Is there a double pathway for the secretion of thyroglobulin: a "short circuit" weakly glycosyl-iodinated and a "long circuit" strongly glycosyl-iodinated?

Intact rat thyroid lobes incubated in vitro release recently synthesized thyroglobulin (Tg) into the media at a faster rate than they release thyroglobulin stored in follicular structures. Differential release of this Tg fraction cannot be explained by morphological alterations in thyroid architecture during incubation. This rapidly excreted fraction exhibits a low density on rubidium chloride gradients characteristic of poorly sialylated and poorly iodinated thyroglobulin, comigrating on rubidium chloride gradients with thyroglobulin isolated from tunicamycin treated glands. This poorly sialylated and poorly iodinated thyroglobulin is itself unaffected in its density or release into the media by tunicamycin treatment. Tg isolated from the media of tunicamycin treated glands has nearly the same low iodine and low sialic acid content as rat serum thyroglobulin and does not incorporate radiolabelled glucosamine. This fraction thus appear to duplicate properties of low glycosylated-low iodinated thyroglobulin released from thyroid cells in organisms that have no follicular structures and no follicular storage process as well as from thyroid tissue in patients with thyroid disease states, particularly thyroid tumors. Thus it is proposed a "short loop" pathway of low-glycosylated low-iodinated thyroglobulin directly into circulation, that bypasses and is not stored in the follicular lumen, the "long loop".     

High-performance anion-exchange chromatography of oligosaccharides using pellicular resins and pulsed amperometric detection

High-performance liquid chromatography using pellicular quaternary amine-bonded resins was used to separate a variety of neutral, sialylated, and phosphorylated oligosaccharides. At pH 4.6, sialylated compounds were separated according to number of negative charges, sialic acid linkage [alpha(2,3) compared to alpha(2,6)], and position of sialic acid linkage along a linear saccharide chain. At pH 13, the neutral sugar portion of the sialylated chain had a significant effect on the separation, due to oxyanion formation. Specifically, sialylated tetrasaccharides containing the Gal beta(1,3)GlcNAc sequence were retained much more than their Gal beta(1,4)GlcNAc- or Gal-beta(1,4)GalNAc-sialylated counterparts. Linear phosphorylated oligosaccharides could be completely separated according to number of charges and net carbohydrate content. Partial separation of linear-chain positional isomers, differing in either location of Man-6-PO4 in the chain or linkage position of Man or Man-6-PO4, was accomplished. Branched-chain phosphorylated compounds could be completely separated according to which antennae contained the Man-6-PO4. The electrochemical current generated by oxidation of sialylated, phosphorylated, and neutral oligosaccharides was compared to that of a glucose. The relative molar response factors for neutral, sialylated, and phosphorylated oligosaccharides ranged from 0.2 to 3.2. Neutral oligosaccharides gave the following molar responses for each group of structurally related compounds: (1) mono- and disaccharide, 1-1.3; (2) linear tri- and tetrasaccharides, 1.5-2.0; and (3) branched pentasaccharide-nonasaccharides, 2.4-3.1. Response factors for the sialyated compounds were not as consistent and were affected by linkage position of sialic acid. For oligosaccharides of the same size, increasing phosphorylation resulted in a twofold decrease in response factor for each added phosphate group. Therefore, conversion of sialylated and phosphorylated oligosaccharides to their neutral counterparts, using alkaline phosphatase or neuraminidase, respectively, was required for quantitative analysis of oligosaccharide mixtures using electrochemical response. Using this approach, complete separation of the parent neutral structures was obtained, the relative proportions of the neutral species were quantified, and the amount of sialic acid released was easily determined in a neuraminidase digest.     

