CoA-independent transfer of arachidonic acid from 1,2-diacyl-sn-glycero-3-phosphocholine to 1-O-alkyl-sn-glycero-3-phosphocholine (lyso platelet-activating factor) by macrophage microsomes

Coenzyme-A-Independent Transacylation System; Possible Involvement of Phospholipase A2 in Transacylation

The coenzyme A (CoA)-independent transacylation system catalyzes fatty acid transfer from phospholipids to lysophospholipids in the absence of cofactors such as CoA. It prefers to use C20 and C22 polyunsaturated fatty acids such as arachidonic acid, which are esterified in the glycerophospholipid at the sn-2 position. This system can also acylate alkyl ether-linked lysophospholipids, is involved in the enrichment of arachidonic acid in alkyl ether-linked glycerophospholipids, and is critical for the metabolism of eicosanoids and platelet-activating factor. Despite their importance, the enzymes responsible for these reactions have yet to be identified. In this review, we describe the features of the Ca²⁺-independent, membrane-bound CoA-independent transacylation system and its selectivity for arachidonic acid. We also speculate on the involvement of phospholipase A2 in the CoA-independent transacylation reaction.

Differential effects of arachidonoyl trifluoromethyl ketone on arachidonic acid release and lipid mediator biosynthesis by human neutrophils. Evidence for different arachidonate pools

The goal of this study was to determine the effects of a putative specific cytosolic phospholipase A2 inhibitor, arachidonyl trifluoromethyl ketone (AACOCF3), on arachidonic acid (AA) release and lipid mediator biosynthesis by ionophore-stimulated human neutrophils. Initial studies indicated that AACOCF3 at concentrations 0-10 micro m did not affect AA release from neutrophils. In contrast, AACOCF3 potently inhibited leukotriene B4 formation by ionophore-stimulated neutrophils (IC50 approximately 2.5 micro m). Likewise, AACOCF3 significantly inhibited the biosynthesis of platelet activating factor. In cell-free assay systems, 10 micro m AACOCF3 inhibited 5-lipoxygenase and CoA-independent transacylase activities. [3H]AA labeling studies indicated that the specific activities of cell-associated AA mimicked that of leukotriene B4 and PtdCho/PtdIns, while the specific activities of AA released into the supernatant fluid closely mimicked that of PtdEtn. Taken together, these data argue for the existence of segregated pools of arachidonate in human neutrophils. One pool of AA is linked to lipid mediator biosynthesis while another pool provides free AA that is released from cells. Additionally, the data suggest that AACOCF3 is also an inhibitor of CoA-independent transacylase and 5-lipoxygenase. Thus, caution should be exercised in using AACOCF3 as an inhibitor of cytosolic phospholipase A2 in whole cell assays because of the complexity of AA metabolism.

Involvement of calcium-independent phospholipase A2 in uterine stromal cell phospholipid remodelling

The role of Ca2+-independent phospholipase A2 (iPLA2) in arachidonic (AA) and docosahexaenoic (DHA) acid incorporation and phospholipid remodelling in rat uterine stromal cells (UIII cells) was studied. Incorporation of AA and DHA into UIII cell phospholipids was Ca2+-independent. Bromoenollactone (BEL), a potent inhibitor of iPLA2, reduced lysophosphatidylcholine level and AA incorporation into phospholipids by approximately 20%. DHA incorporation was not affected by BEL, indicating that the pathways for AA and DHA incorporation are partially different. In control cells, the transfer of AA occurred mainly from diacyl-glycerophosphocholine (GroPCho) to alkenylacyl-glycerophosphoethanolamine (GroPEtn) and to a lesser extent from diacyl-GroPCho to diacyl-GroPEtn. [3H]DHA was redistributed from diacyl-GroPCho and alkylacyl-GroPEtn to alkenylacyl-GroPEtn. BEL treatment inhibited completely the redistributrion of AA within diacyl-GroPCho and diacyl -GroPEtn and reduced the [3H]DHA content of diacyl-GroPEtn, indicating that a BEL-sensitive iPLA2 controls the redistribution of polyunsaturated fatty acids to diacyl-GroPEtn. In contrast the redistribution of radioactive AA and DHA to alkenylacyl-GroPEtn was almost insensitive to BEL. The analysis of substrate specificity and BEL sensitivity of iPLA2 activity indicates that UIII cells exhibit at least two isoforms of iPLA2, one of which is BEL-sensitive and quite selective of diacyl species, and another one that is insensitive to BEL and selective for alkenylacyl-GroPEtn. Taken together, these results suggest that several iPLA2 participate independently in the remodelling of UIII cell phospholipids.

Subcellular localization of enzyme activities involved in the metabolism of platelet-activating factor in rainbow trout leukocytes

The subcellular distribution of an alkyllyso-GPC: acetyl-CoA acetyltransferase (EC 2.3.1.67) and transacylase, two important enzyme activities involved in the remodeling pathway for the biosynthesis of platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF) have been examined in leukocytes isolated from the pronephros of the rainbow trout, Oncorhynchus mykiss. Contrary to mammalian systems, in which the acetyltransferase is localized to intracellular membranes, the subcellular distribution of an acetyltransferase activity in rainbow trout leukocytes was localized to the plasma membrane. Analysis of the acetyltransferase products by thin-layer chromatography (TLC) and high performance liquid chromatography (HPLC) confirmed synthesis of two subclasses of PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine and 1-acyl-2-acetyl-sn-glycero-3-phosphocholine. The transacylase activity in this study was detected in membrane fractions in two domains of the intermediate density region which also contained the NADH dehydrogenase activity, a marker enzyme for the endoplasmic reticulum. Acylation of lysoPAF (1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine) exhibited approximately 95% specificity for omega-3 fatty acids. Acylation patterns were not significantly different in either domain of the endoplasmic reticulum. A model is proposed herein for the metabolism of PAF in rainbow trout leukocytes.

