Glycoprotein biosynthesis in calf pituitary. Chemical characterization of mannose- and glucose-radiolabeled oligosaccharides

Fluorescence method for the structural analysis of oligomannose-type sugar chains by partial acetolysis

Fluorescence labeling was used in the analysis of partial acetolysis products of oligomannose-type sugar chains with five to nine mannose residues. The principle of the method was the pyridylamination of fragments obtained by the partial acetolysis of pyridylamino sugar chains and the identification of the fragments with an HPLC apparatus equipped with a fluorescence spectrophotometer. The method was tested by analysis of eight oligomannose-type sugar chains with known chemical structures and was found to be effective for analysis of branching structures with samples of 0.5 nmol.

Characterization of the oligosaccharides of prolyl hydroxylase, a microsomal glycoprotein

Prolyl hydroxylase is a tetrameric glycoprotein that catalyzes a vital posttranslational modification in the biosynthesis of collagen. The enzyme purified from whole chick embryos (WCE) possesses two nonidentical subunits, alpha and beta, and has been shown by several techniques to reside in the endoplasmic reticulum of chick embryo fibroblasts. The studies described here demonstrate that the larger of the two subunits (alpha) exists in two forms in chick embryo fibroblasts (CEF); these two forms differ in carbohydrate content. The larger alpha subunit, alpha', contains two N-linked high mannose oligosaccharides, each containing eight mannose units; the smaller subunit, alpha, contains a single seven-mannose N-linked oligosaccharide. Both oligosaccharides could be cleaved by endo-beta-N-acetylglucosaminidase H and completely digested with alpha-mannosidase to yield mannosyl-N-acetylglucosamine.

Purification by affinity chromatography of glucosidase I, an endoplasmic reticulum hydrolase involved in the processing of asparagine-linked oligosaccharides

Trimming glucosidase I and II have been solubilized from crude calf liver microsomes and partially enriched by a fractionated extraction procedure applying different concentrations of nonionic detergent and salt. The pH optimum of both enzymes was found to be close to 6.2, which discriminates them from hydrolases of lysosomal origin acting on p-nitrophenyl glycosides with the highest rate at more acidic pH. Glucosidase I and II and the nonspecific alpha-glucosidase(s) were inhibited by 1-deoxynojirimycin with median inhibitory concentration of 3 microM, 20 microM, 12 microM, respectively. Discrimination between these enzymes was strongly enhanced by N-alkylation of 1-deoxynojirimycin and formed the basis for the design of the affinity ligand. Glucosidase I has been purified to homogeneity by affinity chromatography on AH-Sepharose 4B with N-carboxypentyl-1-deoxynojirimycin as ligand. Sodium dodecyl sulfate gel electrophoresis of the purified enzyme revealed a subunit molecular mass of about 85 kDa. The molecular mass of the native enzyme, determined by gel chromatography, was approximately equal to 320-350 kDa, pointing to the association of subunits to a tetramer. Glucosidase I is rather stable when stored at 4 degrees C in the presence of detergent (t 1/2 approximately equal to 20 days) and showed high specificity for the hydrolysis of the terminal (alpha 1,2)-linked glucose residue in the natural substrate Glc3-Man9-(GlcNAc)2.

Biosynthesis of glycoproteins in human placenta: differential labeling of mannose and heterogeneity of oligosaccharide lipid intermediates

The biosynthesis of lipid-linked oligosaccharides has been studied in first trimester human placentas. Tissue was pulsed with [2-3H]mannose, [1-3H]glucosamine, and [1-3H]galactose (for [3H]glucose incorporation). The lipid-linked oligosaccharides (OSL) were purified on DEAE-cellulose. After cleaving the lipid from OSL the oligosaccharides were purified by paper chromatography and borate high-voltage electrophoresis. Four major oligosaccharides with the composition Glc(0-3)Man9GlcNAc2 thus obtained were characterized enzymatically by digestion with endo-beta-N-acetylglucosaminidase H and alpha-mannosidase, and chemically by methylation, acetolysis, and Smith degradation. While identical Glc(1-3)Man9GlcNAc2 oligosaccharides have been isolated from other in vivo systems, the presence of Man9Glc2 is novel for human placenta. Furthermore, Man9GlcNAc2 is present in appreciable amounts in placenta. Second, one mannose in Man9GlcNAc2 had a significantly higher specific radioactivity than the other mannosyl residues of the Glc(0-3)Man9GlcNAc2 oligosaccharides. Third, the differential labeling in Man9GlcNAc2 and our pulse-chase studies indicate that Man9GlcNAc2 is not a major precursor of Glc3Man9GlcNAc2. The data also suggest that the ninth mannose, which has the highest radioactivity, may be incorporated in a different subcellular compartment and may serve as a regulatory step in the biosynthesis of lipid-linked oligosaccharides. A model is proposed which outlines possible multiple pathways of lipid-linked oligosaccharide biosynthesis in human placenta.

Biosynthesis of lutropin in ovine pituitary slices: incorporation of [35S]sulfate in carbohydrate units

Sulfate incorporation into carbohydrate of lutropin (LH) has been studied in sheep pituitary slices using H2(35)SO4. Labeled ovine LH was purified to homogeneity by Sephadex G-100 and carboxymethyl-Sephadex chromatography from both the incubation medium and tissue extract. Autoradiography of the gel showed only two protein bands which comigrated with the alpha and beta subunits of ovine LH in both the purified ovine LH and the immunoprecipitate obtained with LH-specific rabbit antiserum. Furthermore, [35S]sulfate was also incorporated into several other proteins in addition to LH. The location of 35SO2-(4) in the oligosaccharides of ovine LH was evidenced by its presence in the glycopeptides obtained by exhaustive Pronase digestion. The location and the point of attachment of sulfate in the carbohydrate unit were established by the isolation of 4-O-[35S]sulfo-N-acetylhexosaminyl-glycerols and 4-O-[35S] sulfo-N-acetylglucosaminitol from the Smith degradation products and by the release of 35SO2-(4) by chondro-4-sulfatase. Thus, the present line of experimentation indicates the presence of sulfate on both the terminal N-acetylglucosamine and N-acetylgalactosamine in the oligosaccharide chains of the labeled ovine LH.

Effect of glucose deprivation on the incorporation of 3H precursors into pituitary glycoproteins and on basal LH, FSH and prolactin release

Glucose can serve as a precursor for all of the components of the oligosaccharides of glycoproteins. The incorporation of various 3H precursors into glycoproteins was examined by incubating rat anterior pituitaries in vitro for 4h in the presence and absence of glucose. Substituting succinate for glucose enhanced the incorporation of D-(2-3H)mannose six-fold, the incorporation of D-(1,6-3H)-glucosamine two-fold and the incorporation of D-(6-3H) galactose was enhanced slightly. The incorporation of other 3H precursors was unaltered or reduced by glucose deprivation. Rat anterior pituitaries incorporated minimal quantities of N-acetyl-D-(6-3H)mannosamine, the immediate precursor of N-acetyl-D-neuraminic acid, into glycoproteins. Glucose deprivation also stimulated the 4h basal release of LH and FSH by 58% and 95%, respectively, as well as stimulated net intracellular degradation of prolactin. Thus, glucose deprivation enhances the incorporation of some, but not all, 3H precursors into glycoproteins as well as stimulates the basal release of LH and FSH.