Relative degradation of different arachidonoyl molecular species of choline glycerophospholipids in opsonized zymosan-stimulated rabbit alveolar macrophages

Platelet-activating factor and related lipid mediators

Platelet-activating factor (PAF) is a phospholipid with potent, diverse physiological actions, particularly as a mediator of inflammation. The synthesis, transport, and degradation of PAF are tightly regulated, and the biochemical basis for many of these processes has been elucidated in recent years. Many of the actions of PAF can be mimicked by structurally related phospholipids that are derived from nonenzymatic oxidation, because such compounds can bind to the PAF receptor. This process circumvents much of the biochemical control and presumably is regulated primarily by the rate of degradation, which is catalyzed by PAF acetylhydrolase. The isolation of cDNA clones encoding most of the key proteins involved in regulating PAF has allowed substantial recent progress and will facilitate studies to determine the structural basis for substrate specificity and the precise role of PAF in physiological events.

Secreted and intracellular phospholipases A2 inhibition by 1-decyl 2-octyl-glycerophosphocholine in rat peritoneal macrophages

Compounds able to inhibit phospholipases A2 can be considered as potential anti-inflammatory drugs. In this respect, the inhibitory effect of the phospholipid analogue 1-decyl 2-octyl-rac-glycero-3-phosphocholine (decyloctyl-GPC) added to the culture medium of rat peritoneal macrophages stimulated with ionophore A23187 was determined. (a) The substrate of phospholipase A2 1-octadecanoyl 2-[14C]eicosatetraenoyl-sn-glycero-3-phosphocholine ([14C]20:4-GPC) was added to the culture medium. In macrophages + extracellular fluids, its hydrolysis at the 2-position, produced [14C]non-phosphorous lipids which reached 12% of the dose at 0.14 microM, 73% at 0.9 and > 90% at 1.6 microM; in experiments where macrophages and extracellular fluids were analyzed separately, decyloctyl-GPC initially added at 4 microM, significantly inhibited the release of [14C]fatty acids and the eicosanoid synthesis, demonstrating its ability to inhibit secreted and/or intracellular phospholipases A2. (b) Extracellular fluids were separated from the macrophages and incubated with [14C]20:4-GPC: 48% of the dose was hydrolyzed by extracellular fluid-associated secreted phospholipase A2 and decyloctyl-GPC at 3 microM, reduced this hydrolysis by 50%. (c) [3H]arachidonic acid ([3H]20:4) was added to the culture medium and was esterified in the macrophage membrane phospholipids. Activation of intracellular phospholipase A2 induced the release of [3H] fatty acids and eicosanoid synthesis. These releases were inhibited by 50% with decyloctyl-GPC added at 4 microM. (d) [3H]20:4 and [14C]20:4-GPC were added to the culture medium of the macrophages. [3H] and [14C] fatty acids and eicosanoids were released in macrophages or extracellular fluids. They were significantly reduced by the phospholipid analogue added at 4 microM. It is concluded that secreted and intracellular phospholipases A2 were both inhibited by decyloctyl-GPC which extensively reduced the 20:4 release from exogenous and membrane phospholipids and therefore eicosanoid synthesis.

Influence of alkyl chain lengths in dialkylglycerophosphocholines towards phospholipase A2 inhibition in macrophages

Rat peritoneal macrophages were cultured with a specific and potent phospholipase A2 activator A 23187, with 1-stearoyl-2-[3H]arachidonoyl-sn-GPC as source of [3H] arachidonic acid, and with a dialkyl-GPC, at 2, 10 or 20 microM. Four dialkyl-GPCs were prepared by chemical synthesis. Position 2 of rac-glycerol was alkylated with an alkane chain of 8 carbons and position 1 was alkylated with various alkane chains (8, 10, 12, or 16 carbons). [3H] arachidonic acid was split, then recovered with cell nonesterified fatty acids and nonphosphorous glycerolipids after endocellular phospholipase A2 activity. It was also recovered with fatty acids and eicosanoids isolated from culture medium. Inhibition of fatty acid release and eicosanoid synthesis depended on mixed chain dialkyl-GPC structures. The highest inhibitory effect on arachidonic acid release was reached with 1-decyl-2octyl-GPC and was practically as high in culture medium (IC50 at 5 microM) as in cells (IC50 at 4 microM). 1,2-di-octyl-GPC and 1-dodecyl-2-octyl-GPC had weaker inhibitory effects (but higher in culture medium than in cells). The asymmetrical 1-hexadecyl-2-octyl-GPC poorly affected enzyme activity.