Regulation of thyroglobulin glycosylation. A comparative study of the thyroglobulins from porcine thyroid glands and follicles in serum-free culture

Porcine thyroid cells were cultured in serum-free medium and thyrotropin was or was not added at day 4 and [3H]glucosamine at day 6 for 24 h. The major glycoprotein secreted outside the follicles proved to be thyroglobulin by immunoprecipitation, polyacrylamide gel electrophoresis, and amino acid composition. Thyroglobulin glycopeptides were analyzed by sequential affinity chromatography on immobilized lectins and compared to chemically labeled carbohydrate chains released from thyroid-derived thyroglobulin by hydrazinolysis. 82% and 85% of the glucosamine-labeled oligosaccharides of thyroglobulin from control and stimulated cells, respectively, were unretained on concanavalin A (ConA)-Sepharose compared to 46% only for in vivo thyroglobulin. 35-42% and 33-35% of the ConA-unbound glycopeptides were retarded on erythrophytohemagglutinin and leukophytohemagglutinin under basal or stimulatory conditions, respectively, while none of the triantennary structures of in vivo thyroglobulin was. Moreover, binding to Bandieraea-agarose showed that 20% of these complex structures contained alpha-linked galactose in thyroglobulin secreted by control cells, but only 10% in the molecules derived from thyroid. When analyzed on ricin-agarose after neuraminidase treatment, the ConA-unbound glycopeptides were retained to an extent of 65% for those from control cells and 98% for those from stimulated cells. Furthermore, 15% of desialylated ConA-unbound glycopeptides from cellular origin were also found to bind to wheat germ agglutinin. Carbohydrate composition, gel chromatography, and exoglycosidase treatment further demonstrated that thyroglobulin carbohydrate chains synthesized under serum-free cell culture were essentially composed of heterogeneous multiantennary structures instead of usual biantennary and high mannose type species. Under thyrotropin stimulation, 85% of the carbohydrate chains of thyroglobulin was shown to be sialylated by high performance liquid chromatography analysis instead of 65% under basal conditions, suggesting that thyrotropin may shift terminal glycosylation of thyroglobulin from alpha-galactose to sialic acid.     

Characterization of a unique glycoprotein antigen expressed on the surface of human neuroblastoma cells

In order to develop a molecular probe to delineate chemical and biological characteristics of human neuroblastoma cells, a murine monoclonal antibody (Mab 5G3) was produced that is directed to a glycoprotein, preferentially expressed on the surface of such cells. This antibody is of IgG2a isotype, has an association constant of 8 X 10(9) M-1, and reacts preferentially with human neuroblastoma cell lines and fresh frozen tissue sections in enzyme-linked immunosorbent assay and immunoperoxidase assays, respectively. Minimal reactivity is observed with a variety of lymphoblastoid cell lines and normal fetal and adult tissues. Mab 5G3 specifically recognizes a neuroblastoma target glycoprotein antigen of 215 kDa that is derived from a 200-kDa precursor, as evident from pulse-chase biosynthetic studies. Treatment with tunicamycin revealed that both molecules contain N-asparagine-linked oligosaccharides; however, only the 215-kDa species is resistant to treatment with endo-beta-N-acetylglucosaminidase H and sensitive to neuraminidase, indicating that it contains trimmed and terminally sialylated oligosaccharides of the "complex" type. In contrast, the 200-kDa precursor is sensitive to endo-beta-N-acetylglucosaminidase H and resistant to neuraminidase treatment indicating that it contains high-mannose non-processed oligosaccharides. The 215-kDa molecule is sulfated, phosphorylated at serine residues, and expressed on the cell surface. A molecule of 200 kDa is detected by Mab 5G3 in spent culture medium of human neuroblastoma cells which is neither sulfated nor phosphorylated.     

