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

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.

Biochemical, Cellular, Physiological, and Pathological Consequences of Human Loss of N-Glycolylneuraminic Acid

About 2-3 million years ago, Alu-mediated deletion of a critical exon in the CMAH gene became fixed in the hominin lineage ancestral to humans, possibly through a stepwise process of selection by pathogen targeting of the CMAH product (the sialic acid Neu5Gc), followed by reproductive isolation through female anti-Neu5Gc antibodies. Loss of CMAH has occurred independently in some other lineages, but is functionally intact in Old World primates, including our closest relatives, the chimpanzee. Although the biophysical and biochemical ramifications of losing tens of millions of Neu5Gc hydroxy groups at most cell surfaces remains poorly understood, we do know that there are multiscale effects functionally relevant to both sides of the host-pathogen interface. Hominin CMAH loss might also contribute to understanding human evolution, at the time when our ancestors were starting to use stone tools, increasing their consumption of meat, and possibly hunting. Comparisons with chimpanzees within ethical and practical limitations have revealed some consequences of human CMAH loss, but more has been learned by using a mouse model with a human-like Cmah inactivation. For example, such mice can develop antibodies against Neu5Gc that could affect inflammatory processes like cancer progression in the face of Neu5Gc metabolic incorporation from red meats, display a hyper-reactive immune system, a human-like tendency for delayed wound healing, late-onset hearing loss, insulin resistance, susceptibility to muscular dystrophy pathologies, and increased sensitivity to multiple human-adapted pathogens involving sialic acids. Further studies in such mice could provide a model for other human-specific processes and pathologies involving sialic acid biology that have yet to be explored.

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.

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 deficiency in humans

Classic studies suggested that the common mammalian sialic acid N-glycolylneuraminic acid (Neu5Gc) is an oncofetal antigen in humans, being immunogenic in adult humans and yet apparently expressed in human fetuses and tumors. We and others have recently found that the human deficiency of Neu5Gc can be explained by an inactivating mutation in the gene encoding CMP-N-acetylneuraminic acid hydroxylase. Thus, Neu5Gc is not an oncofetal antigen in the classical sense, and other explanations must be found for the observed expression pattern. This review provides an update on this matter, and considers a variety of other old and new questions that arise from it.

A comprehensive phylogenetic analysis of Rieske and Rieske-type iron-sulfur proteins

The Rieske iron-sulfur center consists of a [2Fe-2S] cluster liganded to a protein via two histidine and two cysteine residues present in conserved sequences called Rieske motifs. Two protein families possessing Rieske centers have been defined. The Rieske proteins occur as subunits in the cytochrome bc1 and cytochrome b6f complexes of prokaryotes and eukaryotes or form components of archaeal electron transport systems. The Rieske-type proteins encompass a group of bacterial oxygenases and ferredoxins. Recent studies have uncovered several new proteins containing Rieske centers, including archaeal Rieske proteins, bacterial oxygenases, bacterial ferredoxins, and, intriguingly, eukaryotic Rieske oxygenases. Since all these proteins contain a Rieske motif, they probably form a superfamily with one common ancestor. Phylogenetic analyses have, however, been generally limited to similar sequences, providing little information about relationships within the whole group of these proteins. The aim of this work is, therefore, to construct a dendrogram including representatives from all Rieske and Rieske-type protein classes in order to gain insight into their evolutionary relationships and to further define the phylogenetic niches occupied by the recently discovered proteins mentioned above.

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.

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.

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.

CMP-N-acetylneuraminic acid hydroxylase: the first cytosolic Rieske iron-sulphur protein to be described in Eukarya

Electron paramagnetic resonance (EPR) spectroscopy and analysis of the primary structure of the CMP-N-acetylneuraminic acid hydroxylase revealed that this enzyme is the first iron-sulphur protein of the Rieske type to be found in the cytosol of Eukarya. The dithionite-reduced hydroxylase exhibited an EPR signal known to be characteristic for a Rieske iron-sulphur centre (2Fe-2S), the g-values being 1.78, 1.91 and 2.01, respectively. An analysis of the primary structure of the hydroxylase led to the identification of an amino acid sequence, known to be characteristic for Rieske proteins. Furthermore, possible binding sites for cytochrome b5, the substrate CMP-Neu5Ac and a mononuclear iron centre were also identified.