Developmental changes in enzyme activities in fetal and neonatal rabbit lung. Cytidylyltransferase, cholinephosphotransferase, phospholipases A1 and A2, beta-galactosidase, and beta-glucuronidase

Lysosomal enzymes in preterm infants with bronchopulmonary dysplasia: a potential diagnostic marker

Some lysosomal glycohydrolases (N-acetyl-beta-D-glucosaminidase and their major isoenzymes, beta-D-glucuronidase, alpha-D-galactosidase, beta-D-galactosidase and alpha-D-glucosidase) were investigated in the plasma of 36 preterm infants with respiratory distress, 11 of whom developed bronchopulmonary dysplasia (BPD), in order to evaluate the role of the lysosomal apparatus in the disease. Enzyme activity was assayed fluorimetrically; the major N-acetyl-beta-D-glucosaminidase (NAG) isoenzymes were separated using a routine chromatofocusing procedure; the diagnostic efficiency was evaluated by Bayes theorem. The mean levels of almost all glycohydrolases considered were significantly higher in BPD than in non-BPD infants. Among NAG major isoenzymes, an increase was found only in form A. No variation was evident in the plasma levels of glycohydrolases during dexamethasone therapy. Data from a retrospective analysis performed in all preterms considered, show that alpha-D-galactosidase and beta-D-galactosidase differentiate a posteriori BPD and non-BPD subjects. These enzymes, after a priori verification of their diagnostic potential in preterm infants at risk of BPD development, could acquire an important predictive value.

Maturational changes in rabbit renal cortical phospholipase A2 activity

Several studies have demonstrated that the neonatal kidney has a markedly attenuated response to parathyroid hormone (PTH); however, the cause for this blunted response is unknown. PTH stimulated cAMP production by 215 +/- 18% in neonatal proximal tubule suspensions compared to a 35 +/- 7% increase in adult proximal tubules. Thus, neonatal proximal tubules have functioning PTH receptors and a greater adenylate cyclase response than the adult segment. In adult proximal tubules, PTH stimulates phospholipase A2 (PLA2) activity and the inhibition of Na,K-ATPase activity by PTH is blocked by inhibitors of PLA2. We examined whether maturational changes in renal cortical activity could play a role in the attenuated response to PTH in the neonatal proximal tubule. Compared to adults, neonates had a lower renal cortical cytosolic PLA2 (cPLA2) activity, assessed as the release of 14C-arachidonic acid (AA) from labeled phosphatidyl choline (0.44 +/- 0.10 vs. 0.74 +/- 0.06% 14C-AA released/min/mg protein, P < 0.05) and microsomal PLA2 activity (0.32 +/- 0.03 vs. 1.20 +/- 0.13% 14C-AA released/min/mg protein, P < 0.001). The protein abundance of cPLA2 was not different between the neonatal and adult renal cortex as assessed by immunoblot assay. Thus, the difference in activities must be due to a difference in regulation of cPLA2. Annexin 1 (lipocortin 1) has been shown to inhibit PLA2 activity by binding to phospholipid substrate. Annexin 1 protein abundance was higher in neonatal than in adult renal cortex (P < 0.001). Thus, the lower activity of PLA2 in the neonatal tubules may be due in part to higher expression of annexin 1. PLA2 activation by PTH, -8-bromo-cAMP and PMA was assessed as 3H-AA release from prelabeled suspensions of neonatal and adult proximal tubules. PTH (10(-7) M), 8-bromo-cAMP (10(-4) M) and PMA (5 x 10(-8) M) significantly increased 3H-AA release from adult tubules (P < 0.05) but had no effect on neonatal tubules (P = NS). Thus, PTH, 8-bromo-cAMP and PMA stimulated PLA2 in adult but not neonatal proximal tubules. In conclusion, the maturational changes in renal cortical PLA2 activity may be a factor in the blunted response of neonatal proximal tubules to PTH.

