The readers NIH.
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Financing, Government↗
Biomedical subjects
A Varki
Publications and source records attributed to A Varki.
Structural and immunological characterization of O-acetylated GD2. Evidence that GD2 is an acceptor for ganglioside O-acetyltransferase in human melanoma cells.
We have shown previously that Golgi-enriched vesicles from the human melanoma cell line Melur can transfer [3H]acetate from [acetyl-3H]acetyl-CoA to endogenous GD3 to form [acetyl-3H]O-acetyl-GD3 (Manzi, A. E., Sjoberg, E. R., Diaz, S., and Varki, A. (1990) J. Biol. Chem. 265, 13091-13103). Applying the same approach in the human melanoma cell line M21, label was found in [acetyl-3H]O-acetyl-GD3 and also in a species co-migrating with unsubstituted GD3 on TLC. Both were sialidase-sensitive and alkali-labile, indicating incorporation as [3H]O-acetyl esters on sialic acids. Immunological reactivity, sialidase sensitivity, chromatographic behavior, and the known ganglioside pattern of M21 cells suggested that the slower migrating species might be [acetyl-3H]O-acetyl-GD2. Sialic acids released from this labeled molecule by sialidase showed esterification with [3H]acetate at both C7 and C9 hydroxyls. Lipid extracts from cells metabolically labeled with [3H]galactose showed a corresponding ganglioside, which upon alkali treatment yielded a species migrating with GD2. Analysis of purified ganglioside by high performance thin layer chromatography immuno-overlays, fast atom bombardment-mass spectrometry in positive and negative ion modes, periodate oxidation resistance, linkage analysis by permethylation and gas chromatography-mass spectrometry, and 500 MHz 1H NMR was consistent with the following structure: 9-O Ac-Neu5Ac alpha 2-8Neu5Ac alpha 2-3(GalNAc beta 1-4) Gal beta 1-4Gluc beta 1-1' ceramide Total gangliosides from M21 were analyzed by high performance thin layer chromatography immuno-overlay with monoclonal antibodies D1.1, JONES, 27A, and 8A2, all known to, or suspected of reacting with 9-O-acetylated gangliosides. The first three bound well to 9-O-acetyl-GD3 and a slower migrating 9-O-acetylated ganglioside, which was distinct from 9-O-acetyl-GD2. Antibody 8A2 reacted weakly with purified 9-O-acetyl-GD2 and strongly with two other 9-O-acetylated gangliosides migrating slower than 9-O-acetyl-GD2. Thus, the family of O-acetylated gangliosides in melanoma cells is much more complex than previously appreciated.
Acetyl Coenzyme A↗
Of pride, prejudice, and discrimination. Why generalizations can be unfair to the individual.
The categorization of individuals into groups can serve some useful purposes. If the individuals being grouped are heterogeneous, however, the categories used are likely to be flawed. Thus, any resulting generalizations are also flawed, especially when applied to a given individual in a group. The most serious attendant dangers can be misplaced pride, misguided prejudice, and unfair discrimination. The categorization of physicians according to their original medical training and the generalizations that result are examined here. The principles advanced could also be applied to many other forms of categorization.
Foreign Medical Graduates↗
The spectrum of incomplete N-linked oligosaccharides synthesized by endothelial cells in the presence of brefeldin A.
