Synthesis of a naphthylvinyl-labeled glycerol ether analog of phosphatidylcholine and its use in the assay of phospholipase A2

A continuous fluorometric assay for phospholipase A(2) activity in brain cytosol

Alterations in phospholipase A(2) (PLA(2)) activity have been implicated in Alzheimer disease and other neurological disorders, although brain PLA(2) activity is currently measured using lengthy, non-continuous assays. We describe herein a rapid, continuous assay in which we measured PLA(2) activity in mouse brain cytosol (CB-57). Brains were homogenized in HEPES buffer (pH 7.5) and the cytosolic fraction was prepared by centrifugation at 25000xg for 20 min, followed by centrifugation of the supernatant at 100000xg for 60 min. Cytosolic protein content was determined using the Bradford assay. Pyrene labeled phosphatidylcholine was added to 50 microg of cytosolic protein in Tris buffer (pH 8.0) containing fatty acid free-bovine serum albumin for a final assay volume of 2 ml. Assay temperature was maintained at 30+/-1 degrees C. The excitation wavelength was 345 nm and emission was measured at 377 nm. Fluorescence intensity was converted to molar concentrations using a standard curve. Under these conditions, bromoenol lactone inhibited up to 58% of the PLA(2) activity with an IC(50) of 0.5 microM. In a separate experiment, lack of appreciable alternative acylhydrolase activity was verified chromatographically. Using this method, brain PLA(2) activity can be measured in a continuous, rapid, and sensitive manner.

Phospholipase A2: its usefulness in laboratory diagnostics

Phospholipase A2 (PLA2) is an enzyme that catalyzes the hydrolysis of membrane phospholipids. This article reviews the source and structure of PLA2, the involvement of the enzyme in various biological and pathological phenomena, and the usefulness of PLA2 assays in laboratory diagnostics. Of particular importance is the role of PLA2 in the cellular production of mediators of inflammatory response to various stimuli. Assays for PLA2 activity and mass concentration are discussed, and the results of enzyme determinations in plasma from patients with different pathological conditions are presented. The determination of activity and mass concentration in plasma is particularly useful in the diagnosis and prognosis of pancreatitis, multiple organ failure, septic shock, and rheumatoid arthritis. A very important result is the demonstration that PLA2 is an acute phase protein, like CRP. Indeed, there is a close correlation between PLA2 mass concentration and CRP levels in several pathological conditions. Although the determination of C-reactive protein is much easier to perform and is routinely carried out in most clinical laboratories, the assessment of PLA2 activity or mass concentration has to be considered as a reliable approach to obtain a deeper understanding of some pathological conditions and may offer additional information concerning the prognosis of several disorders.

Quantitative analysis of the kinetics of phospholipase A2 using fast atom bombardment mass spectrometry

Fast atom bombardment mass spectrometry that can directly analyze lysophospholipids was used to quantitatively determine the kinetics of phospholipase A2. This method is 1250 times more sensitive than the colorimetric assay.

Carbonothioate phospholipids as substrate for a spectrophotometric assay of phospholipase A2

A continuous spectrophotometric assay for phospholipase A2 (PLA2) was developed using novel carbonothioate phospholipids. These phospholipid analogues contain a carbonothioate bond in the place of the sn-2 ester of the natural substrates of phospholipase A2 and were synthesized in a one-pot two-step reaction. Phospholipase A2 from cobra venom (Naja naja atra) hydrolyzes carbonothioate phospholipids and liberates a free thiol, alkylmercaptan, which is reacted with 5,5'-dithiobis(2-nitrobenzoic acid) to yield a product that absorbs at 412 nm. The kinetic studies on PLA2 hydrolysis of carbonothioate phospholipids were carried out in pure phospholipid forms and in Triton X-100 mixed micelles. The hydrolysis of pure carbonothioate phospholipids exhibits an interfacial activation phenomenon. The hydrolysis of phospholipid in mixed Triton X-100 micelles follows classical Michaelis-Menten kinetics. In a mixed micellar system, the catalytic efficiency observed with this series of substrates is two orders of magnitude lower than that of the hydrolysis of the natural substrate dipalmitoyl phosphocholine. However, these substrates bind to the enzyme over 10 times tighter than does the natural substrate. Application of this carbonothioate assay to screen both reversible and irreversible enzyme inhibitors of phospholipase A2 is also demonstrated.