Human alpha-N-acetylgalactosaminidase: site occupancy and structure of N-linked oligosaccharides

Human alpha-N-acetylgalactosaminidase (alpha-GalNAc; also known as alpha-galactosidase B) is the lysosomal exoglycohydrolase that cleaves alpha-N-acetylgalactosaminyl moieties in glycoconjugates. Mutagenesis studies indicated that the first five (N124, N177, N201, N359, and N385) of the six potential N-glycosylation sites were occupied. Site 3 occupancy was important for enzyme function and stability. Characterization of the N-linked oligosaccharide structures on the secreted enzyme overexpressed in Chinese hamster ovary cells revealed highly heterogeneous structures consisting of complex (approximately 53%), hybrid (approximately 12%), and high mannose-type (approximately 33%) oligosaccharides. The complex structures were mono-, bi-, 2,4-tri-, 2,6-tri-, and tetraantennary, among which the biantennary structures were most predominant (approximately 53%). Approximately 80% of the complex oligo-saccharides had a core-region fucose and 50% of the complex oligosaccharides were sialylated exclusively with alpha-2,3-linked sialic acid residues. The majority of hybrid type oligo-saccharides were GalGlcNAcMan(6)GlcNAc-Fuc(0-1)GlcNAc. Approximately 54% of the hybrid oligosaccharide were phosphorylated and one-third of these structures were further sialylated, the latter representing unique phosphorylated and sialylated structures. Of the high mannose oligosaccharides, Man(5-7)GlcNAc(2) were the predominant species (approximately 90%) and about 50% of the high mannose oligosaccharides were phosphorylated, exclusively as monoesters whose positions were determined. Comparison of the oligosaccharide structures of alpha-GalNAc and alpha-galactosidase A, an evolutionary-related and highly homologous exoglycosidase, indicated that alpha-GalNAc had more completed complex chains, presumably due to differences in enzyme structure/domains, rate of biosynthesis, and/or aggregation of the overexpressed recombinant enzymes.     

Depletion of divalent cations within the secretory pathway inhibits the terminal glycosylation of complex carbohydrates of thyroglobulin.

Newly synthesized thyroglobulin transiting the secretory pathway is posttranslationally modified by addition of oligosaccharides to asparagine N-linked residues. The effect of divalent cation depletion on oligosaccharide processing of Tg was studied in FRTL-5 cells. Treatment with an ionophore, A23187, or thapsigargin, an inhibitor of the sarcoplasmic/endoplasmic reticulum ATPases delayed Tg secretion. These effects were accompanied by a normal distribution of the marker of the endoplasmic reticulum protein disulfide isomerase. Analysis of the thyroglobulin oligosaccharides by Bio-gel P4 chromatography showed that in the presence of A23187 and thapsigargin the addition of peripheral sialic acid and possibly galactose is inhibited. These findings were strengthened by experiments of exoglycosidase digestion and SDS-PAGE analysis of the resulting products. These results reveal a cellular mechanism of production of thyroglobulin with incompletely processed complex chains, i.e., the ligand of the recently described GlcNAc and asialoglycoprotein receptors of the thyroid. Since A23187 and thapsigargin inhibit biosynthetically the addition of peripheral sugars on N-linked oligosaccharides chains, the thyroglobulin molecules secreted in the presence of A23187 and thapsigargin should greatly facilitate studies on the function of the GlcNAc and asialoglycoprotein receptors of the thyroid.     

Glycosylation of human thyroglobulin and characterization by lectin affinity electrophoresis

Thyroglobulin, a 660 kDa glycoprotein, is the major product of protein synthesis in the thyroid gland. It has been suggested that modifications of thyroglobulin glycosylation occur in various thyroid disorders. In order to study possible changes in glycosylation of tissue thyroglobulin associated with thyroid disease, we have developed a lectin affinity electrophoresis system which allows characterization of small (less than 1 microgram) quantities of thyroglobulin. Human thyroglobulin was extracted and purified. Agarose gels were cast containing concanavalin A, Ricinus communis agglutinin, L-phytohaemagglutinin and pokeweed mitogen at various concentrations. Purified human thyroglobulin was serially diluted, loaded onto lectin gels and electrophoresed. Concanavalin A, R. communis agglutinin and phytohaemagglutinin all bound thyroglobulin in a concentration-dependent manner. Pokeweed mitogen did not bind thyroglobulin. Purified thyroglobulin was treated with neuraminidase and endoglycosidase H. Two-dimensional immunoelectrophoresis revealed the migration of thyroglobulin to be modified by neuraminidase but not by endoglycosidase H. Lectin affinity electrophoresis of purified human thyroglobulin with and without enzyme treatment indicated the presence of: oligomannose structures as shown by concanavalin A reactivity and modification by endoglycosidase H, and complex oligosaccharides as shown by affinity for R. communis agglutinin and modification by neuraminidase. These structures are in keeping with the proposed patterns of glycosylation of human thyroglobulin and indicate suitability of the method for characterizing the glycosylation of small quantities of thyroglobulin.     

