Biomedical subjects

R G Spiro

Publications and source records attributed to R G Spiro.

At least 55 records · Page 3Linked to original sources

PubMed1991

Potential regulation of N-glycosylation precursor through oligosaccharide-lipid hydrolase action and glucosyltransferase-glucosidase shuttle.

The potential role of degradative mechanisms in controlling the level of the dolichyl pyrophosphate-linked Glc3Man9GlcNAc2 required for protein N-glycosylation has been explored in thyroid slices and endoplasmic reticulum (ER) vesicles, focusing on cleavage of the oligosaccharide from its lipid attachment and on the enzymatic removal of peripheral monosaccharide residues. Vesicle incubations demonstrated a substantial release of free Glc3Man9GlcNAc2 (at 30 min approximately 35% of that transferred to protein) which was inhibited in the presence of exogenous peptide acceptor and was sensitive to disruption of membrane integrity by detergent. In thyroid slices glucosylated oligosaccharides terminating in the di-N-acetylchitobiose sequence were also noted and these continued to be formed even during inhibition by puromycin of both protein synthesis and the attendant N-glycosylation. These observations indicated that the oligosaccharide originated from the lipid donor and suggested, together with previously reported similarities in substrate specificity and cofactor requirements, that the oligosaccharyltransferase can carry out in vivo both the hydrolytic and transfer functions. In addition to the release of the intact Glc3Man9GlcNAc2, we also obtained evidence that the lipid-linked oligosaccharide can be modified by the in vivo action of ER glycosidases. Since radiolabeling of the oligosaccharide-lipid in thyroid slices indicated a preferential turnover of the glucose residues, the possible existence of a glucosyltransferase-glucosidase shuttle was explored with the use of castanospermine. In the presence of this glucosidase inhibitor, the formation of under-glucosylated and nonglucosylated oligosaccharides was not observed, even under conditions of energy deprivation in which they accumulate. Glucosidase inhibition in ER vesicle incubations likewise prevented the appearance of incompletely glucosylated oligosaccharide-lipids. Studies employing the mannosidase inhibitor 1-deoxymannojirimycin in thyroid slices furthermore indicated that in vivo removal of at least one mannose residue from the dolichyl pyrophosphate-linked oligosaccharide can occur.

Animals↗

PubMed1990

Characterization of novel sequences containing 3-O-sulfated glucosamine in glomerular basement membrane heparan sulfate and localization of sulfated disaccharides to a peripheral domain.

Fragmentation of the heparan sulfate chains from bovine glomerular basement membrane (GBM) by hydrazine/nitrous acid treatment followed by NaB3H4-reduction yielded a mixture of six sulfated disaccharides containing D-glucuronic (GlcUA) or L-iduronic acid (IdUA) and terminating in 2,5-anhydro[3H]mannitol (AnManH2), in addition to the nonsulfated component GlcUA beta 1----4AnManH2. Among these products two novel disaccharide units were identified as IdUA alpha 1----4AnManH2(3-SO4) and IdUA(2-SO4)alpha 1----4AnManH2(3-SO4); these accounted for 22% of the total sulfated species indicating that there are 2-3 residues of 3-O-sulfated glucosamine/heparan sulfate chain. The disulfated disaccharide was shown through its release by direct nitrous acid treatment to be situated in a GlcNSO3-IdUA(2-SO4)-GlcNSO3(3-SO4) sequence which is distinct from that in which 3-O-sulfated glucosamine is located in the antithrombin-binding region of heparins. Analyses of heparan sulfate from lens capsule, a nonvascular basement membrane, indicated the absence of sequences containing 3-O-sulfated glucosamine, although otherwise the sulfated disaccharides produced by hydrazine/nitrous acid/Na-B3H4 treatment (GlcUA beta 1----4AnManH2(6-SO4), IdUA alpha 1----4AnManH2(6-SO4), IdUA(2-SO4)alpha 1----4AnManH2 and IdUA(2-SO4)alpha 1----4AnManH2(6-SO4] were the same as from GBM. Examination of the GBM heparan sulfate domains after nitrous acid treatment indicated that the O- as well as N-sulfate groups are clustered in an iduronic acid-rich 10-disaccharide peripheral segment, while the internal region (approximately 20 disaccharides) is composed primarily of repeating GlcUA beta 1----4GlcNAc units. The localization of chain diversity to the outer region may facilitate interactions of the heparan sulfate with other macromolecular components.

