Lysophospholipase and transacylase activities of rat gastric mucosa

Phospholipase and lysophospholipase activity of rat eosinophil leukocytes

Previous studies have shown the high lysophospholipase activity of rat eosinophilic leukocytes and used this enzyme to measure the rise in eosinophilic population of peripheral tissues caused by parasitic infections. This report details the methods and results of an investigation showing the presence in the same cells of high phospholipase (PLA) activity. Unfractionated and metrizamide-purified peritoneal eosinophil preparations were assayed using a mixed micelle substrate (6/15 mM lecithin/Triton X-100) at experimentally determined pH (6.4) and ionic strength (I=0.2) optima: the attendant reaction products included free fatty acids and organic P in a 2/1 molar proportion with a correspondent loss in the initial phospholipid concentration. The organic P fragment was further characterized as GPC (glycerylphosphorylcholine) by quantitative precipitation and acid hydrolysis. Estimates of PLA activity averaged 5 micromol/h/10(6) unfractionated eosinophils and metrizamide-purified eosinophil preparations. Paired tests for PLA and LysoPLA on unfractionated and enriched cell preparations, cytosolic extracts, and chromatographic fractions yielded similar activity ratios, supporting the inference of a close association of the two activities which could also be confirmed for the major tissues of eosinophil production and distribution.

Cloning and expression of cDNA encoding rat liver 60-kDa lysophospholipase containing an asparaginase-like region and ankyrin repeat

Mammalian tissues contain small form and large form lysophospholipases. Here we report the cloning, sequence, and expression of cDNA encoding the latter form of lysophospholipase using antibody raised against the enzyme purified from rat liver supernatant (Sugimoto, H., and Yamashita, S. (1994) J. Biol. Chem. 269, 6252-6258). The 2,539-base pair cDNA encoded 564 amino acid residues with a calculated Mr of 60,794. The amino-terminal two-thirds of the deduced amino acid sequence significantly resembled Escherichia coli asparaginase I with the putative asparaginase catalytic triad Thr-Asp-Lys and was followed by leucine zipper motif. The carboxyl-terminal region carried ankyrin repeat. When the cDNA was transfected into HEK293 cells, not only lysophospholipase activity but also asparaginase and platelet-activating factor acetylhydrolase activities were expressed. Reverse transcription-polymerase chain reaction revealed that the transcript occurred at high levels in liver and kidney but was hardly detectable in lung and heart from which large form lysophospholipases had been purified, suggesting the presence of multiple forms of large form lysophospholipase in mammalian tissues.

Purification, cDNA cloning, and regulation of lysophospholipase from rat liver

A lysophospholipase was purified 506-fold from rat liver supernatant. The preparation gave a single 24-kDa protein band on SDS-polyacrylamide gel electrophoresis. The enzyme hydrolyzed lysophosphatidylcholine, lysophosphatidylethanolamine, lysophosphatidylinositol, lysophosphatidylserine, and 1-oleoyl-2-acetyl-sn-glycero-3-phosphocholine at pH 6-8. The purified enzyme was used for the preparation of antibody and peptide sequencing. A cDNA clone was isolated by screening a rat liver lambda gt11 cDNA library with the antibody, followed by the selection of further extended clones from a lambda gt10 library. The isolated cDNA was 2,362 base pairs in length and contained an open reading frame encoding 230 amino acids with a Mr of 24,708. The peptide sequences determined were found in the reading frame. When the cDNA was expressed in Escherichia coli cells as the beta-galactosidase fusion, lysophosphatidylcholine-hydrolyzing activity was markedly increased. The deduced amino acid sequence showed significant similarity to Pseudomonas fluorescence esterase A and Spirulina platensis esterase. The three sequences contained the GXSXG consensus at similar positions. The transcript was found in various tissues with the following order of abundance: spleen, heart, kidney, brain, lung, stomach, and testis = liver. In contrast, the enzyme protein was abundant in the following order: testis, liver, kidney, heart, stomach, lung, brain, and spleen. Thus the mRNA abundance disagreed with the level of the enzyme protein in liver, testis, and spleen. When HL-60 cells were induced to differentiate into granulocytes with dimethyl sulfoxide, the 24-kDa lysophospholipase protein increased significantly, but the mRNA abundance remained essentially unchanged. Thus a posttranscriptional control mechanism is present for the regulation of 24-kDa lysophospholipase.

