The binding of fucose-containing glycoproteins by hepatic lectins. Re-examination of the clearance from blood and the binding to membrane receptors and pure lectins

The nature of the hepatic receptors that bind glycoproteins through fucose at the non-reducing termini of oligosaccharides in glycoproteins has been examined by three different approaches. First, the clearance from blood of intravenously injected glycoproteins was examined in mice with the aid of neoglycoproteins of bovine serum albumin (BSA). The clearance of fucosyl-BSA was rapid and was not strongly inhibited by glycoproteins that inhibit clearance mediated by the galactose or the mannose/N-acetylglucosamine receptors of liver. The clearance of Fuc alpha 1,3(Gal beta 1,4)GlcNAc-BSA (where Fuc is fucose) was inhibited weakly by either Fuc-BSA or Gal beta 1,4GlcNAc-BSA but strongly by a mixture of the two neoglycoproteins, suggesting that its clearance was mediated by hepatic galactose receptors as well as by a fucose-binding receptor. Second, the binding of neoglycoproteins to a membrane fraction of mouse liver was examined. Fuc-BSA binding to membranes was Ca2+ dependent but was not inhibited by glycoproteins that would inhibit the galactose or the mannose/N-acetylglucosamine receptors. In addition, the binding of Fuc-BSA and Gal beta 1,4GlcNAc-BSA differed as a function of pH, in accord with binding of Fuc-BSA through fucose-specific hepatic receptors. Finally, the binding of neoglycoproteins to the pure galactose lectin from rat liver was examined. Neither Fuc-BSA nor Fuc alpha 1,2Gal beta 1,4GlcNAc-BSA bound the galactose lectin, although Fuc alpha 1,3(Gal beta-1,4) GlcNAc-BSA bound avidly. Taken together, these studies suggest that a fucose-binding receptor that differs from the galactose and the mannose/N-acetylglucosamine receptors may exist in rat and mouse liver.     

Isolation and characterization of a mannose/N-acetylglucosamine/fucose-binding protein from rat liver.

A rat liver mannan-binding protein was isolated by affinity chromatography on invertase--Sepharose by a modification of the method of Kawasaki, Etoh & Yamashina [(1978) Biochem. Biophys. Res. Commun. 81, 1018-1024] and by a new method involving chromatography on mannose-Sepharose. The binding protein appears as a single band on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis with an apparent mol.wt. of approx. 30000. Binding of 125I-labelled mannan is saturable and inhibited by mannose, N-acetylglucosamine, or L-fucose but not by galactose or mannose 6-phosphate. Neoglycoproteins containing mannose, N-acetylglucosamine, or L-fucose, but not galactose, are inhibitory. The neoglycoproteins are 10000-fold more effective (based on moles of sugar) than are free monosaccharides as inhibitors. 125I-labelled mannan binding to the binding protein is calcium-dependent.     

Mannose-receptor-mediated clearance of lysosomal alpha-mannosidase in scavenger endothelium of cod endocardium.

Mannose-receptor-mediated clearance of circulating glycoproteins was studied in Atlantic cod (Gadus morhua). Distribution studies with radioiodinated and fluorescently labelled ligands showed that cod liver lysosomal alpha-mannosidase and yeast invertase were rapidly eliminated from blood via a mannose specific pathway in liver parenchymal cells and endocardial endothelial cells of atrium and ventricle. Asialo-orosomucoid, a galactose-terminated glycoprotein, was cleared by liver only. In vitro studies were performed with primary cultures of atrial-endocardial endothelial cells (AEC), incubated at 12 degrees C in a serum free medium. Cod AEC endocytosed mannose-terminated glycoproteins (125I-alpha-mannosidase, 125I-invertase, 125I-mannan, 125I-ovalbumin and unlabelled lysosomal alpha-mannosidase), whereas 125I-asialo-orosomucoid was not recognised. Uptake of radiolabelled mannose-terminated ligands was inhibited 80-100% in the presence of excess amounts of mannan, invertase, D-mannose, L-fucose or EGTA. Our results suggest that the cod endocardial endothelial cells express a specific Ca(2+)-dependent mannose receptor, analogous to the mannose receptor on mammalian macrophages and liver sinusoidal endothelial cells.     

