Construction and characterization of stably transfected BHK-21 cells with human-type sialylation characteristic

The human Golgi enzyme CMP-NeuAc:Gal(beta1-4)GlcNAc-R alpha2,6-sialyltransferase (ST6N) was stably coexpressed with human erythropoietin (EPO) from a BHK-21A cell line. The cell line was characterized with respect to the expression and in vitro activity of the ST6N and the endogenous alpha2,3-sialyltransferase. Detailed structural analysis of the N-linked carbohydrates of the rhuEPO expressed from the new cell line was performed by HPAE-PAD-mapping, MALDI/TOF-MS and methylation analysis after purification of the recombinant protein by immunoaffinity chromatography. This is the first report describing that the human alpha2,6-sialyltransferase is capable of sialylating, apart from Gal(beta1-4)GlcNAc-R, also GalNAc(beta1-4)GlcNAc-R motifs in vivo, which is not the case for the endogenous BHK-cell alpha2,3-sialyltransferase.

Process development of a recombinant antibody/interleukin-2 fusion protein expressed in protein-free medium by BHK cells

The production, purification and stability of quality (in terms of integrity and glycosylation) of an antibody/interleukin-2 fusion protein with potential application in tumour-targeted therapy expressed in BHK21 cells are described. Consistency of the product throughout time was determined by analysis of glycosylation of the fusion protein using MALDI-TOF mass spectroscopy and HPAEC-PAD combined with product integrity studies by SDS-PAGE and Western blotting. These investigations showed consistent expression in terms of integrity and of three major oligosaccharide structures of the fusion protein after 62 generations. The data obtained at this stage indicated the suitability of the cell line for production purposes. Different approaches for the production of this protein were subsequently carried out. The relative productivity of the recombinant fusion protein and general performance of the cells in two different protein-free medium (PFM) culture systems, continuous chemostat and continuous perfusion using a Centritech centrifuge as a cell retention device, were studied. The results indicate that the chemostat culture resulted in more stable and controllable nutrient environment, which could indicate better product consistency, in accordance with what has been observed under serum-containing conditions, in relation to the perfusion culture. Finally, product obtained from the chemostat culture was analysed and purified. The purification process was optimised with an increase in the overall yield from 38 to 70% being obtained, a significant improvement with important consequences for the implementation of an industrial-scale culture system. In conclusion, it was possible to produce and purify the recombinant antibody/interleukin-2 fusion protein assuring the quality and stability of the product in terms of integrity and glycosylation. Therefore, a candidate production process was established.

Structural analysis and antibody response to the extracellular glutathione S-transferases from Onchocerca volvulus

Onchocerca volvulus is a human pathogenic filarial parasite which, like other parasitic nematodes, is capable of surviving in an immunologically competent host by employing a variety of immune evasion strategies and defense mechanisms including the detoxification and repair mechanisms of the glutathione S-transferases (GSTs). In this study we analyzed the glycosylation pattern and the immunological properties of extracellular O. volvulus GST1a and -1b (OvGST1a and -1b). The enzymes differ in only 10 amino acids, and both are glycoproteins that have cleavable signal peptides and unusual N-terminal extensions. These characteristics have not been described for other GSTs so far. Mass spectrometry analyses indicate that both enzymes carry high-mannose type oligosaccharides on at least four glycosylation sites. Glycosylation sites 1 to 3 of OvGST1a (OvGST1b sites 2 to 4) are occupied by truncated N-glycans (Man(2)GlcNAc2 to Man(5)GlcNAc(2)), and N glycosylation site 4 of OvGST1a (OvGST1b site 5) carries Man(5)GlcNAc2 to Man(9)GlcNAc(2). To analyze the capacity of these secretory GSTs to stimulate host immune responses, we studied the antibody responses of onchocerciasis patients against the native affinity-purified OvGST1a and -1b. By enzyme-linked immunosorbent assay we showed that OvGST1a and -1b are immunodominant antigens, with less than 7% nonresponder patients. A direct comparison of the antibody responses to the glycosylated and deglycosylated forms demonstrates the high immunogenicity of the N-glycans. Analyses of the antibody responses to the unusual N-terminal extension show an enhanced recognition of this portion by patients as opposed to recognition of the recombinant protein without extension.

Production and molecular characterization of clinical phase i anti-melanoma mouse IgG3 monoclonal antibody R24

R24 is a mouse IgG3 monoclonal antibody (mab) that reacts with the ganglioside GD3 expressed by cells of neuroectodermal origin. The anti-tumor activity of R24 has been demonstrated in initial phase I and pilot trials in patients suffering from metastatic melanoma. The purpose of this study was to investigate the biotechnological production and particularly the glycosylation of this clinically important antibody. Growth, metabolism, and IgG production of R24 secreting hybridoma cells were analyzed on 1 L bioreactor bench scale using repeated-batch mode. The amount of 57 mg of pure mab was obtained from 1.6 L crude supernatant by protein A chromatography. Western blot binding assays with sugar-specific lectins revealed glycosylation of the heavy chains, whereas no carbohydrates were detectable on the light chains. Because glycosylation is essential for antibody effector functions in vivo (such as complement fixation or binding to macrophage Fc receptors), mab R24 was subjected to both enzymatic deglycosylation using PNGase F and chemical deglycosylation by hydrazinolysis. Released glycans were structurally characterized by high pH anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD), matrix assisted laser desorption ionization time-of-flight (MALDI-TOF), and electrospray ionization quadrupole time-of-flight (ESI-QTOF) mass spectrometry. Six major biantennary chains of the complex glycosylation phenotype were found with variations in galactosylation and core fucosylation. The predominant N-linked structure, indicating the high degree of agalactosyl glycoforms, was the agalacto biantennary chain with a relative percentage of 57% (51% core-fucosylated, 6% nonfucosylated). The second most abundant oligosaccharide was the monogalacto biantennary chain amounting to 30% (26% core- and 4% nonfucosylated). The antibody contained 0.46 microg sialic acid per mg protein, which splits into 0.243 microg Neu5Gc and 0.217 microg Neu5Ac, corresponding to a Neu5Ac:Neu5Gc ratio of 1:1.06. Furthermore, the antigen specificity of R24 was determined by immunodetection of GD3 on thin-layer chromatograms, and real time GD3-antibody binding interactions were measured with an optical biosensor (BIAcore). From the structural data obtained in this study it is concluded that glycosylation of the antibody may be important in the clinical outcome of targeted anti-cancer immunotherapy.