Comparison of macrophage phospholipid radiolabelling methods for measuring phospholipase A2 inhibition

Rat peritoneal macrophages activated by ionophore A 23187, were labelled after introduction in the culture medium of 1-0-stearoyl 2-0-[3H] arachidonylglycero-3-phosphocholine (as unique source of tritiated arachidonic acid), or [3H] arachidonic acid which was esterified by cells in phospholipids and triglycerides or remained non esterified. With either cell-labelling method, stimulated macrophages produced tritiated nonesterified fatty acids and eicosanoids which were isolated from cell and medium lipids. When introduced into the culture medium at 1, 5 or 10 microM, the membrane phospholipid analogue 1,2 di-O-hexadecylglycerophosphocholine (dihexadecyl-GPC), but not the lysolecithin analogue 1-0-octadecyl 2-0-methylglycero phosphocholine, lowered phospholipase A2 activity with either labelling method. Dihexadecyl-GPC had an inhibitory effect on the release of arachidonic acid and eicosanoids. Moreover, this effect, as measured by tritiated nonesterified fatty acid formation, was greater in activated cells labelled with tritiated phospholipid (IC50 6 microM) than with [3H] arachidonic acid (IC50 60 microM). This is attributable to the inhibitory effect of dihexadecyl-GPC on endogenous phospholipase A2 and the endogenous enzyme excreted together with lysosomes into the medium. It may be concluded that radioactive phospholipid labelling is a sensitive method for measuring phospholipase A2 activity and assessing the effects of potential phospholipase inhibitors.

Contributing factors in the trafficking of [3H]arachidonate between phospholipids

Cultured human promyelocytic leukemia cells (HL-60), depleted of arachidonic acid by continued growth in serum-free media, were used as a model system to examine various factors that control the incorporation and distribution of [3H]arachidonic acid into classes and subclasses of cellular lipids. Increasing the culture media concentration of [3H]arachidonic acid from 1 x 10(-8) M to 1 x 10(-5) M caused a greater percentage of the cellular tritium to be distributed into triacylglycerols (from less than 1% at 1 x 10(-8) M to 38% at 1 x 10(-5) M) with a corresponding decrease in cellular [3H]diradylglycerophosphoethanolamine (from 53% at 1 x 10(-8) M to 12% at 1 x 10(-5) M) during 2 h incubations. A greater proportion of the tritium present in diradylglycerophosphoethanolamine and diradylglycerophosphocholine, at the higher media concentration of [3H]arachidonic acid (1 x 10(-5) M), was found in the diacyl subclasses of these two lipids than was observed at the lower concentrations (less than 1 x 10(-6) M) of [3H]arachidonic acid. Significant amounts of diarachidonoyl molecular species were found in the phosphatidylethanolamine (10%) and phosphatidylcholine (15%) of HL-60 cells that were labeled for 2 h with 1 x 10(-5) M [3H]arachidonic acid. This was the only molecular species of phosphatidylcholine to completely disappear when prelabeled cells were placed in arachidonate-free media for 22 h. Prelabeling-chase experiments with 1 x 10(-5) M [3H]arachidonic acid were consistent with movement of [3H]arachidonate from triacylglycerols into diradylglycerophosphatides and from diacylphospholipids into ether-linked phospholipids. Increasing the concentration of HL-60 cells in the incubations influenced the distribution of [3H]arachidonic acid in cellular lipid classes in a manner analogous to decreasing the concentration of [3H]arachidonic acid in the media. Increasing the endogenous level of cellular arachidonate in phospholipid classes with supplements of unlabeled arachidonic acid changed the subsequent lipid class distribution of a low concentration (1 x 10(-8) M) of [3H]arachidonic acid to resemble results obtained with a much higher mass level of [3H]arachidonate in arachidonate depleted cells. HL-60 cells differentiated into granulocytes by treatment with dimethyl sulfoxide incorporated less [3H]arachidonic acid but had a greater proportion associated with alkylacylglycerophosphocholine and alk-1-enylacylglycerophosphoethanolamine than undifferentiated HL-60 cells.

