Oligosaccharides on cathepsin D from porcine spleen

Aspartic cathepsin D endopeptidase contributes to extracellular digestion in clawed lobsters Homarus americanus and Homarus gammarus

Acid digestive proteinases were studied in the gastric fluids of two species of clawed lobster (Homarus americanus and Homarus gammarus). An active protein was identified in both species as aspartic proteinase by specific inhibition with pepstatin A. It was confirmed as cathepsin D by mass mapping, N-terminal, and full-length cDNA sequencing. Both lobster species transcribed two cathepsin D mRNAs: cathepsin D1 and cathepsin D2. Cathepsin D1 mRNA was detected only in the midgut gland, suggesting its function as a digestive enzyme. Cathepsin D2 mRNA was found in the midgut gland, gonads, and muscle. The deduced amino acid sequence of cathepsin D1 and cathepsin D2 possesses two catalytic DTG active-site motifs, the hallmark of aspartic proteinases. The putatively active cathepsin D1 has a molecular mass of 36.4 kDa and a calculated pI of 4.14 and possesses three potential glycosylation sites. The sequences showed highest similarities with cathepsin D from insects but also with another crustacean cathepsin D. Cathepsin D1 transcripts were quantified during a starvation period using real-time qPCR. In H. americanus, 15 days of starvation did not cause significant changes, but subsequent feeding caused a 2.5-fold increase. In H. gammarus, starvation caused a 40% reduction in cathepsin D1 mRNA, and no effect was observed with subsequent feeding.

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.

Human alpha-galactosidase A: characterization of the N-linked oligosaccharides on the intracellular and secreted glycoforms overexpressed by Chinese hamster ovary cells

Human alpha-galactosidase A (alpha-Gal A) is the lysosomal glycohydrolase that cleaves the terminal alpha-galactosyl moieties of various glycoconjugates. Overexpression of the enzyme in Chinese hamster ovary (CHO) cells results in high intracellular enzyme accumulation and the selective secretion of active enzyme. Structural analysis of the N -linked oligosaccharides of the intracellular and secreted glycoforms revealed that the secreted enzyme's oligosaccharides were remarkably heterogeneous, having high mannose (63%), complex (30%), and hybrid (5%) structures. The major high mannose oligosaccharides were Man5-7GlcNAc2 species. Approximately 40% of the high mannose and 30% of the hybrid oligosaccharides had phosphate monoester groups. The complex oligosaccharides were mono-, bi-, 2,4-tri-, 2,6-tri- and tetraantennary with or without core-region fucose, many of which had incomplete outer chains. Approximately 30% of the complex oligosaccharides were mono- or disialylated. Sialic acids were mostly N -acetylneuraminic acid and occurred exclusively in alpha2, 3-linkage. In contrast, the intracellular enzyme had only small amounts of complex chains (7.7%) and had predominantly high mannose oligosaccharides (92%), mostly Man5GlcNAc2 and smaller species, of which only 3% were phosphorylated. The complex oligosaccharides were fucosylated and had the same antennary structures as the secreted enzyme. Although most had mature outer chains, none were sialylated. Thus, the overexpression of human alpha-Gal A in CHO cells resulted in different oligosaccharide structures on the secreted and intracellular glycoforms, the highly heterogeneous secreted forms presumably due to the high level expression and impaired glycosylation in the trans- Golgi network, and the predominately Man5-7GlcNAc2 cellular glycoforms resulting from carbohydrate trimming in the lysosome.

The early and late processing of lysosomal enzymes: proteolysis and compartmentation

Lysosomal enzymes are subjected to a number of modifications including carbohydrate restructuring and proteolytic maturation. Some of these reactions support lysosomal targeting, others are necessary for activation or keeping the enzyme inactive before being segregated, while still others may be adventitious. The non-segregated fraction of the enzyme is secreted and can be isolated from the medium. It is considered that the secreted lysosomal enzymes fulfill certain physiological and pathophysiological roles. By comparing the secreted and the intracellular enzymes it is possible to distinguish between the reactions that occur before and after the segregation. In this review the reactions that may influence the segregation are referred to as the early processing and those characteristic for the enzymes isolated from lysosomal compartments as the late processing. The early processing is characterized mainly by modifications of carbohydrate side chains. In the late processing, proteolytic fragmentation represents the most conspicuous changes. The review focuses on the compartmentation of the reactions and the proteolytic fragmentation of lysosomal enzyme precursors. While a plethora of proteolytic reactions are involved, our knowledge of the proteinases responsible for the particular maturation reactions remains very limited. The review points also to work with cells from patients affected with lysosomal storage disorders, which contributed to our understanding of the lysosomal apparatus.

Isolation of cathepsin D using three-phase partitioning in t-butanol/water/ammonium sulfate

A 6-h procedure for the isolation of bovine cathepsin D is described. The procedure involves essentially only two steps; three-phase partitioning in t-butanol/water/ammonium sulfate followed by affinity chromatography on pepstatin-agarose. The major advantage of this new method over previous methods is the greatly reduced time required to obtain comparably pure cathepsin D.

