1-O-alkyl-linked phosphoglycerides of human platelets: distribution of arachidonate and other acyl residues in the ether-linked and diacyl species

Ether lipid deficiency disrupts lipid homeostasis leading to ferroptosis sensitivity

Ferroptosis is an iron-dependent form of regulated cell death associated with uncontrolled membrane lipid peroxidation and destruction. Previously, we showed that dietary dihomo-gamma-linolenic acid (DGLA; 20: 3(n-6)) triggers ferroptosis in the germ cells of the model organism, Caenorhabditis elegans. We also demonstrated that ether lipid-deficient mutant strains are sensitive to DGLA-induced ferroptosis, suggesting a protective role for ether lipids. The vinyl ether bond unique to plasmalogen lipids has been hypothesized to function as an antioxidant, but this has not been tested in animal models. In this study, we used C. elegans mutants to test the hypothesis that the vinyl ether bond in plasmalogens acts as an antioxidant to protect against germ cell ferroptosis as well as to protect from whole-body tert-butyl hydroperoxide (TBHP)-induced oxidative stress. We found no role for plasmalogens in either process. Instead, we demonstrate that ether lipid-deficiency disrupts lipid homeostasis in C. elegans, leading to altered ratios of saturated and monounsaturated fatty acid (MUFA) content in cellular membranes. We demonstrate that ferroptosis sensitivity in both wild type and ether-lipid deficient mutants can be rescued in several ways that change the relative abundance of saturated fats, MUFAs and specific polyunsaturated fatty acids (PUFAs). Specifically, we reduced ferroptosis sensitivity by (1) using mutant strains unable to synthesize DGLA, (2) using a strain carrying a gain-of-function mutation in the transcriptional mediator MDT-15, or (3) by dietary supplementation of MUFAs. Furthermore, our studies reveal important differences in how dietary lipids influence germ cell ferroptosis versus whole-body peroxide-induced oxidative stress. These studies highlight a potentially beneficial role for endogenous and dietary MUFAs in the prevention of ferroptosis.

Lipidomic dysregulation within the lung parenchyma following whole-thorax lung irradiation: Markers of injury, inflammation and fibrosis detected by MALDI-MSI

Radiation-induced lung injury (RILI) is a delayed effect of acute radiation exposure that can limit curative cancer treatment therapies and cause lethality following high-dose whole-thorax lung irradiation (WTLI). To date, the exact mechanisms of injury development following insult remain ill-defined and there are no FDA approved pharmaceutical agents or medical countermeasures. Traditionally, RILI development is considered as three phases, the clinically latent period, the intermediate acute pneumonitis phase and the later fibrotic stage. Utilizing matrix-assisted laser desorption ionization mass spectrometry imaging, we identified a number of lipids that were reflective of disease state or injury. Lipids play central roles in metabolism and cell signaling, and thus reflect the phenotype of the tissue environment, making these molecules pivotal biomarkers in many disease processes. We detected decreases in specific surfactant lipids irrespective of the different pathologies that presented within each sample at 180 days post whole-thorax lung irradiation. We also detected regional increases in ether-linked phospholipids that are the precursors of PAF, and global decreases in lipids that were reflective of severe fibrosis. Taken together our results provide panels of lipids that can differentiate between naïve and irradiated samples, as well as providing potential markers of inflammation and fibrosis.

Microtubule formation and activities of antioxidative enzymes in PC12 cells exposed to phosphatidylcholine hydroperoxides

Aging increases free radical generation and lipid oxidation and, thereby, mediates neurodegenerative diseases. As the brain is rich in lipids (polyunsaturated fatty acids), the antioxidative system plays an important role in protecting brain tissues from oxidative injury. The changes in microtubule formation and antioxidative enzyme activities have been investigated in rat pheochromocytoma PC12 cells exposed to various concentrations of phosphatidylcholine hydroperoxides (PCOOH). We measured three typical antioxidative enzymes, superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT). The microtubule assembly system was dependent on the antioxidative enzyme system in cells exposed to oxidative stress. The activities of the three enzymes increased in a PCOOH exposure-dependent manner. In particular, the changes in the activity as a result of PCOOH exposure were similar in the three antioxidative enzymes. This is the first report indicating the compatibility between the tubulin-microtubule and antioxidative enzyme systems in cells that deteriorate as a result of phospholipid hydroperoxide administration from an exterior source. The descending order of sensitivity of the three enzymes to PCOOH is also discussed.

Structural characterization of plasmenylcholine photooxidation products

Oxidative damage to plasmenyl-type lipids contributes to decreased membrane barrier function, loss of membrane structure and formation of nonlamellar defects in membrane bilayers. Previous results from this laboratory have shown that membrane-soluble sensitizers (e.g. zinc phthalocyanine and bacteriochlorophyll a) mediate the photooxidation of palmitoyl plasmenylcholine (1-O-alk-1'-Z-enyl-2-palmitoyl-sn-glycero-3-phosphocholine; PPlsC) vesicles with the subsequent creation of lamellar defect structures, vesicle contents leakage and membrane-membrane fusion. Because plasmalogen lipids are significant components of sarcoplasma and myelin membranes, we sought to characterize the products of their photooxidation. This study focuses on the photooxidation of PPlsC vesicles in the presence of the water-soluble sensitizer, aluminum phthalocyanine tetrasulfonate (AlPcS4(4-)). Attack of photogenerated singlet oxygen on the 1-O-alkenyl ether linkage of PPlsC lipids was expected to generate dioxetane- and ene-type photoproducts. The products formed during continuous aerobic irradiation (28 mW/cm2, (610 nm) of PPlsC vesicles in the presence of AlPcS4(4-) were separated via reverse-phase high-performance liquid chromatography (HPLC) with electrochemical detection (ECD) or evaporative light-scattering detection (ELSD). Photooxidized dipalmitoyl-phosphatidylcholine-cholesterol vesicles (control) were used to optimize the HPLC-ECD conditions, using 7 alpha-hydroperoxy-cholesterol as standard. HPLC-ECD was found to be most sensitive for PPlsC hydroperoxides, whereas HPLC-ELSD was more sensitive for nonhydroperoxide photoproducts. The three major photoproducts formed during vesicle irradiation were isolated via preparative HPLC and then characterized by 1H-nuclear magnetic resonance and mass spectrometry. 1-Formyl-2-palmitoyl-sn-glycero-3-phosphocholine and 1-hydroxy-2-palmitoyl-sn-glycero-3-phosphocholine were identified as dioxetane cleavage products that coeluted at approximately 3 min. The second fraction (retention time [RT] = 48 min) was identified as a PPlsC allylic hydroperoxide. The third photoproduct, eluting at RT = 64 min, is tentatively identified as an oxidation product arising from allylic hydroperoxide degradation via Hock rearrangement or free radical decomposition.

