Subcellular distribution of N-ethylmaleimide-sensitive and -insensitive phosphatidic acid phosphohydrolase in rat brain

The Metabolism of 2-arachidonoylglycerol in Rod Outer Segments Is Modulated by Proteins Involved in the Phototransduction Process

We previously reported that 2-arachidonoylglycerol (2-AG) synthesis by diacylglycerol lipase (DAGL) and lysophosphatidate phosphohydrolase (LPAP) and hydrolysis by monoacylglycerol lipase (MAGL) in rod outer segments (ROS) from bovine retina were differently modified by light applied to the retina. Based on these findings, the aim of the present research was to evaluate whether 2-AG metabolism could be modulated by proteins involved in the visual process. To this end, ROS kept in darkness (DROS) or obtained in darkness and then subjected to light (BROS) were treated with GTPγS and GDPβS, or with low and moderate ionic strength buffers for detaching soluble and peripheral proteins, or soluble proteins, respectively. Only DAGL activity was stimulated by the application of light to the ROS. GTPγS-stimulated DAGL activity in DROS reached similar values to that observed in BROS. The studies using different ionic strength show that (1) the highest decrease in DROS DAGL activity was observed when both phosphodiesterase (PDE) and transducin α (Tα) are totally membrane-associated; (2) the decrease in BROS DAGL activity does not depend on PDE association to membrane, and that (3) MAGL activity decreases, both in DROS and BROS, when PDE is not associated to the membrane. Our results indicate that the bioavailability of 2-AG under light conditions is favored by G protein-stimulated increase in DAGL activity and hindered principally by Tα/PDE association with the ROS membrane, which decreases DAGL activity.

The Endocannabinoid System Is Present in Rod Outer Segments from Retina and Is Modulated by Light

The aim of the present research was to evaluate if the endocannabinoid system (enzymes and receptors) could be modulated by light in rod outer segment (ROS) from bovine retina. First, we analyzed endocannabinoid 2-arachidonoylglycerol (2-AG) metabolism in purified ROS obtained from dark-adapted (DROS) or light-adapted (LROS) retinas. To this end, diacylglycerol lipase (DAGL), monoacylglycerol lipase (MAGL), and lysophosphatidate phosphohydrolase (LPAP) enzymatic activities were analyzed using radioactive substrates. The protein content of these enzymes and of the receptors to which cannabinoids bind was determined by immunoblotting under light stimulus. Our results indicate that whereas DAGL and MAGL activities were stimulated in retinas exposed to light, no changes were observed in LPAP activity. Interestingly, the protein content of the main enzymes involved in 2-AG metabolism, phospholipase C β₁ (PLCβ₁), and DAGLα (synthesis), and MAGL (hydrolysis), was also modified by light. PLCβ₁ content was increased, while that of lipases was decreased. On the other hand, light produced an increase in the cannabinoid receptors CB1 and CB2 and a decrease in GPR55 protein levels. Taken together, our results indicate that the endocannabinoid system (enzymes and receptors) depends on the illumination state of the retina, suggesting that proteins related to phototransduction phenomena could be involved in the effects observed.

Cannabinoid receptor-dependent metabolism of 2-arachidonoylglycerol during aging

2-Arachidonoylglycerol (2-AG) is one of the principal endocannabinoids involved in the protection against neurodegenerative processes. Cannabinoids primarily interact with the seven-segment transmembrane cannabinoid receptor 1 (CB1) and cannabinoid receptor 2 (CB2), both of which are expressed in the central nervous system (CNS). The level of 2-AG is controlled through key enzymes responsible for its synthesis or degradation. We have previously observed a deregulation of 2-AG metabolism in physiological aging. The aim of this study was to analyze how 2-AG metabolism is modulated by CB1/CB2 receptors during aging. To this end, both CB1 and CB2 receptor expression and the enzymatic activities (diacylglycerol lipase (DAGL), lysophosphatidate phosphohydrolase (LPAase) and monoacylglycerol lipase (MAGL)) involved in 2-AG metabolism were analyzed in the presence of cannabinoid receptor (CBR) agonists (WIN and JWH) and/or antagonists (SR1 and SR2) in synaptosomes from adult and aged rat cerebral cortex (CC). Our results demonstrate that: (a) aging decreases the expression of both CBRs; (b) LPAase inhibition, due to the individual action of SR1 or SR2, is reverted in the presence of both antagonists together; (c) LPAase activity is regulated mainly by the CB1 receptor in adult and in aged synaptosomes while the CB2 receptor acquires importance when CB1 is blocked; (d) modulation via CBRs of DAGL and MAGL by both antagonists occurs only in aged synaptosomes, stimulating DAGL and inhibiting MAGL activities; (e) only DAGL stimulation is reverted by WIN. Taken together, the results of the present study show that CB1 and/or CB2 receptor antagonists trigger a significant modulation of 2-AG metabolism, underlining their relevance as therapeutic strategy for controlling endocannabinoid levels in physiological aging.

