Use of naturally fluorescent triacylglycerols from Parinari glaberrimum to detect low lipase activities from Arabidopsis thaliana seedlings

A simple, rapid, and sensitive fluorescence-based method to assess triacylglycerol hydrolase activity

Lipases constitute an important class of water-soluble enzymes that catalyze the hydrolysis of hydrophobic triacylglycerol (TAG). Their enzymatic activity is typically measured using multistep procedures involving isolation and quantification of the hydrolyzed products. We report here a new fluorescence method to measure lipase activity in real time that does not require the separation of substrates from products. We developed this method using adipose triglyceride lipase (ATGL) and lipoprotein lipase (LpL) as model lipases. We first incubated a source of ATGL or LpL with substrate vesicles containing nitrobenzoxadiazole (NBD)-labeled TAG, then measured increases in NBD fluorescence, and calculated enzyme activities. Incorporation of NBD-TAG into phosphatidylcholine (PC) vesicles resulted in some hydrolysis; however, incorporation of phosphatidylinositol into these NBD-TAG/PC vesicles and increasing the ratio of NBD-TAG to PC greatly enhanced substrate hydrolysis. This assay was also useful in measuring the activity of pancreatic lipase and hormone-sensitive lipase. Next, we tested several small-molecule lipase inhibitors and found that orlistat inhibits all lipases, indicating that it is a pan-lipase inhibitor. In short, we describe a simple, rapid, fluorescence-based triacylglycerol hydrolysis assay to assess four major TAG hydrolases: intracellular ATGL and hormone-sensitive lipase, LpL localized at the extracellular endothelium, and pancreatic lipase present in the intestinal lumen. The major advantages of this method are its speed, simplicity, and elimination of product isolation. This assay is potentially applicable to a wide range of lipases, is amenable to high-throughput screening to discover novel modulators of triacylglycerol hydrolases, and can be used for diagnostic purposes.

Quantitative and sensitive detection of lipase using a liquid crystal microfiber biosensor based on the whispering-gallery mode

We demonstrate a quantitative and sensitive strategy for monitoring the lipase concentration using a liquid crystal microfiber biosensor based on the whispering-gallery mode.

IR spectroscopy analysis of pancreatic lipase-related protein 2 interaction with phospholipids: 3. Monitoring DPPC lipolysis in mixed micelles

Usual methods for the continuous assay of lipolytic enzyme activities are mainly based on the titration of free fatty acids, surface pressure monitoring or spectrophotometry using substrates labeled with specific probes. These approaches only give a partial information on the chemistry of the lipolysis reaction and additional end-point analyses are often required to quantify both residual substrate and lipolysis products. We used transmission infrared (IR) spectroscopy to monitor simultaneously the hydrolysis of phospholipids by guinea pig pancreatic lipase-related protein 2 (GPLRP2) and the release of lipolysis products. The substrate (DPPC, 1,2-Dipalmitoyl phosphatidylcholine) was mixed with sodium taurodeoxycholate (NaTDC) to form mixed micelles in D₂O buffer at pD 6 and 8. After hydrogen/deuterium exchange, DPPC hydrolysis by GPLRP2 (100nM) was monitored at 35°C in a liquid cell by recording IR spectra and time-course variations in the CO stretching region. These changes were correlated to variations in the concentrations of DPPC, lysophospholipids (lysoPC) and palmitic acid (Pam) using calibration curves established with these compounds individually mixed with NaTDC. We were thus able to quantify each compound and its time-course variations during the phospholipolysis reaction and to estimate the enzyme activity. To validate the IR analysis, variations in residual DPPC, lysoPC and Pam were also quantified by thin-layer chromatography coupled to densitometry and similar hydrolysis profiles were obtained using both methods. IR spectroscopy can therefore be used to monitor the enzymatic hydrolysis of phospholipids and obtain simultaneously chemical and physicochemical information on substrate and all reaction products (H-bonding, hydration, acyl chain mobility).

