Bile salt-stimulated lipase in human milk and carboxyl ester hydrolase in pancreatic juice: are they identical enzymes?

Common single-base insertions in the VNTR of the carboxyl ester lipase (CEL) gene are benign and also likely to arise somatically in the exocrine pancreas

The CEL gene encodes carboxyl ester lipase, a pancreatic digestive enzyme. CEL is extremely polymorphic due to a variable number tandem repeat (VNTR) located in the last exon. Single-base deletions within this VNTR cause the inherited disorder MODY8, whereas little is known about VNTR single-base insertions in pancreatic disease. We therefore mapped CEL insertion variants (CEL-INS) in 200 Norwegian patients with pancreatic neoplastic disorders. Twenty-eight samples (14.0%) carried CEL-INS alleles. Most common were insertions in repeat 9 (9.5%), which always associated with a VNTR length of 13 repeats. The combined INS allele frequency (0.078) was similar to that observed in a control material of 416 subjects (0.075). We performed functional testing in HEK293T cells of a set of CEL-INS variants, in which the insertion site varied from the first to the 12th VNTR repeat. Lipase activity showed little difference among the variants. However, CEL-INS variants with insertions occurring in the most proximal repeats led to protein aggregation and endoplasmic reticulum stress, which upregulated the unfolded protein response. Moreover, by using a CEL-INS-specific antibody, we observed patchy signals in pancreatic tissue from humans without any CEL-INS variant in the germline. Similar pancreatic staining was seen in knock-in mice expressing the most common human CEL VNTR with 16 repeats. CEL-INS proteins may therefore be constantly produced from somatic events in the normal pancreatic parenchyma. This observation along with the high population frequency of CEL-INS alleles strongly suggests that these variants are benign, with a possible exception for insertions in VNTR repeats 1-4.

Effects of bile salt-stimulated lipase on blood cells and associations with disease activity in human inflammatory joint disorders

The bile salt-stimulated lipase (BSSL) was originally recognized as a lipolytic enzyme expressed by the exocrine pancreas and in some species, notably humans, the lactating mammary gland, being secreted into the duodenum and with the mother's milk, respectively. However, BSSL is also present in the blood and has been assigned additional functions, even beyond the gastrointestinal tract. Conventional BSSL knockout mice are protected from developing disease in animal models of arthritis, and antibodies directed towards BSSL prevent or mitigate disease in similar models. The aim of this study was to investigate the role of BSSL as a newly discovered player in inflammation and specifically in inflammatory joint disorders. As part of mechanism of action, we here show that BSSL is secreted by neutrophils, interacts with monocytes and stimulates their migration in vitro. An anti-BSSL antibody that blocks the human BSSL-monocyte interaction was shown to simultaneously prevent the signaling pathway by which BSSL induce cell migration. Moreover, in this cohort study we show that BSSL levels are significantly higher in blood samples from patients with rheumatoid arthritis and psoriatic arthritis compared to healthy controls. The BSSL levels in patients' blood also correlated with disease activity scores and established inflammatory markers. Hence, although the mode of action is not yet fully clarified, we conclude that BSSL could be considered a proinflammatory component in the innate immune system and thus a possible novel target for treatment of chronic inflammation.

Identification and characterization of novel carboxyl ester lipase gene variants in patients with different subtypes of diabetes

INTRODUCTION

Mutations of CEL gene were first reported to cause a new type of maturity-onset diabetes of the young (MODY) denoted as MODY8 and then were also found in patients with type 1 (T1D) and type 2 diabetes (T2D). However, its genotype-phenotype relationship has not been fully determined and how carboxyl ester lipase (CEL) variants result in diabetes remains unclear. The aim of our study was to identify pathogenic variants of CEL in patients with diabetes and confirm their pathogenicity.

RESEARCH DESIGN AND METHODS

All five patients enrolled in our study were admitted to Shandong Provincial Hospital and diagnosed with diabetes in the past year. Whole-exome sequencing was performed to identify pathogenic variants in three patients with MODY-like diabetes, one newborn baby with T1D and one patient with atypical T2D, as well as their immediate family members. Then the consequences of the identified variants were predicted by bioinformatic analysis. Furthermore, pathogenic effects of two novel CEL variants were evaluated in HEK293 cells transfected with wild-type and mutant plasmids. Finally, we summarized all CEL gene variants recorded in Human Gene Mutation Database and analyzed the mutation distribution of CEL.

RESULTS

Five novel heterozygous variants were identified in CEL gene and they were predicted to be pathogenic by bioinformatic analysis. Moreover, in vitro studies indicated that the expression of CEL^R540C was remarkably increased, while p.G729_T739del variant did not significantly affect the expression of CEL. Both novel variants obviously abrogated the secretion of CEL. Furthermore, we summarized all reported CEL variants and found that 74.3% of missense mutations were located in exons 1, 3, 4, 10 and 11 and most missense variants clustered near catalytic triad, Arg-83 and Arg-443.

CONCLUSION

Our study identified five novel CEL variants in patients with different subtypes of diabetes, expanding the gene mutation spectrum of CEL and confirmed the pathogenicity of several novel variants.

Two New Mutations in the CEL Gene Causing Diabetes and Hereditary Pancreatitis: How to Correctly Identify MODY8 Cases

CONTEXT

Maturity onset diabetes of the young, type 8 (MODY8) is associated with mutations in the CEL gene, which encodes the digestive enzyme carboxyl ester lipase. Several diabetes cases and families have in recent years been attributed to mutations in CEL without any functional or clinical evidence provided.

OBJECTIVE

To facilitate correct MODY8 diagnostics, we screened 2 cohorts of diabetes patients and delineated the phenotype.

METHODS

Young, lean Swedish and Finnish patients with a diagnosis of type 2 diabetes (352 cases, 406 controls) were screened for mutations in the CEL gene. We also screened 58 Czech MODY cases who had tested negative for common MODY genes. For CEL mutation-positive subjects, family history was recorded, and clinical investigations and pancreatic imaging performed.

RESULTS

Two cases (1 Swedish and 1 Czech) with germline mutation in CEL were identified. Clinical and radiological investigations of these 2 probands and their families revealed dominantly inherited insulin-dependent diabetes, pancreatic exocrine dysfunction, and atrophic pancreas with lipomatosis and cysts. Notably, hereditary pancreatitis was the predominant phenotype in 1 pedigree. Both families carried single-base pair deletions in the proximal part of the CEL variable number of tandem repeat (VNTR) region in exon 11. The mutations are predicted to lead to aberrant protein tails that make the CEL protein susceptible to aggregation.

CONCLUSION

The diagnosis of MODY8 requires a pancreatic exocrine phenotype and a deletion in the CEL VNTR in addition to dominantly inherited diabetes. CEL screening may be warranted also in families with hereditary pancreatitis of unknown genetic etiology.