Regulation of leukotriene and platelet-activating factor synthesis in human alveolar macrophages

It has been suggested that phospholipase A2 (PLA2) contributes to the regulation of leukotriene (LT) and platelet-activating factor (PAF) synthesis by controlling the release of their precursors, arachidonic acid (AA) and lysophosphatidylcholine (lysoPC), from membrane phospholipids. In rat alveolar macrophages (AMs), PLA2 appears to have a major role in LT synthesis but a more limited role in PAF synthesis. The present study was designed to define the role of PLA2 in LT and PAF synthesis in human AMs and determine whether differences exist between AMs obtained from normal subjects and those from patients with asthma. In the normal subjects, the calcium ionophore A23187 (Cal) increased AM PAF synthesis (percent incorporation of tritiated acetate) by 135% (p < 0.01) and LTB4 synthesis 88-fold (p < 0.001). Phorbol myristate acetate (PMA) had little effect alone, but it had a synergistic effect with Cal, increasing PAF synthesis by 466% and LTB4 synthesis to 229-fold above the control values (p < 0.001 for both). Ro 25-4331, a combined cytosolic (c) and secretory (s) PLA2 inhibitor, had little effect on the Cal-stimulated PAF synthesis, but it completely blocked the effect of PMA. It also blocked the Cal- and Cal+PMA-stimulated LTB4 synthesis. AACOCF3, a cPLA2 inhibitor, had no effect on either Cal or Cal+PMA-stimulated PAF synthesis. It reduced LTB4 synthesis, but it did so less effectively than Ro 25-4331. CoA-independent transacylase (CoAI-TA) activity in the AMs increased after stimulation and exposure to Ro 25-4331. SK&F 45905, a CoAI-TA inhibitor, reduced stimulated PAF synthesis by 30% to 40%. Patients with asthma had similar results except that cPLA2 had a greater role in stimulated LTB4 synthesis. These data indicate that PLA2 plays a direct role in human AM LT synthesis; both the cytosolic and secretory forms contribute to LT synthesis; PLA2 appears to have a more limited role in PAF synthesis, although it mediates the synergistic effect of PMA, probably via sPLA2; and CoAI-TA contributes to PAF synthesis during PLA2 inhibition. With the exception of the greater role for cPLA2 in stimulated LTB4 synthesis in the patients with asthma, the contributions of PLA2 and CoAI-TA to AM LT and PAF synthesis appear to be similar in normal subjects and patients with asthma.

The biochemical role of platelet-activating factor in reproduction

The presence of Co-A independent transacylase activity in amnion cells and the preferential transfer of arachidonic acid to acceptor-ethanolamine plasmalogen provide a satisfactory explanation to the questions raised by the observation that arachidonate-enriched ethanolamine plasmalogen increases in amnion late in gestation without alteration in the total amount of ethanolamine glycerophospholipids. The proposed mechanism also serves as a link between the observed changes in glycerophospholipid composition and the generation of PAF. We have emphasized a role for PAF in fetal lung maturation, the initiation and maintenance of parturition, and in certain complications associated with a premature delivery. Although PAF is known to be the most potent lipid mediator yet described and its importance in reproductive biology is well documented, it is our view that these events cannot be attributed solely to PAF and in all likelihood a number of autacoids participate in these processes.

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.

Molecular species of ethanolamine plasmalogens and transacylase activity in rat tissues are altered by fish oil diets

Effects of dietary fish oil ethyl esters and alkyldiacetylglycerols (an ether-linked lipid) on the distribution of subclasses of choline- and ethanolamine-glycerophospholipids as well as effects on highly unsaturated molecular species of ethanolamine plasmalogens from brain, spleen, kidney, lung, and testis of rats were examined. Supplementation of ethyl ester concentrates of n-3 fatty acids had no effect on the distribution of subclasses in any of the tissues. However, the supplements of 1-O-octadec-9'-enyl-2,3-diacetyl-sn-glycerol (diacetates of selachyl alcohol) caused significant increases in the alkylacylglycerophosphocholine and alkylacylglycerophosphoethanolamine subclasses from spleen and lung and in the alkylacylglycerophosphoethanolamine subclass from kidney. Dietary supplements of fish oil ethyl esters reduced the arachidonate-containing species of ethanolamine plasmalogens whereas molecular species having 20:5(n-3), 22:6(n-3), and/or 22:5(n-3) acyl groups were increased in the spleen, lung, and kidneys, but not brain. In testicular tissue from rats fed the fish oil diets, the molecular species of ethanolamine plasmalogens containing 22:5(n-6) acyl groups were reduced. An increase of ethanolamine plasmalogens with 18:1 alk-1-enyl moieties paired with highly unsaturated sn-2 acyl groups were found in the tissues of rats fed the fish oil plus selachyl alcohol diacetate supplements. Rats on the diet containing fish oil ethyl esters had significantly lower [3H]alkyllysoglycerophosphocholine CoA-independent transacylase activity in spleen microsomes than controls. This suggests that supplements of n-3 fatty acids interferes with the transacylation of arachidonate, an event that could seriously impair the release of arachidonate and lysophospholipids (e.g., lyso-PAF) that are precursors of potent bioactive lipid derivatives.

