Consumption of Enriched Yogurt with PAF Inhibitors from Olive Pomace Affects the Major Enzymes of PAF Metabolism: A Randomized, Double Blind, Three Arm Trial

Platelet-activating factor (PAF), a proinflammatory lipid mediator, plays a crucial role in the formation of the atherosclerotic plaque. Therefore, the inhibition of endothelium inflammation by nutraceuticals, such as PAF inhibitors, is a promising alternative for preventing cardiovascular diseases. The aim of the present study was to evaluate the impact of a new functional yogurt enriched with PAF inhibitors of natural origin from olive oil by-products on PAF metabolism. Ninety-two apparently healthy, but mainly overweight volunteers (35-65 years) were randomly allocated into three groups by block-randomization. The activities of PAF's biosynthetic and catabolic enzymes were measured, specifically two isoforms of acetyl-CoA:lyso-PAF acetyltransferase (LPCATs), cytidine 5'-diphospho-choline:1-alkyl-2-acetyl-sn-glycerol cholinephosphotransferase (PAF-CPT) and two isoforms of platelet activating factor acetylhydrolase in leucocytes (PAF-AH) and plasma (lipoprotein associated phospholipase-A₂, LpPLA₂). The intake of the enriched yogurt resulted in reduced PAF-CPT and LpPLA₂ activities. No difference was observed in the activities of the two isoforms of lyso PAF-AT. In conclusion, intake of yogurt enriched in PAF inhibitors could favorably modulate PAF biosynthetic and catabolic pathways.

Leukocyte-Mediated Combined Targeted Chemo and Gene Therapy for Esophageal Cancer

Poor prognosis of esophageal cancer is associated with limited clinical treatment efficacy and lack of targeted therapies. With advances in nanomedicine, nanoparticle drug delivery systems play increasingly important roles in tumor treatment by enabling the simultaneous delivery of multiple therapeutic agents. We here propose a novel nanovector for targeted combination gene therapy and chemotherapy in esophageal cancer. A novel lipid nanovector (EYLN) was designed to carry the chemotherapy drug doxorubicin (Dox) and small interfering RNA against the lipid anabolic metabolism gene LPCAT1, which we previously showed to be significantly overexpressed in esophageal cancer tissues, and its interference inhibited the proliferation, invasion, and metastasis of esophageal cancer cells. This vector, EYLN-Dox/siLPCAT1, was further coated with leukocyte membranes to obtain mEYLNs-Dox/siLPCAT1. The particle size of the coated nanovector was approximately 136 nm, and the surface zeta potential was -21.18 mV. Compared with EYLNs-Dox/siLPCAT1, mEYLNs-Dox/siLPCAT1 were more easily internalized by esophageal cancer cells due to the LFA-1 highly expressed leukocyte membrane coating and showed significant inhibition of the proliferation, migration, and metastasis of esophageal cancer cells, along with their LPCAT1 expression, through more effective delivery of the drugs. Moreover, the nanovectors showed improved blood circulation time, tissue distribution, tumor targeting, and tumor suppression in a mouse model. Thus, combining chemo and gene therapy with this new nanodelivery system achieved greater therapeutic efficacy, providing a new strategy for the treatment of esophageal cancer.

Down-regulation of LPCAT expression increases platelet-activating factor level in cirrhotic rat liver: potential antiinflammatory effect of silybin

Cholestasis is one of the major causes of liver diseases. A chronic accumulation of toxic bile acids in the liver, which occurs in this condition, can induce fibrosis and cirrhosis. Inflammation is a fundamental component of acute and chronic cholestatic liver injury. Platelet-activating factor (PAF) is a proinflammatory lipid which may be generated by two independent pathways called the de novo and remodeling pathway being the last responsible for the synthesis of PAF during inflammation. In recent years a key role in PAF remodeling has been attributed to lysophosphatidylcholine acyltransferase (LPCAT) enzymes. Although the knowledge on their characteristic is growing, the exact mechanism of LPCAT in pathological conditions remains still unknown. Here, we reported that the level of lyso-PAF and PAF significantly increased in the liver of cirrhotic vs. control rats together with a significant decrease in both mRNA abundance and protein level of both LPCAT1 and LPCAT2. Acyltransferase activities of both LPCAT1 and LPCAT2 were parallel decreased in the liver of cirrhotic animals. Interestingly, treatment with silybin strongly decreased the level of both pro-inflammatory lipids and restored the activity and expression of both LPCAT1 and LPCAT2 of cirrhotic liver. Silybin effect was specific for LPCAT1 and LPCAT2 since it did not affect LPCAT3 mRNA abundance of cirrhotic liver.

A unique lysophospholipid acyltransferase (LPAT) antagonist, CI-976, affects secretory and endocytic membrane trafficking pathways

Previous studies have shown that inhibition of a Golgi-complex-associated lysophospholipid acyltransferase (LPAT) activity by the drug CI-976 stimulates Golgi tubule formation and subsequent redistribution of resident Golgi proteins to the endoplasmic reticulum (ER). Here, we show that CI-976 stimulates tubule formation from all subcompartments of the Golgi complex, and often these tubules formed independently, i.e. individual tubules usually did not contain markers from different subcompartments. Whereas the cis, medial and trans Golgi membranes redistributed to the ER, the trans Golgi network (TGN) collapsed back to a compact juxtanuclear position similar to that seen with brefeldin A (BFA) treatment. Also similar to BFA, CI-976 induced the formation of endosome tubules, but unlike BFA, these tubules did not fuse with TGN tubules. Finally, CI-976 produced an apparently irreversible block in the endocytic recycling pathway of transferrin (Tf) and Tf receptors (TfRs) but had no direct effect on Tf uptake from the cell surface. Tf and TfRs accumulated in centrally located, Rab11-positive vesicles indicating that CI-976 inhibits export of cargo from the central endocytic recycling compartment. These results, together with previous studies, demonstrate that CI-976 inhibits multiple membrane trafficking steps, including ones found in the endocytic and secretory pathways, and imply a wider role for lysophospholipid acyltransferases in membrane trafficking.

