Sequence identity between human pancreatic cholesterol esterase and bile salt-stimulated milk lipase

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.

Variations in gastrointestinal lipases, pH and bile acid levels with food intake, age and diseases: Possible impact on oral lipid-based drug delivery systems

The lipids and some surfactants present in oral lipid-based drug delivery systems are potential substrates for the various lipases involved in gastrointestinal (GI) lipolysis. The levels of these enzymes, together with pH and biliairy secretion, are important parameters that condition the fate of lipid-based formulations (LBF) and the dispersion, solubilization and absorption of lipophilic drugs in the GI tract. Since in vitro methods of digestion are now combined with dissolution assays for a better assessment of LBF performance, it is essential to have a basic knowledge on lipase, pH and bile acid (BA) levels in vivo to develop relevant in vitro models. While these parameters and their variations in healthy subjects are today well documented, in vivo data on specific populations (age groups, patients with various diseases, patients with treatment affecting GI tract parameters, …) are scarce and obtaining them from clinical studies is sometimes difficult due to ethical limitations. Here we collected some in vivo data already available on the levels of digestive lipases, gastric and intestinal pH, and BAs at various ages and in patients with exocrine pancreatic insufficiency, a pathological situation that leads to drastic changes in GI tract parameters and impacts pharmacological treatments.

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.

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.

Understanding the lipid-digestion processes in the GI tract before designing lipid-based drug-delivery systems

Many of the compounds present in lipid-based drug-delivery systems are esters, such as acylglycerols, phospholipids, polyethyleneglycol mono- and di-esters and polysorbate, which can be hydrolyzed by the various lipolytic enzymes present in the GI tract. Lipolysis of these compounds, along with dietary fats, affects the solubility, dispersion and bioavailibity of poorly water-soluble drugs. Pharmaceutical scientists have been taking a new interest in fat digestion in this context, and several studies presenting in vitro gastrointestinal lipolysis models have been published. In most models, it is generally assumed that pancreatic lipase is the main enzyme involved in the gastrointestinal lipolysis of lipid formulations. It was established, however, that gastric lipase, pancreatic carboxyl ester hydrolaze and pancreatic lipase-related protein 2 are the major players involved in the lipolysis of lipid excipients containing acylglycerols and polyethyleneglycol esters. These findings have shown that the lipolysis of lipid excipients may actually start in the stomach and involve several lipolytic enzymes. These findings should therefore be taken into account when testing in vitro the dispersion and bioavailability of poorly water-soluble drugs formulated with lipids. In this review, we present the latest data available about the lipolytic enzymes involved in gastrointestinal lipolysis and suggest tracks for designing physiologically relevant in vitro digestion models.

Comparative Structures and Evolution of Vertebrate Carboxyl Ester Lipase (CEL) Genes and Proteins with a Major Role in Reverse Cholesterol Transport

Bile-salt activated carboxylic ester lipase (CEL) is a major triglyceride, cholesterol ester and vitamin ester hydrolytic enzyme contained within pancreatic and lactating mammary gland secretions. Bioinformatic methods were used to predict the amino acid sequences, secondary and tertiary structures and gene locations for CEL genes, and encoded proteins using data from several vertebrate genome projects. A proline-rich and O-glycosylated 11-amino acid C-terminal repeat sequence (VNTR) previously reported for human and other higher primate CEL proteins was also observed for other eutherian mammalian CEL sequences examined. In contrast, opossum CEL contained a single C-terminal copy of this sequence whereas CEL proteins from platypus, chicken, lizard, frog and several fish species lacked the VNTR sequence. Vertebrate CEL genes contained 11 coding exons. Evidence is presented for tandem duplicated CEL genes for the zebrafish genome. Vertebrate CEL protein subunits shared 53-97% sequence identities; demonstrated sequence alignments and identities for key CEL amino acid residues; and conservation of predicted secondary and tertiary structures with those previously reported for human CEL. Phylogenetic analyses demonstrated the relationships and potential evolutionary origins of the vertebrate CEL family of genes which were related to a nematode carboxylesterase (CES) gene and five mammalian CES gene families.

Macrophage cholesteryl ester mobilization and atherosclerosis

Accumulation of cholesteryl esters (CE) stored as cytoplasmic lipid droplets is the main characteristic of macrophage foam cells that are central to the development of atherosclerotic plaques. Since only unesterified or free cholesterol (FC) can be effluxed from the cells to extracellular cholesterol acceptors, hydrolysis of CE is the obligatory first step in CE mobilization from macrophages. This reaction, catalyzed by neutral cholesteryl ester hydrolase (CEH), is increasingly being recognized as the rate-limiting step in FC efflux. CEH, therefore, regulates the process of reverse cholesterol transport and ultimate elimination of cholesterol from the body. In this review, we summarize the earlier controversies surrounding the identity of CEH in macrophages, discuss the characteristics of the various candidates recognized to date and examine their role in mobilizing cellular CE and thus regulating atherogenesis. In addition, physiological requirements to hydrolyze lipid droplet-associated substrate and complexities of interfacial catalysis are also discussed to emphasize the importance of evaluating the biochemical characteristics of candidate enzymes that may be targeted in the future to attenuate atherosclerosis.