The cytoplasmic, transmembrane, and stem regions of glycosyltransferases specify their in vivo functional sublocalization and stability in the Golgi

We provide evidence for the presence of targeting signals in the cytoplasmic, transmembrane, and stem (CTS) regions of Golgi glycosyltransferases that mediate sorting of their intracellular catalytic activity into different functional subcompartmental areas of the Golgi. We have constructed chimeras of human alpha1, 3-fucosyltransferase VI (FT6) by replacement of its CTS region with those of late and early acting Golgi glycosyltransferases and have stably coexpressed these constructs in BHK-21 cells together with the secretory reporter glycoprotein human beta-trace protein. The sialyl Lewis X:Lewis X ratios detected in beta-trace protein indicate that the CTS regions of the early acting GlcNAc-transferases I (GnT-I) and III (GnT-III) specify backward targeting of the FT6 catalytic domain, whereas the CTS region of the late acting human alpha1,3-fucosyltransferase VII (FT7) causes forward targeting of the FT6 in vivo activity in the biosynthetic glycosylation pathway. The analysis of the in vivo functional activity of nine different CTS chimeras toward beta-trace protein allowed for a mapping of the CTS donor glycosyltransferases within the Golgi/trans-Golgi network: GnT-I < (ST6Gal I, ST3Gal III) < GnT-III < ST8Sia IV < GalT-I < (FT3, FT6) < ST3Gal IV < FT7. The sensitivity or resistance of the donor glycosyltransferases toward intracellular proteolysis is transferred to the chimeric enzymes together with their CTS regions. Apparently, there are at least three different signals contained in the CTS regions of glycosyltransferases mediating: first, their Golgi retention; second, their targeting to specific in vivo functional areas; and third, their susceptibility toward intracellular proteolysis as a tool for the regulation of the intracellular turnover.

Recombinant glycoprotein product quality in proliferation-controlled BHK-21 cells

We analyzed product quality to determine the applicability of proliferation-controlled mammalian cells for recombinant pharmaceutical protein production. Baby hamster kidney (BHK)-21 cells were engineered to express a dicistronic, stabilized, self-selecting growth control system consisting of a beta-estradiol-activatable transcription factor IRF-1 fusion protein. IRF-1 activity led to a reduced growth rate, whereas productivity, protein integrity, and glycosylation pattern of the industrially relevant secreted pharmaceutical glycoprotein erythropoietin remained consistent, showing that this technique has the potential for improving the consistency of high-quality pharmaceutical products and thus warrants further development.

Male-specific modification of human CD52

CD52 is an unusually short, bipolar glycopeptide bearing a highly charged N-linked carbohydrate moiety and a glycosylphosphatidylinositol membrane anchor. It is exclusively expressed on lymphocytes and in the male genital tract where it is shed into the seminal plasma and inserts into the sperm membrane. The sperm surface molecule has potential significance as a target for antibodies that inhibit sperm function and gamete interaction. Western blot analyses suggested cell type-specific modifications of the antigen. It was purified from seminal plasma and a detailed structural analysis performed. The majority of anchor structures in male genital tract CD52 showed 2-inositol palmitoylation, rendering molecules insensitive toward phospholipase C, and a sn-1-alkyl-2-lyso-glycerol structure in place of the diacylated anchor described by Treumann et al. (Treumann, A., Lifely, M. R., Schneider, P., and Ferguson, M. A. (1995) J. Biol. Chem. 270, 6088-6099). N-Glycans of the male genital tract product were based on bi-, tri-, and tetraantennary structures of highly charged (up to -7), terminally sialylated complex-type sugars. A substantial proportion carried varying numbers of lactosamine repeats of which nearly 30% were branched. Different from lymphocytes, 10-15% of all N-glycans of the male genital tract antigen also contained peripheral fucose. These data confirm that male genital tract CD52 is distinct from the lymphocyte form by both N-linked glycans and COOH-terminal attached lipid anchor.

Structural characterization of the oligosaccharide chains of native and crystallized boar seminal plasma spermadhesin PSP-I and PSP-II glycoforms

The PSP-I/PSP-II heterodimer is the major protein of boar seminal plasma. Both subunits are glycoproteins of the spermadhesin family and each contains a single N-glycosylation site. After enzymatic release of the oligosaccharides from isolated PSP-I and PSP-II, mainly neutral and monosialylated oligosaccharides, and small amounts of disialylated oligosaccharides, were recovered from both proteins. Twenty-two neutral oligosaccharides, 11 monosialylated glycans and three disialylated carbohydrate chains were characterized using mass spectrometric and NMR techniques. PSP-I and PSP-II share the same glycans but differ in their relative molar ratios. Most glycan structures are proximally alpha1-6-fucosylated, diantennary complex-type bearing nonsialylated or alpha2-6-sialylated N-acetyllactosamine or di-N-acetyllactosamine antennae. The majority of nonsialylated N-acetyllactosamine antennae bear terminal alpha1-3-linked Gal residues. In addition, the N-acetylglucosamine residue of nonsialylated N-acetyl and di-N-acetyllactosamine antennae can be modified by an alpha1-3-linked fucose residue. Structures of higher antennarity, as well as structures 3,6-branched at galactose residues, were found in smaller amounts. In one oligosaccharide, N-acetylneuraminic acid is substituted by N-glycolylneuraminic acid. Mass spectrometric analysis of PSP-I and PSP-II glycoforms isolated from crystallized PSP-I/PSP-II heterodimer showed the coexistence of major PSP-I and PSP-II glycoforms in the hexagonal crystals. Oligosaccharides with the NeuNAcalpha2-6GalNAcbeta1-4GlcNAc-R motif block adhesive and activation-related events mediated by CD22, suggesting a possible immunoregulatory activity for PSP-I/PSP-II.

beta-Trace protein in human cerebrospinal fluid: a diagnostic marker for N-glycosylation defects in brain

As carbohydrate-deficient glycoprotein syndromes (CDGS) are multisystemic disorders with impaired central nervous function in nearly all cases, we tested isoforms of beta-trace protein (beta TP), a 'brain-type' glycosylated protein in cerebrospinal fluid (CSF) of nine patients with the characteristic CDGS type I pattern of serum transferrin. Whereas the serum transferrin pattern did not discriminate between the various subtypes of CDGS type I (CDGS type Ia, type Ic, and patients with unknown defect), beta TP isoforms of CDGS type Ia patients differed from that of the other CDGS type I patients. The percentage of abnormal beta TP isoforms correlated with the severity of the neurological symptoms. Furthermore, two patients are described, who illustrate that abnormal protein N-glycosylation can occur restricted to either the 'peripheral' serum or the central nervous system compartment. This is the first report presenting evidence for an N-glycosylation defect restricted to the brain. Testing beta TP isoforms is a useful tool to detect protein N-glycosylation disorders in the central nervous system.