Bile salt-dependent lipase promotes the barrier integrity of Caco-2 cells by activating Wnt/β-catenin signaling via LRP6 receptor

Bile salt-dependent lipase (BSDL) within intestinal lumen can be endocytosed by enterocytes and support the intestinal barrier function. However, the epithelial-supporting effect of this protein has not been verified in a human cell line and neither the direct signaling pathway nor the function of endocytosis in this process has been clearly identified. We sought to investigate the signaling pathway and the membrane receptor through which BSDL might exert these effects using intestinal epithelial cells. Caco-2 cells were treated with recombinant BSDL, and the barrier function, cell proliferation, and activation of the Wnt signaling pathway were assessed. The effect of Wnt signaling activation induced by BSDL and BSDL endocytosis was investigated in LRP6-silenced and non-silenced cells. Moreover, caveolae- and clathrin-dependent endocytosis inhibitors were also applied respectively to analyze their effects on Wnt signaling activation induced by BSDL. BSDL treatment increased the barrier function but not proliferation of Caco-2 cells. It also induced β-catenin nuclear translocation and activated Wnt target gene transcription. Moreover, in the Wnt pathway, BSDL increased the levels of non-phosphorylated-β-catenin (Ser33/37/Thr41) and phosphorylated-β-catenin (Ser552). Notably, the silencing of LRP6 expression impaired BSDL endocytosis and decreased BSDL-induced β-catenin nuclear translocation. The inhibition of BSDL endocytosis induced by caveolae-mediated endocytosis inhibitor was stronger than that by clathrin-mediated endocytosis inhibitor, and the Wnt signaling activation associated with its endocytosis was also most likely caveolae-dependent. Our findings suggested that LRP6, a canonical Wnt pathway co-receptor, can mediate BSDL endocytosis and then activate Wnt signaling in Caco-2 cells.

Digestion of human milk fat in healthy infants

Lipid digestion is critical for infant development, and yet, the interconnection between lipid digestion and the microbiota is largely understudied. This review focuses on digestion of the human milk fat globule and summarizes the current understanding of the mechanisms underlying this process in infants. We first discuss the partial hydrolysis of milk fat in the stomach, which leads to rearrangement of lipid droplets, creating a lipid-water interface necessary for duodenal lipolysis. In the first few months of life, secretion of pancreatic triglyceride lipase, phospholipase A₂, and bile salts is immature. The dominant lipases aiding fat digestion in the newborn small intestine are therefore pancreatic lipase-related protein 2 and bile salt-stimulated lipase from both the exocrine pancreas and milk. We summarize the interaction between ionic fatty acids and cations to form insoluble fatty acid soaps and how it is influenced by various factors, including cation availability, pH, and bile salt concentration, as well as saturation and chain length of fatty acids. We further argue that the formation of the soap complex does not contribute to lipid bioavailability. Next, the possible roles that the gut microbiota plays in lipid digestion and absorption are discussed. Finally, we provide a perspective on how the manufacturing process of infant formula and dairy products may alter the physical properties and structure of lipid droplets, thereby altering the rate of lipolysis.

Molecular Genetic Study in a Cohort of Iranian Families Suspected to Maturity-Onset Diabetes of the Young, Reveals a Recurrent Mutation and a High-Risk Variant in the CEL Gene

Background

Diabetes mellitus (DM) is a group of metabolic disorders in the body, accompanied with increasing blood sugar levels. Diabetes is classified into three groups: Type 1 DM (T1DM), Type 2 DM (T2DM), and monogenic diabetes. Maturity-onset diabetes of the young (MODY) is a monogenic diabetes that is frequently mistaken for T1D or T2D. The aim of this study was to diagnose MODY and its subtype frequency in a diabetic population in Iran.

Materials and Methods

In this study among ten diabetic families that were highly suspected to MODY by nongenetic biomarkers and without any pathogenic mutation in GCK and HNF1A genes, two patients from two unrelated families were examined via whole-exome sequencing (WES) in order to detect the causative gene of diabetes. Co-segregation analysis of the identified variant was performed using Sanger sequencing.

Results

In this study, no pathogenic variant was found in GCK and HNF1A genes (MODY2 and MODY3), while these two types of MODY were introduced as the most frequent in other studies. By using WES, a pathogenic variant (p.I488T) was found in one of the patients in CEL gene causing MODY8 that its frequency is very rare in other studied populations. A high-risk variant associated with diabetes was found in another patient.

Conclusion

WES was applied in this study to reveal the cause of MODY in 1 family. This pathogenic mutation was previously reported as a disease causing mutation.

Association of a new 99-bp indel of the CEL gene promoter region with phenotypic traits in chickens

Carboxyl ester lipase (CEL) encodes a cholesterol ester hydrolase that is secreted into the duodenum as a component of pancreatic juice. The objective of this study was to characterize the CEL gene, investigate the association between the CEL promoter variants and chicken phenotypic traits, and explore the CEL gene regulatory mechanism. An insertion/deletion (indel) caused by a 99-bp insertion fragment was shown for the first time in the chicken CEL promoter, and large differences in allelic frequency were found among commercial breeds, indigenous and feral birds. Association analysis demonstrated that this indel site had significant effects on shank length, shank girth, chest breadth at 8 weeks (p < 0.01), evisceration weight, sebum weight, breast muscle weight, and leg weight (p < 0.05). Tissue expression profiles showed extremely high levels of the CEL gene in pancreatic tissue. Moreover, the expression levels of the genes APOB, MTTP, APOV1 and SREBF1, which are involved in lipid transport, were significantly reduced by adding a 4% oxidized soybean oil diet treatment at the individual level and transfecting the embryonic primary hepatocytes with a CEL-overexpression vector. Interestingly, the results showed that the expression level of the II homozygous genotype was significantly higher than that of the ID and DD genotypes, while individuals with DD genotypes had higher phenotypic values. Therefore, these data suggested that the CEL gene might affect body growth by participating in hepatic lipoprotein metabolism and that the 99-bp indel polymorphism could be a potentially useful genetic marker for improving the economically important traits of chickens.

The role of the carboxyl ester lipase (CEL) gene in pancreatic disease

The enzyme carboxyl ester lipase (CEL), also known as bile salt-dependent or -stimulated lipase (BSDL, BSSL), hydrolyzes dietary fat, cholesteryl esters and fat-soluble vitamins in the duodenum. CEL is mainly expressed in pancreatic acinar cells and lactating mammary glands. The human CEL gene resides on chromosome 9q34.3 and contains a variable number of tandem repeats (VNTR) region that encodes a mucin-like protein tail. Although the number of normal repeats does not appear to significantly influence the risk for pancreatic disease, single-base pair deletions in the first VNTR repeat cause a syndrome of endocrine and exocrine dysfunction denoted MODY8. Hallmarks are low fecal elastase levels and pancreatic lipomatosis manifesting before the age of twenty, followed by development of diabetes and pancreatic cysts later in life. The mutant protein forms intracellular and extracellular aggregates, suggesting that MODY8 is a protein misfolding disease. Recently, a recombined allele between CEL and its pseudogene CELP was discovered. This allele (CEL-HYB) encodes a chimeric protein with impaired secretion increasing five-fold the risk for chronic pancreatitis. The CEL gene has proven to be exceptionally polymorphic due to copy number variants of the CEL-CELP locus and alterations involving the VNTR. Genome-wide association studies or deep sequencing cannot easily pick up this wealth of genetic variation. CEL is therefore an attractive candidate gene for further exploration of links to pancreatic disease.

