Terminal glycosylation of bovine uroplakin III, one of the major integral-membrane glycoproteins of mammalian bladder

Uroplakin III (UPIII) is one of the major transmembrane glycoproteins exposed at the luminal face of mammalian bladder. We investigated the terminal glycosylation of bovine UPIII in order to ascertain whether it contains the alpha 2,3-sialylated sequence thus potentially serving as a receptor for uropathogenic Escherichia coli expressing type S adhesins. We report the occurrence of sialic acid in alpha 2,3- and alpha 2,6-linkage to galactose in bovine UPIII glycans as evidenced by the sensitivity of UPIII to both Vibrio cholera and Newcastle disease virus neuraminidase and by the colocalization of UPIII antigen and material detected by lectins of Sambucus nigra and Maackia amurensis on the luminal face of the bladder. We also present evidence that UPIII glycans are capped by Gal-alpha 1,3-Gal epitope. Consistently, alpha 2,3- and alpha 2, 6-sialyltransferase, as well as alpha 1,3-galactosyltransferase were found to be present in the cells detached from the luminal side of bovine bladder, which are responsible for the UPIII biosynthesis. The putative role of UPIII sialylated glycans in enhancing the uropathogenicity of E. coli expressing type S adhesins is discussed.     

Histochemical analysis of glycoconjugates in the domestic cat testis

The localization and characterization of oligosaccharide sequences in the cat testis was investigated using 12 lectins in combination with the beta-elimination reaction, N-Glycosidase F and sialidase digestion. Leydig cells expressed O-linked glycans with terminal alphaGalNAc (HPA reactivity) and N-glycans with terminal/internal alphaMan (Con A affinity). The basement membrane showed terminal Neu5Acalpha2,6Gal/GalNAc, Galbeta1,3GalNAc, alpha/betaGalNAc, and GlcNAc (SNA, PNA, HPA, SBA, GSA II reactivity) in O-linked oligosaccharides, terminal Galbeta1,4GlcNAc (RCA120 staining) and alphaMan in N-linked oligosaccharides; in addition, terminal Neu5acalpha2,3Galbeta1,4GlcNac, Forssman pentasaccharide, alphaGal, alphaL-Fuc and internal GlcNAc (MAL II, DBA, GSA I-B4, UEA I, KOH-sialidase-WGA affinity) formed both O- and N-linked oligosaccharides. The Sertoli cells cytoplasm contained terminal Neu5Ac-Galbeta1,4GlcNAc, Neu5Ac-betaGalNAc as well as internal GlcNAc in O-linked glycans, alphaMan in N-linked glycoproteins and terminal Neu5Acalpha2,6Gal/ GalNAc in both O- and N-linked oligosaccharides. Spermatogonia exhibited cytoplasmic N-linked glycoproteins with alphaMan residues. The spermatocytes cytoplasm expressed terminal Neu5Acalpha2,3Galbeta1,4 GlcNAc and Galbeta1,3GalNAc in O-linked oligosaccharides, terminal Galbeta1,4GlcNAc and alpha/betaGalNAc in N-linked glycoconjugates. The Golgi region showed terminal Neu5Acalpha2,3Galbeta1,4GlcNac, Galbeta1,4GlcNAc, Forssman pentasaccharide, and alphaGalNAc in O-linked oligosaccharides, alphaMan and terminal betaGal in N-linked oligosaccharides. The acrosomes of Golgi-phase spermatids expressed terminal Galbeta1,3GalNAc, Galbeta1,4GlcNAc, Forssmann pentasaccharide, alpha/betaGalNAc, alphaGal and internal GlcNAc in O-linked oligosaccharides, terminal alpha/betaGalNAc, alphaGal and terminal/internal alphaMan in N-linked glycoproteins. The acrosomes of cap-phase spermatids lacked internal Forssman pentasaccharide and alphaGal, while having increased alpha/betaGalNAc. The acrosomes of elongated spermatids did not show terminal Galbeta1,3GalNAc, displayed terminal Galbeta1,4GlcNAc and alpha/betaGalNAc in N-glycans and Neu5Ac-Galbeta1,3GalNAc in O-linked oligosaccharides.     