An electron microscope autoradiographic study of the carbohydrate recognition systems in rat liver. I. Distribution of 125I-ligands among the liver cell types

Electron microscope autoradiography was used to study the cellular localization of seven glycoproteins rapidly cleared from the circulating plasma of rats and taken up by the liver. 1 and 15 min after intravenous administration of the 125I-glycoproteins, livers were fixed in situ by perfusion and processed for autoradiography. Autoradiographic grains in the developed sections were found to represent the intact 125I-ligand. A quantitative analysis of the distribution and concentration (density) of autoradiographic grains over the three major cell types of the liver was then performed. Three molecules, asialo-fetuin, asialo-orosomucoid, and lactosaminated RNase A dimer, the oligosaccharide chains of which terminate in galactose residues, were bound and internalized almost exclusively (greater than 90%) by hepatocytes. Conversely, four molecules, the oligosaccharide chains of which terminate in either N-acetyl-glucosamine (agalacto-orosomucoid) or mannose (ahexosamino-orosomucoid, preputial beta-glucuronidase, and mannobiosaminated RNase A dimer), were specifically bound and internalized by cells lining the blood sinusoids--that is, by Kupffer cells and endothelial cells. Endothelial cells were two to six times more active (on a cell volume basis) than were Kupffer cells in the internalization of these four 125I-ligands. Mannose and N-acetylglucosamine-terminated glycoproteins competed with each other for uptake into either endothelial cells or Kupffer cells, indicating that a single system recognized mannose or N-acetyl-glucosamine residues. Finally, agalacto-orosomucoid and ahexosamino-orosomucoid were also associated with hepatocytes, but competition experiments utilizing excess asialo-orosomucoid demonstrated that residual galactosyl residues were responsible for this association.     

Human β-glucuronidase: In vivo clearance and in vitro uptake by a glycoprotein recognition system on reticuloendothelial cells

Previously reported studies demonstrated that infused human placental β-glucuronidase is rapidly cleared from rat plasma and localizes predominantly in rat liver. Prior treatment of the enzyme with sodium metaperiodate converted the enzyme to a very slow clearance form. This suggested that the clearance system recognized the carbohydrate structure of the glycoprotein hydrolase. This report defines the glycosyl specificity and the cell type(s) involved in the clearance process. Clearance of infused human β-glucuronidase was blocked by simultaneous infusion of glycoproteins which have mannose or N-acetylglucosamine in their exposed nonreducing position, or by some simple sugars (α-methylmannoside, mannose or L-fucose) which block clearance of these glycoproteins. Two immunohistochemical techniques demonstrated preferential localization of human β-glucuronidase in sinusoidal lining cells (nonparenchymal cells) in rat liver. Human placental β-glucuronidase was also taken up by isolated rat alveolar macrophages by the carbohydrate-mediated glycoprotein uptake system recently demonstrated in these cells. This isolated cell uptake system appears to have the same specificity as the system for plasma clearance of infused human placental β-glucuronidase in the intact rat.The combined data from in vivo clearance studies and from studies of enzyme uptake by isolated rat macrophages suggest that a mannose/N-acetylglucosamine-glycoprotein uptake system is expressed on fixed tissue macrophages in the rat, and that this system mediates plasma clearance of infused human placental β-glucuronidase.     

Isolation and characterization of a mannan-binding protein from rabbit liver

A membrane protein which binds mannan has been isolated from rabbit liver by affinity chromatography. Upon polyacrylamide gel electrophoresis, a single major band was recovered with an estimated molecular weight of 31,000. When assayed as inhibitors, N-acetylmannosamine, N-acetylglucosamine, and mannose were potent inhibitors of mannan binding; N-acetylgalactosamine and mannose-6-phosphate were inert. Glycoproteins with terminal N-acetylglucosamine and/or mannose residues which are cleared rapidly from the circulation by the liver were the most potent inhibitors. On the basis of these results, it is proposed that the mannan-binding protein is the principle mediating the hepatic uptake of glycoproteins with terminal N-acetylglucosamine and/or mannose residues.     

In vivo and in vitro interaction of high and low molecular weight single-chain urokinase-type plasminogen activator with rat liver cells.