Purification and properties of lysophospholipase isoenzymes from pig gastric mucosa

Two lysophospholipases, named gastric lysophospholipases I and II (enzymes I and II), were purified 3730- and 2680-fold from pig gastric mucosa. The preparations showed 22 and 23 kDa single protein bands on SDS/PAGE respectively. Both enzymes lacked transacylase activity and appeared to exist as monomers. Their activities were not affected by Ca2+, Mg2+ or EDTA. Enzyme I was most active at pH 8.5 and hydrolysed a variety of lysophospholipids including acidic lysophospholipids and the acyl analogue of platelet-activating factor, whereas enzyme II was most active at pH 8 and its activity was confined to lysophosphatidylcholine and lysophosphatidylethanolamine. When 1-palmitoylglycerophosphocholine was used as substrate, enzymes I and II showed half-maximal activities at 11 and 12 microM respectively. The enzymes exhibited no phospholipase B, lipase or general esterase activity. Enzyme II was significantly inhibited by lysophosphatidic acid whereas enzyme I was only moderately inhibited. Peptide mapping with V8 protease and papain revealed structural dissimilarity between the two enzymes. Antiserum raised against enzyme I did not recognize enzyme II, but did recognize the small-sized lysophospholipase purified from rat liver. Anti-(enzyme II) consistently did not cross-react with enzyme I or the liver enzyme. These antisera specifically recognized neither the 60 kDa lysophospholipase transacylase purified from liver nor any peritoneal macrophage protein. Thus gastric mucosa contains two different small-sized lysophospholipases: one is closely related to the small-sized lysophospholipase of liver, but the other appears to be a novel isoform.

Purification and characterization of lysophospholipase-transacylase of pathogenic fungus Candida albicans

A lysophospholipase-transacylase was purified to homogeneity from the culture broth of Candida albicans by ammonium sulfate precipitation and chromatographs on DEAE-cellulose, Ultrogel AcA-44, first Mono Q, hydroxyapatite, TSKgel-3000 and second Mono Q columns. The purified protein was a single band (Mr 41,000) as inferred by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It had a specific activity of 78 mumol/min per mg protein for fatty acid release and 320 mumol/min per mg protein for phosphatidylcholine formation. Fatty acid release obeyed Michaelis-Menten kinetics and the apparent Km was 76 microM of 1-palmitoyl-sn-glycero-3-phosphatidylcholine, but Lineweaver-Burk plots of transacylase activity was parabolic. The ratio of hydrolase to transacylase activity of the purified enzyme was varied depending upon the concentration of lysophosphatidylcholine. Transacylation was prominent at high concentration of substrate and the ratio of hydrolase to transacylase was 0.24. Low concentration of palmitoylcarnitine (50 microM) inhibited markedly phosphatidylcholine formation but stimulated fatty acid release. The degree of esterification of 1-acyllysophosphatidylcholine was altered with mixtures of different molecular species of substrate, demonstrating acyl chain selectivity in the transfer process. These results suggest that C. albicans lysophospholipase-transacylase is different from the corresponding mammalian enzymes in enzymatic properties.

Occurrence of phospholipase A2 and lysophospholipase in a gastric H,K-ATPase-containing membrane fraction, and the formation of lysophosphatidylcholine in stimulated pig parietal cells