Transfer of arachidonyl groups within the lipids of two human neuroblastoma cell lines

The incorporation and mobilization of [3H]arachidonic acid in lipids of human neuroblastoma cell lines, SK-N-SHF and LA-N-5, was studied. Essentially similar results were obtained with these two cell lines. Except for phosphatidylinositol which displayed the highest specific activity, the incorporation patterns within phospholipid classes tended to reflect phospholipid composition initially. However at later stages, counts in the acid-stable phosphatidylcholine plateaued and/or decreased while those of plasmenylethaniolamine and acid-stable phosphatidylethanolamine increased steadily. When cells were pulse-labelled with [3H]arachidonic acid and chased with fresh medium, there was a movement of label from diacyl (acid-stable) phosphatidylcholine to plasmenylethanolamine and diacyl (acid-stable) phosphatidylethanolamine. Plasmenylcholine did not appear to be involved in the arachidonyl group transfer. Under these chase conditions there was extensive turnover in the 32P-labelled polar headgroup of phosphatidylinositol but not in that of the other phospholipids. In both incorporation and chase studies involving [3H]arachidonic acid, a movement of arachidonyl groups from triacylglycerol to phospholipid could be observed. The results indicated that the patterns of incorporation and redistribution of arachidonic acid in human neuroblastoma cells were effectively regulated to favor lipids such as phosphatidylinositol and the subclasses of phosphatidylethanolamine. Possible mechanisms involved in these enrichment processes are discussed.

Induction of coenzyme A-dependent transacylation activity in rat liver microsomes by administration of clofibrate

The effect of administration of clofibrate on the activity of coenzyme A-dependent (CoA-dependent) transacylation of 1-acyl-glycerophosphocholine (1-acyl-GPC) was examined in rat liver microsomes. Administration of clofibrate to rats increased the activity of Co-A-dependent transacylation of 1-[14C]acyl-GPC and the activity reached a value (8.37 nmol/min per mg protein) twice that in control rats (3.95 nmol/min per mg protein) without any changes in apparent Km values for CoA (1.2 microM in control and 1.0 microM in clofibrate-treated) and 1-acyl-GPC (33.4 microM in control and 27.8 microM in clofibrate-treated). The rate of CoA-dependent transfer of [14C]arachidonic acid (20:4) from 1-acyl-2-[14C]20:4-glycerophosphoethanolamine (GPE) or 1-acyl-2-[14C]20:4-glycerophosphoinositol (GPI) to 1-acyl-GPC (synthesis of 1-acyl-2-[14C]20:4-GPC) was also increased by treatment with clofibrate (1.9-fold and 1.5-fold increases, respectively). These results suggest that a CoA-dependent transacylation system of 1-acyl-GPC was induced by treatment with clofibrate.

Porcine aortic endothelial cell membranes contain a LPAF: CoA-independent transacylase

Membranes isolated from porcine aortic endothelial cells (PAEC) contain a CoA-independent transacylase enzyme (CoA-IT). CoA-IT, an integral membrane protein, transfers an acyl moiety to added [3H]alkylhydroxyglycerophosphocholine (LPAF) to generate [3H]alkylacylglycerophosphocholine (alkylacyl-GPC). This enzyme exhibits an apparent Km of 0.7 microM and a Vmax of 0.8 nmol/min per mg for the transfer of an acyl group to added [3H]LPAF. The addition of the nonionic detergent Triton X-100 (TX-100) (0.5 mg/ml), the sulfhydryl reagents N-ethylmaleimide (NEM) (200 microM) or thimerosal (200 microM), or pre-incubating the membranes at 95 degrees C for 10 min all decreased LPAF: CoA-IT activity by more than 95%. The inhibitory action of NEM or thimerosal suggests that sulfhydryl group(s) are involved in or are close to the catalytic site of LPAF: CoA-IT.

Effect of ozone on platelet activating factor metabolism in phorbol-differentiated HL60 cells

The mechanisms of ozone (O3) toxicity in the lung may involve the formation of lipid inflammatory mediators. We have previously demonstrated that exposure to O3 in vitro results in increased accumulation and release of platelet activating factor (PAF) in the macrophage-like cell line HL60 differentiated with phorbol ester (dHL60). In the present study we have examined possible biochemical mechanisms responsible for the O3-induced increase in PAF levels in dHL60 cells. Specifically, we studied the effect of O3 on phospholipase A2 (PLA2), acetyltransferase, acetylhydrolase, and reacylation activities. dHL60 cells were exposed to 1.0 ppm O3 or air alone. O3 exposure was found to significantly decrease dHL60 cell acetylhydrolase activity by 36%. Additional experiments demonstrated that extracellular acetylhydrolase activity, but not intracellular acetylhydrolase activity, was inhibited by O3 exposure of dHL60 cells. O3 exposure resulted in a small (13%) but statistically significant reduction in reacylation activity in dHL60 cells. In addition, a significant (22%) contribution of PLA2 activation to the O3-induced increase in PAF levels was also found. Basal and calcium ionophore-induced acetyltransferase activity was found to be unaffected by exposure of dHL60 cells to O3. These data suggest that in vitro exposure to O3 affects both synthetic and degradative pathways of PAF metabolism in dHL60 cells.