Purification and characterization of recombinant human bile salt-stimulated lipase expressed in milk of transgenic cloned cows

Bile salt-stimulated lipase (BSSL) is a lipolytic digestive enzyme with broad substrate specificity secreted from exocrine pancreas into the intestinal lumen in all species and from the lactating mammary gland into the milk of some species, notably humans but not cows. BSSL in breast milk facilitates digestion and absorption of milk fat and promotes growth of small for gestational age preterm infants. Thus, purified recombinant human BSSL (rhBSSL) can be used for treatment of patients with fat malabsorption and expressing rhBSSL in the milk of transgenic cloned cows would therefore be a mean to meet a medical need. In the present study, a vector pBAC-hLF-hBSSL was constructed, which efficiently expressed active rhBSSL in milk of transgenic cloned cows to a concentration of 9.8 mg/ml. The rhBSSL purified from cow milk had the same enzymatic activity, N-terminal amino acid sequence, amino acid composition and isoelectric point and similar physicochemical characteristics as human native BSSL. Our study supports the use of transgenic cattle for the cost-competitive, large-scale production of therapeutic rhBSSL.

Expression of truncated bile salt-dependent lipase variant in pancreatic pre-neoplastic lesions

Pancreatic adenocarcinoma (PDAC) is a dismal disease. The lack of specific symptoms still leads to a delay in diagnosis followed by death within months for most patients. Exon 11 of the bile salt-dependent lipase (BSDL) gene encoding variable number of tandem repeated (VNTR) sequences has been involved in pancreatic pathologies. We hypothesized that BSDL VNTR sequences may be mutated in PDAC. The amplification of BSDL VNTR from RNA extracted from pancreatic SOJ-6 cells allowed us to identify a BSDL amplicon in which a cytosine residue is inserted in a VNTR sequence. This insertion gives rise to a premature stop codon, resulting in a truncated protein and to a modification of the C-terminal amino-acid sequence; that is PRAAHG instead of PAVIRF. We produced antibodies directed against these sequences and examined pancreatic tissues from patients with PDAC and PanIN. Albeit all tissues were positive to anti-PAVIRF antibodies, 72.2% of patient tissues gave positive reaction with anti-PRAAHG antibodies, particularly in dysplastic areas of the tumor. Neoplastic cells with ductal differentiation were not reactive to anti-PRAAHG antibodies. Some 70% of PanIN tissues were also reactive to anti-PRAAHG antibodies, suggesting that the C insertion occurs early during pancreatic carcinogenesis. Data suggest that anti-PRAAHG antibodies were uniquely reactive with a short isoform of BSDL specifically expressed in pre-neoplastic lesions of the pancreas. The detection of truncated BSDL reactive to antibodies against the PRAAHG C-terminal sequence in pancreatic juice or in pancreatic biopsies may be a new tool in the early diagnosis of PDAC.

Diabetes and pancreatic exocrine dysfunction due to mutations in the carboxyl ester lipase gene-maturity onset diabetes of the young (CEL-MODY): a protein misfolding disease

CEL-maturity onset diabetes of the young (MODY), diabetes with pancreatic lipomatosis and exocrine dysfunction, is due to dominant frameshift mutations in the acinar cell carboxyl ester lipase gene (CEL). As Cel knock-out mice do not express the phenotype and the mutant protein has an altered and intrinsically disordered tandem repeat domain, we hypothesized that the disease mechanism might involve a negative effect of the mutant protein. In silico analysis showed that the pI of the tandem repeat was markedly increased from pH 3.3 in wild-type (WT) to 11.8 in mutant (MUT) human CEL. By stably overexpressing CEL-WT and CEL-MUT in HEK293 cells, we found similar glycosylation, ubiquitination, constitutive secretion, and quality control of the two proteins. The CEL-MUT protein demonstrated, however, a high propensity to form aggregates found intracellularly and extracellularly. Different physicochemical properties of the intrinsically disordered tandem repeat domains of WT and MUT proteins may contribute to different short and long range interactions with the globular core domain and other macromolecules, including cell membranes. Thus, we propose that CEL-MODY is a protein misfolding disease caused by a negative gain-of-function effect of the mutant proteins in pancreatic tissues.

Human bile salt-dependent lipase efficiency on medium-chain acyl-containing substrates: control by sodium taurocholate

Bile salt-dependent lipase was purified to homogeneity from lyophilized human milk and used to screen the influence of the acyl chain length (2-16 carbon atoms) on the kinetic constants k(cat) and K(m) of the hydrolysis of para-nitrophenyl (pnp) ester substrates in the presence or absence of sodium taurocholate (NaTC: 0.02-20 mM). The highest k(cat) value (∼3,500 s(-1)) was obtained with pnpC(8) as substrate, whereas the lowest K(m) (<10 µM) was that recorded with pnpC(10). In the absence of NaTC, the maximal catalytic efficiency (k(cat)/K(m)) was obtained with pnpC(8), while in the presence of NaTC k(cat)/K(m) was maximal with pnpC(8), pnpC(10) or pnpC(12). The bile salt activated the enzyme in two successive saturation phases occurring at a micromolar and a millimolar concentration range, respectively. The present data emphasize the suitability of this enzyme for the hydrolysis of medium-chain acyl-containing substrates and throw additional light on how BSDL is activated by NaTC.

Plasma carboxyl ester lipase activity modulates apolipoprotein B-containing lipoprotein metabolism in a transgenic mouse model

Pancreatic carboxyl ester lipase (CEL) is in the plasma of many mammals, including humans and rats, but not mice. In vitro, CEL hydrolyzes cholesterol esters of apolipoprotein B-containing lipoproteins (apo B-Lp). To study the effect of CEL on metabolism of apo B-Lp and atherosclerosis in vivo, apo E-knockout (EKO) mice, which have high plasma levels of apo B-Lp and are prone to atherosclerosis, were made to secrete CEL into plasma by introducing a transgene containing a liver-specific promoter and rat CEL complementary DNA. Plasma CEL activity in EKO-CEL mice was comparable with that found in rats. Evidence of modification of apo B-Lp by plasma CEL in vivo was an increase in the free cholesterol to cholesterol ester ratio of apo B-Lp from mice on chow or a Western-type diet. In addition, plasma total cholesterol levels were elevated in EKO-CEL mice, with the elevation found exclusively in the apo B-Lp fraction. Associated with the increase in steady-state apo B-Lp levels was an increase in the plasma half-life of very low-density lipoproteins (VLDL) in EKO-CEL mice, measured by the clearance rate of injected VLDL. Interestingly, despite the increase of apo B-Lp, the atherosclerotic lesion did not differ between EKO and EKO-CEL mice on a Western-type diet. In summary, our results demonstrate that plasma CEL modulates apo B-Lp metabolism in vivo, resulting in reduced VLDL clearance and elevated plasma cholesterol levels.