A membrane fraction containing H,K-ATPase (EC 3.6.1.36) was prepared from pig gastric mucosa and found to contain phospholipase A2 (EC 3.1.1.4) and lysophospholipase (EC 3.1.1.5) activities. Washing the membranes decreased their protein content by 25%. Recovery profiles of H,K-ATPase, phospholipase A2 and lysophospholipase were similar for membranes washed either with water or with 0.15 or 1.5 M KCl. Nearly identical distribution profiles were obtained for the three enzyme activities after centrifugation of washed vesicle membranes on a linear sucrose gradient. The phospholipase A2 activity was stimulated by calcium and increased further in the presence of calmodulin. The amount of cellular radioactively labelled lysophosphatidylcholine was doubled upon cholinergic stimulation of isolated parietal cells prelabelled with [3H]glycerol or 32Pi. The liberated lyso[32P]phosphatidylcholine had its acyl chain in the sn-1 position, which implies an activation of a phospholipase A2. These findings indicate that secretagogues which increase the cytosolic Ca2+ concentration, i.e. acetylcholine, histamine and gastrin, may activate a phospholipase A2 in the parietal cell.

Purification and characterization of phospholipase A2 from rat stomach

Phospholipase A2, which is localized in the mucosal part of the corpus of rat stomach (Hirohara et al. (1987) Biochim. Biophys. Acta 919, 231-238), was purified 990-fold from the supernatant of a tissue homogenate by heat treatment at acidic pH, ammonium sulfate fractionation, ion-exchange chromatography, gel-filtration and reverse-phase high-performance liquid chromatography (reverse-phase HPLC). The purified enzyme gave a single protein band on sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis with a molecular mass of approx. 17 kDa. The enzyme had a pH optimum of 8.0 and hydrolyzed the 2-arachidonoyl residue of phosphatidylcholine preferentially to the 2-oleoyl residue, the Vmax and Km values for the two being 227 and 29 mumol/min per mg protein and 0.037 and 0.019 mM, respectively. The activity was calcium-dependent and was markedly increased by SDS and dimethyl sulfoxide (DMSO). The enzyme showed typical product inhibition. Free unsaturated fatty acids (oleic, arachidonic and docosahexaenoic acids), which are supposedly the main enzymatic products in vivo, inhibited the activity. Arachidonic acid caused noncompetitive inhibition and its concentration for its maximal inhibition (50% inhibition) was 5 x 10(-5) M. Lysophosphatidylcholine, free saturated fatty acids (palmitic and stearic acids) and arachidonic acid metabolites (leukotrienes and prostaglandins) had no effect on the activity.

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 of lysophosphatidylcholine transacylase from bovine heart muscle microsomes and regulation of activity by lipids and coenzyme A

Heart muscle microsomes catalyze the transacylation of lysophosphatidylcholine (lyso PC) to produce phosphatidylcholine (PC). The enzyme which catalyzes this reaction, lyso PC:lyso PC transacylase, has been isolated and characterized from bovine heart muscle microsomes. The purification of the enzyme was achieved by a procedure involving extraction with 3-[3-cholamidopropyl)dimethylammonio)-1-propanesulfonate (CHAPS) detergent and chromatography on DEAE-cellulose, Reactive blue agarose, and Matrex gel green A. The purified enzyme was nearly homogeneous and consisted of a single molecular species of 128 kDa as determined by polyacrylamide gel electrophoresis in the presence of dodecyl sulfate. The catalytic activity of the enzyme was dependent on the presence of either CoA or acyl-CoA, both of which maximally stimulated at concentrations of approx. 10 microM. Analysis of the PC produced in the reaction showed that the enzyme catalyzed a transacylation in which both acyl groups arose from lyso PC. Furthermore, the enzyme did not possess acyl-CoA:lyso PC acyltransferase activity, lysophospholipase or acyl-CoA hydrolase activity, nor did it catalyze transacylation from lyso PC to lysophosphatidylethanolamine, lysophosphatidylinositol or lysophosphatidylserine. Although transacylation was highly specific for lyso PC as the substrate, various unsaturated fatty acyl-CoA derivatives served as activators. Palmitoyl-CoA and stearoyl-CoA did not significantly activate, although acetyl-CoA was an effective activator. Further modulation of activity was produced by palmitic acid and PC, both of which further activated the enzyme in the presence of oleoyl-CoA, whereas arachidonic acid, oleic acid, phosphatidylethanolamine and phosphatidylserine had no effect on activity. The high activity of this transacylase and its regulation by lipids suggests an important role for disaturated PC species in membranes and a mechanism for controlling the metabolism of lyso PC.

