Biosynthesis of pulmonary surfactant: comparison of 1-palmitoyl-sn-glycero-3-phosphocholine and palmitate as precursors of dipalmitoyl-sn-glycero-3-phosphocholine in adenoma alveolar type II cells

Metabolic Profiling Revealed Prediction Biomarkers for Infantile Hemangioma in Umbilical Cord Blood Sera: A Prospective Study

Infantile hemangioma (IH), the most common benign tumor in infancy, mostly arises and has rapid growth before 3 months of age. Because irreversible skin changes occur in the early proliferative stage, early medical treatment is essential to reduce the permanent sequelae caused by IH. Yet there are still no early screening biomarkers for IH before its visible emergence. This study aimed to explore prediction biomarkers using noninvasive umbilical cord blood (UCB). A prospective study of the metabolic profiling approach was performed on UCB sera from 28 infants with IH and 132 matched healthy controls from a UCB population comprising over 1500 infants (PeptideAtlas: PASS01675) using liquid chromatography-mass spectrometry. The metabolic profiling results exhibited the characteristic metabolic aberrance of IH. Machine learning suggested a panel of biomarkers to predict the occurrence of IH, with the area under curve (AUC) values in the receiver operating characteristic analysis all >0.943. Phenylacetic acid had potential to predict infants with large IH (diameter >2 cm) from those with small IH (diameter <2 cm), with an AUC of 0.756. The novel biomarkers in noninvasive UCB sera for predicting IH before its emergence might lead to a revolutionary clinical utility.

Flow cytometric detection of transbilayer movement of fluorescent phospholipid analogues across the boar sperm plasma membrane: elimination of labeling artifacts

Reliable protocols were established for investigating asymmetric distributions of 6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino-caproyl (C6NBD) phospholipids in the plasma membrane of boar sperm cells under physiological conditions. A method based on fluorescence resonance energy transfer was used to ensure that incorporation of the fluorescent phospholipids into the sperm proceeded via monomeric transfer. The total amount of incorporated phospholipid fluorescence and the proportion of translocated phospholipid fluorescence were determined by flow cytometric analysis before, and after, dithionite destruction of outer leaflet fluorescence. Catabolism of incorporated fluorescent phospholipids was blocked with phenylmethylsulfonyl fluoride. Membrane-damaged cells were detected with impermeant DNA stains, thereby enabling their exclusion from subsequent analyses of the flow cytometric data, whence it could be demonstrated that the labeled phospholipids were incorporated only via the outer plasma membrane leaflet in living sperm cells. Phospholipid uptake and internalization was followed at 38 degrees C. After 1 hr of labeling, about 96% of the incorporated C6NBD-phosphatidylserine, 80% of C6NBD-phosphatidylethanolamine, 18% of C6NBD-phosphatidylcholine, and 4% of C6NBD-sphingomyelin were found to have moved across the plasma membrane bilayer to the interior of the spermatozoa. These inward movements of fluorescent phospholipids were ATP-dependent and could be blocked with sulfhydryl reagents. Movements from the inner to the outer leaflet of the sperm plasma membrane were minimal for intact fluorescent phospholipids, but were rapid and ATP-independent for fluorescent lipid metabolites. The described method enables, for the first time, assessment of changes in lipid asymmetry under fertilizing conditions.

Transmembrane redistribution of phospholipids of the human red cell membrane during hypotonic hemolysis

The transmembrane distribution of spin-labeled phospholipids was measured in human erythrocytes before and after hypotonic hemolysis by electron paramagnetic resonance. With a first series of partially water soluble probes a complete randomization of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and sphingomyelin analogues was achieved when cells were resealed in the absence of Mg-ATP or when the aminophospholipid translocase was inhibited by vanadate or calcium. If the ghosts were resealed with Mg-ATP inside, the transmembrane asymmetry of the aminophospholipids was reestablished. With long chain insoluble spin-labeled lipids complete randomization was obtained with the phosphatidylcholine analogue but even in the presence of vanadate only a small percentage (approx. 15%) of the spin-labeled phosphatidylserine flopped to the outer monolayer and comparable percentage of the spin-labeled sphingomyelin flipped to the inner monolayer, indicating a hierarchy in the phospholipid redistribution for these water insoluble lipids during hemolysis. The mechanism by which a selective randomization takes place is not known. It may involve phosphatidylserine-protein interactions in the inner leaflet and sphingomyelin-cholesterol or sphingomyelin-sphingomyelin interaction in the outer leaflet.