Measurement of phospholipase A2 and 1-alkylglycerophosphocholine acetyltransferase activities in stimulated alveolar macrophages by HPLC analysis of NBD-labeled ether lipids

The importance of phospholipases in cellular signaling and 1-alkylglycerophosphocholine acetyltransferase in the formation of platelet-activating factor (PAF) has stimulated demand for methods to measure these enzyme activities in inflammatory cells. Most of the assays currently used rely on radiolabeled substrates. We have synthesized NBD-labeled ether lipids as substrates for measuring enzyme activities of the PAF cycle and of lysosomal phospholipase A2 (PLA2). The fluorescent lipids were incubated with homogenates of stimulated bovine alveolar macrophages. The generated products were separated from the substrates by HPLC on a normal phase and monitored with a fluorescence detector. NBD-lyso-PAF was well accepted by acetyl- and acyltransferases of the cell-free preparations, which metabolized the substrate into NBD-PAF and NBD-alkyl-acylglycerophosphocholines. Homogenates of stimulated cells showed an enhanced production of NBD-PAF. The increased formation of the biological mediator was dependent on the nature of the stimuli and the time of stimulation. Lysosomal PLA2 was measured with 1-O-(12-NBD-aminododecyl)-2-acyl-sn-glycero-3-phosphocholine as substrate. By varying the pH and the calcium concentration, it was possible to distinguish between the cytosolic PLA2 and the lysosomal PLA2 activity. Optimal conditions for the determination of the lysosomal PLA2 were obtained at pH 4.5 and in the presence of EDTA. Stimulation with particulate agonists induced an enhancement of the lysosomal PLA2 activity in macrophages.

Critical analysis of phospholipid hydrolyzing activities in ripening tomato fruits. Study by spectrofluorimetry and high-performance liquid chromatography

Using the spectrofluorimetric method described by Wittenauer et al. [Wittenauer, L.A., Shirai, K., Jackson, R.L., and Johnson, J.D. (1984) Biochem. Biophys. Res. Commun. 118, 894-901] for phospholipase A2 (PLA2) measurement, we have detected a phospholipase activity in Ailsa Craig and in mutant rin tomatoes at their normal harvest time (mature green stage). This activity in Ailsa Craig tomatoes increased at the beginning of fruit ripening (green-orange stage) and then decreased slowly. The decrease in activity, however, was greater when ripening occurred after tomato picking at normal harvest time than when ripening occurred on tomato plants. This phospholipase activity was always higher in rin tomatoes than in normal ones. Thin-layer chromatography of compounds obtained after incubation of tomato extract demonstrated a decrease in the substrate 1-acyl-2-(6[(7-nitro-2,1,3 benzoxadiazol-4-yl)amino]-caproyl)-sn-glycero-3-phosphocholine (C6-NBD-PC), and an increase in one product (NBD-aminohexanoic acid), but failed to detect the second product (1-acyl-sn-glycero-3-phosphocholine). We, therefore, developed a new one-step method for separation and quantification of a mixture of phospholipids and other lipids, using straight-phase-high-performance liquid chromatography with light-scattering detection. This method detected another fatty acid-releasing activity in enzyme extract from green-orange tomatoes. This lipolytic enzyme (or family of enzymes) slowly produced free fatty acids when 1-oleoyl-sn-glycero-3-phosphocholine was added as substrate. The production of fatty acids was stoichiometric and more rapid when 1-oleoyl-sn-glycero-3-phosphate and 1-oleoyl-sn-glycerol were used as substrates.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of C6-NBD-PC for assaying phospholipase A2-activity: scope and limitations

