Rapid intramolecular turnover of N-linked glycans in plasma membrane glycoproteins. Extension of intramolecular turnover to the core sugars in plasma membrane glycoproteins of hepatoma

Chemical glycomics enrichment: imaging the recycling of sialic acid in living cells

The development of metabolic oligosaccharide engineering (MOE) over the past two decades enabled the bioimaging studies of glycosylation processes in physio-pathological contexts. Herein, we successfully applied the chemical reporter strategy to image the fate of sialylated glycoconjugates in healthy and sialin-deficient patient fibroblasts. This chemical glycomics enrichment is a powerful tool for tracking sialylated glycoconjugates and probing lysosomal recycling capacities. Thus, such strategies appear fundamental for the characterization of lysosomal storage diseases.

Metabolic Glycoengineering with N-Acyl Side Chain Modified Mannosamines

In metabolic glycoengineering (MGE), cells or animals are treated with unnatural derivatives of monosaccharides. After entering the cytosol, these sugar analogues are metabolized and subsequently expressed on newly synthesized glycoconjugates. The feasibility of MGE was first discovered for sialylated glycans, by using N-acyl-modified mannosamines as precursor molecules for unnatural sialic acids. Prerequisite is the promiscuity of the enzymes of the Roseman-Warren biosynthetic pathway. These enzymes were shown to tolerate specific modifications of the N-acyl side chain of mannosamine analogues, for example, elongation by one or more methylene groups (aliphatic modifications) or by insertion of reactive groups (bioorthogonal modifications). Unnatural sialic acids are incorporated into glycoconjugates of cells and organs. MGE has intriguing biological consequences for treated cells (aliphatic MGE) and offers the opportunity to visualize the topography and dynamics of sialylated glycans in vitro, ex vivo, and in vivo (bioorthogonal MGE).

Inhibitors of the human neuraminidase enzymes

A review of known small molecule inhibitors and substrates of the human neuraminidase enzymes.

Identification of selective inhibitors for human neuraminidase isoenzymes using C4,C7-modified 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (DANA) analogues

In the past two decades, human neuraminidases (human sialidases, hNEUs) have been found to be involved in numerous pathways in biology. The development of selective and potent inhibitors of these enzymes will provide critical tools for glycobiology, help to avoid undesired side effects of antivirals, and may reveal new small-molecule therapeutic targets for human cancers. However, because of the high active site homology of the hNEU isoenzymes, little progress in the design and synthesis of selective inhibitors has been realized. Guided by our previous studies of human NEU3 inhibitors, we designed a series of C4,C7-modified analogues of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (DANA) and tested them against the full panel of hNEU isoenzymes (NEU1, NEU2, NEU3, NEU4). We identified inhibitors with up to 38-fold selectivity for NEU3 and 12-fold selectivity for NEU2 over all other isoenzymes. We also identified compounds that targeted NEU2 and NEU3 with similar potency.

The schistosome in the mammalian host: understanding the mechanisms of adaptation

In this review, we envisage the host environment, not as a hostile one, since the schistosome thrives there, but as one in which the relationship between the two organisms consists of constant communication, through signalling mechanisms involving sense organs, surface glycocalyx, surface membrane and internal organs of the parasite, with host fluids and cells. The surface and secretions of the schistosome egg have very different properties from those of other parasite stages, but adapted for the dispersal of the eggs and for the preservation of host liver function. We draw from studies of mammalian cells and other organisms to indicate how further work might be carried out on the signalling function of the surface glycocalyx, the raft structure of the surface and existence of pores in the surface membrane, the repair of the surface membrane, the role of the membrane structure in ion channel function (including recent work on the actin cytoskeleton and calcium channels) and the possible role of P-glycoproteins in the adaptation of the parasite to its environment. We are speculative in some areas, such as the suggestions that variability in surface properties of schistosomes may relate to the existence of membrane rafts and that parasite communities may exhibit quorum sensing. This speculative approach is adopted with the hope that future work on the whole organisms and their interactions will be encouraged.