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.

Leukotriene B4 level in neutrophils from allergic and healthy subjects stimulated by low concentration of calcium ionophore A23187. Effect of exogenous arachidonic acid and possible endogenous source

Peripheral blood neutrophils from patients with allergic rhinitis and from normal subjects were incubated for 5 min at 37 degrees C with 0.15 microM calcium ionophore A23187 in the absence or presence of exogenous arachidonic acid (2.5 to 10 microM). In neutrophils from allergic patients, the leukotriene B4 (LTB4) level was significantly increased by exogenous arachidonic acid in a concentration-dependent manner (16.2 +/- 4.2 and 38.1 +/- 6.8 pmol/5 min per 2 X 10(6) cells in the absence and presence of 10 microM arachidonic acid, respectively; P less than 0.005; n = 8). The LTB4 level in neutrophils from healthy subjects was only 0.97 +/- 0.17 pmol/5 min per 2 x 10(6) cells (n = 5) and was not enhanced by exogenous arachidonate. When cells from allergic patients were challenged in the presence of exogenous [1-14C]arachidonic acid, released LTB4 was radiolabeled and the incorporated radioactivity increased with the labeled arachidonate concentration. Labeled LTB4 was never detectable after incubating neutrophils from normal donors with exogenous labeled arachidonate. When neutrophils were incubated with [1-14C]arachidonate for 1 h, the different lipid pools of the two cell populations were labeled but both types of neutrophils produced unlabeled LTB4 in response to ionophore stimulation. The hydrolysis of choline and ethanolamine phospholipids into diacyl-, alkenylacyl- and alkylacyl-species revealed that solely the alkylacyl-subclass of phosphatidylcholine was unlabeled. We conclude (i) that neutrophils from allergic patients stimulated by low ionophore concentration produce more LTB4 than neutrophils from healthy subjects and incorporate exogenous arachidonate, (ii) that endogenous arachidonate converted to LTB4 by the 5-lipoxygenase pathway may provide only from 1-O-alkyl-2-arachidonoyl-glycero-3-phosphocholine.

Ether lysophospholipid-induced production of platelet-activating factor in human polymorphonuclear leukocytes

Human polymorphonuclear leukocytes (PMN) produced considerable amounts of platelet-activating factor (PAF) when exposed to various concentrations of lyso-PAF, especially in the absence of albumin. The amount of produced PAF in the presence of 5 microM lyso-PAF (without albumin) was 1.1 pmol/10 min per 2.5 X 10(6) cells, which was close to the level in the case of opsonized zymosan stimulation. We found that the activity of neither acetyltransferase nor acetylhydrolase was affected markedly by the treatment of cells with lyso-PAF, suggesting that the increased availability of lyso-PAF could be responsible for the induction of PAF synthesis. We also found that PAF synthesis was induced not only by lyso-PAF but also by ether-containing ethanolamine lysophospholipids, 1-alkenyl(alkyl)-sn-glycero-3-phosphoethanolamine (GPE). The addition of 1-alkenyl(alkyl)-GPE caused the degradation of pre-existing 1-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine (GPC) and an increased level of lyso-PAF, followed by the formation of PAF. By contrast, 1-acyl-GPC and 1-acyl-GPE failed to induce PAF production. These results suggest a possible key role of the availability of lyso-PAF in triggering the biosynthesis of PAF in human PMN.