Transmembrane diffusion of fluorescent phospholipids in human erythrocytes

The outside-inside passage and transmembrane equilibrium distribution of several amphiphilic fluorescent phospholipids were examined in human erythrocytes. The results were compared with previous kinetic data obtained with spin-labeled phospholipids and with the equilibrium distribution of endogenous lipids in erythrocytes. When a nitro benzoxadiazole (NBD) was at the terminal position of a 6 carbon beta-chain, the outside-inside diffusion of the fluorescent phosphatidylserine (PS) analogue was slower, and the plateau lower than with long chain radioactive PS or spin-labeled PS. The corresponding phosphatidylethanolamine (PE) did not flip nor did the phosphatidylcholine (PC) analogue. With a NBD at the 12th carbon of a 18C alpha-chain, the amino-derivatives behaved more like endogenous PS and PE, i.e. they accumulated rapidly on the inner monolayer; however, the phosphatidylcholine analogue reached a plateau corresponding to 50% inside within 2 h at 37 degrees C, indicative of an abnormal rapid diffusion. In the latter case, changing the beta-chain from four to eight carbons had no influence on this rapid diffusion. We conclude that when the NBD is close to the glycerol moiety, it diminishes the affinity of the aminophospholipids for the aminophospholipid translocase. When it is close to the methyl terminal of an acyl chain, there is an acceleration of the spontaneous flip-flop. Presumably the polarity of the NBD is responsible for an unconventional orientation of the flexible acyl chain, thereby causing the transmembrane destabilization of the phospholipid. Overall these results illustrate the respective roles of spontaneous diffusion and translocase activity on transmembrane equilibrium distribution of phospholipids. They also show that NBD derivatives should be used cautiously as indicators of endogenous phospholipids.

Alteration of the aminophospholipid translocase activity during in vivo and artificial aging of human erythrocytes

Human erythrocytes were separated into three density groups representing different age groups. Phospholipid outside-inside translocation rates and equilibrium distribution were determined in each group with spin-labeled phosphatidylserine (PS*), phosphatidylethanolamine (PE*), and phosphatidylcholine (PC*), at 37 degrees C and 4 degrees C. At both temperatures, the initial velocity of aminolipid translocation was reduced in the more dense (older) cells. The equilibrium distribution was not significantly modified for PS*, but a larger fraction of PE* remained on the outer monolayer of the more dense cells. PC* transmembrane diffusion was identical in the three fractions. Cytosolic ATP, which is required for aminophospholipid translocation, was not responsible for the variability of the density separated cells since ATP enrichment did not cancel the differences between top and bottom fractions, although it equalized the ATP concentration of the various fractions. Variations in the level of intracellular Ca2+ could also be excluded. Thus, the enzyme aminophospholipid translocase seemed to be directly altered in aged cells, possibly due to oxidation caused by lipid peroxidation products. Experiments with malonyldialdehyde or H2O2 treated cells confirmed this interpretation and suggest that defects in endogenous lipid asymmetry observed in aged human erythrocytes may be due to altered activity of the translocase.

Theoretical approach to the steady-state kinetics of a bi-substrate acyl-transfer enzyme reaction that follows a hydrolysable-acyl-enzyme-based mechanism. Application to the study of lysophosphatidylcholine:lysophosphatidylcholine acyltransferase from rabbit lung