Role of monocytes in atherogenesis

This review focuses on the role of monocytes in the early phase of atherogenesis, before foam cell formation. An emerging consensus underscores the importance of the cellular inflammatory system in atherogenesis. Initiation of the process apparently hinges on accumulating low-density lipoproteins (LDL) undergoing oxidation and glycation, providing stimuli for the release of monocyte attracting chemokines and for the upregulation of endothelial adhesive molecules. These conditions favor monocyte transmigration to the intima, where chemically modified, aggregated, or proteoglycan- or antibody-complexed LDL may be endocytotically internalized via scavenger receptors present on the emergent macrophage surface. The differentiating monocytes in concert with T lymphocytes exert a modulating effect on lipoproteins. These events propagate a series of reactions entailing generation of lipid peroxides and expression of chemokines, adhesion molecules, cytokines, and growth factors, thereby sustaining an ongoing inflammatory process leading ultimately to lesion formation. New data emerging from studies using transgenic animals, notably mice, have provided novel insights into many of the cellular interactions and signaling mechanisms involving monocytes/macrophages in the atherogenic processes. A number of these studies, focusing on mechanisms for monocyte activation and the roles of adhesive molecules, chemokines, cytokines and growth factors, are addressed in this review.

Alterations in arachidonic acid metabolism in mouse mast cells induced to undergo maturation in vitro in response to stem cell factor

We studied arachidonic acid (AA) metabolism during the maturation of bone marrow-derived cultured mast cells (BMCMCs) into mast cells with phenotypic characteristics, which were more similar to those of connective tissue-type mast cells. BMCMCs were maintained in medium containing 100 ng/ml recombinant rat stem cell factor (SCF) for 1 to 6 weeks. After 3 to 4 weeks in SCF, BMCMCs acquired many phenotypic characteristics of maturation, including enlarged size, numerous electron-dense cytoplasmic granules, and a 50-fold elevation in histamine content. Maintenance in SCF for 6 weeks did not significantly alter the amounts or species of eicosanoids that were produced by BMCMCs stimulated with calcium ionophore A23187. However, SCF-treated mast cells released 2.6 +/- 0.13 times more free AA and accumulated 6.4 +/- 1.0 times higher levels of intracellular free AA than did immature BMCMCs not exposed to SCF. There was no increase in the mobilization of other fatty acids (e.g., linoleic or oleic acid), indicating specificity for AA. Moreover, there were no differences between the 5-lipoxygenase activities of SCF-treated or untreated cells, as assayed in cell homogenates prepared by nitrogen cavitation. Although the total AA content in SCF-treated cells was significantly elevated, the distribution of AA in phospholipid and neutral lipid classes was not altered by SCF treatment. Total phospholipase (PL)A2 activity increased 85% +/- 11.5% in SCF-treated cells. In homogenates of immature BMCMCs, 51.0% +/- 13.7% of the PLA2 activity was inhibited by 0.5 mmol/L dithiothreitol, whereas the same concentration of dithiothreitol caused only a 2.2% +/- 10.7% reduction in the PLA2 activity in homogenates of SCF-treated BMCMCs (p < or = 0.05, n = 4). These findings suggest that SCF treatment induces a dithiothreitol-resistant PLA2 and that this PLA2 may contribute to the mobilization of AA that is not further metabolized to eicosanoids.

Comparison of acceptor and donor substrates in the CoA-independent transacylase reaction in human neutrophils

In human neutrophils (PMN) the ethanolamine-containing phosphoglyceride fraction (PE), principally plasmalogen-linked PE (1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine), is the major store of arachidonic acid (AA). Exogenous AA is initially incorporated into 1-acyl-linked phosphoglycerides and is believed to be transferred into the 1-ether-linked phosphoglycerides via the action of a CoA-independent transacylase (CoA-IT). We have investigated the selectivity for both the "acceptor' lysophospholipids and "donor' AA-containing phospholipid substrates in the CoA-IT reaction. Evidence suggests CoA-IT may also participate in the synthesis of platelet activating factor. The transfer of [3H]AA from endogenously labeled choline-containing phosphoglycerides (PC) to exogenously added alkenyl-lyso-PE (0-50 microM) was examined in saponin-permeabilized PMN. In these "donor' studies, we observed that [3H]AA was transferred from both alkyl- and diacyl-linked PC in a proportional manner. More detailed molecular species analysis showed that [3H]AA was deacylated from all the major AA-containing molecular species in both the alkyl and diacyl subclasses with no selectivity for either subclass. To investigate the "acceptor' selectivity, membrane fractions prelabeled with either [3H]alkyl-arachidonoyl-PE or -PC were utilized as donor substrates. Various unlabeled lysophospholipids (10 microM) were added and the generation of [3H]lyso-PE or -PC was monitored as a measure of CoA-IT activity. Significant subclass preference was observed upon addition of lyso-PE species (1-alkenyl > 1-alkyl > 1-acyl) however, little selectivity was seen with the corresponding lyso-PC species. On the other hand, lysophosphatidylserine, lysophosphatidylinositol, and lysophosphatidic acid all served as poor acceptor substrates in the reaction. These data from PMN are consistent with other evidence that the CoA-IT plays a pivotal role in the enrichment of AA into plasmalogen-linked PE.

The cleavage of plasmenylethanolamine by phospholipase A2 appears to be mediated by the low affinity binding site of the TxA2/PGH2 receptor in U46619-stimulated human platelets

Two TxA2/PGH2 receptor binding sites linked to different effector systems have recently been identified. Since plasmenylethanolamine represents the major phospholipid reservoir of arachidonic acid (AA) in resting human platelets, we assessed the differential role of these binding sites on plasmenylethanolamine hydrolysis by phospholipase A2 activity upon platelet activation by determining the generation of the corresponding [3H]lysoplasmenylethanolamine. Ethanolamine-containing phospholipids in platelets were pre-labelled with [3H]ethanolamine prior to platelet stimulation with U46619 (1 microM), a TxA2 mimetic, in the presence or absence of S-145, an antagonist of the low affinity TxA2/PGH2 receptor. Labelled platelets were also treated with the TxA2/PGH2 receptor antagonist, GR32191B, prior to washing (which blocks the low affinity site of the receptor) and subsequent stimulation. The above conditions provided for blockage of platelet aggregation but not shape change with U46619. The rise in [3H]lysoplasmenylethanolamine accumulation (170% of unstimulated controls) with U46619 as the agonist was inhibited in platelets pre-treated with S-145 and in platelets washed from GR32191B. Similar findings were also obtained for [3H]lysophosphatidylethanolamine accumulation. The present results indicate that the TxA2-dependent activation of plasmenylethanolamine cleavage by phospholipase A2 in intact human platelets is predominantly linked to the low affinity site of the TxA2/PGH2 receptor and may be important for platelet aggregation but not shape change.