Composition of mouse peritoneal macrophage phospholipid molecular species

The individual molecular species composition of diacyl, alkylacyl and alkenylacyl glycerophospholipids was determined in mouse peritoneal macrophages. A marked heterogeneity in the relative composition (mol%) of macrophage ether and ester phospholipid individual species was noted. High concentrations of 16:0-20:4 were found in ether phospholipids such as alkenylacyl glycerophosphoethanolamine (GPE; 27.5 mol%) and alkylacyl glycerophosphocholine (GPC; 16.6%) as compared to mol% levels of 16:0-20:4 in diacyl GPE (5.7%) and diacyl GPC (8.1%), respectively. Interestingly, alkenylacyl GPE was highly enriched in 1-ether (16:0) relative to alkylacyl GPC. The predominant diacyl molecular species in glycerophosphoinositol (GPI) and glycerophosphoserine (GPS) were 18:0-20:4 (59.1%) and 16:0-18:1 (41.1%), respectively. It is noteworthy that the level of 18:0-20:4 was several times higher in diacyl GPI (59.1%) than in diacyl GPS (11.1%), diacyl GPE (25.7%), and diacyl GPC (3.7%). The most abundant molecular species in diacyl GPC and diacyl GPE were 16:0-18:1 (29.9%) and 18:0-20:4 (25.7%), respectively. The abundance of 20:4 in ether phospholipids, specifically 16:0-20:4 and 18:0-20:4, in alkylacyl GPC is significant in view of the role these antecedents play in the biosynthesis of platelet-activating factor (PAF) and 20:4-derived eicosanoids in stimulated macrophages. The unique molecular species composition of the peritoneal macrophage distinguishes this cell type from others.

Incorporation of arachidonic, dihomogamma linolenic and eicosapentaenoic acids into cultured V79 cells

The uptake and distribution of three common dietary polyunsaturated fatty acids was studied using Chinese hamster lung fibroblasts (V79 cells). Treatment of V79 cells with arachidonic (20:4), eicosapentaenoic (20:5) and dihomogammalinolenic (20.3) acids for 24 hr produced a marked uptake of 20:3 and 20:4, both of which were assimilated to a considerably greater degree than 20:5. All polyunsaturated fatty acids were incorporated primarily into phospholipids; however, there were considerable differences in their distribution into individual phospholipid species. Although 20:4 was incorporated primarily into phosphatidylcholine, 20:3 entered largely into phosphatidylethanolamine and phosphatidylglycerol, and 20:5 was distributed about equally between phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol. A marked conversion of 20:3 to 20:4 was found after 24 hr and, in several phospholipids, there was as much derived-radiolabeled 20:4 as there was radiolabeled 20:3. There was little evidence of 20:4 and 20:5 metabolism. V79 cells undergo substantial changes in phospholipid fatty acid composition following supplementation with these polyunsaturated fatty acids; however, these fatty acids are assimilated to different degrees and their distribution among cellular phospholipids is distinct, suggesting incorporation via independent mechanisms.

The de novo synthesis of molecular species of phosphatidylinositol from endogenously labeled CDP diacylglycerol in alveolar macrophage microsomes

The de novo synthesis of molecular species of phosphatidylinositol (PI) from endogenously labeled CDP diacylglycerol (CDP-DG) and phosphatidic acid (PA), with [14C]-glycerol 3-phosphate, in microsomes of macrophages was studied using a recently developed HPLC technique. Endogenously labeled PA, CDP-DG, and PI were sequentially formed from labeled glycerol 3-phosphate through the addition of CoA, CTP, and then inositol into microsomes. The rate of formation of CDP-DG from endogenously labeled PA was low as compared with those of PA and PI. The low rate of CDP-DG synthesis suggests that it may be the rate-limiting step in the de novo synthesis of PI. Analysis of newly synthesized molecular species of PI by HPLC revealed that large proportions of radioactivity were associated with the 16:0-18:1, 16:0-18:2, 18:1-18:2, and 18:2-18:2 species, and a small amount, 2-3%, of radioactivity was associated with the 18:0-20:4 species. The profiles of newly synthesized PA and CDP-DG species were quite similar to those of PI species. This suggests that the enzymes involved in the formation of PI species from glycerol 3-phosphate show little specificity toward different molecular species of substrates. The results of the present study also suggest that free fatty acid composition in microsomes greatly affect the composition of the molecular species of PI synthesized through the de novo pathway, since the proportion of fatty acids utilized for the de novo synthesis of PI species was similar to that of free fatty acids in the microsomal membrane.