Heterogeneity of arachidonate and paf-acether precursor pools in mast cells

In mammalian cells, arachidonate release and paf-acether formation are frequently associated. The alkyl-acyl-GPC has been proposed as an important source for released arachidonic acid and arachidonate-containing alkylacyl-GPC species as unique precursor for paf-acether. However, the specificity of precursor pools either concerning arachidonic acid or paf-acether is still a matter of controversy. We studied the relationship between the precursor pools for both autacoids in antigenically-stimulated cultured mast cells. We took advantage of the particular arachidonate turnover rate in each phospholipid to investigate the role of alkyl-arachidonyl-GPC in the supply of arachidonic acid by using newly and previously [14C]arachidonate-labeled cells. The specific activity of the released arachidonate was reduced 2-fold following overnight cell incubation, whereas labeling in alkyl-arachidonoyl-GPC was only slightly modified and never corresponded to that of released arachidonate when newly or previously labeled cells were triggered with the antigen. These results are not in favor of a major role for alkyl-arachidonoyl-GPC in supplying arachidonate. In contrast, by using previously labeled cells, we demonstrated that all arachidonate-containing phospholipids were involved in the release of arachidonic acid. The pattern of alkyl chains in alkyl-arachidonoyl-GPC, as well as in total alkylacyl-GPC, is unique since it consists mainly of 18:1 (more than 55%), whereas the 16:0 represents only about 30% of total alkyl chains. Therefore, we analyzed paf-acether molecular composition in order to compare it to the alkyl composition of the precursor pools. The content in 18:1 species of paf-acether, as measured by bioassay (aggregation of rabbit platelets), was always lower than that of 16:0 species and then did not correspond to the alkyl composition of the precursor. These data suggest that the enzymes involved in paf synthesis might be specific for 16:0 alkyl chains of precursor pool.

Regulation of the biosynthesis of platelet-activating factor in alveolar macrophages

Activities of enzymes which metabolize lysoplatelet-activating factor (lysoPAF) and platelet-activating factor (PAF) were studied in rabbit alveolar macrophage lysates. Substantial acetyltransferase activity was noted in the presence of 100 microM acetyl-coenzyme A (CoA), and this activity was increased in A23187-stimulated cell lysate. On the other hand, in the absence of exogenous acetyl-CoA, lysoPAF was mainly acylated through a transacylation pathway rather than by acetyltransferase in both control and A23187-stimulated cell lysates. We confirmed that the intracellular concentration of acetyl-CoA is relatively low. The observations suggest that the transacylation system may play an equally important role in the regulation of the availability of lysoPAF in intact cells. Intracellular lysoPAF was also maintained at relatively low levels. Interestingly, large amounts of PAF were produced even in unstimulated cells upon addition of an excess of exogenous lysoPAF, suggesting that generation of an adequate amount of lysoPAF within cells may be sufficient to trigger PAF synthesis in this type of cells.

Transmembrane signalling and paf-acether biosynthesis

Expression of lyso paf-acether (lyso paf):acetyl-CoA acetyltransferase and its activation above basal levels by specific agonists controls the rate of paf biosynthesis in proinflammatory cells. Acetyltransferase activation in these cells is due to the rapid postranslational modification of an inactive precursor by phosphorylation, most probably catalyzed by a cAMP-dependent kinase. However, the possibility exists that a calcium/calmodulin-dependent kinase can be implicated as well. Unlike murine cultured mast cells, human neutrophils form paf when stimulated with phorbol myristate acetate (PMA) or diacylglycerol. In both cell types, acetyltransferase is activated by PMA. Controversy exists as to whether PMA activates the remodeling pathway, i.e. the activation of phospholipase A2 and acetyltransferase, or the de novo route through CDPcholine cholinephosphotransferase action on alkylacetylglycerol. There is some indication that PKC might regulate paf biosynthesis. The implication of a GTP-regulated protein has also been postulated in signal transduction leading to paf formation in endothelial cells, neutrophils, and mast cells. The topography of paf formation is discussed in light of the subcellular distribution of acetyltransferase in neutrophils and Krebs II 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.

Acylation of alkyllysophospholipids by Fischer sarcoma microsomes

Acylation of alkyllysophospholipids in most cells occurs by: (a) CoA-independent transacylation, (b) CoA-dependent transacylation, and (c) acyl-CoA-dependent acylation. Using a recently developed high-performance liquid chromatography method, we have investigated the factors that influence the molecular species composition of the acylated products formed via these pathways with 1-hexadecyl-2-lyso-sn-glycero-3-phosphocholine (alkyllyso-GPC) or 1-hexadecyl-2-lyso-sn-glycero-3-phospho-ethanolamine (alkyllyso-GPE) as substrates for the enzymes in Fischer R-3259 sarcoma microsomes. We found that short incubation times and low substrate concentrations favored the formation of polyunsaturated molecular species, i.e., 16:0-22:6, 16:0-22:5 (n - 3), and 16:0-20:4. Also, in agreement with results from other systems, CoA-independent transacylation produced a high percentage of polyunsaturated molecular species; acyl-CoA-dependent acylations generated the least polyunsaturated molecular species and CoA-dependent transacylation gave intermediate values. Furthermore, no substrate selectivity occurred with respect to alkyl chain lengths of alkyllyso-GPE; similar molecular species composition was obtained with either hexadecyllyso-GPE or octadecyllyso-GPE as substrates. Responses to N-ethylmaleimide inhibition and heat inactivation as well as pH optima suggest the same enzyme catalyzes the CoA-independent transacylation of both alkyllyso-GPC and alkyllyso-GPE.