Asparagine-linked oligosaccharides on lutropin, follitropin, and thyrotropin. I. Structural elucidation of the sulfated and sialylated oligosaccharides on bovine, ovine, and human pituitary glycoprotein hormones

We have elucidated the structures of the anionic asparagine-linked oligosaccharides present on the glycoprotein hormones lutropin (luteinizing hormone), follitropin (follicle-stimulating hormone), and thyrotropin (thyroid-stimulating hormone). Purified hormones, isolated from bovine, ovine, and human pituitaries, were digested with N-glycanase, and the released oligosaccharides were reduced with NaB[3H]4. The 3H-labeled oligosaccharides from each hormone were then fractionated by anion-exchange high performance liquid chromatography (HPLC) into populations differing in the number of sulfate and/or sialic acid moieties. The anionic oligosaccharides were further purified as well as structurally characterized using a variety of preparative and analytical techniques, including HPLC, endo- and exoglycosidase digestions, and lectin affinity chromatography. The sulfated, sialylated, and sulfated/sialylated structures, which together comprised 67-90% of the asparagine-linked oligosaccharides on the pituitary glycoprotein hormones, were highly heterogeneous and displayed hormone- as well as animal species-specific features. The sulfated oligosaccharides consisted of hybrid and complex type oligosaccharides with one or two branches terminating in SO4-4GalNAc beta 1,4. In contrast, the sialylated oligosaccharides consisted of a wide array of differing structures containing two or three peripheral branches as well as one, two, or three sialic acid moieties. A previously uncharacterized dibranched oligosaccharide, bearing one residue each of sulfate and sialic acid, was found on all of the hormones except bovine lutropin. In this study, we describe the purification and detailed structural characterizations of the sulfated, sialylated, and sulfated/sialylated oligosaccharides found on lutropin, follitropin, and thyrotropin from several animal species. In the accompanying paper (Green, E.D., and Baenziger, J.U.(1987) J. Biol. Chem. 262, 36-44) we demonstrate the marked quantitative differences among the pituitary glycoprotein hormones in terms of sulfation, sialylation, and underlying oligosaccharide structures, as well as provide evidence for site-specific synthesis of oligosaccharides on individual hormones.     

Modulation of the carbohydrate moiety of thyroglobulin by thyrotropin and calcium in Fisher rat thyroid line-5 cells.

Thyroglobulin secreted in the medium by Fisher rat thyroid line-5 (FRTL-5) cells cultured in the presence of thyroid stimulating hormone (TSH) shows a slower electrophoretic mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and a higher density position in a CsCl gradient than thyroglobulin secreted by FRTL-5 cells cultured in the absence of TSH for 5-7 days. Such a TSH effect is much less or not evident when secreted thyroglobulin is digested with peptide N-glycohydrolase F or when intracellular thyroglobulin is compared. Intracellular thyroglobulin migrates faster than thyroglobulin secreted either in the presence or in the absence of TSH. Evaluation of the mannose and galactose content of thyroglobulin demonstrates that intracellular thyroglobulin has more mannose and less galactose than extracellular thyroglobulin; it also shows that TSH decreases the mannose content of thyroglobulin while increasing its galactose content. Bio-Gel P6 chromatography shows that TSH increases the complex type carbohydrate chains while decreasing the high mannose chains in the secreted thyroglobulin. High mannose type oligosaccharides were characterized by fast atom bombardment-mass spectrometry analysis. Treatment with the calcium ionophore A23187 (5 microM) of FRTL-5 cells cultured with or without TSH causes the appearance of a "fast" migrating form of thyroglobulinin in the culture medium. Bio-Gel P6 chromatography shows that A23187 causes a dramatic decrease of the complex carbohydrate chains of the secreted thyroglobulin.     

