Mucin synthesis. Conversion of R1-beta 1-3Gal-R2 to R1-beta 1-3(GlcNAc beta 1-6)Gal-R2 and of R1-beta 1-3GalNAc-R2 to R1-beta 1-3(GlcNAc beta 1-6)GalNAc-R2 by a beta 6-N-acetylglucosaminyltransferase in pig gastric mucosa

A UDP-GlcNAc:R1-beta 1-3Gal(NAc)-R2 [GlcNAc to Gal(NAc)] beta 6-N-acetylglucosaminyltransferase activity from pig gastric mucosa microsomes catalyzes the formation of GlcNAc beta 1-3(GlcNAc beta 1-6)Gal-R from GlcNAc beta 1-3Gal-R where -R is -beta 1-3GalNAc-alpha-benzyl or -beta 1-3(GlcNAc beta 1-6)GalNAc-alpha-benzyl. This enzyme is therefore involved in the synthesis of the I antigenic determinant in mucin-type oligosaccharides. The enzyme also converts Gal beta 1-3Gal beta 1-4Glc to Gal beta 1-3(GlcNAc beta 1-6)Gal beta 1-4Glc. The enzyme was stimulated by Triton X-100 at concentrations between 0 and 0.2% and was inhibited by Triton X-100 at 0.5%. There is no requirement for Mn2+ and the enzyme activity is reduced to 65% in the presence of 10 mM EDTA. Enzyme products were purified and identified by proton NMR, methylation analysis and beta-galactosidase digestion. Competition studies suggest that this pig gastric mucosal beta 6-GlcNAc-transferase activity is due to the same enzyme that converts Gal beta 1-3GalNAc-R to mucin core 2, Gal beta 1-3(GlcNAc beta 1-6)GalNAc-R, and GlcNAc beta 1-3GalNAc-R to mucin core 4, GlcNAc beta 1-3(GlcNAc beta 1-6)GalNAc-R. Substrate specificity studies indicate that the enzyme attaches GlcNAc to either Gal or GalNAc in beta (1-6) linkage, provided these residues are substituted in beta (1-3) linkage by either GlcNAc or Gal. The insertion of a GlcNAc beta 1-3 residue into Gal beta 1-3GalNAc-R to form GlcNAc beta 1-3Gal beta 1-3GalNAc-R prevents insertion of GlcNAc into GalNAc. These studies establish several novel pathways in mucin-type oligosaccharide biosynthesis.

The effect of a "bisecting" N-acetylglucosaminyl group on the binding of biantennary, complex oligosaccharides to concanavalin A, Phaseolus vulgaris erythroagglutinin (E-PHA), and Ricinus communis agglutinin (RCA-120) immobilized on agarose

The effect of a "bisecting" 2-acetamido-2-deoxy-beta-D-glucopyranosyl group, linked (1----4) to the beta-D-mannopyranosyl group of asparagine-linked complex and hybrid oligosaccharides, on the binding of [14C]acetylated glycopeptides to columns of immobilized concanavalin A (Con A), Phaseolus vulgaris erythroagglutinin (E-PHA), and Ricinus communis agglutinin-120 (RCA-120) was investigated. The presence of this "bisecting" GlcNAc group caused significant inhibition of the binding to ConA-agarose of biantennary complex glycopeptides in which the two branches are terminated at their nonreducing ends by two GlcNAc groups, or by a Gal and a GlcNAc group, or by two Gal groups, or by a Man and a GlcNAc group. Binding of biantennary, complex glycopeptides to E-PHA-agarose required a "bisecting" GlcNAc group, a Gal group at the nonreducing terminus of the alpha-D-Man-p-(1----6) branch, and a terminal or internal GlcNAc residue linked beta-(1----2) to the alpha-D-Manp-(1----3) branch. Binding to RCA-120-agarose occurred only when at least one nonreducing terminal Gal group was present, and increased as the proportion of terminal Gal groups increased; the presence of a "bisecting" GlcNAc group caused either enhancement or inhibition of these binding patterns. It is concluded that a "bisecting" GlcNAc group affects the binding of glycopeptides to all three lectin columns.

Biosynthetic controls that determine the branching and microheterogeneity of protein-bound oligosaccharides

Detailed studies on the enzyme machinery responsible for the biosynthesis of protein-bound oligosaccharides of the Asn-GlcNAc and Ser(Thr)-GalNAc linkage types have allowed the formulation of some general rules which explain, at least in part, the branching patterns and microheterogeneity of these structures. These rules are discussed under the following headings: competition of two or more enzymes for a common substrate; controls at the level of enzyme substrate specificity (e.g., critical sugar residues which turn enzyme activity on or off, branch specificity, and the role of the polypeptide in the glycoprotein substrate); substrate availability.