Isolation and determination of bile acids

In this article, the methods of isolation and determination of bile acids are reviewed. Methods for separation of bile acids from cattle and pig bile are given in detail. Isolation of a mixture of cholic acid and deoxycholic acids from cattle bile and their subsequent purification are described. The isolation and purification of hyodeoxycholic acid and other components of pig bile are also included. Methods for the determination of bile acids in various biological samples are reviewed, including enzyme assays, radioimmunoassay, enzyme immunoassay and chromatographic methods. Among chromatographic methods, separation and determination of bile acids by thin-layer chromatography, gas chromatography and high performance liquid chromatography are reviewed. Particular attention is given to the use of high performance liquid chromatography since this has recently been the most commonly applied method for the separation and determination of bile acids.

Carboxyl ester lipase: structure-function relationship and physiological role in lipoprotein metabolism and atherosclerosis

Carboxyl ester lipase (CEL), previously named cholesterol esterase or bile salt-stimulated (or dependent) lipase, is a lipolytic enzyme capable of hydrolyzing cholesteryl esters, tri-, di-, and mono-acylglycerols, phospholipids, lysophospholipids, and ceramide. The active site catalytic triad of serine-histidine-aspartate is centrally located within the enzyme structure and is partially covered by a surface loop. The carboxyl terminus of the protein regulates enzymatic activity by forming hydrogen bonds with the surface loop to partially shield the active site. Bile salt binding to the loop domain frees the active site for accessibility by water-insoluble substrates. CEL is synthesized primarily in the pancreas and lactating mammary gland, but the enzyme is also expressed in liver, macrophages, and in the vessel wall. In the gastrointestinal tract, CEL serves as a compensatory protein to other lipolytic enzymes for complete digestion and absorption of lipid nutrients. Importantly, CEL also participates in chylomicron assembly and secretion, in a mechanism mediated through its ceramide hydrolytic activity. Cell culture studies suggest a role for CEL in lipoprotein metabolism and oxidized LDL-induced atherosclerosis. Thus, this enzyme, which has a wide substrate reactivity and diffuse anatomic distribution, may have multiple functions in lipid and lipoprotein metabolism, and atherosclerosis.

Site-directed mutagenesis of the basic N-terminal cluster of pancreatic bile salt-dependent lipase. Functional significance

Previous studies have postulated the presence of a heparin-binding site on the bile salt-dependent lipase (BSDL), whereas two bile salt-binding sites regulate the enzyme activity. One of these sites may overlap with the tentative heparin-binding site at the level of an N-terminal basic cluster consisting of positive residues Lys(32), Lys(56), Lys(61), Lys(62), and Arg(63). The present study uses specific site-directed mutagenesis to determine the functional significance of this basic cluster. Mutations in this sequence resulted in recombinant enzymes that were able to bind to immobilized and to cell-associated heparin before moving throughout intestinal cells. Recombinant BSDL was fully active on soluble substrate, but mutants were less active on micellar cholesteryl oleate in comparison with the wild-type enzyme. Activation studies by primary (sodium taurocholate) and by secondary (sodium taurodeoxycholate) bile salts revealed that the activation of BSDL by sodium taurocholate at concentrations below the critical micellar concentration, and not that evoked by micellar bile salts, was affected by substitutions, suggesting that this N-terminal basic cluster likely represents the specific bile salt-binding site of BSDL. Substitutions also affected the activation of the enzyme promoted by anionic phospholipids, extending the function of this site to that of a cationic regulatory site susceptible to accommodate anionic ligands.

Bile salt-stimulated carboxyl ester lipase influences lipoprotein assembly and secretion in intestine: a process mediated via ceramide hydrolysis

Bile salt-stimulated carboxyl ester lipase (CEL), also called cholesterol esterase, is one of the major proteins secreted by the pancreas. The physiological role of CEL was originally thought to be its mediation of dietary cholesterol absorption. However, recent studies showed no difference between wild type and CEL knockout mice in the total amount of cholesterol absorbed in a single meal. The current study tests the hypothesis that CEL in the intestinal lumen may influence the type of lipoproteins produced. A lipid emulsion containing 4 mm phospholipid, 13.33 mm [(3)H]triolein, and 2.6 mm [(14)C]cholesterol in 19 mm taurocholate was infused into the duodenum of lymph fistula CEL(+/+) and CEL(-/-) mice at a rate of 0.3 ml/h. Results showed no difference between CEL(+/+) and CEL(-/-) mice in the rate of cholesterol and triglyceride transport from the intestinal lumen to the lymph. However, CEL(-/-) mice produced predominantly smaller lipoproteins, whereas the CEL(+/+) mice produced primarily large chylomicrons and very low density lipoprotein. The proximal intestine of CEL(-/-) mice was also found to possess significantly less ceramide hydrolytic activity than that present in CEL(+/+) mice. By using Caco2 cells grown on Transwell membranes as a model, sphingomyelinase treatment inhibited the secretion of larger chylomicron-like lipoproteins without affecting total cholesterol secretion. In contrast, the addition of CEL to the apical medium increased the amount of large lipoproteins produced and alleviated the inhibition induced by sphingomyelinase. Taken together, this study identified a novel and physiologically significant role for CEL, namely the promotion of large chylomicron production in the intestine. The mechanism appears to be mediated through CEL hydrolysis of ceramide generated during the lipid absorption process.

Transcytosis of pancreatic bile salt-dependent lipase through human Int407 intestinal cells

In previous studies, we have shown that the bile-salt-dependent-lipase (BSDL), secreted by pancreatic acinar cells and secreted into the duodenal lumen, can be transcytosed through intestinal cells up to the lamina propria. In this study, we used an in vitro system to provide insights into the apical to basolateral transport of BSDL, across the intestinal barrier. The Int407 human epithelial cell line, grown under conditions that optimize polarity, was used as a tight epithelium model. We attempted to delineate uptake mechanisms and the transcytotic pathway followed by this pancreatic enzyme within the intestinal Int407 cells, which do not produce BSDL. When added to the apical reservoir of Transwell-grown Int407 cells, BSDL was shown to first interact with the apical membrane. Further, BSDL forms clusters that are internalized via clathrin-coated pits. Following endocytosis, BSDL is directed to a nocodazole- and colchicin-sensitive multivesicular compartment. Interestingly, this protein transits through the Golgi apparatus, where it was found to colocalize with the KDEL retrieval-receptor. Finally, enzymatically active intact BSDL was released at the basolateral membrane level. This is the first demonstration for an apical-to-basolateral transcytotic pathway of a secreted pancreatic digestive enzyme through polarized intestinal cells.