Previous studies in many cell lines have shown that Brefeldin A (BFA) inhibits the forward movement of newly synthesized glycoconjugates by fusing the cis-, medial-, and trans-Golgi compartments with the rough endoplasmic reticulum. Studies on the oligosaccharide processing of individual glycoproteins have yielded confusing and incomplete results regarding the location of the block. Assuming that all glycoproteins with N-linked oligosaccharides follow the same endoplasmic reticulum to the Golgi pathway, a more complete picture on the location and nature of the block can be determined by analyzing N-linked oligosaccharides synthesized in the presence of BFA. In bovine pulmonary artery endothelial cells, BFA (0.1 microgram/ml) reversibly inhibits the secretion of greater than 95% of Tran35S and [3H]Man-labeled glycoproteins without affecting protein synthesis or N-linked glycosylation. In addition, BFA inhibits the synthesis and secretion of 35SO4-labeled oligosaccharides. Initial oligosaccharide trimming is uninhibited, but further processing is affected since the majority (65%) of the chains terminate only in beta-GlcNAc residues. Concomitantly, the proportion of [3H]Man-labeled N-linked anionic oligosaccharides is reduced from 60 to 20%, and the great majority of the charge is due to one sialic acid. The rate-limiting step for sialylation appears to be the branch selective addition of beta-Gal residues. The remaining charge is due to sulfate esters (0.6%) which normally account for greater than 10% of the anionic substituents. BFA also reduces the amount of phosphorylated chains by 80% and greatly diminishes further phosphodiester processing since the majority of these oligosaccharides (60%) contain a Man-6-PO4 residue in an acid-sensitive diester linkage. The addition of all polylactosamine chains, outer-branch fucose and terminal alpha-Gal residues are completely inhibited by BFA. Secretion, fucosylation, and sialylation are completely restored when BFA is removed, but the other modification steps are only partially restored. Our results indicate that addition of sulfate esters, terminal alpha-Gal residues, polylactosamine chains, outer-branch fucose residues, some initial phosphorylation, and most phosphodiester processing may occur beyond a compartment where some beta-Gal and sialic acid residues can be added. Essentially, all of the effects on oligosaccharide processing are partially or completely reversible.
Amidohydrolases↗
The synthesis of high-specific-activity UDP-[6-3H]galactose, UDP-N-[6-3H]acetylgalactosamine, and their corresponding monosaccharides.
Tritiated uridine-5'-diphosphogalactose (UDP-[3H]Gal) has been widely used to study oligosaccharide biosynthesis and structure. It can be synthesized either chemically or enzymatically using galactose oxidase to oxidize the hydroxyl moiety at C-6 to an aldehyde (6-aldo-UDP-Gal), which is then reduced back to the alcohol with tritiated sodium borohydride. Although the enzymatic approach is simple and efficient, there are several problems associated with it. First, incomplete oxidation to the aldehyde reduces the final specific activity. Second, if the galactose oxidase is not removed from the 6-aldo-UDP-Gal prior to reduction, the resulting UDP-[6-3H]Gal can be reoxidized to 6-aldo-UDP-[6-3H]Gal. We present evidence for the occurrence of this compound in one commercially obtained preparation of UDP-[6-3H]Gal. Finally, if an excess of 6-aldo-UDP-Gal is used for good yield, it is necessary to quench the reduction with nonradioactive borohydride, again reducing the final specific activity. We have devised a rapid, inexpensive, and efficient synthesis of UDP-[6-3H]Gal that circumvents all of these problems. Galactose oxidase is used to produce 6-aldo-UDP-Gal and the completeness of this reaction is confirmed on polyethyleneimine (PEI) cellulose TLC plates. The 6-aldo-UDP-Gal is purified on silica gel 60 TLC plates. This purified compound is then reduced with tritiated sodium borohydride, with the aldehyde present in excess. Unreacted 6-aldo-UDP-Gal is then purified away from the product UDP-[6-3H]Gal by chromatography on PEI cellulose. Radiochemically pure UDP-[6-3H]Gal with a specific activity of 10 Ci/mmol was obtained using the above scheme.(ABSTRACT TRUNCATED AT 250 WORDS)
Chromatography, High Pressure Liquid↗
Selectins and other mammalian sialic acid-binding lectins.
Several recently discovered mammalian cell adhesion proteins recognize and bind to sialic acid-containing ligands. Reports concerning the molecular specificities of these interactions have been intriguing but somewhat confusing, partly because of pitfalls in methodology or interpretation. Nevertheless, these protein-carbohydrate recognition phenomena are important in the normal biology of blood cells and in the pathophysiology of many diseases.
Animals↗
Identification of a specific glycoprotein ligand for P-selectin (CD62) on myeloid cells.