Gestational age dependence of 11 beta-hydroxysteroid dehydrogenase and its relationship to the enzymes of phosphatidylcholine synthesis in lung and liver of fetal rat

Increase in fetal surfactant synthesis and lung maturity is caused by the glucocorticoidal induction of enzymes required for phosphatidylcholine (PC) synthesis towards the end of gestation. The regulation of gestational age-dependent induction of PC synthesis by glucocorticoids is still unclear. Since 11-beta-hydroxysteroid dehydrogenase (11 beta-HSD) activity and its metabolising capacity for glucocorticoids have been suggested to play a central role in this regulation, we measured the gestational age-dependent changes in 11 beta-HSD and PC synthesizing enzymes in lung and liver of fetal rat. The activity of cholinephosphate cytidyltransferase (CCT; key enzyme in PC synthesis), choline phosphotransferase (CPT) and lysolecithin acyltransferase (LAT) were found to increase gradually in the lung towards the end of gestation, reached peak values at term followed by a decrease of activity reaching finally adult levels. Only CK activity exhibited constant levels until term followed by a slight increase after the birth. In comparison with the lung, the liver enzymes followed a similar pattern, but at a higher rate of activity except for CCT which was higher in the lung. The activity of 11 beta-HSD in fetal lung microsomes was detectable from day 20 and increased towards the end of gestation in the lung and liver of the rat. Oxidase activity was always found to exceed the reductase activity. The activity of 11 beta-HSD continued to increase after delivery and reached peak levels in adult animals in both organs. In order to test the hypothesis, whether 11 beta-HSD activity and PC synthesis are induced by increasing endogenous glucocorticoidal levels, we examined on day 19 of gestation the effect of dexamethasone (DEXA) on enzymatic activities (11 beta-HSD, CCT) and on [14C]choline incorporation in phosphatidylcholine in fetal lung organoid cultures. Additionally, changes in CCT activity in fetal lungs after maternal administration of DEXA were measured. DEXA accelerated 11 beta-HSD and CCT activities as well as [14C]choline incorporation. We conclude, that endogenous glucocorticoids induce PC synthesis as well as 11 beta-HSD activity in lung and liver of the fetal rat. Fetal PC synthesis is not altered by increasing 11 beta-HSD levels, because the increase of free serum corticosterone levels apparently exceeds the metabolising capacity of 11 beta-HSD towards term.

Metabolism of endogenous arachidonic acid by isolated lungs of neonatal and adult rabbits stimulated with calcium ionophore: effect of hypoxia

We have determined the effect of hypoxia on arachidonic acid metabolism by rabbit lungs stimulated with calcium ionophore A23187. Isolated lungs of neonatal and adult rabbits were perfused during normoxia (pO2 greater than 100 torr) or hypoxia (pO2 less than 40 torr) and arachidonic acid metabolism stimulated by the addition of A23187 (5 microM) to the perfusate. Cyclooxygenase metabolites PGE2, TxA2, and PGI2 were measured by radioimmunoassay and lipoxygenase metabolites LTB4, LTC4, LTD4, and LTE4 by HPLC. During normoxia, neonatal lungs synthesized the three cyclooxygenase metabolites that we measured in similar amounts. LTC4 constituted 56% of the leukotrienes produced. Hypoxia caused a 100% increase in the amount of PGI2 synthesized by neonatal lungs; however, total TxB2 and PGE2 production was not altered significantly. LTC4 production decreased significantly during hypoxia, whereas LTE4 production increased. Adult lungs synthesized significantly lower amounts of both cyclooxygenase and 5-lipoxygenase products than neonatal lungs. During normoxia, PGE2 was measured in highest amount in adult lungs. Similar to the neonatal lungs, LTC4 was the predominant leukotriene (56%) measured. During hypoxia, there was a 100% increase in PGI2 production by adult lungs, as was observed in neonatal lungs. There was also a small but significant increase in PGE2 production, with no change in TxA2 production. LTC4 production also decreased, but there was a marked increase in synthesis of LTB4 by adult lungs. Our data demonstrate that hypoxia alters the profile of arachidonic acid metabolites produced by rabbit lungs stimulated with A23187. Also, the age of the rabbit significantly affects both the amount and profile of metabolites synthesized by stimulated lungs during normoxia and hypoxia.