The structure of truncated recombinant human bile salt-stimulated lipase reveals bile salt-independent conformational flexibility at the active-site loop and provides insights into heparin binding

Human bile salt-stimulated lipase (BSSL), which is secreted from the pancreas into the digestive tract and from the lactating mammary gland into human milk, is important for the effective absorption of dietary lipids. The dependence of BSSL on bile acids for activity with water-insoluble substrates differentiates it from other lipases. We have determined the crystal structure of a truncated variant of human BSSL (residues 1-5.8) and refined it at 2.60 A resolution, to an R-factor of 0.238 and R(free) of 0.275. This variant lacks the C-terminal alpha-helix and tandem C-terminal repeat region of native BSSL, but retains full catalytic activity. A short loop (residues 115-126) capable of occluding the active-site (the active site loop) is highly mobile and exists in two conformations, the most predominant of which leaves the active-site open for interactions with substrate. The bile salt analogue 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonic acid (CHAPS) was present in the crystallisation medium, but was not observed bound to the enzyme. However, the structure reveals a sulfonate group from the buffer piperizine ethane sulfonic acid (PIPES), making interactions with Arg63 and His115. His115 is part of the active-site loop, indicating that the loop could participate in the binding of a sulphate group from either the glycosaminoglycan heparin (known to bind BSSL) or a bile acid such as deoxycholate. Opening of the 115-126 active-site loop may be cooperatively linked to a sulphate anion binding at this site. The helix bundle domain of BSSL (residues 319-398) exhibits weak electron density and high temperature factors, indicating considerable structural mobility. This domain contains an unusual Asp:Glu pair buried in a hydrophobic pocket between helices alpha(H) and alpha(K) that may be functionally important. We have also solved the structure of full-length glycosylated human BSSL at 4.1 A resolution, using the refined coordinates of the truncated molecule as a search model. This structure reveals the position of the C-terminal helix, missing in the truncated variant, and also shows the active-site loop to be in a closed conformation.

Intestinal absorption of dietary cholesteryl ester is decreased but retinyl ester absorption is normal in carboxyl ester lipase knockout mice

Carboxyl ester lipase (CEL; EC 3.1.1.13) hydrolyzes cholesteryl esters and retinyl esters in vitro. In vivo, pancreatic CEL is thought to liberate cholesterol and retinol from their esters prior to absorption in the intestine. CEL is also a major lipase in the breast milk of many mammals, including humans and mice, and is thought to participate in the processing of triglycerides to provide energy for growth and development while the pancreas of the neonate matures. Other suggested roles for CEL include the direct facilitation of the intestinal absorption of free cholesterol and the modification of plasma lipoproteins. Mice with different CEL genotypes [wild type (WT), knockout (CELKO), heterozygote] were generated to study the functions of CEL in a physiological system. Mice grew and developed normally, independent of the CEL genotype of the pup or nursing mother. Consistent with this was the normal absorption of triglyceride in CELKO mice. The absorption of free cholesterol was also not significantly different between CELKO (87 +/- 26%, mean +/- SD) and WT littermates (76 +/- 10%). Compared to WT mice, however, CELKO mice absorbed only about 50% of the cholesterol provided as cholesteryl ester (CE). There was no evidence for the direct intestinal uptake of CE or for intestinal bacterial enzymes that hydrolyze it, suggesting that another enzyme besides CEL can hydrolyze dietary CE in mice. Surprisingly, CELKO and WT mice absorbed similar amounts of retinol provided as retinyl ester (RE). RE hydrolysis, however, was required for absorption, implying that CEL was not the responsible enzyme. The changes in plasma lipid and lipoprotein levels to diets with increasing lipid content were similar in mice of all three CEL genotypes. Overall, the data indicate that in the mouse, other enzymes besides CEL participate in the hydrolysis of dietary cholesteryl esters, retinyl esters, and triglycerides.

An intracellular role for pancreatic bile salt-dependent lipase: evidence for modification of lipid turnover in transfected CHO cells

Pancreatic bile salt-dependent lipase (BSDL) hydrolyzes cholesteryl esters, triglycerides and phospholipids. BSDL is also capable of transferring free fatty acid to cholesterol. BSDL has been detected in many cells including fetal and tumor cells, hepatocytes, macrophages and eosinophils and in tissues such as adrenal glands and testes. The enzyme may be secreted or located within subcellular compartments such as the endoplasmic reticulum or the cytosol. Although the role of the secreted enzyme is well documented, that of the intracellular form(s) is still hypothetical. In the present study, we addressed the effects of BSDL on cell lipid metabolism. For that purpose, the cDNA of rat BSDL was transfected into CHO K1 cells (CHO K1-BSDL clone) which were then loaded with [3H]oleic acid. The results demonstrate that the transfected BSDL is secreted; in spite of that, a large fraction of catalytically active BSDL is found in cell lysate. The lipid metabolism of transfected cells is affected and BSDL induces an enhanced incorporation of [3H]oleic acid in cholesteryl esters whereas fatty acid incorporation in phosphatidylcholine is decreased. These effects were particularly important in the cytosol of transfected cells where transfected BSDL preferentially locates. These data suggested that BSDL could be implicated in the cycle of the cellular homeostasis of cholesterol which is particularly affected in tumoral cells leading to cholesteryl ester storage within cytosolic lipid droplets.

Molecular cloning of the bile salt-dependent lipase of ferret lactating mammary gland: an overview of functional residues

Ferret lactating mammary gland bile salt-dependent lipase (BSDL, EC 3.1.1.-) has been cloned by RT-PCR. The open reading frame consists of 1869 nucleotides which encode 623 amino acids of the functional enzyme. When compared to other species, the greatest homology is observed between residues 1 and 484, with little or no homology at the C-terminal end where seven repeated segments of similar sequence are located. Ferret mammary gland BSDL retains residues involved in the active site and the tentative heparin binding site at similar positions in comparison to other milk or pancreatic BSDL. Other important items, such as binding peptide to chaperone molecular, phosphorylation site(s) or bile salt binding sites, were also tentatively located in well conserved regions of seven available BSDL sequences.

Carboxyl ester lipase (bile salt-stimulated lipase), colipase, lipase, and phospholipase A2 levels in pancreatic enzyme supplements

BACKGROUND

Pancreatic lipolytic activity originates from lipase (LIP) and its cofactor colipase (COL), carboxyl ester lipase (CEL), and phospholipase A2 (PLA2). Yet there are few data on the levels of individual lipolytic enzymes in pancreatic enzyme supplements (PES). This study determines activity and immunoreactive mass in some commonly used PES and thus contributes to the understanding of the poor relationship between 'lipase dose' and clinical improvements.

METHODS

Recommended doses of each PES were incubated at 37 degrees C for 2 h in a 1-mM Tris-maleate buffer, pH 7.0, containing 150 mM NaCl and 1 mM CaCl2. Aliquots for determinations of enzyme activities and for immunochemical mass were taken every half hour. For comparison a standard dose was defined as 10,000 declared lipase units.

