Role of lamellar bodies in the biosynthesis of phosphatidylcholine in mouse lung

A novel lysophosphatidylcholine acyl transferase activity is expressed by peroxiredoxin 6

The phospholipase A2(PLA2) activity of peroxiredoxin (Prdx)6 has important physiological roles in the synthesis of lung surfactant and in the repair of peroxidized cell membranes. These functions require the activity of a lysophospholipid acyl transferase as a critical component of the phospholipid remodeling pathway. We now describe a lysophosphatidylcholine acyl transferase (LPCAT) activity for Prdx6 that showed a strong preference for lysophosphatidylcholine (LPC) as the head group and for palmitoyl CoA in the acylation reaction. The calculated kinetic constants for acylation wereKm18 μM andVmax30 nmol/min/mg protein; theVmaxwas increased 25-fold by phosphorylation of the protein whileKmwas unchanged. Study of recombinant protein in vitro and in mouse pulmonary microvascular endothelial cells infected with a lentiviral vector construct indicated that amino acid D31 is crucial for LPCAT activity. A linear incorporation of labeled fatty acyl CoA into dipalmitoyl phosphatidylcholine (PC) indicated that LPC generated by Prdx6 PLA2activity remained bound to the enzyme for the reacylation reaction. Prdx6 is the first LPCAT enzyme with demonstrated cytoplasmic localization. Thus, Prdx6 is a complete enzyme comprising both PLA2and LPCAT activities for the remodeling pathway of PC synthesis or for repair of membrane lipid peroxidation.

Altered lung phospholipid metabolism in mice with targeted deletion of lysosomal-type phospholipase A2

Lung surfactant dipalmitoylphosphatidylcholine (DPPC) is endocytosed by alveolar epithelial cells and degraded by lysosomal-type phospholipase A2 (aiPLA2). This enzyme is identical to peroxiredoxin 6 (Prdx6), a bifunctional protein with PLA2 and GSH peroxidase activities. Lung phospholipid was studied in Prdx6 knockout (Prdx6-/-) mice. The normalized content of total phospholipid, phosphatidylcholine (PC), and disaturated phosphatidylcholine (DSPC) in bronchoalveolar lavage fluid, lung lamellar bodies, and lung homogenate was unchanged with age in wild-type mice but increased progressively in Prdx6-/- animals. Degradation of internalized [3H]DPPC in isolated mouse lungs after endotracheal instillation of unilamellar liposomes labeled with [3H]DPPC was significantly decreased at 2 h in Prdx6-/- mice (13.6 +/- 0.3% vs. 26.8 +/- 0.8% in the wild type), reflected by decreased dpm in the lysophosphatidylcholine and the unsaturated PC fractions. Incorporation of [14C]palmitate into DSPC at 24 h after intravenous injection was decreased by 73% in lamellar bodies and by 54% in alveolar lavage surfactant in Prdx6-/- mice, whereas incorporation of [3H]choline was decreased only slightly. Phospholipid metabolism in Prdx6-/- lungs was similar to that in wild-type lungs treated with MJ33, an inhibitor of aiPLA2 activity. These results confirm an important role for Prdx6 in lung surfactant DPPC degradation and synthesis by the reacylation pathway.

Surfactant lipid synthesis and lamellar body formation in glycogen-laden type II cells

Pulmonary surfactant is a lipoprotein complex that functions to reduce surface tension at the air liquid interface in the alveolus of the mature lung. In late gestation glycogen-laden type II cells shift their metabolic program toward the synthesis of surfactant, of which phosphatidylcholine (PC) is by far the most abundant lipid. To investigate the cellular site of surfactant PC synthesis in these cells we determined the subcellular localization of two key enzymes for PC biosynthesis, fatty acid synthase (FAS) and CTP:phosphocholine cytidylyltransferase-alpha (CCT-alpha), and compared their localization with that of surfactant storage organelles, the lamellar bodies (LBs), and surfactant proteins (SPs) in fetal mouse lung. Ultrastructural analysis showed that immature and mature LBs were present within the glycogen pools of fetal type II cells. Multivesicular bodies were noted only in the cytoplasm. Immunogold electron microscopy (EM) revealed that the glycogen pools were the prominent cellular sites for FAS and CCT-alpha. Energy-filtering EM demonstrated that CCT-alpha bound to phosphorus-rich (phospholipid) structures in the glycogen. SP-B and SP-C, but not SP-A, localized predominantly to the glycogen stores. Collectively, these data suggest that the glycogen stores in fetal type II cells are a cellular site for surfactant PC synthesis and LB formation/maturation consistent with the idea that the glycogen is a unique substrate for surfactant lipids.