A kinetic model is proposed for catalysis by an enzyme that has several special characteristics: (i) it catalyses an acyl-transfer bi-substrate reaction between two identical molecules of substrate, (ii) the substrate is an amphiphilic molecule that can be present in two physical forms, namely monomers and micelles, and (iii) the reaction progresses through an acyl-enzyme-based mechanism and the covalent intermediate can react also with water to yield a secondary hydrolytic reaction. The theoretical kinetic equations for both reactions were deduced according to steady-state assumptions and the theoretical plots were predicted. The experimental kinetics of lysophosphatidylcholine:lysophosphatidylcholine acyltransferase from rabbit lung fitted the proposed equations with great accuracy. Also, kinetics of inhibition by products behaved as expected. It was concluded that the competition between two nucleophiles for the covalent acyl-enzyme intermediate, and not a different enzyme action depending on the physical state of the substrate, is responsible for the differences in kinetic pattern for the two activities of the enzyme. This conclusion, together with the fact that the kinetic equation for the transacylation is quadratic, generates a 'hysteretic' pattern that can provide the basis of self-regulatory properties for enzymes to which this model could be applied.

Aminophospholipid translocase of human erythrocytes: phospholipid substrate specificity and effect of cholesterol

The outside-inside translocation rate and transmembrane equilibrium distribution, at 37 degrees C, of 16 different amphiphilic spin-labeled phospholipids have been determined in human erythrocytes. The transmembrane distribution was assessed by bovine serum albumin extraction of the spin-labels present in the outer monolayer. Within 15 min, more than 90% of the phosphatidylserine analogue was found in the inner monolayer; the equilibrium distribution of phosphatidylethanolamine spin-label was approximately 85-90% inside, with a half-time for translocation of approximately 50 min. In contrast, phosphatidylcholine reached a distribution corresponding to approximately 30% of the labels inside with a half-time of approximately 8 h, and only traces of sphingomyelin were found in the inner monolayer after 16 h. Thus, the spin-label analogues distributed themselves like endogenous phospholipids in red cells with a spontaneous segregation between the amino lipids and the choline-containing phospholipids. Progressive methylation of the amine group of phosphatidylethanolamine resulted in a stepwise decrease of the specific transport; modification of the beta-carbon of the serine also decreased the efficiency of the rapid translocation without abolishing it. Phosphatidyl-propanolamine was not transported. Substitution of the glyceride group by a ceramide abolished the rapid outside-inside translocation even with a molecule bearing a serine head group. Also it was found that esterification of the sn-2 position of the glycerol component was necessary for a rapid translocation since lysophosphatidylserine was only slowly transported from outside to inside.(ABSTRACT TRUNCATED AT 250 WORDS)

Acylation of lysophosphatidylcholine in bovine heart muscle microsomes: purification and kinetic properties of acyl-CoA:1-acyl-sn-glycero-3-phosphocholine O-acyltransferase

Bovine heart muscle microsomes rapidly convert lysophosphatidylcholine (LPC) into phosphatidylcholine (PC) in the presence of oleoyl-CoA. Both substrates are incorporated into the product, although the rate of incorporation of radiolabel into PC from 1-[14C]palmitoyl-LPC was approximately threefold higher than the rate of incorporation from [14C]oleoyl-CoA. Furthermore, the rate of incorporation of radiolabel from [14C]LPC was stimulated fivefold by the presence of oleoyl-CoA. These results demonstrate the presence of both acyl-CoA:1-acyl-sn-glycero-3-phosphocholine O-acyltransferase (EC 2.3.1.23) and an LPC:LPC transacylase (EC 3.1.1.5) in microsomes. Separation of the two enzymatic activities and purification of the acyltransferase was achieved by a procedure involving extraction with 3-[3-cholamidopropyl)dimethylammonio)-1-propanesulfonate detergent and chromatography on DEAE-cellulose, Reactive blue agarose, and Matrex gel green A. The isolated acyltransferase was a single species of 64,000 Da as judged by polyacrylamide gel electrophoresis in the presence of dodecyl sulfate. The substrate specificity of the enzyme was studied by using a series of lysophospholipids as acyl acceptors and acyl-CoA derivatives as acyl donors. The enzyme was catalytically active with LPC as acyl acceptor but displayed little or no activity with lysophosphatidylethanolamine, lysophosphatidylinositol, or lysophosphatidylserine. Of the LPC derivatives tested, the highest activity was obtained with 1-palmitoyl-LPC. Wider specificity was exhibited for the nature of the acyl donor, for which arachidonoyl-CoA, linoleoyl-CoA, and oleoyl-CoA were highly active substrates. These properties of the acyltransferase are in accord with a role of the enzyme in determining the composition of PC in myocardium.