Incorporation of ammonium into intracellular UDP-activated N-acetylhexosamines and into carbohydrate structures in glycoproteins

The negative effects of ammonia on animal cells, especially in vitro cultures, are well known, but the mechanism of how ammonia inhibits cell growth and influences the glycosylation of proteins is not completely understood. We investigated the ammonium action on the synthesis of the intracellular UDP-N-acetylhexos- amines (UDPGNAc), which are precursors of glycosylation as well as on N-linked oligosaccharides of a recombinant human IL-2 mutant variant model glycoprotein expressed in BHK-21 cells under defined and controlled culture conditions in a continuously perfused bioreactor. The examinations were based on our previous observations that increased ammonia concentrations in the medium lead to the intracellular formation and accumulation of UDPGNAc (Ryll et al., 1994). The kinetics of formation of the UDPGNAc pool after adding ammonia and its reconstitution to normal conditions are shown. To study the pathway leading to the intracellular increase of UDPGNAc, the uptake and incorporation of 15NH4+ was confirmed by the detection of 15N in UDP-N-acetylglucosamine (UDP-GlcNAc). UDP-GlcNAc was purified using high pH anion-exchange chromatography with pulsed amperometric detection and analyzed by GC/MS. The proportion of UDP-GlcNAc containing 15N was approximately 60% and corresponds quantitatively to the increased intracellular concentration of UDP-GlcNAc. In order to confirm the direct influence of ammonia on protein glycosylation, the human IL-2 mutant glycoprotein variant IL-Mu6, bearing a novel N-glycosylation site, has been produced under defined protein-free medium conditions in the presence of 15NH4Cl. IL-Mu6 glycoprotein was purified and N-glycans released were analyzed by matrix-assisted laser desorption ionization time of flight mass spectroscopy. Maximally 60-80% of N-acetylated sugars in N-glycan structures contained 15N indicating that ammonium is used as a building block during synthesis of the carbohydrate structures expressed from in vitro cultivated mammalian cells.

Genetic engineering of recombinant glycoproteins and the glycosylation pathway in mammalian host cells

The analysis of many natural glycoproteins and their recombinant counterparts from mammalian hosts has revealed that the basic oligosaccharide structures and the site occupancy of glycosylated polypeptides are primarily dictated by the protein conformation. The equipment of many frequently used host cells (e.g. BHK-21 and CHO-cells) with glycosyltransferases, nucleotide-sugar synthases and transporters appears to be sufficient to guarantee complex-type glycosylation of recombinant proteins with a high degree of terminal alpha2-3 sialylation even under high expression conditions. Some human tissue-specific terminal carbohydrate motifs are not synthesized by these cells since they lack the proper sugar-transferring enzymes (e.g. alpha1-3/4 fucosyltransferases, alpha2-6 sialyltransferases). Glycosylation engineering of these hosts by stable transfection with genes encoding terminal human glycosyltransferases allows to obtain products with tailored (human tissue-specific) glycosylation in high yields. Using site-directed mutagenesis, unglycosylated polypeptides can be successfully converted in N- and/or O-glycoproteins by transferring glycosylation domains (consisting of 7-17 amino acids) from donor glycoproteins to different loop regions of acceptor proteins. The genetic engineering of glycoproteins and of host cell lines are considered to provide a versatile tool to obtain therapeutic glyco-products with novel/improved in-vivo properties, e.g. by introduction of specific tissue-targeting signals by a rational design of terminal glycosylation motifs.

In vivo specificity of human alpha1,3/4-fucosyltransferases III-VII in the biosynthesis of LewisX and Sialyl LewisX motifs on complex-type N-glycans. Coexpression studies from bhk-21 cells together with human beta-trace protein

Each of the five human alpha1,3/4-fucosyltransferases (FT3 to FT7) has been stably expressed in BHK-21 cells together with human beta-trace protein (beta-TP) as a secretory reporter glycoprotein. In order to study their in vivo properties for the transfer of peripheral Fuc onto N-linked complex-type glycans, detailed structural analysis was performed on the purified glycoprotein. All fucosyltransferases were found to peripherally fucosylate 19-52% of the diantennary beta-TP N-glycans, and all enzymes were capable of synthesizing the sialyl LewisX (sLex) motif. However, each enzyme produced its own characteristic ratio of sLex/Lex antennae as follows: FT7 (only sLex), FT3 (14:1), FT5 (3:1), FT6 (1.1:1), and FT4 (1:7). Fucose transfer onto beta-TP N-glycans was low in FT3 cells (11% of total antennae), whereas the values for FT7, FT5, FT4, and FT6 cells were 21, 25, 35, and 47%, respectively. FT3, FT4, FT5, and FT7 transfer preponderantly one Fuc per diantennary N-glycan. FT4 preferentially synthesizes di-Lex on asialo diantennary N-glycans and mono-Lex with monosialo chains. In contrast, FT6 forms mostly alpha1,3-difucosylated chains with no, one, or two NeuAc residues. FT3, FT4, and FT6 were proteolytically cleaved and released into the culture medium in significant amounts, whereas FT7 and FT5 were found to be largely resistant toward proteolysis. Studies on engineered soluble variants of FT6 indicate that these forms do not significantly contribute to the in vivo fucose transfer activity of the enzyme when expressed at activity levels comparable to those obtained for the wild-type Golgi form of FT6 in the recombinant host cells.

In vitro alpha1-3 or alpha1-4 fucosylation of type I and II oligosaccharides with secreted forms of recombinant human fucosyltransferases III and VI

Transgalactosylation of chitobiose and chitotriose employing beta-galactosidase from bovine testes yielded mixtures with beta1-3 linked galactose (type I) and beta1-4 linked galactose (type II) in a final ratio of 1:1 for the tri- and 1:1.4 for the tetrasaccharide. After 24 h incubations of the two purified oligosaccharide mixtures with large amounts (20-fold increase compared with standard conditions) of human alpha1,3/4-fucosyltransferase III (FucT III), the type I tri-/tetrasaccharides were completely converted to the Lewis(a) structure, whereas approximately 10% fucosylation of the type II isomers to the Lewis(x) oligosaccharides was observed in long-term incubations. Employing large amounts of human alpha1,3-fucosyltransferase VI (FucT VI), the type I trisaccharide substrate was exclusively fucosylated at the proximal O-4 substituted N-acetylglucosamine (GlcNAc) (20%) whereas almost all of the type II isomers was converted to the corresponding Lewis(x) product. 45% of the type I tetrasaccharide was fucosylated at the second GlcNAc solely by FucT VI. The type II isomer was almost completely alpha1-3 fucosylated to yield the Lewisx derivative with traces of a structure that contained an additional fucose at the reducing GlcNAc. The results obtained in the present study employing high amounts of enzyme confirmed our previous results that FucT III acts preponderantly as a beta1-4 fucosyltransferase onto GlcNAc in vitro. Human FucT VI attaches fucose exclusively in an alpha1-3 linkage to 4-substituted GlcNAc in vitro and does not modify any 3-substituted GlcNAc to yield Lewis(a) oligosaccharides. With 8-methoxycarbonyloctyl glycoside acceptors used under standard conditions, FucT III acts exclusively on the type I and FucT VI only on the type II derivative. With lacto-N-tetraose, lacto-N-fucopentraose I, or LS-tetrasaccharide as substrates, FucT III modified the 3-substituted GlcNAc and the reducing glucose; FucT VI recognized only lacto-N-neotetraose as a substrate.