Generation of serum-stabilized retroviruses: reduction of alpha1,3gal-epitope synthesis in a murine NIH3T3-derived packaging cell line by expression of chimeric glycosyltransferases

Retroviral vectors released from mouse-derived packaging cell lines are inactivated in human sera by naturally occurring antibodies due to the recognition of Galalpha1,3Galbeta1,4GlcNAc (alphagal-epitope) decorated surface proteins. In this study, an extensive analysis of the glycosylation potential of NIH3T3-derived PA317 packaging cells using combined MALDI/TOF-MS and HPAE-PAD reveals that 34% of the N-glycan moiety represents alphagal-epitope containing structures. Stable expression of glycosyltransferases and transport signal chimeras has been demonstrated to represent an efficient tool to alter cell- and species-specific glycosylation (Grabenhorst and Conradt, 1999. J. Biol. Chem. 274, 36107-36116). In order to reduce alphagal-epitope synthesis selected chimeric glycosyltransferases were constructed by fusing Golgi-signal sequences for compartment-specific localization with the catalytic domain of alpha2,3-sialyltransferase (ST3). Stable expression of these constructs in these cells resulted in a significant reduced alphagal-epitope synthesis, and moreover, a release of retroviral vectors showing an up to 3.5-fold increase in serum stability. Thus, our results suggest that the stably transfected cells stably transfected with chimeric glycosyltransferases compete efficiently with endogenous alpha1,3-galactosyltransferase. This approach allows favored glycodesign and we anticipate the applicability of such improved retroviral vectors produced by glycosylation engineered host cells for in vivo gene therapy and, furthermore, suggest the therapeutic benefit of this technology for xenotransplantation.     

Photobacterium sp. JT-ISH-224 produces two sialyltransferases, alpha-/beta-galactoside alpha2,3-sialyltransferase and beta-galactoside alpha2,6-sialyltransferase

A novel bacterium, Photobacterium sp. JT-ISH-224, that produces alpha-/beta-galactoside alpha2,3-sialyltransferase and beta-galactoside alpha2,6-sialyltransferase, was isolated from the gut of a Japanese barracuda. The genes that encode the enzymes were cloned from the genomic library of the bacterium using the genes encoding alpha-/beta-galactoside alpha2,3-sialyltransferase from P. phosphoreum and beta-galactoside alpha2,6-sialyltransferase from P. damselae as probes. The nucleotide sequences were determined, and open reading frames of 1,230 and 1,545 bp for encoding an alpha2,3-sialyltransferase and an alpha2,6-sialyltransferase of 409- and 514-amino acid residues, respectively, were identified. The alpha2,3-sialyltransferase had 92% amino acid sequence identity with the P. phosphoreum alpha2,3-sialyltransferase, whereas the alpha2,6-sialyltransferase had 54% amino acid sequence identity with the P. damselae alpha2,6-sialyltransferase. For both enzymes, the DNA fragments that encoded the full-length protein and its truncated form lacking the putative signal peptide sequence were amplified by a polymerase chain reaction and cloned into an expression vector. Each gene was expressed in Escherichia coli, and the lysate from each strain had enzymatic activity. The alpha2,3-sialyltransferase catalysed the transfer of N-acetylneuraminic acid (NeuAc) from CMP-NeuAc to lactose, alpha-methyl-galactopyranoside and beta-methyl-galactopyranoside with low apparent K(m) and the alpha2,6-sialyltransferase catalysed the transfer of NeuAc from CMP-NeuAc to lactose with low apparent K(m).     