Biochemical characterization of serum thyroglobulin from patients with bone metastases from follicular carcinoma of the thyroid.

The biochemical properties of serum thyroglobulin obtained from six patients with follicular carcinoma of the thyroid and distant metastases to bone(s) have been studied. Since it is difficult to isolate sufficient thyroglobulin from serum samples, in vivo radioiodinated serum thyroglobulin obtained after radioiodine administration was used. In contrast to a sharp salting-out pattern observed with native thyroglobulin isolated from normal thyroid tissue, a broad salting-out curve was seen with metastatic serum thyroglobulin. The metastatic serum thyroglobulin eluted with low ionic strength from ion-exchange column. More than 95% of metastatic serum thyroglobulin could be bound to concanavalin-A sepharose and be eluted with 0.5 M alpha-methyl mannoside. The reactivity of metastatic serum thyroglobulin and native thyroglobulin towards concanavalin-A was comparable. Both types of thyroglobulins showed identical mobilities on sucrose linear density gradient centrifugation. The metastatic serum thyroglobulin from follicular carcinoma of the thyroid thus appears to be 19 S thyroglobulin with near normal concanavalin-A binding sugars but altered surface charges.     

Demonstration of a heterogeneous transcription pattern of thyroglobulin mRNA in human thyroid tissues.

Previous reports on human thyroglobulin (hTg) modifications in thyroid carcinomas prompted us to study hTg mRNA in thyroid adenomas and carcinomas. The quantification of hTg mRNA showed a decrease in its levels of expression in both pathological conditions which differed by a factor of 2 between adenomas and carcinomas. Furthermore, PCR was used to analyse the characteristics of hTg mRNA by amplifying 4 regions of the hTg mRNA. When applied to 2 normal, 17 benign and 13 malignant pathological tissue specimens, PCR showed no modification in the size of Tg mRNA. However, abnormal sized cDNAs appeared in all tissues with no distinction between the pathologies; the Restriction Fragment Length Polymorphism study of these cDNAs suggests the existence of alternate splicing patterns in thyroglobulin mRNAs.     

Afrikander cattle congenital goiter: size heterogeneity in thyroglobulin mRNA

The mRNA coding for thyroglobulin in cattle homozygous for an autosomal recessive defect of thyroglobulin synthesis was investigated using a recombinant plasmid containing bovine mRNA coding sequence. Total RNA preparations from goiter contained one third of the thyroglobulin mRNA sequences found in normal thyroid tissue. This mRNA was not translated into thyroglobulin by Xenopus oocytes. Northern transfer analysis revealed both a normal sized and a smaller thyroglobulin mRNA in the goiter.     

In vitro studies of the thyroglobulin degradation pathway: endocytosis and delivery of thyroglobulin to lysosomes, release of thyroglobulin cleavage products--iodotyrosines and iodothyronines