Aging modifies the enzymatic activities involved in 2-arachidonoylglycerol metabolism

One of the principal monoacylglycerol (MAG) species in animal tissues is 2-arachidonoylglycerol (2-AG), and the diacylglycerol lipase (DAGL) pathway is the most important 2-AG biosynthetic pathway proposed to date. Lysophosphatidate phosphatase (LPAase) activity is part of another 2-AG-forming pathway in which monoacylglycerol lipase (MAGL) is the major degrading enzyme. The purpose of this study was to analyze the manner in which DAGL, LPAase, and MAGL enzymes are modified in the central nervous system (CNS) during aging. To this end, diacylglycerols (DAGs) and MAGs of different composition were used as substrates of DAGL and MAGL, respectively. All enzymatic activities were evaluated in membrane and soluble fractions as well as in synaptic terminals from the cerebral cortex (CC) of adult and aged rats. Results related to 2-AG metabolism show that aging: (a) decreases DAGL-α expression in the membrane fraction whereas in synaptosomes it increases DAGL-β and decreases MAGL expression; (b) decreases LPAase activity in both membrane and soluble fractions; (c) decreases DAGL and stimulates LPAase activities in CC synaptic terminals; (d) stimulates membrane-associated MAGL-coupled DAGL activity; and (e) stimulates MAGL activity in CC synaptosomes. Our results also reveal that during aging the net balance between the enzymatic activities involved in 2-AG synthesis and breakdown is low availability of 2-AG in CC membrane fractions and synaptic terminals. Taken together, our results lead us to conclude that these enzymes play crucial roles in the regulation of 2-AG tissue levels during aging.

Metabolic pathways for the degradation of phosphatidic acid in isolated nuclei from cerebellar cells

The aim of the present research was to analyse the pathways for phosphatidic acid metabolism in purified nuclei from cerebellar cells. Lipid phosphate phosphatase and diacylglyceride lipase activities were detected in nuclei from cerebellar cells. It was observed that DAGL activity makes up 50% of LPP activity and that PtdOH can also be metabolised to lysophosphatidic acid. With a nuclear protein content of approximately 40 μg, the production of diacylglycerol and monoacylglycerol was linear for 30 min and 5 min, respectively, whereas it increased with PtdOH concentrations of up to 250 μM. LysoPtdOH, sphingosine 1-phosphate and ceramide 1-phosphate, which are alternative substrates for LPP, significantly reduced DAG production from PA. DAG and MAG production increased in the presence of Triton X-100 (1 mM) whereas no modifications were observed in the presence of ionic detergent sodium deoxycholate. Ca²+ and Mg²+ stimulated MAG production without affecting DAG formation whereas fluoride and vanadate inhibited the generation of both products. Specific PtdOH-phospholipase A1 and PtdOH-phospholipase A2 were also detected in nuclei. Our findings constitute the first reported evidence of active PtdOH metabolism involving LPP, DAGL and PtdOH-selective PLA activities in purified nuclei prepared from cerebellar cells.