Mucin synthesis. UDP-GlcNAc:GalNAc-R beta 3-N-acetylglucosaminyltransferase and UDP-GlcNAc:GlcNAc beta 1-3GalNAc-R (GlcNAc to GalNAc) beta 6-N-acetylglucosaminyltransferase from pig and rat colon mucosa

Pig and rat colon mucosal membrane preparations catalyze the in vitro transfer of N-acetyl-D-glucosamine (GlcNAc) from UDP-GlcNAc to GalNAc-ovine submaxillary mucin to form GlcNAc beta 1-3GalNAc-mucin. Rat colon also catalyzes the in vitro transfer of GlcNAc from UDP-GlcNAc to GlcNAc beta 1-3GalNAc-mucin to form GlcNAc beta 1-3(GlcNAc beta 1-6) GalNAc-mucin. This is the first demonstration of in vitro synthesis of the GlcNAc beta 1-3GalNAc disaccharide and of the GlcNAc beta 1-3-(GlcNAc beta 1-6)GalNAc trisaccharide, two of the four major core types found in mammalian glycoproteins of the mucin type, i.e., those containing oligosaccharides with GalNAc-alpha-serine (threonine) linkages. The activity catalyzing synthesis of the disaccharide has been named UDP-GlcNAc:GalNAc-R beta 3-N-acetylglucosaminyltransferase (mucin core 3 beta 3-GlcNAc-transferase), while the activity responsible for synthesizing the trisaccharide has been named UDP-GlcNAc:GlcNAc beta 1-3GalNAc-R (GlcNAc to GalNAc) beta 6-N-acetylglucosaminyltransferase (mucin core 4 beta 6-GlcNAc-transferase). The beta 3-GlcNAc-transferase from pig colon is activated by Triton X-100, has an absolute requirement for Mn2+, and transfers GlcNAc to GalNAc-alpha-phenyl, GalNAc-alpha-benzyl, and GalNAc-ovine submaxillary mucin with apparent Km values of 5, 2, and 3 mM and Vmax values of 59, 62, and 37 nmol h-1 (mg of protein)-1, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of glycoprotein synthesis. Bovine milk UDPgalactose:N-acetylglucosamine beta-4-galactosyltransferase catalyzes the preferential transfer of galactose to the GlcNAc beta 1,2Man alpha 1,3- branch of both bisected and nonbisected complex biantennary asparagine-linked oligosaccharides

Bovine milk UDPgalactose:N-acetylglucosamine beta-4-galactosyltransferase has been used to investigate the effect of a bisecting GlcNAc residue (linked beta 1,4 to the beta-linked mannose of the trimannosyl core of asparagine-linked complex oligosaccharides) on galactosylation of biantennary complex oligosaccharides. Columns of immobilized lectins (concanavalin A, erythroagglutinating phytohemagglutinin, and Ricinus communis agglutinin 120) were used to separate the various products of the reactions. Preferential galactosylation of the GlcNAc beta 1,2Man alpha 1,3 arm occurred both in the absence and in the presence of a bisecting GlcNAc residue; the ratio of the rates of galactosylation of the Man alpha 1,3 arm to the Man alpha 1,6 arm was 6.5 in the absence of a bisecting GlcNAc and 2.8 in its presence. The bisecting GlcNAc residue reduced galactosylation of the Man alpha 1,3 arm by about 78% probably due to steric hindrance of the GlcNAc beta 1,2Man alpha 1,3 beta 1,4 region of the substrate by the bisecting GlcNAc. This steric hindrance prevents the action of four other enzymes involved in assembly of complex asparagine-linked oligosaccharides and indicates the importance of the bisecting GlcNAc residue in the control of glycoprotein biosynthesis. The Man alpha 1,3 arm of biantennary oligosaccharides is believed to be freely accessible to enzyme action whereas the Man alpha 1,6 arm is believed to be folded back toward the core. This may explain the preferential action of Gal-transferase on the Man alpha 1,3 arm of both bisected and nonbisected oligosaccharides.