The plasma clearance and the interaction of high (HMW) and low (LMW) molecular weight single-chain urokinase-type plasminogen activator (scu-PA) with rat liver cells was determined. 125I-Labeled HMW- and LMW-scu-PA were rapidly cleared from plasma with a half-life of 0.45 min and a maximal liver uptake of 55% of the injected dose. Liver uptake of scu-PA was mediated by parenchymal cells. Excess of unlabeled HMW-scu-PA reduced the liver uptake of 125I-HMW-scu-PA strongly. In vivo liver degradation of scu-PA was reduced by inhibitors of the lysosomal pathway. A high affinity binding site (Kd 45 nM, Bmax 1.7 pmol/mg cell protein) for both HMW- and LMW-scu-PA was determined on isolated parenchymal liver cells. Cross-competition binding studies showed that LMW- and HMW-scu-PA bind to the same site. Tissue-type plasminogen activator, mannose- or galactose-terminated glycoproteins did not affect the scu-PA binding to parenchymal liver cells. It is concluded that LMW- and HMW-scu-PA are taken up in the liver by a common, newly identified recognition site on parenchymal liver cells and are subsequently degraded in the lysosomes. It is suggested that this site is important for the regulation of the turnover of scu-PA.     

Effect of inhibition of carbohydrate-mediated endocytosis on catabolism of equine haptoglobin and its complex with haemoglobin in hen

Equine haptoglobin (Hp) and haptoglobin-haemoglobin complex (Hp-Hb) are metabolised in the hen parenchymal cells of the liver in which endocytosis was inhibited by agalacto-orosomucoid (AGOR) or native orosomucoid (OR). The intravenous administration of AGOR or OR together with [125I]Hp slightly decreased clearance of Hp from circulation. This suggested that heterologous Hp could be catabolised by the alternative pathway following the uptake by the liver RES cells. Administration of the [125I]Hp-Hb complex and AGOR was of no effect on the clearance of the Hp-Hb complex.     

Clearance of lysosomal hydrolases following intravenous infusion. Kinetic and competition experiments with beta-glucuronidase and N-acetyl-beta-D-glucosaminidase.

Clearance experiments with highly purified lysosomal glycosidases, β-glucuronidase and N-acetyl-β-d-glucosaminidase, following intravenous infusion revealed widely varying clearance profiles which depended on the tissue source of the enzyme. Normal rat serum β-glucuronidase and epididymal N-acetyl-β-d-glucosaminidase were cleared slowly from the circulation when compared with rat preputial gland β-glucuronidase, liver lysosomal β-glucuronidase, and liver lysosomal N-acetyl-β-d-glucosaminidase, respectively, which were cleared rapidly. Experiments comparing the catalytic properties and molecular dimensions of the enzymes revealed no differences between rapid and slow clearance forms. Kinetic analysis using the rapid clearance forms of β-glucuronidase has allowed the resolution of at least two components, rapid and slow. Clearance of the rapid component is saturable and appears to reflect binding or uptake by a limited number of sites. By contrast, the clearance rate of the slow component increased linearly with respect to dose and may be due to nonspecific or low-affinity binding. Competition experiments with β-glucuronidase-free lysosomal extract and highly purified lysosomal enzymes, but not serum glycoproteins or colloidal silver, suggest that one lysosomal enzyme inhibits clearance of others and that a common mechanism may be involved in their binding.     

Glycoprotein catabolism in rat liver. Lysosomal digestion of iodinated asialo-fetuin

(125)I-labelled asialo-fetuin, administered intravenously, rapidly accumulates in rat liver and the radioactivity is subsequently cleared from the liver within 60min. Plasma radioactivity reaches a minimum between 10 and 15 min after injection and rises slightly during the period of liver clearance. Free iodide is the only radioactive compound found in plasma during this latter period. Fractionation of rat liver at 5 and 13min after injection of (125)I-labelled asialo-fetuin supports the hypothesis that asialo-glycoprotein is taken into liver by pinocytosis after binding to the plasma membrane and is then hydrolysed by lysosomal enzymes. At 5min, radioactivity was concentrated 23-fold in a membrane fraction similarly enriched in phosphodiesterase I, a plasma-membrane marker enzyme, whereas at 13min the radioactivity appeared to be localized within lysosomes. Separation of three liver fractions (heavy mitochondrial, light mitochondrial and microsomal) on sucrose gradients revealed the presence of two populations of radioactive particles. One population banded in a region coincident with a lysosomal marker enzyme. The other, more abundant, population of radioactive particles had a density of 1.13 and contained some phosphodiesterase, but very little lysosomal enzyme. These latter particles appear to be pinocytotic vesicles produced after uptake of the asialo-fetuin bound by the plasma membrane. Lysosomal extracts extensively hydrolyse asialo-fetuin during incubation in vitro at pH4.7 and iodotyrosine is completely released from the iodinated glycoprotein. Protein digestion within lysosomes was demonstrated by incubating intact lysosomes containing (125)I-labelled asialo-fetuin in iso-osmotic sucrose, pH7.2. The radioactive hydrolysis product, iodotyrosine, readily passed through the lysosomal membrane and was found in the external medium. These results are not sufficient to account for the presence of free iodide in plasma, but this was explained by the observation that iodotyrosines are deiodinated by microsomal enzymes in the presence of NADPH.     