Intracellular UDP-N-acetylhexosamine pool affects N-glycan complexity: a mechanism of ammonium action on protein glycosylation

Understanding the mechanisms by which ammonium ions affect glycosylation may suggest strategies for producing glycoproteins with homogeneous biological activity in the presence of undesirable byproducts of cellular energy metabolism. We have previously shown that ammonium ions cause an increase in the intracellular UDP-N-acetylhexosamine (UDPGNAc) pool, which may be responsible for the ammonium-induced increase in complexity and decrease in sialylation state of the N-linked oligosaccharide. To investigate this novel hypothesis, we induced an artificial increase in UDPGNAc pool by treating recombinant BHK cells expressing an IL-2 variant that features an artificial site for N-glycosylation, with glucosamine (1:2 molar ratio to glucose) and uridine (2 mmol L-1) in the absence of ammonium ions or glutamine. The product fractions collected during this treatment showed increased antennarity compared to product collected under control conditions. When this pool was returned to normal levels, the glycosylation pattern regained its original (control) features. However, the sialylation state remained unaffected, suggesting that the decreased sialylation observed under ammonium treatment is due to a different mechanism of action, possibly involving changes in intracellular pH. By pretreating the cells with 0.5 mmol L-1 adenosine, and exposing them continuously to NH4Cl and adenosine we were able to prevent the ammonium-induced increase in UDPGNAc. Product fractions collected during this treatment showed unchanged antennarity but decreased sialylation of the N-linked oligosaccharide, thus conclusively demonstrating that ammonium ions act on protein glycosylation by at least two independent mechanisms, one of which involves an increase in the UDPGNAc pool.

Hypoglycosylation of a brain glycoprotein (beta-trace protein) in CDG syndromes due to phosphomannomutase deficiency and N-acetylglucosaminyl-transferase II deficiency

Human beta-trace protein is a major intrathecally synthesized polypeptide constituent of human cerebrospinal fluid. We have previously shown that this protein is almost quantitatively modified with biantennary complex-type N-linked oligosaccharides which show "brain-type" glycosylation characteristics (Hoffmann,A. et al., J. Neurochem., 63, pp. 2185-2191, 1994). In the present study human beta-trace protein from the cerebrospinal fluid (CSF) of patients with carbohydrate-deficient glycoprotein syndrome (CDGS) due to phosphomannomutase (PMM) deficiency and N-acetyl-glucosaminyltransferase II (GlcNAc-T II) deficiency as well as from control individuals was studied by Western blot analysis. The protein from pooled CSFs was purified by immunoaffinity chromatography. The protein from the five patients with CDGS PMM deficiency showed three protein bands upon SDS-PAGE analysis corresponding to the di-, mono-, and unglycosylated polypeptide forms. Carbohydrate structural analysis of the enzymatically liberated N-glycans was performed applying mapping by HPAEC-PAD, methylation analysis as well as MALD/TOF-MS. Essentially identical oligosaccharide structures were detected in beta-TP from type I patients and control adult pooled CSF. The beta-trace protein from two patients with GlcNAc-T II deficiency showed a single di-N-glycosylated protein band with a significantly lower molecular weight than the di-glycosylated polypeptide from control patients and the beta-trace protein from pooled adult CSF. Beta-TP from GlcNAc-T II deficiency patients shared only three oligosaccharides out of the 13 observed in beta-TP from controls or patients with PMM deficiency. The major oligosaccharide structures of the glycoprotein from patients with GlcNAc-T II deficiency were found to be monoantennary asialo- or monosialylated lactosamine-type chains with proximal fucose and bisecting GlcNAc.

Identification of the human Lewis(a) carbohydrate motif in a secretory peroxidase from a plant cell suspension culture (Vaccinium myrtillus L.)

This paper reports for the first time the presence of the human Lewis(a) type determinant in glycoproteins secreted by plant cells. A single glycopeptide was identified in the tryptic hydrolysis of the peroxidase VMPxC1 from Vaccinium myrtillus L. by HPLC/ESI-MS. The oligosaccharide structures were elucidated by ESI-MS-MS and by methylation analysis before and after removal of fucose by mild acid hydrolysis. The major structure determined is of the biantennary plant complex type containing the outer chain motif Lewis(a) [structure in text]. A corresponding fucosyltransferase activity catalyzing the formation of Lewis(a) type structures in vitro was identified in cellular extracts of the suspension cultures.

Molecular characterization of beta-trace protein in human serum and urine: a potential diagnostic marker for renal diseases

We have isolated beta-trace protein from cerebrospinal fluid, serum, plasma, and urine samples of normal volunteers and sera and hemofiltrate of patients with chronic renal failure. Blood-derived and urinary beta-trace have significantly higher molecular weights than their cerebrospinal fluid counterpart, the amino acid sequences being identical. Oligosaccharide structural analysis revealed these molecular weight differences to be due to different N-glycosylation. beta-Trace from hemofiltrate and urine has larger sugar chains and concurrently significantly higher sialylation than cerebrospinal fluid-beta-trace which bears truncated "brain-type" oligosaccharide chains (published previously). beta-Trace concentrations were about 40 ng/ml for normal sera and plasma. 2000-6000 ng/ml were measured in sera of dialysis patients whereas in normal human cerebrospinal fluid, beta-trace concentration was about 8000 ng/ml. A reduced amount of 900 ng/ml was found in a single case of hydrocephalus cerebri. The sialylated glycoforms of beta-trace detected in the blood are presumably derived from resorbed cerebrospinal fluid protein whereas beta-TP-molecules bearing asialo-oligosaccharides are absent due to their hepatic clearance. The residual, sialylated beta-TP-species are probably eliminated from the blood via the kidney. This physiological clearance mechanism for the sialylated glycoforms is disturbed in hemodialysis patients resulting in about 100-fold elevated serum concentrations. These results let us suggest beta-trace may become a useful novel diagnostic protein in renal diseases.

Stable expression of the Golgi form and secretory variants of human fucosyltransferase III from BHK-21 cells. Purification and characterization of an engineered truncated form from the culture medium

Stable BHK-21 cell lines were constructed expressing the Golgi membrane-bound form and two secretory forms of the human alpha1, 3/4-fucosyltransferase (amino acids 35-361 and 46-361). It was found that 40% of the enzyme activity synthesized by cells transfected with the Golgi form of the fucosyltransferase was constitutively secreted into the medium. The corresponding enzyme detected by Western blot had an apparent molecular mass similar to those of the truncated secretory forms. The secretory variant (amino acids 46-361) was purified by a single affinity-chromatography step on GDP-Fractogel resin with a 20% final recovery. The purified enzyme had a unique NH2 terminus and contained N-linked endo H sensitive carbohydrate chains at its two glycosylation sites. The fucosyltransferase transferred fucose to the O-4 position of GlcNAc in small oligosaccharides, glycolipids, glycopeptides, and glycoproteins containing the type I Galbeta1-3GlcNAc motif. The acceptor oligosaccharide in bovine asialofetuin was identified as the Man-3 branched triantennary isomer with one Galbeta1-3GlcNAc. The type II motif Galbeta1-4GlcNAc in bi-, tri-, or tetraantennary neutral or alpha2-3/alpha2-6 sialylated oligosaccharides with or without N-acetyllactosamine repeats and in native glycoproteins were not modified. The soluble forms of fucosyltransferase III secreted by stably transfected cells may be used for in vitro synthesis of the Lewisa determinant on carbohydrates and glycoproteins, whereas Lewisx and sialyl-Lewisx structures cannot be synthesized.

