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PubMed1988

Membrane phospholipid turnover as an intermediary step in insulin secretion. Putative roles of phospholipases in cell signaling.

One or more phospholipases of the C and A2 types exist in rodent islets and may play a pivotal role in the cell signaling cascade culminating in exocytotic insulin release. Phospholipase C generates myo-inositol-1,4,5-trisphosphate, which mobilizes a "pool" of calcium in the endoplasmic reticulum and which may also secondarily facilitate calcium (Ca++) influx from the extracellular space to replenish that pool. Diacylglycerol is also generated by phospholipase C action and activates protein kinase C; it may thereby potentiate the cellular response to elevations in cytosolic free Ca++ concentration. Arachidonic acid may be released during the degradation of diacylglycerol and may also contribute to islet activation. Phospholipase C is activated by glucose, cholinergic agonists, and probably by Ca++ fluxes. Phospholipase A2 action generates arachidonic acid and lysophospholipids. Certain lysophospholipids mobilize cellular Ca++, at least in part from superficial, plasmalemmal stores. Native (unoxygenated) arachidonic acid also has the capability of mobilizing cellular Ca++ from membrane-bound stores; it may, in addition, activate protein kinase C, as suggested by recent indirect studies. The further metabolism of arachidonic acid via lipoxygenase and cyclo-oxygenase appears to provide positive and negative modulation, respectively, of stimulated insulin secretion. Many pieces of the puzzle remain, however, to be supplied. For example, it has not yet been unequivocally demonstrated that phospholipase A2 is activated by physiologic stimuli in intact islets. Furthermore, the absence of truly specific pharmacologic stimulators or inhibitors of these processes currently precludes precise delineation of the respective physiologic roles of each potential mediator in stimulus-secretion coupling. When such roles are elucidated, it can be asked whether the defects in insulin secretion in diabetes mellitus may be due in part to abnormalities in the turnover of beta-cell membrane phospholipids and the generation of intracellular lipid-derived signals.

Animals↗

PubMed1987

An enzymatic colorimetric assay of calcium-dependent phospholipases A.

In this paper we describe a new method for the assay of calcium-dependent phospholipases A. In this method released fatty acids are quantitated by an enzymatic colorimetric reagent kit which is commercially available. We have tested the applicability of this assay with enzymes from porcine pancreas (phospholipase A2), snake venom (phospholipase A2), and Rhizopus arrhizus (a lipase with phospholipase A1-like activity) as well as with a phospholipase A2 activity of bovine seminal vesicle fluid. We conclude that the kit procedure is a convenient, rapid and sensitive endpoint assay for calcium-dependent phospholipases A.

Calcium↗

PubMed1988

Involvement of a pertussis toxin-sensitive G-protein-coupled phospholipase A2 in lipopolysaccharide-stimulated prostaglandin E2 synthesis in cultured rat mesangial cells.

E. coli lipopolysaccharide (LPS) stimulated a dose- and time-dependent release of prostaglandin E2 (PGE2) in cultured rat glomerular mesangial cells. Pertussis toxin, an inhibitor of several GTP-binding proteins (G proteins), blocked nearly 80% of the LPS-stimulated PGE2 formation, while having virtually no effect on calcium ionophore-stimulated PGE2 production. We tested the possibility that a G protein-coupled activation of phospholipase A2 mediated the LPS-stimulated PGE2 production. Evidence for LPS activation of phospholipase A2 included a time-dependent LPS-induced generation of [32P]lysophosphatidylcholine and the inhibitory effects of a phospholipase A2 inhibitor, mepacrine, on LPS-induced PGE2 formation. Possible roles for phospholipase C-dependent activation of PGE2 synthesis by LPS seemed unlikely, as LPS did not elevate the cytosolic free calcium concentration or augment the appearance of water-soluble inositol phosphates. We conclude that LPS-induced PGE2 synthesis in rat glomerular mesangial cells is mediated through a G-protein-coupled phospholipase A2 activation. The activation of phospholipase A2 releases arachidonic acid and stimulates PGE2 synthesis preferentially, thereby improving glomerular hemodynamic events in endotoxemia.

Animals↗

PubMed1990

Stimulation of CTP: phosphocholine cytidylyltransferase and phosphatidylcholine synthesis by incubation of rat hepatocytes with phospholipase A2.