Novel family of cholesterol esterases produced by actinomycetes bacteria

Although cholesterol esterase (CHE; EC 3.1.1.13) is widespread in nature, CHEs from Streptomyces lavendulae and Streptomyces sp. X9 are the only known CHEs produced by actinomycetes. We purified CHEs from S. avermitilis JCM5070, and S. griseus IFO13350 and identified four new CHEs from actinomycetes. The enzymic properties of the CHEs from Streptomyces sp. X9, S. avermitilis, and S. griseus including substrate specificity, sensitivity to inhibitors and optimal conditions for catalysis were similar. We identified genes for the CHEs from Streptomyces sp. X9 and S. avermitilis and the encoded predicted sequences comprised 217 and 214 amino acid residues, respectively, with 64% similarity. The CHEs from Streptomyces sp. X9 and S. avermitilis were also 54 and 57% similar, respectively, to S. lavendulae CHE, indicating that these CHEs are orthologs. Phylogenetic analysis showed that they are distantly related to the conventional lipase/esterase type CHEs from mammals, yeasts and other bacteria. The actinomycetes CHEs did not have the Gly-Xaa-Ser-Xaa-Gly sequence that is conserved in the lipase/esterase family. A database search showed that orthologs of this type of CHE were restricted to actinomycetes. These findings imply that the actinomycetes CHEs constitute a novel family of cholesterol esterases.

Novel cholesterol esterase secreted by Streptomyces persists during aqueous long-term storage

We isolated a moderate thermophilic actinomycete, Streptomyces sp. X9, from soil and purified cholesterol esterase (CHE) from the culture medium to homogeneity. The molecular masses of the purified CHE estimated by sodium dodecyl sulfate polyacrylamide gel electrophoresis and gel filtration chromatography were 23.6 and 163 kDa, respectively, indicating that the enzyme assumes an oligomeric form. Heavy metals such as Hg2+ and Ag+ similarly inhibited the activity of the CHE in the same manner as those of other bacterial CHEs. The activity of Streptomyces sp. X9 CHE was susceptible to dithiothreitol, beta-mercaptoethanol and p-chloromercuribenzoate, but resistant to phenylmethylsulfonyl fluoride, unlike those of other bacterial CHEs. The purified CHE could utilize both cholesteryl and p-nitrophenyl (pNP) esters of fatty acids as substrates. Steady-state kinetics revealed respective Km values for cholesteryl myristate and pNP-myristate of 0.34 and 1.1 mM, indicating that the cholesteryl residue is important for catalysis. We also found that the Km for the pNP esters are dependent on the chain length of the substrate fatty acid residues. These results indicate that the novel CHE specifically hydrolyzes substrates by recognizing both cholesteryl and fatty acid moieties. The enzyme was stable during long-term aqueous storage at room temperature, indicating its potential application as a diagnostic reagent.

Cloning and expression of a novel esterase gene cpoA from Burkholderia cepacia

AIMS

To screen and clone a novel enzyme with specific activity for the resolution of (R)-beta-acetylmercaptoisobutyrate (RAM) from (R,S)-beta-acetylmercaptoisobutyrate [(R,S)-ester].

METHODS AND RESULTS

A micro-organism that produces a novel esterase was isolated and identified as the bacterium Burkholderia cepacia by using the analysis of cellular fatty acids, Biolog automated microbial identification/characterization system, and 16S rRNA gene sequence analysis. A novel esterase gene was cloned from the chromosomal DNA of B. cepacia and was designated as cpoA. The cpoA encodes a polypeptide of 273 amino acids which shows a strong sequence homology with many bacterial nonhaeme chloroperoxidases. In addition, a typical serine-hydrolase motif, Gly-X-Ser-X-Gly, and the highly conserved catalytic triad, Ser95, Asp224, and His253, were identified in the deduced amino acid sequence of cpoA by multiple sequence alignment.

CONCLUSION

The cpoA cloned from B. cepacia encodes a novel esterase which is highly related to the nonhaeme chloroperoxidases.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report that describes the isolation and cloning of a serine esterase gene from B. cepacia, which is useful in the chiral resolution of (R,S)-ester. The cloned gene will allow additional research on the bifunctionality of the enzyme with esterase and chloroperoxidase activity at the structural and functional levels.