Mucin synthesis. The action of pig gastric mucosal UDP-GlcNAc:Gal beta 1-3(R1)GalNAc-R2 (GlcNAc to Gal) beta 3-N-acetylglucosaminyltransferase on high molecular weight substrates

Membrane preparations from pig gastric mucosa were shown to transfer [14C]GlcNAc from UDP-[14C]GlcNAc to blood group A-negative porcine submaxillary mucin previously subjected to mild acid hydrolysis to remove terminal sialyl and fucosyl residues. O-Glycosyl oligosaccharides were removed from enzyme product by alkaline borohydride treatment and, after purification, were subjected to high resolution proton nuclear magnetic resonance spectroscopy and methylation analysis. Two trisaccharide products were detected: [14C]GlcNAc beta 1-3Gal beta 1-3GalNAcOH and Gal beta 1-3[( 14C]GlcNAc beta 1-6)GalNAcOH. We have previously reported the in vitro synthesis of the latter compound, a branched trisaccharide, by UDP-GlcNAc:Gal beta 1-3GalNAc-R (GlcNAc to GalNAc) beta 6-N-acetylglucosaminyltransferase from canine submaxillary glands. However, this is the first report of the in vitro synthesis of the linear trisaccharide GlcNAc beta 1-3Gal beta 1-3GalNAc. Pig gastric mucosal beta 3-N-acetylglucosaminyltransferase catalyzed the formation of this trisaccharide by incorporation of GlcNAc into the terminal Gal of Gal beta 1-3GalNAc-alpha-R when R was a polypeptide from either mucin or antifreeze glycoprotein, but not when R was o-nitrophenyl. We have previously reported the in vitro synthesis by pig gastric mucosa of the tetrasaccharide GlcNAc beta 1-3Gal beta 1-3(GlcNAc beta 1-6)GalNAc-alpha-R when R was o-nitrophenyl or benzyl. We show in this report that pig gastric mucosa can synthesize this tetrasaccharide in vitro when R is a polypeptide from either porcine submaxillary mucin or antifreeze glycoprotein. Pig gastric mucosa therefore contains a beta 6-N-acetylglucosaminyltransferase capable of converting Gal beta 1-3GalNAc-alpha-R to Gal beta 1-3(GlcNAc beta 1-6)GalNAc-alpha-R and one or more beta 3-N-acetylglucosaminyltransferases which can add GlcNAc in beta 1-3 linkage to a terminal Gal residue to form either GlcNAc beta 1-3Gal beta 1-3GalNAc-alpha-R or GlcNAc beta 1-3Gal beta 1-3(GlcNAc beta 1-6)GalNAc-alpha-R where R is the polypeptide backbone of either mucin or antifreeze glycoprotein.

Asparagine-linked oligosaccharides in murine tumor cells: comparison of a WGA-resistant (WGAr) nonmetastatic mutant and a related WGA-sensitive (WGAs) metastatic line

MDW40, a wheat germ agglutinin-resistant (WGAr) mutant of the highly metastatic tumor cell line called MDAY-D2, is restricted to local growth at the subcutaneous site of inoculation. The WGAr tumor cells acquire metastatic ability by fusing spontaneously with a normal host cell followed by chromosome segregation, a process accompanied by reversion of the WGAr phenotype (i.e., WGAs). Since lectin-resistant mutant cell lines often have oligosaccharide alterations that may affect membrane function and consequently metastatic capacity, we compared the major Asn-linked glycopeptides in WGAr and WGAs cell lines. [2-3H]mannose-labeled glycopeptides were separated into four fractions on a DEAE-cellulose column and then further fractionated on a concanavalin A-Sepharose column. Glycopeptide structures were determined by: (a) sequential exoglycosidase digestion followed by chromatography on lectin/agarose and Bio-Gel P-4 columns and (b) proton nuclear magnetic resonance analysis. The metastatic WGAs cells had a sialylated poly-N-acetyllactosamine-containing glycopeptide which was absent in the nonmetastatic mutant cell line. Unique to the mutant was a neutral triantennary class of glycopeptide lacking sialic acid and galactose; the WGAr lesion therefore appeared to be a premature truncation of the antennae of the poly-N-acetyllactosamine-containing glycopeptide found in the WGAs cells. High mannose glycopeptides containing five to nine mannose residues constituted a major class in both WGAr and WGAs cells. Lysates of both wild-type and mutant cells had similar levels of galactosyltransferase activity capable of adding galactose to the N-acetylglucosamine-terminated glycopeptide isolated from mutant cells; the basis of the WGAr lesion remains to be determined.