A pancreatic-type phospholipase A2 in rat gastric mucosa

A phospholipase A2, which is immuno-crossreactive with the anti-rat pancreatic phospholipase A2 antibody, is present in rat gastric mucosa. The content of the enzyme in the gastric mucosa was comparable to that in the pancreas, but the specific activity in the gastric mucosa homogenate (60.7 +/- 19.5 nmol/min/mg) was higher than that in the pancreas homogenate (3.16 +/- 0.77 nmol/min/mg). A greater proportion of the enzyme was found in the particulate fraction. The gastric enzyme and its proenzyme were purified from the supernatant. The amino acid sequence of the N-terminal 15 residues of the gastric enzyme was determined and found to be identical with that of rat pancreatic phospholipase A2. Like the pancreatic proenzyme, the gastric proenzyme was activated on trypsin treatment.

Effects of taurocholic acid/HCl alone or after pretreatment with geranylgeranylacetone on phospholipid metabolism in rat gastric mucosa

Changes in phospholipid metabolism in gastric mucosa caused by instillation of taurocholic acid (TCA)/HCl (80 mM/300 mM) into the stomach of rats and the effects of pretreatment with an antiulcer agent, geranylgeranylacetone (GGA), were studied after intravenous injection of radioisotope-labeled precursors. The instillation of TCA/HCl rapidly reduced the incorporation of labeled fatty acids and glycerol into phosphatidylcholine and phosphatidylethanolamine, indicating the inhibition of de novo synthesis of phospholipids. These changes were restored by 120-150 min after the TCA/HCl treatment. Pretreatment with GGA enhanced the incorporation of precursors into phosphatidylcholine immediately after the instillation of TCA/HCl. Experiments in which the mucosal lipids were labeled with fatty acids prior to the instillation of TCA/HCl showed that the degradation of cellular lipids and release of the products into the gastric lumen were induced by TCA/HCl and that these changes were not prevented by GGA. Since GGA almost completely inhibited the gastric lesions induced by TCA/HCl, the enhancement of synthesis of mucosal phosphatidylcholine induced by GGA may be involved in the prevention of gastric damage. The incorporation of labeled fatty acids into free fatty acid fraction and diacylglycerol was increased quickly by the TCA/HCl treatment, suggesting early damage to the blood vessels of the gastric mucosa; these changes were inhibited significantly by GGA.

Phospholipase A2 activity of rat stomach

Phospholipase A activity in rat stomach wall and in gastric content was studied using [1-14C]dioleoylphosphatidylcholine as substrate. The optimum activity of the stomach wall was found to take place at pH 7.0. During optimal phospholipase action about 40% of the [1-14C]oleic acid released was due to an active intracellular lysophospholipase. The gastric phospholipase required 5 mM Ca2+ for full activity and is inhibited by EDTA. It specifically hydrolyzed the sn-2 position of the phospholipid molecule. The enzyme was heat labile and inactivated by acidification at pH 3.0. The gastric content enzyme had a lower specific activity and an optimum pH of 8.0. It was heat stable and was not inactivated by acidification. These results indicate that gastric content phospholipase A is of pancreatic origin, via a duodenal reflux. By ligating the stomach we were able to further confirm that the gastric wall phospholipase was different from that of the gastric content. It originated from the stomach mucosa. Subcellular fractionation suggests that the gastric phospholipase A2 is essentially bound to the plasma membrane. About 6% of the activity was found to be soluble. Biopsies of human gastric mucosa displayed a phospholipase A activity which had similar properties to that of rat gastric enzyme. The physiological function of this enzyme is discussed in terms of prostaglandin synthesis via the release of arachidonic acid.