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.

Human bile salt-stimulated lipase has a high frequency of size variation due to a hypervariable region in exon 11

The apparent molecular mass of human milk bile salt-stimulated lipase (BSSL) varies between mothers. The molecular basis for this is unknown, but indirect evidence has suggested the differences to reside in a region of repeats located in the C-terminal part of the protein. We here report that a polymorphism within exon 11 of the BSSL gene is the explanation for the molecular variants of BSSL found in milk. By Southern blot hybridization we analyzed the BSSL gene from mothers known to have BSSL of different molecular masses in their milk. A polymorphism was found within exon 11, previously shown to consist of 16 near identical repeats of 33 bp each. We detected deletions or, in one case, an insertion corresponding to the variation in molecular mass of the BSSL protein found in milk from the respective woman. Furthermore, we found that 56%, out of 295 individuals studied, carry deletions or insertions within exon 11 in one or both alleles of the BSSL gene. Hence, this is a hypervariable region and the current understanding that exon 11 in the human BSSL gene encodes 16 repeats is an oversimplification and needs to be revisited. Natural variation in the molecular mass of BSSL may have clinical implications.

Production of recombinant human bile salt-stimulated lipase in Pichia pastoris

Recombinant human bile salt-stimulated lipase (rhBSSL) was efficiently expressed under the control of the AOX1 gene promoter in Pichia pastoris. Human BSSL has 16 successively repeated sequences in the carboxy terminal region. The sequence consists of 11 amino acid residues. The coding sequence for the middle 11 of the 16 repeats was removed from hBSSL cDNA to facilitate efficient secretory expression. The clone used for fermentation was a transformant of GS115 (his4) integrated with four copies of the expression cassette containing the modified hBSSL cDNA. Unique fermentation conditions were required for efficient expressions of rhBSSL in the high cell-density fermentation. A sufficient glycerol feed at 30 degrees C and pH 4 under an adequate concentration of dissolved oxygen in the growth phase make the cells active over a long induction period of approximately 15 days. On methanol induction, the concentration of dissolved oxygen should be maintained very low in the presence of sorbitol and skimmed milk at 20 degrees C and pH 5.7. Under these conditions, 0.8-1 g of rhBSSL was secreted in 1 liter of the medium. By immunoelectron microscopy, rhBSSL-tagged gold particles were located in secretion microbodies after the beginning of methanol induction. The secreted rhBSSL was efficiently captured and purified by expanded bed adsorption chromatography.

Crystal structure of the catalytic domain of human bile salt activated lipase

Bile-salt activated lipase (BAL) is a pancreatic enzyme that digests a variety of lipids in the small intestine. A distinct property of BAL is its dependency on bile salts in hydrolyzing substrates of long acyl chains or bulky alcoholic motifs. A crystal structure of the catalytic domain of human BAL (residues 1-538) with two surface mutations (N186D and A298D), which were introduced in attempting to facilitate crystallization, has been determined at 2.3 A resolution. The crystal form belongs to space group P2(1)2(1)2(1) with one monomer per asymmetric unit, and the protein shows an alpha/beta hydrolase fold. In the absence of bound bile salt molecules, the protein possesses a preformed catalytic triad and a functional oxyanion hole. Several surface loops around the active site are mobile, including two loops potentially involved in substrate binding (residues 115-125 and 270-285).

Importance of arginines 63 and 423 in modulating the bile salt-dependent and bile salt-independent hydrolytic activities of rat carboxyl ester lipase

Previous studies using chemical modification approach have shown the importance of arginine residues in bile salt activation of carboxyl ester lipase (CEL) activity. However, the x-ray crystal structure of CEL failed to show the involvement of arginine residues in CEL-bile salt interaction. The current study used a site-specific mutagenesis approach to determine the role of arginine residues 63 and 423 in bile salt-dependent and bile salt-independent hydrolytic activities of rat CEL. Mutations of Arg(63) to Ala(63) (R63A) and Arg(423) to Gly(423) (R423G) resulted in enzymes with increased bile salt-independent hydrolytic activity against lysophosphatidylcholine, having 6.5- and 2-fold higher k(cat) values, respectively, in comparison to wild type CEL. In contrast, the R63A and R423A mutant enzymes displayed 5- and 11-fold decreases in k(cat), in comparison with wild type CEL, for bile salt-dependent cholesteryl ester hydrolysis. Although taurocholate induced similar changes in circular dichroism spectra for wild type, R63A, and R423G proteins, this bile salt was less efficient in protecting the mutant enzymes against thermal inactivation in comparison with control CEL. Lipid binding studies revealed less R63A and R423G mutant CEL were bound to 1,2-diolein monolayer at saturation compared with wild type CEL. These results, along with computer modeling of the CEL protein, indicated that Arg(63) and Arg(423) are not involved directly with monomeric bile salt binding. However, these residues participate in micellar bile salt modulation of CEL enzymatic activity through intramolecular hydrogen bonding with the C-terminal domain. These residues are also important, probably through similar intramolecular hydrogen bond formation, in stabilizing the enzyme in solution and at the lipid-water interface.