Control of glycoprotein synthesis. The in vitro synthesis by hen oviduct membrane preparations of hybrid asparagine-linked oligosaccharides containing 5 mannose residues

Hen oviduct membranes were incubated with UDP-N-acetyl-D-[14C]glucosamine and [3H]GlcNAc beta 1-2Man alpha 1-3[Man alpha 1-6(Man alpha 1-3)Man alpha 1-6]Man beta 1-4GlcNAc beta 1-4GlcNAc-Asn (glycopeptide Gn(I)M5). Two double labeled products were obtained, both containing 5 Man and 4 GlcNAc residues. In order to separate these isomeric components, the mixture was treated with rat liver Golgi-rich membranes as a source of mannosidase II. One of the isomers was degraded by mannosidase action while the other was not, thereby allowing separation of two products (A and B). Product A was shown to be [3H]GlcNAc beta 1-2[( 14C] GlcNAc beta 1-3,4, or 6)Man alpha 1- 3Man beta 1-4GlcNAc beta 1-4GlcNAc, proving that hen oviduct membranes were capable of incorporating GlcNAc in beta-linkage into the Man alpha 1-3- residue of Gn(I)M5. Product B was identified as [3H]GlcNAc beta 1-2Man alpha 1-3[( 14C]GlcNAc beta 1-4)-[Man alpha 1-3)Man alpha 1-6]Man beta 1-4GlcNAc beta 1- f4GlcNAc , showing that hen oviduct membranes could incorporate a bisecting GlcNAc residue (linked beta 1-4 to the beta-linked Man) into Gn(I)M5. The ability of hen oviduct to carry out these two reactions in vitro supports the hypothesis first suggested by Harpaz and Schachter ( Harpaz , N., and Schachter , H. (1980) J. Biol. Chem. 255, 4894-4902) that the synthesis of bisected hybrid oligosaccharides is controlled by the insertion of a bisecting GlcNAc residue into Gn(I)M5. The presence of a bisecting GlcNAc residue prevents mannosidase II action and the synthetic pathway is therefore committed to hybrid oligosaccharide synthesis.

Control of glycoprotein synthesis. IX. A terminal Man alpha l-3Man beta 1- sequence in the substrate is the minimum requirement for UDP-N-acetyl-D-glucosamine: alpha-D-mannoside (GlcNAc to Man alpha 1-3) beta 2-N-acetylglucosaminyltransferase I

Twenty low molecular weight compounds were tested as substrates for UDP-GlcNAc:alpha-D-mannoside (GlcNAc to Man alpha 1-3) beta 2-N-acetylglucosaminyltransferase I (GlcNAc-transferase I) purified from bovine colostrum. This enzyme is at a key control point in the biosynthetic path leading to complex Asn-linked oligosaccharides. The highest activity was obtained with the substrate Man alpha 1-3(R1 alpha 1-6)Man beta 1-R2 where R1 was Man alpha 1-3(Man alpha 1-6)Man- (Km = 0.20 mM) and R2 was -4GlcNAc beta 1-4GlcNAc-Asn. Somewhat less effective were substrates in which R1 was Man- (Km = 0.4-0.6 mM) and R2 was either-4GlcNAc or -4GlcNAc beta 1-4(Fuc alpha 1-6)GlcNAc-Asn. Removal of the Man alpha 1-6 arm (R1 = H-) or replacing R2 with an isopropyl group had no effect on Vmax but increased the Km about 10-fold, thereby leading to an 85% reduction in enzyme activity as measured under standard conditions. An 85% reduction in activity was also observed if R2 was replaced with N-acetylglucosaminitol. Enzyme activity was reduced 33% if R1 was Gal beta 1-4GlcNAc beta 1-2Man-. Any compounds lacking a Man alpha 1-3- terminus or in which the beta-linked Man had been replaced with an alpha-linked Man were totally inactive. It was concluded that a terminal Man alpha 1-3Man beta 1-sequence is a minimal structural requirement for a GlcNAc-transferase I substrate. The only effective substrate for partially purified UDP-GlcNAc:alpha-D-mannoside (GlcNAc to Man alpha 1-6) beta 2-N-acetylglucosaminyltransferase II (GlcNAc-transferase II) from bovine colostrum was R1-GlcNAc beta 1-2Man alpha 1-3(Man alpha 1-6)Man beta 1-R2 where R1 = H-. The absence of a terminal GlcNAc beta 1-2- residue or masking this residue by making R1 = Gal beta 1-4-, both prevented enzyme activity, indicating that GlcNAc-transferase I action must precede GlcNAc-transferase II action during biosynthesis of complex Asn-linked oligosaccharides.