Differential recognition of core and terminal portions of oligosaccharide ligands by carbohydrate-recognition domains of two mannose-binding proteins

Two different mannose-binding proteins (MBP-A and MBP-C), which show 56% sequence identity, are present in rat serum and liver. It has previously been shown that MBP-A binds to a range of monosaccharide-bovine serum albumin conjugates, and that, among oligosaccharide ligands tested, preferential binding is to terminal nonreducing N-acetylglucosamine residues of complex type N-linked oligosaccharides. In order to compare the binding specificity of MBP-C, an expression system has been developed for production of a fragment of this protein which contains the COOH-terminal carbohydrate-recognition domain. After radioiodination, the domain has been used to probe natural glycoproteins, neoglycoproteins, and neoglycolipids. Like MBP-A, MBP-C binds several different monosaccharides conjugated to bovine serum albumin, including mannose, fucose, and N-acetylglucosamine, although binding to the last of these is relatively weaker than observed for MBP-A. The results of binding to natural glycoproteins and to neoglycolipids containing oligosaccharides derived from these proteins are most compatible with the interpretation that MBP-C interacts primarily with the trimannosyl core of complex N-linked oligosaccharides, with additional ligands being terminal fucose and perhaps also peripheral mannose residues of high mannose type oligosaccharides. This binding specificity is thus quite distinct from that of MBP-A. The presence of multiple MBPs with distinct binding specificities in preparations derived from serum and liver explains conflicting conclusions which have been reached about carbohydrate recognition by these proteins.     

Carbohydrate composition of variant-specific surface antigen glycoproteins from Trypanosoma brucei

The carbohydrate of variant-specific surface antigen glycoproteins from bloodstream forms of 13 cloned variants of Trypanosoma brucei was analyzed by gas-liquid chromatography. The glycoproteins contained from 6 to 17% carbohydrate by weight, and all contained the same 4 sugars: mannose, galactose, glucose, and glucosamine (probably as N-acetylglucosamine). The glycoprotein from variant 048, strain 427 contained (+20%) 11 mannose, 4 galactose, 4 glucose, and 5 glucosamine residues/mole of glycoprotein (molecular weight 65,000). Glucose was an intergral component of the glycoproteins, not dissociable by sodium dodecyl sulphate, 8 M urea, or 1 M acetic acid. Some of the glucose was dissociated by trichloroacetic acid. Most of the glycoproteins formed precipitin with concanavalin A in Ouchterlony double diffusion, but none formed such bands with wheat germ agglutinin or Ricinus communis lectin (molecular weight 120, 000).     

Receptor-mediated endocytosis of fucosylated neoglycoprotein by macrophages

The characteristics of the recognition system involved in the receptor mediated endocytosis of the neoglycoprotein, fucose human serum albumin (HSA) were studied. It was found that (i) fucose-HSA showed strong affinity binding and uptake by various macrophages; (ii) binding was specific for L-fucose and D-mannose; (iii) binding was found to be inhibited by oxidant like H₂O₂ and swainsonine whereas it was elevated by dexamethasone; (iv) clearance of¹²⁵I-fucose-HSA was rapid and strongly inhibited by unlabelled fucose-HSA. Greater than 70% of fucose-HSA was found in liver and more than 60% of this was found in liver lysosomes; (v) uptake of fucose-HSA was thirty-fold more efficient in liver macrophages (Kupffer cells) than in hepatocytes; (vi) moreover, mannose-HSA and ovalbumin which are potent inhibitors of mannose/N-acetylglucosamine receptors inhibited clearance and uptake of fucose-HSA by liver as well as by isolated Kupffer cells suggesting the involvement of both fucose and mannose receptors or a single type of receptor having greater affinity for fucose-HSA than for mannose-HSA. These results emphasize the important role of fucose-terminated glycoproteins in site-specific drug targeting.     