Platelet-activating factor and related acetylated lipids as potent biologically active cellular mediators

Platelet-activating factor (PAF or 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is the most potent lipid mediator yet discovered. It is known to stimulate a wide span of biological responses ranging from aggregation and degranulation of platelets and neutrophils to a variety of cellular effects involving the stimulation of chemotaxis; chemokinesis; superoxide formation; protein phosphorylation; activation of protein kinase C, arachidonic acid, and phosphoinositide metabolites; glycogenolysis; and tumor necrosis factor production. Obviously, with such a diversity of biological activities, it is not surprising that PAF has been considered to be a key component in numerous diseases related to hypersensitivity and inflammatory responses. Evidence has also been presented for the role of PAF in physiological processes, particularly those involving reproduction and fetal development. Furthermore, because of its potent hypotensive action, PAF has been implicated as a contributing factor in blood pressure regulation. PAF is produced by two independent enzymatic pathways. The remodeling route involves the structural modification of a membrane lipid (1-alkyl-2-acyl-sn-glycero-3-phosphocholine) by replacement of the acyl moiety with an acetate group. An alternate route is the de novo synthesis of PAF from an O-alkyl analogue of a lysophosphatidic acid that requires a reaction sequence of acetylation, dephosphorylation, and phosphocholine addition steps. Hypersensitivity and other pathophysiological reactions are thought to be caused by activation of the remodeling pathway, whereas the de novo route is believed to be the source of endogenous levels of PAF required for physiological functions. Inactivation of PAF occurs when the acetate group is hydrolyzed by an acetylhydrolase that is present in both extra- and intracellular compartments, although the catalytic activity of the two forms of acetylhydrolase are identical, some of their properties differ. The control of PAF metabolism is very complex, but acetylhydrolase, Ca2+, phosphorylation/dephosphorylation of enzymes, and fatty acids (especially polyunsaturates) appear to be important regulatory factors. Specific PAF receptors have clearly been demonstrated on several different types of cells, and although the mechanism of PAF actions is poorly understood, it appears that the PAF/receptor-induced responses are closely associated with the signal transduction process; both G proteins and adenyl cyclase appear to be involved. Because significant quantities of PAF are often retained within certain cells, the possibility of PAF serving as an intracellular mediator has also been proposed.

Very high proportion of disaturated molecular species in rat platelet diacyl-glycerophosphocholine: involvement of CoA-dependent transacylation reactions

The molecular species composition of rat platelet diacyl-glycerophosphocholine (GPC) was investigated by reverse-phase HPLC and by mass spectrometry. The two methods gave the same very high proportion of fully saturated phospholipids, the 16:0-16:0 and 16:0-18:0 species representing together about 40% of the overall molecular species. [14C]Palmitoyllyso-GPC was found to be acylated by resting platelets in equal amounts into 16:0-16:0 and into 16:0-20:4 species. The acylation rate of this lysophospholipid was increased by 3-fold and 14-fold when platelets were stimulated for 10 min with thrombin and the ionophore A23187, respectively. Essentially the same two molecular species were synthesized upon stimulation but with a higher preference for arachidonate than for palmitate. We investigated the mechanisms responsible for the incorporation of palmitate and arachidonate by examining the enzymatic acylation of [14C]palmitoyllyso-GPC by platelet homogenates. The percentage of the various molecular species formed when CoA, ATP, and Mg2+ were added excludes the CoA, ATP-dependent pathway as being involved in the acylation reactions previously observed. In the absence of ATP, CoA-independent transacylations appear to play a crucial role in the synthesis of the 16:0-20:4 species whereas the addition of CoA greatly favored dipalmitoyl-GPC synthesis. The involvement of CoA-dependent mechanisms in the synthesis of dipalmitoyl-GPC was demonstrated as follows: (i) the labeling in the sn-2 position of the dipalmitoyl-GPC synthesized in the presence of CoA was not modified when free unlabeled palmitic acid was added to the incubation medium and (ii) platelet homogenates were unable to esterify lysolecithin with added labeled palmitic acid in the presence of CoA only.

Arachidonic acid incorporation and redistribution in human neuroblastoma (SK-N-BE) cell phospholipids

The incorporation and redistribution of [1-14C]arachidonic acid in SK-N-BE human neuroblastoma cell phospholipids were investigated. By continuous labelling in serum-enriched medium, a rapid radioactivity incorporation into phosphatidylcholine (PtdCho), phosphatidylinositol, and phosphatidylserine was observed; initially, phosphatidylethanolamine (PtdEtn) was poorly labelled, but at later stages it displayed the highest level of arachidonic acid incorporation, in comparison with other phospholipid classes. Labelling of triacylglycerols was also observed. When cells were pulse-labelled with [1-14C]arachidonic acid and then reincubated in label-free medium, a decrease of the radioactivity in triacylglycerols was observed initially, paralleled by an increase of phospholipid labelling; thereafter, arachidonic acid redistribution was consistent with a net decrease of the radioactivity associated with PtdCho acid-stable forms (i.e., diacyl plus alkylacyl forms), concomitantly with a net labelling increase of both acid-stable PtdEtn and alkenylacyl-PtdEtn. Data indicate the following: (a) neuroblastoma cells incorporate arachidonic acid into phospholipids through complex kinetics involving transfer of the fatty acid from acid-stable PtdCho to both alkenylacyl-PtdEtn and acid-stable PtdEtn; and (b) triacylglycerols act as storage molecules for arachidonic acid which is subsequently incorporated into phospholipids. The possibility that arachidonic acid transfer to PtdEtn subclasses is driven by distinct mechanisms is discussed.