Transfer of pyrene-labelled diacyl-, alkylacyl-, and alkenylacyl-glycerophospholipids from vesicles to human blood platelets

The present study was aimed at investigating the spontaneous transfer of fluorescently labelled serine- and choline-glycerophospholipids from unilamellar vesicles to resting human blood platelets. The most effectively transferred phospholipids were pyrene-phosphatidylserine (PS) and the ether analogues of choline-glycerophospholipids, e.g., pyrene-alkylacyl- and pyrene-1'-alkenylacyl-glycerophosphocholines (plasmalogens). Transfer of pyrene-diacyl-glycerophosphocholine and pyrene-phosphatidic acid was almost not detectable under the same experimental conditions. The fast intermembrane PS-transfer could be explained by the very high degree of adsorption of PS donor vesicles to the platelet plasma membrane. The short halftime of transfer rate (12-14 min) and the high incorporation (1.08-2.16% of total platelet glycerophosphocholines) observed for ether choline-phospholipids in contrast to pyrene-PS (20 min, 0.8% of total platelet PS), could be interpreted in terms of their bulk membrane properties.

The platelet-activating factor precursor of the injured cornea is selectively implicated in arachidonate and eicosanoid release

The purpose of this study was to isolate the platelet-activating factor (PAF) precursor and other choline phosphoglycerides (GPC) i.e. the alkenylacyl and diacyl lipids from the rabbit cornea, to analyze their fatty acid content and to determine which pool was the most susceptible to arachidonate depletion when activated corneal tissue released arachidonic acid (AA) and metabolites. Rabbit iridal GPC was also analyzed for comparative purposes. The fatty acid methyl esters of the GPC components extracted from the rabbit cornea and iris-ciliary body, isolated by high performance liquid chromatography (HPLC), were determined by capillary gas liquid chromatography. Rabbit corneas were labelled in vivo by intracameral injection of 3H-AA (1 microCi, specific activity = 218 Ci/mmol) and cryogenically injured 18 h later. Corneas were incubated in vitro and the AA and eicosanoids released into the medium were extracted and separated by HPLC. The GPC was extracted from the tissues and the labeling of the three GPC constituents was quantified by liquid scintillation counting. The corneal and iridal PAF precursor represented 4.1 +/- 0.2% and 2.9 +/- 0.2% respectively of total GPC in those tissues. On a mole basis, the alkyl arachidonoyl species constituted 12.7 +/- 0.7% of the corneal and 38 +/- 0.6% of the iridal PAF precursors respectively. The release of AA and prostaglandins by the cornea was linear until 15 min; whereas 12-HETE levels continuously increased until 60 min. All GPC components lost label but 1-O-alkyl-2-arachidonoyl was the most affected, with its labeled content 50% less than the non-injured control.(ABSTRACT TRUNCATED AT 250 WORDS)

Eicosanoid/thromboxane A2-independent and -dependent generation of lysoplasmenylethanolamine via phospholipase A2 in collagen-stimulated human platelets

Collagen-induced human platelet stimulation is dependent on the release of arachidonic acid (AA) from membrane phospholipid and the formation of thromboxane A2 (TxA2) for TxA2-induced platelet activation. Since plasmenylethanolamine represents the single major phospholipid reservoir of AA in resting human platelets, we assessed its hydrolysis via phospholipase A2 upon platelet stimulation with low levels of collagen by determining the generation of [3H]lysoplasmenylethanolamine via eicosanoid/TxA2-independent and -dependent processes. Ethanolamine phospholipids in platelets were prelabelled with [3H]ethanolamine before stimulation with either collagen or the TxA2 mimetic U46619, in the presence or absence of BW755C, a dual inhibitor of the cyclooxygenase/lipoxygenase activities, or GR32191B, a TxA2-receptor antagonist. Collagen stimulation promoted a marked generation of [3H]lysoplasmenylethanolamine, which was only moderately decreased when TxA2 synthesis or TxA2 receptors were blocked by BW755C or GR32191B respectively. The moderate rise in [3H]lysoplasmenylethanolamine formation with U46619 as the agonist was only slightly affected by BW755C and blocked by GR32191B. Evidence for eicosanoid/TxA2-independent and -dependent generation of [3H]lysophosphatidylethanolamine was also obtained. A significant quantitative loss of AA from plasmenylethanolamine was also demonstrated in collagen-stimulated platelets. The present findings indicate the activation of plasmenylethanolamine cleavage via phospholipase A2 in collagen-stimulated human platelets, which, to a considerable extent, does not depend on eicosanoid/TxA2 synthesis. This may represent an important source of releasable AA for TxA2 generation and the promotion of further liberation of AA and phospholipid-mediated signalling pathways.

Incorporation and turnover of eicosapentaenoic and docosahexaenoic acids in human blood platelets in vitro

Mass changes in the incorporation of linoleic (C18:2), eicosapentaenoic (C20:5) and docosahexaenoic (C22:6) acids in human blood platelet phospholipids were induced by incubating the cells and these fatty acids complexed to albumin. The remodelling of [14C]C18:2, [14C]C20:5 and [14C]C22:6 in classes, subclasses and molecular species of platelet phospholipids was studied in resting and thrombin-stimulated cells. More than 85% of the incorporation was located in phospholipids, representing 5-fold and 2.5-fold increases in the phospholipid C20:5 and C22:6 endogenous content respectively. Thrombin stimulation induced a 30% degradation of 1-acyl-2-C20:5-glycerophosphocholine (GPC) and 1-acyl-2-C22:6-GPC, but did not induce significant release of C18:2 from 1-acyl-2-C18:2-GPC. There was no change in the [14C]fatty acid composition of 1-alkyl-2-acyl-GPC. Thrombin-dependent increases in 1-alkenyl-2-C20:5-glycerophosphoethanolamine (GPE) and 1-alkenyl-2-C22:6-GPE of 2.1-fold and 2.5-fold respectively accounted for the rise in GPE radioactivity and partly compensated for the loss of these fatty acids from 1,2-diacyl-GPC: transfer to 1-alkenyl-2-acyl-GPE was 0.4 and 1.5 nmol/10(9) platelets for C20:5 and C22:6 respectively. [14C]C20:5 and [14C]C22:6 were incorporated into six different species of 1,2-diacyl-GPC, with acylation in the major endogenous forms (C18:1 +C16:0 and C18:0 species) representing 76% and 66% respectively of the total radioactivity present in 1,2-diacyl-GPC. Stimulation by thrombin induced significant release of these fatty acids from the main molecular species of 1,2-diacyl-GPC, but significantly stimulated the synthesis of alkenyl forms of GPE containing C18:1/C22:6 +C16:0/C22:6, C18:0/C22:6 and C18:0/C20:5. C18:0/C18:2, the major endogenous C18:2 molecular species, represented only 10.5% of the incorporation; none of the [14C]C18:2 molecular species was a substrate for transfer towards 1-alkenyl-2-acyl-GPE. It is concluded that when C20:5 and C22:6, but not C18:2, are acylated in 1,2-diacyl-GPC, they participate in thrombin-dependent phospholipid remodelling, and might compete with the turnover and release of arachidonic acid from platelet phospholipids and the subsequent activation of the cells.