Animals↗

PubMed1990

Demonstration that Golgi endo-alpha-D-mannosidase provides a glucosidase-independent pathway for the formation of complex N-linked oligosaccharides of glycoproteins.

Studies on N-linked oligosaccharide processing were undertaken in HepG2 cells and calf thyroid slices to explore the possibility that the recently described Golgi endo-alpha-D-mannosidase (Lubas, W.A., and Spiro, R.G. (1987) J. Biol. Chem. 262, 3775-3781) is responsible for the frequently noted failure of glucosidase inhibitors to achieve complete cessation of complex carbohydrate unit synthesis. We have found that in the presence of the glucosidase inhibitors, castanospermine (CST) or 1-deoxynojirimycin, there is a substantial production of the glucosylated mannose saccharides (Glc3Man, Glc2Man, and Glc1Man) which are the characteristic products of endomannosidase action. Furthermore, in HepG2 cells, a secretion of these components into the medium could be demonstrated. Characterization of the N-linked polymannose oligosaccharides produced by HepG2 cells in the presence of CST (as well as 1-deoxymannojirimycin to prevent processing by alpha-mannosidase I) indicated the occurrence, in addition to the expected glucosylated species, of substantial amounts of Man8GlcNAc and Man7GlcNAc. Since Man9GlcNAc was almost completely absent and the Man8GlcNAc isomer was shown to be identical with that formed by the in vitro action of endomannosidase on glucosylated polymannose oligosaccharides, we concluded that this enzyme was actively functioning in the intact cells and could provide a pathway for circumventing the glucosidase blockade. Indeed, quantitative studies in HepG2 cells supported this contention as the continued formation of complex carbohydrate units (50% of control) during CST inhibition could be accounted for by the deglucosylation effected by endomannosidase.

1-Deoxynojirimycin↗

PubMed1990

Ultrastructural immunogold studies of heparan sulphate proteoglycan in normal human glomeruli and glomerulonephritis.

The distribution of heparan sulphate proteoglycans (HSPG) has been investigated in normal human glomeruli, membranous glomerulonephritis, mesangial IgA disease, and anti-glomerular basement membrane disease. HSPG was localized using anti-bovine HSPG antibody and 10 nm gold-labelled secondary antibody on paraformaldehyde-fixed, Lowicryl K4M resin-embedded kidneys. HSPG was present in all glomeruli and there was a zonation of its distribution in that it was predominantly on the epithelial aspect of the glomerular basement membrane (GBM) and mesangium with little in the central regions of the mesangial matrix. In the cases of immune complex glomerulonephritis, no HSPG was found in the electron-dense deposits. These findings contrast with our previous studies using the same technique in which type IV collagen and fibronectin were found predominantly on the endothelial aspect of the GBM.

Adult↗

PubMed1990

Insulin receptor carbohydrate units contain poly-N-acetyllactosamine chains.

The insulin receptor was immunoprecipitated from cultured human lymphocytes (IM-9) and rat hepatocytes (Fao) after biosynthetic labeling with [3H]glucosamine or [3H]mannose, and the nature of the carbohydrate units was investigated. Digestion of the receptor from IM-9 lymphocytes with E. freundii endo-beta-galactosidase increased the migration of the insulin receptor alpha- and beta-subunits on sodium dodecyl sulfate-polyacrylamide gels and sharpened the electrophoretic bands; the alpha-subunit was converted from an apparent mol wt (Mr) of 123,000 to a Mr of 118,000, and the beta-subunit from a Mr of 92,000 to 89,000. The susceptibility of the insulin receptor to this enzyme indicates that its carbohydrate units contain poly-N-acetyllactosamine sequences. Affinity chromatography of receptor glycopeptides on Concanavalin-A-Sepharose revealed that the poly-N-acetyllactosamine units were attached to multiantennary glycopeptides that accounted for over 75% of the [3H]glucosamine incorporated into the IM-9 lymphocyte insulin receptor; the remaining radioactivity was present in polymannose units (primarily Man8GlcNAc2) and biantennary complex saccharides. Several differences in the carbohydrate chains of the insulin receptor from the Fao and IM-9 cells indicated that glycosylation was cell specific despite the occurrence of poly-N-acetyllactosamine chains in both cell types. The IM-9 lymphocyte receptor glycopeptides were larger (Mr, 3,200-9,500) and more susceptible to endo-beta-galactosidase than those from the Fao receptor (Mr, 3,000-5,000). Moreover, the released saccharides from the Fao receptor were found by exoglycosidase digestions and chromatographic comparison to standards to contain terminal sialic acid in both alpha 2----3 and alpha 2----6 linkage to galactose, whereas the IM-9 carbohydrate units contained only alpha 2----3-linked sialic acid.