Nature, origin and fates of membranous lamellae in the lung of the neonate rat

This study explores the basic nature and formation of lamellar accumulations in the vertebrate lung, and the problematical interrelationship of the lamellae with mitochondria. Autolysosomes are a constant feature of the type II alveolar pneumonocyte of the 2-day-old rat. They are characterized by a single boundary membrane, enclosing a heterogeneous collection of vesicles and membraneous lamellae. The autolysosomes result from repeated episodes of glycogen catabolism, and eventually transform into osmiophilic lamellated bodies. Membranous lamellae within autolysosomes and lamellated bodies represent isolating membranes of cellular autophagy, emptied of their digested contents. Proliferated isolating membranes themselves undergo lysis, providing recycled constituents for the differentiating or dividing cell. Mitochondria of the type II alveolar cell often display invaginations occupied by membranous masses; these masses are demonstrated by high magnification electron micrographs to be continuous with and derived from lamellated bodies, a new finding. Inner mitochondrial membranes lining the invaginations are thinned and crista-free. It is concluded that undergraded or partially lysed isolating membranes follow either of two cellular pathways: they may be eliminated from the cell into the alveolar cavity, or may fill mitochondrial indentations.

Evidence for a high activity of sphingomyelin biosynthesis by phosphocholine transfer from phosphatidylcholine to ceramides in lung lamellar bodies

Biosynthesis of sphingomyelin from ceramides was investigated in lung subcellular fractions by incubating a lyophilized mixture of albumin and subcellular fraction (0.1-0.2 mg of protein) coated with [acyl-14C]-ceramide and phosphatidyl[methyl-3H]choline in Tris-buffer. The lamellar-body-rich fraction exhibited the highest specific activity for sphingomyelin biosynthesis measured by 14C incorporation into sphingomyelins or by [3H]phosphocholine transfer from phosphatidylcholines. Plasma membranes formed the next most active fraction, followed by the 'smooth' and, then, the 'rough' endoplasmic reticulum. Sphingomyelin biosynthesis by lamellar bodies was optimum at pH 7.4 and was inhibited by sphingomyelins formed. Slight inhibitory effects were also observed with Mn2+, Ca2+ and lysophosphatidylcholine. Phosphocholine transfer from CDPcholine was not observed under the reaction conditions employed. Ceramide conversion and phosphocholine transfer increased with ceramide concentration to reach a maximum at about 0.06 mM. The highest conversion rate was observed when 18:1 ceramide was used as an acceptor. When 1-palmitoyl-2-oleoylphosphatidylcholine was the phosphocholine donor, the overall biosynthesis of sphingomyelin was much higher than when using dipalmitoylphosphatidylcholine. These results suggest the possible involvement of the studied reaction in the control of the degree of saturation of the surfactant phosphatidylcholine.

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.

Acyl-chain specificity and membrane fluidity. Factors which influence the activity of a purified phospholipid-transfer protein from lung

A purified phospholipid-transfer protein from rat lung has been characterized in terms of the specificity of the protein for phosphatidylcholine molecules with different apolar moieties. The study demonstrated that the lung-phospholipid-transfer protein discriminates between dipalmitoylphosphatidylcholine and molecular species of phosphatidylcholine with unsaturated acyl chains. The initial rate of transfer of dipalmitoylphosphatidylcholine is 1.5-fold greater than the rate of transfer of dioleoylphosphatidylcholine, 1-palmitoyl-2- arachidonylphosphatidylcholine , or egg phosphatidylcholine under most assay conditions. Although the protein preferentially transfers dipalmitoylphosphatidylcholine, the incorporation of increasing mole percentages of dipalmitoylphosphatidylcholine into unilamellar phosphatidylcholine vesicles profoundly affects their effectiveness as donors for phosphatidylcholine transfer by the transfer protein. At 60 mol% dipalmitoylphosphatidylcholine, the rate of transfer is one-third that observed when vesicles are composed of 100% egg phosphatidylcholine. Decreases in membrane fluidity as estimated by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene correlate with decreases in the effectiveness of the vesicles as donors in the phospholipid-transfer reaction. The conclusion from these studies is that the rate of transfer of phosphatidylcholine by the purified phospholipid-transfer protein from lung is determined by physical properties of membrane interfaces with which the protein interacts, as well as by the specificity of the phospholipid-transfer protein for different molecular species of phosphatidylcholine.

