The structure of N-linked oligosaccharides of human pancreatic bile-salt-dependent lipase

Structure of dimeric lipoprotein lipase reveals a pore adjacent to the active site

Lipoprotein lipase (LPL) hydrolyzes triglycerides from circulating lipoproteins, releasing free fatty acids. Active LPL is needed to prevent hypertriglyceridemia, which is a risk factor for cardiovascular disease (CVD). Using cryogenic electron microscopy (cryoEM), we determined the structure of an active LPL dimer at 3.9 Å resolution. This structure reveals an open hydrophobic pore adjacent to the active site residues. Using modeling, we demonstrate that this pore can accommodate an acyl chain from a triglyceride. Known LPL mutations that lead to hypertriglyceridemia localize to the end of the pore and cause defective substrate hydrolysis. The pore may provide additional substrate specificity and/or allow unidirectional acyl chain release from LPL. This structure also revises previous models on how LPL dimerizes, revealing a C-terminal to C-terminal interface. We hypothesize that this active C-terminal to C-terminal conformation is adopted by LPL when associated with lipoproteins in capillaries.

Structure of Dimeric Lipoprotein Lipase Reveals a Pore for Hydrolysis of Acyl Chains

Lipoprotein lipase (LPL) hydrolyzes triglycerides from circulating lipoproteins, releasing free fatty acids. Active LPL is needed to prevent hypertriglyceridemia, which is a risk factor for cardiovascular disease (CVD). Using cryogenic electron microscopy (cryoEM), we determined the structure of an active LPL dimer at 3.9 Ã… resolution. This is the first structure of a mammalian lipase with an open, hydrophobic pore adjacent to the active site. We demonstrate that the pore can accommodate an acyl chain from a triglyceride. Previously, it was thought that an open lipase conformation was defined by a displaced lid peptide, exposing the hydrophobic pocket surrounding the active site. With these previous models after the lid opened, the substrate would enter the active site, be hydrolyzed and then released in a bidirectional manner. It was assumed that the hydrophobic pocket provided the only ligand selectivity. Based on our structure, we propose a new model for lipid hydrolysis, in which the free fatty acid product travels unidirectionally through the active site pore, entering and exiting opposite sides of the protein. By this new model, the hydrophobic pore provides additional substrate specificity and provides insight into how LPL mutations in the active site pore may negatively impact LPL activity, leading to chylomicronemia. Structural similarity of LPL to other human lipases suggests that this unidirectional mechanism could be conserved but has not been observed due to the difficulty of studying lipase structure in the presence of an activating substrate. We hypothesize that the air/water interface formed during creation of samples for cryoEM triggered interfacial activation, allowing us to capture, for the first time, a fully open state of a mammalian lipase. Our new structure also revises previous models on how LPL dimerizes, revealing an unexpected C-terminal to C-terminal interface. The elucidation of a dimeric LPL structure highlights the oligomeric diversity of LPL, as now LPL homodimer, heterodimer, and helical filament structures have been elucidated. This diversity of oligomerization may provide a form of regulation as LPL travels from secretory vesicles in the cell, to the capillary, and eventually to the liver for lipoprotein remnant uptake. We hypothesize that LPL dimerizes in this active C-terminal to C-terminal conformation when associated with mobile lipoproteins in the capillary.

The mucinous domain of pancreatic carboxyl-ester lipase (CEL) contains core 1/core 2 O-glycans that can be modified by ABO blood group determinants