Influence of immunologic activation and cellular fatty acid levels on the catabolism of platelet-activating factor within the murine mast cell (PT-18)

The present study has examined the catabolism of 1-O-[3H]hexadecyl-2-acetyl-GPC (C16-PAF) and of 1-O-octadecyl-2-acetyl-GPC (C18-PAF) in spleen-derived PT-18 murine mast cells (mast cells). Mast cells catabolized exogenous PAF into two inactive metabolites, 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine (lysoPAF) and 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine (1-O-alkyl-2-acyl-GPC). The rate of conversion of C16-PAF to metabolites was more rapid than that of C18-PAF. Analysis of the acyl composition of 1-O-alkyl-2-acyl-GPC formed during the metabolism of PAF revealed that arachidonic acid (20:4) was the major fatty acyl chain incorporated at the sn-2 position. However, 25% of newly formed 1-O-alkyl-2-acyl-GPC was reacylated with docosahexaenoic acid (22:6). The influence of cellular fatty acid content on PAF catabolism was further explored in mast cells in which the ratio of fatty acids within cellular phosphoglycerides had been altered by supplementing the cells with various fatty acids in culture. Mast cells supplemented with 20:4 or 22:6 converted PAF to 1-O-alkyl-2-acyl-GPC at a significantly higher rate than non-supplemented cells. In contrast, cells supplemented with linoleic acid (18:2) metabolized PAF at rates similar to non-supplemented cells. Analysis of the acyl composition of 1-O-alkyl-2-acyl-GPC derived from the metabolism of PAF in 20:4-supplemented cells indicated that 20:4 was incorporated exclusively into the sn-2 position. Conversely, 22:6-supplemented cells incorporated predominantly 22:6 at the sn-2 position of 1-alkyl-2-lyso-GPC. Supplementation with 18:2 had no effect on the acylation pattern seen in newly formed 1-O-alkyl-2-acyl-GPC. Activation of passively sensitized mast cells with antigen or with ionophore A23187 significantly enhanced the rate of catabolism of exogenously-provided PAF but had no effect on the acylation pattern of 1-O-alkyl-2-acyl-GPC. Experiments performed with the soluble fraction of the cells showed that acetyl hydrolase activity was increased in mast cells stimulated with antigen. In addition, supernatant fluids from antigen or ionophore-treated mast cells converted PAF to lysoPAF, suggesting that acetyl hydrolase activity was released during cell activation. These data indicate that the ability of mast cells to catabolize PAF to inactive metabolites is influenced by cell activation and by the cellular levels of certain fatty acids.

Selective acyl transfer in the reacylation of brain glycerophospholipids. Comparison of three acylation systems for 1-alk-1'-enylglycero-3-phosphoethanolamine, 1-acylglycero-3-phosphoethanolamine and 1-acylglycero-3-phosphocholine in rat brain microsomes

The activities of three acylation systems for 1-alkenylglycerophosphoethanolamine (1-alkenyl-GPE), 1-acyl-GPE and 1-acylglycerophosphocholine (1-acyl-GPC) were compared in rat brain microsomes and the acyl selectivity of each system was clarified. The rate of CoA-independent transacylation of 1-[3H]alkenyl-GPE (approx. 4.5 nmol/10 min per mg protein) was about twice as high as in the case of 1-[3H]acyl-GPE and 1-[14C]acyl-GPC. On the other hand, the rates of CoA-dependent transacylation and CoA + ATP-dependent acylation (acylation of free fatty acids by acyl-CoA synthetase and acyl-CoA acyltransferase) of lysophospholipids were in the order 1-acyl-GPC greater than 1-acyl-GPE much greater than 1-alkenyl-GPE. HPLC analysis of newly synthesized molecular species revealed that the CoA-independent transacylation system exclusively esterified docosahexaenoate and arachidonate, regardless of the lysophospholipid class. The CoA-dependent transacylation and CoA + ATP-dependent acylation systems were almost the same with respect to the selectivities for unsaturated fatty acids when the same acceptor lysophospholipid was used, but some distinctive acyl selectivities were observed with different acceptor lysophospholipids. 1-Alkenyl-GPE selectively acquired only oleate in these two systems. 1-Acyl-GPE and 1-acyl-GPC showed selectivities for both arachidonate and oleate. In addition, an appreciable amount of palmitate was transferred to 1-acyl-GPC, not to 1-acyl-GPE, in CoA- or CoA + ATP-dependent manner. The acylation of exogenously added acyl-CoA revealed that the acyl selectivities of the CoA-dependent transacylation and CoA + ATP-dependent acylation systems may be mainly governed through the selective action of acyl-CoA acyltransferase. The preferential utilization of oleoyl-CoA by all acceptors and the different utilization of arachidonoyl-CoA between alkenyl and acyllysophospholipids indicated that there might be two distinct acyl-CoA:lysophospholipid acyltransferases that discriminate between oleoyl-CoA and arachidonoyl-CoA, respectively. Our present results clearly show that all three microsomal acylation systems can be active in the reacylation of three major brain glycerophospholipids and that the higher contribution of the CoA-independent system in the reacylation of ethanolamine glycerophospholipids, especially alkenylacyl-GPE, may tend to enrich docosahexaenoate in these phospholipids, as compared with in the case of diacyl-GPC.