Synthesis and characterization of a new fluorogenic substrate for monoacylglycerol lipase and application to inhibition studies

Human monoacylglycerol lipase (MAGL), a soluble serine hydrolase that belongs to the α/β hydrolase fold superfamily, regulates 2-arachidonoyl glycerol level in the endocannabinoid system, which is implicated in a number of severe diseases, and therefore, inhibition of MAGL activity is crucial in the treatment of these diseases. We have synthesized a red fluorogenic substrate, 7-hydroxyresorufinyl-arachidonate (7-HRA), for a new MAGL assay. This assay is simple, sensitive, and reliable and useful for identifying compounds that modulate MAGL activity. In addition, the assay emits red fluorescence, which can significantly reduce interference due to compound fluorescence and dust or lint, all of which fluoresce in the blue wavelength. MAGL catalyzes the hydrolysis of 7-HRA to generate arachidonic acid and a highly red fluorescent resorufin, excitation at 571 nm and emission at 588 nm, with a Km of 0.87 μM and Vmax of 26 nmol min(-1) mg protein(-1). The known MAGL inhibitors URB602, methyl arachidonyl fluorophosphonate, and JZL184 were used to validate the test assay. The assay was highly reproducible with an overall average Z' value of 0.80. This new red fluorogenic substrate and the resulting enzyme assay could be used in high-throughput screening to identify and develop new potential MAGL inhibitors.

A broad pH range indicator-based spectrophotometric assay for true lipases using tributyrin and tricaprylin

A continuous assay is proposed for the screening of acidic, neutral, or alkaline lipases using microtiter plates, emulsified short- and medium-chain TGs, and a pH indicator. The lipase activity measurement is based on the decrease of the pH indicator optical density due to protonation which is caused by the release of FFAs during the hydrolysis of TGs and thus acidification. Purified lipases with distinct pH optima and an esterase were used to validate the method. The rate of lipolysis was found to be linear with time and proportional to the amount of enzyme added in each case. Specific activities measured with this microplate assay method were lower than those obtained by the pH-stat technique. Nevertheless, the pH-dependent profiles of enzymatic activity were similar with both assays. In addition, the substrate preference of each enzyme tested was not modified and this allowed discriminating lipase and esterase activities using tributyrin (low water solubility) and tricaprylin (not water soluble) as substrates. This continuous lipase assay is compatible with a high sample throughput and can be applied for the screening of lipases and lipase inhibitors from biological samples.

Development of a high-throughput assay for measuring phospholipase A activity using synthetic 1,2-α-eleostearoyl-sn-glycero-3-phosphocholine coated on microtiter plates

To date, several sensitive methods, based on radiolabeled elements or sterically hindered fluorochrome groups, are usually employed to screen phospholipase A (PLA) activities. With the aim of developing a convenient, specific, sensitive, and continuous new ultraviolet (UV) spectrophotometric assay for PLA, we have synthesized a specific glycerophosphatidylcholine (PC) esterified at the sn-1 and sn-2 positions, with α-eleostearic acid (9Z, 11E, 13E-octadecatrienoic acid) purified from Aleurites fordii seed oil. The conjugated triene present in α-eleostearic acid constitutes an intrinsic chromophore and, consequently, confers the strong UV absorption properties of this free fatty acid as well as of the glycerophospholipids harboring it. This coated PC film cannot be desorbed by the various buffers used during PLA assays. Following the action of PLA at the oil-water interface, α-eleostearic acid is freed and desorbed from the film and then solubilized with β-cyclodextrin. The UV absorbance of the α-eleostearic acid is considerably enhanced due to the transformation from an adsorbed to a water-soluble state. The PLA activity can be measured continuously by recording the variations with time of the UV absorption spectra. The rate of lipolysis was monitored by measuring the increase of absorption at 272 nm, which was found to be linear with time and proportional to the amount of added PLA. This continuous high-throughput PLA assay could be used to screen new PLA and/or PLA inhibitors present in various biological samples.

Liquid crystal based sensors monitoring lipase activity: a new rapid and sensitive method for cytotoxicity assays

In this work we present liquid crystal (LC) based sensor devices to monitor cell viability. The sensing layer is composed by the LC and a planar monolayer of phospholipids. In the presence of minute traces of phospholipases, which hydrolyze enzymatically phospholipids, the LC-lipid interface is disintegrated. This event causes a change in orientation of the LC, which was followed in a polarized microscope. The lipase activity can be used to measure the cell viability, since members of this enzyme family are released by cells, as they undergo necrosis. The described sensor was used to monitor the presence of the lipases released from three different cell lines, which were either exposed to highly cytotoxic model compounds (sodium azide and paracetamol) or subjected to freeze-thaw cycles to induce cell death by a non-chemical based inducer for apoptosis, such as temperature. Finally, the comparison of lipase activity detected by a state-of-the-art fluorescence assay to the LC based system resulted in the superiority of the LC system concerning incubation time and sensitivity.

