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

In vitro induction of type II pneumocyte-related differentiation in a clonal fetal bronchiolo-alveolar epithelial cell line (M3E3/C3)

The aim of the present study is to investigate the differentiation of a cloned fetal Syrian hamster lung epithelial cell line, M3E3/C3, to assume morphological and biochemical features of Type II pneumocytes (phospholipid synthesis). The use of a soft agar overlay and a differentiation medium, based on RPMI 1640 combined with hormone supplements, increased the cellular content of phosphatidylcholine (PC) from 48.6% in the conventional culture without any of these factors (referred to as 'control') to 64.7% (p < 0.02). The other cell membrane-associated components, phosphatidylethanolamine (p < 0.05), sphingomyelin (p < 0.001), phosphatidylserine (n. s.), phosphatidic acid (p < 0.02) and phosphatidylinositol (p < 0.02) decreased. The content of phosphatidylglycerol showed no essential change (from 11.2% to 8.4%) and the content of disaturated phospholipids decreased from 32.0 to 23.4 micrograms/10(6) cells (p < 0.002). The phospholipid pattern of these differentiated cells is in rough accordance with that of primary isolated Type II pneumocytes. They incorporated 3H-choline over a period of four hours at a higher rate in the Type II pneumocyte-specific phospholipids, PC and dipalmitoyl-phosphatidylcholine (DPPC), than the undifferentiated control. The radiolabelling of PC and DPPC in the differentiated cells, after 3 hours of incubation with 3H-choline, was about 3.2-fold and 2.2-fold, respectively, higher than that in the control cells (p < 0.001). Intracytoplasmatic phospholipid granules were evident in the differentiated cells by light and fluorescence microscopy (modified Papanicolaou stain, Phosphin 3 R fluorescence). Furthermore, the differentiated cells had a high activity of alkaline phosphatase, whereas the control cells showed only little activity of this enzyme. Ultrastructurally, many concentric multilayered osmiophilic bodies, well developed Golgi apparatuses and many cytoplasmic protrusions comparable to microvilli, were detectable in the cuboidal shaped differentiated cells. The control cells remained wide and flattened on the plastic surface and produced a fibrillar extracellular matrix. In the simultaneously studied fetal lung fibroblasts none of these specific features were noted. These results indicate a specific differentiation capacity of the clonal fetal cell line, M3E3/C3, by closely resembling Type II pneumocytes.

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.

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.

Mechanism of the reaction catalyzed by acyl-CoA: lysolecithin acyltransferase from rabbit lung. pH studies and chemical modification

This paper deals with the first attempt to elucidate the chemical mechanism of acyl-CoA: lysolecithin acyltransferase from rabbit lung, a key enzyme in the metabolism of lung surfactant. For this purpose, the pH dependence of kinetic constants as well as the chemical modification of the protein have been studied on a partially-purified preparation. From these experiments, the pKs on which the activity of the enzyme relies have been calculated, giving values of pK1 congruent to 5.5 and pK2 congruent to 10. Analysis of the effect of organic solvents on these pKs and the calculation of the enthalpies of ionization, together with the chemical modification experiments, lead to the conclusion that pK1 is due to an histidine residue, whereas pK2 arises from the amino group of the adenine ring of palmitoyl-CoA. Moreover, chemical modification demonstrated an essential cysteine. A tentative chemical mechanism, in accordance with these results, is proposed and it is hypothesized, in view of other results obtained in our laboratory and from the literature, that the chemical mechanism of acyl transfer to sn-2 position may be common to other enzymes of glycerolipid metabolism.

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.

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.

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.

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.

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.