Transcriptional regulation of the human carboxyl ester lipase gene in exocrine pancreas. Evidence for a unique tissue-specific enhancer

The human carboxyl ester lipase (CEL) is an important enzyme for the intestinal absorption of dietary lipids. The gene is highly expressed in exocrine pancreas and in the mammary gland during pregnancy and lactation. In this paper, we have focused on its transcriptional regulation in exocrine pancreas. Reporter gene analysis in cell cultures reveals that a high level of tissue-specific expression is established by the proximal 839 base pairs of the 5'-flanking region. This is due to a strong enhancer, located at -672 to -637. Transfections in mammary gland-derived cells reveal that the enhancer is pancreas-specific and does not contribute to the mammary gland expression. This indicates that the expression of the CEL gene in the mammary gland and pancreas, respectively, is due to two different regulatory systems. Further characterizations of the enhancer reveal that it is composed of two closely located cis-elements. The proximal element mediates a positive effect, whereas the distal element exerts a silencing effect on the positive proximal element. The functional enhancer complex is composed of ubiquitously expressed factors, since similar interactions are achieved with nuclear extracts from cells derived from other tissues. However, no enhancer activity is achieved in such cells. Hence, the net enhancer activity is the result of a tissue-specific balance between factors interacting with the two elements. Since none of the described cis-elements show any clear homology to known cis-elements, we propose that the interacting complex is composed of yet unidentified transcription factors.

Synthesis and secretion of the pancreatic-type carboxyl ester lipase by human endothelial cells

Human aortic extracts contain significant cholesteryl ester hydrolytic activity. The enzymic activity was shown to be activated by trihydroxylated bile salt, but not by dihydroxylated bile salt. Monospecific antibodies prepared against rat pancreatic carboxyl ester lipase (CEL, cholesterol esterase) immunoprecipitated cholesteryl ester hydrolytic activity from human aorta, demonstrating that the neutral CEL in aorta is highly similar to and probably identical with the pancreatic enzyme. Reverse transcriptase PCR amplification of mRNA from human aortic endothelial cells revealed de novo synthesis of the pancreatic-type CEL by these cells. Preincubating human aortic endothelial cells with oxidized or native low-density lipoprotein resulted in an 8- and 3-fold increase in CEL activity secreted into the culture medium respectively. A potential physiological role for the endothelial CEL was demonstrated by studies showing its ability to confer partial protection against the cytotoxic effects of lysophosphatidylcholine. The protective effect of CEL is related to its bile-salt-independent lysophospholipase activity. However, CEL hydrolysis of lysophosphatidylcholine can be inhibited by excess cholesterol. Taken together, these results indicate that pancreatic-type CEL is synthesized by cells lining the vessel wall. Moreover, vascular CEL may interact with cholesterol and oxidized lipoproteins to modulate the progression of atherosclerosis.

Modified low density lipoprotein enhances the secretion of bile salt-stimulated cholesterol esterase by human monocyte-macrophages. species-specific difference in macrophage cholesteryl ester hydrolase

Reverse transcriptase-polymerase chain reaction was used to study the biosynthesis of two different cholesteryl ester hydrolases by human and mouse macrophages. Oligonucleotide primers for bile salt-stimulated cholesterol esterase yielded positive reactions with RNA isolated from human peripheral blood monocytes, monocyte-derived macrophages, the human monocytic THP-1 cells, and phorbol ester-induced THP-1 macrophages. In contrast, oligonucleotide primers for hormone-sensitive lipase yielded positive reactions only with RNA isolated from non-differentiated human THP-1 monocytic cells and peripheral blood monocytes, but not those obtained from differentiated THP-1 macrophages or monocyte-derived macrophages. Thus, while human monocytes were capable of synthesizing both enzymes, human macrophages synthesized only bile salt-stimulated cholesterol esterase and not the hormone-sensitive lipase. The synthesis of bile salt-stimulated cholesterol esterase by human macrophages was confirmed by detection of bile salt-stimulated cholesteryl ester hydrolytic activity in conditioned media of differentiated THP-1 cells and human peripheral blood monocyte-derived macrophages. Moreover, incubating human macrophages with oxidized low density lipoprotein (LDL) or acetylated LDL increased bile salt-stimulated cholesterol esterase activity in the conditioned media of these cells. These results with human macrophages were contrasted with results of studies with mouse macrophages, which showed the presence of hormone-sensitive lipase mRNA but not the bile salt-stimulated cholesterol esterase mRNA. Taken together, these results demonstrated species-specific differences in expression of cholesteryl ester hydrolytic enzymes in macrophages. The expression of bile salt-stimulated cholesterol esterase by human macrophages, in a process inducible by modified LDL, suggests a role of this protein in atherogenesis.

