Lysophospholipase and lysophosphatidylcholine:lysophosphatidylcholine transacylase from rat lung: evidence for a single enzyme and some aspects of its specificity

Transacylase formation of bis(monoacylglycerol)phosphate

Recent work within our laboratory has focused on the enzymes we hypothesize are involved in the biosynthesis of bis(monoacylglycerol)phosphate from phosphatidylglycerol. Here we describe a transacylase, active at acidic pH values, isolated from a macrophage-like cell line, RAW 264.7. This enzyme acylates the head group glycerol of sn-3:sn-1' lysophosphatidylglycerol to form sn-3:sn-1' bis(monoacylglycerol)phosphate. Here we demonstrate that this enzyme uses two lysophosphatidylglycerol molecules, one as an acyl donor and another as an acyl acceptor, and that the acyl contributions from all other lipids tested are comparatively minor. This enzyme prefers saturated acyl chains to monounsaturates, 16 and 18 carbon fatty acids over 14 carbon fatty acids, and saturated acyl chains at the sn-1 position to monounsaturated acyl chains on the sn-2 carbon of lysophosphatidylglycerol. We present data which show the transacylase activity depends on the presence of a lipid-water interface and the lipid polymorphic state.

Cytosolic lysophosphatidylcholine/transacylase in the production of dipolyunsaturated phosphatidylcholine in bonito muscle

Phosphatidylcholine with docosahexaenoic acid at both sn-1 and sn-2 positions occurs in relatively high abundance in bonito muscle. To explore a possible route for the dipolyunsaturated molecular species, phosphatidylcholine formation from 2-[1-14C]linoleoyl lysophosphatidylcholine was examined using a cytosolic fraction from bonito muscle. The formation of radiolabeled phosphatidylcholine was greatest at 15 degrees C and did not require the presence of cofactors such as CoA and calcium. By DEAE-cellulofine column chromatography, the activity to form phosphatidylcholine was separated from that of phospholipase A1, and the specific activity increased by about 100-fold. The possible involvement of cytosolic lysophosphatidylcholine/transacylase in synthesis of dipolyunsaturated phosphatidylcholine is discussed.

Mechanisms of hepatic phosphatidylcholine synthesis in adult rat: effects of pregnancy

Late pregnancy in the rat (gestational ages 16-21 days) was accompanied by a specific increase in hepatic phosphatidylcholine (PC) and phosphatidylethanolamine (PE) molecular species containing C16:0 at the sn-1 position and polyunsaturated essential fatty acids (PUFA), in particular C22:6(n-3), at the sn-2 position. Incorporation of either CDP:[Me-14C]choline or CDP:[1,2-14C]-ethanolamine into hepatic microsomal sn-1 C16:0 PC or PE molecular species in vitro was greater at term than in non-pregnant animals, suggesting modifications to the composition of specific diacylglycerol (DAG) pools destined for synthesis of either PC or PE. Also, incorporation of [Me-14C]choline or [Me-14C]methionine into hepatic PC in vivo over 6 h in term pregnant rats was consistent with decreased phospholipase A1-dependent acyl remodelling of sn-1 C16:0 to sn-1 C18:0 molecular species. There was, however, no evidence to support any change to the specificity of acyl remodelling. The rate of PC synthesis by the de novo pathway in vivo was increased in term liver compared with non-pregnant animals, accompanied by increased choline-phosphotransferase activity in vitro in d21 liver microsomes. The rate of PC synthesis by PE N-methylation did not appear to change during pregnancy. Changes in composition of plasma PC species at term reflected those of newly synthesized hepatic PC. Our data suggest supply of PUFA to the developing fetal rat is the result of specific adaptations to maternal hepatic phospholipid biosynthesis rather than passive transfer from the maternal diet.