Properties and localization of phospholipase A2 activity in rat stomach

In the glandular stomach of rat, phospholipase A2 activity was detected and characterized by the use of sonicated 1-acyl-2-[1-14C]oleoyl-sn-glycero-3-phosphocholine (1 mM in 200 mM glycylglycine buffer/2 mM CaCl2) as substrate. The specific activity was 4.9 X 10(-2) mumol/min per mg protein in the whole glandular stomach homogenate (n = 8). It showed individual variation among rats. The optimal pH was 8.0. The activity was relatively heat-resistant and calcium-dependent. More than 90% of phospholipase A2 activity was located in the corpus and less than 10% was in the antrum. In the corpus wall, which consists of mucosa, submucosa, muscularis externa and serosa, the mucosal part (mucosa and submucosa) contained 80-90% of total activity. The effect of some clinical agents (ulcerogenic and anti-ulcer agents) on the phospholipase A2 activity was examined. The activity in the corpus was inhibited by cimetidine (50% inhibition at 5 X 10(-3) M) and stimulated by indomethacin (50% stimulation at 5 X 10(-5) M). Cetraxate hydrochloride (10(-6)-10(-3) M), 16,16-dimethylprostaglandin E2 (10(-7)-10(-4) M) and dexamethasone (10(-7)-10(-3) M) had no effect.

Substrate selectivity of lysophosphatidylcholine: lysophosphatidylcholine acyltransferase from rabbit lung

The influence of both polar group and acyl chain of lysophospholipids on the lysophosphatidylcholine: lysophosphatidylcholine acyltransferase from rabbit lung was studied. Both, transacylase and hydrolase activities of this enzyme, utilize selectively 1-[1-14C]palmitoyl-sn-glycero-3-phosphocholine when compared with 1-[9,10-3H2]palmitoyl-sn-glycero-3-phosphoethanolamine. Transacylase activity is more selective for lysophosphatidylcholine as acyl acceptor than as acyl donor. The amount of dipalmitoylphosphatidylcholine/min/mg protein synthesized from mixed lysophosphatidylcholine/lysophosphatidylethanolamine micelles does not change with increasing molar percentages of lysophosphatidylethanolamine in the mixture and is similar to that formed with pure lysophosphatidylcholine micelles. Transacylation reaction takes place preferentially with long and saturated acyl chains whereas hydrolysis reaction does more efficiently with longer acyl chains, independently of their insaturation degree.

Inhibition of gastric (H+ + K+)-ATPase by unsaturated long-chain fatty acids

Arachidonic acid and unsaturated C18 fatty acids at concentrations near 10(-5) M markedly inhibited (H+ + K+)-ATPase in hog or rat gastric membranes. Arachidonic acid was a more potent inhibitor than unsaturated C18 fatty acids, but the involvement of the metabolites of arachidonic acid cascade was ruled out. Linolenic acid inhibited the formation of phosphoenzyme and the K+ -dependent p-nitrophenylphosphatase activity of the hog ATPase. Treatment with fatty acid-free bovine serum albumin abolished only the inhibitory effect of the fatty acid on the phosphatase activity without restoring the overall ATPase action. These data suggest the existence of at least two groups of hydrophobic binding sites in the gastric ATPase for unsaturated long-chain fatty acids which affect differentially the catalytic reactions of the ATPase. (H+ + K+)-ATPase in rat gastric membranes was found more susceptible to the fatty acid inhibition and also more unstable than the ATPase in hog gastric membranes. The presence of a millimolar level of lanthanum chloride or ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid stabilized the rat ATPase probably via the inhibition of Ca2+ -dependent phospholipases in the gastric membranes.

Age-dependency of vascular phospholipid deacylation-reacylation in spontaneously hypertensive rats

We have studied the temporal relation of phospholipid turnover and prostaglandin synthesis to the evolution of hypertensive vascular disease in the spontaneously hypertensive rat. The incorporation of arachidonate into aortic phospholipids, its release by phospholipase A2 and its utilization for prostaglandin synthesis were compared in spontaneously hypertensive and Wistar-Kyoto rats aged 7, 20 and 42 weeks. When expressed per mg of protein in the assay medium, arachidonate incorporation into aortic phospholipids decreased, while prostaglandin synthesis increased, with age in both rat strains. No significant differences were noted between hypertensive and normotensive animals at 7 weeks of age whereas both enhanced phospholipid turnover and prostaglandin synthesis was demonstrated in hypertensive rats at 20 and 42 weeks of age. The higher phospholipase activity in hypertensive aortas was associated with a significant increase in the capacity for exogenous lysophosphatide hydrolysis. Transacylation and reacylation of lysolecithin, however, were not significantly enhanced in hypertensive aortas. These biochemical changes accompany, and may be related to, structural modifications of the aortic wall in the course of hypertension.