Direct utilization of mannose for mammalian glycoprotein biosynthesis

Direct utilization of mannose for glycoprotein biosynthesis has not been studied because cellular mannose is assumed to be derived entirely from glucose. However, animal sera contain sufficient mannose to force uptake through glucose-tolerant, mannose-specific transporters. Under physiological conditions this transport system provides 75% of the mannose for protein glycosylation in human hepatoma cells despite a 50- to 100-fold higher concentration of glucose. This suggests that direct use of mannose is more important than conversion from glucose. Consistent with this finding the liver is low in phosphomannose isomerase activity (fructose-6-P<->mannose-6-P), the key enzyme for supplying glucose-derived mannose to the N-glycosylation pathway. [2- 3H] Mannose is rapidly absorbed from the intestine of anesthetized rats and cleared from the blood with a t1/2of 30 min. After a 30 min lag, label is incorporated into plasma glycoproteins, and into glycoproteins of all organs during the first hour. Most (87%) of the initial incorporation occurs in the liver, but this decreases as radiolabeled plasma glycoproteins increase. Radiolabel in glycoproteins also increases 2- to 6-fold in other organs between 1-8 h, especially in lung, skeletal muscle, and heart. These organs may take up hepatic- derived radiolabeled plasma glycoproteins. Significantly, the brain, which is not exposed to plasma glycoproteins, shows essentially no increase in radiolabel. These results suggest that mammals use mannose transporters to deliver mannose from blood to the liver and other organs for glycoprotein biosynthesis. Additionally, contrary to expectations, most of the mannose for glycoprotein biosynthesis in cultured hepatoma cells is derived from mannose, not glucose. Extracellular mannose may also make a significant contribution to glycoprotein biosynthesis in the intact organism.      

Uptake and degradation of 125I-labelled high density lipoproteins in rat liver cells in vivo and in vitro.

1. The uptake of 125I-labelled high density lipoproteins (HDL) in various organs of the rat was determined after an intravenous injection. The uptake of 125I-labelled polyvinylpyrrolidone in the same organs was determined in order to assess uptake by fluid endocytosis. The uptake/organ was highest for the liver. The adrenals showed the highest uptake/unit weight of the organs studied. The liver, the kidneys and the spleen showed comparable values for uptake/g of tissue. The uptake of 125I-labelled HDL exceeded by far that of 125I-labelled polyvinylpyrrolidone in the liver, the kidneys, the spleen and the adrenals, indicating that the uptake of 125I-labelled HDL was mediated by adsorptive endocytosis. 2. The in vivo uptake of 125I-labelled HDL was determined in purified hepatocytes and non-parenchymal cells prepared by collagenase perfusion of livers from animals after intravenous injections of 125I-labelled HDL. When expressed per cell, the hepatocytes and the non-parenchymal liver cells took up about the same amount of 125I-labelled HDL. 3. The in vitro uptake and degradation of 125I-labelled HDL in isolated rat hepatocytes was studied. The uptake at increasing concentrations of 125I-labelled HDL was saturable indicating uptake mediated through binding sites. 125I-labelled HDL were easily degraded by contaminating proteases from the perfusate. 4. Subcellular fractionation by isopycnic centrifugation indicated that the accumulation of 125I-labelled HDL did not take place in the lysosomes, but rather on the plasma membrane and possibly in the endosomes (phagosomes). 5. 125I-labelled HDL were internalized into the cells and degraded in the lysosomes. Leupetin and chloroquine, inhibitors of the lysosomal function effectively inhibited the formation of 125I-labelled acid-soluble radioactivity by the cells. Chloroquine, but not the protease inhibitor leupeptin, reduced the hydrolysis of the cholesteryl ester moiety of HDL.     

Studies in vivo and in vitro on the uptake and degradation of soluble collagen alpha 1(I) chains in rat liver endothelial and Kupffer cells.