The effect of phospholipase A2 treatment of rat hepatocytes on CTP: phosphocholine cytidylyltransferase and phosphatidylcholine synthesis was investigated. Cytidylyltransferase is recovered from the cytosol and in a membrane-bound form with the microsomes. Digitonin treatment of cells causes rapid release into the medium of the cytosolic, but not the microsomal form of the cytidylyltransferase. Incubation of hepatocytes for 10 min with phospholipase A2 (0.9 units/dish) in the medium, resulted in a 33% decrease in the cytidylyltransferase activity released by digitonin treatment (2.5 +/- 0.15 nmol/min per mg compared to 3.9 +/- 0.10 nmol/min per mg in the control). In agreement with the digitonin experiments, incubation with 0.9 units/dish of phospholipase A2 resulted in a decrease in the cytidylyltransferase activity in the cytosol (from 4.3 +/- 0.10 nmol/min per mg to 2.6 +/- 0.14 nmol/min per mg) and a corresponding increase in the microsomal fraction (from 0.9 +/- 0.16 nmol/min per mg to 1.8 +/- 0.20 nmol/min per mg). The effect of phospholipase A2 on cytidylyltransferase translocation was concentration- and time-dependent. Incubation of hepatocytes in the presence of phospholipase A2 (0.9 units/dish) for 10 min prior to pulse-chase experiments resulted in an increase in radiolabel incorporation into phosphatidylcholine (from 2.4 +/- 0.02.10(-5) dpm/dish to 3.1 +/- 0.1.10(-5) dpm/dish) and a corresponding decrease in radiolabel associated with the choline (from 2.5 +/- 0.05.10(-5) to 1.4 +/- 0.03.10(-5) dpm) and phosphocholine fractions (from 8.5 +/- 0.07.10(-5) to 6.9 +/- 0.05.10(-5) dpm). We conclude that phospholipase A2 can cause a stimulation of CTP: phosphocholine cytidylyltransferase activity and phosphatidylcholine synthesis in cultured rat hepatocytes.

Animals↗

PubMed1986

Stabilization of synaptic membranes by alpha-tocopherol against the damaging action of phospholipases. Possible mechanism of biological action of vitamin E.

Using fluorescent and ESR-spin probes, the effects of phospholipases A2, C and D on rat brain synaptosomal membranes were investigated. It was shown that the exposure of synaptosomal membranes to phospholipases A2, C and D results in their depolarization and an increase of the negative surface potential. In the case of phospholipases A2 and C, these changes are associated with a decrease of the microviscosity of the membrane lipid bilayer. alpha-Tocopherol stabilizes synaptosomal membranes against the damaging action of the phospholipases. This stabilization consists in the reconstitution of the transmembrane potential and an increase of microviscosity of the phospholipase-treated membranes. The stabilizing effect of alpha-tocopherol is due to the binding of phospholipid hydrolysis products, but not to the inhibition of phospholipases. The observed stabilization of synaptosomal membranes by alpha-tocopherol is regarded as a possible mechanism of biological action of vitamin E on biomembranes.

Animals↗

PubMedDate not supplied

Hydrolysis of N-acylated glycerophospholipids by phospholipases A2 and D: a method of identification and analysis.

We have previously identified N-acylethanolamine phospholipids in infarcted dog heart and in normal fish brain by chemical and enzymatic degradation. We now report that hydrolysis with phospholipase D from Streptomyces chromofuscus removes N-acylethanolamine from N-acylethanolamine phospholipids and lyso N-acylethanolamine phospholipids, or N-acylserine from lyso N-acylserine phospholipids. At acidic pH, a phosphatase present in the phospholipase D preparation further hydrolyzes the resulting phosphatidic acid (PA) or lyso-PA to diacyl- or monoacylglycerol. Because N-acylserine phospholipids are a poor substrate for the phospholipase D, pretreatment with phospholipase A2 (Trimeresurus flavoviridis venom) is used to remove the 2-O-acyl group. Thus, both types of N-acylated phospholipids can be analyzed by consecutive phospholipase A2 and phospholipase D treatment. Reaction products, i.e., free fatty acids, monoacylglycerols and N-acylethanolamine or N-acylserine, are separable by thin-layer chromatography. Both N-acyl components can be further characterized by conversion to the t-butyldimethylsilyl derivatives. The method was used to identify and analyze the N-acylserine phospholipids of bovine brain.