P-selectin (CD62, GMP-140, PADGEM), a Ca(2+)-dependent lectin on activated platelets and endothelium, functions as a receptor for myeloid cells by interacting with sialylated, fucosylated lactosaminoglycans. P-selectin binds to a limited number of protease-sensitive sites on myeloid cells, but the protein(s) that carry the glycans recognized by P-selectin are unknown. Blotting of neutrophil or HL-60 cell membrane extracts with [125I]P-selectin and affinity chromatography of [3H]glucosamine-labeled HL-60 cell extracts were used to identify P-selectin ligands. A major ligand was identified with an approximately 250,000 M(r) under nonreducing conditions and approximately 120,000 under reducing conditions. Binding of P-selectin to the ligand was Ca2+ dependent and was blocked by mAbs to P-selectin. Brief sialidase digestion of the ligand increased its apparent molecular weight; however, prolonged digestion abolished binding of P-selectin. Peptide:N-glycosidase F treatment reduced the apparent molecular weight of the ligand by approximately 3,000 but did not affect P-selectin binding. Western blot and immunodepletion experiments indicated that the ligand was not lamp-1, lamp-2, or L-selectin, which carry sialyl Le(x), nor was it leukosialin, a heavily sialylated glycoprotein of similar molecular weight. The preferential interaction of the ligand with P-selectin suggests that it may play a role in adhesion of myeloid cells to activated platelets and endothelial cells.
Antibodies, Monoclonal↗
Diversity in the sialic acids.
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Acetylation↗
The times they are a'changin': changing with the times.
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Publishing↗
Developmental abnormalities in transgenic mice expressing a sialic acid-specific 9-O-acetylesterase.
9-O-acetylation of sialic acids is tissue specific and developmentally regulated. We have selectively destroyed these O-acetyl groups during murine embryogenesis by expressing the 9-O-acetyl-sialic acid-specific esterase of influenza C. DNA constructs driven by the metallothionein promoter arrested development at the 2-cell stage and gave a markedly decreased yield of live mice. A similar construct driven by the phenylethanolamine-N-methyltransferase promoter did not cause this block, but gave transgenic mice with selective expression of esterase in the retina and the adrenal gland. These organs showed variable abnormalities in organization, while all other tissues examined appeared normal. The ganglioside 9-O-acetyl-GD3 was selectively destroyed in target tissues. Thus, 9-O-acetylated sialic acids may play an role in murine development at the 2-cell stage and in certain differentiated tissues.
Acetylation↗
GMP-140 binds to a glycoprotein receptor on human neutrophils: evidence for a lectin-like interaction.
GMP-140 is a rapidly inducible receptor for neutrophils and monocytes expressed on activated platelets and endothelial cells. It is a member of the selectin family of lectin-like cell surface molecules that mediate leukocyte adhesion. We used a radioligand binding assay to characterize the interaction of purified GMP-140 with human neutrophils. Unstimulated neutrophils rapidly bound [125I]GMP-140 at 4 degrees C, reaching equilibrium in 10-15 min. Binding was Ca2+ dependent, reversible, and saturable at 3-6 nM free GMP-140 with half-maximal binding at approximately 1.5 nM. Receptor density and apparent affinity were not altered when neutrophils were stimulated with 4 beta-phorbol 12-myristate 13-acetate. Treatment of neutrophils with proteases abolished specific binding of [125I]GMP-140. Binding was also diminished when neutrophils were treated with neuraminidase from Vibrio cholerae, which cleaves alpha 2-3-, alpha 2-6-, and alpha 2-8-linked sialic acids, or from Newcastle disease virus, which cleaves only alpha 2-3- and alpha 2-8-linked sialic acids. Binding was not inhibited by an mAb to the abundant myeloid oligosaccharide, Lex (CD15), or by the neoglycoproteins Lex-BSA and sialyl-Lex-BSA. We conclude that neutrophils constitutively express a glycoprotein receptor for GMP-140, which contains sialic acid residues that are essential for function. These findings support the concept that GMP-140 interacts with leukocytes by a lectin-like mechanism.
Antibodies, Monoclonal↗
The selectin GMP-140 binds to sialylated, fucosylated lactosaminoglycans on both myeloid and nonmyeloid cells.