Intravenously administered 125I-labelled monomeric alpha 1 chains (125I-alpha 1) of collagen type I were rapidly cleared and degraded by the liver of rats. Isolation of the liver cells after injection of the label revealed that the uptake per liver endothelial cell equalled the uptake per Kupffer cell, whereas the amount taken up per hepatocyte was negligible. The uptake of 125I-alpha 1 in cultured cells was 10 times higher per liver endothelial cell than per Kupffer cell. The ligand was efficiently degraded by cultures of both cell types. However, spent medium from cultures of Kupffer cells, unlike that from cultures of other cells, contained gelatinolytic activity which degraded 125I-alpha 1. The presence of hyaluronic acid, chondroitin sulphate or mannose/N-acetylglucosamine-terminal glycoproteins, which are endocytosed by the liver endothelial cells via specific receptors, did not interfere with binding, uptake or degradation of 125I-alpha 1 by these cells. Unlabelled alpha 1 and heat-denatured collagen inhibited the binding to a much greater extent than did native collagen. The presence of fibronectin or F(ab')2 fragments of anti-fibronectin antibodies did not affect the interaction of the liver endothelial cells, or of other types of liver cells, with 125I-alpha 1. The accumulation of fluorescein-labelled heat-denatured collagen in vesicles of cultured liver endothelial cells is evidence that the protein is internalized. Moreover, chloroquine, 5-dimethylaminonaphthalene-1-sulphonylcadaverine (dansylcadaverine), monensin and cytochalasin B, which impede one or more steps of the endocytic process, inhibited the uptake of 125I-alpha 1 by the liver endothelial cells. Leupeptin, an inhibitor of cathepsin B and 'collagenolytic cathepsins', inhibited the intralysosomal degradation of 125I-alpha 1, but had no effect on the rate of uptake of the ligand. The current data are interpreted as follows. (1) The ability of the liver endothelial cells and the Kupffer cells to sequester circulating 125I-alpha 1 efficiently may indicate a physiological pathway for the breakdown of connective-tissue collagen. (2) The liver endothelial cells express receptors that specifically recognize and mediate the endocytosis of collagen alpha 1(I) monomers. (3) The receptors also recognize denatured collagen (gelatin). (4) Fibronectin is not involved in the binding of alpha 1 to the receptors. (5) Degradation occurs intralysosomally by leupeptin-inhibitable cathepsins.     

Glycosidase inhibitors: inhibitors of N-linked oligosaccharide processing.

The biosynthesis of the various types of N-linked oligosaccharide structures involves two series of reactions: 1) the formation of the lipid-linked saccharide precursor, Glc3Man9(GlcNAc)2-pyrophosphoryl-dolichol, by the stepwise addition of GlcNAc, mannose and glucose to dolichyl-P, and 2) the removal of glucose and mannose by membrane-bound glycosidases and the addition of GlcNAc, galactose, sialic acid, and fucose by Golgi-localized glycosyltransferases to produce different complex oligosaccharide structures. For most glycoproteins, the precise role of the carbohydrate is still not known, but specific N-linked oligosaccharide structures are key players in targeting of lysosomal hydrolases to the lysosomes, in the clearance of asialoglycoproteins from the serum, and in some cases of cell:cell adhesion. Furthermore, many glycoproteins have more than one N-linked oligosaccharide, and these oligosaccharides on the same protein frequently have different structures. Thus, one oligosaccharide may be of the high-mannose type whereas another may be a complex chain. One approach to determining the role of specific structures in glycoprotein function is to use inhibitors that block the modification reactions at different steps, causing the cell to produce glycoproteins with altered carbohydrate structures. The function of these glycoproteins can then be assessed. A number of alkaloid-like compounds have been identified that are specific inhibitors of the glucosidases and mannosidases involved in glycoprotein processing. These compounds cause the formation of glycoproteins with glucose-containing high mannose structures, or various high-mannose or hybrid chains, depending on the site of inhibition. These inhibitors have also been useful for studying the processing pathway and for comparing processing enzymes from different organisms.     

Identification of a defence mechanism in vivo against the leakage of enterokinase into the blood.