Branch specificity of purified rat liver Golgi UDP-galactose: N-acetylglucosamine beta-1,4-galactosyltransferase. Preferential transfer of of galactose on the GlcNAc beta 1,2-Man alpha 1,3-branch of a complex biantennary Asn-linked oligosaccharide

In the final stages of the terminal glycosylation of N-linked complex oligosaccharides, UDP-galactose: N-acetylglucosamine beta-1,4-galactosyltransferase (galactosyltransferase) transfers galactose (Gal) onto the N-acetylglucosamine (GlcNAc) residue of each branch of a biantennary oligosaccharide. Purified rat liver Golgi galactosyltransferase was used with GlcNAc beta 1,2-Man alpha 1,6-(GlcNAc beta 1,2-Man alpha 1,3-)-Man beta 1,4-GlcNAc beta 1,4-(Fuc alpha 1,6-)-GlcNAc-Asn in order to determine the sequence of addition of Gal residues to the biantennary oligosaccharide. The different galactosylated products were separated by concanavalin A affinity chromatography and high voltage paper electrophoresis in borate. It was found that Gal was transferred at a much faster rate to the GlcNAc beta 1,2-Man alpha 1,3-branch than to the GlcNAc beta 1,2-Man alpha 1,6-branch, i.e. k1 was at least 5 times larger than k2. Also, k3 was larger than k4, indicating that most of the digalactosylated product "GG" was formed by the sequential addition of Gal to the Man alpha 1,3-branch followed by addition to the Man alpha 1,6-branch. The preferential galactosylation of the GlcNAc beta 1,2-Man alpha 1,3-branch may explain the formation of the asymmetrical oligosaccharides found in bovine and human IgG.

Glycoproteins: their structure, biosynthesis and possible clinical implications

Glycoproteins carrying asparagine-linked N-glycosyl oligosaccharides have many diverse biological functions. The role of the carbohydrate in these functions is often obscure. However, there is evidence that carbohydrate is involved in stabilization of glycoproteins during passage from the rough endoplasmic reticulum to the cell surface, and in recognition phenomena such as receptor-mediated endocytosis, routing of lysosomal hydrolases to the lysosomes, and the spread of cancer cells to secondary sites. The cell surface carbohydrate of some transformed cell lines tends to be more highly branched than that of the non-transformed controls. The control of branching during synthesis of N-glycosyl oligosaccharides resides in the N-acetylglucosaminyltransferases (GlcNAc-transferases) which initiate these branches. There must be at least seven such GlcNAc-transferases to account for the diversity of structures that have been observed. Our laboratory has developed assays for four of these enzymes. Substrate specificity studies on these enzymes have shed light on some of the control mechanisms involved in the synthesis of highly branched structures. Alterations in these control mechanisms may be important in the pathogenesis of cancer and other disease.

Coordination between enzyme specificity and intracellular compartmentation in the control of protein-bound oligosaccharide biosynthesis

This laboratory has developed schemes for the control of biosynthesis of N- and O-glycosyl oligosaccharides based on studies in cell-free systems of glycosyl-transferase substrate specificities. These schemes are based on assumptions that may not be universally correct. For example, we have ignored the possible compartmentation of reactions in different cells or in different organelles within a cell. Recent evidence has indicated that the Golgi apparatus has at least three functionally distinct regions (cis, medial and trans). The addition of galactosyl and sialyl residues to the antennae of complex and hybrid N-glycans probably occurs entirely within the trans-cisternae while the N-acetylglucosaminyl-transferases which initiate these antennae appear to be located in a denser region of the Golgi (cis and/or medial cisternae). We have constructed a modified scheme for the biosynthesis of the antennae of N-glycans. This scheme combines our substrate specificity data (H. Schachter, S. Narasimhan, P. Gleeson and G. Vella, 1983, Can. J. Biochem. Cell Biol., 61, 1049-1066) with compartmentation data. It provides a basis for understanding the control of glycoprotein synthesis in normal tissues and in certain lectin-resistant mutant cell lines.