Animals↗

PubMed1990

Occurrence of type VI collagen in extracellular matrix of renal glomeruli and its increase in diabetes.

Human and bovine glomerular basement membrane (GBM) preparations, representing the extracellular matrix of the renal filtration units, were found to contain type VI collagen. This protein was solubilized by guanidine and guanidine-dithiothreitol extractions and characterized after polyacrylamide gel electrophoretic resolution by immunoblotting with an antiserum directed against the alpha 1(VI) and alpha 2(VI) polypeptide chains and by its insensitivity to collagenase digestion in the nonreduced state. In contrast to GBM, which is the product of three distinct cells, type VI collagen could not be detected in extracts from calf lens capsule, an epithelial cell-derived basement membrane. Quantitation by radioimmunoassay of the type VI collagen content of GBM from 17 diabetic and 15 nondiabetic human subjects indicated a 2.8-fold higher level (P less than 0.001) in the diabetic preparations. Because in the glomerulus type VI collagen is considered on the basis of immunohistochemistry to be localized to the mesangium, we believe that measurement of this protein in GBM preparations can provide a valuable index of mesangial expansion in diabetic and other glomerulopathies.

Adult↗

PubMed1989

Characterization of a thyroid sulfotransferase responsible for the 3-O-sulfation of terminal beta-D-galactosyl residues in N-linked carbohydrate units.

Calf thyroid microsomes were found to contain an enzyme which catalyzes the transfer of sulfate from 3'-phosphoadenosine 5'-phospho[35S]sulfate (PAPS) to C-3 of terminal galactose residues in beta 1----4 linkage to GlcNAc. This sulfotransferase is believed to be involved in the biosynthesis of the recently described Gal(3-SO4) capping groups present in the N-linked oligosaccharides of thyroglobulin (Spiro, R.G., and Bhoyroo, V. D. (1988) J. Biol. Chem. 263, 14351-14358). Assays with various native and modified glycopeptides indicated that the enzyme acted optimally on complex-type carbohydrate units in which beta-linked Gal has been uncovered by desulfation or brought into a terminal position by removal of sialyl and/or alpha-galactosyl residues. With fetuin asialoglycopeptides as acceptors (Km = 0.1 mM) the transfer of sulfate from PAPS (Km = 6.3 microM) had a pH optimum of approximately 7.0, required Mn2+ ions (10-50 mM) and was markedly stimulated by Triton X-100 (0.1%) and ATP (2 mM). The same enzyme apparently sulfated free N-acetyllactosamine (LacNAc; Km = 0.69 mM) and its ethyl glycoside, indicating that it had no absolute requirement for a peptide recognition site. Studies with a number of disaccharides related to LacNAc provided information relating to the specifying role of the beta 1----4 galactosyl linkage and the configuration at C-2 of the sugar to which it is attached. Hydrazine-nitrous acid-NaBH4 treatment of the 35S-labeled products from sulfotransferase action on asialoglycopeptides as well as on the ethyl glycoside of LacNAc yielded the same disaccharide, Gal(3-SO4) beta 1----4 anhydromannitol, as is obtained from a similar treatment of thyroglobulin. Subcellular distribution studies indicated that the PAPS:galactose 3-O-sulfotransferase is located in the Golgi compartment which is consistent with the late occurrence of the requisite beta-galactosylation step. It is proposed that in certain tissues the ultimate nature of the capping groups attached to glycoproteins containing terminal Gal beta 1----4GlcNAc sequences could be the result of a competition between this 3-O-sulfotransferase and sialyl- and/or alpha-galactosyltransferases.

Acetylglucosamine↗

PubMed1988

Effect of phospholipids on thyroid oligosaccharyltransferase activity and orientation. Evaluation of structural determinants for stimulation of N-glycosylation.