Studies on the incorporation and transacylation of various fatty acids in choline and ethanolamine-containing phosphoacylglycerol subclasses in human neutrophils

The incorporation of eight 14C-labeled fatty acids into human neutrophil phospholipids was investigated and the results were expressed as percent of the total phospholipid associated 14C-labeled substrate incorporated after an initial 15 min labeling and a subsequent 2 h reincubation in fatty acid-free buffer. In all cases, the PC fraction accounted for more than 40% of the total phospholipid radioactivity. The inositol-containing phosphoacylglycerols were also labeled well by all the fatty acids except 22:6(n - 3) and 16:0; however, a greater percentage of [14C]22:6(n - 3) was found in PE than that of any other labeled fatty acid substrate. In all cases, most of the radioactivity in PC after 15 min was in the diacyl subclass. After 2 h, there was a shift of [14C]20:4(n - 6), [14C]20:5(n - 3), [14C]22:6(n - 3) and [14C]18:4(n - 4) into the ether-linked subclass. No such shift was observed for [14C]16:0 or [14C]18:2(n - 6) and, although there was an increase in the percent radioactive 20:3(n - 6) and 20:3(n - 9) in ether-linked PC after 2 h, the total radioactivity in this fraction remained low by comparison. A similar shift in label occurred in the plasmalogenic-linked PE subspecies in cells labeled with [14C]20:4(n - 6), [14C]20:5(n - 3) and [14C]22:6(n - 3).

Synthesis of platelet activating factor by tissues from the rainbow trout, Salmo gairdneri

In mammals, platelet activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a lipid mediator with biological activity at concentrations in the subnanomolar range. Although PAF is known to have many activities in mammals, little is known about its synthesis and importance in other vertebrate groups. We demonstrate here the synthesis of PAF from [3H]acetate by slices of trout gill, kidney, liver and spleen. PAF synthesis was stimulated by the calcium ionophore A23187 and was time-dependent. The radiolabeled PAF produced was characterized by TLC, HPLC, derivatization and by saponification and phospholipase A2 hydrolysis. These findings suggest that PAF may be an important mediator in fish.

Sources for brain arachidonic acid uptake and turnover in glycerophospholipids

Brain arachidonic acid comes from linoleic acid and arachidonic acid in the blood. Part of the brain arachidonic acid is elongated to adrenic acid, 22:4 (n = 6), especially in higher animals. With labeled arachidonic acid injected into cerebral ventricles of mice, the highest specific radioactivity was in triacylglycerols. The highest labeling in PtdCho and PtdIns was found at 15 to 60 minutes. Labeling of PtdEtn was much less. The molecular species with 16:0 and 18:1 were labeled better than those with 18:0. Adrenic acid was preferred by alkylacyl-GroPEtn. The highest level of labeling by arachidonic acid was found in the choline plasmalogens and the alkylacyl-GroPCho at 24 hours after injection. The PtdCho arachidonic acid turned over several times within 24 hours. Part of this turnover probably represents the transfer of labeled arachidonic acid to unlabeled ether-linked choline glycerophospholipids, including 1-alkyl-2-lyso-GroPCho, also known as lyso platelet activating factor. The energy-independent transfer of arachidonic acid from PtdCho to ether-linked choline glycerophospholipids may follow removal of their arachidonic acid by phospholipase A2 due to receptor activation. The lack of pulse labeling of ether-linked choline glycerophospholipids complicates the study of their function.

Distribution of omega-3 and omega-6 fatty acids in the ether- and ester-linked phosphoglycerides from tissues of the rainbow trout, Salmo gairdneri

1. Data presented here demonstrate that polyunsaturated fatty acids in the phospholipids of rainbow trout tissues are compartmentalized differently than in mammalian tissues. 2. We have determined the distribution of omega-3 (n-3) and omega-6 (n-6) fatty acids in the alkyl-, alk-1-enyl-, and diacyl- subclasses of phosphatidylcholines (PC), phosphatidyl-ethanolamines (PE), phosphatidylinositols (PI), and phosphatidylserines (PS) from gill, kidney and spleen of rainbow trout. 3. Alkyl-linked PC and alk-1-enyl-linked PE were the most abundant ether-containing phospholipids, amounting to 10-15% of each class; no ether-linked PI or PS was detected. 4. C20:4 n-6 was found in high concentrations only in PI; the n-3 fatty acids were found in highest concentration in the ether-linked phospholipids as compared with the diacyl subclasses and C20:5 n-3 was especially prevalent in 1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine and C22:6 n-3 was prevalent in PS.