Expression of functional soluble forms of human beta-1, 4-galactosyltransferase I, alpha-2,6-sialyltransferase, and alpha-1, 3-fucosyltransferase VI in the methylotrophic yeast Pichia pastoris

The cDNAs encoding soluble forms of human beta-1, 4-galactosyltransferase I (EC 2.4.1.22), alpha-2,6-sialyltransferase (EC 2.4.99.1), and alpha-1,3-fucosyltransferase VI (EC 2.4.1.65), respectively, have been expressed in the methylotrophic yeast Pichia pastoris. The vector pPIC9 was used, which contains the N-terminal signal sequence of Saccharomyces cerevisiae alpha-factor to allow entry into the secretory pathway. The recombinant enzymes had similar kinetic properties as their native counterparts. Their identity was confirmed by Western blotting. Recombinant enzymes may be used for in vitro synthesis of oligosaccharides.     

Glycoengineering of therapeutic glycoproteins: in vitro galactosylation and sialylation of glycoproteins with terminal N-acetylglucosamine and galactose residues

Therapeutic glycoproteins produced in different host cells by recombinant DNA technology often contain terminal GlcNAc and Gal residues. Such glycoproteins clear rapidly from the serum as a consequence of binding to the mannose receptor and/or the asialoglycoprotein receptor in the liver. To increase the serum half-life of these glycoproteins, we carried out in vitro glycosylation experiments using TNFR-IgG, an immunoadhesin molecule, as a model therapeutic glycoprotein. TNFR-IgG is a disulfide-linked dimer of a polypeptide composed of the extracellular portion of the human type 1 (p55) tumor necrosis factor receptor (TNFR) fused to the hinge and Fc regions of the human IgG(1) heavy chain. This bivalent antibody-like molecule contains four N-glycosylation sites per polypeptide, three in the receptor portion and one in the Fc. The heterogeneous N-linked oligosaccharides of TNFR-IgG contain sialic acid (Sia), Gal, and GlcNAc as terminal sugar residues. To increase the level of terminal sialylation, we regalactosylated and/or resialylated TNFR-IgG using beta-1,4-galactosyltransferase (beta1,4GT) and/or alpha-2,3-sialyltransferase (alpha2,3ST). Treatment of TNFR-IgG with beta1,4GT and UDP-Gal, in the presence of MnCl(2), followed by MALDI-TOF-MS analysis of PNGase F-released N-glycans showed that the number of oligosaccharides with terminal GlcNAc residues was significantly decreased with a concomitant increase in the number of terminal Gal residues. Similar treatment of TNFR-IgG with alpha2,3ST and CMP-sialic acid (CMP-Sia), in the presence of MnCl(2), produced a molecule with an approximately 11% increase in the level of terminal sialylation but still contained oligosaccharides with terminal GlcNAc residues. When TNFR-IgG was treated with a combination of beta1,4GT and alpha2,3ST (either in a single step or in a stepwise fashion), the level of terminal sialylation was increased by approximately 20-23%. These results suggest that in vitro galactosylation and sialylation of therapeutic glycoproteins with terminal GlcNAc and Gal residues can be achieved in a single step, and the results are similar to those for the stepwise reaction. This type of in vitro glycosylation is applicable to other glycoproteins containing terminal GlcNAc and Gal residues and could prove to be useful in increasing the serum half-life of therapeutic glycoproteins.     

Production of mouse monoclonal antibody with galactose-extended sugar chain by suspension cultured tobacco BY2 cells expressing human beta(1,4)-galactosyltransferase

Previously, we developed a transgenic tobacco BY2 cell line (GT6) in which glycosylation was modified by expressing human beta(1,4)-galactosyltransferase (betaGalT). In this study, we produced a mouse monoclonal antibody in GT6 cells, and determined the sugar chain structures of plant-produced antibodies. Galactose-extended N-linked glycans comprised 16.7%, and high-mannose-type and complex-type glycans comprised 38.5% and 35.0% of the total number of glycans, respectively. N-linked glycans with the plant-specific sugars beta(1,2)-xylose and alpha(1,3)-fucose comprised 9.8%. The introduction of human betaGalT into suspension cultured tobacco cells resulted in the production of recombinant proteins with galactose-extended glycans and decreased contents of beta(1,2)-xylose and alpha(1,3)-fucose.     