Involvement of acyl exchange between acyl-CoA and phosphatidylcholine in the remodelling of phosphatidylcholine in microsomal preparations of rat lung

Microsomal membrane preparations from rat lung catalyse the incorporation of radioactive linolenic acid from [14C]linolenoyl-CoA into position 2 of sn-phosphatidylcholine. The incorporation was stimulated by bovine serum albumin and free CoA. Free fatty acids in the incubation mixtures were not utilised in the incorporation into complex lipids. Fatty acids were transferred to the acyl-CoA pool during the incorporation of linolenic acid into phosphatidylcholine. An increase in lysophosphatidylcholine occurred in incubations containing both bovine serum albumin and free CoA and in the absence of acyl-CoA. The results were consistent with an acyl-CoA: lysophosphatidylcholine acyltransferase operating in both a forwards and backwards direction and thus catalysing the acyl exchange between acyl-CoA and position 2 of sn-phosphatidylcholine. In incubations with mixed species of acyl-CoAs, palmitic acid was the major fatty acid substrate transferred to phosphatidylcholine in acyl exchange, whereas this acid was completely selected against in the acylation of added lysophosphatidylcholine. The selectivity for palmitoyl-CoA was particularly enhanced when the mixed acyl-CoA substrate was presented to the microsomes in molar concentrations equivalent to the molar ratios of the fatty acids in position 2 of sn-phosphatidylcholine. During acyl exchange, the predominant fatty acid transferred to phosphatidylcholine from acyl-CoA was palmitic acid, whereas arachidonic acid was particularly selected for in the reverse reaction from phosphatidylcholine to acyl-CoA. A hypothesis is presented to explain the differential selectivity for acyl species between the forward and backward reactions of the acyltransferase that is based upon different affinities of the enzyme for substrates at high and low concentrations of acyl donor. Acyl exchange between acyl-CoA and phosphatidylcholine offers, therefore, a possible mechanism for the acyl-remodelling of phosphatidylcholine for the production of lung surfactant.

Synthesis of saturated phosphatidylcholine and phosphatidylglycerol by freshly isolated rat alveolar type II cells

Saturated phosphatidylcholine and phosphatidylglycerol are important components of pulmonary surface active material, but the relative contributions of different pathways for the synthesis of these two classes of phospholipids by alveolar type II cells are not established. We purified freshly isolated rat type II cells by centrifugal elutriation and incubated them with [1-14C]palmitate as the sole exogenous fatty acid in one series of experiments or with [9,10-3H]palmitate, mixed fatty acids (16:0, 18:1 and 18:2), and [U-14C]glucose in another series of experiments. Type II cells readily incorporated [1-14C]palmitate into saturated phosphatidic acid (55-59% of total phosphatidic acid), saturated diacylglycerol (82-87% of total diacylglycerol), saturated phosphatidylcholine (69-76% of total phosphatidylcholine), and saturated phosphatidylglycerol (55-59% of total phosphatidylglycerol). Saturated phosphatidic acid, diacylglycerol and phosphatidylglycerol were nearly equally labeled in the sn-1 and sn-2 positions, whereas saturated phosphatidylcholine was preferentially labeled in the sn-2 position. With [9,10-3H]palmitate and [U-14C]glucose, the labeling patterns of phosphatidic acid, diacylglycerol and phosphatidylglycerol were similar to each other but different from that of phosphatidylcholine. The glucose label was found predominantly in the unsaturated phosphatidylcholines at early times (3-10 min) and in the saturated phosphatidylcholines at later times (30-90 min). Similarly, the 3H/14C ratio was very high in saturated phosphatidylcholine and always above that in saturated diacylglycerol. We conclude that freshly isolated type II cells synthesize saturated phosphatidic acid, diacylglycerol, phosphatidylcholine and phosphatidylglycerol and that under our in vitro conditions the deacylation-reacylation pathway is important for the synthesis of saturated phosphatidylcholine but is less important for the synthesis of saturated phosphatidylglycerol. By the assumptions stated in the text during the pulse chase experiment de novo synthesis of saturated phosphatidylcholine from saturated diacylglycerol accounted for 25% of the total synthesis of saturated phosphatidylcholine.