Insulin effect on some biochemical and biophysical characteristics of lung surfactant

The incorporation of [14C]palmitic acid into rat alveolar wash total phospholipids and phospholipid fractions has been followed for 6, 8, 10 and 12 hr after insulin administration, indicating a considerable enhancement. The fatty acid profiles of phosphatidylcholines, phosphatidylethanolamines and phosphatidylglycerols were found changed after the hormone administration. Eight hours post insulin treatment the precursor incorporation was highest in all phospholipid fractions studied, as well as the contribution of long chain fatty acids. Dynamic monolayer studies of the lung wash lipid extracts indicated a maximally expanded lipid film corresponding to the highly unsaturated phospholipids present.

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.

Isolation and characterization of rabbit lung lamellar bodies

A method has been devised for the isolation of a highly purified preparation of lamellar bodies from rabbit lung. The purity of the preparation was confirmed by electron microscopy, marker enzymes, phospholipid composition, and isopycnic centrifugation on continuous density sucrose gradients. Contamination of the lamellar bodies by such subcellular components as mitochondria, nuclei, lysosomes and plasma membranes could be excluded; however, reduced nicotinamide adenine dinucleotide phosphate (NADPH) cytochrome c reductase, an enzyme specific for the endoplasmic reticulum components was a persistent contaminant in the preparation of the isolated lamellar bodies. When the lamellar bodies were subject to isopycnic centrifugation, all of the NADPH cytochrome c reductase activity was associated with the lamellar bodies in the low density peak; no reductase activity could be detected in the region of the density gradient demonstrated to localize microsomes. Use of 3H-radiolabeled microsomes confirmed that all of the NADPH cytochrome c reductase activity present in the lamellar body preparations could be accounted for by microsomal contamination. When lamellar bodies or liposomal membranes synthesized from the total phospholipid fraction of lamellar bodies were analyzed by the electron paramagnetic resonance probe, 5-dioxyl-methylstearate, they exhibited a high degree of fluidity at physiological temperature. This was in contrast to the low fluidity of liposomal membranes composed of pure dipalmitoylphosphatidylcholine, the major component (50%) of rabbit lamellar body phospholipids. Furthermore, the major temperature-dependent phase transition in lamellar body membranes occurred at a different temperature (30.5 degrees C) from that of dipalmitoyl-phosphatidylcholine (41.0 degrees C). It is clear, therefore, that the membrane fluidity of lamellar bodies must be highly influenced by the minor lipid component.

Phosphatidic acid phosphatase activity in subcellular fractions derived from adult rat type II pneumocytes in primary culture

The development of a method for the subcellular isolation of lamellar body, microsomal, mitochondrial and cytosolic enriched fractions from the adult rat type II pneumocyte is presented. These fractions were used to investigate the distribution and specificity of phosphatidic acid phosphatase (EC 3.1.3.4). The cytosolic enriched fraction possessed the greatest proportion of total phosphatidic acid phosphatase activity and the highest relative specific activity. The small amounts of phosphatase activity in the lamellar body, microsomal and mitochondrial enriched fractions correlated best with a nonspecific phosphatase activity. 1,2-Disaturated phosphatidic acid served as a slightly better substrate than did 1-saturated, 2-unsaturated phosphatidic acid in the cytosolic enriched fraction. This observation indicates that the de novo pathway could contribute to dipalmitoylphosphatidylcholine synthesis in the rat type II pneumocyte.