Constitutive secretion of beta-trace protein by cultivated porcine choroid plexus epithelial cells: elucidation of its complete amino acid and cDNA sequences

Primary porcine choroid plexus epithelial cells cultivated in chemically defined medium maintain their epithelial characteristics and form confluent monolayers. They produce a fluid the composition of which resembles cerebrospinal fluid. The present study demonstrates constitutive secretion of large amounts of beta-trace protein. This intrathecally synthesized protein is a prominent polypeptide constituent of natural cerebrospinal fluid. According to the identity of amino acid sequences it has previously been tentatively identified as a prostaglandin-D synthase and as a member of the lipocalin protein family. beta-Trace was purified from cell culture supernatants and was subjected to tryptic digestion and amino acid sequencing of the resulting peptides. The complete primary structure of the protein was obtained by additional isolation of the cDNA from cultured epithelial cells. The porcine 163-amino acid polypeptide showed 69% identity with the human beta-trace and contained two N-glycosylation sites occupied by complex-type oligosaccharides as is the case for the human protein. The amino acid sequences around the N-glycosylation sites of mammalian beta-trace proteins (porcine, human, murine, and rat) were highly conserved. The nucleotide sequence was found to be less conserved; the porcine cDNA had a strikingly high GC-content (67%). The constitutive secretion of beta-trace protein from the in vitro cultivated porcine choroid plexus epithelial cells demonstrates that the cells have retained their major in vivo physiological properties: secretion of cerebrospinal fluid proteins. Therefore, this in vitro culture system may be used as a versatile tool for studying the regulation of the formation of cerebrospinal fluid.

Characterization of changes in the glycosylation pattern of recombinant proteins from BHK-21 cells due to different culture conditions

The N-glycosylation patterns of a genetically engineered human interleukin-2 variant glycoprotein (IL-Mu6), produced by BHK-21 cells from long-term suspension and microcarrier cultures in the presence and absence of fetal calf serum were compared. IL-Mu6 was used as a model protein in studying the effect of different controlled cell culture conditions on the expression of N-glycans in recombinant glycoproteins. IL-Mu6 contains a single amino acid substitution (Glu100<==>Asn) generating a potential N-glycosylation recognition site (Asn100-Xxx-Thr/Ser) in addition to the natural O-glycosylation at position Thr3. Parallel cell cultivations were carried out in two continuously perfused 2.5-liter stirred bioreactors under defined culture conditions. Major differences were found in the glycoprotein products obtained during these different cultivation conditions. Serum-free cultures resulted in a higher level of terminal sialylation and proximal alpha 1-6 fucosylation. The ratio of O- to N-glycans as well as the amount of nonglycosylated product and the antennarity of N-linked carbohydrates in the model protein exhibited major differences depending on the presence or absence of serum, the condition of growth and the cultivation procedure.

Construction of stable BHK-21 cells coexpressing human secretory glycoproteins and human Gal(beta 1-4)GlcNAc-R alpha 2,6-sialyltransferase alpha 2,6-linked NeuAc is preferentially attached to the Gal(beta 1-4)GlcNAc(beta 1-2)Man(alpha 1-3)-branch of diantennary oligosaccharides from secreted recombinant beta-trace protein

The human beta-trace protein has been cloned and has been expressed for the first time in a mammalian host cell line. Stable BHK-21 cell lines exhibiting altered terminal sialylation properties were constructed by cotransfection of cells with the plasmids pMT-beta TP or pAB3-1 which contain the cDNAs encoding the human secretory glycoproteins beta-trace protein or antithrombin III and pABSial containing the human Golgi enzyme CMP-NeuAc:Gal(beta 1-4)GlcNAc-R alpha 2,6-sialyltransferase (ST6N) gene. The beta-trace protein was purified by immunoaffinity chromatography and N-linked oligosaccharides were subjected to carbohydrate structural analysis. The enzymically liberated oligosaccharides were found to consist of 90% of diantennary chains as is the case for natural beta-trace protein from human cerebrospinal fluid. About 90% of the total oligosaccharides were recovered in the monosialo and disialo fractions in a ratio of 1:5. The monosialylated oligosaccharides of beta-trace protein coexpressed with human ST6N were found to contain NeuAc in alpha 2,6- or alpha 2,3-linkage in the same ratio. From 1H-NMR analysis as well as calculations of peak areas obtained by HPLC, 60% of the molecules of the disialo fraction were found to contain NeuAc in both alpha 2,3- and alpha 2,6-linkage to Gal beta(1-4)GlcNAc-R, whereas 40% of the molecules of this fraction contained NeuAc in only alpha 2,3-linkage to Gal(beta 1-4)GlcNAc-R. The alpha 2,6-linked NeuAc was shown to be attached preferentially to the Gal(beta 1-4)GlcNAc(beta 1-2)Man(alpha 1-3) branch of the diantennary structure. Therefore the in vivo specificity of the newly introduced recombinant human ST6N observed in this study supports the previously reported in vitro branch specificity of the bovine colostrum ST6N activity. Furthermore, these studies demonstrate the suitability of genetically engineered mammalian host cell lines with novel glycosylation properties for the production of human-type glycosylated secretory recombinant polypeptides.

Identification and structural characterization of a mannose-6-phosphate containing oligomannosidic N-glycan from human erythropoietin secreted by recombinant BHK-21 cells

A sialidase resistant mono-charged N-glycan was isolated from glycosylation site I (Asn-24) of recombinant human erythropoietin expressed from baby hamster kidney (BHK-21) cells and constituted approximately 2-4% of the oligosaccharide material at this glycosylation site. Mass spectrometry and both 1- and 2-dimensional NMR techniques revealed a high mannose type structure (Man6) with a phospho-diesterbridged N-acetylglucosamine as follows: [formula: see text]

'Brain-type' N-glycosylation of asialo-transferrin from human cerebrospinal fluid

Asialo-transferrin from human cerebrospinal fluid was purified to homogeneity. Investigation of the structural characteristics of its oligosaccharides support our hypothesis of 'brain-type' glycosylation of intrathecally synthesized cerebrospinal fluid proteins. For carbohydrate structural analysis, high-pH anion-exchange chromatography, methylation analysis, liquid secondary ion- and matrix-assisted laser desorption/ ionization mass spectrometry of the permethylated derivatives were used. The major structure turned out to be a complex-type agalactodiantennary oligosaccharide with bisecting N-acetylglucosamine and proximal fucose. Analysis of a second transferrin preparation containing both asialo- and sialo-transferrin revealed another major glycan species derived from the sialylated transferrin variant which is galactosylated and lacks bisecting N-acetylglucosamine and fucose.