Lysophospholipase activity in rabbit skeletal muscle

Skeletal muscle contains appreciable lysophospholipase activity which is differentiated by muscle type with red muscle subcellular fractions having greater activity than the corresponding white ones.

Comparison of ACYL-CoA : lysophosphatidylcholine acyltransferase and lysophosphatidylcholine : lysophosphatidylcholine transacylase activity in various mammalian lungs

Lung disaturated phosphatidylcholine is thought to by synthesized, at least partially, by remodelling of unsaturated phosphatidylcholines. In view of the putative role of acyl-CoA : lysophosphatidylcholine acyltransferase (EC 2.3.1.23) and lysophosphatidylcholine : lysophosphatidylcholine transacylase in this remodelling process the activity of these enzymes in the lung of various mammals was investigated. The acyl-CoA-dependent acylation pathway was easily detected in the microsomal fractions of all animals investigated. By contrast, the enzyme catalyzing a transacylation between two molecules of lysophosphatidylcholine was only present in the 100 000 X g supernatants of rat, rabbit and mouse lung. It appeared to be undetectable in these fractions from guinea pig, bovine, pig, horse and sheep lung. These results provide and additional argument for the idea that the transacylation does not contribute to the synthesis of dipalmitoyl phosphatidylcholine in vivo.

Degradation of phosphatidylinositol by soluble enzymes of rat gastric mucosa

Rat gastric mucosa homogenates contain two enzymatic systems for hydrolyzing phosphatidylinositol: a deacylation activity yielding lysophosphatidylinositol and free fatty acid, and a phospholipase C-like activity producing 1,2-diacylglycerol and inositol phosphates. These activities were found mainly in the 105 000 x g supernatant and could be distinguished by differential stabilities, metal requirements and the action of deoxycholate and mepacrine. Each lipolytic reaction displayed a major pH optimum at 7.5 and a minor pH optimum at 5.5. The deacylation system was 8-10 times as active as the phospholipase C, with an apparent Km of 0.63 mM towards 1-acyl-2-arachidonylphosphatidylinositol at pH 7.5. The phospholipase C activity, on the other hand, hydrolyzed 1-acyl-2-arachidonylphosphatidylinositol or 1-acyl-2-arachidonylphosphatidylethanolamine and yielded 1-acyl-2-arachidonyl-sn-glycerol. This 1,2-diacylglycerol could be phosphorylated to form 1-acyl-2-[14C]arachidonyl-sn-phosphoglycerol (phosphatidic acid), but could not be hydrolyzed to produce free [14C]arachidonic acid using stomach mucosal microsomes. Phospholipase A2 and phospholipase C attack 1-acyl-2-arachidonylphosphatidylethanolamine and phosphatidylinositol equally well, but hydrolyze 1-acyl-2-arachidonylphosphatidylcholine poorly.

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.

Molecular species of phosphatidylcholine from rat gastric mucosa

Phosphatidylcholine of rat gastric mucosa were found to constitute about half of the total phospholipids. The composition of 20 molecular species accounting for approx. 90% of the total phosphatidylcholine was determined by specific enzymic hydrolyses and AgNO3 thin-layer and gas-liquid chromatography. Disaturated (dipalmitoyl) phosphatidylcholine made up about 31% of the total phosphatidylcholines. Other species which occurred in significant concentrations included 16:0/18:1, 18:0/18:1, 16:0/18:2, 18:0/18:2, 16:0/18:3, 18:0/18:3, 16:0/20:4, and 18:0/20:4. These results indicate that rat gastric mucosa is similar to lung in that both contain elevated amounts of dipalmitoyl phosphatidylcholine. Other similarities between these two tissues are discussed.