Sialic acids: fascinating sugars in higher animals and man

Sialic acids are acidic monosaccharides, which are among the most important molecules of higher animals, and occur in some microorganisms. They are bound to complex carbohydrates and occupy prominent positions, especially in cell membranes. Their structural diversity is high and, correspondingly, the mechanisms for their biosynthesis are complex. Sialic acid substituents strongly influence the activity of catabolic enzymes, in particular the sialidases, and thus the turnover rate of glycoconjugates. These sugars are involved in manifold cell functions. Due to the surface location of the acidic molecules they shield macromolecules and cells from enzymatic and immunological attacks. But they also represent recognition sites for various physiological receptors as well as for toxins and microorganisms, and thus allow their colonization. Many viruses use sialic acids for the infection of cells. As sialic acids also play a decisive role in tumor biology they prove to be rather versatile molecules that modulate cell biological events in a sensitive way. It is discussed that their evolvement may have stimulated evolution and rendered organisms less vulnerable to environmental attacks. However, disturbance of their metabolism may cause diseases.

Activity and properties of alpha-L-fucosidase are dependent on the state of enterocytic differentiation of HT-29 colon cancer cells

Previously we have demonstrated an impairment in the activity of alpha-L-fucosidase in colon tumours. In order to establish an in vitro model to study this enzyme in colon cancer, we have determined the activity and properties of the enzyme during the differentiation of HT-29 colon cancer cells. Cultures were committed to differentiate into enterocyte-like cells by placing them in a culture medium without glucose for 18-21 days. The state of differentiation was evaluated by assaying the activity of enterocytic marker enzymes, and the acquisition of enterocyte morphology was assessed by electron microscopy. The alpha-L-fucosidase activity was determined using a fluorometric method. Intracellular levels of alpha-L-fucosidase activity are lower in non-differentiated cells (3.0 +/- 1.01 U/mg) than in differentiated ones (9.2 +/- 4.09 U/mg) (P < 0.001). This variation is not due to a greater secretion of the enzyme to the culture medium, and properties such as pH optimum or the affinity towards substrate are not dependent on differentiation. The enzyme however, is more stable at acidic pH and at high temperatures, and V(max) is higher in differentiated cells. Moreover, in undifferentiated cells the enzyme is mainly in a monomeric form whereas multimeric forms of the enzyme appear only upon differentiation. Most of these changes are very similar to those previously observed between normal colon tissue and colon tumours. Thus, we suggest that differentiation of HT-29 colon cancer cells could be used as a model to study the alterations of the enzyme alpha-L-fucosidase during the progression of the tumoural process.

Identification and characterization of the prolactin-binding protein in human serum and milk

The actions of prolactin (PRL) are mediated by its receptor, a member of the superfamily of single transmembrane cytokine receptors. High affinity binding proteins for the closely related growth hormone have been found in the sera of several species including humans and are generated by alternative splicing or proteolysis of the growth hormone receptor extracellular domain (ECD). In contrast, no conclusive evidence has been presented that an analogous prolactin-binding protein (PRLBP) is expressed in human serum. Using both monoclonal and polyclonal antibodies generated against hPRL and the ECD of the human prolactin receptor, co-immunoprecipitation analyses of human serum identified a 32-kDa hPRLBP capable of binding both hPRL and human growth hormone. A measurable fraction of circulating PRL (36%) was associated with the hPRLBP. Despite well documented sex differences in serum hPRL levels, there were no significant differences in the levels of hPRLBP found in the sera of normal adult males and females (15.3 +/- 1.3 ng/ml versus 13.4 +/- 0.8 ng/ml, respectively (mean +/- S.E.)). Immunoprecipitation studies also detected the PRLBP in human milk albeit at lower concentrations than found in sera. Deglycosylation did not alter its electrophoretic mobility, indicating an absence of carbohydrate moieties and suggesting that the hPRLBP spans most of the PRLR ECD, a result confirmed by limited proteolysis and mass spectrometry. The potential function of this serum chaperone was assessed in vitro by the addition of recombinant hPRLBP to the culture medium of the PRL-dependent Nb2 T-cell line. These studies revealed that the hPRLBP antagonizes PRL action, inhibiting PRL-driven growth in a dose-dependent manner.