Phospholipid-transfer activities in cytosols from lung, isolated alveolar type II cells and alveolar type II cell-derived adenomas

We have examined phospholipid-transfer activities in cytosols from rat and mouse whole lung, isolated rat alveolar type II cells and alveolar type II cell-derived mouse pulmonary adenomas. We report an enrichment in phosphatidylcholine and phosphatidylglycerol (but not phosphatidylinositol) protein-catalysed transfer in the type II cell and adenoma cytosols compared with the whole-lung cytosols. The activities from these cytosols were resolved using column chromatofocusing, which clearly demonstrated the presence of a phosphatidylcholine-specific transfer protein in each of the four tissues. In addition, two proteins (rat) or three proteins (mouse) catalysing both phosphatidylcholine and phosphatidylglycerol transfer were resolved from whole lung, whereas in both the rat isolated alveolar type II cells and the mouse type II cell-derived adenomas one of these less specific proteins is not present.

Lysolecithin acyltransferase and alveolar phosphatidylcholine palmitate in experimental acute alveolar injury in the dog lung

Lysolecithin acyltransferase (EC 2.3.1.23) activities in lung homogenates and in subcellular fractions, and fatty acid composition of phosphatidylcholine (PC) in lung lavage were studied in dogs with acute alveolar injury induced by N-nitroso-N-methylurethane. The specific activity in the microsomal fraction was 10 and 3 times higher than those of homogenate and mitochondrial fractions, respectively. Both the lysolecithin acyltransferase activities and the proportions of palmitate in alveolar lavage PC increased during the early phase of injury (days 2-4), and decreased during peak injury (days 6-8). Such correlation was not found during the recovery period (day 15). During recovery, specific and total activities of the enzyme were nearly 2- and 3-fold, respectively, those of controls. Nevertheless, the palmitate proportions in PC were normal, indicating that the increased enzyme activity in vitro was not reflected in increased PC palmitate during recovery. This finding indicates that the enzyme activity per cell was normal during recovery and suggests that the increase in specific and total activities is due to massive regeneration of type II cells and that the enzyme is localized mainly in these cells. The decrease in the proportion of palmitate in lavage PC during peak injury may lead to abnormality of surfactant function.

Synthesis of disaturated phosphatidylcholine by cholinephosphotransferase in rat lung microsomes

1. Incubation of rat lung microsomes with cytidine diphospho[methyl-14C]choline resulted in synthesis of radioactively labeled phosphatidylcholine. 2. 10-15% of this phosphatidylcholine appeared to be disaturated species. In similar experiments with rat liver microsomes only 2-3% of the radioactivity was present in the disaturated species. 3. When de novo synthesis was blocked after 5 min by addition of Ca2+ no increase in the proportion of disaturated phosphatidylcholine was observed upon further incubation of lung microsomes. Under these conditions the enzymes involved in a remodeling mechanism, i.e. phospholipase A and acyl-CoA: lysophosphatidyl-choline acyltransferase, remain fully active. 4. Addition of diacylglycerols from egg phosphatidylcholine containing trace amounts of di[1-14C]palmitoyl glycerol resulted in direct incorporation of 14C label into phosphatidylcholine. The rate of phosphatidylcholine synthesis measured from incorporation of di[1-14C]palmitoyl glycerol equalled that observed with labeled CDP choline. 5. These results support the conclusion that disaturated phosphatidylcholine in lung can be formed by direct utilization of disaturated diacylglycerol and is not produced exclusively via remodelling of de novo synthesized unsaturated species.