Determination of phospholipase A2 (PLA2) activity is of special interest in view of the abundance of this enzyme in various organelles of all cells, and its role in many cell functions, especially in eicosanoid production. Assaying PLA2 activity is therefore of special importance to cell biology. However, it is a complicated and non-trivial task for several reasons including the critical dependence of PLA2 activity on the physical state of the lipid substrate, the complex kinetics of its action, the low activity of most intercellular and membrane-bound enzymes and the metabolism of the fatty acid products, when applied to intact cell. In recent years the fluorescent analogue of phosphatidylcholine, C6-NBD-PC, has been used for determination of the activity of soluble and membrane-bound PLA2. In the present study we evaluate the use of this method for continuous monitoring of PLA2 activity, based on time-dependent changes in the fluorescence intensity which results from the hydrolysis of C6-NBD-PC into NBD-caproic acid and lysolecithin. The fluorescence intensity of aggregated C6-NBD-PC is reduced due to self-quenching which is maximal in systems containing no additional lipids, when NBD-PC forms micelles. In these systems the hydrolysis increase the fluorescence intensity due to de-quenching, since the NBD caproic acid products dissolves in water in the form of monomers. In contrast, in the presence of additional lipids (mixed micelles, membrane vesicles, lipid emulsion particles or lipoproteins), the probe partitions into lipidic compartments where its fluorescence is only partially quenched (if at all) and its quantum yield is much higher. Consequently, the hydrolysis is accompanied by a decrease in fluorescence. The time course of this change is a complex function of the additional lipid concentration(s) and of physical processes which follow the hydrolysis. Due to this complexity, the assay of PLA2 activity by continuous monitoring of fluorescence is ambiguous. Furthermore, the rate of NBD-PC hydrolysis is very different from that of the 'host' lipid bilayer or monolayer and is less sensitive to the physical state of the lipids. Under various conditions it follows very different kinetics, depending on the ratio of NBD-PC to the host PC. Therefore, it can not be used as a general assay for PLA2 in lipid-containing systems.

Assay strategies and methods for phospholipases

Of the general considerations discussed, the two issues which are most important in choosing an assay are (1) what sensitivity is required to assay a particular enzyme and (2) whether the assay must be continuous. One can narrow the options further by considering substrate availability, enzyme specificity, assay convenience, or the presence of incompatible side reactions. In addition, the specific preference of a particular phospholipase for polar head group, micellar versus vesicular substrates, and anionic versus nonionic detergents may further restrict the options. Of the many assays described in this chapter, several have limited applicability or serious drawbacks and are not commonly employed. The most commonly used phospholipase assays are the radioactive TLC assay and the pH-stat assay. The TLC assay is probably the most accurate, sensitive assay available. These aspects often outweigh the disadvantages of being discontinuous, tedious, and expensive. The radioactive E. coli assay has become popular recently as an alternative to the TLC assay for the purification of the mammalian nonpancreatic phospholipases. The assay is less time consuming and less expensive than the TLC assay, but it is not appropriate when careful kinetics are required. Where less sensitivity is needed, or when a continuous assay is necessary, the pH-stat assay is often employed. With purified enzymes, when free thiol groups are not present, a spectrophotometric thiol assay can be used. This assay is approximately as sensitive as the pH-stat assay but is more convenient and more reproducible, although the substrate is not available commercially. Despite the many assay choices available, the search continues for a convenient, generally applicable assay that is both sensitive and continuous. The spectrophotometric SIBLINKS assay and some of the fluorescent assays show promise of filling this need.

A facile asymmetric synthesis of glycerol phospholipids via tritylglycidol prepared by the asymmetric epoxidation of allyl alcohol. Thiolester and thioether analogs of phosphatidylcholine

Trityl-glycidol was synthesized by in situ derivatization of glycidol, which was prepared by the catalytic asymmetric epoxidation of allyl alcohol. Depending on the enantiomer of diisopropyl tartrate used with the titanium catalyst, either (R)- or (S)-trityl-glycidol was obtained in a "one pot" synthesis in about 50% overall yield. The optical purity, determined by NMR spectroscopy of a Mosher ester, was greater than 98% ee. Nucleophilic opening of the chiral epoxide with dodecyl mercaptan gave optically active 1-S-dodecyl-3-O-trityl-1-thio-glycerol, which was used to synthesize 1-S-dodecyl-2-O-decanoyl-thio-sn-glycero-3-phosphocholine. Opening of the epoxide with methyl xanthate gave a 1,2-trithiocarbonate derivative of trityl glycerol which can be used to synthesize 1,2-bis(S-decanoyl)-1,2-dithio-sn-glycero-3-phosphocholine. Opening of the epoxide with thiodecanoic acid gave 1-S-decanoyl-3-O-trityl-1-thio-glycerol which was used to synthesize 1-S-decanoyl-2-O-decanoyl-1-thio-sn-glycero-3-phosphocholine.