Synthesis of phosphatidylcholine in guinea-pig fetal lung involves acyl remodelling and differential turnover of individual molecular species

The mechanisms for accumulation of disaturated phosphatidylcholine (PC) molecular species in developing fetal guinea-pig lung during the period of surfactant synthesis, between day (d) 55 and term (d68), were determined by the incorporation of 50 mu Ci [methyl-14C]choline into lung PC in utero over 3 h. Comparison of the pattern of PC synthesis de novo with the composition of the total PC pool indicated that approx. 50% of the total PC16:0/16:0 was synthesized by acyl remodelling of PC16:0/18:2 by the actions of phospholipase A2 and acyltransferases. Acyl remodelling was established before the onset of surfactant synthesis (d55) and so was not specific for this process. Between d55 and term the concentration of lung PC increased significantly. Conversely, the incorporation of [14C]choline into lung tissue and the rate of PC synthesis decreased over this period. Calculation of turnover times of lung PC species suggested that the increase in lung disaturated PC concentration during surfactant production might be due to a differential decrease in catabolism rather than increased PC synthesis.

Mechanisms of phosphatidylcholine acyl remodeling by human fetal lung

The molecular specificity of phosphatidylcholine (PC) synthesis by the de novo pathway in postmortem samples of human fetal lung (15 to 20 wk of gestation) was determined from the incorporation pattern in isolated microsomal preparations of CDP:[14C]choline into individual molecular species of PC. These analyses are based on the assumption that the molecular species composition of the pool of endogenous diacylglycerol used for PC synthesis by isolated microsomes reflects that of the authentic pool of diacylglycerol converted to PC by intact cells. Comparison of this microsomal incorporation pattern of radiolabel into PC with tissue PC composition suggested that even at this early stage of gestation 50% of lung dipalmitoyl PC was derived from synthesis de novo, with the remainder coming from acyl remodeling mechanisms. Analysis of PC synthesis de novo by organ cultures of human fetal lung showed that these acyl remodeling mechanisms were lost in culture. Despite evidence for differentiation of type II alveolar epithelial cells in culture, equilibrium labeling of PC with [14C]choline over 18 h resulted in a progressive decline in fractional incorporation into dipalmitoyl PC with time in culture. By 4 days in culture, this value was no different from the fractional incorporation of CDP:[14C]choline into microsomal PC in vitro over 3 h. The pattern of PC synthesized was not altered when total PC synthesis was stimulated by exposure of cultures to dexamethasone and tri-iodothyronine but was readily manipulated by exposure to exogenous fatty acids. These results demonstrate for the first time the activity of PC acyl remodeling mechanisms in human fetal lung, well before the initiation of surfactant production.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Metabolism of lysophospholipids in intact rat islets. The insulin secretagogue p-hydroxymercuribenzoic acid impairs lysophosphatidylcholine catabolism and permits its accumulation

Although recent studies implicate lysophospholipids (lyso-PLs) in stimulus-secretion coupling in the pancreatic islet, almost no data on lyso-PL metabolism therein exist. Therefore, intact rat islets were loaded with insulinotropic and non-toxic concentrations of 1-[14C]palmitoyl-lysophosphatidylcholine (lyso-PC) via transbilayer movement, and its metabolic fate was studied. The time-dependent hydrolysis of lyso-PC to fatty acid (lysophospholipase activity), its conversion to phosphatidylcholine (putative acyltransferase activity) and, to a lesser degree, the appearance of label in phosphatidylethanolamine (putative transacylase or base exchange activity) were observed. p-Hydroxymercuribenzoic acid (PHMB) at 100 microM (a concentration previously demonstrated to elicit potent exocytotic insulin release) inhibited all three activities (by 56, 46 and 75%, respectively) and led to the intracellular accumulation of lyso-PC. Antimycin A inhibited phosphatidylcholine formation but not lysophospholipase activity; lyso-PC did not accumulate, implying that blockade of both of the major metabolic pathways is required to induce a detectable increment in lyso-PC levels. Calculations derived from data using the lowest effective insulinotropic concentration of lyso-PC suggested that increments in lyso-PC accumulation at critical membrane sites of less than 10-15% above basal values are sufficient to trigger insulin release. Since PHMB elicited increments of 50-100% in lyso-PC after its translocation into islets, support is provided for the earlier contention that lyso-PLs mediate the insulinotropic effect of PHMB. In addition, these studies may provide a more precise experimental paradigm for future studies of islet lyso-PL metabolism.