The metabolic importance of unabsorbed dietary lipids in the colon

Digestion and absorption of lipids is a highly efficient process. From Western diets about 95% will be absorbed. This implies that together with lipids from endogenous sources 6-8 g of lipids will enter the colon daily. This input significantly increases during various lipid malabsorption syndromes. It has long been assumed that the biological fate of unabsorbed lipids is physiologically not relevant. However, significant microbial lipid metabolism occurs. Circumstantial evidence is arising which supports a role of unabsorbed lipid metabolites in the development of colonic diseases. Lipid metabolites may act as detergents in the colon, leading to mucosal injury and reactive hyperproliferation, which in its turn could promote tumour development. Lipid metabolites could also be transformed in biological active metabolites, which have a tumour promoting potency. More mechanistic information is needed on the colonic metabolic fate of lipids in order to develop strategies for manipulating colonic flora in the prevention of lipid related colonic diseases.

Molecular cloning and expression of rat hepatic neutral cholesteryl ester hydrolase

The 1923 bp cDNA for rat hepatic cholesteryl ester hydrolase (CEH) was cloned by screening a lambda gt11 expression library with an oligonucleotide containing the consensus active site sequence for cholesteryl esterases. Expression of a fusion protein, cross-reacting with antibody to the purified liver CEH, was demonstrated by Western blot analysis. The cDNA was sequenced and found to have only 44% homology with pancreatic CEH. Although unique, the cDNA sequence exhibited much greater overall homology with liver carboxylesterases, in both coding and 5'/3' non-coding regions. In Northern blot analysis, the cDNA hybridized with a single band from liver mRNA but not with pancreatic mRNA. The 1.7 kb coding sequence, predicting a 62 kDa protein, was cloned into an Escherichia coli expression system with an inducible promoter and into COS-7 cells. Both expression systems produced a protein which comigrated with liver CEH (66 kDa) on SDS-PAGE and immunoreacted with antibodies to liver CEH on Western blots. Whereas the prokaryotic system produced an inactive protein, expression in COS-7 cells was accompanied by a 5-fold increase in CEH activity and a corresponding increase in immunoreactive protein.

Lipid metabolism in pediatric nutrition

The abrupt transition from carbohydrate to fat as the main energy source that occurs at birth is not matched by commensurate endogenous fat-digesting capacity in the newborn. Newborn infants are, however, able to digest fat efficiently through the activities of gastric lipase and the exogenous digestive lipase of human milk, which compensate for the low activity of pancreatic lipase. Fat absorption is well-developed at birth and is commensurate with the high fat intake of the infant. Tissue uptake of dietary fat is also adequate, based on sufficient lipoprotein lipase (above 26 to 27 weeks' gestation) and rapid postnatal increase of lecithin:cholesterol acyl transferase, the enzymes that regulate tissue uptake of circulatory lipoprotein triglyceride and cholesterol.

Structural organisation of human bile-salt-activated lipase probed by limited proteolysis and expression of a recombinant truncated variant

Bile-salt-activated lipase belongs to the cholinesterase alpha/beta-hydrolase-fold family of proteins. Here, we have investigated the structural organisation of the human isoform by mapping tryptic cleavage sites using limited proteolysis and by expression studies using a recombinant truncated variant. Two accessible regions in the tertiary structure were identified. The first is defined by a tryptic cleavage at Lys429 and lies within the alpha/beta-hydrolase fold in bile-salt-activated lipase between a central beta-sheet and an active-site histidine residue, as deduced from sequence similarity across the cholinesterases and known structural properties. This region exhibits a proteolytic and topological similarity to the lid region in pancreatic lipase. The other accessible region in the tertiary structure is defined by a tryptic cleavage at Arg520 and occurs within a catalytically non-essential segment Leu519-Gln535, as identified by expression of a truncated variant which lacks the C-terminus starting from Leu519. This region is consistent with an interdomain region between the cholinesterase-related part of the protein structure and the unique proline-rich C-terminal repeats. Both protease-sensitive regions appear to occur at domain borders, and, therefore, are consistent with a multi-domain structure. The truncated variant was fully functional as a lipase and as a bile-salt-stimulated esterase. However, compared to the full-length enzyme, the truncated variant showed an increased susceptibility to limited proteolysis, suggesting that the C-terminal repeats may regulate proteolytic degradation of the protein.