Carboxyl-ester lipase (CEL) is a pancreatic fat-digesting enzyme associated with human disease. Rare mutations in the CEL gene cause a syndrome of pancreatic exocrine and endocrine dysfunction denoted MODY8, whereas a recombined CEL allele increases the risk for chronic pancreatitis. Moreover, CEL has been linked to pancreatic ductal adenocarcinoma (PDAC) through a postulated oncofetal CEL variant termed feto-acinar pancreatic protein (FAPP). The monoclonal antibody mAb16D10 was previously reported to detect a glycotope in the highly O-glycosylated, mucin-like C terminus of CEL/FAPP. We here assessed the expression of human CEL in malignant pancreatic lesions and cell lines. CEL was not detectably expressed in neoplastic cells, implying that FAPP is unlikely to be a glycoisoform of CEL in pancreatic cancer. Testing of the mAb16D10 antibody in glycan microarrays then demonstrated that it recognized structures containing terminal GalNAc-α1,3(Fuc-α1,2)Gal (blood group A antigen) and also repeated protein sequences containing GalNAc residues linked to Ser/Thr (Tn antigen), findings that were supported by immunostainings of human pancreatic tissue. To examine whether the CEL glycoprotein might be modified by blood group antigens, we used high-sensitivity MALDI-TOF MS to characterize the released O-glycan pool of CEL immunoprecipitated from human pancreatic juice. We found that the O-glycome of CEL consisted mainly of core 1/core 2 structures with a composition depending on the subject's FUT2 and ABO gene polymorphisms. Thus, among digestive enzymes secreted by the pancreas, CEL is a glycoprotein with some unique characteristics, supporting the view that it could serve additional biological functions to its cholesteryl esterase activity in the duodenum.

Pancreatic adenocarcinoma, chronic pancreatitis, and MODY-8 diabetes: is bile salt-dependent lipase (or carboxyl ester lipase) at the crossroads of pancreatic pathologies?

Pancreatic adenocarcinomas and diabetes mellitus are responsible for the deaths of around two million people each year worldwide. Patients with chronic pancreatitis do not die directly of this disease, except where the pathology is hereditary. Much current literature supports the involvement of bile salt-dependent lipase (BSDL), also known as carboxyl ester lipase (CEL), in the pathophysiology of these pancreatic diseases. The purpose of this review is to shed light on connections between chronic pancreatitis, diabetes, and pancreatic adenocarcinomas by gaining an insight into BSDL and its variants. This enzyme is normally secreted by the exocrine pancreas, and is diverted within the intestinal lumen to participate in the hydrolysis of dietary lipids. However, BSDL is also expressed by other cells and tissues, where it participates in lipid homeostasis. Variants of BSDL resulting from germline and/or somatic mutations (nucleotide insertion/deletion or nonallelic homologous recombination) are expressed in the pancreas of patients with pancreatic pathologies such as chronic pancreatitis, MODY-8, and pancreatic adenocarcinomas. We discuss the possible link between the expression of BSDL variants and these dramatic pancreatic pathologies, putting forward the suggestion that BSDL and its variants are implicated in the cell lipid metabolism/reprogramming that leads to the dyslipidemia observed in chronic pancreatitis, MODY-8, and pancreatic adenocarcinomas. We also propose potential strategies for translation to therapeutic applications.

Bile salt-stimulated lipase from human milk binds DC-SIGN and inhibits human immunodeficiency virus type 1 transfer to CD4+ T cells

A wide range of pathogens, including human immunodeficiency virus type 1 (HIV-1), hepatitis C virus, Ebola virus, cytomegalovirus, dengue virus, Mycobacterium, Leishmania, and Helicobacter pylori, can interact with dendritic cell (DC)-specific ICAM3-grabbing nonintegrin (DC-SIGN), expressed on DCs and a subset of B cells. More specifically, the interaction of the gp120 envelope protein of HIV-1 with DC-SIGN can facilitate the transfer of virus to CD4+ T lymphocytes in trans and enhance infection. We have previously demonstrated that a multimeric LeX component in human milk binds to DC-SIGN, preventing HIV-1 from interacting with this receptor. Biochemical analysis reveals that the compound is heat resistant, trypsin sensitive, and larger than 100 kDa, indicating a specific glycoprotein as the inhibitory compound. By testing human milk from three different mothers, we found the levels of DC-SIGN binding and viral inhibition to vary between samples. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, Western blotting, and matrix-assisted laser desorption ionization analysis, we identified bile salt-stimulated lipase (BSSL), a Lewis X (LeX)-containing glycoprotein found in human milk, to be the major variant protein between the samples. BSSL isolated from human milk bound to DC-SIGN and inhibited the transfer of HIV-1 to CD4+ T lymphocytes. Two BSSL isoforms isolated from the same human milk sample showed differences in DC-SIGN binding, illustrating that alterations in the BSSL forms explain the differences observed. These results indicate that variations in BSSL lead to alterations in LeX expression by the protein, which subsequently alters the DC-SIGN binding capacity and the inhibitory effect on HIV-1 transfer. Identifying the specific molecular interaction between the different forms may aid in the future design of antimicrobial agents.