Isolation of protein components from rat lung lamellar bodies

Lamellar bodies isolated from 10% (w/v) rat lung homogenates by discontinuous sucrose gradient centrifugation were shown to contain variable amounts of adhering proteins. These contaminating proteins could be removed by either Sepharose 4B gel filtration or precipitation of the crude preparation at pH 11.5. Both purification methods yielded membrane preparations with a phospholipid-to-protein ratio of 10.0 mumol/mg. Nearly complete separation of lamellar body phospholipid and protein could be achieved upon application of the purified membranes to DEAE-cellulose in the presence of 0.2% (v/v) Triton X-100. Phospholipid analyses showed that 83% of total lipid phosphorus was recovered in phosphatidylcholine. In phosphatidylethanolamine, phosphatidylglycerol, phosphatidylserine and phosphatidylinositol recoveries amounted to 4, 8, 2 and 2%, respectively. Molecular mass determinations of the isolated protein component of lamellar bodies by means of SDS polyacrylamide gel electrophoresis and staining with Coomassie brilliant blue revealed the presence of three protein bands with molecular masses of 64, 33 and 31 kDa. Upon staining with silver a 16 kDa protein was also visible. Sephadex G-100 gel filtration showed only one protein peak corresponding to a molecular mass of 64 kDa when protein was assayed with Coomassie brilliant blue.

Phospholipid-protein interactions in rat lung lamellar bodies

We investigated the specificity of the cytosol-mediated phosphatidylcholine transfer between isolated rat lung microsomes and rat lung lamellar bodies. For that purpose we labeled the microsomes with 1-acyl-2-[1-14C]palmitoyl- and 1-acyl-2-[9,10-3H]oleoylphosphatidylcholine through protein-catalyzed phosphatidylcholine exchange. Incubation in buffer resulted in 3-5% transfer of label from microsomes to lamellar bodies. Lung cytosol stimulated this transfer about 2-fold and the presence of 12 micrograms/ml phosphatidylcholine-transfer protein from bovine liver resulted in a 30 to 35% recovery of radioactivity in the lamellar bodies. When microsomal donor membranes with a 3H/14C ratio of 2.6 were used, the 3H/14C ratios of the lamellar bodies were 3.9, 3.7 and 3.7, after incubation in buffer, with cytosol and with bovine liver exchange protein, respectively. Doubling the amount of lamellar body acceptor membranes resulted in 3H/14C ratios in the lamellar bodies of 4.6 and 4.1, after incubation in buffer and with cytosol, respectively. Furthermore, we isolated the protein component from rat lung lamellar bodies and performed reconstitution experiments with phospholipids. Reconstituted and non-reconstituted phospholipid and protein were separated by either Sepharose 4B gel filtration or discontinuous sucrose gradient centrifugation. The presence of lamellar body protein in the reconstitution mixture resulted in the formation of larger structures with higher density than those formed in control experiments without protein. When 1-acyl-2-[1-14C]palmitoyl- and 1-acyl-2-[9,10-3H]oleoylphosphatidylcholine were included in the reconstitution mixture, the structures containing lamellar body protein had 2- to 4-fold lower 3H/14C ratios than initially present in the incubation. These results suggest that lamellar body proteins associate preferentially with disaturated phosphatidylcholine species.