Structural studies on the carbohydrate moieties of human liver alpha-L-fucosidase

alpha-L-Fucosidase was purified from human liver to apparent homogeneity and subjected to exhaustive digestion with Pronase. The resulting glycopeptides were isolated by gel filtration on Sephadex G-50 and further fractionated by Bio-Gel P-4 chromatography. Five glycopeptide fractions were obtained. The structures of the carbohydrate portions of all glycopeptide components were fully characterized by a combination of 500-MHz 1H NMR spectroscopy and carbohydrate composition analysis. Fraction I contained disialyl diantennary glycopeptides of the N-acetyllactosamine type. Fractions II and III contained predominantly mono(sialyl-N-acetyllactosaminyl) diantennary glycopeptides with the NeuAc alpha(2----6)Gal beta(1----4)GlcNAc beta(1----2) branch attached to alpha(1----3)-linked Man in II and to alpha(1----6)-linked Man in III. The N-acetyllactosamine-type glycopeptides in fractions I to III have a small portion (10-15%) of their Asn-linked GlcNAc residues substituted by additional alpha(1----6)-linked Fuc. Also, a minor portion of the NeuAc residues appeared to be attached to Gal in alpha(2----3) rather than alpha(2----6) linkage. Fraction IV contained a mixture of larger-size oligomannoside-type glycopeptides with a variable number (6 to 9) of Man residues. Smaller-size oligomannoside-type glycopeptides were found in fraction V, containing 3 or 5 Man residues; a small portion (10%) of the Man3GlcNAc2Asn component appeared to contain in addition a Fuc residue in alpha(1----6) linkage to the Asn-bound GlcNAc. The overall ratio of oligomannoside-type to N-acetyllactosamine-type carbohydrate structures was found to be 5:4. This article is the first account of the complete characterization of the oligomannoside-type structures in alpha-L-fucosidase; furthermore, the occurrence in alpha-L-fucosidase of mono(sialyl-N-acetyllactosaminyl) structures, Fuc-containing oligosaccharides, and NeuAc alpha(2----3) linked to Gal are reported for the first time.

Isolation and characterization of endogenous ligands for liver mannan-binding protein

Endogenous ligands for the hepatic lectin which is specific for mannose and N-acetylglucosamine (mannan-binding protein, MBP) were isolated from rat liver rough microsomes and primary cultured hepatocytes by affinity chromatography on an immobilized MBP column. Western blotting using specific antisera revealed that serum glycoproteins, alpha 1-macroglobulin, alpha 1-antitrypsin, and alpha 1-acid glycoprotein, and a lysosomal enzyme, beta-glucuronidase were the major constituents of the endogenous ligands. These endogenous ligands consisted of high mannose-type oligosaccharides of Man9GlcNAc2 and Man8GlcNAc2, and had rapid turnover rates with an average half-life of 45 min, indicating that they were mainly composed of biosynthetic intermediates of glycoproteins. In view of the identification of the endogenous ligands as the biosynthetic intermediates of glycoproteins, the possible functions of the intracellular lectin are discussed in relation to the intracellular transport of glycoproteins.

The distribution of 215-kilodalton mannose 6-phosphate receptors within cis (heavy) and trans (light) Golgi subfractions varies in different cell types

The distribution of mannose 6-phosphate (Man-6-P) receptors for lysosomal enzymes was investigated in Golgi subfractions prepared from three different cultured cell lines. Total microsomal fractions from clone 9 hepatocytes, normal rat kidney, or Chinese hamster ovary cells were subfractionated by flotation in sucrose density gradients, which resolves Golgi membranes into heavy (cis), intermediate, and light (trans) subfractions. The distribution of Man-6-P receptors within the subfractions was assessed by quantitative immunoprecipitation, and the results were compared to those obtained by immunoperoxidase localization of the receptors in Golgi cisternae of intact cells. In all cases, the results obtained by Golgi subfractionation and by immunoelectron microscopy were in agreement. In clone 9 cells, Man-6-P receptors were enriched in heavy (cis) Golgi subfractions, whose peak density (rho = 1.17) was greater than those containing either galactosyltransferase activity, a trans Golgi marker, or alpha-mannosidase II, a middle Golgi marker. By immunoelectron microscopy, the receptors were localized to a single cis Golgi cisterna. In Chinese hamster ovary cells, Man-6-P receptors were concentrated in Golgi membranes of low density (1.12 g/ml) overlapping the peak of galactosyltransferase activity. By the immunoperoxidase technique, the receptors were usually localized to a single trans Golgi cisterna. In normal rat kidney cells, Man-6-P receptors were found to be broadly distributed across Golgi membranes (rho = 1.12-1.17), and by immunoperoxidase localization they were found to be broadly distributed across the stacked Golgi cisternae. It is concluded that the distribution of Man-6-P receptors within the Golgi complex varies from one cell type to another. These differences in receptor distribution may reflect variations in lysosomal enzyme trafficking among different cell types.