Inhibition of a Golgi complex lysophospholipid acyltransferase induces membrane tubule formation and retrograde trafficking

Recent studies have suggested that formation of Golgi membrane tubules involves the generation of membrane-associated lysophospholipids by a cytoplasmic Ca2+-independent phospholipase A2 (PLA2). Herein, we provide additional support for this idea by showing that inhibition of lysophospholipid reacylation by a novel Golgi-associated lysophosphatidylcholine acyltransferase (LPAT) induces the rapid tubulation of Golgi membranes, leading in their retrograde movement to the endoplasmic reticulum. Inhibition of the Golgi LPAT was achieved by 2,2-dimethyl-N-(2,4,6-trimethoxyphenyl)dodecanamide (CI-976), a previously characterized antagonist of acyl-CoA cholesterol acyltransferase. The effect of CI-976 was similar to that of brefeldin A, except that the coatomer subunit beta-COP remained on Golgi-derived membrane tubules. CI-976 also enhanced the cytosol-dependent formation of tubules from Golgi complexes in vitro and increased the levels of lysophosphatidylcholine in Golgi membranes. Moreover, preincubation of cells with PLA2 antagonists inhibited the ability of CI-976 to induce tubules. These results suggest that Golgi membrane tubule formation can result from increasing the content of lysophospholipids in membranes, either by stimulation of a PLA2 or by inhibition of an LPAT. These two opposing enzyme activities may help to coordinately regulate Golgi membrane shape and tubule formation.

Acylation of lysophosphatidylcholine plays a key role in the response of monocytes to lipopolysaccharide

Mononuclear phagocytes play a pivotal role in the progression of septic shock by producing tumor necrosis factor-alpha (TNF-alpha) and other inflammatory mediators in response to lipopolysaccharide (LPS) from Gram-negative bacteria. Our previous studies have shown monocyte and macrophage activation correlate with changes in membrane phospholipid composition, mediated by acyltransferases. Interferon-gamma (IFN-gamma), which activates and primes these cells for enhanced inflammatory responses to LPS, was found to selectively activate lysophosphatidylcholine acyltransferase (LPCAT) (P < 0.05) but not lysophosphatidic acid acyltransferase (LPAAT) activity. When used to prime the human monocytic cell line MonoMac 6, the production of TNF-alpha and interleukin-6 (IL-6) was approximately five times greater in cells primed with IFN-gamma than unprimed cells. Two LPCAT inhibitors SK&F 98625 (diethyl 7-(3,4,5-triphenyl-2-oxo2,3-dihydro-imidazole-1-yl)heptane phosphonate) and YM 50201 (3-hydroxyethyl 5,3'-thiophenyl pyridine) strongly inhibited (up to 90%) TNF-alpha and IL-6 production in response to LPS in both unprimed MonoMac-6 cells and in cells primed with IFN-gamma. In similar experiments, these inhibitors also substantially decreased the response of both primed and unprimed peripheral blood mononuclear cells to LPS. Sequence-based amplification methods showed that SK&F 98625 inhibited TNF-alpha production by decreasing TNF-alpha mRNA levels in MonoMac-6 cells. Taken together, the data from these studies suggest that LPCAT is a key enzyme in both the pathways of activation (priming) and the inflammatory response to LPS in monocytes.

Sesamin inhibits lysophosphatidylcholine acyltransferase in Mortierella alpina

The filamentous fungus, Mortierella alpina, accumulates complex lipids relatively rich in arachidonic acid (C(20:4) Delta(5,8,11,14)). The lignan, sesamin, has been used to reduce arachidonic acid production by specifically inhibiting Delta(5)-desaturation [Shimizu, Akimoto, Shinmen, Kawashima, Sugano and Yamada (1991) Lipids 26, 512-516]. Microsomal membrane preparations from M. alpina exhibit acyl-CoA:1-acyl lysophosphatidylcholine acyltransferase (LPCAT) activity. LPCAT is an enzyme involved in channelling fatty acid substrates to phosphatidylcholine for subsequent desaturation. Sesamin was found to inhibit this enzyme as measured in both spectrophotometric and radioactive assays. The inhibitory effect of sesamin on LPCAT was only evident in species of Mortierella and could not be demonstrated in other organisms.

Effects of inhibitors of arachidonic acid turnover on the production of prostaglandins by the guinea-pig uterus

The supply of free arachidonic acid from phospholipids is generally regarded as the rate-limiting step for prostaglandin (PG) synthesis by tissues. Two enzymes involved in arachidonic acid uptake into, and release from, phospholipids are acyl-CoA:lysophospholipid acyltransferase (ACLAT) and phospholipase A2 (PLA2), respectively. PGF2 alpha produced by the endometrium induces luteolysis in several species including guinea-pigs. Thimerosal, an inhibitor of ACLAT, and aristolochic acid, an inhibitor of PLA2, both reduced, in a concentration-dependent manner, the output of PGF2 alpha from guinea-pig endometrium cultured for 24 h on days 7 and 15 of the oestrous cycle. This study showed that the continual production of PGF 2 alpha by guinea-pig endometrium is not only dependent upon the activity of PLA2 for releasing free arachidonic acid for PGF2 alpha synthesis, but also on the incorporation of arachidonic acid into the phospholipid pool by the activity of ACLAT. The inhibitory effects of thimerosal and aristolochic acid on the outputs of PGE2 and 6-keto-PGF1 alpha were less marked, particularly on day 7 when the low output of PGE2 was unaffected and the output of 6-keto-PGF1 alpha was increased at the lower concentrations of thimerosal. This finding indicates that there are different pools of arachidonic acid bound as phospholipid for the syntheses of PGF2 alpha and 6-keto-PGF1 alpha by guinea-pig endometrium.