Identification and characterization of molecular targets of natural products by mass spectrometry

Natural products, and their derivatives and mimics, have contributed to the development of important therapeutics to combat diseases such as infections and cancers over the past decades. The value of natural products to modern drug discovery is still considerable. However, its development is hampered by a lack of a mechanistic understanding of their molecular action, as opposed to the emerging molecule-targeted therapeutics that are tailored to a specific protein target(s). Recent advances in the mass spectrometry-based proteomic approaches have the potential to offer unprecedented insights into the molecular action of natural products. Chemical proteomics is established as an invaluable tool for the identification of protein targets of natural products. Small-molecule affinity selection combined with mass spectrometry is a successful strategy to "fish" cellular targets from the entire proteome. Mass spectrometry-based profiling of protein expression is also routinely employed to elucidate molecular pathways involved in the therapeutic and possible toxicological responses upon treatment with natural products. In addition, mass spectrometry is increasingly utilized to probe structural aspects of natural products-protein interactions. Limited proteolysis, photoaffinity labeling, and hydrogen/deuterium exchange in conjunction with mass spectrometry are sensitive and high-throughput strategies that provide low-resolution structural information of non-covalent natural product-protein complexes. In this review, we provide an overview on the applications of mass spectrometry-based techniques in the identification and characterization of natural product-protein interactions, and we describe how these applications might revolutionize natural product-based drug discovery.

A simple activity staining protocol for lipases and esterases

A simple activity staining protocol for rapid detection and differentiation of lipases and esterases was developed based on pH drop due to fatty acids released following lipolysis. Though the detection of lipolysis as a function of drop in pH is not new, the present method has been made more sensitive by the judicious selection of the initial pH of the chromogenic substrate, which has been set near the end point of the dye so that even a slight drop in pH results in immediate color change. In the present case, the dye phenol red was taken, which has the end point at pH 7.3-7.4 where the color is pink. A slight drop due to fatty acid release results in yellow coloration. The assay has high reproducibility and can detect as low as 0.5 p-NPP enzyme units within 15 min. In addition, this method can be used for various lipidic substrates such as oils and tributyrin, making it suitable for both lipases and esterases.

Sensitive assay for hormone-sensitive lipase using NBD-labeled monoacylglycerol to detect low activities in rat adipocytes

The recent finding that p-nitrobenzofurazan (NBD)-FA is incorporated into and released from the acylglycerols of isolated rat adipocytes in an insulin-sensitive manner [G. Muller, H. Jordan, C. Jung, H. Kleine, and S. Petry. 2003. Biochimie. 85: 1245-1246] suggests that NBD-FA-labeled acylglycerols are cleaved by rat adipocyte hormone-sensitive lipase (HSL) in vivo. In the present study, we developed a continuous, sensitive in vitro lipase assay using a monoacylglycerol (MAG) containing NBD (NBD-MAG). NBD-MAG was found to provide an efficient substrate for rat adipocyte and human recombinant HSL. Ultrasonic treatment applied in the presence of phospholipids leads to the incorporation of NBD-MAG into the phospholipid liposomes and to a concomitant change of its spectrophotometric properties. The enzymatic release of NBD-FA and its dissociation from the carrier liposomes is accompanied by the recovery of the original spectrophotometric characteristics. The rate of lipolysis was monitored by measuring the increase in optical density at 481 nm, which was found to be linear with time and linearly proportional to the amount of lipase added. To assess the specific activity of recombinant HSL, we determined the molar extinction coefficient of NBD-FA under the assay conditions. This convenient assay procedure based on NBD-MAG should facilitate the search for small molecule HSL inhibitors.

Protein composition of oil bodies in Arabidopsis thaliana ecotype WS

Till now, only scattered data are available in the literature, which describes the protein content of plant oil bodies. Especially, the proteins closely associated with the model plant Arabidopsis thaliana oil bodies have never been previously purified and characterized. Oil bodies have been purified using flotation techniques, combined with incubations under high salt concentration, in the presence of detergents and urea in order to remove non-specifically trapped proteins. The identity and integrity of the oil bodies have been characterized. Oil bodies exhibited hydrodynamic diameters close to 2.6 microm, and a ratio fatty acid-protein content near 20. The proteins composing these organelles were extracted, separated by SDS-PAGE, digested by trypsin, and their peptides were subsequently analyzed by nano-chromatography-mass spectrometry (nano-LC-MS/MS). This led to the identification of a limited number of proteins: four different oleosins, ATS1, a protein homologous to calcium binding protein, a 11-beta-hydroxysteroid dehydrogenase-like protein, a probable aquaporin and a glycosylphosphatidylinositol-anchored protein with no known function. The two last proteins were till now never identified in plant oil bodies. Structural proteins (oleosins) represented up to 79% of oil body proteins and the 18.5 kDa oleosin was the most abundant among them.