Pancreatic carboxylester lipase from Atlantic salmon (Salmo salar). cDNA sequence and computer-assisted modelling of tertiary structure

We report the isolation and characterization of a 1795-bp cDNA fragment encoding Atlantic salmon pancreatic carboxylester lipase from salmon pancreas mRNA. The nearly full-length cDNA contained a 540-amino-acid open-reading frame, encompassing the mature protein (by similarity to mammalian carboxylester lipase enzymes). The salmon carboxylester lipase primary structure shared 58% identity with mammalian carboxylester lipases, lacking the proline-rich C-terminal repeats found in human and rat carboxylester lipases. Congruent with other esterase B type enzymes, the salmon carboxylester lipase contained a canonical serine-esterase catalytic triad motif consisting of serine, histidine and aspartic acid. Computer-assisted modelling of the tertiary structure for salmon carboxylester lipase was conducted using acetylcholine esterase (Torpedo californica) as a template structure. The model, in conjunction with sequence comparisons and available enzymological data, has been used to locate putative bile-salt-binding and lipid-binding sites. The carboxylester lipase enzymes contain a unique, highly conserved insert region that may be associated with bile-salt binding. In the model structure, this region is located close to the active site, and contains a tyrosine residue with an adjacent carboxylester-lipase-conserved arginine. These traits have previously been predicted for the non-specific (regarding bile-salt hydroxylation) bile-salt-binding site in carboxylester lipase enzymes. At this site, a dihydroxy or trihydroxy bile-salt molecule may bind the tyrosine via hydrophobic interactions, the anionic bile-salt head group may bind the arginine, while hydrogen bonding between the bile-salt 12 alpha hydroxy group and an adjacent aspargine residue is possible. The model does not contain an active site 'lid' structure as found in other lipases. The carboxylester lipase structural homolog to the 'flap' of the lipases from Geotrichum candidum and Candida rugosa contains a carboxylester-lipase-conserved deletion that renders this region unable to cover the active site. Instead, the shortening of this loop leads to solvent exposure of the carboxylester lipase insert region, an additional indication of the functional importance of this region.

The synthesis of fatty acid ethyl ester by carboxylester lipase

Carboxylester lipase obtained from pig pancreas is associated with fatty acid ethyl ester synthase as judged by their elution in the same fraction from a heparin-Sepharose column, coprecipitations by antibody against purified carboxylester lipase and identical profiles of inhibition by diisopropyl fluorophosphate. Only one polypeptide of molecular mass 74-kDa in purified carboxylester lipase was labeled by immunostaining and affinity labeling with [3H]diisopropyl fluorosphate. Bovine serum albumin decreased the fatty-acid-ethyl-ester-synthesizing activity in a concentration-dependent manner. On incubation of purified carboxylester lipase with trioleylglycerol in an ethanol/water mixture, fatty acid ethyl ester was formed in the presence of a high concentration of bovine serum albumin. The acyltransfer activities from trioleylglycerol to ethanol (ethanolysis) were approximately 25-30 times higher than the acyltransfer activities to water (hydrolysis). When cholesterol was used as an acceptor, acyltransfer activity from trioleylglycerol to cholesterol (cholesterolysis) was also observed. We propose the following mechanism of fatty acid ethyl ester formation from triacyl glycerol. The enzyme attacks triacyl glycerol forming an acyl-enzyme intermediate, and during the deacylation process, alcohol binds to fatty acid as an acceptor. These results suggest that during lipid (triacyl glycerol) degradation, carboxylester lipase contributes to non-oxidative ethanol metabolism in the intestinal lumen.

Proline-rich domain and glycosylation are not essential for the enzymic activity of bile salt-activated lipase. Kinetic studies of T-BAL, a truncated form of the enzyme, expressed in Escherichia coli