Granule membrane protein-140 (GMP-140) is an inducible receptor for myeloid leukocytes on activated platelets and endothelium. Like other selectins, GMP-140 recognizes specific oligosaccharide ligands. However, prior data on the nature of these ligands are contradictory. We investigated the structural features required for ligand interaction with GMP-140 using purified GMP-140, cells naturally expressing specific oligosaccharides, and cells expressing cloned glycosyltransferases. Like the related selectin endothelial leukocyte adhesion molecule-1 (ELAM-1), GMP-140 recognizes alpha(2-3)sialylated, alpha(1-3)fucosylated lactosaminoglycans on both myeloid and nonmyeloid cells, including the sequence Neu5Ac alpha 2-3Gal beta 1-4(Fuc alpha 1-3)GlcNac beta-R (sialyl Lewis x). Recognition requires sialic acid, because cells expressing large amounts of Lewis x, but not sialyl Lewis x, do not interact with GMP-140. Although sialyl Lewis x is expressed by both myeloid HL-60 cells and CHO cells transfected with an alpha 1-3/4 fucosyltransferase, GMP-140 binds with significantly higher affinity to HL-60 cells. Thus, the sialyl Lewis x tetrasaccharide may require additional structural modifications or specific presentations in order for leukocytes in flowing blood to interact rapidly and with high affinity to GMP-140 on activated platelets or endothelium.
Amino Sugars↗
Radioactive tracer techniques in the sequencing of glycoprotein oligosaccharides.
Complete sequencing of the oligosaccharide units of glycoproteins can be performed by conventional physical techniques if nanomole quantities of pure molecules are available. However, isolation of sufficient quantities of a glycoprotein may not be technically feasible (e.g., the analysis of biosynthetic intermediates, or rare molecules). Alternatively, partial structural analyses may answer the biological question at hand. In both instances, radioactive sugars can be used to metabolically label the oligosaccharide units of a glycoprotein, permitting substantial structural characterization. Several aspects of this approach are discussed in this overview, including selection of the labeled precursor, maximization of uptake and incorporation, determinants of the specificity of labeling, and general principles for the release and structural analysis of labeled oligosaccharides. Particular advantages include simplicity, ease of use without sophisticated instrumentation, and the fact that purification to radiometric homogeneity is sufficient. Radioactive tracer techniques cannot replace conventional approaches to sequencing oligosaccharides. However, they do provide a rapid, relatively simple approach to obtaining considerable information from limited amounts of material. For molecules such as short-lived biosynthetic intermediates, there is no substitute for these techniques. This approach has been responsible for the initial identification and characterization of many novel oligosaccharides of biological interest.
Animals↗
Biosynthesis and turnover of O-acetyl and N-acetyl groups in the gangliosides of human melanoma cells.
We and others previously described the melanoma-associated oncofetal glycosphingolipid antigen 9-O-acetyl-GD3, a disialoganglioside O-acetylated at the 9-position of the outer sialic acid residue. We have now developed methods to examine the biosynthesis and turnover of disialogangliosides in cultured melanoma cells and in Golgi-enriched vesicles from these cells. O-Acetylation was selectively expressed on di- and trisialogangliosides, but not on monosialogangliosides, nor on glycoprotein-bound sialic acids. Double-labeling of cells with [3H]acetate and [14C]glucosamine introduced easily detectable labels into each of the components of the ganglioside molecules. Pulse-chase studies of such doubly labeled molecules indicated that the O-acetyl groups turn over faster than the parent molecule. When Golgi-enriched vesicles from these cells were incubated with [acetyl-3H]acetyl-coenzyme A, the major labeled products were disialogangliosides. [Acetyl-3H]O-acetyl groups were found at both the 7- and the 9-positions, indicating that both 7-O-acetyl GD3 and 9-O-acetyl GD3 were synthesized by the action of O-acetyltransferase(s) on endogenous GD3. Analysis of the metabolically labeled molecules confirmed the existence of both 7- and 9-O-acetylated GD3 in the intact cells. Surprisingly, the major 3H-labeled product of the in vitro labeling reaction was not O-acetyl-GD3, but GD3, with the label exclusively in the sialic acid residues. Fragmentation of the labeled sialic acids by enzymatic and chemical methods showed that the 3H-label was exclusively in [3H]N-acetyl groups. Analyses of the double-labeled sialic acids from intact cells also showed that the 3H-label from [3H]acetate was exclusively in the form of [3H]N-acetyl groups, whereas the 14C-label was at the 4-position. Pulse-chase analysis of the 3H/14C ratio showed that the N-acetyl groups of both GD3 and of the monosialoganglioside GM3 were turning over faster than the parent molecules. Selective periodate oxidation showed that both the inner and outer sialic acid residues of GD3 incorporated 3H-label in the in vitro reaction, and showed similar turnover of N-acetylation in the pulse-chase study. Taken together, these results indicate that both the O- and N-acetyl groups of the sialic acid residues of gangliosides turn over faster than the parent molecules. They also demonstrate a novel re-N-acetylation reaction that predicts the existence of de-N-acetyl gangliosides in melanoma cells.