Role of an acidic compartment in synthesis of disaturated phosphatidylcholine by rat granular pneumocytes

A possible role for an acidic subcellular compartment in biosynthesis of lung surfactant phospholipids was evaluated with granular pneumocytes in primary culture. Incubation with chloroquine (100mum) was used to perturb this compartment. With control cells, incorporation of [9,10-(3)H]palmitic acid into total lipids and into total phosphatidylcholines increased linearly with time up to 4h. Total incorporation into phosphatidylcholine during a 1h incubation was 999+85pmol of [9,10-(3)H]palmitic acid, 458+/-18pmol of [1-(14)C]oleic acid and 252+/-15pmol of [U-(14)C]glucose per mug of phosphatidylcholine phosphorus. The cellular content of either disaturated phosphatidylcholine or total phosphatidylcholines did not change during a 2h incubation with chloroquine. In the presence of chloroquine, the specific radioactivity of [(3)H]palmitic acid in disaturated phosphatidylcholine increased by 40%, and that of disaturated-phosphatidylcholine fatty acids from [U-(14)C]glucose increased by 125%. Incorporation of [1-(14)C]oleic acid into phosphatidylcholine was decreased by chloroquine by 79% and 33% in the presence or absence of palmitic acid respectively. Chloroquine stimulated phospholipase activity in intact cells, and in sonicated cells at pH4.0, but not at pH8.5. The observations indicate that chloroquine stimulates synthesis of disaturated phosphatidylcholine in granular pneumocytes from fatty acids, both exogenous and synthesized de novo, which can be due to stimulation of acidic phospholipase. This stimulation of acidic phospholipase A activity by chloroquine appears to be coupled to the synthesis of disaturated phosphatidylcholine, thereby enhancing remodelling of phosphatidylcholine synthesized de novo. Our findings, therefore, implicate the involvement of an acidic subcellular compartment in the remodelling pathway of disaturated phosphatidylcholine synthesis by granular pneumocytes.

Concerning the coidentity of phosphatidic acid phosphohydrolase and phosphatidylglycerophosphate phosphohydrolase in rat lung lamellar bodies

The properties of the phosphatidylglycerophosphate phosphohydrolase activity in lamellar bodies from rat lung have been compared with the properties of the activities responsible for the degradation of aqueously-dispersed phosphatidic acid (PAaq) and membrane-bound phosphatidic acid (PAmb). Subcellular fractionation studies revealed that the phosphatidylglycerophosphate phosphohydrolase activity and the PAaq-dependent phosphatidic acid phosphohydrolase activity were predominantly associated with the mitochondrial and microsomal fractions, while the PAmb-dependent phosphohydrolase activity was associated with the cytosol. Although the lamellar body fraction contained less than 1% of the total activity, the phosphatidic acid phosphohydrolase activities associated with this fraction could not be explained by contamination with microsomes or cytosol. The three activities exhibited similar heat inactivation profiles at 55 degree C. However, differences in the responses of these activities to the presence of iodoacetate, p-chloromercuriphenyl sulphonate, mercaptoethanol, mercaptoethanol plus MgCl2, and Triton X-100 indicated that the enzymes responsible for these activities may be distinct. Furthermore, addition of up to a ten-fold greater amount of PAaq did not seriously affect the hydrolysis of phosphatidylglycerophosphate. These results indicate that the phosphatidic acid phosphohydrolase and phosphatidylglycerophosphate phosphohydrolase activities in rat lung lamellar bodies are not necessarily catalyzed by the same protein.