Differential expression of Galalpha1,3Gal epitope in polymeric and monomeric IgM secreted by mouse myeloma cells deficient in alpha2, 6- sialyltransferase

IgM are glycoproteins secreted by plasma cells as (mu2L2)5+J or (mu2L2)6 polymers. In most species, mu- and J-chains bear five and one N -glycans, respectively. Here we compare the terminal glycosylation patterns of 4-hydroxy-3-nitrophenylacetyl (NP)-specific IgM secreted by transfectants of the J558L mouse myeloma deficient in the alpha2,6 sialyltransferase [alpha2,6ST(N)] or by a hybridoma expressing this enzyme (B1.8 cells). The absence of alpha2,6-sialylation results in an increased addition of alpha1, 3-galactosyl residues to mu- and J-chain N-glycans. Since alpha1, 3-galactosyltransferase (alpha1,3Gal-T) is similarly expressed in the two cell lines, these results indicate that a competition reaction occurs in vivo between alpha2,6ST(N) and alpha1,3Gal-T. In the alpha2,6ST(N) deficient transfectants, mu-chains lacking the C-subterminal Cys575 residue, which are secreted mainly in the form of mu2L2 monomers, are more efficiently capped by alpha1, 3- galactosyl residues, confirming that polymerization significantly reduces the accessibility of mu-chain glycans to the Golgi processing enzymes involved in the biogenesis of antennary sugars. Functional assays indicate that IgM sialylation affects antigen-binding and complement-dependent hemolysis of haptenated red blood cells.      

Mammalian glycosyltransferase expression allows sialoglycoprotein production by baculovirus-infected insect cells.

The baculovirus-insect cell expression system is widely used to produce recombinant mammalian glycoproteins, but the glycosylated end products are rarely authentic. This is because insect cells are typically unable to produce glycoprotein glycans containing terminal sialic acid residues. In this study, we examined the influence of two mammalian glycosyltransferases on N-glycoprotein sialylation by the baculovirus-insect cell system. This was accomplished by using a novel baculovirus vector designed to express a mammalian alpha2,6-sialyltransferase early in infection and a new insect cell line stably transformed to constitutively express a mammalian beta1,4-galactosyltransferase. Various biochemical assays showed that a foreign glycoprotein was sialylated by this virus-host combination, but not by a control virus-host combination, which lacked the mammalian glycosyltransferase genes. Thus, this study demonstrates that the baculovirus-insect cell expression system can be metabolically engineered for N-glycoprotein sialylation by the addition of two mammalian glycosyltransferase genes.     

Differential lectin binding patterns in the oviductal ampulla of the horse during oestrus

We investigated the oligosaccharide sequence of glycoconjugates, mainly sialoglycoconjugates, in the horse oviductal ampulla during oestrus by means of lectin and pre-lectin methods such as the KOH-neuraminidase procedure to remove sialic acid residues and incubation with N-glycosidase F to cleave N-linked glycans. Ciliated cells displayed N-linked oligosaccharides throughout the cytoplasm. The cilia glycocalyx expressed both N- and O-linked (mucin-type) oligosaccharides, both showing a high variety of terminal sequences. In the most non-ciliated cells, the whole cytoplasm contained N-linked oligosaccharides with terminal alphaGal as well as mucin-type glycans with terminal Forssman pentasaccharides. In a few scattered non-ciliated cells, the whole cytoplasm displayed sialylated N-linked oligosaccharides with terminal Neu5Ac-GalNAc and O-linked glycans terminating with neutral and/or alphaGalNAc, Neu5Ac alpha2,6Gal/GalNAc, Neu5AcGal beta1,3GalNAc. Supra-nuclear granules, probably Golgi zones, of non-ciliated cells showed mainly O-linked glycans rich in sialic acid residues. The luminal surface of non-ciliated cells showed N-linked oligosaccharides, containing terminal/internal alphaMan/alphaGlc, betaGlcNAc and terminal alphaGal, as well as mucin-type oligosaccharides terminating with a large variety of either neutral saccharides or sialylated sequences. Apical protrusions containing O-linked oligosaccharides with terminal Forssman pentasaccharide, Neu5Ac-Gal beta1,4GlcNAc, Neu5Ac-GalNAc were seen in non-ciliated cells scattered along the epithelium. These findings show the presence of sialoglycoconjugates in the oviductal ampulla epithelium of the mare and the existence of different lectin binding profiles between ciliated and non-ciliated (secretory) cells, as well as the presence of non-ciliated cell sub-types which might determine functional differences along the ampullary epithelium of mare oviduct.     