Acetates↗
Studies of naturally occurring modifications of sialic acids by fast-atom bombardment-mass spectrometry. Analysis of positional isomers by periodate cleavage.
A variety of modifications of sialic acids have been described in nature. There are currently many difficulties in the detection and quantitation of these modified sialic acids from biological sources. We report here that fast-atom bombardment-mass-spectrometry (FAB-MS) of native sialic acids provides specific detection and quantitation of many previously known compounds. Derivatization of the sialic acids by reduction and peracylation under acidic conditions prior to FAB-MS provides further confirmation of their identity and improves the sensitivity of detection. Samples containing as little as 100 ng of a derivatized sialic acid loaded onto the FAB target allowed accurate identification. Mixtures of sialic acids could be analyzed, and minor components were seen, at levels undetectable by other currently known techniques. Analysis of known mixtures of different sialic acids gave reproducible relative signal intensities, indicating that quantitative data can be derived from the FAB-MS spectra. After reduction and peracylation, each sialic acid gave two major molecular ions, corresponding to the fully derivatized linear species and a lactone form, and a minor ion, corresponding to an anhydro form. Lactone formation was minimal in the case of four substituted sialic acids, indicating that the hydroxyl group at the 4-position is involved in lactonization. Differentiation between different positional isomers of the modified sialic acids could be achieved using controlled degradation with periodate, tagging of the fragments with p-aminobenzoic acid ethyl ester under acid reducing conditions, peracylation, and FAB-MS of the derivatized products. We used this FAB-MS strategy to identify a novel sialic acid, 8-O-methyl-7,9-di-O-acetyl-N-glycolyl-neuraminic acid from the starfish Pisaster brevispinus, and to demonstrate the presence of a previously undetected sialic acid, 4,8-anhydro-N-acetyl-neuraminic acid in acid hydrolysates of horse serum. We also use FAB-MS to show that the alkaline conditions traditionally used for analytical de-O-acetylation of sialic acids causes substantial conversion of 4-O-acetylated sialic acids into the same anhydro compound.
Acetylation↗
High-pressure liquid chromatography of sialic acids on a pellicular resin anion-exchange column with pulsed amperometric detection: a comparison with six other systems.
A wide variety of different sialic acids have been reported in nature. Following their release and purification, detection and quantitation of these molecules is now possible by a number of techniques. We and others have previously reported high-pressure liquid chromatography separation of sialic acids with several different columns, elution methods, and detection techniques. We report here a new method for the separation of sialic acids at neutral pH on a Carbopac PA-1 anion-exchange column of pellicular resin, with pulsed amperometric detection following postcolumn addition of alkali. The major advantages of this system are the separation of a variety of sialic acids, sensitive detection (into the picomole range), and the relative ease of use for preparative purposes. Using a set of defined sialic acid standards, this method is compared and contrasted with six other HPLC methods previously described by us and by others. The advantages and disadvantages of each system are also addressed. In the final analysis, no single method is adequate to completely separate and quantitate all of the known sialic acids. However, used in appropriate combinations, these methods allow exploration of the biology of sialic acids in a manner heretofore not possible.
Acetates↗
Biosynthesis of N-glycolyneuraminic acid. The primary site of hydroxylation of N-acetylneuraminic acid is the cytosolic sugar nucleotide pool.