Transmembrane orientation of palmitoyl-CoA: lysophosphatidylcholine acyltransferase in microsomes isolated from an alveolar type II cell adenoma and rat liver

1. The transverse localization of palmitoyl-CoA : lysophosphatidylcholine acyltransferase in the membrane of microsomal vesicles isolated from mouse lung adenomas and rat liver was studied by treating intact and deoxycholate-disrupted microsomes with trypsin and pronase. 2. The latency of mannose-6-phosphatase was preserved during protease treatment, suggesting that membrane integrity was not affected. 3. In adenoma microsomes 35-50% and in liver microsomes 35% of lysophosphatidylcholine acyltransferase activity is accessible to the action of the proteases. Our results suggest that at least a sizable portion of the active center of the enzyme that is responsible for remodeling phospholipids is embedded in the membrane interior. 4. Since enzymes involved in de novo lipid synthesis are reported to be located at the cytoplasmic surface of the microsomal membrane, our results support the notion that in lipid metabolism distinct metabolic pools might exist at opposite sides of the microsomal membrane.

Phospholipids of the lung in normal, toxic, and diseased states

The highly pulmonary concentration of 1,2-dipalmitoyl-sn-glycerol-3-phosphorylcholine (dipalmitoyllecithin) and its implication as an important component of lung surfactant have promoted investigation of phospholipid metabolism in the lung. This review will set the contents including recent informations for better understanding of phospholipid metabolism of the lung in normal state (physiological significances of lung phospholipids, characteristics of phospholipids in lung tissue and alveolar washing, biosynthetic pathways of dipalmitoyllecithin, etc.) as well as in toxic states (pulmonary oxygen toxicity, etc.) and in diseased states (idiopathic respiratory distress syndrome, pulmonary alveolar proteinosis, etc.) Since our main concern has been to clarify the most important route for supplying dipalmitoyllecithin, this review will be focused upon the various biosynthetic pathways leading to the formation of different molecular species of lecithin and their potential significance in the normal, toxic, and diseased lungs.

Selective utilization of palmitoyl lysophosphatidylcholine in this synthesis of disaturated phosphatidylcholine in rat lung: a combined in vitro and in vivo approach

1. The acyl-CoA:lysophosphatidylcholine acyltransferase system in rat lung microsomes was found to utilize selectively 1-[1-14C]palmitoyl-sn-glycero-3-phosphocholine when compared with 1-[9,10-3H2]stearoyl-sn-glycero-3-phosphocholine. This result was found with either palmitoyl-CoA, linoleoyl-CoA or an equimolar mixture of these acyl donors and confirms recent data reported by Holub, Piekarski and Possmayer (Can. J. Biochem. 58 (1980) 434-439). 2. The selective utilization of palmitoyl lysophosphatidylcholine from a mixture of lysophosphatidylcholine species may cause an increased isotopic ratio in phosphatidylcholine when compared with that of total lysophosphatidylcholine. Thus, when rats were injected with a single doubly labelled species, i.e. 1-[9,10-3H2]palmitoyl-sn-glycero-3-phospho[methyl-14C]choline, the isotopic ratio in both total and disaturated phosphatidylcholine from lung was nearly identical to that of the injected substrate. This suggested a direct acylation by lung acyl-CoA:lysophosphatidylcholine acyltransferases. By contrast, when a mixture of 1-[9,10-3H2]palmitoyl-sn-glycero-3-phospho[methyl-14C]choline and 1-stearoyl-sn-glycero-3-phospho[methyl-14C]choline was injected, the 3H/14C ratio in disaturated lung phosphatidylcholine increased to about 1.4-fold that of the injected substrate. 3. These data indicate that increased isotopic ratios in disaturated phosphatidylcholine of lung tissue, after intravenous injection of lysophosphatidylcholine, do not necessarily point to the involvement of lysophosphatidylcholine:lysophosphatidylcholine transacylase in disaturated phosphatidylcholine formation.

Pathways of palmitate metabolism in the isolated rat lung

Plasma fatty acids represent major precursors of lung lipids. In this study, the pathways of palmitate metabolism were measured in an isolated perfused rat lung. Lungs were ventilated with 5% CO2 in air and perfused with Krebs-Ringer bicarbonate containing 3% serum albumin and 0.25 mM [U-14C] and [9, 10(-3)H] palmitate. Fatty acid utilization was estimated by recovery of radiolabel in products of metabolism. Fourteen percent of a total 14C-fatty-acid utilization of 4.5 mumol fatty acid/100 min/g dry wt. was recovered as 14CO2. Degradation of fatty acid to acetyl CoA was indicated by a 3H2O production that was twice fatty acid oxidation to CO2. The majority of palmitate was recovered in lung phosphatidylcholines with a 14C to 3H ratio of 1.4 accounting for differences between 14C and 3H2O productions. Addition of glucose to the perfusate decreased fatty acid oxidation to CO2 by 32% but had no effect on 14C recovery in phospholipids. Perfusion with the uncoupler of oxidative phosphorylation 2,4-dinitrophenol stimulated fatty acid oxidation twofold but decreased 14C incorporation into lipids. These data together with estimates of fatty acid synthesis based on 3H2O incorporation into lipids, suggested that exogenous fatty acids and glucose both represent sources of carbon for de novo fatty acid synthesis and energy production.