Chemoenzymatic synthesis of diverse asparagine-linked α-(2,3)-sialyloligosaccharides

Partial sialyl transfer reaction by alpha-(2,3)-sialyltransferase toward (Gal-beta-1,4-GlcNAc-beta-1,2-Man-alpha-1,6/1,3-)(2)Man-beta-1,4-GlcNAc-beta-1,4-GlcNAc-beta-1-asparagine-Fmoc 1 was examined to obtain mono-alpha-(2,3)-sialyloligosaccharides and then branch-specific exo-glycosidase digestion (beta-D-galactosidase, N -acetyl-beta-D-glucosaminidase and alpha-D-mannosidase) toward the asialo-branch was performed to obtain diverse asparagine-linked complex type alpha-(2,3)-sialyloligosaccharides. In addition, two kinds of disialyloligosaccharides in which the sialyl linkage was a mixture of alpha-(2,3)- and alpha-(2,6)-types were also specifically prepared by an additional alpha-(2,6)-sialyltransferase reaction toward mono-alpha-(2,3)-sialyloligosaccharides thus obtained.     

Effect of genistein on topoisomerase activity and on the growth of [Val 12]Ha-ras-transformed NIH 3T3 cells

Genistein inhibited topoisomerase II and I; it increased the enzyme-DNA complex in L1210 cells at 1 micrograms/ml, and interfered with pBR322 DNA relaxation by the enzymes. To test the role of topoisomerase in the transformation by oncogenes, the effect of genistein on the transformation of NIH 3T3 cells by transfection with [Val 12]Ha-ras was compared with that of N-alpha-tosyl-L-lysyl-chloromethyl ketone (TLCK), since genistein inhibits tyrosine kinase as well as TLCK. Genistein reduced the number of foci of the transformed cells, and suppressed selectively the growth of ras-transformed NIH 3T3 cells but not normal NIH 3T3 cells. In contrast, TLCK did not affect the transformation. It inhibited the growth of the normal cells but not the transformed cells.     

Transfection of 2,6 and 2,3-sialyltransferase genes and GlcNAc-transferase genes into human glioma cell line U-373 MG affects glycoconjugate expression and enhances cell death

Human glioma cell line U-373 MG expresses CMP-NeuAc : Galbeta1,3GlcNAc alpha2,3-sialyltransferase [EC No. 2.4.99.6] (alpha2,3ST), UDP-GlcNAc : beta-d-mannoside beta1,6-N-acetylglucosaminyltransferase V [EC 2.4.1.155] (GnT-V) and UDP-GlcNAc3: beta-d-mannoside beta1,4-N-acetylglucosaminyltransferase III [EC 2.4.1.144] (GnT-III) but not CMP-NeuAc : Galbeta1,4GlcNAc alpha2,6-sialyltransferase [EC 2.4.99.1] (alpha2,6ST) under normal culture conditions. We have previously shown that transfection of the alpha2,6ST gene into U-373 cells replaced alpha2,3-linked sialic acids with alpha2,6 sialic acids, resulting in a marked inhibition of glioma cell invasivity and a significant reduction in adhesivity. We now show that U-373 cells, which are typically highly resistant to cell death induced by chemotherapeutic agents (< 10% death in 18 h), become more sensitive to apoptosis following overexpression of these four glycoprotein glycosyltransferases. U-373 cell viability showed a three-fold decrease (from 20 to 60% cell death) following treatment with staurosporine, C2-ceramide or etoposide, when either alpha2,6ST and GnT-V genes were stably overexpressed. Even glycosyltransferases typically raised in cancer cells, such as alpha2,3ST and GnT-III, were able to decrease viability two-fold (from 20 to 40% cell death) following stable overexpression. The increased susceptibility of glycosyltransferase-transfected U-373 cells to pro-apoptotic drugs was associated with increased ceramide levels in Rafts, increased caspase-3 activity and increased DNA fragmentation. In contrast, the same glycosyltransferase overexpression protected U-373 cells against a different class of apoptotic drugs, namely the phosphatidylinositol 3-kinase inhibitor LY294002. Thus altered surface protein glycosylation of a human glioblastoma cell line can lead to lowered resistance to chemotherapeutic agents.     