Acylation↗

PubMed1997

Inhibitory effect on curcumin on mammalian phospholipase D activity.

Curcumin, the major yellow pigment of turmeric (Curcuma longa), has strong anti-carcinogenic and anti-inflammatory activities. We examined the effects of curcumin on enzyme activities of the following phospholipases in a cell-free system: G protein-mediated phospholipase D (PLD), phosphatidylinositol-specific phospholipase C, and phospholipase A2 from mouse macrophage-like cell line J774.1 cells, sphingomyelinase from bovine brain, and phosphatidylcholine-phospholipase C from Bacillus cereus. Curcumin inhibited several types of phospholipases, most effectively PLD among those tested. It also inhibited 12-O-tetradecanoylphorbol-13-acetate-induced PLD activation in intact J774.1 cells in a dose-dependent manner. These results suggest that the anti-inflammatory and anti-carcinogenic action of curcumin is partly due to the inhibition of PLD.

Animals↗

PubMed1987

Enzymatic hydrolysis of short-chain lecithin/long-chain phospholipid unilamellar vesicles: sensitivity of phospholipases to matrix phase state.

Short-chain lecithin/long-chain phospholipid unilamellar vesicles (SLUVs), unlike pure long-chain lecithin vesicles, are excellent substrates for water-soluble phospholipases. Hemolysis assays show that greater than 99.5% of the short-chain lecithin is partitioned in the bilayer. In these binary component vesicles, the short-chain species is the preferred substrate, while the long-chain phospholipid can be treated as an inhibitor (phospholipase C) or poor substrate (phospholipase A2). For phospholipase C Bacillus cereus, apparent Km and Vmax values show that bilayer-solubilized diheptanoylphosphatidylcholine (diheptanoyl-PC) is nearly as good a substrate as pure micellar diheptanoyl-PC, although the extent of short-chain lecithin hydrolysis depends on the phase state of the long-chain lipid. For phospholipase A2 Naja naja naja, both Km and Vmax values show a greater range: in a gel-state matrix, diheptanoyl-PC is hydrolyzed with micellelike kinetic parameters; in a liquid-crystalline matrix, the short-chain lecithin becomes comparable to the long-chain component. Both enzymes also show an anomalous increase in specific activity toward diheptanoyl-PC around the phase transition temperature of the long-chain phospholipid. Since the short-chain lecithin does not exhibit a phase transition, this must reflect fluctuations in head-group area or vertical motions of the short-chain lecithin caused by surrounding long-chain lecithin molecules. These results are discussed in terms of a specific model for SLUV hydrolysis and a general explanation for the "interfacial activation" observed with water-soluble phospholipases.

Bacillus cereus↗

PubMed1989

Kinetic characterization of Escherichia coli outer membrane phospholipase A using mixed detergent-lipid micelles.

The substrate specificity of Escherichia coli outer membrane phospholipase A was analyzed in mixed micelles of lipid with deoxycholate or Triton X-100. Diglycerides, monoglycerides, and Tweens 40 and 85 in Triton X-100 are hydrolyzed at rates comparable to those of phospholipids and lysophospholipids. p-Nitrophenyl esters of fatty acids with different chain lengths and triglycerides are not hydrolyzed. The minimal substrate characteristics consist of a long acyl chain esterified to a more or less hydrophilic headgroup as is the case for the substrate monopalmitoylglycol. Binding occurs via the hydrocarbon chain of the substrate; diacyl compounds are bound three to five times better than monoacyl compounds. When acting on lecithins, phospholipase A1 activity is six times higher than phospholipase A2 activity or 1-acyl lysophospholipase activity. Activity on the 2-acyl lyso compound is about two times less than that on the 1-acyl lysophospholipid. The enzyme therefore has a clear preference for the primary ester bond of phospholipids. In contrast to phospholipase A1 activity, phospholipase A2 activity is stereospecific. Only the L isomer of a lecithin analogue in which the primary acyl chain was replaced by an alkyl ether group is hydrolyzed. The D isomer of this analogue is a competitive inhibitor, bound with the same affinity as the L isomer. On these ether analogues the enzyme shows the same preference for the primary acyl chain as with the natural diester phospholipids. Despite its broad specificity, the enzyme will initially act as a phospholipase A1 in the E. coli envelope where it is embedded in phospholipids.