Iodinated thyroglobulin stored in the thyroid follicular lumen is subjected to an internalization process and thought to be transferred into the lysosomal compartment for proteolytic cleavage and thyroid hormone release. In the present study, we have designed in vitro models to study: 1) the transfer of endocytosed thyroglobulin into lysosomes, and 2) the intracellular fate of free thyroid hormones and iodinated precursors generated by intralysosomal proteolysis of thyroglobulin. Open follicles prepared from pig thyroid tissue by collagenase treatment were used to probe the delivery of exogenous thyroglobulin to lysosomes via the differentiated apical cell membrane. Open follicles were incubated with pure [125I]thyroglobulin with or without unlabeled thyroglobulin in the presence or in the absence of chloroquine. Subcellular fractionation on a Percoll gradient showed that [125I]thyroglobulin was internalized and present in low (for the major part) and high density thyroid vesicles. In chloroquine-treated open follicles, we observed the appearance of a definite fraction of [125I]thyroglobulin in a lysosome subpopulation having the expected properties of phagolysosomes or secondary lysosomes. In contrast, in control open follicles, the amount of [125I]thyroglobulin or degradation products found in high density vesicles was lower and associated with the bulk of lysosomes, i.e., primary lysosomes. The content in thyroglobulin and degradation products of lysosomes at steady-state was analyzed by Western blot using polyclonal anti-pig thyroglobulin antibodies. Under reducing conditions, immunoreactive thyroglobulin species correspond to polypeptides with molecular weights ranging from 130,000 to less than 20,000. The presence of free thyroid hormones and iodotyrosines inside lysosomes and their intracellular fate was studied in dispersed thyroid cells labeled with [125I]iodide. Neo-iodinated [125I]thyroglobulin gave rise to free [125I]T4 which was secreted into the medium. In addition to released [125I]T4, a fraction of free [125I]T4 was identified inside the cells. Lysosomes isolated from dispersed thyroid cells did not contain significant amounts of free [125I]T4. The free intracellular [125I]T4 fraction seems to represent an intermediate 'hormonal pool' between thyroglobulin-bound T4 and secreted T4. Evidence for such a precursor-product relationship was obtained from pulse-chase experiments. In conclusion: 1) open thyroid follicles have the ability to internalize thyroglobulin by a mechanism of limited capacity and to address the endocytosed ligand to lysosomes.(ABSTRACT TRUNCATED AT 400 WORDS)     

Insulin/IGF-I hybrid receptors play a major role in IGF-I signaling in thyroid cancer.

The insulin-like growth factor-I (IGF-I) plays an important role in determining the biological behavior of a variety of malignancies. We measured IGF-I, its receptor and related receptors in thyroid cancer. IGF-I was present both in normal thyroid tissue and in thyroid cancer tissue and it was produced by stromal cells but not by thyrocytes. Values were significantly higher in malignant than in normal tissue. IGF-I receptors (IGF-I-Rs) and the homologous insulin receptors (IRs) were found overexpressed in both thyroid cancer cell lines (n = 4) and specimens (n = 17) as compared to normal values. In addition, high levels of hybrid IGF-I/insulin receptors (IR/IGF-I-Rs) were present in both thyroid cancer specimens and cell lines. IR/IGF-I-R hybrids were the most represented type of receptor in 14/17 specimens and exceeded the IGF-I-R content in all cases. Hybrid content correlated with the IR and IGF-I-R content, suggesting that in thyroid tissue hybrid formation occurs by random assembly of IR and IGF-I-R half receptors. Hybrid receptor autophosphorylation was stimulated by IGF-I with high affinity. In cells with a high IR/IGF-I-Rs content, blocking antibodies specific to these receptors substantially inhibited IGF-I induced cell growth. These data indicate that the IGF-I system is overactivated in thyroid cancer and that IR/IGF-I-R hybrid receptors play an important role in IGF-I mitogenic signaling in these tumors.     

The asparagine-linked oligosaccharides on bovine fetuin. Structural analysis of N-glycanase-released oligosaccharides by 500-megahertz 1H NMR spectroscopy