Synthesis of azidophospholipids and labeling of lysophosphatidylcholine acyltransferase from developing soybean cotyledons

A photoreactive substrate analog of lysophosphatidylcholine (LPC), 1-([(4-azidosalicyl)-12-amino)]dodecanoyl-sn-glycerol-3-phospho cholin e (azido-LPC) was synthesized. Fast atom bombardment mass spectrometry was employed to confirm the structures of azido-LPC and its intermediates. Azido-LPC was used to label putative acyl-CoA:LPC acyltransferase from microsomal membranes of developing soybean cotyledons. The synthesized substrate analog acts as a substrate for the target acyltransferases and phospholipases in the dark. When the microsomal membranes were incubated with the acyl acceptor analog and immediately photolyzed, LPC acyltransferase was irreversibly inhibited. Photoinactivation of the enzyme by the photoprobe decreased in the presence of LPC. Microsomal membranes were photolyzed with 125I-labeled azido-LPC and analyzed by SDS-PAGE followed by autoradiography. These revealed that the analog preferentially labeled 54- and 114-kDa polypeptides. Substrate protected the labeling of both the polypeptides. In our earlier report, the same polypeptides were also labeled with photoreactive acyl-CoA analogs, suggesting that these polypeptides could be putative LPC acyltransferase(s). These results demonstrated that the photoreactive phospholipid analog could be a powerful tool to label acyltransferases involved in lipid biosynthesis.

Cholesterol-loading of membranes of normal erythrocytes inhibits phospholipid repair and arachidonoyl-CoA:1-palmitoyl-sn-glycero-3-phosphocholine acyl transferase. A model of spur cell anemia

Spur cell anemia may occur in severe liver disease including alcoholic cirrhosis. Spur cell anemia red blood cells (RBCs) have a characteristic morphology, with irregular projections, an increased ratio of membrane cholesterol (Ch) to phospholipid, evidence of oxidative damage, and shortened survival resulting in hemolytic anemia. Normal RBCs may acquire many of the features of spur cells either by transfusion into a spur cell patient or in an in vitro model system that loads the RBC membrane with Ch relative to phospholipid by means of Ch-rich, phospholipid-Ch sonicates. We found evidence of abnormal phospholipid repair metabolism in spur cell anemia RBCs characterized by decreased arachidonate (Ar) uptake into phospholipids and by increased uptake into a fatty acid membrane repair intermediate, acylcarnitine (AcylCn). To study the possible modulation of phospholipid repair metabolism in spur cells by Ch-loading, we compared the Ar metabolism of RBCs loaded with Ch in vitro with that of control cells incubated in autologous serum. Ar, a polyunsaturated fatty acid, is especially sensitive to peroxidation and, thus, is likely to be involved in phospholipid repair. Ch-loading decreased the incorporation of [14C]Ar into total lipids (Ch-loaded, 1,113 +/- 48 pmol/10(10) RBCs; control, 1,525 +/- 48 pmol/10(10) RBCs) including phosphatidylethanolamine, phosphatidylserine, and phosphatidylcholine. Uptake of [14C]Ar into AcylCn increased (control AcylCn, 169 +/- 31 pmol/10(10) RBCs; Ch-loaded AcylCn, 196 +/- 35 pmol/10(10) RBCs; P = .0012). Thimerosal, an inhibitor of arachidonoyl- CoA:l-palmitoyl-sn-glycero-3-phosphocholine acyl transferase or lysophosphocholine acyl transferase (LAT), produced a similar pattern of metabolic abnormality, with decreased incorporation into phospholipid but relative increase into AcylCn. We assayed LAT in RBC membranes from Ch-loaded RBCs, using [14C]arachidonoyl CoA as precursor, and found similar decreased LAT activity at concentrations of 1-palmitoyllysophosphatidylcholine (LPC) from 1 to 30 micromol/L. Similar LAT assay results were obtained using [14C]palmitoyl LPC as the precursor. We conclude that Ch-loading of RBC membranes results in inhibition of LAT in the cell-free system in vitro and may account for the inhibited phospholipid repair in Ch-loaded intact RBCs in vitro and in spur cell anemia RBCs in vivo. Decreased ability to replace peroxidized membrane fatty acid by this metabolic pathway may contribute to the hemolytic process in spur cell anemia.

Uptake, incorporation and redistribution of arachidonic acid in isolated salivary glands of the lone star tick

The ability of isolated salivary glands from the lone star tick, Amblyomma americanum, to take up, incorporate and redistribute [3H]arachidonic acid was examined. Uptake of arachidonic acid was concentration dependent--a single salivary gland incorporated up to approximately 2.8 micrograms arachidonic acid in 60 min. Over 90% of the [3H]arachidonate entering the glands was esterified and found only in the phospholipid (approximately 80%) and triglyceride (approximately 10%). Essentially no radioactivity was associated with the diglyceride fraction and none with phosphatidic acid indicating de novo phospholipid synthesis was negligible. Phospholipid synthesis via acylation of lysophospholipids (the Lands pathway) was indicated by the rapidity of the synthesis (< 2 min) and the sensitivity to sulfhydryl-blocking agents. Within the phospholipids, [3H]arachidonate was incorporated only into phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Initially [3H]arachidonate was incorporated primarily into PC, but as the incubation proceeded PE contained an increasing proportion of the label. The proportion of [3H]arachidonate incorporated into triglyceride increased at higher media concentrations of arachidonic acid. The roles of lysophosphatide acyltransferase, transacylase and diglycerol acyltransferase in the distribution of arachidonate in tick salivary glands are discussed.