Studies on the selectivity of enzymes involved in platelet-activating factor formation in stimulated cells

The present studies were undertaken to obtain further insight into the selectivities of the enzymes, i.e., phospholipase A2 and acetyltransferase, involved in platelet-activating factor (PAF) production upon stimulation of human polymorphonuclear leukocytes (PMN) and platelets. After appropriate stimulation of the cells in the presence of [3H]acetate the total PAF and analogs, i.e., 1-alkyl-2-acetyl-, 1-alkenyl-2-acetyl-, and 1-acyl-2-acetyl-glycero-3- phosphocholine were isolated by high performance liquid chromatography. The isolated mixture was subjected to treatment with phospholipase A1 to differentiate acetate incorporation into 1-ether linked and 1-ester linked species. The ratio of acetate incorporation into 1-ether linked vs 1-ester linked PAF analogs amounted to 13.8 +/- 1.0 and 1.3 +/- 0.1 for PMN and platelets, respectively. When compared to the ratio of 1-ether linked and 1-ester linked species in the diradylglycerophosphocholine precursors in each cell type, i.e., 1.13 for PMN and 0.22 for platelets, these data suggested a pronounced selectivity for the phospholipase A2 and/or acetyltransferase in the process of PAF production. When the experiments were repeated with cells that had been pretreated with phenylmethanesulfonylfluoride (PMSF) to block the acetylhydrolase, the most dramatic effects were observed on acetate incorporation into 1-acyl-2-acetyl-glycero-3-phosphocholine, which increased much more than that into 1-alk(en)yl-2-acetyl-glycero-3-phosphocholine. Under these conditions, the ratio of acetate incorporation into 1-ether linked vs 1-ester linked PAF analogs became 1.4 +/- 0.2 and 0.17 +/- 0.02 for PMN and platelets, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Recent advances in our understanding of the biochemical interactions between platelet-activating factor and arachidonic acid

In the last few years, it has become increasingly apparent that the biochemistry of PAF (platelet-activating factor) and that of arachidonic acid are interrelated in a number of inflammatory cells. Experiments presented here further point out that arachidonic acid plays a crucial role in the catabolism and biosynthesis of PAF. In addition, they suggest that the same phospholipid molecular species may serve as a source for both arachidonic acid and 1-alkyl-2-lyso-sn-glycero-3-phosphocholine during cell activation. Finally, they reveal that there may be common regulatory mechanisms for the biosynthesis of PAF and arachidonic acid metabolites. Taken together, studies examining the relationship between PAF and arachidonic acid suggest it may be difficult to consider the biochemistry of PAF without considering arachidonic acid metabolism and vice versa.

Phospholipid composition of rat megakaryocytes and its rearrangement in platelets

Rat platelets and their megakaryocyte precursors were examined for phospholipid composition. (1) The phospholipid composition of rat megakaryocytes, which were enriched and prepared from bone marrow cells, was almost identical to that of platelets. (2) The subclass composition of choline-containing glycerophospholipids (CGP) of rat megakaryocytes differed significantly from that of platelets: 1-alkenyl-2-acyl glycerophosphocholine (GPC) in megakaryocytes accounted for 29% of the total, whereas that in platelets was only 7%. (3) Rat platelets contained a larger amount of arachidonic acid than megakaryocytes, especially in ethanolamine-containing glycerophospholipids (EGP). (4) [32P]Phosphoric acid was significantly incorporated into megakaryocytes, whereas platelets showed little incorporation. On the other hand, the uptake of [3H]arachidonic acid into platelet phospholipids was about 15-times higher than that observed with megakaryocytes. (5) As reported previously for other blood cells, such as neutrophils and macrophages, the radioactivity of labeled arachidonic acid incorporated into CGP of platelets decreased, whereas that incorporated into EGP increased during a subsequent chase period. Hardly any such change was observed with megakaryocytes. These results suggest that the phospholipid composition of rat platelets is mainly determined at the time of thrombopoiesis, whereas the composition of molecular species is remodeled during circulation after thrombopoiesis.

Thrombin induces a biphasic 1,2-diacylglycerol production in human platelets

The 1,2-diacylglycerol (DAG) mass content was measured in thrombin-stimulated human platelets. Thrombin stimulates a biphasic accumulation of DAG, with an early phase reaching a peak at 10 s and a later phase reaching a peak at 2-3 min. The time course of first-phase DAG production corresponded well to that of Ins(1,4,5)P3 formation, which was rapid and transient. The second phase of DAG accumulation occurred after the level of Ins(1,4,5)P3 returned to nearly basal. Thrombin stimulated the decrease in PtdIns and phosphatidylcholine contents. The source of second-phase DAG was examined in platelets prelabelled with three radioactive fatty acids, i.e. arachidonic, palmitic and myristic. Thrombin stimulated the increase in radioactivity of DAG with decline of PtdIns in platelets labelled with [3H]arachidonic acid or [3H]palmitic acid, in which PtdIns was considerably labelled. In contrast, significant accumulation of [3H]DAG was not observed in [3H]myristic acid-labelled platelets, in which PtdIns was poorly labelled. In platelets prelabelled with [3H]inositol, an increase in InsP in response to thrombin was seen for more than 5 min. In contrast, upon stimulation, significant increases in [3H]phosphocholine and [3H]choline were not observed in [methyl-3H]choline-labelled platelets. Thrombin induced a small production of phosphatidylethanol, when ethanol was present during stimulation. However, the formation of DAG and phosphatidic acid was not significantly affected by ethanol. These results suggest that thrombin stimulates a biphasic accumulation of DAG, initially from PtdInsP2 and later from PtdIns in human platelets.