N-Glycolylneuraminic acid (Neu5Gc) is an oncofetal antigen in humans and is developmentally regulated in rodents. We have explored the biology of N-acetylneuraminic acid hydroxylase, the enzyme responsible for conversion of the parent sialic acid, N-acetylneuraminic acid (Neu5Ac) to Neu5Gc. We show that the major sialic acid in all compartments of murine myeloma cell lines is Neu5Gc. Pulse-chase analysis in these cells with the sialic acid precursor [6-3H]N-acetylmannosamine demonstrates that most of the newly synthesized Neu5Gc appears initially in the cytosolic low-molecular weight pool bound to CMP. The percentage of Neu5Gc on membrane-bound sialic acids closely parallels that in the CMP-bound pool at various times of chase, whereas that in the free sialic acid pool is very low initially, and rises only later during the chase. This implies that conversion from Neu5Ac to Neu5Gc occurs primarily while Neu5Ac is in its sugar nucleotide form. In support of this, the hydroxylase enzyme from a variety of tissues and cells converted CMP-Neu5Ac to CMP-Neu5Gc, but showed no activity towards free or alpha-glycosidically bound Neu5Ac. Furthermore, the majority of the enzyme activity is found in the cytosol. Studies with isolated intact Golgi vesicles indicate that CMP-Neu5Gc can be transported and utilized for transfer of Neu5Gc to glycoconjugates. The general properties of the enzyme have also been investigated. The Km for CMP-Neu5Ac is in the range of 0.6-2.5 microM. No activity can be detected against the beta-methylglycoside of Neu5Ac. On the other hand, inhibition studies suggest that the enzyme recognizes both the 5'-phosphate group and the pyrimidine base of the substrate. Taken together, the data allow us to propose pathways for the biosynthesis and reutilization of Neu5Gc, with initial conversion from Neu5Ac occurring primarily at the level of the sugar nucleotide. Subsequent release and reutilization of Neu5Gc could then account for the higher steady-state level of Neu5Gc found in all of the sialic acid pools of the cell.
Animals↗
O-acetylation and de-O-acetylation of sialic acids. O-acetylation of sialic acids in the rat liver Golgi apparatus involves an acetyl intermediate and essential histidine and lysine residues--a transmembrane reaction?
Isolated intact rat liver Golgi vesicles utilize [acetyl-3H]coenzyme A to add 3H-O-acetyl esters to sialic acids of internally facing endogenous glycoproteins. During this reaction, [3H]acetate also accumulates in the vesicles, even though the vesicles are impermeant to free acetate. On the other hand, entry of intact AcCoA into the lumen of the vesicles could not be demonstrated, and permeabilization of the vesicles did not alter the reaction substantially (Diaz, S., Higa, H. H., Hayes, B. K., and Varki, A. (1989) J. Biol. Chem. 264, 19416-19426). When vesicles prelabeled with [acetyl-3H] coenzyme A are permeabilized with saponin, we can demonstrate a [3H]acetyl intermediate in the membrane that can transfer label to the 7- and 9-positions of exogenously added free N-acetylneuraminic acid but not to glucuronic acid or CMP-N-acetylneuraminic acid. This labeled acetyl intermediate represents a significant portion of the radioactivity incorporated into the membranes during the initial incubation and cannot be accounted for by nonspecifically "trapped" acetyl-CoA in the permeabilized vesicles. There was no evidence for involvement of acetylcarnitine or acetyl phosphate as an intermediate. The overall acetylation reaction appears to involve two steps. The first step (utilization of exogenous acetyl-CoA to form the acetyl intermediate) is inhibited by coenzyme A-SH (apparent Ki = 24-29 microM), whereas the second (transfer from the acetyl intermediate to sialic acid) is not affected by millimolar concentrations of the nucleotide. Studies with amino acid-modifying reagents indicate that 1 or more histidine residues are involved in the first step of the acetylation reaction. Diethylpyrocarbonate (which can react with both nonsubstituted and singly acetylated histidine residues) also blocks the second reaction, indicating that the acetyl intermediate on both sides of the membrane involves histidine residue(s). Taken together with data presented in the preceding paper, these results indicate that the acetylation of sialic acids in Golgi vesicles may occur by a transmembrane reaction, similar to that described for the acetylation of glucosamine in lysosomes (Bame, K. J., and Rome, L. H. (1985) J. Biol. Chem. 260, 11293-11299). However, several features of this Golgi reaction distinguish it from the lysosomal one, including the nature and kinetics of the reaction and the additional involvement of an essential lysine residue. The accumulation of free acetate in the lumen of the vesicles during the reaction may occur by abortive acetylation (viz. transfer of label from the acetyl intermediate to water). It is not clear if this is an artifact that occurs only in the in vitro reaction.
Acetates↗