Phospholipase D and phosphatidate phosphohydrolase activities in rat cerebellum during aging

Aging is a process that affects different organs, of which the brain is particularly susceptible. PA and DAG are central intermediates in the phosphoglyceride as well as in the neutral lipid biosynthetic pathway, and they have also been implicated in signal transduction. Phospholipase D (PLD) and phosphatidate phosphohydrolase (PAP) are the enzymes that generate PA and DAG. The latter can be transformed into MAG by diacylglycerol lipase (DGL). In the present study, we examine how aging modulates the PLD, PAP, and DGL isoforms in cerebellar subcellular fractions from 4- (adult), 28-, and 33-mon-old (aged) rats. PI-4,5-bisphosphonate (PIP2)-dependent PLD, PAP1, and DGL1 were distributed in different percentages in all cerebellum subcellular fractions. On the other hand, PAP2 and DGL2 activities were observed in all subcellular fractions except in the cytosolic fraction. Aging modified the enzyme distribution pattern. In addition, aging decreased nuclear (45%), mitochondrial-synaptosomal (55%), and cytosolic (71%) PAP1 activity and increased (28%) microsomal PAP1 activity. DGL1 activity was decreased in nuclear (85%) and mitochondrial-synaptosomal (63%) fractions by aging. On the other hand, PIP2-dependent PLD activities were increased in the mitochondrial-synaptosomal fraction. PAP2 and DGL2 were increased in the microsomal fraction by 87 and 114%, respectively, and they were decreased in the nuclear fraction. The changes observed in cerebellum PAP1 and DGL1 activities from aged rats with respect to adult rats could be related to modifications in lipid metabolism. Differential PA metabolization during aging through PIP2-dependent PLD/PAP2/DGL2 activities could be related to alterations in the neural signal transduction mechanisms.

Age-associated changes in central nervous system glycerolipid composition and metabolism

In this review, changes in brain lipid composition and metabolism due to aging are outlined. The most striking changes in cerebral cortex and cerebellum lipid composition involve an increase in acidic phospholipid synthesis. The most important changes with respect to fatty acyl composition involve a decreased content in polyunsaturated fatty acids (20:4n-6, 22:4n-6, 22:6n-3) and an increased content in monounsaturated fatty acids (18:1n-9 and 20:1n-9), mainly in ethanolamine and serineglycerophospholipids. Changes in the activity of the enzymes modifying the phospholipid headgroup occur during aging. Serine incorporation into phosphatidylserine through base-exchange reactions and phosphatidylcholine synthesis through phosphatidylethanolamine methylation increases in the aged brain. Phosphatidate phosphohydrolase and phospholipase D activities are also altered in the aged brain thus producing changes in the lipid second messengers diacylglycerol and phosphatidic acid.

Differential modulation of phospholipase D and phosphatidate phosphohydrolase during aging in rat cerebral cortex synaptosomes

Phosphatidylcholine (PC) hydrolysis generates two important second messengers: phosphatidic acid (PA) and diacylglycerol (DAG). Phospholipase D (PLD) and phosphatidate phosphohydrolase (PAPase) are involved in their generation and therefore are key enzymes in signal transduction. Specific isoforms of these enzymes are activated by receptor occupancy in brain. Phosphatidylinositol 4,5-bisphosphate-dependent PLD (PIP2-PLD) and N-ethylmaleimide-insensitive PAPase (PAP2) have been suggested to act in series to generate the biologically active lipids PA and DAG. In the present study we examine age-induced changes mainly in PIP2-PLD and PAP2 activities in cerebrocortical synaptosomes from adult (4 months) and aged (28 months) Wistar rats. Aging increases the activity of both enzymes. Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) and cytosol (from cerebral cortex) stimulate PLD activity in adult and senescent synaptosomal membranes, the effect being greater in the latter. Under the same experimental conditions PAP2 activity was stimulated in aged membranes whereas in adult membranes GTPgammaS had no effect and cytosol showed a slight inhibitory effect. Diacylglycerol lipase (DGL) activity differs from that of PAP2 in aged rats and it was 21% inhibited with respect to synaptosomal membranes from adult rats. Increased sinaptosomal PLD activity in aged membranes appears to be independent of G protein regulation, whereas PAP2 activity is differentially regulated by GTPgammaS in aged membranes with respect to adult membranes. Our results suggest that under G-protein activation conditions, DAG production by the serial activation of PLD and PAP2 activities is increased in synaptosomal membranes from aged brain. The present paper demonstrates that PA generation (PLD activity) and degradation (PAPase activity) are differentially modulated during the aging process.