Oligosaccharyltransferase solubilized by Nonidet P-40 was found to have a highly specific lipid requirement which is consistent with the lability of the enzyme when removed from its membrane association. Enzyme activity as measured by the N-glycosylation of a hexapeptide acceptor was greatly stimulated and stabilized by phosphatidylcholine (PC) while other naturally occurring phosphoglycerides had minimal effect. The quaternary ammonium group of PC was observed to be involved in the interaction with the enzyme as modification of the choline moiety by removal of methyl groups resulted in a progressive loss of the stimulatory effect (choline greater than N,N-dimethylethanolamine greater than N-monomethylethanolamine greater than ethanolamine) which was reflected primarily in the Vmax rather than the Km values. Evaluation of a number of PC and choline derivatives indicated that the nonpolar domain of the lipid also played an important specifying role. Two hydrophobic chains attached to the phosphoglycerol backbone were found to be essential, and furthermore the length and degree of unsaturation of the fatty acid substituents as well as their position of attachment on the glycerol moiety greatly affected the extent of activation. Since the L-isomer of PC brought about a 3-fold greater stimulation than the D-isomer the interaction of the enzyme with the phospholipid appears to be stereoselective. Upon chromatography of the PC-stabilized enzyme on concanavalin A-agarose almost complete retention occurred at 0.4% Nonidet P-40, while no binding took place at a detergent concentration of 0.075%; this suggested that upon dilution in the presence of PC, the oligosaccharyltransferase was reconstituted into vesicles in an asymmetric fashion with its N-linked carbohydrate located internally. Enzymatic assay of these vesicles demonstrated that the active site of the enzyme was also oriented toward the interior. These studies indicate that the activity as well as the membrane insertion of the oligosaccharyltransferase are to a large measure influenced by its interaction with PC.

Animals↗

PubMed1988

Occurrence of sulfate in the asparagine-linked complex carbohydrate units of thyroglobulin. Identification and localization of galactose 3-sulfate and N-acetylglucosamine 6-sulfate residues in the human and calf proteins.

Human thyroglobulin glycopeptides representing the multiple asparagine-linked complex (unit B) carbohydrate units of this protein were found to contain substantial amounts of sulfate (ranging from 0.5 to 2.5 mol/mol of oligosaccharide); this substituent was shown to occur primarily in the form of terminal beta-linked Gal-3-SO4 residues which represent novel capping groups occurring alternatively to sialic acid and in comparable amounts. Upon hydrazine/nitrous acid fragmentation and radiolabeling with NaB3H4, all human unit B DEAE-resolved glycopeptide fractions yielded an acidic disaccharide which was characterized as Gal-3-SO4 beta 1----4-anhydromannitol. Studies on glycopeptides modified by desialylation, desulfation, and beta-galactosidase treatment indicated that the majority (approximately 70%) of the complex carbohydrate units contain sulfate groups and that Gal-3-SO4 and sialic acid residues can coexist in terminal positions on the same N-linked oligosaccharide. In addition to Gal-3-SO4, the most acidic unit B variants were found to contain GlcNAc-6-SO4 which was recovered as Gal beta 1----4-anhydromannitol-6-SO4 after hydrazine/nitrous acid treatment and NaB3H4 reduction. On the basis of chromatography on immobilized concanavalin A, it was determined that whereas the Gal-3-SO4 groups occur on biantennary as well as more highly branched carbohydrate units, GlcNAc-6-SO4 is exclusively present in the latter oligosaccharides. In contrast to the N-linked carbohydrate units, the previously described O-linked glycosaminoglycan chain of human thyroglobulin yielded GlcA beta 1----3-anhydrotalitol-6-SO4 upon hydrazine/nitrous acid/NaB3H4 treatment, indicating that it is a chrondroitin 6-sulfate-like polymer. The distribution of sulfate in the complex oligosaccharides of calf thyroglobulin was quite different from that in the human protein; sulfate was not detectable in most of the glycopeptides and was sequestered in a single multibranched complex-type glycopeptide fraction (1.6 mol of sulfate/mol of oligosaccharide) which contained about equal amounts of Gal-3-SO4 and GlcNAc-6-SO4. The difference in galactose sulfation between human and calf thyroglobulins may be related to the substitution in the latter protein of some of the galactose residues by alpha-D-Gal capping groups.