Application of a specific and sensitive radiometric assay for microbial lipase activities in marine water samples from the lagoon of nouméa

Marine microbiologists commonly assay lipase activities by using a synthetic fluorescent analog, 4-methylumbelliferyl (MUF)-oleate. The technique is convenient, but it is considered to be unspecific because of the structure of this analog. This study reports the design of a new specific and sensitive lipase assay based on the use of a radiolabeled triglyceride, [3H]triolein. Free fatty acids (FFA) resulting from its hydrolysis are isolated as a function of time in a one-step liquid-liquid extraction and then radioassayed. MUF-oleate and [3H]triolein techniques were compared by measuring lipase activities at similar substrate concentrations along a trophic gradient in the Southwest Lagoon of New Caledonia, near Nouméa. Hydrolysis rates decreased from the nearshore station to the offshore station and showed similar trends regardless of the technique used. Rates decreased from 5.83 to 0.88 nmol of FFA. liter-1. h-1 and from 0.76 to 0.23 nmol of 3H-FFA. liter-1. h-1, respectively. These results appeared to be consistent with bacterial production results, which also decreased similarly (from 0.59 to 0.26 micrograms of C. liter-1. h-1). However, the ratio of MUF-oleate activities to [3H]triolein activities, which was constant at the offshore stations (3.8 +/- 0.1), gradually increased at the nearshore stations (from 4.1 to 7.6). This result shows that the two assays respond in different ways to changes in environmental conditions and validates the need to set up more specific enzymatic assays.

An ultraviolet spectrophotometric assay for measuring lipase activity using long-chain triacyglycerols from Aleurites fordii seeds

In this study, we designed a specific, continuous, and sensitive UV spectrophotometric lipase assay using natural triacylglycerols (TAGs) from the Aleurites fordii seed oil (tung oil). alpha-Eleostearic acid (9,11,13-cis, trans,trans-octadecatrienoic acid) is the main fatty acid component (it accounts for up to 70%) of the TAGs from tung oil. The conjugated triene present in alpha-eleostearic acid constitutes an intrinsic chromophore, which confers strong UV absorption properties on both the free fatty acid and the TAGs from tung oil. The lipase assay is based on the difference between the apparent molar extinction coefficients of the two types of alpha-eleostearic acid present, that which is esterified into TAGs and that which is released into the reaction medium. This difference is responsible for the variations in the UV absorption spectrum of the reaction medium occurring upon enzymatic TAGs hydrolysis. Using the purified lipase from Thermomyces lanuginosa (TLL) and the detergent sodium taurodeoxycholate (NaTDC, 4 mM), it was established that the most suitable method of measuring lipolysis consisted of monitoring the decrease in the OD at 292 nm, which was linear with time and proportional to the amount of lipase added. In order to be able to estimate the specific activity of TLL, we determined an apparent molar extinction coefficient of alpha-eleostearic acid (epsilon = 13,900 M(-1) cm(-1)) under the assay conditions. Amounts of pure TLL as small as 1 ng can be easily detected in the presence of 4 mM NaTDC. Interestingly, the NaTDC concentration can be decreased as far as 0.05 mM. In comparison with other well-known methods of lipase assay, the detection limit of this new method is 100-fold lower than with the pH-stat method and similar to that of a fluorescent assay recently developed at our laboratory.

Binding of Thermomyces (Humicola) lanuginosa lipase to the mixed micelles of cis-parinaric acid/NaTDC

The binding of Thermomyces lanuginosa lipase and its mutants [TLL(S146A), TLL(W89L), TLL(W117F, W221H, W260H)] to the mixed micelles of cis-parinaric acid/sodium taurodeoxycholate at pH 5.0 led to the quenching of the intrinsic tryptophan fluorescence emission (300-380 nm) and to a simultaneous increase in the cis-parinaric acid fluorescence emission (380-500 nm). These findings were used to characterize the Thermomyces lanuginosa lipase/cis-parinaric acid interactions occurring in the presence of sodium taurodeoxycholate. The fluorescence resonance energy transfer and Stern-Volmer quenching constant values obtained were correlated with the accessibility of the tryptophan residues to the cis-parinaric acid and with the lid opening ability of Thermomyces lanuginosa lipase (and its mutants). TLL(S146A) was found to have the highest fluorescence resonance energy transfer. In addition, a TLL(S146A)/oleic acid complex was crystallised and its three-dimensional structure was solved. Surprisingly, two possible binding modes (sn-1 and antisn1) were found to exist between oleic acid and the catalytic cleft of the open conformation of TLL(S146A). Both binding modes involved an interaction with tryptophan 89 of the lipase lid, in agreement with fluorescence resonance energy transfer experiments. As a consequence, we concluded that TLL(S146A) mutant is not an appropriate substitute for the wild-type Thermomyces lanuginosa lipase for mimicking the interaction between the wild-type enzyme and lipids.