Determination of the structure of the carbohydrate chains of acid alpha-glucosidase from human placenta

Acid alpha-glucosidase (alpha-D-glucoside glucohydrolase, EC 3.2.1.20) from human placenta (70 and 76 kDa) was found to contain 4 N-glycosidic carbohydrate chains per molecule. Sugar analysis of purified enzyme revealed the presence of mannose, N-acetylglucosamine and fucose at a molar ratio of 5.0:2.0:0.6. In addition, trace amounts of galactose and N-acetylneuraminic acid were detected. The sugar chains were liberated from the polypeptides by the hydrazinolysis procedure and subsequently fractionated by gel filtration and HPLC. Purified compounds were investigated by 500-MHz 1H-NMR spectroscopy. Oligomannoside-type chains of intermediate size, e.g., Man5GlcNAcGlcNAc-ol and Man7GlcNAcGlcNAc-ol, and N-type chains of smaller size e.g., Man2-3GlcNAc[Fuc]0-1GlcNAc-ol, were demonstrated to be present at a ratio of 2:3. In addition, a small amount of sialylated N-acetyllactosamine-type chains has been found. The possible biosynthetic route of the fucose-containing small-size chains is discussed.

Structural analysis of the carbohydrate chains of beta-N-acetylhexosaminidases from bovine brain

The oligosaccharide structures of bovine brain beta-N-acetylhexosaminidases A and B (EC 3.2.1.30) were studied at the glycopeptide level by employing 500 MHz 1H-n.m.r. spectroscopy and methylation analysis involving g.l.c.-m.s. More than 90% of the chains were found to be of the oligomannoside type, containing, on average, five to six mannose residues. Biantennary N-acetyl-lactosamine-type chains terminated in N-acetylneuraminic acid were found to comprise the remaining 5-10% of the total carbohydrate. The isoenzyme forms A and B do not differ from each other in the structure of their carbohydrate moiety, but do deviate in carbohydrate content and, in consequence, in the number of carbohydrate chains per molecule.

Determination of the structure of the carbohydrate chains of prostaglandin endoperoxide synthase from sheep

Prostaglandin endoperoxide synthase was isolated from sheep seminal vesicles. Sugar analysis of the glycoprotein revealed the presence of mannose and N-acetylglucosamine only. The carbohydrate moiety was released from the polypeptide backbone by hydrazinolysis. After re-N-acetylation and reduction, the resulting mixture of oligosaccharide-alditols was fractionated on Bio-Gel P-4 and their structures were investigated by 500-MHz1H-NMR spectroscopy. The carbohydrate chains turned out to be of the oligomannoside type containing six to nine mannose residues. The largest and most abundant compound was established to be: (formula; see text) For the smaller structures heterogeneity occurs with respect to the outer alpha(1----2)-linked mannose residues. Furthermore, a small amount of Man6GlcNAc-ol (artefact of the hydrazinolysis procedure) was detected by 1H-NMR spectroscopy and fast atom bombardment mass spectrometry.

Subcellular distribution of the mannan-binding protein and its endogenous inhibitors in rat liver

The subcellular distribution of the mannan-binding protein from rat liver, a lectin specific for mannose and N-acetylglucosamine, was studied. Approximately 75% of the binding activity of the homogenate was recovered in microsomes, approximately 76% of which was accounted for by rough microsomes. Rough microsomes had the highest specific activity of binding, followed by the Golgi apparatus and smooth microsomes, whereas plasma membranes, lysosomes, mitochondria, and the soluble fraction had little or no binding activity. A topographical survey indicated that the binding protein was localized exclusively on the cisternal surface of microsomal vesicles. Thus, the binding protein of microsomal vesicles was protected from protease digestion and was released from the vesicles by mild detergent treatment. Competitive inhibitors, which presumably represent endogenous ligands of the binding protein, were found among subcellular fractions. More than 50% of the inhibitory activity of the homogenate was recovered in rough microsomes, while the highest specific activity of inhibition was found in lysosomes. The Ki values estimated for rough microsomes and lysosomes were 25.9 and 8.67 micrograms/ml, respectively. The distribution profiles of inhibitors were correlated roughly with those of the binding protein, resulting in masking of the binding activity in organelles up to the level of 86%. On the basis of the known localization and topology of the binding protein and endogenous inhibitors (ligands), possible physiological functions of the binding protein relevant to the transport of biosynthetic intermediates of glycoproteins from the rough endoplasmic reticulum to the Golgi apparatus and from the Golgi apparatus to lysosomes were discussed.

Primary structure of the major glycans of the N-acetyllactosamine type derived from the human immunoglobulins M from two patients with Waldenström's macroglobulinemia

The carbohydrate chains of the pathological human immunoglobulins M from two patients with Waldenström's macroglobulinemia were released by hydrazinolysis. The N-acetyllactosamine-type glycans were obtained by affinity chromatography on concanavalin A and fractionated by high-voltage paper electrophoresis. The primary structure of the major compounds was elucidated on the basis of carbohydrate analysis, methylation analysis, including mass-spectrometry, and 500 MHz 1H-NMR spectroscopy. For both patients, this appeared to be a monosialyl monofucosyl biantennary structure; the compounds differed by the presence of an intersecting N-acetylglucosamine residue.