Cell Membrane↗

PubMed1987

Characterization of antigen association with accessory cells: specific removal of processed antigens from the cell surface by phospholipases.

To characterize the basis for the cell surface association of processed antigen with the antigen-presenting cell (APC) we analyzed its sensitivity to enzymatic digestion. Antigen-exposed APC that are treated with phospholipase and then immediately fixed lose their ability to stimulate antigen-plus-Ia-specific T-T hybridomas. This effect is seen with highly purified phospholipase A2 and phospholipase C. In addition it is observed with three distinct antigens--ovalbumin, bovine insulin, and poly(LGlu56LLys35LPhe9) [(GluLysPhe)n]. The effect of phospholipases is highly specific. Identically treated APC are equivalent to controls in their ability to stimulate alloreactive hybridomas specific for precisely the same Ia molecule that is corecognized by antigen-plus-Ia-specific hybrids. Furthermore, the antigen-presenting function of enzyme-treated, fixed APC can be reconstituted by the addition of exogenous in vitro processed or "processing independent" antigens. In parallel studies 125I-labeled avidin was shown to specifically bind to APC that were previously exposed and allowed to process biotin-insulin. Biotin-insulin-exposed APC that are pretreated with phospholipase bind significantly less 125I-labeled avidin than do untreated, exposed APC. Identical enzyme treatment does not reduce the binding of avidin to a biotinylated antibody already bound to class II major histocompatibility complex molecules of APC. At least some of the biotin-insulin surface sites are immunologically relevant, because the presentation of processed biotin-insulin by fixed APC is blocked by avidin. This effect is specific. Avidin binding to biotin-insulin-exposed APC does not inhibit allospecific stimulation nor the presentation of unconjugated insulin. These studies demonstrate that phospholipase effectively removes processed cell surface antigen.

Animals↗

PubMed1991

Phospholipase activity in the endometrium of women with normal menstrual blood loss and women with proven ovulatory menorrhagia.

The activity of phospholipase A2 types 1 and 2 and phospholipase C was measured in the endometrium of women with ovulatory menorrhagia and in those with normal menstrual blood loss. In both groups of subjects phospholipase A2 type 1 activity was significantly higher in the secretory phase than in the proliferative phase (P less than 0.001). The median activity (pmol/mg protein/min) for the proliferative phase was 27.6 in normal subjects and 40.4 in women with ovulatory menorrhagia and for the secretory phase the median activity was 144.5 in normal women and 138.1 in women with ovulatory menorrhagia. There was no difference between the two groups of women at either stage of the cycle. Phospholipase A2 type 2 activity was also higher in the secretory phase than in the proliferative phase (P less than 0.05 for normal subjects and P less than 0.001 for women with menorrhagia). The median activity (pmol/mg protein/min) for the proliferative phase was 94.4 (normal subjects) and 56.6 (women with menorrhagia) and for the secretory phase 148.3 (normal subjects) and 142.5 (women with menorrhagia). The activity of phospholipase A2 type 2 was significantly lower in the proliferative phase of women with ovulatory menorrhagia compared with normal subjects (P less than 0.05). Phospholipase C activity (nmol/mg protein/min) was significantly higher in women with ovulatory menorrhagia (median 8.2) compared with women with normal blood loss (median 5.5) (P less than 0.01).

Endometrium↗

PubMed1983

Effects of phospholipases on the kinetic properties of rat striatal membrane-bound tyrosine hydroxylase.

Rat striatal tyrosine hydroxylase can be isolated in both a soluble and a synaptic membrane-bound form. The membrane-bound enzyme, which exhibits lower Kms for both tyrosine (7 microM) and reduced pterin cofactor (110 microM) relative to the soluble enzyme (47 microM and 940 microM, respectively), can be released from the membrane fraction with mild detergent, and concomitantly its kinetic properties revert to those of the soluble enzyme. Treatment of membrane-bound tyrosine hydroxylase with C. perfringens phospholipase C increased the Km of the enzyme for tyrosine to 27 microM and the Vmax by 60% without changing the Km for cofactor. In contrast, treatment of membrane-bound tyrosine hydroxylase with V. russelli phospholipase A2 increased the Km for tyrosine to 48 microM, increased the Vmax, and increased the Km for cofactor to 560 microM. The enzyme remained bound to the membrane fraction following both phospholipase treatments. Addition of phospholipids to treated enzyme could partially reverse the effects of phospholipase A2 treatment, but not the effects of phospholipase C treatment. The kinetic properties of phospholipase-treated, detergent-solubilized tyrosine hydroxylase were identical to those of the control solubilized enzyme. Tyrosine hydroxylase appears to interact with synaptic membrane components to produce at least two separately determined consequences for the kinetic properties of the enzyme.