The structures of the entire population of sialylated asparagine-linked oligosaccharides present on bovine fetuin were elucidated. Asparagine-linked oligosaccharides were released from fetuin with N-glycanase, radiolabeled by reduction with NaB[3H]4, and fractionated by anion-exchange high performance liquid chromatography (HPLC), ion-suppression amine adsorption HPLC, and concanavalin A affinity chromatography. The 3H-labeled oligosaccharide fractions obtained were analyzed by 500-MHz 1H nuclear magnetic resonance spectroscopy, revealing the presence of 23 distinct oligosaccharide structures. These oligosaccharides differed in extent of sialylation (3% mono-, 35% di-, 54% tri-, and 8% tetrasialylated), number of peripheral branches (17% di- and 83% tribranched), linkage (alpha 2,3 versus alpha 2,6) and location of sialic acid moieties, and linkage (beta 1,4 versus beta 1,3) of galactose residues. This represents the first time that the asparagine-linked oligosaccharides of fetuin have been successfully fractionated and characterized as sialylated species. The sialylated oligosaccharides derived from fetuin were also used to further define the specificities of the lectins leukoagglutinating phytohemagglutinin and Ricinus communis agglutinin I. The behavior of these oligosaccharides during lectin affinity HPLC further establishes the structural features which predominate in the interaction of oligosaccharides with leukoagglutinating phytohemagglutinin and R. communis agglutinin I.     

Carbohydrate structure of glycoprotein 65 encoded by the polycythemia-inducing strain of Friend spleen focus-forming virus

The secondary envelope-gene product, glycoprotein 65 (gp65), of the polycythemia-inducing variant of Friend spleen focus-forming virus (F-SFFVp) was isolated from F-SFFVp-infected normal rat kidney cells cultivated in the presence or absence (-Glc) of glucose. Oligosaccharide side chains present were sequentially liberated by treatment of tryptic glycopeptides with endo-beta-N-acetylglucosaminidase H and peptide N-glycosidase F and fractionated by high-performance liquid chromatography. The glycans were characterized by digestion with exoglycosidases, by chromatographic comparison with oligosaccharide standards and by methylation analysis. The results demonstrate that gp65 contains oligomannosidic, hybrid and N-acetyllactosaminic glycans. The oligomannosidic glycans represent the same partially glucosylated species with six to nine mannose residues present in F-SFFVp gp52, the biosynthetic precursor of gp65 [Strube, K.-H. Schott, H.-H. and Geyer, R. (1988) J. Biol. Chem. 263, 3762-3771]. Oligosaccharides of the hybrid type were found to comprise one sialylated lactosamine unit and three or four alpha-linked mannose residues. Analysis of the N-acetyllactosaminic glycans revealed that gp65 carries fucosylated, partially sialylated bi-antennary, tri-antennary and tetra-antennary oligosaccharides, in addition to incomplete species. The glycosylation of gp65(-Glc) is characterized by the presence of oligomannosidic glycans with five to nine mannose residues, similar hybrid-type species and by increased amounts of incomplete N-acetyllactosaminic oligosaccharides, a decrease in sialylation and the lack of tetra-antennary species.     

Structure, pathology and function of the N-linked sugar chains of human chorionic gonadotropin

Human chorionic gonadotropin (hCG) contains five acidic N-linked sugar chains, which are derived from three neutral oligosaccharides by sialylation. Each of the two subunits (hCGalpha and hCGbeta) of hCG contain two glycosylated Asn residues. Glycopeptides, each containing a single glycosylated Asn, were obtained by digestion of hCGalpha with trypsin, and of hCGbeta with chymotrypsin and lysyl endopeptidase. Comparative study of the sugar chains of the four glycopeptides revealed the occurrence of site-directed glycosylation. Studies of the sugar chains of hCGs, purified from urine of patients with various trophoblastic diseases, revealed that choriocarcinoma hCGs contain sialylated or non-sialylated forms of eight neutral oligosaccharides. In contrast, hCGs from invasive mole patients contain sialyl derivatives of five neutral oligosaccharides. The structural characteristics of the five neutral oligosaccharides, detected in choriocarcinoma hCGs but not in normal placental hCGs, indicate that N-acetylglucosaminyltransferase IV (GnT-IV) is abnormally expressed in the malignant cells. This supposition was confirmed by molecular biological study of GnT-IV in placenta and choriocarcinoma cell lines. The appearance of tumor-specific sugar chains in hCG has been used to develop a diagnostic method of searching for malignant trophoblastic diseases. In addition, a summary of the current knowledge concerning the functional role of N-linked sugar chains in the expression of the hormonal activity of hCG has been presented.     