Mucin synthesis. III. UDP-GlcNAc:Gal beta 1-3(GlcNAc beta 1-6)GalNAc-R (GlcNAc to Gal) beta 3-N-acetylglucosaminyltransferase, an enzyme in porcine gastric mucosa involved in the elongation of mucin-type oligosaccharides

Pig gastric mucosa microsomes have been shown to catalyze the following reaction: UDP-GlcNAc + Gal beta 1-3(GlcNAc beta 1-6)-GalNAc-alpha-R----GlcNAc beta 1-3Gal beta 1-3 (GlcNAc beta 1-6)GalNAc-alpha-R + UDP, where R is o-nitrophenyl or benzyl. The enzyme catalyzing this reaction has been named UDP-GlcNAc:Gal beta 1-3(GlcNAc beta 1-6)GalNAc-R (GlcNAc-R (GlcNAc to Gal) beta 3-N-acetylglucosaminyltransferase. The beta 3-GlcNAc-transferase does not act on Gal beta 1-3GalNAc-alpha-o-nitrophenyl. The beta 3-GlcNAc-transferase requires Mn2+ and Triton X-100 for optimal activity. The Vmax for the microsomal enzyme is 8.7 nmol/mg protein per hour and the Km values are 1.6, 0.9, and 1.2 mM for UDP-GlcNAc and the alpha-o-nitrophenyl and alpha-benzyl derivatives of Gal beta 1-3(GlcNAc beta 1-6)GalNAc, respectively. Pig gastric mucosa microsomes catalyze the transfer of GlcNAc to lactose to form GlcNAc beta 1-3Gal beta 1-4Glc, but fail to transfer GlcNAc to lactosyl ceramide, Gal beta 1-4GlcNAc, or Gal beta 1-4GlcNAc-beta-benzyl.

Control of branching during the biosynthesis of asparagine-linked oligosaccharides

Many mammalian and avian complex carbohydrates (glycoproteins and glycolipids) have highly branched oligosaccharides. Although the function of complex carbohydrates is not known, there is evidence to suggest that oligosaccharide branching may be an important factor in the process by which cells recognize one another and their environment. Asparagine-linked (N-glycosyl) oligosaccharides can be subdivided into at least 12 classes according to their branching patterns. It is presently believed that these classes all stem from a common precursor oligosaccharide containing three D-glucose, nine D-mannose, and two N-acetyl-D-glucosamine residues. This precursor is incorporated into the protein backbone in the rough endoplasmic reticulum and is then processed within the endoplasmic reticulum and Golgi apparatus by a series of highly specific glycosidases and glycosyltransferases to yield the various classes of N-glycosyl oligosaccharides. The branches that occur in N-glycosyl oligosaccharides are usually initiated by the incorporation of a N-acetylglucosamine (GlcNAc) residue. Our laboratory has studied four of the N-acetylglucosaminyltransferases (GlcNAc-transferases) involved in this initiation process. We have defined various factors which determine the synthetic pathway. There are at least three types of control that are commonly found. (i) Tissues differ in the relative activities of the different glycosyltransferases and glycosidases and, therefore, competition between two or more enzymes for a common intermediate often determines the synthetic route. (ii) The incorporation of a key glycosyl residue into an oligosaccharide may convert a nonsubstrate to a substrate for either a glycosyltransferase or a glycosidase. (iii) Conversely, the incorporation of a key residue may convert a substrate into a nonsubstrate. Other controls are undoubtedly operative during glycoprotein synthesis: e.g., the effect of the polypeptide sequence on transferase specificity, the distribution of transferases along the endomembrane system, and compartmentation and the availability of substrates and cofactors. These factors have not been studied in our laboratory. However, the oligosaccharides made by the hen oviduct correlate quite well with the control factors elucidated by our approach; other tissues are presently under investigation. Recent studies on the three-dimensional structures of N-glycosyl oligosaccharides have enabled us to explain certain features of glycosyltransferase substrate specificity on the basis of steric factors.

The separation by liquid chromatography (under elevated pressure) of phenyl, benzyl, and O-nitrophenyl glycosides of oligosaccharides. Analysis of substrates and products for four N-acetyl-D-glucosaminyl-transferases involved in mucin synthesis