Carbohydrate structures of beta-trace protein from human cerebrospinal fluid: evidence for "brain-type" N-glycosylation

The carbohydrate structures of beta-trace protein from human cerebrospinal fluid have been elucidated. This protein carries exclusively N-linked oligosaccharides at two sites (Asn29 and Asn56). Enzymatically released N-glycans were studied by compositional and methylation analyses, high-pH anion-exchange chromatography, and liquid secondary ion mass spectrometry. All glycans were found to be of the complex type, and most (90%) of them were biantennary with no (40%), one (40%), or two (20%) N-acetylneuraminic acid residues. The rest were triantennary chains or biantennary chains with intact or truncated lactosamine repeats. The innermost N-acetylglucosamine residues of nearly all structures were found to be alpha 1,6-fucosylated. Peripheral fucose (about 20% alpha 1,3-linked to N-acetylglucosamine) was also detected. Seventy percent of the oligosaccharides contained a bisecting N-acetylglucosamine. Especially in the neutral, but also in the monosialylated oligosaccharide fractions, many incomplete antennae consisting of N-acetylglucosamine only were present. At least 20 different N-glycans were identified. Analysis of the site-specific glycosylation patterns at Asn29 and Asn56 revealed only minor differences. According to the structural features (a high degree of fucosylation, high amounts of bisecting N-acetylglucosamine, as well as terminal N-acetylglucosamine and galactose residues, and significant amounts of N-acetylneuraminic acid in alpha 2,3 linkage), this protein can be classified as "brain-type" glycosylated.

A strategy for the mapping of N-glycans by high-performance capillary electrophoresis

We have evaluated high-performance capillary electrophoresis (HPCE) with respect to its suitability for use in establishing a carbohydrate-mapping database that would enable a carbohydrate structural analysis by mere comparison of migration times. The suitability of HPCE for carbohydrate structural assignments was ascertained by validation experiments. The migration times of distinct N-glycans, prepared and measured on different days, were shown to be highly reproducible, with a coefficient of variation of usually less than 0.20%, requiring only femtomoles of N-glycan per injection for reliable measurements. By including mesityl oxide and sialic acid as internal standards and a triple-correction method, HPCE fulfills the analytical requirements with respect to accuracy, precision, reproducibility, and sensitivity. The N-glycan-mapping database was established using a newly developed and optimized buffer system containing 1,5-diaminopentane as an organic modifier. Approximately 80 different sialylated N-glycans of known structure, which have thus far been measured and characterized, have been entered into our Lotus 1-2-3 mapping database. The database for structural determinations was tested using the N-linked carbohydrates released from recombinant human urinary erythropoietin (baby hamster kidney) by PNGase F treatment and from bovine serum fetuin and alpha 1-acid glycoprotein by automated and manual (large-scale) hydrazinolysis, respectively. The efficiency of the database and of the triple-correction method was further confirmed by HPCE measurements performed in a different laboratory and by a different analyst who used the HPCE system of a different manufacturer.

Purification and chemical characterization of beta-trace protein from human cerebrospinal fluid: its identification as prostaglandin D synthase

beta-Trace protein from pooled human CSF was purified to homogeneity. An apparent molecular mass of 23-29 kDa was determined for the polypeptide on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Amino-terminal sequencing of the polypeptide yielded the unique amino acid sequence APEAQVSVQPNFQQDKFLGRWFSA. Alignment of amino acid sequences obtained from tryptic peptides with the sequence previously deduced from a cDNA clone isolated by other investigators allowed the identification of beta-trace protein as prostaglandin D synthase [prostaglandin-H2 D-isomerase; (5Z, 13E)-(15S)-9 alpha, 11 alpha-epidioxy-15-hydroxyprosta-5,13-dienoate D-isomerase; EC 5.3.99.2]. A conservative amino acid exchange (Thr instead of Ser) was detected at amino acid position 154 of the beta-trace polypeptide chain in the corresponding tryptic peptide. The two N-glycosylation sites of the polypeptide were shown to be almost quantitatively occupied by carbohydrate. Carbohydrate compositional as well as methylation analysis indicated that Asn29 and Asn56 bear exclusively complex-type oligosaccharide structures (partially sialylated with alpha 2-3- and/or alpha 2-6-linked N-acetylneuraminic acid) that are almost quantitatively alpha 1-6 fucosylated at the proximal N-acetylglucosamine; approximately 70% of these molecules contain a bisecting N-acetylglucosamine. Agalacto structures as well as those with a peripheral fucose are also present.

Biosynthesis and secretion of human interleukin 2 glycoprotein variants from baculovirus-infected Sf21 cells. Characterization of polypeptides and posttranslational modifications

Human interleukin 2 (IL-2) and human IL-2 mutant proteins, with artificially introduced N-glycosylation or O-glycosylation sites, have been expressed in a lepidopteran cell line (Sf21, Spodoptera frugiperda) using recombinant baculovirus vectors. Only approximately 25% of the total recombinant IL-2 protein synthesized by Sf21 cells was secreted into the culture medium. Significant N-terminal truncations were detected in the secreted polypeptides (up to 85% of the molecules). Alanine and proline were absent in the major truncated forms; the first 3-5 amino acids were also absent in a small proportion of the purified proteins. The introduction of potential artificial O-glycosylation peptide sequences (..GGKAPTPPPK..), to the C-terminus or between positions 80 and 81 of the IL-2 polypeptide chain, resulted in the secretion of unglycosylated and O-glycosylated variant forms. Fast atom bombardment mass spectrometry, compositional analysis and methylation analysis, of the tryptic glycopeptide APTPPPK, revealed the presence of either GalNAc or the disaccharide Gal(beta 1-3)GalNAc as the only carbohydrate constituents attached exclusively to Thr in this peptide, in a specific ratio for each individual IL-2 mutant protein. The Gal(beta 1-3)GalNAc protein forms could be partially altered in vitro to mammalian-type glycoforms by porcine liver beta-galactoside alpha-2,3-sialyltransferase in the presence of CMP-N-acetylneuraminic acid. An IL-2 mutant form, with an 11-amino-acid peptide of human interferon-beta at position 4, which includes its only N-glycosylation site, had exclusively truncated proximally fucosylated oligomannosidic glycans; Man3GlcNAc[Fuc(alpha 1-6)]GlcNAc or Man2GlcNAc[Fuc(alpha 1-6)]GlcNAc structures, in a ratio of 3:1, were detected in the secreted proteins. No evidence was obtained for the presence of secreted proteins with complex oligosaccharide chains, irrespective of the cell-culture conditions used or the harvesting time, for infected cells with recombinant baculovirus constructs.

