparation of porcine liver (2-3)-alpha-sialyltransferase, roughly purified by a chromatography on Cibacron Blue-agarose. alpha-Neu5Gc-(2-3)-beta-D-Gal-(1-3)-beta-D-GalNac-OBn 13 isolated in 56% yield was deprotected to give the non-reducing terminal sequence of GM1b glycolylated ganglioside, which might be expressed in human tumors.

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.

Catalytic properties of the CMP-N-acetylneuraminic acid hydroxylase from the starfish Asterias rubens: comparison with the mammalian enzyme

The biosynthesis of N-glycolylneuraminic acid (Neu5Gc) was investigated in cell-free extracts of the starfish Asterias rubens, which is one of the evolutionarily least-advanced species known to possess Neu5Gc-containing glycoconjugates. As in higher animals, Neu5Gc is synthesised in Asterias rubens by the action of a CMP-Neu5Ac hydroxylase. Enzyme activity was detected in all starfish tissues tested, the turnover being the greatest in the gonads. The enzyme from this tissue has a temperature optimum between 25 and 33 degrees C and a pH optimum between pH 6.0 and 6.4. This hydroxylase exhibits many characteristics in common with the mammalian enzyme. For example, the enzyme is extracted in a predominantly soluble form. Oxygen and a reduced pyridine nucleotide are necessary for activity, with NADH being the most effective cofactor. Furthermore, the activation of the hydroxylase by exogenously added iron salts and the potent inhibitory effects of several iron ligands point to the involvement of a non-haem iron cofactor. The enzyme has a high affinity for the substrate CMP-Neu5Ac, the apparent Km being 18 microM. In contrast to the mammalian enzyme, the hydroxylase from Asterias rubens is not inhibited by increased ionic strength and cannot be activated by non-ionic detergents. Moreover, the CMP-Neu5Ac turnover increased linearily with increasing protein concentration. In accordance with other enzymes in starfish, seasonal changes in the CMP-Neu5Ac hydroxylase activity were also observed.

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.

Comparison of the N-glycoloylneuraminic and N-acetylneuraminic acid content of platelets and their precursors using high performance anion exchange chromatography

N-Acetylneuraminic acid (Neu5Ac) and N-glycoloylneuraminic acid (Neu5Gc) are distributed widely in nature. Using a Carbopac PA-1 anion exchange column, we have determined the ratios of Neu5Ac and Neu5Gc in hydrolysates of platelets and their precursors: a rat promegakaryoblastic (RPM) cell line and a human megakaryoblastic leukemia cell line (MEG-01). The ratio of Neu5Gc:Neu5Ac in cultured RPM cells is 16:1, whereas in platelet rich plasma and cultured MEG-01 cells it is 1:38 and 1:28, respectively. The nature of these sialic acids from RPM cells was verified using thin layer chromatography and liquid secondary ion mass spectrometry. The relevance of increased Neu5Gc levels in early stages of development is discussed.

Mouse liver cytidine-5'-monophosphate-N-acetylneuraminic acid hydroxylase. Catalytic function and regulation

In this paper, we present the results of an investigation into the catalytic properties of CMP-Neu5Ac hydroxylase (Neu5Ac: N-acetylneuraminic acid) in high-speed supernatants of mouse liver. The enzyme was most active in Hepes/NaOH pH 7.4 and was markedly inhibited by relatively small increases in ionic strength, though the inhibition was abolished by desalting procedures. Several nonionic detergents could activate the hydroxylase to various degrees in a concentration-dependent manner. Ionic detergents and a number of phospholipids were, however, generally inert or inhibitory. The lack of inhibitory influence of a wide range of nucleotides revealed that CMP-Neu5Ac hydroxylase binds its sugar-nucleotide substrate with a high degree of specificity. Thus, even millimolar concentrations of several cytidine nucleotides elicited virtually negligible inhibition, though the reaction product, CMP-Neu5Gc (Neu5Gc: N-glycoloylneuraminic acid), was a weak inhibitor. The results also indicate that the enzyme is not regulated by any nucleotides or sugar-nucleotides. Dilution of high-speed supernatants with buffer gave rise to a decrease in the specific activity of the hydroxylase, implicating the involvement of more than one component in catalysis. Activity could be restored by the addition of a heat extract of the supernatant. The active principle in this extract was found to be a heat-stable protein with a molecular mass of about 17 kDa. Immunochemical studies allowed this protein to be identified as cytochrome b5 and it was shown that this electron carrier is essential for the activity of CMP-Neu5Ac hydroxylase. Inhibition studies using iron ligands and activation by exogenous iron salts suggest the involvement of a non-haem iron cofactor in the catalytic cycle of this hydroxylase. Cytochrome b5 may thus serve as an electron donor for this postulated cofactor.

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.

CMP-N-acetylneuraminic acid hydroxylase activity determines the wheat germ agglutinin-binding phenotype in two mutants of the lymphoma cell line MDAY-D2

The dominant glycosylation mutants of MDAY-D2 mouse lymphoma cells, designated class 2 (D33W25 and D34W25) were selected for their resistance to the toxic effects of wheat germ agglutinin (WGA) and shown to express elevated levels of Neu5Gc. In accordance with this, the activity of CMP-Neu5Ac hydroxylase was found to be substantially higher in the mutant cells. The hydroxylase in the D33W25 mutant cells exhibited kinetic properties identical to those of the same enzyme from mouse liver. Growth rate experiments in vivo and in vitro, where the mutant cells grew more slowly at low cell densities in serum-free medium and also formed slower growing tumours in syngeneic mice, indicate that CMP-Neu5Ac hydroxylase expression may be associated with altered growth of the mutant cells.

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.