Engineering lepidopteran insect cells for sialoglycoprotein production by genetic transformation with mammalian beta 1,4-galactosyltransferase and alpha 2,6-sialyltransferase genes

Recombinant mammalian glycoproteins produced by the baculovirus-insect cell expression system usually do not have structurally authentic glycans. One reason for this limitation is the virtual absence in insect cells of certain glycosyltransferases, which are required for the biosynthesis of complex, terminally sialylated glycoproteins by mammalian cells. In this study, we genetically transformed insect cells with mammalian beta 1,4-galactosyltransferase and alpha 2,6-sialyltransferase genes. This produced a new insect cell line that can express both genes, serve as hosts for baculovirus infection, and produce foreign glycoproteins with terminally sialylated N-glycans.     

Amplification and overexpression of the met gene in spontaneously transformed NIH3T3 mouse fibroblasts.

We have identified a class of transformed NIH3T3 mouse fibroblasts that arise at low frequencies in transfection experiments with DNA from both neoplastic and non-neoplastic cells and that may result from a low level of spontaneous transformation of NIH3T3 cells. DNA from the transformed cells was unable to transform NIH3T3 cells in a second cycle of transfection and, where examined, the cells showed no evidence for the uptake of the transfected DNA sequences. The results of Southern analyses demonstrate that a mouse homologue of the human met oncogene is amplified 4- to 8-fold in 7 of 10 lines of these transformed NIH3T3 mouse fibroblasts. The cells containing the amplified gene also exhibit at least a 20-fold overexpression of an 8.5-kb mRNA that is homologous to met. To test the hypothesis that met encodes a growth factor receptor, we examined the binding of platelet-derived growth factor, epidermal growth factor, insulin-like growth factor I and gastrin-releasing peptide to transformed and non-transformed NIH3T3 cells. The results show that there is no significant elevation of the binding of these growth factors to cells containing amplification and overexpression of met.     

Engineering of Sialylated Mucin-type O-Glycosylation in Plants

Proper N- and O-glycosylation of recombinant proteins is important for their biological function. Although the N-glycan processing pathway of different expression hosts has been successfully modified in the past, comparatively little attention has been paid to the generation of customized O-linked glycans. Plants are attractive hosts for engineering of O-glycosylation steps, as they contain no endogenous glycosyltransferases that perform mammalian-type Ser/Thr glycosylation and could interfere with the production of defined O-glycans. Here, we produced mucin-type O-GalNAc and core 1 O-linked glycan structures on recombinant human erythropoietin fused to an IgG heavy chain fragment (EPO-Fc) by transient expression in Nicotiana benthamiana plants. Furthermore, for the generation of sialylated core 1 structures constructs encoding human polypeptide:N-acetylgalactosaminyltransferase 2, Drosophila melanogaster core 1 β1,3-galactosyltransferase, human α2,3-sialyltransferase, and Mus musculus α2,6-sialyltransferase were transiently co-expressed in N. benthamiana together with EPO-Fc and the machinery for sialylation of N-glycans. The formation of significant amounts of mono- and disialylated O-linked glycans was confirmed by liquid chromatography-electrospray ionization-mass spectrometry. Analysis of the three EPO glycopeptides carrying N-glycans revealed the presence of biantennary structures with terminal sialic acid residues. Our data demonstrate that N. benthamiana plants are amenable to engineering of the O-glycosylation pathway and can produce well defined human-type O- and N-linked glycans on recombinant therapeutics.     