Acetylglucosamine↗

PubMed1988

Evaluation of the role of rat liver Golgi endo-alpha-D-mannosidase in processing N-linked oligosaccharides.

Golgi membranes from rat liver have been shown to contain an endo-alpha-D-mannosidase which can convert Glc1Man9GlcNAc to Man8GlcNAc with the release of Glc alpha 1----3Man (Lubas, W. A., and Spiro, R. G. (1987) J. Biol. Chem. 262, 3775-3781). We now report that this enzyme has the capacity to cleave the alpha 1----2 linkage between the glucose-substituted mannose residue and the remainder of the polymannose branch in a wide range of oligosaccharides (Glc3Man9GlcNAc to Glc1Man4GlcNAc) as well as glycopeptides and oligosaccharide-lipids. Whereas the tri- and diglucosylated species (Glc3Man9GlcNAc and Glc2Man9GlcNAc), which yielded Glc3Man and Glc2Man, respectively, were processed more slowly than Glc1Man9GlcNAc, the monoglucosylated components with truncated mannose chains (Glc1Man8GlcNAc to Glc1Man4GlcNAc) were trimmed at an increased rate which was inversely related to the number of mannose residues present. The endomannosidase was not inhibited by a number of agents which are known to interfere with N-linked oligosaccharide processing by exoglycosidases, including 1-deoxynojirimycin, castanospermine, bromoconduritol, 1-deoxymannojirimycin, swainsonine, and EDTA. However, Tris and other buffers containing primary hydroxyl groups substantially decreased its activity. After Triton solubilization, the endomannosidase was observed to be bound to immobilized wheat germ agglutinin, indicating the presence of a type of carbohydrate unit consistent with Golgi localization of the enzyme. The Man8GlcNAc isomer produced by endomannosidase action was found to be processed by Golgi enzymes through a different sequence of intermediates than the rough endoplasmic reticulum-generated Man8GlcNAc variant, in which the terminal mannose of the middle branch is absent. Whereas the latter oligosaccharide is converted to Man5GlcNAc via Man7GlcNAc and Man6GlcNAc at an even rate, the processing of the endomannosidase-derived Man8GlcNAc stalls at the Man6GlcNAc stage due to the apparent resistance to Golgi mannosidase I of the alpha 1,2-linked mannose of the middle branch. The results of our study suggest that the Golgi endomannosidase takes part in a processing route for N-linked oligosaccharides which have retained glucose beyond the rough endoplasmic reticulum; the distinctive nature of this pathway may influence the ultimate structure of the resulting carbohydrate units.

Animals↗

PubMed1988

Biosynthesis of sulfated asparagine-linked complex carbohydrate units of calf thyroglobulin.

Thyroglobulin from colloid as well as from membrane fractions became radiolabeled upon incubation of calf thyroid slices with [35S]sulfate. The identity of the sulfate-labeled molecule was established by immunoprecipitation, polyacrylamide gel electrophoresis, Bio-Gel A-5m filtration, and DEAE-cellulose chromatography. Size analysis by gel filtration of [35S]glycopeptides and hydrazine-released oligosaccharides indicated that the sulfate was primarily located in the complex (unit B) carbohydrate units of thyroglobulin. Moreover, although [35S]sulfate-labeled oligosaccharides were cleaved by N-glycanase to the same extent as those labeled with [3H]mannose, they were not released by endo-beta-N-acetylglucosaminidase under conditions that led to the complete removal of polymannose carbohydrate (unit A). The failure of 35S-labeled glycopeptides and oligosaccharides to bind to immobilized Concanavalin-A indicated that the sulfate residues in calf thyroglobulin are located in carbohydrate units with three or more branches. No evidence for the occurrence of tyrosine sulfate was found upon examination of Pronase digests of radiolabeled thyroglobulin, and chemical analyses excluded the presence of this amino acid down to a level of 0.5 residues/polypeptide subunit. Studies with density gradient-separated membrane fractions as well as with puromycin indicated that sulfate addition is a late event in thyroglobulin biosynthesis which occurs in the Golgi compartment. Furthermore, it was observed that the nondimerized thyroglobulin subunit was much less sulfate labeled than the mature molecule. The location of the sulfated carbohydrate in a terminal portion of the calf thyroglobulin peptide chain was suggested by the observation that the subunit [mol wt (Mr) = 330,000] can undergo a transformation, presumably mediated by an endogenous protease, to a sulfate-free component (Mr = approximately 270,000) with the appearance of a 35S-labeled 60,000 Mr fragment; the release of a single sulfate-labeled peptide (Mr = 60,000) by mild trypsin treatment was consistent with a sequestration of sulfate groups in the thyroglobulin molecule.