Insulin, glucagon and fatty acid treatment of hepatocytes does not result in phosphorylation or changes in activity of triacylglycerol hydrolase

It is recognized that the majority of very low density lipoprotein (VLDL) associated triacylglycerol (TG) is synthesized from fatty acids and partial acylglycerols generated by lipolysis of intra-hepatic storage rather than made de novo. Triacylglycerol hydrolase (TGH) is involved in mobilizing stored TG. Modulating the ability of TGH to hydrolyze stored lipids represents a potentially regulated and rate limiting step in VLDL assembly. Phosphorylation of lipases and carboxylesterases trigger diverse but functionally significant events. We explored the potential for regulating the mobilization of hepatic TG through phosphorylation of TGH. Insulin is known to suppress VLDL secretion from liver, and glucagon can be considered an opposing hormone. However, neither insulin nor glucagon treatment of hepatocytes led to phosphorylation of TGH or changes in its activity. Augmenting intracellular TG stores by incubations with oleic acid also did not lead to changes in TGH activity. Therefore, changes in phosphorylation state are not a mechanism for regulating TGH activity, access to TG substrate pools or for TGH-mediated contributions to VLDL assembly and secretion.

Lectin-like Ox-LDL receptor is expressed in human INT-407 intestinal cells: involvement in the transcytosis of pancreatic bile salt-dependent lipase

We have recently shown that the pancreatic bile salt-dependent lipase (BSDL) can be taken up by intestinal cells and transported to the blood circulation. This mechanism likely involves (specific) receptor(s) able to bind BSDL and located at the apical intestinal cell membrane. In this study, using Int407 human intestinal cells cultured to form a tight epithelium, we attempted to characterize (the) BSDL receptor(s). We found that an apical 50-kDa protein was able to bind BSDL. Further, we have demonstrated that Int407 cells expressed the lectin-like oxidized-LDL receptor (LOX-1), the upregulation of which by oxidized-LDL potentiates the transcytosis of BSDL, whereas carrageenan and to a lesser extent polyinosinic acid and fucoidan decrease the enzyme transcytosis. The mAb JTX92, which blocks the LOX-1 receptor function, also impaired the BSDL transcytosis. To confirm these results, the cDNA encoding the human intestinal receptor LOX-1 has been cloned, inserted into vectors, and transfected into Int407 cells. Overexpression of LOX-1 by these cells leads to a substantial increase in the BSDL transcytosis. Globally, these data support the view that LOX-1 could be an intestinal receptor for BSDL, which is implicated in the transcytosis of this enzyme throughout Int407 cells.

Impairment of bile salt-dependent lipase secretion in AR4-2J rat pancreatic cells induces its degradation by the proteasome

Bile salt-dependent lipase (BSDL, EC 3.1.1.13) is a lipolytic enzyme normally secreted by the pancreatic acinar cell. Co- and post-translational modifications, such as N- and O-linked glycosylation, regulate the secretion of this enzyme; therefore it was of first importance to determine the behaviour of BSDL under conditions that impaired its secretion. Using AR4-2J pancreatic cells as model, we showed, particularly when BSDL secretion is impaired, that proteasome inhibitors increased the amount of intracellular BSDL, suggesting that the proteasome is involved in the degradation of this protein. This was strengthened by the detection of ubiquitinated BSDL and of degradation product. Our results suggested that both ubiquitination and degradation of the enzyme occurred at the level of the cytosolic side of microsome membranes. ATP hydrolysis appears essential in ubiquitinated BSDL association with membranes and degradation. Furthermore, under normal secretory conditions, we have shown that a fraction of ubiquitinated BSDL is neither O-glycosylated nor N-glycosylated, suggesting that the N-glycosylation-deficient proteasome substrate does not reach the Golgi and could be degraded by the ER-associated degradation machinery. However, another fraction of ubiquitinated BSDL that is deficient in O-glycosylation, carries out endoglycosidase H-insensitive N-linked glycans, meaning that a second system, that detects abnormal BSDL molecules, could also operate at the level of the Golgi compartment. Consequently, it appears that impairment of BSDL secretion consecutive to secretion inhibition or to a deficient glycosylation leads to the proteasome-ubiquitin-dependent degradation of the protein. Therefore, this pathway is part of the quality control involved in BSDL secretion.