Liquid chromatography under elevated pressure (h.p.l.c.) has been applied to the separation of the phenyl, benzyl, and O-nitrophenyl glycosides of 2-acetamido-2-deoxy-D-galactopyranose and of various mucin-type, di-, tri-, and tetra-saccharides. The separations were carried out with a Whatman Partisil PXS 5/25 PAC column and various proportions of acetonitrile and water in the mobile phase. These methods were subsequently used to separate the substrates and products of the following N-acetylglucosaminyltransferase reactions: UDP-GlcNAc + beta-Gal-(1 leads to 3)-GalNAc-R leads to beta-Gal-(1 leads to 3)-[beta-GlcNAc-(1 leads to 6)]-GalNAc-R + UDP (1); UDP-GlcNAc + beta-Gal-(1 leads to 3)-[beta-GlcNAc-(1 leads to 6)]-GalNAc-R leads to beta-GlcNAc-(1 leads to 3)-beta-Gal-(1 leads to 3)-[beta-GlcNAc-(1 leads to 6)]-GalNAc-R + UDP (2); UDP-GlcNAc + GalNAc-R' leads to beta-GlcNAc-(1 leads to 3)-GalNAc-R' + UDP (3); and UDP-GlcNAc + beta-GlcNAc-(1 leads to 3)-GalNAc-R' leads to beta-GlcNAc-(1 leads to 6)-[beta-GlcNAc-(1 leads to 3)]-GalNAc-R' + UDP (4), where R is = benzyl or o-nitrophenyl, and R' = benzyl or phenyl alpha-D-glycoside. Reaction 1 is catalyzed by a transferase in canine submaxillary glands and porcine gastric mucosa, and reaction 2 by an enzyme in porcine gastric mucosa. Enzyme activities catalyzing reactions 3 and 4 have recently been demonstrated in rat colonic mucosa. Liquid chromatography can be used at the preparative level for the purification and identification of the transferase products, and at the analytical level in the assay of glycosyltransferases.

Control of glycoprotein synthesis

Hen oviduct membranes have been shown to catalyze the transfer of GlcNAc from UDP-GlcNAc to GlcNAc-beta 1-2Man alpha 1-6(GlcNAc beta 1-2 Man alpha 1-3) Man beta 1-4GlcNAc beta 1-4GlcNAc-Asn-X (GnGn) to form the triantennary structure GlcNAc beta 1-2Man alpha 1-6[GlcNAc beta 1-2(GlcNAc beta 1-4)Man alpha 1-3]Man beta 1-4GlcNAc beta 1-4GlcNAc-Asn-X. The enzyme has been named UDP-GlcNAc:GnGn (GlcNAc to Man alpha 1-3) beta 4-N-acetylglucosaminyltransferase IV (GlcNAc-transferase IV) to distinguish it from three other hen oviduct GlcNAc-transferases designated I, II, and III. Since GlcNAc-transferases III and IV both act on the same substrate, concanavalin A/Sepharose was used to separate the products of the two enzymes. At pH 7.0 and at a Triton X-100 concentration of 0.125% (v/v), GlcNAc-transferase IV activity in hen oviduct membranes is 7 nmol/mg of protein/h. The product was characterized by high resolution proton NMR spectroscopy at 360 MHz and by methylation analysis. In addition to triantennary oligosaccharide, hen oviduct membranes produced about 20% of bisected triantennary material, GlcNAc beta 1-2Man alpha 1-6[GlcNAc beta 1-2(GlcNAc beta 1-4)Man alpha 1-3] [GlcNAc beta 1-4]Man beta 1-4GlcNAc beta 1-4GlcNAc-Asn-X. Maximal GlcNAc-transferase IV activity requires the presence of both terminal beta 1-2-linked GlcNAc residues in the substrate. Removal of the GlcNAc residue on the Man alpha 1-6 arm or of both GlcNAc residues reduces activity by at least 80%. A Gal beta 1-4GlcNAc disaccharide on the Man alpha 1-6 arm reduces activity by 68% while the presence of this disaccharide on the Man alpha 1-3 arm reduces activity to negligible levels. A similar substrate specificity was found for GlcNAc-transferase III, the enzyme which adds a bisecting GlcNAc in beta 1-4 linkage to the beta-linked Man residue. Since a bisecting GlcNAc was found to prevent GlcNAc-transferase IV action, the bisected triantennary material found in the incubation must have been formed by the sequential action of GlcNAc-transferase IV followed by GlcNAc-transferase III. Activities similar to GlcNAc-transferase IV were also detected in rat liver Golgi-rich membranes (0.4 nmol/mg/h) and pig thyroid microsomes (0.1 nmol/mg/h).