5-Hydroperoxyeicosatetraenoic acid (5-HPETE) enhances the synthesis of 1-O-alkyl-2-sn-acetyl-glycero-3-phosphocholine (PAF) in fMet-Leu-Phe-stimulated HL-60 cells: key role of diacylglycerol (DAG) in activation of protein kinase C (PKC)

We investigated the effect of 5-hydroperoxyeicosatetraenoic acid (5-HPETE) on the PAF formation in fMet-Leu-Phe-stimulated HL-60 cells. 5-HPETE was found to enhance the PAF synthesis in fmlp-stimulated cells without causing additional mobilization of intracellular calcium. However, a significant increase in diacylglycerol (DAG) levels due to 5-HPETE was observed, which in turn activated the protein kinase C (PKC). Obviously, PKC is responsible for the activation of phospholipase A2 and the release of lyso-PAF and AA from complex lipid stores. Further, the dose-dependent increase in DAG production in absence of simultaneous increase in total inositol phosphates is indicative of an additional source for DAG besides PIP2.

Properties and purification of an arachidonoyl-hydrolyzing phospholipase A2 from a macrophage cell line, RAW 264.7

The lipid mediators, platelet activating factor (PAF) and the eicosanoids, can be coordinately produced from the common phospholipid precursor, 1-O-alkyl-2-arachidonoylglycerophosphocholine (1-O-alkyl-2-arachidonoyl-GPC), through the initial action of a phospholipase A2 that cleaves arachidonic acid from the sn-2 position. The mouse macrophage cell line RAW 264.7, which was used as a model macrophage system to study the arachidonoyl-hydrolyzing phospholipase A2 enzyme(s), could be induced to release arachidonic acid in response to inflammatory stimuli. A phospholipase A2 that hydrolyzed 1-O-hexadecyl-2-[3H]arachidonoyl-GPC was identified in the cytosolic fraction of these macrophages. This phospholipase activity was optimal at pH 8 and dependent on calcium. Enzyme activity could be stimulated 3-fold by heparin, suggesting the presence of phospholipase inhibitory proteins in the macrophage cytosol. Compared to 1-alkyl-2-arachidonoyl-GPC, the enzyme hydrolyzed 1-acyl-2-arachidonoylglycerophosphoethanolamine (1-acyl-2-arachidonoyl-GPE) with similar activity but showed slightly greater activity against 1-acyl-2-arachidonoyl-GPC, suggesting no specificity for the sn-1 linkage or the phospholipid base group. Although comparable activity against 1-acyl-2-arachidonoylglycerophosphoinositol (1-acyl-2-arachidonoyl-GPI) could be achieved, the enzyme exhibited much lower affinity for the inositol-containing substrate. The enzyme did, however, show apparent specificity for arachidonic acid at the sn-2 position, since much lower activity was observed against choline-containing substrates with either linoleic or oleic acids at the sn-2 position. The cytosolic phospholipase A2 was purified by first precipitating the enzyme with ammonium sulfate followed by chromatography over Sephadex G150, where the phospholipase A2 eluted between molecular weight markers of 67,000 and 150,000. The active peak was then chromatographed over DEAE-cellulose, phenyl-Sepharose, Q-Sepharose, Sephadex G150 and finally hydroxylapatite. The purification scheme has resulted in over a 1000-fold increase in specific activity (2 mumol/min per mg protein). Under non-reducing conditions, a major band on SDS-polyacrylamide gels at 70 kDa was observed, which shifted to a lower molecular weight, 60,000, under reducing conditions. The properties of the purified enzyme including the specificity for sn-2-arachidonoyl-containing phospholipids was similar to that observed for the crude enzyme. The results demonstrate the presence of a phospholipase A2 in the macrophage cell line. RAW 264.7, that preferentially hydrolyzes arachidonoyl-containing phospholipid substrates.