Cell surface glycoproteins undergo postbiosynthetic modification of their N-glycans by stepwise demannosylation

Primary rat hepatocytes and two hepatoma cell lines have been used to study whether high mannose-type N-glycans of plasma membrane glycoproteins may be modified by the removal of mannose residues even after transport to the cell surface. To examine glycan remodeling of cell surface glycoproteins, high mannose-type glycoforms were generated by adding the reversible mannosidase I inhibitor deoxymannojirimycin during metabolic labeling with [3H]mannose, thereby preventing further processing of high mannose-type N-glycans to complex structures. Upon transport to the cell surface, glycoproteins were additionally labeled with sulfosuccinimidyl-2-(biotinamido)ethyl-1,3-dithiopropionate. This strategy allowed us to follow selectively the fate of cell surface glycoproteins. Postbiosynthetic demannosylation was monitored by determining the conversion of Man8-9GlcNAc2 to smaller structures during reculture of cells in the absence of deoxymannojirimycin. The results show that high mannose-type N-glycans of selected cell surface glycoproteins are trimmed from Man8-9GlcNAc2 to Man5GlcNAc2 with Man7GlcNAc2 and Man6GlcNAc2 formed as intermediates. It could be clearly shown in MH 7777 as well as in HepG2 cells that demannosylation affects plasma membrane glycoproteins after they are routed to the cell surface. As was determined for total cell surface glycoproteins in HepG2 cells, this process occurs with a half-time of 6.7 h. By analyzing the size of high mannose-type glycans of glycoproteins isolated from the cell surface at the end of the reculture period, i.e. after trimming had occurred, we were able to demonstrate that glycoproteins carrying trimmed high mannose glycans become exposed at the cell surface. From these data we conclude that cell surface glycoproteins can be trimmed by mannosidases at sites peripheral to N-acetylglucosaminyltransferase I without further processing of their glycans to the complex form. This glycan remodeling may occur at the cell surface or during endocytosis and recycling back to the cell surface.

Alterations of glycosidases in human colonic adenocarcinoma

OBJECTIVES

We have carried out a detailed study of some glycosidases in an attempt to explain the differential profile of enzyme activity between human colonic adenocarcinoma and normal mucosa.

DESIGN AND METHODS

Several glycosidase activities associated with human colonic adenocarcinoma and control tissues were submitted to a detailed structural and functional characterization.

RESULTS

Tumoral and control samples were assayed for beta-D-galactosidase, beta-D-glucuronidase, alpha-D-mannosidase, beta-NAc-D-glucosaminidase and beta-NAc-D-galactosaminidase activities. Tumoral tissue showed higher beta-D-galactosidase, beta-NAc-D-glucosaminidase, and beta-NAc-D-galactosaminidase activities than control tissue. Glycosidases from tumoral and control tissues demonstrated no differences in optimum pH, subcellular distribution, pH and thermal stability. However, the kinetic analysis showed a statistically significant increased Vmax in tumoral colon with respect to the control for beta-D-galactosidase, beta-NAc-D-glucosaminidase, and beta-NAc-D-galactosaminidase activities. The Km remained unaltered.

CONCLUSIONS

The increased Vmax detected for some glycosidase activities in human colonic adenocarcinoma could correspond with a greater presence of enzyme proteins in the tumoral cells, and not to changes in protein and/or active site structure.

Selective reentry of recycling cell surface glycoproteins to the biosynthetic pathway in human hepatocarcinoma HepG2 cells

Return of cell surface glycoproteins to compartments of the secretory pathway has been examined in HepG2 cells comparing return to the trans-Golgi network (TGN), the trans/medial- and cis-Golgi. Transport to these sites was studied by example of the transferrin receptor (TfR) and the serine peptidase dipeptidylpeptidase IV (DPPIV) after labeling these proteins with the N-hydroxysulfosuccinimide ester of biotin on the cell surface. This experimental design allowed to distinguish between glycoproteins that return to these biosynthetic compartments from the cell surface and newly synthesized glycoproteins that pass these compartments during biosynthesis en route to the surface. Reentry to the TGN was measured in that surface glycoproteins were desialylated with neuraminidase and were monitored for resialylation during recycling. Return to the trans-Golgi was traced measuring the transfer of [3H]fucose residues to recycling surface proteins by fucosyltransferases. To study return to the cis-Golgi, surface proteins were metabolically labeled in the presence of the mannosidase I inhibitor deoxymannojirimycin (dMM). As a result surface proteins retained N-glycans of the oligomannosidic type. Return to the site of mannosidase I in the medial/cis-Golgi was measured monitoring conversion of these glycans to those of the complex type after washout of dMM. Our data demonstrate that DPPIV does return from the cell surface not only to the TGN, but also to the trans-Golgi thus linking the endocytic to the secretory pathway. In contrast, no reentry to sites of mannosidase I could be detected indicating that the early secretory pathway is not or is only at insignificant rates accessible to recycling DPPIV. In contrast to DPPIV, TfR was very efficiently sorted from endosomes to the cell surface and did not return to the TGN or to other biosynthetic compartments in detectable amounts, indicating that individual surface proteins are subject to different sorting mechanisms or sorting efficiencies during recycling.