Oligosaccharide branching of glycoproteins: biosynthetic mechanisms and possible biological functions

One of the most striking features of N- and O-glycosyl oligosaccharides and of lipid-linked oligosaccharides is the high degree of branching of these complex structures. Both proteins and nucleic acids are essentially linear structures and are synthesized by template mechanisms. The branched nature of complex carbohydrates dictates a totally different mechanism of biosynthetic control. Although there are undoubtedly many factors controlling this assembly (e.g. subcellular compartmentation, availability of substrates, cations), our laboratory has studied primarily the enzymatic factors that control the assembly of branched N-glycosyl (Asn-GlcNAc type) and O-glycosyl (Ser[Thr]-GalNAc type) oligosaccharides. There are three basic types of control points that appear to direct biosynthesis. (a) There may be two or more enzymes capable of acting on a single common substrate. Control at this juncture is exerted by the relative activities of these enzymes in a particular tissue. (b) Addition of a specific sugar to the growing oligosaccharide may shut off one or more subsequent enzyme steps, thereby 'freezing' the structure at a certain stage in its synthesis. (c) Progression of the pathway may be impossible until a certain key sugar residue is inserted into the growing oligosaccharide chain. Examples of all three types of control occur in the assembly of both N- and O-glycosyl oligosaccharides. This paper discusses our work on the N-acetylglucosaminyltransferases, which initiate branches in N-glycosyl oligosaccharides, as well as some studies on glycosyltransferases that control the assembly of the four basic Ser(Thr)-GalNAc cores. Important features at all stages of control are the three-dimensional shape of the oligosaccharide, the effect of certain key sugar residues on this three-dimensional shape and the stereochemistry of the interaction of oligosaccharides with proteins. From a functional point of view, protein-oligosaccharide interaction is of vital importance not only to enzyme control mechanisms but to a variety of biological problems such as malignancy and cell-cell interactions, differentiation and development, and susceptibility of cells to hormones, drugs and toxins.

Structure of the glycopeptides of a human gamma 1-immunoglobulin G (Tem) myeloma protein as determined by 360-megahertz nuclear magnetic resonance spectroscopy

High field magnetic resonance spectroscopy has been utilized to deduce the primary structure of the glycopeptides from a human myeloma gamma 1-immunoglobulin G (Tem). The major structures found belong to the biantennary complex class of glycopeptides, with a minor (5%) fraction belonging to the bisected biantennary complex class. In the biantennary class, three structures were present with different residues at the termini of the alpha Man(1-6) and alpha Man(1-3) arms: (i) with beta Gal(1-4) and alpha NeuNAc(2-6), respectively (33%); (ii) with beta Gal(1-4) and beta Gal(1-4), respectively (45%); and (iii) beta Gal(1-4) and beta GlcNAc(1-2), respectively (17%). In the bisected biantennary class only the latter termini were found for the two arms. These results suggest that the galactosyl transferase in these cells has a preference for the beta GlcNAc(1-2) of the alpha Man(1-6) arm and that the sialyltransferase has a preference for the beta Gal(1-4) of the alpha Man(1-3) arm.

Product-identification and substrate-specificity studies of the GDP-L-fucose:2-acetamido-2-deoxy-beta-D-glucoside (FUC goes to Asn-linked GlcNAc) 6-alpha-L-fucosyltransferase in a Golgi-rich fraction from porcine liver

Golgi-rich membranes from porcine liver have been shown to contain an enzyme that transfers L-fucose in alpha-(1 goes to 6) linkage from GDP-L-fucose to the asparagine linked 2-acetamido-2-deoxy-D-glucose residue of a glycopeptide derived from human alpha 1-acid glycoprotein. Product identification was performed by high resolution, 1H-n.m.r. spectroscopy at 360 MHz and by permethylation analysis. The enzyme has been named GDP-L-fucose: 2-acetamido-2-deoxy-beta-D-glucoside (Fuc goes to Asn-linked GlcNAc) 6-alpha-L-fucosyltransferase, because the substrate requires a terminal beta-(1 goes to 2)-linked GlcNAc residue on the alpha-Man (1 goes to 3) arm of the core. Glycopeptides with this residue were shown to be acceptors whether they contain 3 or 5 Man residues. Substrate-specificity studies have shown that diantennary glycopeptides with two terminal beta-(1 goes to 2)-linked GlcNAc residues and glycopeptides with more than two terminal GlcNAc residues are also excellent acceptors for the fucosyltransferase. An examination of four pairs of glycopeptides differing only by the absence or presence of a bisecting GlcNAc residue in beta-(1 goes to 4) linkage to the beta-linked Man residue of the core showed that the bisecting GlcNAc prevented 6-alpha-L-fucosyltransferase action. These findings probably explain why the oligosaccharides with a high content of mannose and the hybrid oligosaccharides with a bisecting GlCNAc residue that have been isolated to date do not contain a core L-fucosyl residue.