Regulation of phosphatidic acid phosphohydrolase 1 by fatty acids

In the starved state and during metabolic stress, free fatty acids (FFA) are the principal hepatic energy supply, undergoing beta-oxidation. Accordingly, it appears paradoxical that FFA have been reported to increase the liver's esterification capacity by translocating the rate-limiting enzyme phosphatidic acid phosphohydrolase (PAP-1) from the cytosol to the endoplasmic reticulum. We have therefore investigated the regulation of rat liver PAP-1. Oleic acid inhibited PAP activity in all subcellular fractions, with PAP-1 activity in cytosol being the most sensitive. Inhibition was also observed with oleoyl-CoA, linoleate, and palmitate. Fatty acids and their derivatives show detergent effects at high concentrations, and such effects can lead to enzyme inhibition. Inhibition by oleate, however, was reversed by phosphatidic acid and albumin and exhibited sigmoidal kinetics. These results demonstrate that PAP-1 is reversibly inhibited by FFA and their CoA esters, which may play a role in directing hepatic FFA to beta-oxidation during times of increased energy demand.

Aging promotes a different phosphatidic acid utilization in cytosolic and microsomal fractions from brain and liver

Among the morphological and biochemical changes taking place in the membranes of aged tissues, we reported in previous studies on alterations in phospholipid synthesis and phospholipid-specific fatty acid composition. Phosphatidic acid (PA) and diacylglycerol (DAG) are central intermediates in phosphoglyceride and neutral lipid biosynthetic pathways and have also recently been implicated in signal transduction. The present paper shows the effect of aging on phosphatidate phosphohydrolase (PAPase) activiy, which operates on phosphatidic acid to synthesize diacylglycerol. Two forms of mammalian PAPase can be indentified on the basis of subcellular localization and enzyme properties, one involved predominantly in lipid synthesis (PAP 1) and the other in signal transduction (PAP 2). Microsomal and cytosolic fractions of brain and liver from 3.5-month-old (adult) and 28.5-month-old (aged) rats were used. PAPase isoform activities were differentiated on the basis of N-ethylmaleimide (NEM) sensitivity and Mg(2+)-dependency. Our results demonstrate that aging caused PAP 2 to increase in brain microsomal fractions but did not affect PAP 1, whereas in brain cytosolic fractions, it caused a strong decrease in PAP 1 (57%). The distribution of enzymes between microsomes and cytosol changed in aged rats with respect to adult rats, showing a translocation of PAP 1 from cytosol to microsomes. In addition, an increase in the production of monoacylglycerol (MAG) was observed in microsomes from aged brain. PAP 2 activity in liver microsomal fractions from aged rats showed no changes with respect to adult rats whereas PAP 1 activity increased 228% in microsomal fractions and 76% in cytosolic fractions in this tissue. The distribution of PAP 1 activity between microsomal and cytosolic fractions in liver tissue was also affected in aged rats, indicating a translocation of this form of the enzyme from cytosolic to microsomal fractions. The production of monoacylglycerol in liver microsomes also increased, whereas there was a decrease in MAG formation from cytosolic fraction. The changes observed in the two PAPase forms in brain and liver of aged rats with respect to adult rats suggest that PA is differently utilized by the PAPase isoforms, probably generating aging-related DAGs different to those present in adults and required for specific cellular functions. The changes observed in liver PAP 1 from aged with respect to adult rats suggest that such changes could be related with modifications in lipid homeostasis induced by age-altered hormonal balance. However, PA-modified utilization during aging through PAP 2 activity could be related to alterations in neural signal transduction mechanisms.