Proton nuclear magnetic resonance studies on the molecular dynamics of plasmenylcholine/cholesterol and phosphatidylcholine/cholesterol bilayers

Physiologically relevant molecular species of plasmenylcholine and phosphatidylcholine were synthesized and their molecular dynamics and interactions with cholesterol were compared by determination of salient proton spin-lattice relaxation times and apparent activation energies for 1H-NMR observable motion. The molecular dynamics of PA PhosCho (1-hexadecanoyl-2-eicosatetra-5',8',11',14'-enoyl-sn-glycero-3-pho sphocholine) in multiple regions of the bilayer. Furthermore, the fluidity gradient of PA PhosCho was larger than that of PA PlasCho as ascertained by 1H spin-lattice relaxation time measurements. Introduction of cholesterol into each bilayer resulted in disparate effects on the dynamics of each subclass including: (1) increased motional freedom in the polar head group of PA PlasCho without substantial alterations in the dynamics of the polar head group of PA PhosCho; and (2) increased immobilization of the membrane interior in PA PlasCho in comparison to PA PhosCho. Analysis of Arrhenius plots of T1 relaxation times demonstrated that the apparent activation energies for vinyl and bisallylic methylene proton NMR observable motion in PA PhosCho were greater than that in PA PlasCho. Thus, comparisons of spin-lattice relaxation times and apparent activation energies demonstrate that vesicles comprised of PA PlasCho and PA PhosCho possess differential molecular dynamics and distinct interactions with cholesterol. Collectively, these results underscore the significance of the conjoint presence of the vinyl ether linkage and arachidonic acid as an important determinant of membrane dynamics in specialized mammalian membranes.

Eicosanoid-dependent and -independent formation of individual [14C]stearoyl-labelled lysophospholipids in collagen-stimulated human platelets

Low level collagen activation of platelets is mediated via the release of arachidonic acid (AA) from membrane phospholipids and the formation of thromboxane A2 (TxA2). To assess the specific phospholipids undergoing deacylation via phospholipase A2 thereby providing source(s) of releasable AA, we have measured the individual lysophospholipid formations in platelets prelabelled with [14C]stearic acid and incubated with a low level (2 micrograms/ml) or a high level (10 micrograms/ml) of collagen in the absence or presence of BW755C, a dual inhibitor of cyclooxygenase and lipoxygenase activities. Collagen activation resulted in the generation of [14C]stearoyl-labelled lysophosphatidylinositol (lysoPI), lysophosphatidylcholine (lysoPC), lysophosphatidylethanolamine (lysoPE) and lysophosphatidylserine. BW755C significantly inhibited these collagen-induced changes, suggesting that much of the lysophospholipid, and therefore AA release, was eicosanoid-mediated. At the lower level of collagen, considerable generation of [14C]lysoPE was maintained even in the presence of BW755C, suggesting an eicosanoid-independent degradation of phosphatidyl-ethanolamine. The TxA2-dependent release of AA was also investigated in U-46619-stimulated platelets. This TxA2 mimetic induced considerable formation of the 14C-labelled lysophospholipids, including lysoPI and lysoPC, but not lysoPE. These results suggest that an eicosanoid-independent degradation of phosphatidylethanolamine via phospholipase A2 at lower collagen levels may provide a source of the initial AA for conversion to TxA2 and the subsequent deacylation of phosphatidylinositol, phosphatidylcholine, and also phosphatidylserine.

The coumarin derivative AD6 inhibits the release of arachidonic acid by interfering with phospholipase A2 activity in human platelets stimulated with thrombin

AD6 is a coumarin derivative which is able to inhibit platelet aggregation and release due to various agonists as adrenaline, PAF, Ca++ ionophore and others. It has been demonstrated that this compound reduces the production of free arachidonate and diglyceride from human platelets pulse-labeled with radioactive arachidonic acid thus suggesting a possible interference with the activity of phospholipase A2 and/or phospholipase C. The present report indicates that the drug has no effect on the increase of the labeling of phosphatidic acid which takes place when platelets pulse-labeled with arachidonic acid are stimulated with thrombin. Furthermore, AD6 is not able to cause changes on the metabolism of phosphoinositides monitored using platelets pre-labeled with [3H] inositol. These observations exclude the possibility that AD6 interferes with phospholipase C activity. Experiments with platelets pulse-labeled with arachidonate suggest that AD6 inhibits phospholipase(s) A2 activity or modulate negatively one or more processes involved in its activation.

Synthesis of platelet-activating factor by human blood platelets and leucocytes. Evidence against selective utilization of cellular ether-linked phospholipids

Synthesis of platelet activating factor (PAF) in blood platelet suspensions may be due to leucocyte contamination. We therefore investigated PAF synthesis in human blood platelet suspensions and granulocyte- (PMN)-enriched leucocyte suspensions upon stimulation by thrombin and Ca2+-ionophore A23187, both in the presence and absence of the presumed PAF catabolism inhibitor phenylmethylsulfonyl fluoride (PMSF). PAF synthesis was measured by aggregation of washed rabbit platelets and by [3H]acetate incorporation. In contrast to A23187, thrombin was unable to stimulate PAF synthesis by leucocytes. As thrombin did induce PAF synthesis by platelet suspensions, this was evidently not due to leucocyte contamination. A23187 also induced PAF synthesis by platelets, but this was dependent upon the platelet isolation method and possibly associated activation. The ratio of [3H]acetate incorporation into 1-alkyl- versus 1-acyl-2-acetylglycerophosphocholine upon stimulation of non-PMSF-treated leucocytes and platelets amounted to 12.8 and 1.2, respectively. These values are at least 10-fold higher than the ratio of 1-alkyl versus 1-acyl species in the cellular phosphatidylcholine precursor for PAF. By PMSF pretreatment, the distribution of incorporated [3H]acetate between 1-ether- and 1-ester-linked species became similar to that in the precursor phosphatidylcholines of the respective cell type, due to increased recovery of [3H]acetate in the acyl compounds. Both leucocyte and platelet homogenates rapidly degraded acylacetylglycerophosphocholine to (acetyl)glycerophosphocholine, and this deacylation was inhibited by PMSF pretreatment of the cells. We conclude that upon cell stimulation a phospholipase A2 converts both alkylacylglycerophosphocholine and diacylglycerophosphocholine to the 2-lysoanalogs in a ratio similar to the occurrence of the parent compounds. The acetyltransferase subsequently acetylates both compounds to acylacetylglycerophosphocholine and alkylacetylglycerophosphocholine (PAF), respectively. Deacylation of the 1-ester-linked species, either before or after acetylation, gives the impression of selective utilization of 1-ether-linked species for PAF production. It is only after inhibition of the deacylation by pretreatment of the cells with PMSF that a mainly nondiscriminative use of 1-ether- and 1-ester-linked species by both phospholipase A2 and acetyltransferase becomes evident.