Animals↗

PubMed1988

Nonenzymatic glycation of basement membranes from human glomeruli and bovine sources. Effect of diabetes and age.

The nonenzymatic glycation of glomerular basement membranes (GBMs) from 14 diabetic and 19 nondiabetic human subjects was determined after boronic acid affinity and high-performance cation-exchange chromatography of their NaB[3H]4-reduced ketoamine adducts. The glucitol-lysine (Glc-Lys) and the glucitol-hydroxylysine (Glc-Hyl) content of diabetic GBM was found to be about twofold higher than that of nondiabetic samples (P less than .001). The content of these glycated amino acids did not correlate with age over the range examined (20-91 yr) or with the length of disease in diabetic subjects (2-16 yr). However, analyses of Glc-Lys and Glc-Hyl in calf and adult bovine GBM and lens capsules indicated that the levels of these glycated amino acids were several times greater in basement membranes from older animals. We also observed that guanidine-insoluble collagen of bovine GBM is more extensively glycated (approximately 4-fold) than primarily noncollagenous proteins that are extracted by this reagent. In all of the basement membranes examined, the percentage of glycation of lysine was greater than of hydroxylysine. Characterization of the components released by alkaline hydrolysis indicated that O-glycosylated hydroxylysine residues are nonenzymatically N-glycated to the same extent as those without an enzymatically attached carbohydrate unit. Our study indicates that more than a hundred times as many hydroxylysine residues are enzymatically glycosylated in human and bovine GBM as those containing the nonenzymatically formed ketoamine adduct.

Adult↗

PubMed1987

Presence of an O-glycosidically linked hexasaccharide in fetuin.

Examination by gel filtration, thin layer and anion exchange chromatography of the O-linked carbohydrate units released from fetuin by alkaline borohydride treatment indicated the presence in this glycoprotein of an acidic glucosamine-containing hexasaccharide in addition to the previously described tetra- and trisaccharides. The structure of the hexasaccharide was determined to be NeuAc alpha 2----3Gal beta 1----3[NeuAc alpha 2----3Gal beta 1----4GlNAc beta 1----6]GalNAc, on the basis of exoglycosidase digestion, periodate oxidation, and methylation analysis as well as hydrazine-nitrous acid fragmentation. The latter procedure when carried out on the reduced asialohexasaccharide yielded Gal----2-deoxygalactitol and Gal----anhydromannose which were shown to be derived, respectively, from Gal----N-acetylgalactosaminitol and Gal----GlcNAc sequences. Reductive amination of the Gal----anhydromannose disaccharide with [14C] methylamine permitted identification of its linkage as 1----4. While Diplococcus pneumoniae endo-alpha-DN-acetylgalactosaminidase acting on asialofetuin released the sialic acid-free tetra- and trisaccharides (Gal beta 1----3GalNAc), this enzyme did not cleave the peptide attachment of the asialohexasaccharide (Gal beta 1----3 [Gal beta 1----4GlcNAc beta 1----6] GalNAc). The number of O-linked hexa-, tetra-, and trisaccharides per fetuin molecule was determined to be 0.2, 0.7, and 2.1, respectively, on the basis of galactosaminitol analyses. The absence of O-linked N-acetylglucosamine-containing tetra- or pentasaccharides in fetuin suggest that the attachment of this sugar is a rate-limiting step; furthermore, the limited occurrence of the hexasaccharide may indicate that the addition of sialic acid to Gal beta 1----3GalNAc to form the NeuAc alpha 2----3Gal linkage precludes action of the GlcNAc transferase to form the branch point on the GalNAc residue.

Animals↗

PubMed1987

Selective deglycosylation of the heparan sulfate proteoglycan of bovine glomerular basement membrane and identification of the core protein.