Coenzyme A-mediated transacylation of sn-2 fatty acids from phosphatidylcholine in rat lung microsomes

Evidence was obtained for a CoA-dependent transfer of linoleate from rat lung microsomal phosphatidylcholine to lysophosphatidylethanolamine without the intervention of a Ca2+-requiring phospholipase A2 activity and ATP. To study this CoA-mediated transacylation process, microsomes were prepared in which the endogenous phosphatidylcholine was labeled by protein-catalyzed exchange with phosphatidylcholines containing labeled fatty acids in the sn-2-position. The apparent Km for CoA in the transfer of arachidonate from phosphatidylcholine to 1-acyllysophosphatidylethanolamine was 1.5 microM. At saturating lysophosphatidylethanolamine concentrations, the transacylation was linear with the amount of microsomal protein, i.e., a fixed percentage of the labeled fatty acid was transferred independent of the amount of microsomal protein. A maximal transfer of 12.2% for arachidonate and 2.0% for linoleate from the respective phosphatidylcholines to lysophosphatidylethanolamine was observed in 30 min. With 1-acyl-2-[1-14C]arachidonoylphosphatidylcholine as acyl donor, lysophosphatidylethanolamine was the best acceptor followed by lysophosphatidylglycerol and lysophosphatidylserine. Lysophosphatidate barely functioned as acceptor. These data provide further evidence for the widespread occurrence of CoA-mediated transacylation reactions. The arachidonate transacylation from phosphatidylcholine to other phospholipids in lung tissue may contribute to the low level of arachidonate in pulmonary phosphatidylcholine.

Cytosol-stimulated remodeling of phosphatidylcholine in rat lung microsomes

When 600 X g supernatants of 10% (w/v) rat lung homogenates were incubated with CDP[Me-14C]choline both saturated and unsaturated species of phosphatidylcholine were formed from endogenous diacylglycerols. The percentage radioactivity in the disaturated species of total phosphatidylcholine increased with time from 12% after 5 min to 30% after 60 min incubation. In similar experiments with 20 000 X g supernatants, the increase in the disaturated species of microsomal phosphatidylcholine was from 25 to 37% over the same time period. In incubations of isolated microsomes in buffer, the percent of 14C label in disaturated phosphatidylcholine remained constant at a level of 25%. To investigate a possible role of cytosolic factor(s) in the increase in the percentage of disaturated phosphatidylcholine with time, microsomes were prelabeled by incubation in buffer with CDP[Me-14C]choline to give a fixed ratio of radioactive saturated and unsaturated phosphatidylcholine species. When the reisolated microsomes were incubated in buffer, the distribution of radioactivity over saturated and unsaturated species remained constant. In contrast, incubation of prelabeled microsomes in the presence of cytosol caused an increase in the percent radioactivity in saturated phosphatidylcholines from a starting value of 18 to 30% after 60 min incubation, while leaving total phosphatidylcholine radioactivity unaffected. These results indicate a remodeling of phosphatidylcholine under the influence of a cytosolic factor(s). Evidence is presented that suggests that Ca2+-independent-cytosolic phospholipase A2 activity as well as a microsomal ATP-independent CoA-mediated acyltransferase activity might contribute to this remodeling. The cytosol donates the necessary CoA for this acyl transfer as well as saturated acyl-CoA for the reacylation of lysophosphatidylcholine.

Effects of streptozotocin-induced diabetes on phosphoglyceride metabolism of the rat liver

We have studied the effect of streptozotocin (SZ)-induced diabetes on fatty acyltransferase and phospholipase enzyme activities involved in the synthesis and degradation of rat liver phosphoglycerides. Neither mitochondrial nor microsomal acyl-CoA:glycerol 3-phosphate acyltransferase (GPAT) activity was altered, although insulin treatment stimulated mitochondrial GPAT activity. However, microsomal acyl-CoA:1-acylglycerol 3-phosphate acyltransferase (1-acyl-GPAT) activity increased (24-33 per cent, p less than 0.01) in the diabetic animals using 3 different acyl-CoA donors: palmitoyl-CoA, oleoyl-CoA and linoleoyl-CoA. SZ-induced diabetes also increased acyl-CoA;1-acylglycerol 3-phosphorylcholine acyltransferase (GPCAT) activity (38-45 per cent, p less than 0.01) with 3 different acyl-CoA donors: oleoyl-CoA, linoleoyl-CoA and arachidonoyl-CoA. 1-acyl-GPAT and GPCAT activity returned to normal with insulin treatment. In contrast to the increased activity of the microsomal fatty acyl-transferases 1-acyl-GPAT and GPCAT, SZ-induced diabetes decreased mitochondrial phospholipase A2 activity and lysophospholipase activity (49-70 per cent, p less than 0.01). Insulin treatment of the diabetic rats corrected the decreased lysophospholipase and stimulated phospholipase A2 activity 35 per cent higher than controls. Since microsomal 1-acyl-GPAT and GPCAT are known to have higher activity toward unsaturated fatty acyl-CoA donors, the increased GPCAT activity coupled with the decreased lysophospholipase activity and the increased 1-acyl-GPAT activity in diabetes would tend to increase the formation of newly synthesized phospholipids containing unsaturated fatty acids. This mechanism plus the decreased fatty acid desaturase (4) may be the factors which alter the fatty acid composition of phosphoglycerides in diabetic rat liver microsomes.

