Inhibition of human milk bile-salt-dependent lipase by boronic acids. Implication to the bile salts activator effect

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.

Cysteine proteases such as papain are not inhibited by substrate analogue peptidyl boronic acids

Peptidyl boronic acids that are close structural analogues of good substrates have been prepared and evaluated as potential transition state analogue inhibitors of the representative cysteine protease, papain. However, no inhibition could be detected at concentrations up to 10 mM. The reasons for the lack of inhibition were sought from molecular modeling. Molecular mechanics and semi-empirical quantum mechanics calculations indicated that the absence of inhibition was due to boronic acid-cysteine protease tetrahedral complexes being 0.79 kcal mol-1 less stable than their preceding noncovalent EI-complexes. In contrast, an analogous boronic acid-serine protease tetrahedral complex was calculated to be 2.74 kcal mol-1 more stable than its precursor Michaelis EI-complex. It thus appears that boronic acids are ineffective inhibitors of cysteine proteases due to the thermodynamic favoring of a weak EI-complex preceding tetrahedral intermediate formation, and that any oxyanion hole stabilization of the subsequent tetrahedral intermediate cannot overcome this energy handicap.

Inhibition of hepatic lipase by m-aminophenylboronate. Application of phenylboronate affinity chromatography for purification of human postheparin plasma lipases

Phenylboronates are competitive inhibitors of serine. hydrolases including lipases. We studied the effect of m-aminophenylboronate on triglyceride-hydrolyzing activity of hepatic lipase (EC 3.1,1.3). m-Aminophenylboronate inhibited hepatic lipase activity with a Ki value of 55 microM. Furthermore, m-aminophenylboronate protected hepatic lipase activity from inhibition by di-isopropyl fluorophosphate, an irreversible active site inhibitor of serine hydrolases. Inhibition of hepatic lipase activity by m-aminophenylboronate was pH-dependent. The inhibition was maximal at pH 7.5, while at pH 10 it was almost non-existent. These data were used to develop a purification procedure for postheparin plasma hepatic lipase and lipoprotein lipase. The method is a combination of m-aminophenylboronate and heparin-Sepharose affinity chromatographies. Hepatic lipase was purified to homogeneity as analyzed on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The specific activity of purified hepatic lipase was 5.46 mmol free fatty acids h-1 mg-1 protein with a total purification factor of 14,400 and a final recovery of approximately 20%. The recovery of hepatic lipase activity in m-aminophenylboronate affinity chromatography step was 95%. The purified lipoprotein lipase was a homogeneous protein with a specific activity of 8.27 mmol free fatty acids h-1 mg-1. The purification factor was 23,400 and the final recovery approximately 20%. The recovery of lipoprotein lipase activity in the m-aminophenylboronate affinity chromatography step was 87%. The phenylboronate affinity chromatography step can be used for purification of serine hydrolases which interact with boronates.

Characterization of a partially purified diacylglycerol lipase from bovine aorta

A partially-purified diacylglycerol (DG) lipase from bovine aorta has been characterized with respect to the effects of lipid metabolites and two lipase inhibitors, phenylboronic acid and tetrahydrolipstatin (THL). DG lipase activity was determined by the hydrolysis of the sn-1 position of 1-[1-14C]palmitoyl-2-oleoyl-sn-glycerol. The products of the lipase reaction, 2-monoacylglycerol (2-monoolein) and non-esterified fatty acids (oleate, archidonate) produced a concentration-dependent (20-200 microM) inhibition of DG lipase activity. Oleoyl-CoA and dioleoylphosphatidic acid also inhibited aortic DG lipase activity, but lysophosphatidylcholine had little or no effect. The inhibition of aortic DG lipase by phenylboronic acid was competitive, with a Ki of approx. 4 mM. THL was a very potent inhibitor of aortic DG lipase; the concentration required for inhibition to 50% of control was 2-6 nM. THL inhibition was reduced when the concentration of substrate in the assay was increased. Attempts to identify the aortic DG lipase by covalent-labelling with [14C]THL were unsuccessful. Immunoblotting experiments revealed that hormone-sensitive triacylglycerol lipase (HSL) could not be detected in bovine aorta.

Human fetoacinar pancreatic protein: an oncofetal glycoform of the normally secreted pancreatic bile-salt-dependent lipase

A fetoacinar pancreatic protein (FAP) associated with the ontogenesis, differentiation and oncogenic transformation of the human exocrine pancreas has been purified from pancreatic juices of patients suffering from pancreatitis or duodenal cancers invading the pancreas [Escribano and Imperial (1989) J. Biol. Chem. 264, 21865-21871]. This protein has striking similarities, i.e. M(r), amino acid composition and N-terminal sequence, to the bile-salt-dependent lipase (BSDL) of normal human pancreatic secretion. The aim of this study was to gain further insight into the nature of the two proteins. Reactivity with the mouse monoclonal antibody J28 (mAb J28), which characterizes FAP, and enzyme activity could not be dissociated during biochemical purification of BSDL. Furthermore, a polyclonal antiserum raised against purified human BSDL reacted completely with FAP in Western-blot analysis giving additional support to the idea of similar molecular structures for BSDL and FAP. However, by the same technique, mAb J28 reacted with a relatively restricted population of BSDL molecules. The classical BSDL preparation could be separated into molecules bearing the J28 epitope and those devoid of it by immunoaffinity on immobilized mAb J28. The two subpopulations had identical N-terminal sequences and some differences in their amino acid compositions. However, they had different carbohydrate compositions. J28-epitope-bearing molecules were active on BSDL substrates, although their specific activity was decreased. These results are consistent with the existence of two closely related polypeptide chains with different glycan counterparts. Therefore, if the name FAP is reserved for molecules bearing the J28 epitope, which is linked to a carbohydrate-dependent structure. FAP could represent an oncofetal-related variant of BSDL. Our result is the first demonstration of the existence of an oncofetal-type subpopulation of an otherwise normally secreted human pancreatic enzyme.