The heparan sulfate proteoglycan of the bovine glomerular basement membrane (Mr = 200,000, 30% carbohydrate by weight) has been deglycosylated by various chemical and enzymatic procedures to identify the core protein and provide information about the N- and O-linked saccharide units. Heparitinase digestion of the proteoglycan reduced its Mr to 143,000, consistent with the removal of its four glycosaminoglycan chains with the exception of short segments adjacent to the carbohydrate-protein linkage region, whereas nitrous acid treatment brought about a smaller reduction in size (to Mr = 168,000) which was shown to be due to the resistance of the internal portion of the heparan sulfate polymer to this reagent. Incubation of the heparitinase-digested proteoglycan with peptide N-glycosidase F decreased its Mr by about 8,000 and liberated oligosaccharides which were primarily acidic in nature; since endo-beta-N-acetylglucosaminidase H did not bring about any saccharide release, it appears that the N-linked carbohydrate units (three per molecule) occur exclusively as the complex type. Treatment of the proteoglycan with trifluoromethanesulfonic acid, a reagent which cleaves all saccharide units, yielded the core protein which migrated as a single discrete band (Mr = 128,000) on polyacrylamide gel electrophoresis. Although the native and heparitinase-treated proteoglycan reacted with concanavalin A and Bandeiraea simplicifolia I, the core protein had no affinity for these lectins, and this loss of reactivity can be attributed to the removal of the N- and small O-linked saccharides. However, the immunoreactivity of the deglycosylated protein with antiserum directed against the intact proteoglycan was to a large measure (80%) preserved, suggesting that the polyclonal response to this glomerular basement membrane glycoconjugate is primarily directed against determinants on the polypeptide portion.

Animals↗

PubMed1987

Golgi endo-alpha-D-mannosidase from rat liver, a novel N-linked carbohydrate unit processing enzyme.

An enzyme has been found in Triton-treated rat liver Golgi membranes which trims Glc1Man9GlcNAc to Man8GlcNAc with the release of Glc alpha 1-3Man. By removing a glucosylmannose disaccharide and yielding only one Man8GlcNAc isomer, this endo-alpha-D-mannosidase provides a processing route alternative to the sequential actions of alpha-glucosidase II and alpha-mannosidase I. The endomannosidase was fully active in the presence of 1-deoxynojirimycin and EDTA which inhibited exoglycosidase release of glucose and mannose, respectively, and these agents were, therefore, included in the standard assay. The specific activity of the endomannosidase was found to be 69-fold greater in Golgi than in rough endoplasmic reticulum (RER) membranes, and Golgi-RER mixing experiments excluded the possibility that the low activity in the RER was the result of some inhibitor present in this fraction. The neutral pH optimum (approximately 7.0) of the enzyme was consistent with a role in N-linked oligosaccharide processing. The existence of an endo-alpha-D-mannosidase pathway for glucose removal could provide an explanation for the incomplete block in oligosaccharide processing which is observed in cells with inhibited or deficient alpha-glucosidase.

Animals↗

PubMed1987

Studies on macromolecular components of human glomerular basement membrane and alterations in diabetes. Decreased levels of heparan sulfate proteoglycan and laminin.

Treatment of human glomerular basement membrane (GBM) with 4 M guanidine HCl resulted in a preferential extraction of noncollagenous components including laminin, fibronectin, entactin, and heparan sulfate proteoglycan, whereas effective solubilization of type IV collagen required exposure to denaturing solvents in the presence of reducing agents. The guanidine HCl-solubilized constituents were identified by immunochemical procedures after resolution by polyacrylamide gel electrophoresis, CL-6B filtration, and DEAE-cellulose chromatography. Two immunologically related heparan sulfate proteoglycans (Mr approximately 350,000 and 210,000) were observed by electrophoresis, with the higher-molecular-weight form being predominant. An examination of the two proteoglycans after heparitinase digestion or chemical deglycosylation indicated that heparan sulfate chains and other carbohydrate units are attached to core proteins with Mr approximately 140,000 and 110,000, respectively. Radioimmunoassays indicated that human diabetic GBM contained significantly lower (P less than .005) amounts of heparan sulfate proteoglycan and laminin with average values that were 30 and 60%, respectively, of nondiabetic controls; the fibronectin content of the diabetic GBM, however, was not significantly different from the normal. These findings, together with previous studies showing increases in GBM collagen, indicate that an alteration in the macromolecular architecture of this basement membrane occurs in diabetes that may be responsible for the filtration defect and the ultimate glomerular occlusion.