Structures of sialylated oligosaccharides of human erythropoietin expressed in recombinant BHK-21 cells

The native structures of the Asn-linked oligosaccharides and the O-glycans at Ser126 of human erythropoietin expressed from recombinant BHK cells have been elucidated. Enzymatically released N-glycans were studied by methylation analyses, fast-atom-bombardment mass spectrometry as well as one- and two-dimensional 1H-NMR spectrometry at 600 MHz. Many (82.7%) were found to be tetraantennary N-acetyllactosamine-type (22.8% with one, 3.6% with two and 0.4% with three N-acetyllactosamine repeats) being tetrasialylated (41%), trisialylated (29.6%) and disialylated (12.2%). A few (9.7%; 4.1% 2,4-branched, 5.6%, 2,6-branched) of the chains were triantennary (5.4% trisialyl, 4.3% disialyl) and 4.6% were of the disialyl diantennary type. Almost all of the innermost GlcNAc residues were alpha 1-6 fucosylated and NeuAc was exclusively alpha 2-3 linked to Gal beta 1-4GlcNAc-R; 60% of the protein was found to be O-glycosylated at Ser126; structures were monosialylated (70%) or disialylated (30%) forms of the Gal beta 1-3GalNAc core type. Glycosylation patterns at individual Asn-Xaa-Thr/Ser sites were determined by analytical high-pH anion-exchange chromatography with pulsed amperometric detection. Only tetraantennary chains with 0-3 N-acetyllactosamine repeats were detected at Asn38 and Asn83, while almost all of the di- and triantennary oligosaccharides were attached to Asn24. Batch analysis of different preparations of recombinant erythropoietin revealed the high reproducibility of the production procedure. Structures containing terminal GalNAc-GlcNAc were detected in small amounts in a few batches.

Decreased affinity of recombinant antithrombin for heparin due to increased glycosylation

Recombinant antithrombin produced by baby hamster kidney (BHK) or Chinese hamster ovary (CHO) cells was separated into two fractions, containing comparable amounts of protein, by affinity chromatography on matrix-linked heparin. Fluorescence titrations showed that the more tightly binding fraction had a heparin affinity similar to that of plasma antithrombin (Kd approximately 20 nM), whereas the affinity of the more weakly binding fraction was nearly 10-fold lower (Kd approximately 175 nM). Analyses of the heparin-catalysed rate of inhibition of thrombin further showed that the fractions differed only in their affinity for heparin and not in the intrinsic rate constant of either the uncatalysed or the heparin-catalysed inactivation of thrombin. The recombinant antithrombin fraction with lower heparin affinity migrated more slowly than both the fraction with higher affinity and plasma antithrombin in SDS/PAGE under reducing conditions, consistent with a slightly higher apparent relative molecular mass. This apparent size difference was abolished by the enzymic removal of the carbohydrate side chains from the proteins. Such removal also increased the heparin affinity of the weakly binding fraction, so that it eluted from matrix-linked heparin at a similar position to the deglycosylated tightly binding fraction or plasma antithrombin. Analyses of N-linked carbohydrate side chains showed that the weakly binding fraction from CHO cells had a higher proportion of tetra-antennary and a lower proportion of biantennary oligosaccharides than the tightly binding fraction. We conclude that the recombinant antithrombin produced by the two cell lines is heterogeneously glycosylated and that the increased carbohydrate content of a large proportion of the molecules results in a substantial decrease in the affinity of these molecules for heparin. These findings are of particular relevance for studies aimed at characterizing the heparin-binding site of recombinant antithrombin by site-directed mutagenesis.

A strategy for the mapping of N-glycans by high-pH anion-exchange chromatography with pulsed amperometric detection

We have evaluated the high-pH anion-exchange chromatography with pulsed amperometric detection (HPAE-PAD) with respect to its suitability to establish a carbohydrate mapping database that would enable carbohydrate structural analysis by mere comparison of retention times. The suitability of HPAE-PAD for carbohydrate structural analysis was ascertained by validation experiments. The retention times of distinct N-glycans, prepared and measured on different days, were shown to be highly reproducible, with a coefficient of variation (CV) of less than 0.5%, requiring less than 100 pmol of N-glycan per injection for reliable measurements. Including appropriate internal chromatographic standards, such as (Neu5Ac)1, (Neu5Ac)2, (Neu5Ac)3, and Neu5Gc, the HPAE-PAD method fulfills the analytical requirements with respect to accuracy, precision, reproducibility, and sensitivity. The N-glycan mapping database was established, using two optimized linear gradients "S" and "A" for sialylated and asialo N-glycans, respectively. Approximately 100 different N-glycans of known structure, which have thus far been measured and characterized, have entered our Lotus 1-2-3 mapping database. The efficiency of the database for structural determinations was tested, using the N-linked carbohydrates isolated from rhuEPO, expressed in BHK cells. Nine different sialylated N-glycans of rhuEPO (BHK) could be assigned with a deviation of less than +/- 0.5%, using gradient S, and six of the eight asialo N-glycans of rhuEPO (BHK) detected with gradient A could be assigned with an accuracy of less than +/- 1%, three of them even with an accuracy of less than 0.1%, providing the reliability of the established HPAE-PAD mapping database.

Carbohydrate structures of a human tissue plasminogen activator variant expressed in recombinant Chinese hamster ovary cells

The carbohydrate structures of a genetically engineered human tissue plasminogen activator variant bearing a single N-glycosylation site at Asn 448 are reported. After isolation of the tryptic glycopeptide and liberation of the N-linked carbohydrates by polypeptide:N-glycosidase F, 6 major oligosaccharide fractions were separated by HPLC on NH2-bonded phase. Their structures were determined by compositional and methylation analyses combined with fast atom bombardment mass spectrometry. Seventy percent of the carbohydrates were of the biantennary complex type with fucose at the proximal GlcNAc and zero, one or two alpha 2-3 linked NeuAc. The remainder were triantennary structures with one, two or three NeuAc.

Characterization of recombinant human antithrombin III synthesized in Chinese hamster ovary cells

Biochemical and physiochemical properties of recombinant human antithrombin III were examined. This protein, produced in Chinese hamster ovary cells, showed a conformation apparently identical with the natural product isolated from human plasma when examined by circular dichroism, UV absorbance, and fluorescence spectroscopy. Comparison of the NH2-terminal sequences of recombinant and human plasma-derived antithrombin III showed that on synthesis and secretion of the recombinant protein from Chinese hamster ovary cells the signal peptide is correctly cleaved by the corresponding endoplasmic signal peptidase. The recombinant antithrombin III has identical properties in heparin binding and biological activities as determined in vitro by two-dimensional immunoelectrophoresis, progressive inhibitor, and heparin cofactor assays. Analysis of the carbohydrate portion of recombinant antithrombin III synthesized in Chinese hamster ovary cells revealed glycosylation of the complex type. Characterization of the oligosaccharide chains present in the recombinant protein reveals three major fractions, A (20%), B (60%), and C (20%). Fraction A contains tri- and tetraantennary complex-type oligosaccharides, fraction B contains biantennary oligosaccharides, and fraction C partially truncated biantennary structures. Pharmacokinetic studies with recombinant and plasma-derived antithrombin III in rabbits showed that the clearance behavior of both proteins is very similar and can be described by a double exponential decrease with almost identical kinetic parameters.