Effect of light on phosphatidate phosphohydrolase activity of retina rod outer segments: the role of transducin

The aim of the present paper is to evaluate the modulation of phosphatidate phosphohydrolase (PAPase) and diacylglyceride lipase (DGL) activities in bovine rod outer segment (ROS) under dark and light conditions and to evaluate the role of transducin (T) in this phenomenon. In dark-adapted ROS membranes exposed to light, PAPase activity is inhibited by 20% with respect to the activity found under dark conditions. To determine whether the retinal G protein, T, participates in the regulation of PAPase activity in these membranes, the effects of GTPgammaS and GDPbetaS on enzyme activity were examined. Under dark conditions in the presence of GTPgammaS, which stabilizes T in its active form (Talpha + Tbetagamma), enzyme activity was inhibited and approached control values under light conditions. GDPbetaS, on the other hand, which stabilizes the inactive state of T (Talphabetagamma), stimulated PAPase activity by 36% with respect to control light conditions. ADP-ribosylation by cholera and pertussis toxin was also studied. In ADP-rybosilated ROS membranes with pertussis toxin under dark conditions, PAPase activity was 36% higher than the activity found under control light conditions. ADP-ribosylation by CTx, on the other hand, inhibited PAPase activity by 22%, with respect to dark control conditions, mimicking light effect. The effects of GTPgammaS and GDPbetaS and conditions of ADP-ribosylation by PTx and CTx on DGL activity were similar to those of PAPase activities. Based on NEM sensitivity we have also demonstrated that the PAPase present in ROS is the PAP 2 isoform. Our findings therefore suggest that light inhibition of PAP 2 in ROS is a transducin-mediated mechanism.

A metabolic path for the degradation of lysophosphatidic acid, an inhibitor of lysophosphatidylcholine lysophospholipase, in neuronal nuclei of cerebral cortex

Neuronal nuclei isolated from rabbit cerebral cortex were found to be enriched in an NEM-insensitive lysophosphatidic acid (lysoPA) phosphohydrolase activity. LysoPA is an inhibitor of the nuclear lysophosphatidylcholine (lysoPC) lysophospholipase, and by preserving lysoPC levels, lysoPA boosted the nuclear production of the acyl analogue of platelet-activating factor by promoting the acetylation of lysoPC (Baker and Chang, Mol. Cell Biochem., 1999, in press). The nuclear phosphohydrolase converts lysoPA to 1-monoacylglycerol, and thus eliminates this lysoPA inhibition of lysoPC lysophospholipase. The nuclear lysoPA phosphohydrolase specific activity was more than three times that observed for the nuclear lysoPA lysophospholipase (Baker and Chang, Biochim. Biophys. Acta 1438 (1999) 253-263) and represents a more active route for nuclear lysoPA removal. The neuronal nuclear lysoPA phosphohydrolase was inhibited at acidic pH, and also inhibited by calcium ions. The 1-monoacylglycerol product of the phosphohydrolase is rapidly degraded by neuronal monoacylglycerol lipase, an enzyme some sevenfold more active than the phosphohydrolase and sensitive to inhibition by arachidonoyl trifluoromethyl ketone (AACOCF(3)). Both acidic pH and free fatty acid inhibited the lipase. In the absence of AACOCF(3), production of fatty acid from lysoPA substrate could be largely attributed to the sequential actions of the nuclear phosphohydrolase and lipase. This facilitates fatty acid recycling back into phospholipid by lysophospholipid acylation when ATP levels are restored following periods of brain ischemia. At relatively low concentrations, sphingosine-1-phosphate, and alkylglycerophosphate were the most effective phosphohydrolase inhibitors while phosphatidic acid, alkylacetylglycerophosphate and ceramide were without effect. LysoPA is an interesting regulatory molecule that can potentially preserve lysophosphatidylcholine within the nuclear membrane for use in acetylation reactions. Thus conditions relevant to brain ischemia such as falling pH, falling ATP concentrations, rising fatty acid and intracellular calcium levels may, by slowing this metabolic path for lysoPA loss, promote the production of acyl PAF and contribute to the increased levels of the acetylated lipids noted in ischemia.