Animals↗

PubMed1986

Control of N-linked carbohydrate unit synthesis in thyroid endoplasmic reticulum by membrane organization and dolichyl phosphate availability.

Thyroid rough endoplasmic reticulum (ER) has been shown to contain a highly organized multienzyme system capable of carrying out the N-glycosylation of newly synthesized proteins. These reactions were studied in isolated ER vesicles and found to be controlled to a large extent by the availability of a key substrate, dolichyl phosphate (Dol-P), as well as by the amount of endogenous polypeptide acceptor present. Although in intact vesicles UDP-Glc was utilized in an efficient manner to form Dol-P-Glc and glucosylated oligosaccharide-lipid, after disruption of vesicle integrity, even with low concentrations of Triton X-100, the coupling of Dol-P-Glc formation to lipid-linked oligosaccharide assembly and subsequent N-glycosylation was substantially impaired. Increased incubation temperatures also resulted in a decreased effectiveness of glucose transfer from Dol-P-Glc to lipid-oligosaccharide, presumably because of a decline in the extent of structural organization of the ER membranes. The limited availability of endogenous Dol-P was demonstrated by the pronounced stimulation in Dol-P-Glc formation resulting from the addition of this lipid acceptor to Triton-disrupted ER membranes as well as by its generation in intact vesicles. The latter was accomplished by stimulating recycling of endogenous Dol-P through the addition of a peptide (Tyr-Asn-Leu-Thr-Ser-Val) which is an N-glycosylation substrate. The inhibition of Dol-P-Glc synthesis from UDP-Glc observed in the presence of elevated levels of GDP-Man which could be relieved in Triton-disrupted or intact ER vesicles by the addition or generation, respectively, of Dol-P, is considered to be the result of a competing requirement for Dol-P by the mannosyltransferase. Moreover GTP, by selectively inhibiting the mannosyltransferase, prevented the decrease of Dol-P-Glc formation caused by GDP-Man. Since addition of the acceptor peptide to intact vesicles stimulated Dol-P-P-GlcNAc as well as Dol-P-Glc and Dol-P-Man synthesis it would appear that a pool of Dol-P available in common to all three enzymes responsible for dolichol-linked monosaccharide synthesis exists in the ER membranes.

Animals↗

PubMed1986

Macromolecular organization of basement membranes. Characterization and comparison of glomerular basement membrane and lens capsule components by immunochemical and lectin affinity procedures.

The macromolecular components of bovine glomerular basement membrane (GBM) and lens capsules (anterior and posterior) solubilized by sequential extractions with denaturing agents were quantitated and characterized by polyacrylamide gel electrophoresis, CL-6B filtration, and DEAE-cellulose chromatography with the help of immunochemical techniques. Laminin, entactin, fibronectin, and heparan sulfate proteoglycan were primarily recovered (over 80%) from both basement membranes in a guanidine HCl extract which contained only a limited amount of the total protein (10-14%); most of the remainder of these noncollagenous components could be solubilized by the guanidine in the presence of reducing agent. Although a portion of the Type IV collagen could be obtained by these treatments, effective extraction of this protein depended on exposure to sodium dodecyl sulfate under reducing conditions. Immunoblot analysis revealed a remarkably similar pattern for GBM and lens capsule Type IV collagens with prominent bands of Mr = 390,000, 210,000, and 190,000 being evident. Fibronectin was present in much greater amounts in GBM than lens capsule while the reverse was true for entactin. In both GBM and lens capsules, the entactin (Mr = 150,000) exceeded laminin; the latter protein on immunoblotting was found to contain primarily the alpha-subunit (Mr = 200,000). The size of the heparan sulfate proteoglycan from anterior (Mr = 400,000) and posterior lens capsule (Mr greater than 500,000) was substantially larger than that from GBM (Mr = 200,000). During DEAE-cellulose chromatography under nonreducing conditions in a denaturing solvent, a portion of the Type IV collagen coeluted with the proteoglycan from these membranes. Considerable Bandeiraea simplicifolia I binding activity (alpha-D-galactose specific) was observed in GBM and lens capsule extracts and column fractions which could not be accounted for by laminin alone. Several components which reacted with this lectin were seen on transblots and among these Type IV collagen was identified. In contrast to the basement membranes from bovine tissues, the constituents from human GBM did not react with the B. simplicifolia I lectin.

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