Lamellar bodies isolated from adult human lung tissue

Lamellar bodies, the intracellular storage form of pulmonary surfactant, were isolated from adult human lung tissue. As shown by electron microscopy, the isolated human lamellar bodies resembled the lamellar bodies isolated from experimental animals. Chemical analysis revealed that the lamellar bodies consisted largely of lipids, particularly phospholipids (85%). The major phospholipid was phosphatidylcholine, which accounted for 71% of the total phospholipids. Phosphatidylglycerol and phosphatidylethanolamine were 10 and 8%, respectively, of the lamellar body phospholipid. Phosphatidylserine, phosphatidylinositol, lysophosphatidylcholine, and sphingomyelin were minor components. Cholesterol was found to represent 60% of the neutral lipids or 9% of the total lipids. Phosphatidylcholine contained largely saturated fatty acids, of which palmitic acid was the most abundant. Disaturated phosphatidylcholines comprised 67% of the total phosphatidylcholines. Phosphatidylglycerol and phosphatidylethanolamine contained considerably less saturated fatty acids. Only 24% of phosphatidylglycerol was disaturated. The chemical composition of adult human lamellar bodies was very similar to that of lamellar bodies derived from experimental animals. The similarities in morphology and chemical composition of lamellar bodies suggest that surfactant metabolism in human lung may be similar to that of other mammals.

Lung phosphatidate phosphatase: activity during altered physiologic states

Phosphatidic acid phosphatase (PAPase) which catalyzes the conversion of phosphatidic acid to 1,2-diacyl-sn-glycerol was studied in fetal, neonatal, and adult rat lung microsomal fractions from whole lung under normal and altered physiological states. The maximal activity was obtained at pH 7.0 with 1.0 mM phosphatidic acid as the substrate. Twenty-one-day-old fetal rat lung averaged 20.3 +/- 0.6 SE nmol/min/mg microsomal protein compared to 9.9 +/- l.0 nmol/min/mg in liver. Following birth there was a dramatic 53% increase in the PAPase activity. Twenty-one-day-old fetal rat lungs from diabetic mothers (streptozotocin-induced) and from mothers fasted the last four days of gestation did not show altered PAPase activity. Premature breathing for 3-6 hr on day 21 of gestation also did not affect the PAPase activity. These data demonstrate that the microsomal PAPase activity (l) increases dramatically only after birth (2) is not responsive to altered physiologic state (maternal diabetes, maternal fasting, and premature breathing) and (3) may not be an important regulatory enzyme in lung surfactant phospholipid production.

In vivo incorporation of 3H-choline in alveolar type-II pneumocytes in mice treated with butylated hydroxytoluene (BHT)

Butylated hydroxytoluene has been previously shown to damage alveolar type-I cells followed by proliferation of type-II pneumocytes which then redifferentiate into intermediate and newly formed type-I epithelial cells. In relation to this transformation process, in vivo incorporation of H3-choline, a precursor of the main component of lung surfactant, was studied in type-II cells of BHT-treated mice by electron microscopic radioautography. The labeling density was reduced in some type-II cells at 2, 3 and 5 days after a single injection of BHT. Cellular compartment analysis indicated that the transformation of type-II cells is accompanied by a notable loss of their lamellar body component as well as a decreased radioautographic labeling associated with this cell type, possible in relation with modified synthesis of phospholipid material of surfactant.

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.

Pulmonary phosphatidic acid phosphohydrolase. Developmental patterns in rabbit lung

1. The developmental patterns of the phosphatidic acid phosphohydrolase activities in developing rabbit lung were determined using both aqueously dispersed phosphatidic acid (PAaq) and membrane-bound phosphatidic acid (PAmb) as the substrates. 2. The specific activities and the total activities of the PAmb-dependent phosphohydrolase activities in the microsomes and to a lesser extent in the homogenates increased between 26 and 30 days gestation (term 31), but decreased in the adult. The PAaq-dependent activities demonstrated a smaller increase during late gestation and a decrease in the adult. 3. There was little change in either the Paaq- or the Pamb-dependent activities in the cytosol between 25 and 30 days gestation. The total activities per g lung were increased in the adult. 4. Fractionation of adult cytosol on Bio-Gel A5m revealed PAaq-dependent activities in the void volume (Vo) (50% total), a peak with an apparent molecular mass (Mr) = 150 kdaltons (25% total) and a peak with Mr = 110 kdaltons (25% total). The PAaq-dependent peak with Mr = 150 kdaltons was not detected in the fetal cytosols. 5. Gel filtration revealed PAmb-dependent activity in the Vo (15% total), a major peak with an apparent Mr = 390 kdaltons (44% total) and minor peaks with Mr = 240 kdaltons (16% total) and Mr = 110 kdaltons (24% total). Little change was observed during development. 6. Thermal denaturation studies on he PAmb-dependent activities in the cytosols produced biphasic curves with a rapidly inactivated component and a relatively heat-stable component. The thermal denaturation profiles for the PAmb-dependent activities remained relatively unaltered throughout fetal development. The thermal denaturation profiles of the PAaq-dependent activities in the fetal cytosols were also biphasic. In contrast, the inactivation profiles of the PAaq-dependent activities in adult cytosol were monophasic.