Changes in glycosylation of human bile-salt-stimulated lipase during lactation

Bile-salt-stimulated lipase (BSSL) is an enzyme in human milk, which is important for the fat digestion in the newborn infant. BSSL is highly glycosylated and includes one site for N-glycosylation and several sites for O-glycosylation. BSSL has previously been found to express Lewis a, Lewis b, and Lewis x carbohydrate antigens. In this study, glycosylation of BSSL was studied at different times during lactation. BSSL was purified from milk collected individually from four donors at several different times during the first 6 months of lactation. The BSSL glycans were characterized through monosaccharide analysis, high-pH anion-exchange chromatography, matrix-assisted laser desorption-ionization mass spectrometry, and ELISA. Both total carbohydrate content and relative amount of sialic acid were higher in BSSL from the first lactation month as compared to BSSL from milk collected later in lactation. BSSL from the first lactation month also showed a different composition of sialylated O-linked glycans and the N-linked oligosaccharides consisted of lower amounts of fucosylated structures compared to later in lactation. We also found a gradual increase in the expression of the carbohydrate epitope Lewis x on BSSL throughout the lactation period. This study shows that glycosylation of BSSL is dependent on blood group phenotype of the donor and changes substantially during the lactation period.

Expression of a 70-kDa immunoreactive form of bile salt-dependent lipase by human pancreatic tumoral mia PaCa-2 cells

This work describes the characterization of an immunoreactive form of bile salt-dependent lipase (BSDL) expressed by the human pancreatic tumoral Mia PaCa-2 cell line. This BSDL-related protein, which has an M(r) of 70 kDa, is enzymatically active and poorly secreted. Furthermore, a protein with the same electrophoretic migration can also be immunoprecipitated with polyclonal antibodies specific for the human pancreatic BSDL after in vitro translation of RNA isolated from Mia PaCa-2 cells. These data indicated that this BSDL-related protein might be poorly, or not, glycosylated. Reverse transcription and amplification of RNA extracted from Mia PaCa-2 cells using primers able to specifically amplify the full-length mRNA of the human BSDL resulted in a detectable 1.8-kb cDNA product, shorter than that of BSDL (2.2 kb). The sequence of this transcript corresponds to the mRNA sequence that codes for the mature human pancreatic BSDL. However, a deletion of 330 bp is located within the 3'-domain of this cDNA. Therefore data allowed us to conclude that the 70-kDa BSDL-related protein is a 612 amino acid length protein and represents a truncated form of BSDL. The deletion of 110 amino acids occurs in the C-terminal region of the protein, which encompasses 6 tandemly repeated sequences instead of the 16 normally present in the sequence of BSDL. Because feto-acinar pancreatic protein (FAPP), which is the oncofetal counterpart of BSDL, is a C-terminally truncated isoform of BSDL, it is suggested that the 70-kDa BSDL-related protein expressed in MiaPaCa-2 cells could be representative of the protein moiety of FAPP.

Structural characterization of the N-linked oligosaccharides in bile salt-stimulated lipase originated from human breast milk

The detailed structures of N- glycans derived from bile salt-stimulated lipase (BSSL) found in human milk were determined by combining exoglycosidase digestion with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The N- glycan structures were conclusively determined in terms of complexity and degree of fucosylation. Ion-exchange chromatography with pulsed amperometric detection, together with mass-spectral analysis of the esterified N- glycans, indicated the presence of monosialylated structures. The molecular mass profile of esterified N- glycans present in BSSL further permitted the more detailed studies through collision-induced dissociation (CID) and sequential exoglycosidase cleavages. The N- glycan structures were elucidated to be complex/dibranched, fucosylated/complex/dibranched, monosialylated/complex/dibranched, and monosialylated/fucosylated/dibranched entities.