Substrate specificity of lysophospholipase-transacylase from rat lung and its action on various physical forms of lysophosphatidylcholine

Lysophospholipase-transacylase (lysolecithin acylhydrolase, EC 3.1.1.5) from rat lung catalyzes the transfer of acyl groups from lysophosphatidylcholine to either water or another molecule of lysophosphatidylcholine. Studies on the substrate specificity of the purified enzyme showed that a phosphate group in the substrate is essential for enzymatic activity; monoacylglycerol is not hydrolyzed, nor does it serve as an acceptor of acyl groups. The influence of the acyl chain in lysophosphatidylcholine was investigated by using mixtures of differently labelled lysophosphatidylcholine species, or by studying the transfer of [1-14C]Palmitate from [1-14C]palmitoylpropane (1,3)diol-phosphocholine to various 1-acyl-sn-glycero-3-phosphocholines. Lysophosphatidylcholines with acyl chains comprised of ten or more C-atoms were found to serve as acyl acceptors. This finding was used to determine the action of the enzyme on 1-[1-14C]auroyl- and 1[1-14C]myristoyl-sn-glycero-3-phosphocholine both below and above the critical micelle concentration of the substrate. Monomeric substrate was effectively hydrolyzed, but the transacylase activity of the enzyme was only expressed when substrate micelles were present. Likewise, no transacylase activity was found when lysophosphatidylcholine was embedded in liposomal membranes prepared from lung total lipids. These findings, which persist with crude enzyme preparations (100 000 x g supernatant), are discussed in relation to the putative function of the lysophospholipase-transacylase in the synthesis of disaturated phosphatidylcholine in lung.

Specificity of lysophospholipase D

The specificity of lysophospholipase D (1-alkyl-sn-glycero-3-phosphoethanolamine ethanolaminehydrolase, EC 3.1.4.39; also works on choline analogs) for 1-alkyl- and 1-acyl-linked substrates was examined using rat liver microsomes. The microsomes were treated with diisopropylphosphorofluoridate to inhibit the hydrolysis of acyl chains from the acyl-linked compounds (1-palmitoyl-sn-glycero-3-phosphocholine and 1-palmitoyl-sn-glycero-3-phosphoethanolamine) and were treated with p-bromophenacyl bromide to block acylation of the compounds tested. In the presence of the inhibitors, 1-alkyl-sn-glycero-3-phosphocholine and 1-alkyl-sn-glycero-3-phosphoethanolamine were hydrolyzed extensively by lysophospholipase D but the corresponding 1-acyl-linked analogs were only negligibly hydrolyzed. Lysophospholipase D therefore appears to be specific for the ether-linked compounds. 1-Alk-1-'-enyl-sn-glycero-3-phosphoethanolamine (lyso plasmalogen) was also tested as a substrate, but a plasmalogenase in the rat liver microsomes rapidly hydrolyzed the compound and we were unable to determine whether it is a substrate for lysophospholipase D. Alkyl-linked substrates containing long-chain acyl groups at the 2-position are not hydrolyzed by the enzymes. We tested 1-alkyl-2-acetoyl-sn-glycero-3-phosphocholine and 1-alkyl-2-acetoyl-sn-glycero-3-phosphoethanolamine to determine if the less bulky, more hydrophilic acetate group would permit hydrolysis by lysophospholipase D; the derivatives did not appear to be attacked, except after hydrolysis of the acetate group. However, in the absence of inhibitors, the acetate groups were rapidly hydrolyzed by microsomal preparations.