1. The serum proteinase inhibitors alpha 1-antitrypsin, alpha 2-macroglobulin, inter-alpha-trypsin inhibitor and C1-esterase inhibitor were found not to affect the catalytic activity of human enterokinase, whereas bovine trypsin activity was modified essentially as expected. Enterokinase was also not inhibited by Trasylol (trypsin inhibitor from bovine lung) or bovine pancreatic trypsin inhibitor. No other component in human or mouse serum complexing with enterokinase was identified. 2. Human enterokinase administered intravenously into mice was rapidly cleared from the circulation with a half-life of 2.5 min. This removal was not the result of the difference in species, since partially purified mouse enterokinase was cleared at the same rate as the human enzyme. Clearance was mediated by recognition of the carbohydrate portion of enterokinase and not through specific recognition of its catalytic site. Immunofluorescent staining showed that the enzyme accumulated in the liver. Attempts to block the clearance by the simultaneous infusion of competing glycoproteins suggested that enterokinase was taken up by hepatocytes. Of the glycoproteins tested only two, human lactoferrin (terminal fucosyl alpha 1 leads to 3 N-acetylglucosamine) and bovine asialo-fetuin (terminal galactosyl beta 1 leads to 4 N-acetylglucosamine) were weakly competitive. Two inhibitors of endocytosis, Intralipid and Triton WR1339, failed to delay the removal of enterokinase. It is proposed that enterokinase is cleared from the circulation by an as yet uncharacterized hepatocyte receptor.     

Grafting of different glycosides on the surface of liposomes and its effect on the tissue distribution of 125I-labelled γ-globulin encapsulated in liposomes

Different glycosides were grafted on the surface of liposomes containing ¹²⁵I-labelled γ-globulin by two ways: (1) by using glycolipid and (2) by covalent coupling of $\text{p-}\text{aminophenyl-}\text{d}\text{-glycosides}$ to phosphatidylethanolamine liposomes using glutaraldehyde. The distribution of ¹²⁵I-labelled γ-globulin was determined in mouse tissues from 5–60 min after a single injection of these liposomes. The liver uptake of encapsulated ¹²⁵I-labelled γ-globulin was highest from liposomes having galactose and mannose on the surface. Competition experiments and cross-inhibition studies indicate that this uptake are mediated by specific recognition of the surface galactose and mannose residues of liposomes by the receptors present on the plasma membrane of liver cells. Stearylamine-containing liposomes were found to be more efficient in mediating the uptake of ¹²⁵I-labelled γ-globulin by the lung, whereas in the case of spleen, phosphatidylethanolamine liposomes were more efficient. The extent of uptake of ¹²⁵I-labelled γ-globulin from all types of liposome decreases as the amount of given liposomes increases. The uptake of ¹²⁵I-labelled γ-globulin from liposomes containing asialogangliosides depends upon the phospholipid/ glycolipid ratio. These experiments clearly demonstrate that enhanced liposome uptake by liver cells could be achieved by grafting galactose and mannose on the liposomal surface.     

Lysosomal enzyme oligosaccharide phosphorylation in mouse lymphoma cells: specificity and kinetics of binding to the mannose 6-phosphate receptor in vivo

Phosphomannosyl residues on lysosomal enzymes serve as an essential component of the recognition marker necessary for binding to the mannose 6-phosphate (Man 6-P) receptor and translocation to lysosomes. The high mannose-type oligosaccharide units of lysosomal enzymes are phosphorylated by the following mechanism: N-acetylglucosamine 1-phosphate is transferred to the 6 position of a mannose residue to form a phosphodiester; then N- acetylglucosamine is removed to expose a phosphomonoester. We examined the kinetics of this phosphorylation pathway in the murine lymphoma BW5147.3 cell line to determine the state of oligosaccharide phosphorylation at the time the newly synthesized lysosomal enzymes bind to the receptor. Cells were labeled with [2-(3)H]mannose for 20 min and then chased for various times up to 4 h. The binding of newly synthesized glycoproteins to the Man 6-P receptor was followed by eluting the bound ligand with Man 6-P. Receptor-bound material was first detected at 30 min of chase and reached a maximum at 60 min of chase, at which time approximately 10 percent of the total phosphorylated oligosaccharides were associated with the receptor. During longer chase times, the total quantity of cellular phosphorylated oligosaccharides decreased with a half-time of 1.4 h, suggesting that the lysosomal enzymes had reached their destination and had been dephosphorylated. The structures of the phosphorylated aligosaccharides of the eluted ligand were then determined and compared with the phosphorylated oligosaccharides of molecules which were not bond to the receptor. The major phosphorylated oligosaccharide species present in the nonreceptor-bound material contained a single phosphosphodiester at all time examined. In contrast, receptor-bound oligosaccharides were greatly enriched in species possessing one and two phosphomonoesters. These results indicate that binding of newly synthesized lysosomal enzymes to the Man 6-P receptor occurs only after removal of the covering N- acetylglucosamine residues.