Purification of pancreatic cholesterol esterase expressed in recombinant baculovirus-infected Sf9 cells

A cDNA clone encoding the entire coding sequence of rat pancreatic cholesterol esterase (bile salt-stimulated lipase) was subcloned into the Baculovirus transfer vector pVL1392 and used to co-transfect Spodoptera frugiperda (Sf9) insect cells with wild-type Autographa californica nuclear polyhedrosis virus (AcNPV) DNA. Two recombinant proteins (M(r) 74 kDa and 64 kDa) reactive with anti-cholesterol esterase IgG were produced and secreted by the infected Sf9 cells in large quantities in a time-dependent manner. The 74-kDa protein was detectable in the cultured medium at the second day post-infection and increased progressively, reaching a level of 50 micrograms/ml of culture medium after 8 days. Amino-terminal sequencing of this recombinant protein showed that the signal peptide of cholesterol esterase was correctly cleaved, resulting in the production of mature protein. The 64-kDa recombinant protein was not detected in the medium until Day 5 post-infection and accumulated to a level of 25 micrograms/ml at Day 8. Both the 74- and the 64-kDa cholesterol esterases were biologically active and hydrolyzed the artificial substrate p-nitrophenyl butyrate. Results of this study demonstrated that Baculovirus-infected Sf9 cells can be used for high-level expression of pancreatic cholesterol esterase. The recombinant enzyme will be useful for further characterization of this protein.

Pancreatic bile salt dependent lipase from cod (Gadus morhua): purification and properties

The enzymatic basis for cod digestive lipolysis has been investigated. Lipase activity was found in aqueous extracts from pyloric caeca as well as in pancreatic tissue surrounding the caeca and the bile duct. A bile salt-dependent lipase (BSDL) was purified from either defatted powder of cod pyloric caeca or aqueous pancreatic extracts by combined affinity chromatography on cholate-Sepharose and gel filtration on Sephacryl S-200 HR. By SDS-PAGE analysis the molecular weight of purified cod BSDL was estimated to 60 kDa. The enzyme was totally dependent on bile salts for hydrolysis of insoluble fatty acid esters. Antiserum raised against purified cod BSDL reacted specifically with selected mammalian pancreatic BSDLs by Western blot analysis. Results presented in this paper strongly suggest that the bile salt-dependent lipase is the only pancreatic enzyme involved in lipid digestion in cod. The enzyme has been characterized and compared to human pancreatic BSDL with respect to substrate specificity, temperature- and pH-dependence and inhibitors. Both soluble and insoluble fatty acid esters were hydrolysed and the enzyme was 1,3-specific in hydrolysis of triolein. The enzyme was inhibited by di-isopropyl fluorophosphate and phenyl boronic acid, but not significantly by phenyl methyl sulfonyl fluoride. The cod BSDL is probably homologous to mammalian pancreatic BSDLs.

Betaine:Homocysteine methyltransferase from rat liver: Purification and inhibition by a boronic acid substrate analog

Betaine:homocysteine methyltransferase (BHMT) from rat liver has been highly purified by an efficient procedure requiring only two chromatographic steps: Sephadex G-100 chromatography and fast protein liquid chromatography chromatofocusing. A 170-fold purification and 7.5% overall yield were achieved. Chromatofocusing yielded three active forms of BHMT with pI values near 8.0, 7.6, and 7.0. The subunit molecular weight of each active form is 45,000 Da as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the native enzyme has a molecular weight of 270,000 as determined by exclusion chromatography. The stability of the purified enzyme was found to be potentiated by the presence of 1 mM dimethylglycine and 1 mM homocysteine. Boronate analogs of betaine (pinanyl N,N,N-trimethylaminomethaneboronate) (4) and dimethylglycine (pinanyl N,N-dimethylaminomethaneboronate) were synthesized from pinanyl iodomethaneboronate (3) and trimethylamine or dimethylamine, respectively. The free acid of the betaine analog (5) was reversibly generated from (4). The inhibition of BHMT by (5) appears competitive with a Ki = 45 microM. Since the Km for betaine measured with the purified enzyme is near 0.1 mM, the boronic acid analog of betaine appears to function effectively as a substrate analog inhibitor of BHMT. The analog does not appear to act as a methyl donor to homocysteine when (5) is substituted for betaine in the enzyme reaction. In addition, an enzyme assay based upon C3-cyano reverse phase HPLC detection of the o-phthalaldehyde derivative of methionine was developed as an alternative to the standard radiochemical assay. Betaine:homocysteine methyltransferase in the picomole range can be quantitated using this assay as indicated by a linear response of enzyme activity to protein concentration.

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.