Effects of specific dietary sugars on the incorporation of 13C label from dietary glucose into neutral sugars of rat intestine and serum glycoproteins

Although theoretically all glycoprotein sugars can be derived from glucose, it may be hypothesized that specific dietary sugars could be preferential substrates for glycoprotein synthesis. To test this hypothesis, groups of rats received either continuously (continuous-labelling experiment) or for a single nutritional period (pulse-labelling experiment) a 13C-rich diet containing either maize starch or artificially labelled [13C]glucose. Some groups of rats were also provided during a single nutritional period with low amounts (20-200 mg/animal) of low-13C dietary sugars (mannose, galactose, fucose or fructose). If specific dietary sugars were preferentially incorporated into glycoproteins instead of glucose-derived labelled sugars, a decrease would be expected in the intestinal or serum glycoprotein-sugar 13C enrichment monitored by gas chromatography-isotope-ratio mass spectrometry (GC-IRMS). Contrary to this hypothesis the results showed no significant decrease with any of the specific dietary sugars. Furthermore, with dietary low-13C mannose or galactose, a significant increase in 13C enrichment of glycoprotein-sugars was observed compared with some other nutritional groups. Moreover, in the pulse-labelling experiment, dietary mannose and galactose induced similar patterns of 13C enrichment in intestinal and serum glycoprotein-sugars. Therefore, although specific dietary sugars do not appear to be preferential substrates for glycosylation under conditions and doses relevant to current concepts of nutrition, regulatory roles of some specific dietary sugars in relation to glycoprotein-sugar metabolism might be hypothesized. These findings could lead to similar studies using stable-isotope methodology in man which could have practical consequences, especially in parenteral nutrition where glucose is the only sugar provided to the metabolism.

Expression of the erb B oncogene in the Morris hepatoma 7777

Altered expression of protooncogenes/oncogenes is believed to be involved in hepatocarcinogenesis of the chemically induced, transplantable Morris hepatoma 7777. We compared the mRNA expression of c-N-ras and v-erb B mRNA of normal rat liver with that of Morris hepatoma 7777 using Northern blot analysis and in situ hybridization. Northern blot analysis revealed a strong overexpression of the v-erb B related mRNA, while the c-N-ras mRNA was only slightly increased. In situ hybridization using a c-N-ras mRNA probe also showed only a slightly increased number of silver grains in the hepatoma cells compared with normal rat liver. On the other hand, the v-erb B related mRNA was strongly overexpressed in the hepatoma cells, while the connective-tissue capsule, the blood vessels, blood cells and the necrotic foci did not show an elevated v-erb B related gene mRNA expression. Similar results were obtained in liver metastases. The detectable v-erb B hybridization signal was lost by pretreatment with RNase A. We conclude that the c-N-ras gene is of minor importance in the chemically induced, transplantable Morris hepatoma 7777, while the increased expression of the v-erb B related mRNA is due to a selection of ligand-independent tyrosine kinase activity.

Liver-intestine cadherin: molecular cloning and characterization of a novel Ca(2+)-dependent cell adhesion molecule expressed in liver and intestine

A novel member of the cadherin family of cell adhesion molecules has been characterized by cloning from rat liver, sequencing of the corresponding cDNA, and functional analysis after heterologous expression in nonadhesive S2 cells. cDNA clones were isolated using a polyclonal antibody inhibiting Ca(2+)-dependent intercellular adhesion of hepatoma cells. As inferred from the deduced amino acid sequence, the novel molecule has homologies with E-, P-, and N-cadherins, but differs from these classical cadherins in four characteristics. Its extracellular domain is composed of five homologous repeated domains instead of four characteristic for the classical cadherins. Four of the five domains are characterized by the sequence motifs DXNDN and DXD or modifications thereof representing putative Ca(2+)-binding sites of classical cadherins. In its NH2-terminal region, this cadherin lacks both the precursor segment and the endogenous protease cleavage site RXKR found in classical cadherins. In the extracellular EC1 domain, the novel cadherin contains an AAL sequence in place of the HAV sequence motif representing the common cell adhesion recognition sequence of E-, P-, and N-cadherin. In contrast to the conserved cytoplasmic domain of classical cadherins with a length of 150-160 amino acid residues, that of the novel cadherin has only 18 amino acids. Examination of transfected S2 cells showed that despite these structural differences, this cadherin mediates intercellular adhesion in a Ca(2+)-dependent manner. The novel cadherin is solely expressed in liver and intestine and was, hence, assigned the name LI-cadherin. In these tissues, LI-cadherin is localized to the basolateral domain of hepatocytes and enterocytes. These results suggest that LI-cadherin represents a new cadherin subtype and may have a role in the morphological organization of liver and intestine.