Photolabeling of soybean microsomal membrane proteins with photoreactive acyl-CoA analogs

Synthesis of 32P-labeled 12-azidooleoyl-CoA and 125I-labeled 12-[(azidosalicyl)amino]dodecanoyl-CoA (ASD-CoA) was achieved. The synthesized radioactive, photoreactive reagents were tested as photoaffinity labels for acyl-CoA:lysophosphatidylcholine acyltransferase from the microsomal membranes of developing soybean cotyledons. When a mixture of microsomal membranes and the azidooleoyl-CoA or ASD-CoA were incubated in the dark, the analogs were recognized as substrate and competitive inhibitor, respectively. The enzyme preferentially utilizes unsaturated acyl-CoAs rather than saturated acyl-CoAs. Incubation of microsomal membranes with acyl-CoA analogs and immediately followed by photolysis resulted in an irreversible inhibition of lysophosphatidylcholine acyltransferase activity. Analysis of photolyzed microsomal membranes by SDS/PAGE and autoradiography revealed that azidooleoyl-CoA preferentially labeled eight acyl-CoA binding proteins, but ASD-CoA labeled only three polypeptides with molecular masses of 110, 90 and 32 kDa that are commonly labeled by both the analogs. Oleoyl-CoA and dodecanoyl-CoA protect the enzyme against photoinactivation by azidooleoyl-CoA and ASD-CoA, respectively. The protection was profound in 110-kDa polypeptide indicating that this protein could be lysophosphatidylcholine acyltransferase. These results demonstrate that the photoaffinity of acyl-CoA analogs makes them potential probes to identify and characterize lipid biosynthetic enzymes.

Acyl transfer reactions associated with cis Golgi apparatus of rat liver

Isolated Golgi apparatus, highly purified from rat liver, were found to contain an acyl transfer activity capable of restoring the acyl chains of the lysophospholipid products of the action of phospholipase A2 on phosphatidylcholine. The activity was located primarily in cis and medial Golgi apparatus fractions, had a pH optimum of 6.0 to 7.5 and was stimulated by various acyl-CoA derivatives but not by fatty acids plus ATP. The activity, determined from the conversion of [14C]lysophosphatidylcholine to [14C]phosphatidylcholine, was unaffected by EGTA, inhibited by manoalide at high concentrations (0.2 mM), and temperature-dependent. Temperature dependency, however, showed no definite transition temperature over the range 15 to 37 degrees C. The results demonstrated that cis Golgi apparatus membranes have the enzymatic capacity to restore fatty acids lost from phospholipids through the action of phospholipase A. The latter has been previously suggested to occur at the cis Golgi apparatus membranes based on analyses of cell-free transfer of radiolabeled phosphatidylcholine.

Activation of plasma lysolecithin acyltransferase reaction by apolipoproteins A-I, C-I and E

The lysolecithin acyltransferase (LAT) activity of lecithin-cholesterol acyltransferase (LCAT) converts lysophosphatidylcholine (lyso PC) to PC, and requires low-density lipoproteins (LDL). To determine whether LDL can be replaced by defined substrates, we tested proteoliposomes containing egg PC, labeled lyso PC and apoprotein (apo) A-I at molar ratios of 250:12.5:0.8, as substrate for purified enzyme. A significant percent of lyso PC in this substrate was acylated to PC, indicating that apo A-I can substitute for apo B in LAT reaction, and that PC is the acyl donor in the reaction. Apo C-I and apo E were, respectively, 70% and 40% as effective as apo A-I. When both lyso PC and free cholesterol (FC) were incorporated into the same proteoliposome, they competed with each other for the acyl groups, with 72% of the total acylation being directed to FC and 28% to lyso PC, at equimolar concentrations. With the native lipoproteins, the esterification of lyso PC was dominant in LDL, whereas FC esterification was dominant in high-density lipoproteins (HDL). Albumin inhibited LAT and activated LCAT in both lipoproteins, but its effects were more pronounced in HDL. These results indicate that the esterification of lyso PC and FC involve the same mechanism, and that the relative predominance of LAT on LDL is due to higher affinity of lyso PC to LDL, compared to HDL.

Enhanced production of platelet-activating factor in stimulated rat leukocytes caused by the blockade of lysophospholipid acylation

We have previously reported that triacsin C, an acyl-CoA synthetase inhibitor, enhanced the production of platelet-activating factor (PAF) in calcium ionophore-activated rat polymorphonuclear leukocytes (PMNs). In this report, we further demonstrated that the production of PAF by PMNs in response to opsonized zymosan was significantly enhanced by pretreatment with triacsin C and also by the pretreatment with merthiolate, which was reported to be an inhibitor of acyl-CoA/lysolecithin acyltransferase. Pretreatment with triacsin C or merthiolate also enhanced the lyso-PAF content in the stimulated PMNs. Addition of lyso-PAF in the incubation mixture of PMNs in the presence of opsonized zymosan augmented the production of PAF. The enhancement of PAF production by lyso-PAF has been reported by several authors, and the importance of lyso-PAF in the remodeling pathway of PAF synthesis has been generally recognized. Therefore, from the above findings, it is assumed that blockades of the reacylation of lyso-phospholipids, by inhibitors such as triacsin C and merthiolate, might lead to accumulation of lyso-PAF and might result in the enhancement of PAF production when the remodeling pathway is active.