RESULTS

No simple parallelism between LIP, COL, CEL, and/or PLA2 activities was seen. The LIP contents ranged from 135% to 301% of the standard dose. None of the PES were short of COL (227%-504%). The variation in CEL was twentyfold, and in PLA2 sevenfold. Less variations were seen in the mass composition. There was considerable variation in activity to mass ratios (particularly for CEL), declared lipase units per recommended dose (6000-160,000), and cost (0.36-3.52 SEK).

CONCLUSIONS

PES differ considerably in their content of lipolytic enzymes. CEL activities were relatively low and COL and PLA2 activities high compared with normal duodenal content. The manufacturing procedure can be improved to increase the lipolytic activity in PES in a broader meaning. It seems to be most important to increase the amount of CEL. From these in vitro data we advocate a more careful decision in the choice of PES for each patient, depending on the total clinical picture. Money can be saved without disadvantage to the patient.

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.

Recombinant human bile salt-stimulated lipase: an example of defective O-glycosylation of a protein produced in milk of transgenic mice

The expression of recombinant human bile salt-stimulated lipase (bssl) was targeted to the lactating mammary gland of transgenic mice. Expression of recombinant genes comprising bsslcDNA, or alternatively genomic bssl DNA, under control of regulatory elements derived from the murine whey acidic protein (wap) gene was achieved and evaluated. Constructs containing genomic bssl sequences mediated high levels (0.5-1 mg ml-1) of recombinant human BSSL in the milk. The recombinant BSSL produced was purified, biochemically characterized and compared to native BSSL and recombinant BSSL produced in mouse C127 and hamster CHO cells. Recombinant BSSL derived from transgenic mice showed a different migration and distribution after SDS-PAGE electrophoresis, lower apparent molecular mass on size-exclusion chromatography and no detectable interactions with a panel of lectins. These results indicate a significantly lower degree of O-glycosylation of recombinant BSSL in milk from transgenic mice than was found for the native enzyme or recombinant CHO- or C127 cell-produced BSSL. Despite these differences, mouse-milk-derived recombinant BSSL exhibited similar lipase activity, the same stability to low pH and similar sensitivity to elevated temperatures as the native enzyme. The observation that mouse-C127-cell-produced recombinant BSSL is heavily O-glycosylated makes species-related restrictions less attractive as an explanation for the reduced O-glycosylation.

Pancreatic lipase-related protein 2 but not classical pancreatic lipase hydrolyzes galactolipids

The pancreatic lipase family contains three subfamilies, the 'classical' lipases and the pancreatic lipase-related proteins 1 (PLRP1) and 2 (PLRP2). Galactolipids are present in membranes of leaves and vegetables and consist of digalactosyldiacylglycerol (DGalDG) monogalactosyldiacylglycerol (MGalDG) and sulfoquinovosyldiacylglycerol (SQDG). These lipids were incubated with PLRP2 from guinea-pig (GPLRP2) and rat (RPLRP2). In the presence of bile salts DGalDG was efficiently hydrolyzed by GPLRP2 and, although less efficiently, by RPLRP2 to digalactosylmonoacylglycerol (DGalMG), free fatty acids and water-soluble galactose-containing compounds. Also, MGalDG and SQDG were hydrolyzed by GPLRP2 and RPLRP2. These data suggest a possible role of PLRP2 in the digestion of dietary galactolipids.

Digestive lipases of the newborn ferret: compensatory role of milk bile salt-dependent lipase

The amount of mRNA hybridizing to bile salt-dependent lipase and to colipase-dependent lipase probes as well as their translation into active proteins were quantified in the adult and newborn pancreas and lactating mammary gland from the ferret, a species whose milk, similar to that of the human, has bile salt-dependent lipase. The concentration of colipase-dependent lipase mRNA correlated with the amount of activity found in the adult and newborn pancreas, whereas neither mRNA nor activity of this enzyme was detected in the kit pancreas or in the lactating mammary gland. These data indicate that colipase-dependent lipase is actually expressed in adult pancreas and might represent the main lipolytic system in the adult. mRNA hybridizing to the bile salt-dependent lipase probe used in this study were detected in adult and in newborn ferret pancreas as well as in lactating mammary gland. However, the bile salt-dependent lipase activity expressed in the newborn pancreas was very low when compared with the activity expressed either in the mammary gland or in the adult pancreas. These data argue for a compensatory role of milk bile salt-dependent lipase in lipid digestion in the newborn. The hydrolysis of dietary fat might be initiated by preduodenal lipase, the activity of which is only two times lower in the gastric mucosa of the newborn than in the adult ferret. The high concentration of mRNA hybridizing to the bile salt-dependent lipase probe associated with a very poor bile salt-dependent lipase activity and protein suggests either that these mRNA are very unstable or that they are poorly translated into an active pancreatic bile salt-dependent lipase.

Bile salt-dependent lipase transcripts in human fetal tissues

In human fetal pancreas, we identified two cDNA transcripts of the bile salt-dependent lipase (BSDL) using reverse transcription followed by polymerase chain reaction (RT-PCR). The sequence of four overlapping segments obtained by RT-PCR matched the sequence of the 2.2 kb cDNA cloned from human adult pancreas (Reue et al. (1991) J. Lipid Res. 32, 267-276). A second RT-PCR product of approx. 1.1 kb was evidenced, the sequence of which corresponds to that of the BSDL-pseudogene transcript (Nilsson et al. (1993) Genomics, 17, 416-422). The short transcript is present in all tissues examined whereas the former one (2.2 kb) is either poorly (in liver and kidney) or not at all expressed in adult tissues, excepted in the pancreas. On the other hand, the 2.2 kb transcript specific of the BSDL gene was detected in all fetal tissues examined as early as the 6th week of gestation. Results also suggested that the fetal pancreas contains more 2.2 kb transcript than its adult counterpart. Therewith, BSDL was immuno-precipitated from fetal liver. The role of BSDL-gene expression during the fetal life is discussed.

Purification and identification of two distinct isoforms of rabbit pancreatic cholesterol esterase

Cholesterol esterase (CEases; E.C. 3.1.13) has been purified to homogeneity from rabbit pancreas. The method of purification consists of homogenization of total pancreas, high speed centrifugation, anion exchange column chromatography on S-Sepharose, size exclusion on Sephacryl followed by affinity chromatography on heparin agarose. During the purification procedure, two distinct isoforms of CEases have been identified. Both forms are similar in their molecular weights, bile salt requirement and pH optima but differ in their sensitivity to heparin. Isoform-I is resistant and isoform-II is sensitive to heparin. In the normal pancreas of the adult rabbit, the amount of each of the enzymes appears to be in equimolar concentrations. Physiological significance of the existence of heparin sensitive and resistant forms by the same tissue is unclear. In view of the significant role played by heparin in the modulation of CEase activity and several other physiological functions, these two isoforms may have different mechanisms of action on the hydrolysis of carboxyl esters of cholesterol and vitamins.