Synthesis of platelet activating factor by cholinephosphotransferase in developing fetal rabbit lung

Developing fetal lung is a possible source of the platelet activating factor (PAF, 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) present in amniotic fluid of women in labor. We have assayed the microsomal activities of a specific enzyme for the de novo synthesis of PAF in developing fetal and neonatal rabbit lung, 1-alkyl-2-acetyl-glycerol-dependent dithiothreitol-insensitive cholinephosphotransferase. The specific activity of this enzyme increased from 0.92 to 3.60 nmol X min-1 X mg-1 protein between day 21 and day 31 of gestation. In constrast, during this same period the activity of the PAF-biosynthetic cholinephosphotransferase in developing rabbit kidney did not change significantly. The specific activity of the diacyl-glycerol-dependent, dithiothreitol-sensitive cholinephosphotransferase that catalyzes the final step in phosphatidylcholine biosynthesis was not altered during late gestation in either fetal lung or kidney. Previously, increased amounts of pulmonary PAF had been found during the latter stages of gestation (Hoffman, Truong and Johnston (1986) Biochim. Biophys. Acta 879, 88-96) and may be attributed to increased activity of the PAF biosynthetic enzymes found in this investigation. This elevated level of PAF in fetal lung may serve to facilitate breakdown of glycogen that provides, in part, the carbon and energy source for surfactant biosynthesis. In addition, PAF may be secreted in association with surfactant into amniotic fluid in which it may interact with amnion tissue and subsequently participate in the events associated with the initiation of parturition.

Characterization of phospholipase A2 from rabbit lung microsomes

A phospholipase A2 activity associated with the microsomal fraction of rabbit lung homogenates was studied. The enzyme showed specificity for the sn-2 ester bond of phosphatidylcholine, had an alkaline pH optimum and required Ca2+ for activity. Other divalent cations were unable to support hydrolysis. In the absence of detergents, exogenous phosphatidylethanolamine was deacylated at a rate sevenfold higher than phosphatidylcholine. The activity toward both substrates could be enhanced by sodium deoxycholate or, more effectively, by sodium taurodeoxycholate. Phosphatidylethanolamine required higher detergent/phospholipid molar ratios than phosphatidylcholine. Under these conditions, the preference for the former substrate over the latter was nearly abolished. The zwitterionic detergent 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate (CHAPS) and the nonionic detergent Triton X-100 were either ineffective (phosphatidylcholine) or inhibitory (phosphatidylethanolamine). Addition of KCl produced opposite effects on the activity depending on the bile salt used to disperse the substrate. The phospholipase A2 activity was inhibited by p-bromophenacyl bromide but remained unaffected after treatment with diisopropylfluorophosphate or NaF. N-Ethylmaleimide, but not other thiol reagents, partially inhibited the activity.

Profile of phospholipase A2 activity in subcellular fractions and lamellar bodies of developing, neonatal and adult rabbit lung. Correlation with intracellular levels of disaturated phosphatidylcholine