Directed shift of fatty acids from phospholipids to triacylglycerols in HL-60 cells induced by nanomolar concentrations of triethyl lead chloride: involvement of a pertussis toxin-sensitive pathway

Triethyl lead chloride (Et3PbCl) was found to induce a shift of fatty acids from membrane phospholipids to triacylglycerols in the human promyelocytic leukemia cell line HL-60. High concentrations of Et3PbCl (greater than 10 microM) caused a substantial liberation of [14C]arachidonic acid within 10 to 20 min in dimethyl sulfoxide-differentiated cells, comparable to the effect of the calcium ionophore A23187 (10 microM). Following liberation of arachidonic acid, its metabolites could be detected. Prolongation of the incubation time and reduction of Et3PbCl concentration resulted in a shift of fatty acids from phospholipids to triacylglycerols. Deacylation of phospholipids and reacylation into phospholipids and triacylglycerols were in equilibrium when the cells were treated with Et3PbCl at concentrations of less than or equal to 10 microM for 5 hr or less than or equal to 1 microM for 24 hr; no increase of free fatty acids could be observed, and the loss of fatty acids within the phospholipids was equivalent to the increase of fatty acid content within the triacylglycerols. Moreover, under these conditions, no loss of viability was seen after 24 hr, as compared with untreated differentiated cells. This concentration- and time-dependent effect of Et3PbCl might be due to a stimulated liberation of fatty acids via phospholipase A2, because this stimulation could be totally prevented by the phospholipase inhibitors quinacrine and p-bromophenacylbromide. Additionally, pretreatment of differentiated HL-60 cells with pertussis toxin resulted in a drastic reduction of [14C]arachidonic acid liberation when cells were stimulated with Et3PbCl. These results suggest the involvement of a pertussis toxin-sensitive GTP-binding protein and of a signal transduction mechanism during stimulated fatty acid release; release does not seem to be via a direct stimulation of phospholipase activity by the lead compound.

Essential residues in lysolecithin:lysolecithin acyltransferase from rabbit lung: assessment by chemical modification

The inhibition of lysolecithin:lysolecithin acyltransferase by several specific reagents was studied. Diisopropyl fluorophosphate (DFP) completely inhibited both activities at a concentration of 4 mM. Activity was not protected by substrate and the enzyme showed a change in circular dichroism spectrum upon treatment with inhibitor. Phenylmethanesulfonyl fluoride, another serine-specific reagent, did not inhibit either hydrolysis or transacylation. Therefore, we suggest that DFP does not modify an active serine in the catalytic site. p-Hydroxymercury benzoate and N-ethylmaleimide (NEM) abolished both activities of the enzyme. The presence of substrate partially protected against inactivation. Far-uv CD spectrum of NEM-modified enzyme revealed no changes in protein structure. The existence of two classes of essential cysteine residues was deduced from kinetics of NEM inactivation. Both classes differ in NEM reactivity and also in their participation in the catalytic mechanism. A tyrosine-specific reagent, tetranitromethane, also inhibited hydrolysis and transacylation, following first-order kinetics. The partial protection by substrate suggested the possible existence of essential tyrosines near the active site. At pH 5.0 N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline inactivated hydrolysis but not transacylation. However, both of them remained unchanged at pH 6.5. The substrate prevented the loss of hydrolytic ability. Therefore, a carboxyl residue participating just in the catalytic mechanism of hydrolysis is proposed.

Purification and kinetic properties of lysophosphatidylinositol acyltransferase from bovine heart muscle microsomes and comparison with lysophosphatidylcholine acyltransferase

The enzyme acyl-CoA:1-acyl-sn-glycero-3-phosphoinositol acyltransferase (LPI acyltransferase, EC 2.3.1.23) was purified approximately 11,000-fold to near homogeneity from bovine heart muscle microsomes. The purification was effected by extraction with the detergent 3-((3-cholamidopropyl)dimethylammonio)-1-propanesulfonate, followed by chromatography on Cibacron blue agarose, DEAE-cellulose, and Matrex gel green A. The isolated enzyme was a single protein of 58,000 Da as measured by polyacrylamide gel electrophoresis in the presence of dodecyl sulfate. This purification procedure also allows isolation of the related enzyme lysophosphatidylcholine (LPC) acyltransferase, which was separated from LPI acyltransferase at the final chromatographic step. The purified LPI acyltransferase exhibits an absolute specificity for LPI as the acyl acceptor. Broader specificity was found for acyl-CoA derivatives as substrates, although the preferred substrates are long-chain, unsaturated derivatives: measured reactivities were in the order arachidonoyl-CoA greater than oleoyl-CoA greater than eicosadienoyl-CoA greater than linoleoyl-CoA. Little activity was found with palmitoyl-CoA or stearoyl-CoA as potential substrates. These properties are consistent with a role of the enzyme in controlling the acyl group composition of phosphoinositides. Comparison of LPC acyltransferase and LPI acyltransferase shows that these two enzymes have distinct kinetic and physical properties and are affected differently by local anesthetics, which are potent inhibitors.

The kinetic mechanism of acyl-CoA:lysolecithin acyltransferase from rabbit lung

Acyl-CoA:lysolecithin acyltransferase is a key enzyme in the deacylation-reacylation pathway of biosynthesis of molecular species of lecithin. However, the mechanism of the reaction has been little studied. In this paper, the kinetic mechanism of acyl-CoA:lysolecithin acyltransferase, partially purified from rabbit lung, is studied. The double-reciprocal plots of initial velocity vs substrate concentration gave two sets of parallel lines which fitted to a ping-pong equation with the following parameters: Km (palmitoyl-CoA) = 8.5 +/- 2 microM, Km (lysolecithin) = 61 +/- 16 microM, and V = 18 +/- 4 nmol/min/mg protein. Inhibition studies by substrates, alternate substrates, and products supported the ping-pong mechanism, although some nonclassical behavior was observed. Palmitoyl-CoA did not inhibit even at concentrations of 100 Km. In contrast, lysolecithin was a dead-end inhibitor with a dissociation constant of Ki = 930 +/- 40 microM. Alternate substrates and CoA showed alternate pathways for the reaction due to the formation of ternary complexes. Dipalmitoylphosphatidylcholine inhibition pointed to an isomerization of the free enzyme prior to the start of the reaction. From these results, an iso-ping-pong kinetic mechanism for lysolecithin acyltransferase is proposed. The kinetic steps of the reaction are correlated with previous chemical studies of the enzyme.