Specific transfer of dipalmitoyl phosphatidylcholine in rabbit lung

Liposomes were used as a tool to probe the specific transfer of dipalmitoyl phosphatidylcholine in rabbit lung. After incubation of liposomes with lung slices, lamellar bodies took up 30-50% more radioactively labeled dipalmitoyl phosphatidylcholine from egg yolk phosphatidylcholine liposomes than the labeled dioleoyl phosphatidylcholine, whereas mitochondria and microsomes took up both species at relatively equal rates. Furthermore, when 50% egg yolk phosphatidylcholine was replaced by dipalmitoyl phosphatidylcholine, the uptake of [1-14C]dipalmitoyl phosphatidylcholine by lamellar bodies was greatly reduced due to the isotope-dilution effect of the unlabeled dipalmitoyl species. This competitive uptake was not observed in mitochondria and microsomes. Similar results were also observed when liposomes were incubated with the cell-free homogenate. On the other hand, distearoyl or dioleoyl phosphatidylcholine did not compete with the uptake of [1-14C]dipalmitoyl phosphatidylcholine. These results suggest an intracellular specific transfer of dipalmitoyl phosphatidylcholine from liposomes to lamellar bodies, presumably by a specific transfer protein. When liposomes were labeled with [1-14C]dioleoyl phosphatidylcholine, the addition of unlabeled dioleoyl phosphatidylcholine up to 50% of total phosphatidylcholine concentration did not compete with the uptake of [1-14C]dioleoyl phosphatidylcholine in all fractions including lamellar bodies. This indicates that the intracellular uptake of liposomal dioleoyl phosphatidylcholine by lamellar bodies and other subcellular organelles was probably dominated by the fusion mechanism.

Distribution of the phosphatidate phosphohydrolase activity in the lamellar body and lysosomal fractions of lung tissue

Phosphatidate phosphohydrolase (PAPase) activity was measured in lamellar bodies purified from porcine lung tissue. After repeated freeze-thawing, only a negligible amount of PAPase activity was released into the soluble fractions, whereas there was release of 2 lysosomal marker enzyme activities, glucosaminidase and beta-galactosidase into the soluble fraction. In addition, a lysosomal-enriched fraction was prepared from adult rat lung tissue by prior treatment of the rats with Triton WR 1339. Treatment with Triton WR 1339 resulted in the significant shift of the activities of the lysosomal marker enzymes, glucosaminidase and beta-galactosidase, to less dense subcellular fractions. The highest specific activity of PAPase was found in a subcellular fraction which had a density that was intermediate between that of the mitochondrial and microsomal fractions and the distribution of PAPase activity was not affected by the prior treatment of the rats with Triton WR 1339.

Properties of an acid phosphatase in pulmonary surfactant

Lung surfactant, a lipid-protein complex purified from dog lungs, contains a highly active phosphomonoesterase associated with it. This phosphatase is quite specific for the hydrolysis of phosphatidic acid and 1-acyl-2-lysophosphatidic acid. The enzyme possesses many of the characteristics of the microsomal enzyme, phosphatidate phosphohydrolase (EC 3.1.3.4). In addition, we have shown that this enzyme will also convert phosphatidylglycerol phosphate [1-(3-sn-phosphatidyl)-sn-glycerol-1-P] to phosphatidylglycerol [1-(3-sn-phosphatidyl)-sn-glycerol] and Pi. The phosphatidylglycerol phosphate was made available to the surfactant enzyme in a coupled assay by hydrolysis of cardiolipin [1-(3-sn-phosphatidyl)-3-(3-sn-phosphatidyl)-sn-glycerol] by stereospecific cleavage with phospholipase C (phosphatidylcholine cholinephosphohydrolase, EC 3.1.4.3) from Bacillus cereus. This enzyme has been previously shown to generate the naturally occurring isomer of phosphatidylglycerol phosphate because it has specificity for the 3-(3-sn-phosphatidyl) group of cardiolipin. Other properties of the surfactant enzyme are discussed in relation to its presence in lung surface active material.