Recombinant human-milk bile-salt-stimulated lipase. Functional properties are retained in the absence of glycosylation and the unique proline-rich repeats

Human milk bile-salt-stimulated lipase ensures efficient utilization of milk lipid in breast-fed infants. The N-terminal two-thirds of the peptide chain is highly conserved and shows striking similarities to typical esterases. In contrast, the remaining C-terminal part consists of a unique sequence of 16 proline-rich O-glycosylated repeats of 11 residues each. Recently we could show, using recombinant lipase variants, that neither these repeats nor the single N-linked sugar chain are essential for catalytic efficiency. In the present study, we report on the lack of importance of glycosylation and the unique repeats for other important functional properties, i.e. bile-salt activation, heparin binding, heat stability, stability at low pH and resistance to proteolytic inactivation. Compared to native enzyme, recombinant full-length lipase produced in two mammalian cell lines differed slightly in glycosylation pattern with no effects on the functional properties. Moreover, a variant lacking all repeats and the C-terminal tail following the last repeat exhibited the same functional characteristics as purified native milk enzyme. Thus, the structural basis for all the typical and functionally important properties reside in the N-terminal conserved part, in spite of the fact that none of these properties are shared by typical esterases. We could however, demonstrate that the C-terminal repeats are responsible for the unusual behaviour of the enzyme in size-exclusion chromatography, resulting in a considerably higher than expected apparent molecular mass.

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.

Bile salt-stimulated lipase in human milk. Evidence that bile salt induces lipid binding and activation via binding to different sites

Human milk bile salt-stimulated lipase ensures efficient triacylglycerol utilization in breast-fed newborns. For activity against long-chain triacylglycerol, primary bile salts are a prerequisite. Bile salts also protect the enzyme from inactivation by intestinal proteases. We have studied the effect of different bile salts on activation, protease protection, lipid binding, and enzyme inactivation, caused by an arginine modifying agent. Based on the results we propose a model involving two bile salt binding sites; one activation-site specific for primary bile salt, and another, less specific, lipid binding promoting site at which also secondary bile salt binds. Binding to this latter site induces binding of enzyme to emulsified substrates but binding promoting site at which also secondary bile salt binds. Binding to this latter site induces binding of enzyme to emulsified substrates but without subsequent lipolysis.

Is there a specific lysophospholipase in human pancreatic juice?

The existence of a specific lysophospholipase in human pancreatic juice was evaluated. The proteins were separated by a series of chromatographic steps including Sephacryl S-200, cholate-Sepharose 4B, Sephadex G-100 and CM-Sephadex G-50. The enzyme activities against 1-palmitoyl lysolecithin (LL) as well as tributyrin (TB) and p-nitrophenyl butyrate (PNPB) were determined in all the fractions of these purification procedures. Enzyme activity against LL was always eluted in parallel with activities against TB and PNPB, and no unique activity against LL could be found. The specific activity against LL was 40-times lower than that against PNPB and 200-times lower than that against TB. It is concluded that there is no unique lysophospholipase in human pancreatic juice and that the hydrolysis of lysolecithin is most likely performed by carboxyl ester lipase.

Bile salt stimulated lipase: comparative studies in ferret milk and lactating mammary gland

Bile salt stimulated lipase (BSSL) activity is 10-20 times higher in ferret milk than in human milk. We have used the ferret to study BSSL activity in lactating mammary gland and in mammary cells isolated by hyaluronidase-collagenase treatment followed by Ficoll gradient centrifugation. Furthermore, we have compared the characteristics of BSSL in the tissue preparations (homogenate or cells) to BSSL of ferret milk and to BSSL purified from ferret and human milk. The characteristics of BSSL in ferret mammary gland preparations and milk were similar to those of human milk BSSL--absolute requirement of primary bile salts, pH optimum of 7.5-9.0, stability at pH 3-9 and inhibition by eserine (physostigmine) and by serum. Purified ferret milk BSSL had a lower molecular weight (90kD) than did human milk BSSL (125 kD). There was an 86% homology of the N-terminal amino acid sequence between BSSL of ferret and of human milk. The marked similarity in characteristics between BSSL in ferret and human milk and the high activity of BSSL in ferret milk (520 U/mL colostrum and 250 U/mL mature milk) indicate that this species is an ideal animal model for the study of the synthesis and secretion of this digestive lipase which constitutes a significant portion (1-2%) of total milk protein.

Genomic organization, sequence analysis, and chromosomal localization of the human carboxyl ester lipase (CEL) gene and a CEL-like (CELL) gene

The gene encoding human carboxyl ester lipase (CEL), including 1628 bp of the 5'-flanking region, has been isolated and characterized from two overlapping lambda phage clones. The gene spans 9832 bp and contains 11 exons interrupted by 10 introns. The exons range in size from 88 to 204 bp, except for the last exon, which is 841 bp. A major and a minor transcription initiation site were determined 13 and 7 bp, respectively, upstream of the initiator methionine. The nucleotide sequence is identical with that of the previously reported cDNA, except for the third nucleotide in the 5'-untranslated sequence, a C, which in the cDNA is a T. A TAAATA sequence is present 26 nt upstream from the major CAP site, and within the 5'-flanking region there are several putative transcription factor binding sites. Seven Alu repetitive sequence elements are present in the region analyzed. The organization of the human CEL gene is similar to that of the recently reported rat pancreatic cholesterol esterase gene. The CEL gene was assigned to chromosome 9q34-qter, which confirms the recently reported results of Tayler et al. (1991, Genomics 10: 425-431). A previously unknown gene with a striking homology to the human CEL gene, here called the CEL-like gene (CELL), has also been isolated and characterized, including 1724 bp of the 5'-flanking region. The CELL gene, which most likely is a psuedogene, spans 4846 bp, and due to the absence of a 4.8-kb segment, the CEL gene exons 2-7 are not present in the CELL gene. Despite these differences, the CELL gene is transcribed. We have also assigned the CELL gene to a separate locus at chromosome 9q34-qter.

Effect of pancreatic phospholipase A2 and gastric lipase on the action of pancreatic carboxyl ester lipase against lipid substrates in vitro

Preincubation of a triolein/phospholipid/cholesteryl oleate-emulsion in vitro with either pancreatic phospholipase A2 (PLA2) or gastric lipase (GL) resulted in hydrolysis (measured by pH-stat-titration) of cholesteryl [3H]oleate only after human pancreatic carboxyl ester lipase (CEL) was added to the system. No appreciable hydrolysis was observed when CEL was added alone. Consequently, a concerted action either of PLA2 and CEL or of GL and CEL made the substrate cholesteryl oleate available for hydrolysis by CEL. This was the case when cholesteryl oleate was solubilised in a phospholipid-stabilised triglyceride emulsion, which is the physico-chemical form in which the major part of dietary cholesteryl esters are presented to the gastro-intestinal tract of man.