Selective inhibition of free arachidonic acid production in activated alveolar macrophages by calmodulin antagonists

The effect of calmodulin antagonists on the amounts of free fatty acids produced by rabbit alveolar macrophages was determined by fluorometric high-performance liquid chromatography. Opsonized zymosan-induced arachidonic acid production was dramatically suppressed in the presence of W-7 and trifluoperazine without an effect on the production of other fatty acids. Calmodulin antagonists inhibited phospholipase A and abolished the release of arachidonic acid from phospholipids. The present results suggest that a zymosan-sensitive pool of 20:4, which is different from that of other fatty acids, is present in macrophages and that calmodulin antagonists selectively inhibit phospholipase A, which preferentially degrades phospholipids with 20:4.

Phospholipid analysis of human eosinophils: high levels of alkylacylglycerophosphocholine (PAF precursor)

The phospholipid composition and fatty acid profiles of human eosinophils were studied. Extremely high levels of ether phospholipids were found in this type of cell, such as alkylacylglycerophosphocholine (GPC) and alkenylacylglycerophosphoethanolamine (GPE); these two ether phospholipids accounted for about three-fourths of the choline and ethanolamine glycerophospholipids (CGP and EGP), respectively. Fatty acid analyses revealed that very large portions of arachidonic acid (20:4) were esterified to alkylacyl-GPC (92.0% in CGP) and alkenylacyl-GPE (86.6% in EGP), respectively. While high amounts of alkylacyl-GPC and the abundance of 20:4 in this ether phospholipid have been observed in other types of blood cells of various animals, these percentages for human eosinophils are the highest among such cells. These results suggest that alkyl and alkenyl ether phospholipids play essential roles in human eosinophils in various physiological and pathological conditions.

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.

Mobilization of arachidonic acid from diacyl and ether-linked phospholipids in FMLP stimulated alveolar macrophages

Exposure of [1 14C]AA labeled guinea-pig alveolar macrophages to FMLP for 15 min induced an extensive mobilization of AA from phospholipids. PC and PI mainly contributed to the AA release, and labeled PE remained unchanged. Analysis of ether-linked phospholipids showed a significant breakdown of labeled diacyl and alkyl-acyl PC and an increase in labeled alkenyl-acyl PE.

Formation of diacyl and alkylacyl glycerophosphocholine in rabbit alveolar macrophages

The incorporation of various labeled precursors into alkenylacyl, alkylacyl and diacyl phospholipids in rabbit alveolar macrophages was studied. The incorporation rates of the individual precursors were shown to be quite different among the three subclasses of phospholipids. [3H]Glycerol, [14C]16:0, [14C]18:1, [14C]18:2 and [32P]-orthophosphate were preferentially incorporated into choline glycerophospholipids (CGP), especially into diacyl glycerophosphocholine (GPC), indicating that the de novo synthesis of diacyl GPC is extremely high. Considerable portions of the radioactivities of [14C]16:0, [14C]18:1, [14C]18:2 and [32P]orthophosphate were also found in alkylacyl GPC, the incorporation being higher than or comparable to that in the case of diacyl glycerophosphoethanolamine (GPE). We then examined the activities of cholinephosphotransferase and ethanol-aminephosphotransferase, and found that the activity of cholinephosphotransferase was remarkably high in macrophage microsomes compared with that in microsomes from several other tissues. This suggests that diradylglycerols were preferentially utilized by choline-phosphotransferase, which is consistent with the results obtained for intact cells. We confirmed that a considerably higher amount of diacyl GPC as well as alkylacyl GPC was formed through this enzyme reaction with macrophage microsomes than with brain microsomes. The high formation of alkylacyl GPC could be responsible, at least in part, for the accumulation of this unique ether phospholipid, a stored precursor form of platelet-activating factor in macrophages.