Structure of bovine pancreatic cholesterol esterase at 1.6 A: novel structural features involved in lipase activation

The structure of pancreatic cholesterol esterase, an enzyme that hydrolyzes a wide variety of dietary lipids, mediates the absorption of cholesterol esters, and is dependent on bile salts for optimal activity, is determined to 1.6 A resolution. A full-length construct, mutated to eliminate two N-linked glycosylation sites (N187Q/N361Q), was expressed in HEK 293 cells. Enzymatic activity assays show that the purified, recombinant, mutant enzyme has activity identical to that of the native, glycosylated enzyme purified from bovine pancreas. The mutant enzyme is monomeric and exhibits improved homogeneity which aided in the growth of well-diffracting crystals. Crystals of the mutant enzyme grew in space group C2, with the following cell dimensions: a = 100.42 A, b = 54.25 A, c = 106.34 A, and beta = 104.12 degrees, with a monomer in the asymmetric unit. The high-resolution crystal structure of bovine pancreatic cholesterol esterase (Rcryst = 21.1%; Rfree = 25.0% to 1.6 A resolution) shows an alpha-beta hydrolase fold with an unusual active site environment around the catalytic triad. The hydrophobic C terminus of the protein is lodged in the active site, diverting the oxyanion hole away from the productive binding site and the catalytic Ser194. The amphipathic, helical lid found in other triglyceride lipases is truncated in the structure of cholesterol esterase and therefore is not a salient feature of activation of this lipase. These two structural features, along with the bile salt-dependent activity of the enzyme, implicate a new mode of lipase activation.

Glycosylation of bile-salt-stimulated lipase from human milk: comparison of native and recombinant forms

Bile-salt-stimulated lipase (BSSL) is an enzyme present in human milk. BSSL is important for fat digestion in infants. It contains one site for N-glycosylation and a serine/threonine-rich domain which is highly O-glycosylated. Both N- and O-linked sugar chains were studied on native BSSL from three donors and compared to the glycosylation of recombinant BSSL produced in Chinese hamster ovary or mouse fibroblast (C-127) cell lines. The carbohydrate composition of oligosaccharides was mapped using sugar and methylation analyses, enzyme-linked immunosorbant assay, and different separation techniques. Native BSSL was found to be highly glycosylated (19-26%). It contained a high amount of fucosylated oligosaccharides and expressed both Lewis a and Lewis b blood group antigens. None of the recombinant BSSL forms contained fucose. N-linked structures on native BSSL were identified as mainly mono- and disialylated biantennary complex type structures with or without fucose substitution. High-pH anion-exchange chromatography analysis indicated that the recombinant forms of BSSL contained similar types of N-glycan structures differing mainly in their content of sialic acid and by the absence of fucose residues. Native BSSL contained predominantly large O-linked oligosaccharides. This was in contrast to the recombinant forms of BSSL which contained mainly short type O-glycans with a high content of sialic acid. Interestingly, the estimated number of O-glycans attached to native BSSL was lower than that for the recombinant forms.

Investigation of two glycosylated forms of bile-salt-dependent lipase in human pancreatic juice

Pure human pancreatic bile-salt-dependent lipase, devoid of its oncofetal glycoform [Mas, E., Abouakil, N., Roudani, S., Miralles, F., Guy-Crotte., O., Figarella, C., Escribano, M. J. & Lombardo, D. (1993) Biochem. J. 289, 609-615], was analyzed on immobilized concanavalin A (ConA). Two variants were separated: an unabsorbed ConA-unreactive fraction; and an absorbed ConA-reactive fraction. Carbohydrate compositions of ConA-reactive and ConA-unreactive fractions were not significantly different, and analysis of 3H-labelled oligosaccharides liberated from these fractions on the ConA-Sepharose column indicated that the fractionation of the bile-salt-dependent lipase on this column depends upon oligosaccharide structures. The activity of the ConA-reactive fraction was however much lower, independent of the substrate (4-nitrophenyl hexanoate or cholesteryl esters), than that of the ConA-unreactive fraction. Therefore, catalytic constants for the hydrolysis of 4-nitrophenyl hexanoate were determined; both fractions had quite similar Km, while the kcat for the ConA-unreactive fraction was 3-4-fold higher than that of the ConA-reactive fraction. ConA-reactive and ConA-unreactive fractions were shown to have slightly different molecular masses and different amino acid compositions. Cleavage patterns after cyanogen bromide treatment of the ConA-reactive and ConA-unreactive fractions suggested that the ConA-reactive (high Mr form) and ConA-unreactive (low Mr form) forms could be different isoforms of the bile-salt-dependent lipase secreted by the human pancreas.