Potential phospholipid source(s) of arachidonate used for the synthesis of leukotrienes by the human neutrophil

The present study has employed two approaches to address the question of whether there are specific phospholipid sources of arachidonate used for leukotriene biosynthesis in the human neutrophil. Firstly, g.c.-m.s. analysis indicated that arachidonate was lost from all major arachidonate-containing phospholipid subclasses during cell activation with ionophore A23187. On a molar basis, the rank order of breakdown among the three major phospholipids was: 1-alk-1-enyl-2-arachidonoyl-sn-glycero-3-phosphoethanolamine greater than 1-alkyl-2-arachidonoyl-sn-3-phosphocholine greater than 1-acyl-2-arachidonyl-sn-3-phosphoinositol. Leukotrienes released into the supernatant fluid accounted for only 10-35% of the total arachidonate depletion. Phospholipid sources were also identified in labelling experiments where the specific radioactivity of arachidonate in phospholipid subclasses, as well as leukotrienes produced during cell activation, was measured. The specific radioactivity of arachidonate within 1-acyl-linked molecular species of phosphatidylcholine and phosphatidylinositol was initially high relative to the leukotrienes and decreased rapidly with stimulation. By contrast, the specific radioactivity of arachidonate in all three subclasses of phosphatidylethanolamine, 1-acyl, 1-alkyl, and 1-alk-1-enyl, was 3-5-fold below that of the leukotrienes throughout cell activation. Of the six major arachidonate-containing subclasses, only in the case of 1-O-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine did the specific radioactivity correlate well with that of leukotriene B4 and 20-hydroxyleukotriene B4. These data strongly suggest that 1-ether-linked phospholipids are an important source of arachidonate used for leukotriene biosynthesis.

Turnover of eicosanoid precursor fatty acids among phospholipid classes and subclasses of cultured human umbilical vein endothelial cells

Using cultured human umbilical vein endothelial cells, in which phosphatidylcholine (PC) is equally pulse-labelled by various eicosanoid precursor fatty acids (EPFAs), we have studied the remodelling of EPFAs among the phospholipid classes and subclasses with and without activation, and the relationship of this remodelling process to the selective release of arachidonic acid (AA) by phospholipase A2-mediated cell stimulation. When endothelial cells are pulse-incubated with radiolabelled EPFA for 15 min, greater than 80% of cell-associated radioactivity is present in phospholipids, among which greater than 60% is found in 1,2-diacyl-sn-glycero-3-phosphocholine (diacyl PC). After removing unincorporated radioactivity, reincubation of the pulse-labelled cells for up to 6 h results in progressive decrease in EPFA-labelled diacyl PC, increase in AA- or eicosapentaenoic acid (EPA)-labelled 1-O-alk-1-enyl-2-acyl-sn-glycero-3-phosphoethanolamine (plasmalogen PE) and increase only in AA-labelled 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine (alkyl PC). This redistribution of radiolabelled phospholipids is not altered by the presence of excess non-radiolabelled EPFAs. When aspirin-treated EPFA-labelled endothelial cells are stimulated with ionophore A23187, a very selective release of AA is noted in comparison with eicosatrienoate (ETA) or EPA, accompanied by an equivalent decrease in AA-labelled diacyl PC and specific increase in AA-labelled plasmalogen PE and alkyl PC. These selective changes in AA radioactivity induced by A23187 are enhanced 2-fold by pretreating the AA-labelled cells with phorbol 12-myristate 13-acetate, which by itself induces no changes. The changes in radioactivity induced by A23187 without and with phorbol ester among the released AA, the diacyl PC and the plasmalogen PE are significantly correlated with each other. These results indicate that human endothelial cells incorporate EPFAs (AA, ETA, EPA) equally into diacyl PC but selectively release AA esterified into diacyl PC with specific remodelling into plasmalogen PE and alkyl PC.

Phorbol diester enhances calcium ionophore A23187-induced [3H]acetate incorporation into platelet-activating factor in murine macrophages: predominant incorporation into 1-O-acyl-2-acetyl-sn-glycero-3-phosphocholine

Pretreatment of macrophages with 12-O-tetradecanoylphorbol-13-acetate (TPA) has been shown to enhance the release of arachidonic acid from cell phospholipids in response to agonist stimulation. This study describes the ability of TPA to also alter calcium ionophore A23187-induced incorporation of [3H]acetate into platelet activating factor (PAF). Cultured murine peritoneal macrophages were preincubated with [3H]acetate (25 muCi) and TPA (10 ng/ml) for 10 min, and subsequently incubated with 0.1 microM A23187 for 0.5-10 min. Buffer and cells were then extracted and PAF resolved by normal-phase HPLC. Sequential exposure to TPA and A23187 resulted in a greatly enhanced incorporation (11,861 dpm/10(6) cells) of [3H]acetate into PAF compared to TPA alone, which did not significantly influence [3H]acetate incorporation into PAF, and 0.1 microM A23187, which induced minimal incorporation (688 dpm/10(6) cells). Macrophage-produced [3H]PAF was resolved by HPLC, extracted, treated with phospholipase-C, and acetylated to facilitate quantitation of 1-O-alkyl-2-acetyl-GPC (PAF) from 1-O-acyl-2-acetyl-GPC (acylPAF). A23187 alone (1 microM) produced 72% 1-O-acyl-2-[3H]acetyl-GPC, and A23187 (0.1 microM) following TPA pretreatment produced 81% 1-O-acyl-2-[3H]acetyl-GPC. Less than 2% of the radioactivity of acylPAF was in the acyl moiety. These data support a role for protein kinase C in modulating agonist-induced PAF synthesis. The results also suggest that acetyltransferase of murine macrophages does not possess specificity for 1-O-alkyl-2-lyso-GPC, and that availability of specific species of lyso-phospholipid may determine the type of PAF produced.