Animals↗

PubMed1987

Involvement of phospholipase A activity in the plasma membrane of the rat corpus luteum during luteolysis.

Plasma membrane samples prepared from corpora lutea (CL) of control and prostaglandin F2 alpha-treated rats were incubated with radiolabeled phosphatidylcholine for 90 min at 40 C with 1 mM CaCl2 to test for the presence of phospholipase A2 activity. At the end of the incubation period, labeled arachidonic acid cleaved from the 2 position of the phospholipid moiety had accumulated to 10.7 +/- 2.4% (mean +/- SE) of the initially added radioactivity in the prostaglandin F2 alpha-treated samples, with 3.7 +/- 1.2% appearing in the controls. Arachidonic acid production was inhibited by calcium chelation and by the phospholipase A2 inhibitor bromophenacyl bromide, indicating heightened activity of phospholipase A2 in CL plasma membranes undergoing regression. Unexpectedly, radiolabeled lysophosphatidylcholine was produced in regressed membrane samples at a similar rate, suggesting the induction of phospholipase A1 activity as well. To determine if the membrane rigidification that occurs with regressed membrane samples under the same incubation conditions is caused by the hydrolysis products of phospholipase activity, fluorescence polarization experiments with the probe trans-parinaric acid were conducted. Washing of incubated membrane samples with fatty acid-free BSA, which selectively removes free fatty acids and lysophosphatides from the bilayer, resulted in a restoration of the fluidity to levels recorded at the onset of the incubation. These results suggest that the previously described decreases in CL plasma membrane fluidity and hCG binding in vitro during luteolysis are caused by a synergistic effect of calcium ion and hydrolysis products of phospholipase A activity.

Animals↗

PubMed1981

Deacylation of dipalmitoyllecithin by phospholipases A in alveolar macrophages.

Optimal conditions in vitro were established for the assay of phospholipases A of Bacillus Calmette Guerin-induced alveolar macrophages that deacylated dipalmitoyllecithin in the alkaline pH range. Sodium deoxycholate and Ca++ were found to stimulate both phospholipases A1 and A2, whereas sodium dodecylsulfate strongly inhibited both enzyme activities. Other detergents like Triton X-100, Triton WR-1339, and Tween-20 activated phospholipase A1, whereas they had no effect on or slightly inhibited the phospholipase A2 enzyme. These phospholipases preferentially removed palmitic acid regardless of the position of palmitic acid on the phospholipid molecule and were more active on phosphatidylcholines than phosphatidylethanolamines. This suggests a role for macrophage phospholipases A in the clearance of saturated lecithins in the pulmonary surfactant complex.

Animals↗

PubMedDate not supplied

Phospholipase activity in human bile.

To investigate the importance of bacterial infection in the formation of free fatty acids found in brown pigment gallstones, free fatty acids and phospholipase activity in hepatic bile, with or without the presence of bacterial infection, were compared. The concentration of free fatty acids in bile with bacterial infection [0.467 +/- 0.447 mg per ml (mean +/- S.D.)] was significantly higher than when bacterial infection was absent (0.073 +/- 0.041 mg per ml; p less than 0.01). However, there was no significant difference in the composition of free fatty acids in hepatic bile when bacterial infection was present. Biliary phospholipase activity was determined by counting [14C] palmitic acid released from [14C]dipalmitoyl phosphatidylcholine that was incubated with native bile. The biliary phospholipase activity was significantly higher when bacterial infection was present. Furthermore, a positive correlation (p less than 0.001) was found between the activity of biliary phospholipases and the concentration of free fatty acids in hepatic bile. Most bacterial strains isolated from bile were shown to have both phospholipase A1 and A2 activity. On the other hand, human pancreatic juice and human gallbladder epithelial cells contained mainly phospholipase A2. Since fatty acids in the gallstone are mainly palmitic acid and must have been cleaved from first position in the biliary phosphatidylcholine molecule, bacterial phospholipase A1 seems to play an important role in the formation of calcium palmitate found in brown pigment gallstones.