Association of bile-salt-dependent lipase with membranes of human pancreatic microsomes

Immunolocalization studies indicated that, in contrast to other enzyme markers of human pancreatic secretion, bile-salt-dependent lipase (BSDL) was partly but specifically associated with endoplasmic reticulum membranes. In microsomes, temperature-induced phase separation using Triton X-114 elucidated the partition of BSDL between the aqueous phase and the detergent-rich phase containing hydrophilic and membrane proteins, respectively. The size of the membrane-associated BSDL (approx. 100 kDa) is compatible with that of the fully processed enzyme. Fucosylated O- and N-linked oligosaccharide structures were detected by means of specific lectins. The membrane-associated BSDL might therefore be released from membranes between the trans-Golgi compartment (where terminal fucose residues were added) and the zymogen granules where BSDL was mainly found in the soluble fraction. Even though BSDL associated with membranes was enzymically active, it appeared less efficient than the soluble form. The association of BSDL with membranes was pH-dependent and optimal association occurred between pH 5-6. The membrane-associated BSDL was released by KBr which suggests that the association of BSDL with microsomal membranes involves ionic interactions. Lipid-protein interactions are probably not involved in this association as BSDL did not associate with liver microsome membranes. We attempted to characterize the putative ligand and showed that BSDL and a 94-kDa protein, immunologically related to a glucose-regulated protein of 94 kDa (Grp94), were co-immunoprecipitated by specific antibodies directed against each individual species. It is suggested that the biogenesis of the human pancreatic BSDL involves an association with intracellular membranes and that its folding may be assisted by molecular chaperones.

Chaperone function of a Grp 94-related protein for folding and transport of the pancreatic bile salt-dependent lipase

In its fundamental attributes, the secretion pathway of the pancreatic bile salt-dependent lipase (BSDL) followed that described for all enzymes involved in regulated secretion. This route was inhibited by drugs that affect protein synthesis and intracellular transport. In the presence of monensin, BSDL was solely detected in microsome membrane fractions. The association of BSDL with intracellular membranes involved a protein complex, formed by at least two proteins of 94 and 56 kDa. In cells experiencing the metabolic stress due to azetidine-2-carboxylic acid, BSDL was additionally associated with a protein of 46 kDa. Affinity blotting showed that BSDL bound directly to the 94-kDa protein (p94). It was suggested that p94 could be a molecular chaperone, further identified as related to the 94-kDa glucose regulated protein (Grp 94). The membrane-associated BSDL (i.e. BSDL bound to the Grp 94-related p94) was O- and N-glycosylated and consequently appeared released from membranes in the trans-Golgi compartment. Therefore and for the first time, it is suggested that a multiprotein complex including the chaperone Grp 94-related p94 protein may play an essential role in the folding and transport of BSDL. One hypothesis is that the association of BSDL with membrane via the Grp 94-related p94 along its secretion pathway is required for its complete O-glycosylation, which occurs on the extended mucin-like structures present on the C-terminal part of the protein.

The glycan moiety of human pancreatic lithostathine. Structure characterization and possible pathophysiological implications