Inhibitors of acyl-coenzyme A:lysolecithin acyltransferase activate the production of endothelium-derived vascular relaxing factor

Considerable acyl-CoA:lysolecithin acyltransferase (LAT) activity could be demonstrated in homogenates of cultured bovine endothelial cells. This LAT activity was inhibited by thimerosal and p-hydroxymercuribenzoate in a concentration-dependent manner. Preconstricted strips of rabbit aorta were relaxed by acetylcholine or the LAT inhibitors in a concentration-dependent fashion if the endothelium was intact (maximal effect of both LAT inhibitors at 10(-5) M). In rabbit aortic strips thimerosal also induced a concentration-dependent stimulation of the formation of 6-keto-prostaglandin F1 alpha, the major cyclooxygenase metabolite of this tissue. This effect of thimerosal was more pronounced in endothelium-intact than in endothelium-denuded preparations. Inhibition of prostaglandin synthesis with indomethacin (10(-5) M) did not impair the relaxation. Thimerosal and acetylcholine-induced relaxations were abolished when the endothelium was removed or when endothelium-intact preparations were pretreated with nordihydroguaiaretic acid (3 X 10(-5) M), gossypol (5 X 10(-6) M) or dithiothreitol (3 X 10(-4) M). In contrast, mepacrine (3 X 10(-5) M), that abolished the acetylcholine response, had no effect on the thimerosal relaxation. In other experiments bovine endothelial cells were grown to confluence on microcarrier beads and packed into columns. Adding thimerosal (5 X 10(-6) M) or bradykinin (10(-10) to 10(-8) M) to the medium superfusing the columns induced the release of an unstable nonprostanoid factor (or factors) that relaxed endothelium-denuded rabbit femoral artery segments. Bradykinin induced a transient effect whereas there was a strong and long-lasting release of the factor after administration of thimerosal.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of membrane phospholipids in myocardial injury induced by ischemia and reperfusion

Depletion of membrane phospholipids is known to be associated with myocardial ischemia, but its relationship to the injury involved with the reperfusion of ischemic myocardium is not known. The present study was designed to relate phospholipid degradation with reperfusion injury. The isolated in situ pig heart was subjected to 60 min of regional ischemia induced by occluding the left anterior descending (LAD) coronary artery and 60 min of global ischemia by hypothermic cardioplegic arrest followed by 60 min of reperfusion. The pigs were divided into two groups. In the treatment group, the heart was preperfused with mepacrine (0.05 mM), a known phospholipase inhibitor, for 15 min prior to LAD occlusion. In the control group, the total phospholipid content was not significantly decreased during LAD occlusion and arrest, but was reduced appreciably after reperfusion. Phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol followed a similar pattern. The lowering of these phospholipids during reperfusion was accompanied by enhancement of lysophosphatidylcholine. Mepacrine restored the normal levels of these phospholipids. During reperfusion, fatty acyl CoA synthetase, lysophospholipase, and lysophosphatidylcholine acyltransferase were depressed, whereas phospholipase A2 was enhanced. Mepacrine inhibited phospholipase A2, but had no effects on the other enzymes. Mepacrine also provided significant protection against reperfusion injury, as documented by the preservation of high-energy phosphate compounds and inhibition of the appearance of creatine kinase activity in the perfusate. These results suggest that membrane phospholipids play an important role in myocardial injury associated with ischemia and reperfusion, primarily because the deacylation-reacylation cycle of phospholipid biosynthesis becomes defective.

Ether phospholipids as inhibitors of the arachidonoyl-CoA: 1-acyl-sn-glycero-3-phosphocholine acyltransferase in macrophages

1-Alkylglycerophosphatide analogs which are known to activate macrophages to enhanced tumor cytotoxicity are structurally closely related to 1-acyl-sn-glycero-3-phosphocholine. In this study we have examined the influence of some of these compounds and of platelet-activating factor (PAF-acether, 1-0-alkyl-2-0-acetyl-sn-glycero-3-phosphocholine) on the arachidonoyl-CoA: 1-acyl-sn-glycero-3-phosphocholine acyltransferase (EC 2.3.1.23) in homogenate of bone-marrow-derived murine macrophages. This enzyme is suggested to be involved in the control of the availability of the icosanoid precursor, arachidonic acid. Kinetic experiments revealed apparent Km and V values for 1-palmitoyl-sn-glycero-3-phosphocholine of 6.0 microM and 16.10 nmol/mg protein per min, respectively. When the 1-palmitoyl-sn-glycero-3-phosphocholine concentration was equal to Km, the enzyme was dose-dependently inhibited by 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine with a 50% inhibition at 30 microM. The kinetic parameters in the presence of 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (K'm = 10.0 microM, V' = 11.40 nmol X mg-1 X min-1) suggest that this alkyl phospholipid is a mixed-type inhibitor. All other alkyl analogs tested (1-O-methyl-2-O-octadecyl-rac-glycerol-3-phosphocholine, racemic PAF-acether, L-PAF-acether, D-1-O-hexadecyl-sn-glycero-3-phosphocholine, 1-O-octadecyl-rac-glycero-3-phosphocholine) inhibited the enzyme to various degrees. Arachidonic acid transfer to the 1-alkylglycerophosphatide analogs themselves could be ruled out under the assay conditions used. Therefore, we conclude that the arachidonoyl-CoA: 1-acyl-sn-glycero-3-phosphocholine acyltransferase can be inhibited by synthetic and naturally occurring ether phospholipids in homogenate of bone-marrow-derived murine macrophages.