Evidence for protein-catalyzed transfer of platelet activating factor by macrophage cytosol

Platelet activating factor (PAF) is a potent, proinflammatory lipid. PAF is synthesized in response to stimuli and is rapidly destroyed by specific acetylhydrolases. In order to express its biological activity, PAF and its metabolites are transported among subcellular membranes by as yet unexplained mechanisms. We report here an assay system using methylcarbamyl-PAF (CPAF, 1-O-hexadecyl-2-O-(N-methylcarbamyl)-sn-glycero-3-phosphocholine) and a vesicle-extrusion technique for examining protein-catalyzed intermembrane transfer of CPAF, and demonstrate the presence of proteins catalyzing the separate transfer of CPAF and diacyl phosphatidylcholine in macrophage cytosol. The CPAF transfer activity is heat- and trypsin-sensitive and elutes from gel-filtration columns well separated from proteins catalyzing the transfer of phosphatidylcholine.

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.

Acylation of dog heart lysophosphatidylserine by transacylase activity

Dog heart microsomes catalyze the transfer of acyl groups from the sn-2 position of phosphatidylcholine (PC) to lysophosphatidylserine (lysoPS) in the presence of coenzyme A (CoA) at pH optima of 4.5-5.0 and 7.5. Acyl transfer activity at acidic pH is about three times higher than at neutral pH. Transacylation of lysoPS by acyl transfer from PC with dog heart microsomes at neutral pH favors arachidonate over linoleate by a factor of 2.1, whereas free linoleic acid is favored by a factor of 3.7 over arachidonic acid for lysoPS acylation in the presence of acyl-CoA-generating cofactors. Considering the location and acyl composition of myocardial PS, it appears that both acyl transfer from PC and utilization of unesterified fatty acids may be involved in the acylation of lysoPS at its sn-2 position.

Analysis of phospholipid molecular species in rat lung as dinitrobenzoate diglycerides by electron capture negative chemical ionization mass spectrometry

The use of electron capture negative ion desorption chemical ionization mass spectrometry was demonstrated in the analysis of phospholipid molecular species at the 1,3-dinitrobenzoate (DNB) diglyceride derivative. Modification of phosphatidylcholine (PC) or phosphatidylethanolamine (PE) by phospholipase C treatment and acylation of the resultant diglyceride with 3,5-dinitrobenzoylchloride afforded separation of the alkylacyl, alkenylacyl, and diacyl dinitrobenzoate subclasses by thin-layer chromatography (TLC). Separation of alkylacyl DNB into individual molecular species by reverse-phase high-performance liquid chromatography (RPHPLC) was demonstrated. Electron capture desorption chemical ionization of individual molecular species (10-25 ng) from a direct probe yielded a mass spectrum characterized by an intense molecular anion. This molecular anion was the base peak of the spectrum accounting for greater than 80% of the total ionization. From this molecular anion the total carbon number and degree of unsaturation of the fatty chains could be determined. Analysis of fatty acid content of the molecular species allowed unequivocal assignment of structure for the alkyl ether phospholipids. Using selected ion monitoring as little as 0.5 pmol of these species could be detected with a signal-to-noise ratio greater than or equal to 3. This technique was useful in the analysis of low picomolar amounts of molecular species of ether phospholipids in the rat lung. Given an appropriate internal standard, analysis of dynamic changes in turnover, metabolism and precursor product relationships could be undertaken.

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.

The metabolism of lyso-platelet-activating factor (1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine) by a calcium-dependent lysophospholipase D in rabbit kidney medulla

A Ca2+-dependent lysophospholipase D activity in microsomal preparations from the rabbit kidney medulla hydrolyzes the choline moiety from 1-O-[9,10-3H]hexadecyl-2-lyso-sn-glycero-3-phosphocholine (lyso-PAF) to form 1-O-[9,10-3H]hexadecyl-2-lyso-sn-glycero-3-P; the latter is subsequently dephosphorylated by a phosphohydrolase to 1-O-[9,10-3H]hexadecyl-sn-glycerol. Sodium vanadate, which is known to inhibit phosphohydrolases, reduces the proportion of hexadecylglycerol and increases the formation of hexadecyl-lysoglycerophosphate. Essentially no hydrolysis occurs when the sn-2 position of the hexadecyllysoGPC substrate contains an acyl moiety. The lysophospholipase D in rabbit kidney is of microsomal origin and has a broad pH optimum between 8.0 and 8.8, with the activity decreasing sharply from pH 7.6 to 7.2. Wykle et al. (Biochim. Biophys. Acta 619 (1980) 58-67) have previously demonstrated the existence of a microsomal lysophospholipase D (specific for ether lipid substrates) in rat tissues that requires Mg2+ and exhibits a pH optimum of 7.2; high activities of the Mg2+-dependent lysophospholipase D were found in liver and brain, but not in kidney. In contrast to the Mg2+-dependent lysophospholipase D in rat tissues, the renal enzyme from rabbits requires Ca2+ (5 mM), whereas Mg2+ (5 mM) exhibits little stimulatory action. Under optimal assay conditions (0.1 M Tris-HCl (pH 8.4)/5 mM CaCl2), lysophospholipase D in the rabbit kidney medulla has an activity of 2.7 nmol/min per mg protein compared to 0.9 nmol/min per mg protein for the lysophospholipase D in the rat kidney medulla (0.1 M Tris-HCl (pH 7.2)/5 mM MgCl2). The Ca2+-dependent lysophospholipase D is highest in the liver and kidney medulla from rabbits, but is very low in rat tissues; similar activities were found in male and female rabbits. Our data indicate that the divalent metal ion requirements for expression of maximum lysophospholipase D activities can differ markedly among animal species and also suggest the microsomal Ca2+-dependent lysophospholipase D is an important catabolic route for lyso-PAF metabolism in rabbit renomedullary tissue.