Comparative study of high-mannose-type oligosaccharides in membrane glycoproteins of rat hepatocytes and different rat hepatoma cell lines

A comparative study was undertaken to characterize the oligosaccharides released by endo-beta-N-acetylglucosaminidase H (endo H) from the membrane glycoproteins of rat hepatocytes and three different Morris hepatoma cell lines (NA-MH 7777, HTC and MH1C1). It is shown that the membrane glycoproteins of hepatocytes and hepatoma cells contain markedly different quantities and forms of high-mannose-type carbohydrate chains. After radiolabelling of the cells with D-[2-3H]mannose, in the absence and presence of 1 mM 1,5-dideoxy-1,5-imino-D-mannitol (1-deoxymannojirimycin), high-mannose-type oligosaccharides were released from delipidated membrane glycoproteins by enzymic digestion with endo H. The carbohydrate chains were converted to their corresponding oligosaccharide alditols by reduction with sodium borohydride, then further analysed by HPLC using an APS-2 Hypersil column. In the absence of 1-deoxymannojirimycin, up to 10% of the radiolabelled oligosaccharides were released by endo H-treatment of the membrane glycoprotein fraction from rat hepatocytes. In contrast, the quantity of radiolabelled high-mannose-type carbohydrate chains released by endo H-treatment from tumour-cell membrane glycoproteins of hepatoma cell lines NA-MH 7777 (31.5%). MH1C1-MH 7795 (37.2%) and HTC-MH 7288c (48%) was increased up to fivefold. The formation of higher-mannosylated structures after oligosaccharide analysis was observed in all hepatoma cell lines, with Man8GlcNAcOH as the major component, whereas in hepatocytes Man5GlcNAcOH was the predominant high-mannose-type structure. In contrast, in the presence of the Golgi alpha-D-mannosidase I inhibitor, 1-deoxymannojirimycin, no significant differences were observed between the distribution of high-mannose-type oligosaccharides in the membrane glycoproteins of hepatocytes and hepatoma cells. However, in the presence of this inhibitor, the proportion of radiolabelled glycans sensitive to deglycosylation by endo H was greatly increased (> 85%) in all the cell lines investigated, the predominant structures being Man8-9-GlcNAcOH. This study shows that an increased content of high-mannose-type sugar chains is a general characteristic of membrane-bound glycoproteins for malignant transformed hepatocytes.

Use of compounds naturally labeled with stable isotopes for the study of the metabolism of glycoprotein neutral sugars by gas-liquid chromatography-isotope-ratio mass spectrometry. Technical validation in the rat

In order to develop an alternative method to radioactive labeling for the study of the glycoprotein sugar metabolism in man, the possible use of stable isotopes provided by naturally, 13C-enriched dietary compounds has been explored in rat intestine and serum. Rats were fed a semisynthetic diet containing 67% wheat starch (containing 1.08692 13C atom/100 carbon atoms) for a week, and then the same diet containing corn starch (1.10042% 13C) for a week. Neutral sugars were prepared from delipidated, trichloroacetic acid-precipitable material from 200-400 mg of intestinal mucosa or 1 mL of serum, separated, and analyzed as alditol acetates by gas-liquid chromatography coupled to isotope-ratio mass spectrometry. This technique allowed the determination, in a single experiment, of the amount and 13C abundance of six neutral sugars (including xylose used as internal standard). Despite the low abundance of 13C in natural products, the sensitivity of the technique was found to be sufficient to detect isotopic enrichment as low as 0.001% with good accuracy and reproducibility in 2 micrograms of each glycoprotein neutral sugar. As an example, the pattern of labeling by dietary D-glucose from corn starch appears to be very different for fucose, ribose, mannose, galactose, and glucose of intestine or serum macromolecules.