The role of lysophosphatidylcholine in lipid synthesis by developing sunflower (Helianthus annuus L.) seed microsomes

The incorporation of oleate from oleoyl-CoA into lipids by microsomes from developing sunflower (Helianthus annuus L.) seeds has been investigated. Oleate was incorporated mainly into position 2 of phosphatidylcholine or released as free fatty acid. The addition of exogenous 1-acyl-lysophosphatidylcholine increased the incorporation of oleate into position 2 of phosphatidylcholine and decreased the release of free oleate. In the absence of exogenous lysophosphatidylcholine, the incorporation of oleate into phosphatidylcholine was limited by the amount of endogenous acceptor present. DH-990, an inhibitor of acyl-CoA:lysophosphatidylcholine acyltransferase, almost completely inhibited the incorporation of oleate from oleoyl-CoA into phosphatidylcholine at a concentration of 2.5 mM. These results indicate that the incorporation of oleate from oleoyl-CoA into microsomal phosphatidylcholine occurs mainly by the acylation of a 1-acyl-lysophosphatidylcholine acceptor rather than by acyl exchange between oleoyl-CoA and phosphatidylcholine. While the incorporation of oleoyl-CoA was completed within 2 to 5 min, exogenous 1-acyl-lysophosphatidylcholine was incorporated into phosphatidylcholine for up to 30 min. Addition of oleoyl-CoA resulted in an increase in both the rate and magnitude of lysophosphatidylcholine incorporation, which could not be accounted for by a stoichiometric reaction between the two substrates. Evidence is provided that free CoA had an independent stimulatory effect on the incorporation of lysophosphatidylcholine. The implications of this finding are discussed.

Stimulation of prostaglandin synthesis in WI-38 human lung fibroblasts following inhibition of phospholipid acylation by p-hydroxymercuribenzoate

The release of arachidonic acid and its metabolites, prostaglandin E2 and thromboxane A2, from WI-38 human lung fibroblasts was modulated by p-hydroxymercuribenzoate. Exposure to the inhibitor resulted in a dose-dependent decrease in [1-14C]arachidonic acid uptake and incorporation into phospholipids and neutral lipid pools. Activities of lung fibroblast arachidonyl-CoA synthetase and lysolecithin acyltransferase were inhibited by 100 microM p-hydroxymercuribenzoate. [14C]Arachidonic acid labelled fibroblasts exhibited an increased release of [14C]arachidonate and [14C]prostaglandin E2 of 54% and 112%, respectively, when exposed to 100 microM of inhibitor. The stimulatory effects of 8.0 microM delta 1-tetrahydrocannabinol on arachidonate release and prostaglandin E synthesis (Burstein, S., Hunter, S.A., Sedor, C. and Shulman, S. (1982) Biochem. Pharmacol. 31, 2361-2365) were modified by the inclusion of inhibiting agent, resulting in a 608% stimulation in arachidonic acid release, while prostaglandin E2 and thromboxane A2 synthesis increased 894% and 390%, respectively, over levels obtained by untreated cells. The levels of arachidonate metabolites were altered by inhibitor when compared to cells treated with cannabinoid alone. No significant inhibition by delta 1-tetrahydrocannabinol was found on arachidonic uptake in these cells. In unlabelled studies, p-hydroxymercuribenzoate resulted in a profound, dose-dependent stimulation of prostaglandin E synthesis of 1490% at 150 microM inhibitor concentration. These results provide evidence that free arachidonate is reincorporated via acylation, thereby implicating this pathway as a possible control mechanism for the synthesis of arachidonic acid metabolites.

Hormonal control of pulmonary surfactant synthesis

The specific activities of palmitoyl-CoA synthetase, phospholipase A2, and lysophosphatidylcholine acyltransferase enzymes were low in the lungs of diabetic and hypophysectomized rats as compared to those found in the normal controls. Administration of triiodothyronine (T3), to the diabetic and hypophysectomized rats restored the normal activities of these enzymes. Stimulation of the enzyme activities were also observed when normal rats were injected with the above hormone. The enhancement of the enzyme activities was also found to be dependent on the dose and duration of the hormonal treatment. Optimum levels were achieved at a dose of about 100 micrograms/100 g body weight of T3, 3-4 days after the administration of this hormone. Actinomycin D or cycloheximide abolished the hormone-mediated stimulation of these enzymes in diabetic and hypophysectomized rats. Reduced rate of in vivo palmitoyl-CoA synthetase synthesis was observed in the lungs of diabetic and hypophysectomized animals. Administration of T3 stimulated the rate of synthesis of this enzyme indicating increasing synthesis of this enzyme and not of activation of the pre-existing inactive species. Reduced phospholipid contents, specially decreased amount of lecithin and dipalmitoyl lecithin (DPL) were observed in the lungs of the diabetic and hypophysectomized animals as compared to those in the normal animals. T3 also increased the lecithin and DPL content of the normal rat lungs. These results provide evidence for the involvement of the thyroid hormones in the control of the pulmonary surfactant. The results further suggest that T3 was capable of inducing the enzymes of the "deacylation-reacylation" pathway involved in palmitate incorporation into phosphatidylcholine thereby contributing to the stimulation of dipalmitoyl phosphatidylcholine biosynthesis.

The acylation of lysophosphatidylcholine in the rat testicular tissue: the combined activity of acyl-CoA synthetase and lysolecithin acyltransferase

The activity of lysophosphatidylcholine acyltransferase (EC 2.3.1.23) in combination with acyl-CoA synthetase (EC 6.2.1.3) has been determined in the homogenates and subcellular fractions of rat testis. The enzyme activity was found to be maximal at pH 7.4 ATP and CoASH were required for optimal incorporation of [1-14C] oleic acid into phosphatidylcholine. The sulfhydryl-binding reagents showed inhibitory effect on the acyltransferase activity. Dibutyryl cyclic AMP and beta-mercaptoethanol did not affect the enzyme activity. Subcellular distribution patterns of markers, marker enzymes and lysolecithin acyltransferase have shown that the acyltransferase activity was found to be predominantly localized in the microsomal fraction, though significant activity was also present in the mitochondrial fraction. These findings, together with our previous studies on testicular phospholipases A, suggest that the deacylation-reacylation cycle is operative in rat testicular tissue.