Bacterial Infections↗

PubMed1993

Purification and immunological analysis of phospholipase D from castor bean endosperm.

Phospholipase D (EC 3.1.4.4) has been implicated in diverse cellular processes, but its physiological role is not well established in plants. In order to develop immunological and molecular biology approaches to address the problem, we report here the immunological analysis and N-terminal amino acid sequence of a cytosolic phospholipase D from castor bean (Ricinus communis L.). The enzyme was purified to apparent homogeneity from germinating castor bean endosperm. The specific activity of the purified enzyme was enhanced by approximately 670-fold with an overall yield of 4%. Its molecular mass was estimated at 92 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The N-terminal amino acid sequence of this enzyme was KLVENIEETVGFGKG. Polyclonal antibodies were raised against the purified enzyme. The antibodies inhibited the activity of transphosphatidylation more than that of hydrolysis of phospholipase D. The differential effect on the two activities of this enzyme implies that different active sites on this enzyme may be involved in the two reactions. Immunoblot analyses showed that the amounts of phospholipase D protein relative to the total endosperm proteins increased during the first 5 days of germination. The antibodies cross-reacted to proteins from several tested plant species, and those proteins had molecular masses similar to that of castor bean phospholipase D. These results indicate that the expression of phospholipase D in castor bean changes according to growth stages and that phospholipase D enzymes of different plant species are structurally related.

Amino Acid Sequence↗

PubMed1996

Structural heterogeneity of phospholipase D in 10 dicots.

The occurrence of multiple forms of phospholipase D (EC 3.1.4.4) was investigated in different tissues of castor bean (Ricinus communis) and in other plant species. Phospholipase D variants were resolved by nondenaturing and isoelectric focusing polyacrylamide gel electrophoresis and detected by immunoblotting using anti-phospholipase D antibodies and by enzyme activity assay. Three phospholipase D variants were produced differentially in the roots, endosperm, cotyledons, and hypocotyl of 5-day postgermination seedlings of castor bean. Furthermore, different patterns of phospholipase D variants were found in the different regions of hypocotyl (elongated and hook). Multiple phospholipase D forms were found in florets of cauliflower and broccoli, leaves of cabbage, celery, tomato, and potato, and alfalfa sprouts, suggesting that structural heterogeneity of phospholipase D occurs widely in plants.

Electrophoresis, Polyacrylamide Gel↗

PubMed1994

On the Ca(2+)-dependence of inositol-phospholipid-specific phospholipase C of microvillar photoreceptors from Sepia officinalis.

The breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) by the action of a phosphoinositide-specific phospholipase C is a key event in the signal transduction mechanism of microvillar photoreceptors. The enzymatic activity of phospholipase C from photoreceptor cell membranes of the cuttlefish Sepia officinalis was measured at different concentrations of free calcium. A bell-shaped curve with a maximum of enzyme activity at about 0.2 to 0.7 microM of free calcium was obtained. In the range of 10 to 90 microM free Ca2+, the activity of the phospholipase C decreased to about 30%. The activity of the phospholipase was also modulated by strontium ions. The presence of 10 microM Sr2+ strongly reduced the dependence of the phospholipase from a given calcium concentration. Similarly to 10 mM EGTA, the presence of 10 microM La3+ reduced the enzymatic activity more than ten-fold. A comparison of these values with the changes of the calcium concentration measured in ventral photoreceptor cells of Limulus polyphemus in the dark (approximately 0.5 microM Ca2+) and under illumination (up to 30-100 microM Ca2+) indicates that in microvillar photoreceptor cells an elevation of the calcium concentration in the physiological range reduces the activity of the phospholipase C. The IP3 produced by the action of phospholipase C is known to elevate the intracellular Ca2+ concentration transiently, which in turn will be able to reduce the further IP3 formation. A negative feedback in this early part of the enzyme cascade of vision in invertebrates may substantially contribute to the process of adaptation.

Animals↗