Heterogeneity in the metabolism of the arachidonoyl molecular species of glycerophospholipids of rabbit alveolar macrophages. The interrelationship between metabolic activities and chemical structures of the arachidonoyl molecular species

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.

Substrate specificity of acyl-CoA:Lysophospholipid acyltransferase (LAT) from pig spleen

The present investigation was undertaken to gain insights into the nature of both substrate binding sites of acyl-CoA:lysophospholipid acyltransferase (LAT) which could be potentially useful for the identification and purification of this specific acyltransferase. Therefore, we have investigated the specificity of LAT from crude membranes of pig spleen toward various 1-palmitoyl-glycerophospholipids and 1-acyl-glycerophosphocholines (1-acyl-GPC). The enzyme showed the highest specificity toward 1-acyl-GPC and was able to distinguish between the acyl-chain length of the 1-acyl group within the 1-acyl-GPC molecule. We found preferential reactivity in the order C10:0 < C12:0 << C14:0, C18:0, C16:0 < C18:1 of 1-acyl-GPC. Lysophosphatidic acid or 1-O-alkyl-GPC were only poor substrates for the enzyme. In competition studies we could show that palmitic acid, oleic acid, arachidonic acid, and palmitoyl-CoA competitively inhibited LAT activity, whereas the coenzyme A failed to inhibit LAT enzyme activity in a concentration-dependent manner. We concluded that the ligand acyl-CoA is bound via its acyl chain. The finding that palmitoyl-CoA was a poor substrate as well as an inhibitor was the basis for protein purification. When palmitoyl-CoA-agarose was used as matrix for affinity chromatography, LAT enzyme activity was bound and eluted by high salt concentrations yielding an estimated 10-fold purification of the solubilized LAT enzyme.

Changes in arachidonic acid metabolism and the aggregation of polymorphonuclear leukocytes in rats with streptozotocin-induced diabetes

Changes in lipid composition were examined in peritoneal polymorphonuclear leukocytes (PMNL) from rats with streptozotocin-induced diabetes. The largest changes associated with the lipids in PMNL prepared from rats with insulin-dependent diabetes mellitus were found in the composition of fatty acids of phospholipids although no consistent changes were noted in the amounts of phospholipids in individual classes. The relative amount of arachidonic acid was significantly reduced and that of linoleic acid was significantly increased in PMNL from diabetic rats. The extent of A23187-stimulated release of arachidonic acid from CGP and IGP in PMNL from diabetic rats was much smaller than that from normal PMNL. Moreover, there were decreases in the production of free arachidonic acid and its metabolites, such as LTB4 and 5-HETE by A23187-stimulated PMNL from diabetic rats as compared to PMNL from normal rats. Aggregation of PMNL provoked by A23187 was significantly suppressed in PMNL from diabetic rats. However, addition of free arachidonic acid or LTB4 to normal PMNL and to those from diabetic rats resulted in aggregation to similar extents, a result that supports a role for enhanced availability of endogenous arachidonic acid in the induction of the aggregation of PMNL from diabetic rats. The present results suggest that diabetes-associated changes in arachidonic acid metabolism might play a critical role in the modulation of aggregation of PMNL.

Mass determination of the fatty acids released from tannin-stimulated rabbit alveolar macrophages

Previous studies with macrophages that had been prelabeled with [14C]arachidonic acid (20:4) have shown that condensed tannin is a potent agonist for the release of arachidonic acid. However, it has not been demonstrated that the percentage release of [14C]20:4 accurately reflects the metabolic activity of the endogenous 20:4 pool. In order to measure the 20:4 mass release relative to the total cellular 20:4 pool, the free fatty acids of freshly isolated alveolar macrophages were derivatized with a fluorescent reagent, and then separated and quantified by high-performance liquid chromatography. The amounts of esterified fatty acids were measured by gas chromatography of the methyl esters. Free fatty acid levels were compared to those of the total esterified plus unesterified fatty acids to determine the actual percentage release of each fatty acid. Tannin-stimulated release of 20:4 mass reflected that previously reported for the release of [14C]20:4 label but at a slower rate and at a much lower percentage indicating that [14C]20:4 had been incorporated into part of a more reactive pool. The specificity of the fatty acid release induced by tannin and beta-1,3-glucan, a known agonist for 20:4 release, was also examined. Both agonists promoted an increase in the levels of free 20:4 and of other fatty acids. A comparison of the absolute increases of each of the fatty acids indicated that tannin caused a preferential increase in the mass of free 20:4, whereas beta-1,3-glucan evoked a selective increase in the mass of 16:0.

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.

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)

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.

Stimulated production and natural occurrence of 1,2-diarachidonoylglycerophosphocholine in human neutrophils

Incorporation of arachidonic acid into phospholipid molecular species of the human neutrophil was found to be dependent, to a large extent, upon the concentration of arachidonate used during the in vitro incubations. When high concentrations of [3H] arachidonate were employed, only two glycerolipids incorporated label. One glycerolipid was a unique glycerophospholipid characterized by HPLC retention time and fast atom bombardment mass spectrometry as 1,2-diarachidonoyl-sn-glycero-3-phosphocholine. The second and most highly labeled glycerolipid was found to be arachidonoyl triacylglycerol species. Human neutrophils isolated from normal individuals and not previously exposed to arachidonic acid in vitro were found to contain a small but measurable amount of diarachidonoyl-GPC. The dose-dependent increase of diarachidonoyl-GPC and arachidonoyl-labeled triacylglycerol when cells were exposed to increasing concentrations of arachidonic acid implied that these lipid molecular species have the capacity to expand their pools, perhaps in manner regulating levels of endogenous arachidonic acid for further metabolism. These observations point to the importance of the concentration of arachidonic acid employed during in vitro labeling studies.