Modification of blood group A antigen expression in a pancreatic cancer cell line (PC-1) by inhibitors of N-glycan processing

Pancreatic adenocarcinomas induced in Syrian hamsters by treatment with N-nitrosobis(2-oxopropyl) amine express blood group A antigen, which is absent in normal pancreatic cells. On membrane glycoproteins purified from tumors, blood group A antigen has been found to be expressed on multiantennary Asn-linked complex glycans. In this study, we investigated the effect of inhibitors of Asn-glycan processing on blood group A antigen bearing glycan structures in a cell line (PC-1) established from a primary induced pancreatic cancer. Expression of blood group A antigen on cells and in membrane preparations was blocked by treatment with 1-deoxymannojirimycin, an inhibitor of mannosidase I, but was retained after treatment with swainsonine, an inhibitor of mannosidase II. However, swainsonine treatment altered the glycan structure associated with blood group A antigen from an endoglycosidase H resistant type to a sensitive type, indicating that the blood group A structure might shift from a complex type to a hybrid type glycan by this treatment. These results demonstrate that Asn-linked glycans carry the major blood group A antigens in PC-1 cells.

N-glycosylation of membrane glycoproteins in retinol-deficient rat liver

The effect of vitamin A deficiency on N-linked oligosaccharides of membrane glycoproteins was studied in rat liver in order to evaluate the suggested role of retinol in protein N-glycosylation. First, oligosaccharides of newly synthesized glycoproteins from rough endoplasmic reticulum of vitamin A deficient liver were compared with that of pair-fed controls. Oligosaccharides were metabolically labelled with D-[2-3H]mannose, released from the glycoproteins with endoglycosidase H, purified by reversed phase HPLC and ion exchange chromatography, and were reduced with sodium borohydride. HPLC fractionation of the oligosaccharide alditols showed that the glycoproteins carried mainly four oligosaccharide species, Glc1Man9GlcNAc2, Man9GlcNAc2, Man8GlcNAc2 and Man7GlcNAc2, in identical relative amounts in the vitamin A deficient and the control tissue. In particular, no increase in the proportion of short chain oligosaccharides was noted in vitamin A deficient liver. Second, the number of N-linked oligosaccharides was estimated in dipeptidylpeptidase IV (DPP IV), a major glycoprotein constituent of the hepatic plasma membrane, comparing the newly synthesized glycoprotein from rough endoplasmic reticulum and the mature form of DPP IV from the plasma membrane. No evidence was obtained that retinol deficiency caused incomplete glycosylation of this membrane glycoprotein. From these data, the suggested role of retinol as a cofactor involved in the synthesis of N-linked oligosaccharides of glycoproteins must be questioned.

Subcellular distribution in rat liver of a novel broad-specificity (alpha 1----2, alpha 1----3 and alpha 1----6) mannosidase active on oligomannose glycans

Recently, the purification to homogeneity was reported of a novel broad-specificity alpha-mannosidase from rat liver microsomal membranes [P. Bonay and R. C. Hughes (1991) Eur. J. Biochem. 197, 229-238]. The enzyme catalyzed the ordered removal of alpha 1----2-, alpha 1----3- and alpha 1----6-linked mannose residues from MannGlcNAc oligosaccharide substrates where n = 4-9. We now show by subcellular fractionation and immunocytochemistry that the novel mannosidase is present in the endoplasmic reticulum, Golgi apparatus and endosomes. Enzyme activity is enriched in a heavy Golgi membrane fraction and to lesser extent in an intermediate density Golgi membrane fraction containing GlcNAc transferase I activity and in a 'late' endosomal fraction. Low levels of enzyme activity were detectable in endoplasmic reticulum membranes and in 'early' endosomes but not in receptor-enriched and ligand-free endosomes. Assays of enzymic activity using Golgi membrane fractions in the absence and presence of Triton X-100 showed that the active site of the enzyme faces the lumen of the membrane vesicles. Antibodies directed against the purified mannosidase showed no immunological cross-reaction to known endoplasmic reticulum and Golgi mannosidases. Conversely, the purified mannosidase was not recognized by antibodies directed against endoplasmic reticulum mannosidase nor Golgi mannosidase IA.