Selective hydrolysis of ether-containing glycerophospholipids by phospholipase A2 in rabbit lung

The role of phospholipase A2 (PLA2) in the simultaneous generation of lyso-platelet-activating factor and arachidonic acid was investigated by examining the calcium dependency and substrate specificity of PLA2 activities in rabbit lung microsomes. Alkylarachidonoylglycerophosphocholine (alkylarachidonoyl-GPC) was preferentially hydrolyzed as compared to acylarachidonoyl-GPC, and both arachidonate-containing substrates were cleaved to a greater extent as compared to alkyl- and acyl-substrates with oleate at the sn-2 position. Hydrolysis of alkylacyl-GPC substrates was not dependent on calcium in the presence of EGTA (1 mM); however, addition of calcium (2 mM) increased hydrolysis of acylarachidonoyl-GPC 2-fold and hydrolysis of acyloleoyl-GPC 10-fold. Substitution of an alkenyl group in the sn-1 position further enhanced calcium-independent PLA2 hydrolysis, and another substitution of arachidonic acid at the sn-2 position of the plasmalogen substrates substantially increased hydrolysis as compared to hydrolysis of substrates containing oleic acid. Hydrolysis of the choline plasmalogen was 3-fold greater than hydrolysis of the ethanolamine plasmalogen containing arachidonate. Preferential calcium-independent hydrolysis of alkylacyl-GPC substrates was observed in several tissues, including adult and fetal rabbit lung and adult rabbit kidney and human amnion. PLA2 substrate specificity may account for the preferential hydrolysis of arachidonoyl-containing alkyl-GPC in several cell types and explain the simultaneous generation of the precursors of two potent autacoids, platelet-activating factor and eicosanoids.

A simplified approach to the analysis of subclasses of phospholipids: application to human platelets

A procedure for the determination of the proportions of diacyl, alkenylacyl and alkylacyl subclasses of glycerophospholipids was developed. The procedure involves: (1) acid methanolysis of the phospholipid followed by Bligh/Dyer extraction of fatty acid methyl esters (FAME) derived from acyl chain types, dimethylacetals (DMA) derived from alkenyl ether chain types, and lysoalkyl phosphatidic acids (lysoalkyl-PA) derived from alkyl ether chain types; and (2) subsequent acetolysis to convert the lysoalkyl-PA to monoalkyl glycerol diacetates (MAGD). GLC analysis and quantitation (using internal standard, 21:0 FAME) of FAME, DMA and MAGD allowed calculation of the proportions of the three molecular subclasses. The methanolysis/acetolysis procedure gave an overall mean phospholipid recovery of 95 +/- 3%. Analysis of the major phospholipids in four separate preparations of fresh resting human platelets by this procedure showed the following range of molecular subclasses: phosphatidylcholine (PC), 86-92 mol % diacyl, 6-10 mol % alkylacyl and 2-3 mol % alkenylacyl; and phosphatidylethanoline (PE), 39-60 mol % diacyl, 5-8 mol % alkylacyl and 34-55 mol % alkenylacyl. The results of these subclass analyses were in general agreement with those reported in the literature.

Hydrolysis of 1-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine, a common precursor of platelet-activating factor and eicosanoids, by human platelet phospholipase A2

The metabolism of platelet-activating factor (PAF) and arachidonic acid is linked through the common intermediate 1-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine (alkylarachidonoyl-GPC). Hydrolysis of alkylarachidonoyl-GPC by phospholipase A2 may initiate the biosynthesis of both PAF and eicosanoids, since alkyllyso-GPC is formed for acetylation to PAF and arachidonic acid is liberated for conversion to biologically active metabolites. In order to elucidate the regulation and functional role of human platelet phospholipase A2 in the pathway leading to the formation of both classes of lipid mediators, we have characterized its action upon alkylarachidonoyl-GPC. Human platelet phospholipase A2 was solubilized and then partially purified in the presence of n-octyl-beta-D-glucopyranoside (octyl glucoside). Hexadecylarachidonoyl-GPC was prepared biosynthetically using platelet sonicates, purified by two-step high-performance liquid chromatography (HPLC) and suspended in buffer by sonication. Our results indicate that deacylation of alkylarachidonoyl-GPC by platelet phospholipase A2 has an absolute requirement for Ca2+. It occurs at submicromolar concentrations of free Ca2+ and exhibits a biphasic Ca2+-dependence with activity plateaus at 10 microM and 2 mM. Phospholipase A2-mediated hydrolysis of alkylarachidonoyl-GPC is increased 2-fold by albumin and is enhanced 5-fold if 1,2-dioleoylglycerol is incorporated into the substrate dispersion. The substrate dependence and specificity of platelet phospholipase A2 for 1-alkyl- vs. 1-acyl-linked subclasses of arachidonic acid containing phosphatidylcholine was examined with 1-O-hexadecyl-2-arachidonoyl-sn-glycero-3-phosphocholine (hexadecylarachidonoyl-GPC) and 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine (palmitoylarachidonoyl-GPC). We found that the substrates were deacylated equivalently. We conclude that, in stimulated platelets, in the presence of increased levels of cytoplasmic free Ca2+ and newly generated diacylglycerol, alkylarachidonoyl-GPC may be rapidly hydrolyzed by phospholipase A2 and may serve as a precursor of both PAF and eicosanoids.

The alkenylacyl class of ethanolamine phospholipid represents a major form of eicosapentaenoic acid (EPA)-containing phospholipid in the platelets of human subjects consuming a fish oil concentrate

A quantitative assessment was made of the distribution of arachidonic acid (AA), eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), and docosahexaenoic acid (DHA) among the individual sub-classes (diacyl, alkylacyl, alkenylacyl) of the various platelet phospholipids of human subjects consuming a fish oil concentrate (as MaxEPA) enriched in EPA plus DHA. This work was of interest since dietary fish oils provide for a pronounced enrichment of platelet phospholipid in EPA as well as DPA plus DHA to a lesser extent while reducing AA-phospholipid and platelet reactivity. After 42 days of MaxEPA supplementation (providing 3.6 g EPA and 2.4 g DHA per day), the majority of the mass of all four polyunsaturated fatty acids in the choline-containing phospholipid (PC) was found in the diacyl species (76-87% of total PC). In contrast, twice as much of the EPA in the ethanolamine-containing phospholipid (PE) resided in the alkenylacyl species (62.7% of total PE) relative to the diacyl species (32.3%) with minor amounts in the alkylacyl fraction (5.1%). The major single reservoir of total EPA-containing phospholipid was the alkenylacyl PE (38.2% of total) followed by diacyl PC (27.8%), diacyl PE (19.6%), with progressively lesser amounts in the alkylacyl PC, alkylacyl PE, diacyl PS (phosphatidylserine), alkenylacyl PC, and diacyl PI (phosphatidylinositol) at 6.7, 3.1, 2.3, 1.4, and 0.8% of the total mass, respectively. In contrast, only 27.1% of the total mass of AA-phospholipid was represented by the alkenylacyl PE with the bulk of the remainder being distributed in the diacyl species of PE, PC, PI, and PS.(ABSTRACT TRUNCATED AT 250 WORDS)