We have expressed and purified a truncated recombinant human milk bile salt-activated lipase (T-BAL) from the T7 expression system in Escherichia coli. This T-BAL contains the N-terminal 538 residues of the 722-residue native enzyme. The purified T-BAL, when assayed with PANA (p-nitrophenyl acetate), had a specific activity of 64 +/- 2 units/mg (n = 4), as compared to 52 units/mg for the native enzyme. Because the recombinant T-BAL expressed in E. coli is not glycosylated, these results indicated that the highly glycosylated C-terminal region of BAL is not essential for catalytic function. Heat inactivation patterns of native BAL and T-BAL were found to be similar, further suggesting that the folding of T-BAL is similar to that of the catalytic domain of the native enzyme. With the availability of a sufficient amount of recombinant T-BAL, the specificity and kinetics of T-BAL and native BAL were compared. Fluorescence studies of T-BAL indicated that it has a slightly higher affinity for the monomeric form of taurocholate with a dissociation constant (KA) of 0.32 mM, compared with the reported 0.37 mM for the native enzyme. Further kinetic analysis indicated that there are enzyme specificity changes revealed with the use of PANA and PANB (p-nitrophenyl butyrate) as substrates. When assayed in the presence of taurocholate, T-BAL has a higher turnover rate constant with p-nitrophenyl acetate than with p-nitrophenyl butyrate, which was found to be in contrast to native BAL.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipoamidase activity in normal and mutagenized pancreatic cholesterol esterase (bile salt-stimulated lipase)

Purified human milk lipoamidase was digested with endoproteinase Lys-C and the digested peptides were subjected to gasphase microsequence analysis. The sequencing of three isolated peptides of human milk lipoamidase revealed the identity of this protein with human milk bile salt-stimulated lipase (pancreatic cholesterol esterase). The identity of the cholesterol esterase with lipoamidase was confirmed by expressing a recombinant form of rat pancreatic cholesterol esterase and testing for lipoamidase activity of the recombinant protein. The results showed that the recombinant cholesterol esterase displayed both lipolytic and lipoamidase activities and was capable of hydrolysing triacetin and lipoyl-4-aminobenzoate (LPAB). The mechanisms of the esterase and amidase activities of the enzyme were further tested by determining enzyme activity in a mutagenized cholesterol esterase with a His435-->Gln435 substitution. This mutation has been shown previously to abolish enzyme activity against esterase substrates [DiPersio, Fontaine and Hui (1991) J. Biol. Chem. 266, 4033-4036]. We showed that the mutagenized protein was effective in hydrolysing the amidase substrate LPAB and displayed similar enzyme kinetics to those of the native enzyme. These data indicate that the mechanism for the cholesterol esterase hydrolysis of lipoamides is different from that of the hydrolysis of substrates with an ester linkage. The presence of an enzyme in the gastrointestinal tract capable of both ester and amide hydrolysis suggests an important role for this protein in the digestion and absorption processes.

Relationship between sequence conservation and three-dimensional structure in a large family of esterases, lipases, and related proteins

Based on the recently determined X-ray structures of Torpedo californica acetylcholinesterase and Geotrichum candidum lipase and on their three-dimensional superposition, an improved alignment of a collection of 32 related amino acid sequences of other esterases, lipases, and related proteins was obtained. On the basis of this alignment, 24 residues are found to be invariant in 29 sequences of hydrolytic enzymes, and an additional 49 are well conserved. The conservation in the three remaining sequences is somewhat lower. The conserved residues include the active site, disulfide bridges, salt bridges, and residues in the core of the proteins. Most invariant residues are located at the edges of secondary structural elements. A clear structural basis for the preservation of many of these residues can be determined from comparison of the two X-ray structures.

Molecular cloning and expression of rabbit pancreatic cholesterol esterase

Rabbit pancreatic cholesterol esterase (CEase, carboxyl ester lipase, EC 3.1.1.3) has been cloned from a lambda gt11 library of adult rabbit pancreatic cDNA. The open reading frame consists of 1788 nucleotides which encodes 576 amino acids of the functional protein and a 20 amino acid leader peptide. When compared to other species, the greatest homology is observed between residues 82-248 with little or no homology at the C-terminal end where proline-glutamate-serine-threonine (PEST) segments are a characteristic feature of the human CEase. Rabbit CEase (RCEase) retains the active-site serine (gxsxg), the active-site histidine and the tentative heparin binding site (KKRCLQ) at similar positions in comparison to pancreatic CEases of other species. When rabbit CEase cDNA is expressed in monkey kidney (COS-7) cells, enzymatic hydrolytic activity is detected in the growth medium as is a 67 kDa protein by Western blotting with polyclonal anti-CEase antibody. Northern blot analysis shows two mRNA (2.2 and 3.2 kb) species.