Regulatory aspects of rat liver mitochondrial phospholipase A2: effects of calcium ions and calmodulin

A comparative study was made of the metal ion requirement of rat liver mitochondrial phospholipase A2 in purified and membrane-associated forms. Membrane-bound enzyme was assayed using either exogenous or endogenous phosphatidylethanolamine. Although several divalent metal ions caused increased activity of the membrane-associated enzyme, only Ca2+ and Sr2+ activated the purified phospholipase A2. The activity in the presence of Sr2+ amounted to about 25% of that found with Ca2+. When the Ca2+ concentration was varied two activity plateaus were observed. The corresponding dissociation constants varied from 6 to 20 microM Ca2+ and from 1.4 to 12 mM Ca2+ for the high- and low-affinity binding sites, respectively, depending on the assay conditions and whether purified or membrane-bound enzyme was used. A kSr2+ of 60 microM was found for the high-affinity binding site. The effect of calmodulin and its antagonist trifluoperazine was also investigated using purified and membrane-associated enzyme. When membrane-bound enzyme was measured with exogenous phosphatidylethanolamine, small stimulations by calmodulin were found. However, these were not believed to indicate a specific role for calmodulin in the Ca2+ dependency of the phospholipase A2, since trifluoperazine did not lower the activity of the membrane-bound enzyme to levels below those found in the presence of Ca2+ alone. Membrane-bound enzyme in its action toward endogenous phosphatidylethanolamine was neither stimulated by calmodulin nor inhibited by trifluoperazine. Purified enzyme was also not stimulated by calmodulin, while trifluoperazine caused small stimulations, presumably due to interactions at the substrate level. These results indicate that calmodulin involvement in phospholipase A2 activation should not be generalized.

Lysolecithin:lysolecithin acyltransferase from rabbit lung. A conformational study

The enzyme lysolecithin:lysolecithin acyltransferase from rabbit lung has been found to have a relatively disordered conformation in solutions of high ionic strength. The protein exhibited an ordering of structure when salt was suppressed. This conformational change was concomitant with the loss of transacylase activity, the hydrolytic reaction remaining unchanged. Addition of NaCl caused a progressive disordering of structure with a parallel increase of transacylase activity. The acid denaturation of the protein, at low and high ionic strengths, showed that the ionization of groups with pK in the range 5.9-6.4 was essential for denaturation. The structure was stable at basic pH. The addition of lipids resulted in a non-specific stabilization of the disordered conformation, in the same manner as the addition of NaCl. From these results, it is suggested that there are two conformations for this protein which differ in their ability to bind lysolecithin molecules in the enzyme deacylation step of the reaction. This hypothesis agrees with previously published properties of the enzyme, concerning aggregation with other proteins and kinetic data. From the amino acid composition and conformational properties, the authors suggest that this enzyme could be a peripheral membrane protein.