Lipolytic activity of porcine pancreas lipase on fatty acid esters of dialkylglycerols: a structural basis for the design of new substrates for the assay of pancreatic lipases activity

For the design of new synthetic substrates for the assay of pancreatic lipases activity, acyl dialkylglycerols of variable chain length were prepared. Titrimetric assay of these substrates showed the highest lipolytic activity of porcine pancreas lipase (pPL) with butanoyl dibutylglycerol. The activity is lower but comparable to that shown by pPL towards the classical substrate tributyrin. The 4-nitrophenylcarbonate of 1,2-di-O-butylglycerol, has been prepared and proposed as synthetic substrate for a new spectrophotometric assay of pancreatic lipases.

Novel methods for biocatalyst screening

There have been a number of recent advances in catalysis assays applicable for screening biocatalyst libraries in high-throughput format. These include instrumental assays such as high-performance liquid chromatography, mass spectrometry, capillary electrophoresis and IR-thermography, reagent-based assays producing spectroscopic signals (UV/VIS or fluorescence) in response to reaction progress, and assays based on fluorogenic or chromogenic substrates. These fluorogenic substrates enable the assaying of a variety of enzymes in enantioselective and stereoselective manner, including alcohol dehydrogenases, aldolases, lipases, amidases, epoxide hydrolases and phosphatases.

Use of the tape stripping technique for directly quantifying esterase activities in human stratum corneum

The enzymes secreted in the intercellular spaces of stratum corneum (SC), the outermost layer of the epidermis, are thought to be involved in normal desquamation and skin barrier function. Their activity can barely be measured due to the difficulty in isolating enough biological material. Human SC layers were obtained from the forearm of healthy volunteers by the tape stripping technique. Assays for esterase activities were carried out in specially designed plates which contained the SC blotted on tape strips, using various fluorescent methylumbelliferone acyl esters as substrates. Triacylglycerol hydrolase activities were also studied by this method. By using radiolabeled triolein and fluorescent 4-methylumbelliferyl 7-oleate as substrates, true lipase activities could be detected and quantitated in SC at pH 5.5 and 7.5. These activities were shown to be strongly inhibited by tetrahydrolipstatin while this was not the case with 4-methylumbelliferyl 7-heptanoate. The method described here combines the painless tape stripping technique with a sensitive plate assay analysis. Since the whole process needs little manipulation, this method can permit rapid quantitation of multiple enzyme activities from a single strip. Therefore, it will permit the study of the involvement of enzyme activities in epidermis aging and skin pathologies.

A novel extracellular esterase from Bacillus subtilis and its conversion to a monoacylglycerol hydrolase

A novel gene lipB, which encodes an extracellular lipolytic enzyme, was identified in the Bacillus subtilis genomic DNA sequence. We have cloned and overexpressed lipB in B. subtilis and Escherichia coli and have also purified the enzyme from a B. subtilis culture supernatant to electrophoretic homogeneity. Four different lipase assays were used to determine its catalytic activity: pH-stat, spectrophotometry, fluorimetry and the monomolecular film technique. LipB preferentially hydrolysed triacylglycerol-esters and p-nitrophenyl-esters of fatty acids with short chain lengths of </= 10 carbon atoms. Triolein, which is a typical substrate for true lipases, was not hydrolysed at all. These results led us to classify LipB as an esterase rather than a lipase. The catalytic triad of LipB consists of residues Ser78, Asp134, and His157 as demonstrated by amino-acid sequence alignments and site-directed mutagenesis. The nucleophile Ser78 is located in a lipase-specific consensus sequence, which is Ala-X-Ser-X-Gly for most Bacillus lipases. All other bacterial lipases contain a glycine residue instead of the alanine at position-2 with respect to the catalytic serine. We have investigated the role of this alanine residue by constructing LipB variant A76G, thereby restoring the lipase-specific consensus motif. When compared with LipB this variant showed a markedly reduced thermostability but an increased stability at pH 5-7. Determination of the specific activities of wild-type LipB and variant A76G using a monomolecular film of the substrate monoolein revealed an interesting result: the A76G substitution had converted the esterase LipB into a monoacylglycerol hydrolase.