Pulmonary phosphatidic acid phosphatase. Properties of membrane-bound phosphatidate-dependent phosphatidic acid phosphatase in rat lung

1. The membrane-bound phosphatidate-dependent phosphatidic acid phosphatase activity of rat lung has been investigated in cytosol and microsomal fractions using as a substrate [32P]phosphatidate bound to heat inactivated rat liver microsomes. Both activities demonstrated broad pH optima with a maximum of 7.4--8 for the cytosol and a maximum of 6.5--7.5 with microsomal preparations. 2. At low concentrations (0--5 mM) Mg2+ produced a slight stimulation of the cytosol activity but at higher concentrations an inhibition was observed. Low concentrations (1.0--2.0 mM) of EDTA abolished the cytosol activity and reduced the microsomal activity to half. In both cases, the addition of Mg2+ in the presence of EDTA resulted in an activity which was more than 2-fold greater than that observed in the absence of chelator or divalent cation. 3. The cytosol activity was relatively resistant to the addition of ionic and nonionic detergents. In general, the addition of a number of phosphate esters increased rather than decreased the release of 32Pi, indicating a relative specificity for phosphate groups associated with a hydrophobic environment. The addition of aqueous dispersions of phosphatidate, lysophosphatidic acid or phosphatidylglycerophosphate markedly reduced the hydrolysis of membrane-bound [32P]phosphatidate. The cytosol activity was slightly inhibited by the addition of phosphatidylcholine. 4. In an attempt to estimate the relative contributions of the cytosol and microsomal activities in vivo, these activities were assayed using [32P]phosphatidate endogenously generated on rat lung microsomes. With the 32P-labelled microsomes, the hydrolysis remained linear over the 45 min of the experiment. Addition of high speed supernatant produced a rapid release of 32Pi during the first 10 min followed by a more gradual release similar to that oberved with the microsomes alone. The cytosol activity remained greater than the microsomal activity at all times studied. 5. When [14C]phosphatidate-labelled microsomes were incubated in the presence of nonradioactive CDPcholine, the addition of cytosol markedly stimulated the incorporation of radioactivity into phosphatidylcholine. This observation suggests that the phosphatidic acid phosphatase activity associated with the cytosol has a role in phosphatidylcholine (and presumably surfactant) biosynthesis in rat lung.

Pulmonary phosphatidic acid phosphatase. A comparative study of the aqueously dispersed phosphatidate-dependent and membrane-bound phosphatidate-dependent phosphatidic acid phosphatase activities of rat lung

1. The properties of the aqueously dispersed phosphatidate-dependent phosphatidic acid phosphatase (EC 3.1.3.4) activities of rat lung have been studied in microsomal and cytosol preparations and compared with the properties of the membrane-bound phosphatidate-dependent activities. 2. The microsomal phosphatidic acid phosphatase displayed a prominent pH optimum at 6.5 with a minor peak which varied between 7.5--8 in different experiments. With the cytosol, the major activity was at the higher pH (7.5--8.0) but a distinct optimum was also observed at pH 6.0--6.5. With the membrane-bound substrate, a single broad optimum was observed between pH 7.4 and 8.0 with the cytosol and 6.5--7.5 with the microsomal fraction. 3. Subcellular fractionation studies revealed that the microsomal fraction possessed the greatest proportion of the total phosphatidic acid phosphatase activity and the highest relative specific activity. However, studies with marker enzymes indicated that the aqueously dispersed phosphatidate-dependent activity could be present in plasma membrane, lysosomes and osmiophilic lamellar bodies as well as in the endoplasmic reticulum. 4. The aqueously dispersed phosphatidic acid-dependent activities present in the microsomal and supernatant fractions were inhibited by Ca2+, Mn2+, F- and by high concentrations of Mg2+. In contrast to the membrane-bound phosphatidate-dependent activities, there was little Mg2+ stimulation and only a very slight inhibitory effect was noted with EDTA. A small EDTA-dependent Mg2+ stimulation could be observed with the microsomal fraction but only at the lower pH optimum (6.5). 5. The presence of a number of phosphate esters tended to stimulate rather than inhibit the microsomal activity, indicating that the hydrolase is relatively specific for lipid substrates. Marked inhibitions were noted with lysophosphatidic acid and phosphatidylglycerol phosphate. Phosphatidylcholine produced a slight inhibition. 6. The results indicate that the bulk of the aqueously dispersed phosphatidate-dependent phosphatidic acid phosphatase activities of rat lung microsomes and cytosol is not related to the activities observed with membrane-bound phosphatidate. The Mg2+-dependent hydrolase activities may be synonymous. However, unequivocal conclusions will only be possible when the polypeptide or polypeptides responsible for these activities can be purified.