Sialylation of human IgG-Fc carbohydrate by transfected rat alpha2,6-sialyltransferase

A recombinant IgG3 antibody with Phe-243 replaced by Ala (FA243) was expressed in a CHO-K1 parental cell line. The resulting IgG-Fc-linked carbohydrate was significantly alpha2,3-sialylated (53% of glycans), as indicated by normal- and reverse-phase HPLC analyses. Following transfection of a rat alpha2,6-sialyltransferase gene into this parental cell line, IgG-Fc-linked glycans were sialylated (60% of glycans) such that the ratio of alpha2,6- to alpha2,3-linked sialic acid was 0.9:1.0. By comparison, the wild-type IgG3 (F243) is minimally sialylated (2-3% alpha2,3-linked), thus suggesting that sialylation is controlled primarily by the protein structure local to the carbohydrate and that the two sialyltransferases compete to sialylate the nascent oligosaccharide. The additional alpha2,6-sialylation affected the function of the recombinant antibody. FA243 IgG3 having both alpha2,6 and alpha2,3-sialylation restored recognition to wild-type IgG3 levels for human FcgammaRI, FcgammaRII, and target cell lysis by complement. We discuss how sialylation linkage could modulate IgG function.     

Ammonium alters N-glycan structures of recombinant TNFR-IgG: degradative versus biosynthetic mechanisms

The effect of ammonium on the glycosylation pattern of the recombinant immunoadhesin tumor necrosis factor-IgG (TNFR-IgG) produced by Chinese hamster ovary cells is elucidated in this study. TNFR-IgG is a chimeric IgG fusion protein bearing one N-linked glycosylation site in the Fc region and three complex-type N-glycans in the TNF-receptor portion of each monomer. The ammonium concentration of batch suspension cultures was adjusted with glutamine and/or NH(4)Cl. The amount of galactose (Gal) and N-acetylneuraminic acid (NANA) residues on TNFR-IgG correlated in a dose-dependent manner with the ammonium concentration under which the N-linked oligosaccharides were synthesized. As ammonium increased from 1 to 15 mM, a concomitant decrease of up to 40% was observed in terminal galactosylation and sialylation of the molecule. Cell culture supernatants contained measurable beta-galactosidase and sialidase activity, which increased throughout the culture. The beta-galactosidase, but not the sialidase, level was proportional to the ammonium concentration. No loss of N-glycans was observed in incubation studies using beta-galactosidase and sialidase containing cell culture supernatants, suggesting that the ammonium effect was biosynthetic and not degradative. Several biosynthetic mechanisms were investigated. Ammonium (a weak base) is known to affect the pH of acidic intracellular compartments (e.g., trans-Golgi) as well as intracellular nucleotide sugar pools (increases UDP-N-acetylglucosamine and UDP-N-acetylgalactosamine). Ammonium might also affect the expression rates of beta1, 4-galactosyltransferase (beta1,4-GT) and alpha2,3-sialyltransferase (alpha2,3-ST). To separate these mechanisms, experiments were designed using chloroquine (changes intracellular pH) and glucosamine (increases UDP-GNAc pool [sum of UDP-GlcNAc and UDP-GalNAc]). The ammonium effect on TNFR-IgG oligosaccharide structures could be mimicked only by chloroquine, another weak base. No differences in N-glycosylation were found in the product synthesized in the presence of glucosamine. No differences in beta1, 4-galactosyltransferase (beta1,4-GT) and alpha2,3-sialyltransferase (alpha2,3-ST) messenger RNA (mRNA) and enzyme levels were observed in cells cultivated in the presence or absence of 13 mM NH(4)Cl. pH titration of endogenous CHO alpha2,3-ST and beta-1,4-GT revealed a sharp optimum at pH 6.5, the reported trans-Golgi pH. Thus, at pH 7.0 to 7.2, a likely trans-Golgi pH range in the presence of 10 to 15 mM ammonium, activities for both enzymes are reduced to 50% to 60%. Consequently, ammonium seems to alter the carbohydrate biosynthesis of TNFR-IgG by a pH-mediated effect on glycosyltransferase activity.