Studies of the congenitally goitrous sheep. Iodoproteins of the goitre

1. Congenitally goitrous thyroid tissue was obtained from South Australian Merino sheep. Ultrastructural studies of the secretory cells in this tissue showed active cells of normal appearance, containing apical protein droplets. 2. (125)I-labelling in vivo of goitre tissue was used to investigate the iodoproteins, in which the major proportion of (125)I appeared in the cell protein fraction soluble in 0.9% sodium chloride (average 62% in goitres from untreated sheep). 3. Ammonium sulphate fractionation showed two clear peaks of iodoprotein precipitation, one at 35-40% saturation and the other at 50-55% saturation. Both iodoprotein fractions contained iodotyrosines and iodothyronines, which were identified chromatographically after enzymic hydrolysis of the protein. 4. Polyacrylamide-gel electrophoresis at pH9.4, at either 7.5 or 5.0% acrylamide concentration, was used to characterize the iodoproteins. Two major fractions were observed, the fastest-migrating fraction coincident with serum albumin, and a slower-migrating, less-well-defined zone. This fraction migrated in 7.5% acrylamide gel, which excluded normal thyroglobulin. 5. Density-gradient (10-40% sucrose) centrifugation was used to determine the approximate sedimentation coefficients of the iodoproteins, which showed major components at s(20,w) 8-9S and s(20,w)<5S. 6. Immunoprecipitation with rabbit anti-(sheep thyroglobulin) failed to sediment (125)I-labelled proteins from goitre extracts. 7. Ouchterlony-type double diffusion in agar plates demonstrated immunoprecipitation lines between rabbit anti-(sheep thyroglobulin) and both the concentrated goitre extract and its Sephadex G-200-excluded fraction, which were confluent with that obtained on reaction with purified normal thyroglobulin. 8. It was concluded that both major iodoprotein fractions were capable of supplying thyroid hormones to the animal, and that the fraction of s(20,w)<5S was iodinated serum albumin. As (125)I-labelled thyroglobulin was not detected in goitre tissue from untreated or thyroxine-treated animals, it was possible that the genetic defect causing goitre resulted in an abnormal thyroglobulin, incapable of being iodinated but immunologically reactive.     

Quantitative mapping of the N-linked sialyloligosaccharides of recombinant erythropoietin: combination of direct high-performance anion-exchange chromatography and 2-aminopyridine derivatization.

A rapid quantitative analysis of the sialylated N-linked oligosaccharides of recombinant erythropoietin (EPO) expressed in Chinese hamster ovary (CHO) cells has been developed. The procedure utilizes a glycoamidase (glycopeptidase F) to release all of the N-linked oligosaccharides from the native glycoprotein, followed by direct chromatographic analysis using high-performance anion-exchange chromatography (HPAEC) with pulsed amperometric detection. The eight sialyloligosaccharides isolated from HPAEC were characterized by derivatizing with 2-aminopyridine followed by two-dimensional HPLC mapping of the pyridylaminated asialooligosaccharides (Tomiya et al., 1988, Anal. Biochem. 171, 73-90). Seven kinds of complex-type asialooligosaccharides were identified ranging from a biantennary structure to N-acetyllactosamine-extended tetraantennary structure. Approximately 3% of the terminal galactose residues of the oligosaccharides released from EPO were not sialylated whereas 97% contained an alpha(2-->3)-linked sialic acid. Quantitative oligosaccharide mapping of four different lots of EPO from CHO cells was performed to quantify the molar balance and distribution of the N-linked oligosaccharides. The sialyloligosaccharides were distributed with approximately 5% disialylated (single type), 20% trisialylated (six types), and 75% tetrasialylated (four types) oligosaccharides with an average molar recovery of 85% starting from 750 pmol of EPO.