Lithostathine, also known as pancreatic stone protein, pancreatic thread protein or regenerating protein, is a glycoprotein which is normally found in the exocrine pancreas, whereas in other tissues it appears either only under pathological conditions, such as Alzheimer's disease (brain), cancer (colon) or during regeneration (endocrine pancreas). In the latter case, it has been shown recently that it acts as a growth factor which stimulates islet regeneration. Little is known about its glycan moiety, which conceivably might be involved in this tissue specificity and pathophysiological characteristics. Therefore we isolated the major oligosaccharide chains of human pancreatic lithostathine and determined their sequences by means of NMR analysis. We obtained eleven different glycoforms and we were able to determine the sequence of seven of them. They all were from the same site of glycosylation (Thr5) and displayed the same core 2 structure: GlcNAc(beta 1-6)[Gal(beta 1-3)]GalNAc alpha-. They ranged in size from 4 to 9 sugar residues. Elongation was found to proceed from a common tetrasaccharidic core: Gal(beta 1-4)GlcNAc(beta 1-6)[Gal(beta 1-3)]GalNAc-ol through N-acetyllactosamine units. The non-reducing ends of some oligosaccharides carry the antigenic determinant H, with presence of external Fuc linked only in (alpha 1-2) to Gal. All the glycans, except one, carry a sialic acid in (alpha 2-3) linkage to Gal, with one disialylated form which displays a supplementary (alpha 2-6) linkage. These findings are consistent with the polymorphism of the protein, shown by means of SDS gel electrophoresis and isoelectric focusing, either in its native form or after enzymic processing. Moreover, sialylation seems to protect to some extent the Arg11-Ile12 bond from in situ hydrolysis, thus preventing the harmful precipitation of the C-terminal polypeptide in the pancreatic ducts.

Human milk bile salt-stimulated lipase: functional and molecular aspects

In breast-fed infants, digestion of milk triglycerides, the major source of energy and long-chain polyunsaturated fatty acids, is catalyzed by a concerted action of gastric lipase, colipase-dependent pancreatic lipase, and bile salt-stimulated lipase (BSSL). The major part of BSSL is present in the milk and the lesser part originates in the infant's exocrine pancreas. Gastric lipase is important in initiating digestion of milk fat globule triglycerides in the stomach. BSSL shifts the final products of triglyceride digestion from monoglyceride and free fatty acid (the products of colipase-dependent pancreatic lipase) to glycerol and free fatty acid, which may promote efficient absorption. Moreover, BSSL is likely to promote efficient use of milk cholesteryl- and fat-soluble vitaminesters and long-chain polyunsaturated fatty acids (> C18). The cDNA sequence has shown that BSSL has a unique primary structure. The N-terminal half is highly conserved between species and shows striking homology to typical esterases, for example, acetylcholine esterase. In contrast, the C-terminal half, containing 16 proline-rich repeats of 11 amino acid residues, is unique to BSSL. Using several recombinant variants of BSSL, we have found that these unique repeats and the glycosylation are completely dispensable for activity. Thus all typical properties of BSSL reside in the N-terminal half of the molecule.

Variation of the glycosylation of human pancreatic bile-salt-dependent lipase

Glycoproteins of human pancreatic juice were characterized by means of lectins after electrophoresis and electrotransfer to nitrocellulose membranes. For the detected glycoproteins, only a 100-kDa glycoprotein varied in the pancreatic juice from a normal patient (i.e. without any pancreatic disorder) compared to the pancreatic juice from a patient suffering from chronic pancreatitis. This protein, which is the only protein in human pancreatic juice which is O-glycosylated and N-glycosylated, was identified as the bile-salt-dependent lipase. Among the glycosylated proteins present in human pancreatic juice, only the glycosylation of bile-salt-dependent lipase differs between individuals. The enzyme was isolated either from normal or pathological human pancreatic juices. The purified variants have an identical molecular mass and amino-acid composition. As suspected from lectin affinity studies, the oligosaccharide composition differs between the variants. The structure of the N-linked oligosaccharides of the variant from the pancreatic juice of a normal donor correlated with complete processing and maturation of a complex-type N-glycan. Alteration of the maturation process can be detected for a bile-salt-dependent-lipase variant from a patient suffering with chronic pancreatitis, since the carbohydrate composition is compatible with the predominance of hybrid or high-mannose-type structures. The amount of sugar involved in O-glycosylation associated with the peanut agglutinin reactivity suggests the presence of 12-14 minimal Gal beta 1-->3GalNac-->T/S O-glycan structures which are sialylated and fucosylated. The amount of sugar involved in the O-linked oligosaccharide structure appears to be unchanged in the variants isolated from the pathological pancreatic juice.