Modulation of phosphatidylcholine synthesis in vitro. Inhibition of diacylglycerol cholinephosphotransferase and lysophosphatidylcholine acyltransferase by centrophenoxine and neophenoxine

1,2-Diacyl-sn-glycerol : CDPcholine cholinephosphotransferase (EC 2.7.8.2) and acyl-CoA : 1-acyl-sn-glycero-3-phosphocholine acyltransferase (EC 2.3.1.23) activities of rat liver microsomes can be inhibited by centrophenoxine (N,N-dimethylaminoethyl p-chlorophenoxyacetate). This inhibition is brought about by the intact centrophenoxine molecule rather than by the products of hydrolysis. A nonhydrolyzable ether analog of centrophenoxine was synthesized (neophenoxine; N,N-dimethylaminoethyl p-chlorophenoxyethyl ether) and proved most effective in inhibiting the two routes of phosphatidylcholine biosynthesis. While 50% inhibition of the cholinephosphotransferase was attained at 5 mM neophenoxine, 50% inhibition of the acyltransferase required 0.6 mM neophenoxine levels only. Inhibition of the cholinephosphotransferase (Ki approximately 1.5 mM) and the acyltransferase (Ki approximately 1 mM) by neophenoxine was shown to be noncompetitive. Other membrane-bound enzymes, such as glucose-6-phosphatase, monoacylglycerol lipase, alkaline phosphatase or phospholipase A2 were not affected by the inhibitors. Because of this specificity, and because of the high affinity of the microsomal membrane for such agents, centrophenoxine and neophenoxine should prove useful for controlling phosphatidylcholine synthesis and for modulating the phosphatidylcholine deacylation-reacylation cycle.

Inhibition of acyltransferase in lymphocytes by concanavalin A

The effects of concanavalin A and succinylated concanavalin A on the transformation of mouse splenic lymphocytes, and on early biochemical events in the transformation, were compared. 1. The transformation of lymphocytes is biphasic with respect to concanavalin A concentration with optimal activation at about 1 microgram/ml. Activation by succinyl concanavalin A is not biphasic over a range of lectin concentration of 1--16 microgram/ml. 2. In intact lymphocytes cultured for 4 h, the enzyme Acyl-CoA:1-acylglycero-3-phosphocholine O-acyltransferase (EC 2.3.1.23) was not activated by concanavalin A but was inhibited at all concentrations tested, and was about 60% inhibited at 16 micrograms concanavalin A per ml. Succinyl concanavalin A gave little or no inhibition at similar concentrations. 3. Lymphocytes become committed to divide while their acyltransferase activities are markedly inhibited by concanavalin A. 4. The inhibition of acyltransferase by concanavalin A can be lifted by displacing the lectin from the cells by alpha-methylmannoside. Lowered enzyme activity is not caused by cell agglutination or by direct cross-linking of lectin receptors. It is unlikely that the inhibition of acyltransferase is due to indirect cross-linking via the cytoskeleton since colchicine did not reverse the inhibition. 5. The inhibition of acyltransferase and the reduced stimulation of transformation by higher levels of concanavalin A appear to be due to hydrophobic interaction of the lectin with the plasma membrane, as shown by liposome aggregation studies.

Molar volume relationships and the specific inhibition of a synaptosomal enzyme by psychoactive cannabinoids

The ability of a number of lipophilic compounds to inhibit the mouse-brain synaptosomal enzyme acyl coenzyme A:lysophosphatidylcholine acyltransferase has been measured in vitro. Psychoactive cannabinoids inhibit the enzyme at concentrations much lower than is predicted from their capacity to act as lipid-soluble anesthetics. Nonpsychoactive cannabinoids do not show specific inhibition. Molar volume relationships are used to show that, while all lipid-soluble molecules exert some inhibitory effect in proportion to their ability to dissolve in biological membranes, psychoactive cannabinoids have an inhibitory effect greatly in excess of their anesthetic potency. The isoprenoid convulsant thujone has been suggested to have psychoactivity similar to cannabinoids but does not mimic the cannabinoids in inhibiting the synaptosomal enzyme. Molar volumes and specific interactions are used in structure-activity correlations which yield information on the relative concentrations of biophase in drug-responsive systems and the specificity of membrane-active drugs.

Inhibition of a lymphocyte membrane enzyme by delta9-tetrahydrocannabinol in vitro

Delta-9-tetrahydrocannabinol (delta9-THC) inhibited the activity of lysolecithin acyl transferase, a membrane-bound lymphocyte enzyme, at concentrations above 1.3 muM. Stimulation of acyl transferase activity by concanavalin A, an early response in lymphocyte activation, was entirely abolished in the presence of delta9-THC.

Isosteres of natural phosphates. 6. The preparation and properties of lysophosphotidic acid

We herein report the first chemical synthesis of phosphonic acid analogues of lysophosphatidic acid. The racemic isosteric analogues, 4-acyloxy-3-hydroxybutyl-1-phosphonic acids, of lysophosphatidic acid were prepared by both catalytic and hydride reductions of the 4-acyloxy-3-oxobutyl-1-phosphonic acids, a general method for the preparation of the latter having been reported previously. The lysophosphatidic acids have been found to substrates for lysophosphatidic acid acyl transferase, and may be acylated chemically to yield phosphotidic acids. The latter reaction is of use in the preparations of differentially acylated phosphatidic acids.