Preferential incorporation of eicosanoid precursor fatty acids into human umbilical vein endothelial cell phospholipids

We have examined the preferential incorporation of specific fatty acids into phospholipid classes of cultured human umbilical vein endothelial cells. Pulse-labeling of human umbilical vein endothelial cell phospholipids with radiolabeled fatty acids and inhibition of radiolabeled fatty acid incorporation by competition with excess, unlabeled fatty acids in pair-wise combinations revealed two distinct classes of esterification systems into human umbilical vein endothelial cell phospholipids. The eicosanoid precursor fatty acids, including arachidonate, 8,11,14-eicosatrienoate (ETA) and 5,8,11,14,17-eicosapentaenoate (EPA), exhibited high affinity incorporation into total phospholipids, whereas other fatty acids, including docosahexaenoate and monohydroxy eicosatetraenoates, showed low affinity incorporation. The relative degree of incorporation of eicosanoid precursor fatty acids into phospholipid classes was phosphatidylcholine (PC) greater than phosphatidylethanolamine (PE) greater than phosphatidylinositol (PI) greater than phosphatidylserine (PS). The specific activity of [14C]arachidonic acid-labeled PI was two times higher than that of any other radiolabeled phospholipids. When competitive incorporation of eicosanoid precursor fatty acids into phospholipid classes was studied, they were found to be acylated into different phospholipid classes at different rates. Although eicosanoid precursor fatty acids were not preferentially incorporated into PC, arachidonic acid was preferentially incorporated into the other phospholipids and exhibited particular selectivity in comparison with the other eicosanoid precursor fatty acids for incorporation into PI. These results demonstrate that human umbilical vein endothelial cells possess selective incorporation mechanisms for specific fatty acids into various phospholipids via the deacylation-reacylation pathway.

Platelet-activating factor (PAF-acether) formation in rat basophilic leukemia cells stimulated by ionophore A23187

In rat basophilic leukemia cells (2 H3-RBL) stimulated with the calcium ionophore A23187, a rapid build-up of PAF-acether was observed within 5 minutes. Thereafter, a slow and complete catabolism was observed within the next 55 minutes. Accumulation of PAF-acether required calcium in the medium and was increased in the presence of acetyl-CoA. Phenyl methyl sulfonyl fluoride, a serine hydrolase inhibitor active on 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine acetyl hydrolase also produced an increased accumulation of PAF-acether in ionophore-stimulated cells. Quinacrine, a non specific inhibitor of phospholipase A2 impaired PAF-acether formation in a dose-dependent manner. The time-course of PAF-acether formation was compared with the ionophore-induced release of arachidonic acid from these cells.

Studies on the preferential incorporation of [3H]glycerol over [32P]phosphate into major phospholipids of human platelets

It is well known that platelets readily incorporate radioactive glycerol, but not radioactive phosphate into phosphatidylcholine (PC) and phosphatidylethanolamine (PE) in vitro, thus not in accordance with de novo synthesis according to the Kennedy pathway. In attempts to understand the reason for the discrepancy, gel-filtered platelets were incubated simultaneously with [32P]Pi and [3H]glycerol, and the specific and relative radioactivities of products and intermediates were determined. Both precursors were incorporated into phosphatidylinositol (PI) with a 32P/3H ratio similar to that in glycerol 3-phosphate (in accordance with the Kennedy pathway). However, PC and PE obtained a much lower ratio. The specific 32P radioactivity in phosphorylcholine was similar to that of the gamma-phosphoryl of ATP and 650-times higher than that of PC. The specific 32P radioactivity of phosphorylethanolamine was 20-times less than that of phosphorylcholine. Both mass and 32P labelling of CDP-choline were below the detection limits. It is concluded that the incorporation of [32P]Pi into PC via phosphorylcholine is insignificant while the preferential incorporation of [3H]glycerol could be explained by exchange of diacyl[3H]glycerol in the reversible choline phosphotransferase (CDP-choline: 1,2-diacylglycerol cholinephosphotransferase) reaction. The same mechanism would explain the preferential incorporation of 3H over 32P into PE, although dilution of 32P at the phosphorylethanolamine stage would account for part of the feeble 32P incorporation. Although other mechanisms are also possible, our results clearly show that the appearance of [3H]glycerol in PC and PE is not a reliable method of monitoring de novo synthesis of these phospholipids.

Hydrolysis of endogenous phospholipids by rat platelet phospholipase A2: ether or acyl bond and polar head group selectivity

Substrate specificity of platelet phospholipase A2 was investigated following Ca2+-dependent hydrolysis by endogenous enzyme of linoleate- or arachidonate-labelled platelet phospholipids. Alkylacyl, alkenylacyl and diacyl classes of ethanolamine and choline glycerophospholipids (GPE and GPC) were separated after their diacylglycerol derivation, and molecular species of diacyl-GPE were analyzed by HPLC. Hydrolysis of platelet ethanolamine and choline glycerophospholipids was dependent on Ca2+ and was maximal at neutral pH. In the presence of 0.2 mM Ca2+ the hydrolysis rate for [14C]arachidonate-labelled phospholipids was in the order diacyl-GPE greater than alkylacyl-GPE = diacyl-GPC = alkenylacyl-GPE greater than alkylacyl-GPC. In addition to being the best substrate at high Ca2+ concentration, diacyl-GPE could be degraded with Ca2+ concentrations in the micromolar range, concentrations which are unable to induce any degradation of diacyl-GPC. As a function of Ca2+ concentration, the hydrolysis rate of [14C]linoleate- and [14C]arachidonate-labelled diacyl-GPE or diacyl-GPC was identical. The five main molecular species of diacyl-GPE labelled with arachidonate or with linoleate were hydrolyzed at the same rate in the presence of 50 microM Ca2+. This study shows that platelet phospholipase A2 is specific for endogenous diacyl-GPE and is independent of fatty chain composition. These results are discussed in relation to the Ca2+ concentration observed in stimulated platelets and in relation to the lysophospholipid-induced specific transfer of arachidonate. They suggest that diacyl-GPE hydrolysis by phospholipase A2 could play a key role in stimulated platelets.