Phospholipid exchange between subcellular organelles of rabbit lung

A soluble protein fraction (PLEP) prepared from rabbit lung can catalyze the exchange of phospholipids between subcellular organelles of the lung and between these subcellular organelles and synthetic liposomes. Phospholipid exchange between microsomes and synthetic liposomes and between mitochondria and synthetic liposomes was stimulated 8-fold and 2.5-fold, respectively, in the presence of the protein fraction. Lung exchange protein could also catalyzed phospholipid exchange between subcellular organelles of the liver and synthetic liposomes. Phospholipid transfer between microsomes and lamellar bodies of the lung was stimulated 2-fold by the exchange protein. Both radiolabeled phosphatidylcholine (PC) and phosphatidylinositol (PI) were transferred from 32P-labeled microsomes to lamellar bodies, but the exchange protein exhibited no transfer activity for phosphatidylglycerol (PG) and that for phosphatidylethanolamine (PE) was insignificant compared to the transfer activity for phosphatidylcholine and phosphatidylinositol. While the physiological role of the phospholipid exchange proteins in the lung is unknown, it is possible that they participate in the distribution of the newly synthesized phospholipids from the site of synthesis to lamellar bodies and other membrane compartments of cells.

Characterization of phosphatidate phosphohydrolase activity associated with isolated lamellar bodies

It has been demonstrated that lamellar bodies isolated from porcine lung have L-alpha-phosphatidate phosphohydrolase (EC 3.1.3.4) activity which cannot be accounted for by microsomal or mitochondrial contamination. Phosphatidate phosphohydrolase activity associated with the lamellar bodies is relatively insensitive to Mg2+ and more heat labile than the activity associated with whole lung microsomes. The enzyme was found to be the most active phosphohydrolase present in isolated lamellar bodies and is inhibited by 5 mM Be2+. Lamellar bodies isolated from human and rat lung tissue were also found to have this activity, and the functional role of the enzyme in lamellar bodies is proposed in relation to glycerophospholipid metabolism.

Catalase positive particles from pig lung. Biochemical preparations and morphological studies

In pig lung tissue catalase positive particles (CPs) are abundant especially in type II pneumocytes and in Clara cells. In both cell types they occur as circular, oval or elongated membrane profiles surrounding a moderately electron dense matrix lacking a crystalline core. In Clara cells and in part of type II pneumocytes they are located as individual particles without any evident morphological relation to other cell organelles. In part of type II pneumocytes 5-8 particles are forming a group and their close relation to agranular endoplasmic reticulum cisterns is evident. The particles can be purified from lung homogenates by fractionated pelleting and subsequent rate sedimentation in a sucrose gradient using a zonal rotor. The catalase rich fraction bands in the middle of the gradient whereas cytochrome oxidase and part of the acid phosphatase sediments at its heavy end. A second part of acid phosphatase stays at the light end of the gradient and--according to morphological control--seems to correspond to lamellar bodies of the type II pneumocytes. The purified catalase positive particles do not contain hydroxyacid and D-aminoacid oxidases thought to be characteristic H2O2 producing enzymes of peroxisomal systems. The buoyant density of the particles (d = 1.195 g/cm3) is lower than that of liver peroxisomes. Cytochemical controls of the peroxisomal pellets exhibit the particles partly uniformly filled with reaction product, partly irregularly stained.