A simple and versatile affinity column for phospholipase A2

An affinity adsorbent for phospholipase A2 (EC 3.1.1.4) was prepared by reacting 10-O-p-toluenesulfonyldecane-1-O-phosphocholine with AH-Sepharose 4B. Phospholipases A2 bind to the immobilized ligand in the presence of Ca2+ and can be eluted with buffers containing EDTA. This principle held not only for soluble phospholipase A2 from porcine pancreas and Crotalus adamanteus, but proved also effective in the purification of phospholipases A2 solubilized from the membranes of rat liver mitochondria and rat platelets.

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.

Purification of rat liver mitochondrial phospholipase A2

Rat liver mitochondrial phospholipase A2 was purified to near homogeneity by a combination of gel-filtration, hydroxyapatite and Matrex gel Blue A column chromatography. The absolute positional specificity of the enzyme for acylester bonds at the sn-2-position was established in experiments with 1-[9,10-3H2]palmitoyl-2-[1-14C]linoleoylphosphatidylethanolamine. Molecular weight estimations revealed Mr values of 15000 by SDS-polyacrylamide gel electrophoresis and of 9700 gel by gel-filtration over Ultrogel AcA 54 columns. The enzyme is unaffected by diisopropylfluorophosphate and thiol reagents such as 5,5'-dithiobis(2-nitrobenzoic acid), N-ethylmaleimide and iodoacetamide, but is completely inhibited by the alkylating reagent p-bromophenacylbromide.

Utilization of disaturated and unsaturated phosphatidylcholine and diacylglycerols by cholinephosphotransferase in rat lung microsomes

1. Cholinephosphosphotransferase catalyzes the conversion of diacylglycerol and CDPcholine into phosphatidylcholine and CMP. Incubation of rat lung microsomes containing phosphatidyl[Me-14C]choline with CMP resulted in an increase in water-soluble radioactivity, suggesting that also in rat lung microsomes the cholinephosphotransferase reaction is reversible. 2. Microsomes containing 14C-labeled disaturated and 3H-labeled monoenoic phosphatidylcholine were prepared by incubation of these organelles with [1-14C]palmitate and [9,10-3H2]oleate in the presence of 1-palmitoyl-sn-glycero-3-phosphocholine, ATP, coenzyme A and MgCl2. Incubation of these microsomes with CMP resulted in an equal formation of 14C- and 3H-labeled diacylglycerols, indicating that disaturated and monoenoic phosphatidylcholines were used without preference by the backward reaction of the cholinephosphotransferase. When in a similar experiment the phosphatidylcholine was labeled with [9,10-3H2]palmitate and [1-14C]linoleate, somewhat more 14C- than 3H-labeled diacylglycerol was formed. 3. The backward reaction was used to generate membrane-bound mixtures of [1-14C]palmitate- and [9,10-3H2]oleate- or of [9,10-3H2]palmitate- and [1-14C]linoleate-labeled diacylglycerols. When the microsomes containing diacylglycerols were incubated with CDPcholine, both 3H- and 14C-labeled diacylglycerols were used for the formation of phosphatidylcholine, indicating that there is no absolute discrimination against disaturated diacylglycerols. This observation is in line with our previous findings and indicates that also the CDPcholine pathway may contribute to dipalmitoylphosphatidylcholine synthesis in lung.

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.

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.

Influence of septic shock upon phosphatidylcholine remodeling mechanism in rat lung

Septic shock in rats lead to pulmonary disorders associated with alterations of phospholipid metabolism. The ratio between phosphatidylcholine and lysophosphatidylcholine is lowered both in lung tissue and in pulmonary surfactant because enzymes of phosphatidylcholine remodeling mechanism are distinctly affected by septic shock. Specific activity of phospholipase A2 is enhanced 5-fold while specific activities of lysolecithin acyltransferase and lysolecithin : lysolecithin acyltransferase are only slightly increased or remain unchanged. Beyond that, palmitic acid content of lung tissue phosphatidylcholine is significantly reduced and replaced mainly by arachidonic acid. The release of this fatty acid by action of phospholipase A2 may lead via intermediates to the generation of potent mediators such as prostaglandins, thromboxane or slow-reacting substance.

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.