Phospholipase A2 activity was determined in subcellular fractions and lamellar bodies of fetal, neonatal and adult rabbit lungs. Specific activity in most fractions decreased from the 24th to the 28th day of gestation. All fractions except the mitochondrial and the nuclear fractions exhibited a sharp increase in activity in the newborn lung. Specific activity in the adult lung generally declined in comparison to neonatal values. During gestation total enzyme activity per gram of lung was concentrated in the cytosolic fraction. With the exception of the lamellar body fraction, the total content of phospholipase A2 activity increased dramatically in all fractions from the neonatal lung. The lamellar body fractions displayed both low specific activity and low total enzyme activity during gestation. Specific activity increased dramatically in the neonatal and adult lung but still accounted for only a small fraction of the activity in comparison to the other subcellular fractions. The subcellular content of disaturated phosphatidylcholine (PC) appeared to correlate well with the activity of phospholipase A2 in the neonatal mitochondrial, microsomal and cytosolic fractions. Since decreasing prenatal enzyme levels are associated with increasing disaturated PC content, the alkaline and calcium-dependent phospholipase A2 may not be directly involved in disaturated PC synthesis in the fetus. However, postnatally, the correlation between the pattern of production of disaturated PC and the activity of the phospholipase A2 indicates a role for this enzyme in surfactant-related disaturated PC synthesis.

Endogenous activity of phospholipases A and C in Ureaplasma urealyticum

The results of recent studies support the concept that Ureaplasma urealyticum may be a major cause of perinatal infection in both term and preterm infants. It has been postulated that phospholipase degradation of placental phospholipids by microorganisms triggers the onset of premature labor. Since the presence of ureaplasmas in placentas is associated with pregnancy loss, prematurity, and neonatal morbidity, we assayed U. urealyticum for the presence of phospholipase A1, A2, and C activities. Phospholipase A1 activity was low in lysates of exponential-phase cells of U. urealyticum. Phospholipase A2 activity was present and was 100-fold higher than the activity of phospholipase A1 in serotypes 3,4, and 8. The total activity and specific activity of phospholipase A2 in serotype 8 were nearly threefold higher than the activities in serotypes 3 and 4. Cell lysates of all three serotypes showed the presence of phospholipase C activity during the exponential phase of growth, and no significant difference in activity was observed among the three serotypes. In stationary-phase cells the phospholipase C activity was 10-fold lower than the activity in exponential-phase cells. Our results demonstrate that phospholipase activities are present in U. urealyticum cells and that the specific activities of phospholipase A2 differed among the three serotypes tested, while the activities of phospholipases A1 and C were similar.

Regulation of phospholipid synthesis by intracellular phospholipases in fetal rabbit type II pneumocytes

Exposure of fetal type II pneumocytes to phospholipase A2 inhibitors led to significantly reduced choline uptake and decreased synthesis of total and disaturated phosphatidylcholines from both [methyl-14C]choline and [9,10(n)-3H]palmitate precursors. The percentage of the total synthesized phosphatidylcholine recovered as disaturated phosphatidylcholine was increased when compared to that in control cultures, suggesting that unsaturated phosphatidylcholine synthesis was reduced to a greater extent than that of the disaturated species. Synthesis of sphingomyelin and phosphatidylethanolamine from labeled palmitate was also reduced, whereas that of phosphatidylinositol and phosphatidylglycerol was significantly increased. Addition of phospholipase C resulted in increased synthesis of phosphatidylcholine from both labeled precursors; no significant changes were found in synthesis of most of the other 3H-labeled lipids. Added phospholipase A2 did not lead to any changes in either choline or palmitate incorporation. However, when melittin (a phospholipase A2 activator) was added to the cultures, greater incorporation of both palmitate and choline was observed, along with a significant increase in the percentage of total cellular radioactivity in 14C-labeled lipids, indicating also stimulation of phosphatidylcholine synthesis. A marked increase in CTP: phosphorylcholine cytidylyltransferase activity was found after treatment of the cultures with phospholipase C. Exposure to quinacrine also increased the activity of this enzyme. Addition of phospholipase C and melittin to prelabeled pneumocyte cultures accelerated degradation of cell phospholipids and the release of free fatty acids as the main degradation products. These findings suggest that intracellular phospholipases are regulators of synthesis of surfactant phospholipids in fetal type II pneumocytes, and that activation or inhibition of these phospholipases could represent a mechanism through which hormones and pharmacological agents modify surfactant and other phospholipid synthesis.