Phosphatidic acid as the biosynthetic precursor of the endocannabinoid 2-arachidonoylglycerol in intact mouse neuroblastoma cells stimulated with ionomycin

In mouse neuroblastoma N18TG2 cells prelabeled with [3H]arachidonic acid ([3H]AA) the biosynthesis of 2-arachidonoylglycerol (2-AG) is induced by ionomycin in a fashion sensitive to an inhibitor of diacylglycerol (DAG) lipase, RHC 80267, but not to four different phospholipase C (PLC) blockers. Pulse experiments with [3H]AA showed that ionomycin stimulation leads to the sequential formation of [3H]phosphatidic acid ([3H]PA), [3H]DAG, and [3H]2-AG. [3H]2-AG biosynthesis in N18TG2 cells prelabeled with [3H]AA was counteracted by propranolol and N-ethylmaleimide, two inhibitors of the Mg2+/Ca2(+)-dependent brain PA phosphohydrolase. Pretreatment of cells with exogenous phospholipase D (PLD) led to a strong potentiation of ionomycin-induced [3H]2-AG formation. These data indicate that DAG precursors for 2-AG in intact N18TG2 cells are obtained from the hydrolysis of PA and not through the activation of PLC. The presence of 2% ethanol during ionomycin stimulation failed to elicit the synthesis of [3H]phosphatidylethanol and did not counteract the formation of [3H]PA, thus arguing against the activation of PLD by the Ca2+ ionophore. Selective inhibitors of secretory phospholipase A2 and the acyl-CoA acylase inhibitor thimerosal significantly reduced [3H]2-AG biosynthesis. The implications of these latter findings, and of the PA-dependent pathways of 2-AG formation described here, are discussed.

Effect of glutathione deficiency on the adipocyte Mg(2+)-dependent phosphatidate phosphohydrolase

Previous studies from this laboratory [Jamdar S. C. and Cao W. F. (1994) Biochem. J. 301, 793-799] show that the adipocyte Mg(2+)-dependent phosphatidate phosphohydrolase (MGPPH), a major regulatory enzyme in adipose triacylglycerol metabolism, requires an active thiol group for its activity and perturbation of this group results in the loss of enzyme activity. Since glutathione (GSH) is important in maintaining the intracellular thiol state, we have used GSH-deficient animals and adipocytes to test the possibility that intracellular GSH concentration is critical in controlling the MGPPH activity. The MGPPH was measured in the presence of aqueous dispersed phosphatidate, and the release of P1 was taken as a measure of enzyme activity. The GSH deficiency in animals and isolated adipocytes was produced in the presence of diethylmaleate (DEM) or buthionine sulfoximine (BSO). Intraperitoneal administration of BSO into animals (3 mmoles/kg) showed 10-25% reduction in the blood and adipose GSH and 25% decline in the adipose MGPPH activity. However, DEM (0.3 ml/kg) was more effective and caused over 70% reduction of the blood and adipose tissue GSH content and 75% decline in the adipose MGPPH activity within 4 hr of drug administration. After 24 hr, these values returned to normal. Adipocytes incubated with 2.5 mM DEM for 60 min at 37 degrees C also showed a significant reduction in the GSH content and the MGPPH activity present in the cytosol and membrane fractions. The loss of membrane MGPPH was associated with decreased rates of triacylglycerol formation from [14C]palmitate. Pre-incubation of adipocyte homogenates with 1 mM DEM also resulted in > 90% decline in the MGPPH activity, which was preventable in the presence of GSH and dithiothreitol. Therefore, these studies suggest that the sulfhydryl environment offered by glutathione is critical for the maintenance of adipocyte MGPPH activity.

Difficulties in the assay of phosphatidate phosphohydrolase activity. Influence of ionic strength, detergent, and selection of substrate

In the present paper, problems in connection with assay of the activity of magnesium-dependent rat liver phosphatidate phosphohydrolase (PAP) are discussed. PAP activity is usually measured by following the production of diacylglycerol or inorganic phosphate from the substrate phosphatidate. These two methods may give widely different results due to a number of factors that may affect the assay. One such factor is the composition of the substrate. Higher apparent enzyme activity was observed with dioleoyl-phosphatidate than with dipalmitoyl-phosphatidate. This substrate-dependent difference in apparent PAP activity was 2-2.5-fold in the absence and 10-fold in the presence of Triton X-100, respectively. Triton X-100 reduced the activity as measured with the dipalmitoyl-phosphatidate substrate. In contrast, the activity of PAP as measured with dioleoyl-phosphatidate was stimulated by Triton X-100. The stimulatory effect of Triton was reduced or abolished when the ionic strength in the assay mixture was increased. Assays based on 32P-labeled substrate are rapid and sensitive. It is shown here that 33P can be used as an alternative. This radionuclide has a longer half-life and also emits particles with lower energy, thus posing less potential health hazards for the user.