Evaluation of fluorescent and colored phosphatidylcholine analogs as substrates for the assay of phospholipase A2

Two fluorescent phospholipid analogs, 1-O-[12-(2-naphthyl)-dodec-11-enyl]-2-O-decanoyl-sn-glycero-3-phos phocholine and dansyl-PC [rac 1-O-(N-dansyl-11- amino-1-undecyl)-2-O-decanoyl-glycero-3-phosphocholine], and a red-colored analog, dabsyl-PC [rac 1-O-(N-dabsyl-11-amino-1-undecyl)-2-O-decanoyl-glycero-3- phosphocholine], were evaluated as substrates for the assay of phospholipase A2 (PLA2) (Crotalus adamanteus). The assay reaction was monitored by separation of the fluorescent or colored substrate and product by TLC and quantitation by fluorescence or absorption scanning. All three substrates gave similar (within an order of magnitude) activities with PLA2. Dansyl-PC was best suited to this TLC assay. Dabsyl-PC was less sensitive to detection by absorbance, but had the advantage of being red colored and readily detected by the unaided eye.

Fluorophore-labeled ether lipids: substrates for enzymes of the platelet-activating factor cycle in peritoneal polymorphonuclear leukocytes

Cell-free preparations of ionophore-stimulated peritoneal rat polymorphonuclear neutrophils (PMNs) incubated with 1-(N-dansyl-11-amino-1-undecyl)-sn-glycerol-3-phosphorylcholine (dansyllyso-PAF) converted this fluorescent lyso ether lipid into two different classes of products. In the absence of acetyl-CoA 1-(N-dansyl-11-amino-1-undecyl)-2-long chain acyl-sn-glycerol-3-phosphorylcholine (dansylalkyl-2-acyl-GPC) was the only identified new fluorescent phospholipid. In the presence of acetyl-CoA an additional new product, 1-(N-dansyl-11-amino-1-undecyl)-2-acetyl-sn-glycerol-3-phosphorylcholine (dansyl-PAF), was formed. The formation of dansyl-PAF in PMN homogenates was only transient with a maximum after about 4 min. When PMN homogenates were incubated with dansyl-PAF the formation of dansyllyso-PAF was observed prior to the formation of dansyl-2-acyl-GPC. Thus, our data indicate that enzymatically formed dansyl-PAF is completely remodeled into dansylalkyl-2-acyl-GPC by the sequential action of PAF acetylhydrolase and CoA-independent transacylase. These results demonstrate that peritoneal rat PMNs contain lyso-PAF acetyltransferase, PAF acetylhydrolase, and CoA-independent transacylase and that fluorophore-labeled ether lipids provide an easy means to assay enzymes which catalyze important enzymatic reactions involved in the biosynthesis and remodeling of platelet-activating factor.

Spin-label assay for phospholipase A2

A rapid and continuous method for measuring phospholipase A2 activity using electron spin resonance spectroscopy and a spin-labeled phospholipid as a substrate has been developed. The substrate, 1-palmitoyl-2-(4-doxylpentanoyl)glycerophosphocholine, gives rise principally to a broad ESR line in aqueous solution due to strong spin-spin interactions, probably resulting from its micellar formation. Upon addition of bee venom phospholipase A2, the water-soluble product, 4-doxylpentanoic acid, is released which brings about a sharp three-line spectrum. Thus, the kinetics of phospholipase A2 activity can be followed by monitoring the increase in the ESR signal amplitude of the three-line spectrum, which is linearly proportional to the amount of 4-doxylpentanoic acid produced; no separation of the product from the substrate is needed during the measurement. The rate of hydrolysis of 1 nmol min-1 can be accurately measured within a 5-min period of time in a sample volume of 100 microliters. This new method should be useful for assaying phospholipase A2 activities in various biological systems.