Expression of human interleukin-2 in recombinant baby hamster kidney, Ltk-, and Chinese hamster ovary cells. Structure of O-linked carbohydrate chains and their location within the polypeptide

The similarity or identity of O-glycosylation in glycoproteins from natural sources or produced in heterologous cell lines, a central problem for the development of many biotechnologically relevant production processes, was examined using interleukin-2 (IL-2) as a model. Human interleukin-2 was constitutively expressed in several mammalian cell lines in high amounts. The recombinant proteins were purified to homogeneity and their carbohydrate structures were analyzed. Only the NeuAc alpha 2-3Gal beta 1-3[NeuAc alpha 2-6]GalNAc oligosaccharide structure or the NeuAc alpha 2-3Gal beta 1-3GalNAc were found in all IL-2 preparations secreted from recombinant Ltk-, Chinese hamster ovary, and baby hamster kidney cell lines. The O-linked chains were exclusively linked to Thr in position 3 of the polypeptide chain which is the carbohydrate attachment site in natural human IL-2. The proportions of O-glycosylated versus nonglycosylated forms of the protein secreted by each recombinant cell line were independent of productivity or of cell culture conditions. Our results show that O-glycosylated human IL-2 can be produced by applying recombinant DNA technology in heterologous cell lines with the same type of post-translational modification that is observed for the protein secreted from natural T lymphocytes.

Posttranslational modification of interleukin-2 is a late event during activation of human T lymphocytes by ionophore A23187 and phorbol ester

Human peripheral blood lymphocytes secrete high titers of interleukin-2 (IL-2) after stimulation by Ca2+-ionophore A23187/phorbol 12-myristate-13-acetate. During the first 30 hours of incubation cells secrete only the nonglycosylated IL-2 M form of the lymphokine, the glycosylated forms IL-2 N1,2 being detected only after prolonged culture times (30-48 h). After recultivation of cells for a second 48 h period (without additional mitogen), the glycosylated and nonglycosylated IL-2 forms are secreted at a constant ratio of 7:3 throughout. The detection of glycosylated IL-2 is parallelled by an increase in cellular glycosyltransferase activities involved in formation of sialylated oligosaccharides O-linked to proteins.

Structure of the carbohydrate moiety of human interferon-beta secreted by a recombinant Chinese hamster ovary cell line

The carbohydrate structure of the major oligosaccharide of human interferon-beta (IFN-beta) synthesized by a genetically engineered Chinese hamster ovary cell line has been determined. Analysis of the glycopeptidase F-released carbohydrates by sequential exoglycosidase treatment, methylation analysis, and fast atom bombardment-mass spectrometry revealed that 95% of the IFN-beta oligosaccharides had the following structure: (Formula: see text). The remaining 5% of the carbohydrates are probably tri- or higher antennary oligosaccharide chains. The major oligosaccharide of the recombinant IFN-beta is remarkably homogeneous with respect to terminal galactose sialylation. NeuAc, which is alpha 2-3-linked to galactose in the human IFN-beta secreted by Chinese hamster ovary cells, can be re-incorporated with an alpha 2-6 linkage in vitro, into enzymatically desialylated IFN-beta using rat liver Gal beta 1-4GlcNAc alpha 2-6 sialyltransferase. The sugar chain is important for maintaining protein solubility as shown by the fact that IFN-beta protein precipitates after deglycosylation with glycopeptidase F.

Secretion of glycosylated human interleukin-2 by recombinant mammalian cell lines

The production of glycosylated forms of the human T cell growth factor (interleukin-2, IL-2) has been studied after transfection of a mouse L cell line and a chinese hamster ovary cell line with a plasmid containing the human chromosomal interleukin-2 gene. Both cell lines produced IL-2 constitutively. Based on their behavior in reversed-phase l.c. and their sodium dodecyl sulfate-gel-electrophoresis pattern, human IL-2 protein secreted by L cells showed a similar distribution of glycosylated (Mr 16 500) and nonglycosylated (Mr 14 500) forms as the natural protein secreted by human peripheral lymphocytes, whereas the hamster cell line secreted preponderantly the glycosylated forms. Exoglycosidase digestion of the 16 500 Mr IL-2 protein shifted the gel electrophoretic mobility towards the low-molecular weight form as is true for the natural glycosylated IL-2, which contains the usual tetrasaccharide alpha-NeuAc-(2----3)-beta-D-Galp-(1----3)-[alpha-NeuAc-(2----6)]-D-GalNAc (IL-2 N2) and the trisaccharide alpha-NeuAc-(2----3)-beta-D-Galp-(1----3)-D-GalNAc (IL-2 N1) as the major carbohydrate constituents. These results support the applicability of recombinant DNA technology as a tool for studying glycoprotein biosynthesis in mammalian cells.

Structures of the major carbohydrates of natural human interleukin-2

Purified human interleukin-2 secreted by peripheral blood lymphocytes from healthy donors was found to exist in several forms. These forms were (partially) resolved by reversed-phase high-performance liquid chromatography and sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Two major polypeptide species (interleukin-2 N1 and N2, 16.5 kDa) were shown to be glycosylated on the basis of [3H]galactose/[3H]glucosamine incorporation and determination of amino sugars after acid hydrolysis. A third component (interleukin-2 M, 14.5 kDa) represents a nonglycosylated form. The amino acid composition and the NH2-terminal sequence of both forms are consistent with the data deduced from the cDNA coding for interleukin-2 after removal of a leader peptide of 20 amino acids. Carbohydrates are O-linked to the IL-2 protein via threonine-3 of the polypeptide chain. The oligosaccharides were released by reductive beta-elimination and were purified by gel filtration and high-performance liquid chromatography. Applying methylation analysis, exoglycosidase digestion and fast atom bombardment mass spectrometry the following major carbohydrate structures were identified: N1, NeuAc(alpha 2-3)Gal(beta 1-3)GalNAc-ol; and N2, NeuAc(alpha 2-3)Gal(beta 1-3)[NeuAc(alpha 2-6)]GalNAc-ol.

Purification of the T lymphocyte growth factor interleukin-2 from culture media of human peripheral blood leukocytes (buffy coats)

Interleukin 2 was purified 100 000-fold to apparent homogeneity from the supernatants of mitogen-stimulated human blood leukocytes. A sequence of three purification steps was used: affinity chromatography on the bound dye cibacron blue, gel filtration on Ultrogel AcA44, and reversed-phase high-performance liquid chromatography on hexyl phase. The resulting interleukin 2 had a specific activity of 2 X 10(6) U/mg protein, and was free of pyrogenicity in the rabbit test. The final purification product showed two bands in sodium dodecyl sulfate/polyacrylamide gels with apparent molecular masses of 15 kDa and 17 kDa respectively. Both bands were biologically active.