Phosphatidate phosphatases of mammals, yeast, and higher plants

Phosphatidate phosphatase (EC 3.1.3.4) catalyzes the hydrolysis of phosphatidate to yield sn-1,2-diacylglycerol and inorganic phosphate. In mammalian systems, forms of phosphatidate phosphatase involved in glycerolipid synthesis and signal transduction have been identified. Forms of the enzyme involved in signal transduction have been purified and partially characterized. In yeast, phosphatidate phosphatases associated with the endoplasmic reticulum and mitochondria have also been purified and partially characterized. Information on phosphatidate phosphatases from mammals and yeast is useful in characterizing the enzyme from plant systems. This review examines progress on the characterization of phosphatidate phosphatases from mammals, yeast, and higher plants. The purification and characterization of the phosphatidate phosphatase involved in glycerolipid synthesis in developing oilseeds may lead to the identification of the encoding gene. Increasing our understanding of the enzymes of lipid synthesis in developing seeds will aid in the development of biotechnological strategies for seed oil modification.

Purification and characterization of novel plasma membrane phosphatidate phosphohydrolase from rat liver

An N-ethylmaleimide-insensitive phosphatidate phosphohydrolase, which also hydrolyzes lysophosphatidate, was isolated from the plasma membranes of rat liver. The specific activity of an anionic form of the enzyme (53 kDa, pI < 4) was increased 2700-fold. A cationic form of enzyme (51 kDa, pI = 9) was purified to homogeneity, but the -fold purification was low because the activity of the highly purified enzyme was unstable. Immunoprecipitating antibodies raised against the homogeneous protein confirmed the identity of the cationic protein as the phosphohydrolase and were used to identify the anionic enzyme. Both forms are integral membrane glycoproteins that were converted to 28-kDa proteins upon treatment with N-glycanase F. Treatment of the anionic form with neuraminidase allowed it to be purified in the same manner as the cationic enzyme and yielded an immunoreactive protein with a molecular mass identical to the cationic protein. Thus, the two ionic forms most likely represent different sialated states of protein. An immunoreactive 51-53-kDa protein was detected in rat liver, heart, kidney, skeletal muscle, testis, and brain. Little immunoreactive 51-53-kDa protein was detected in rat thymus, spleen, adipose, or lung tissue. This work provides the tools for determining the regulation and function of the phosphatidate phosphohydrolase in signal transduction and cell activation.

Purification and characterization of N-ethylmaleimide-insensitive phosphatidic acid phosphohydrolase (PAP2) from rat liver

N-Ethylmaleimide-insensitive phosphatidic acid phosphohydrolase (PAP; EC 3.1.3.4) was purified 5900-fold from rat liver. The enzyme was solubilized from membranes with octylglucoside, fractionated with (NH4)2SO4, and purified in the presence of Triton X-100 by chromatography on Sephacryl S300, hydroxyapatite, heparin-Sepharose and Affi-Gel Blue. Silver-stained SDS/PAGE indicated that the enzyme was an 83 kDa polypeptide. Sephacryl S-300 gel filtration also produced a second peak of enzyme activity, which was eluted from all of the chromatography columns at a different position from the purified enzyme. SDS/PAGE indicated that it contained three polypeptides (83 kDa, 54 kDa and 34 kDa), and gel filtration suggested that it was not an aggregate of the purified enzyme. Both forms were sensitive to inhibition by amphiphilic amines, Mn2+ and Zn2+, but not by N-ethylmaleimide. Purified PAP required detergent for activity, but was not activated by Mg2+, fatty acids or phospholipids. The enzyme was able to dephosphorylate lysophosphatidic acid or phosphatidic acid, and was inhibited by diacylglycerol and monoacylglycerol. No evidence was obtained for regulation of PAP by reversible phosphorylation.