The content of diacylglycerol, triacylglycerol and monoacylglycerol and a comparison of the structural and metabolic heterogeneity of diacylglycerols and phosphatidylcholine during rat lung development

The content of diacylglycerol in fetal rat lung is approx. 36% of the adult and rapidly increases to adult levels by 1 day after birth. Triacylglycerol content is also low (23%) and increases to adult levels between 1 and 2 days following birth. Monoacylglycerol content is relatively low at all stages of development. The analysis of the molecular species of diacylglycerols showed that the disaturated species accounted for 30-40% of the diacylglycerols and the monoene species 20-28%. Phosphatidylcholine contained 40-45% disaturated and approx. 30% monoene species. The overall pattern of molecular species of phosphatidylcholine was similar to the pattern for diacylglycerol. The in vivo incorporation of [2-3H]glycerol into molecular species of diacylglycerol and phosphatidylcholine in -1-day-fetal (i.e., 1 day before birth) lung showed that the disaturated species of diacylglycerol had the highest incorporation and appeared to have a higher rate of turnover. In contrast, [2-3H]glycerol was incorporated by fetal liver most actively in the monoenoic and dienoic species of diacylglycerol. The relative incorporation of radioactivity into disaturated, monoene and diene species of phosphatidylcholine in fetal lung was very similar to that for the corresponding diacylglycerol species. The rate of the reaction from the disaturated species of diacylglycerol to the disaturated species of phosphatidylcholine, calculated from the in vivo data, was one of the higher rates and indicated considerable potential for the synthesis of disaturated phosphatidylcholine via this route. The overall results suggests that de novo synthesis of disaturated phosphatidylcholine from the disaturated species of diacylglycerol can be a major route for the synthesis of dipalmitoylphosphatidylcholine in fetal lung.

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.

An enzymatically driven membrane reconstitution from solubilized components

Acyltransferase activity is present in a variety of membranes species, including liver microsomes. The substrates of this enzyme are lysophosphatides and acyl CoA derivatives. We have found that the detergent effect of these substrates can be used to solubilize rat liver microsomes. If the solubilized fraction in incubated, the acyltransferase acylates the lysophosphatide and thereby degrades the detergent effect so that vesicular membranes re-form. Gel electrophoresis patterns show that the reconstituted membranes contain all of the major protein components of the original microsomes. A marker enzyme for liver microsomes, NADPH-cytochrome c reductase, was present in the reconstituted membranes at 70% of the specific activity in the original microsomes, and freeze-fracture electron microscopy showed intramembrane particles on all fracture faces. The system may provide a useful model for studies particles on all fracture faces. This system may provide a useful model for studies of certain membrane biogenesis reactions that utilize acyltransferase in vivo.

Kinetics of the in vivo labeling of the acyl groups of rabbit lung phosphatidylcholine and disaturated phosphatidylcholine

The kinetics of labeling of lung phosphatidylcholine and disaturated phosphatidylcholine were studied for periods from 0.75--120 min following intravenous injection of radiolabeled palmitic acid and choline into 3-day-old rabbits. The labeled palmitic acid was cleared rapidly from plasma, and rapidly appeared with identical incorporation kinetics in both phosphatidylcholine and disaturated phosphatidylcholine. The 2-acyl positions of both phosphatidylcholine and disaturated phosphatidylcholine were labeled preferentially soon after [14C]palmitic acid injection. The specific activities of palmitic acid in the 2-acyl positions of phosphatidylcholine and disaturated phosphatidylcholine 0.75 min after injection of labeled palmitic acid were 3.4 and 1.9 times, respectively, the specific activities of palmitic acid in the 1-acyl positions. By 120 min the label had randomized between the 1-acyl and 2-acyl positions, and the kinetics of that randomization were defined for both phosphatidylcholine and disaturated phosphatidylcholine. Choline did not pulse label lung phosphatidylcholine or disaturated phosphatidylcholine. The choline label appeared with equal specific activities in both phosphatidylcholine and disaturated phosphatidylcholine. Thus no analysis of the de novo synthesized product via the CDP-choline pathway was possible.

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.