N-Butyl-N-methyl-4-nitrophenyl carbamate as a specific active site titrator of bile-salt-dependent lipases

The effect of a series of synthetic carbamates on the human (milk or pancreatic) bile-salt-dependent lipase (cholesterol esterase) was examined. N-isopropyl-O-phenyl, N-methyl-O-phenyl, N-butyl-(4-nitrophenyl), N-phenyl-(4-nitrophenyl), N-butyl-N-methyl and N-pentyl-O-phenyl carbamates were inhibitors of the enzyme activity, while O-isopropyl-N-phenyl, O-methyl-N-phenyl, O-benzyl-N-isopropyl and O-cyclohexyl-N-phenyl carbamates were not even recognized by the enzyme. The N-alkyl chain length is essential for the enzyme inhibition and N-butyl-(4-nitrophenyl) or N-pentyl-O-phenyl carbamates are more potent inhibitors than N-methyl-O-phenyl or N-isopropyl carbamates. The inhibition by reactive carbamates fits the criteria for mechanism-based inhibition: the inhibition is first-order with time, shows saturation kinetics with increasing carbamate concentration and leads to an inactive stoichiometric enzyme-inhibitor complex; the enzyme activity can be protected by a competitive inhibitor. Evidence is shown that the enzymatic nucleophilic attack of carbamates is directed at the carbonyl carbon atom and not the nitrogen atom. The inhibition of bile-salt-dependent lipase does not occur consecutive to the formation of a reactive isocyanate derivative of carbamate but via a tetrahedral intermediate involving essential residues implicated in the enzyme catalytic site. This intermediate evolves by liberation of alcohol (or phenol) and formation of an inactive carbamyl enzyme. Among the carbamates tested, N-butyl-N-methyl-(4-nitrophenyl) carbamate specifically inhibits the bile-salt-dependent lipase; the release of 4-nitrophenol from this carbamate is directly proportional to the enzyme inhibition and it may be defined as a specific active-site titrator for bile-salt-dependent lipases.

Change in bile salt dependent lipase in human breast milk during extended lactation

Two hundred eighty-one milk samples collected from Zaïrian nonprivileged, undernourished mothers, in series of nine groups from 1 month to 18 months after parturition, and 66 milk samples collected from French privileged mothers in series of four groups from 2 days to 16 months postpartum, were analyzed for their lactose, lipid and protein contents. In addition, the activity of bile salt-dependent lipase (esterase), which may play an important role in the newborn infant's lipids digestion, was measured. After the first month postpartum, independent of the nutritional state of the mother, sugar and protein concentrations were identical. Lipid content of French mothers' milk was lower in transitional milk, but appeared constant in mature milk with an average value of 29.1 +/- 5.8 mg/mL of milk. In Zaïrian mothers' milk, the lipid content of mature milk plateaued at around 50-55 mg/mL independent of the stage of lactation. Bile salt-dependent lipase showed constant esterase activity within the lactation stage in privileged mothers' milk, but decreased by almost 80-90% during the first four months of lactation in undernourished mothers. The data suggest that milk from nonprivileged mothers may lose some of its ability to hydrolyze milk lipid esters, which could also be of consequence to the infant's normal growth in view of its effect on the esters of the lipid-soluble vitamins A, E and D.

Human milk bile-salt stimulated lipase. Sequence similarity with rat lysophospholipase and homology with the active site region of cholinesterases

To determine the active site residue, human milk bile-salt stimulated lipase (BSSL) was labelled with [3H]diisopropyl fluorophosphate (DFP). Partial sequence analysis of cyanogen bromide fragments (a total of 146 residues from 6 peptides) revealed 84% sequence identity with a putative rat lysophospholipase. Sequence analysis of a [3H]DFP-labelled peptide indicated that the active site serine was contained in the sequence Gly-Glu-Ser-Ala-Gly. In addition to similarity with rat lysophospholipase, this sequence showed homology with regions of human butyrylcholinesterase and electric ray acetylcholinesterase (68% identity). It is concluded that these proteins are members of a new supergene family.

cDNA cloning of human-milk bile-salt-stimulated lipase and evidence for its identity to pancreatic carboxylic ester hydrolase

We have isolated and sequenced cDNA clones covering the entire coding sequence of human-milk bile-salt-stimulated lipase, as well as 996 nucleotides of the 3' end of the pancreatic enzyme carboxylic ester hydrolase. The deduced amino acid sequence of the lipase starts with a 23-residue leader peptide. The open reading frame continues with 722 amino acid residues. The sequence contains in the C-terminal part a proline-rich repeat, 16 repeats of 11 amino acid residues each. The mRNA was estimated to be approximately 2500 nucleotides from Northern blot and of similar size in mammary and pancreatic tissues. Data obtained indicate that the lipase and the carboxylesterase are identical and coded for by the same gene. The cDNA is 2428 bases long, which indicates that a near full-length copy of the transcript has been isolated. Comparisons with other enzymes show that the lipase is a new member of the supergene family of serine hydrolases. It is not only closely related (and in its N-terminal half virtually identical) to lysophospholipase from rat pancreas and cholesterol esterase from bovine pancreas, but also shows a high degree of similarity to several esterases, e.g. acetylcholine esterase. In contrast, no such similarity could be found to typical lipases.

Hydrolysis of fluorescent pyrene-acyl esters by human pancreatic carboxylic ester hydrolase and bile salt-stimulated lipase

Fluorescent esters containing pyrenedecanoic acid (P10) or pyrenebutanoic (P4) acid (P4cholesterol, P10cholesterol, P4- and P10-containing triacylglycerols) were synthesized and used as substrates for human pancreatic carboxylic ester hydrolase and bile salt-stimulated lipase from human milk. Both enzymes were purified by immunoaffinity chromatography. All fluorescent pyrene derivatives were hydrolyzed by pancreatic carboxylic ester hydrolase and bile salt-stimulated lipase, but at different rates. The hydrolytic rates of the "short" acyl esters (P4-containing esters) were higher than those of the "long" ones (P10-containing esters). Conditions were optimized for sensitivity of the assay using fluorescent cholesteryl esters. The pH optimum was 7.5-8.0. Sodium cholate exhibited a stronger activating effect than taurocholate or taurodeoxycholate (maximal activation was achieved with 5 mmol/L cholate and with a molar ratio cholesteryl ester/cholate around 1:10). Both pancreatic carboxylic ester hydrolase and bile salt-stimulated lipase from milk were strongly inhibited by the other amphiphiles tested, namely phosphatidylcholine and Triton X-100, and were inactivated by low concentrations (10 mumol/L) of the serine-reactive diethyl-paranitrophenyl phosphate (E600). Both enzymes were strongly inhibited by relatively low concentrations of plasma low density lipoproteins. These studies indicate that the fluorescent esters containing pyrene fatty acids can be used as substrates for assaying and investigating the properties of pancreatic carboxylic ester hydrolase as well as bile salt-stimulated lipase from milk.