Analysis of the catalytic mechanism of juvenile hormone esterase by site-directed mutagenesis

1. Juvenile hormone esterase (JHE) is a serine hydrolase selective for hydrolysis of the conjugated methyl esters of insect juvenile hormones. 2. We have investigated the mechanism of catalytic action of this enzyme by site-directed mutagenesis of the cloned enzyme and expression of the mutants in a baculovirus system. 3. A series of individual mutations of JHE were made to residues possibly involved in catalysis of juvenile hormones, and which are highly conserved in both esterases and lipases. 4. Mutation of the serine residue at position 201 to glycine (S201G), or aspartate 173 to asparagine (D173N), or histidine 446 to lysine (H446K), removed all detectable activity and these mutagenized enzymes were determined to be at least 10(6)-fold less active than wild type JHE. 5. Mutation of arginine 47 to histidine (R47H) decreased but did not abolish activity, with Km essentially unchanged at 66 nM for R47H compared to 34 nM for wild type JHE. 6. The kcat for R47H was decreased from 103 min-1 for wild type JHE to 1.9 min-1. 7. In addition, glutamate residue 332 was altered to glutamine (E332Q) and expressed in an Escherichia coli system. 8. This mutation was also found to remove all detectable activity. 9. From the results presented in this study and by comparison of JHE to other serine esterases and lipases, we predict that JHE possesses a Ser201-His446-Glu332 catalytic triad. 10. In addition, aspartate 173 and arginine 47 are essential for the efficient functioning of JHE.

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.

Regulation of fatty acid 18O exchange catalyzed by pancreatic carboxylester lipase. 1. Mechanism and kinetic properties

The exchange of 18O between H2O and long-chain free fatty acids is catalyzed by pancreatic carboxylester lipase (EC 1.1.1.13). For palmitic, oleic, and arachidonic acid in aqueous suspension and for 13,16-cis,cis-docosadienoic acid (DA) in monomolecular films, carboxyl oxygens were completely exchanged with water oxygens of the bulk aqueous phase. With enzyme at either substrate or catalytic concentrations in the argon-buffer interface, the exchange of DA oxygens obeyed a random sequential mechanism, i.e., 18O,18O-DA in equilibrium with 18O,16O-DA in equilibrium with 16O,16O-DA. This indicates that the dissociation of the enzyme-DA complex is much faster than the rate-limiting step in the overall exchange reaction. Kinetic analysis of 18O exchange showed a first-order dependence on surface enzyme and DA concentrations, i.e., the reaction was limited by the acylation rate. The values of kcat/Km, 0.118 cm2 pmol-1 s-1, for the exchange reaction was comparable to that for methyl oleate hydrolysis and 5-fold higher than that for cholesteryl oleate hydrolysis in monolayers [Bhat, S., & Brockman, H. L. (1982) Biochemistry 21, 1547]. Thus, fatty acids are good "substrates" for carboxylester lipase. With substrate levels of carboxylester lipase in the interfacial phase, the acylation rate constant kcat/Km was 200-fold lower than that obtained with catalytic levels of enzyme. This suggests a possible restriction of substrate diffusion in the protein-covered substrate monolayer.

Fatty acid specificity of bile salt-dependent lipase: enzyme recognition and super-substrate effects

A putative fatty acid specificity of bile salt-dependent lipases (BSDLs) has been re-investigated. The strategy was to use two evolutionally distant, homologous BSDLs (from human and cod), and to investigate their hydrolysis of different fatty acid esters at different assay conditions affecting the physicochemical phase of the substrate. Depending on assay conditions, large variations were seen in the hydrolysis rate for esters of different fatty acids. The two enzymes displayed similar fatty acid specificity patterns, with small, but significant differences that were maintained at various assay conditions. Compared to the human enzyme, the cod enzyme showed a preference for hydrolysis of long-chain polyunsaturated fatty acyl esters (up to 22 carbons in length). On the other hand, the human enzyme hydrolysed esters of shorter chain saturated fatty acids at significantly higher rates compared to the cod enzyme. Changing physicochemical factors affecting the substrate phase induced large changes in fatty acid specificity that affected both enzymes in similar manners. It is concluded that though the aliphatic chains of the fatty acids may not be recognized by the enzymes, these chains indirectly affect the conformation or interfacial availability of the carboxyl ester bond in the substrate, and the enzymes show minor specificities for variations in these structures.

Structure of the rat pancreatic cholesterol esterase gene

The gene encoding the rat pancreatic cholesterol esterase has been isolated and characterized. Analysis of overlapping genomic clones showed that the cholesterol esterase gene spans approximately 8 kb, containing 11 exons interrupted by 10 introns. The exons ranged in size from 83 to 201 bp except for the last exon, which was 548 bp in length. A TAAATA sequence was present at -31 nucleotides from the transcriptional initiation site. A putative pancreas-specific enhancer sequence was found at -90 bp upstream from the CAP site. Although cholesterol esterase shares three domains of similarity with cholinesterase and acetylcholinesterase, these domains were found to be localized in distinct exons of the cholesterol esterase gene. The organization of the cholesterol esterase gene suggests its divergent evolution with other members of the serine esterase gene family.