Phosphatidylethanolamine methyltransferase and phospholipid methyltransferase activities from Saccharomyces cerevisiae. Enzymological and kinetic properties

In the yeast Saccharomyces cerevisiae, two membrane-associated enzymes catalyze the three-step methylation of phosphatidylethanolamine (PE) to phosphatidylcholine (PC). Phosphatidylethanolamine methyltransferase (PEMT) catalyzes the first methylation reactions (PE----phosphatidylmonomethylethanolamine (PMME] and phospholipid methyltransferase (PLMT) catalyzes the second two methylation reactions (PMME----phosphatidyldimethylethanolamine (PDME)----PC). Using gene disruption mutants of the S. cerevisiae OP13 and CHO2 genes, we independently studied the enzymological properties of microsome-associated PEMT and PLMT, respectively. The enzymological properties of the enzymes differed with respect to their pH optima, cofactor requirements and thermal lability. For the PEMT reactions, the apparent Km values for PE and S-Adenosylmethionine (AdoMet) were 57 microM and 110 microM, respectively. For the PLMT reactions, the apparent Km values for PMME and PDME were 380 microM and 180 microM, respectively. The apparent Km values for AdoMet were 54 microM and 59 microM with PMME and PDME as substrates, respectively. S-Adenosylhomocysteine (AdoHcy) was a competitive inhibitor of PEMT (Ki = 12 microM) and PLMT (Ki = 57 microM and Ki = 54 microM for PMME and PDME, respectively) with respect to AdoMet. AdoHcy was a noncompetitive inhibitor of PEMT (Ki = 160 microM) and PLMT (Ki = 120 microM) with respect to PE and PMME and PDME, respectively.

Purification and properties of phosphatidyl-N-monomethylethanolamine N-methyltransferase, the enzyme catalyzing the second and the third steps in the phosphatidylethanolamine N-methyltransferase system, from mouse liver microsomes

The phosphatidylethanolamine (PE) N-methyltransferase (MT) system is known to convert PE to phosphatidylcholine by three successive N-methylations. Phosphatidyl-N-monomethylethanolamine (PME) MT was purified 1,400-fold from mouse liver microsomes and separated from the PE-MT activity for the first time. This enzyme catalyzes N-methylations of PME and phosphatidyl-N,N-dimethylethanolamine, the intermediates of PE-MT system, but not PE, the initial substrate of the PE-MT system. In addition, a preparation with a different affinity to S-adenosyl-L-homocysteine catalyzing all the three methylations was obtained. These results suggest that at least two enzymes are involved in the PE-MT system.

Cyclic nucleotide phosphodiesterases and methyltransferases in purified lymphocytes, monocytes and polymorphonuclear leucocytes from healthy donors and asthmatic patients

Cyclic nucleotide phosphodiesterase and phospholipid N-methyl-transferase activities were simultaneously measured in purified polymorphonuclear cell-, mononuclear cell-, lymphocyte- and monocyte-homogenates from control subjects, from patients with atopic asthma and from patients with non-atopic asthma. Whereas cyclic AMP and cyclic GMP phosphodiesterase activities were found to be about 10-fold lower in polymorphonuclear than in mononuclear cells, phospholipid N-methyltransferase proved to be rather similar in each cell type from control donors. Cyclic AMP phosphodiesterase and phospholipid N-methyltransferase were significantly decreased in polymorphonuclear cells and monocytes from asthmatic patients compared with the control group while cyclic GMP phosphodiesterase was significantly impaired only in the monocyte subpopulation.

Sterol effects on phospholipid biosynthesis in the yeast strain GL7

Cells of the yeast sterol auxotroph GL7 were grown on either ergosterol or cholesterol to mid-logarithmic phase and total membrane fractions prepared. Activities of phospholipid biosynthetic enzymes in the two cell types were determined. The rates of phosphatidyl-ethanolamine-phosphatidyl-choline-N-methyl transferase and acyl-CoA-alpha-glycerol-3-phosphate transcylase were significantly greater in ergosterol-grown than in cholesterol-grown cells. These reactions were also inhibited by the polyene antibiotic filipin. By contrast the activities of long-chain fatty acyl-CoA synthetase, CTP-phosphatidate-cytidyl transferase, phosphatidylserine decarboxylase and of phosphatidylinositol synthetase were identical in the two (ergosterol and cholesterol) cultures and unaffected by filipin. The ergosterol effect on phosphatidyl-ethanolamine N-methyl transferase was greatest in cells harvested in early log phase, intermediate in the mid-log phase cells, and not significant in stationary phase cells.

Stimulation of phospholipid N-methylation by isoproterenol in rat hearts

Phosphatidylethanolamine (PtdEtn) N-methyltransferase activities were studied in rat heart sarcolemmal and sarcoplasmic reticular fractions after a single intraperitoneal injection of isoproterenol (0.5-5.0 mg/kg). Three active sites (I, II, and III) for PtdEtn N-methylation were assayed by measurement of [3H]methyl group incorporation from 0.055, 10, and 150 microM S-adenosyl-L-[methyl-3H]methionine into membrane PtdEtn molecules. Total methylation activity for catalytic site I of both sarcolemma and sarcoplasmic reticulum was stimulated within 2 minutes by isoproterenol in a dose-dependent manner. Although the increased methyltransferase activity in sarcoplasmic reticulum was normalized at 10 minutes, the enzyme activity in sarcolemma was normalized at 5 minutes but was again increased at 10-30 minutes after isoproterenol injection. No changes in response to isoproterenol were seen for site II and III N-methylation activities in either membrane. Individual N-methylated phospholipids (phosphatidyl-N-monomethylethanolamine, phosphatidyl-N,N-dimethylethanolamine, and phosphatidylcholine), which specifically formed at each site, showed similar behavior. Pretreatment of the animals with a beta-blocking drug, atenolol, for 2 days prevented the isoproterenol-induced changes in hemodynamic parameters and sarcolemmal methylation without affecting the enhanced methylation activities in sarcoplasmic reticulum. In vitro addition of cyclic AMP-dependent protein kinase (catalytic subunit) plus Mg-ATP enhanced methyltransferase activities in sarcolemma and sarcoplasmic reticulum from control hearts by 2.7- and 2.3-fold, respectively; however, under the same in vitro conditions, only about 20% activation was seen in both subcellular membranes isolated from the heart of isoproterenol-injected animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of methyl-group acceptors on the regulation of plasma cholesterol level in rats fed high cholesterol diets

The effects of methyl-group acceptors such as glycine, guanidinoacetic acid, and nicotinamide on cholesterol metabolism and phosphatidylcholine(PC) biosynthesis were investigated with rats fed a 25% casein diet containing cholesterol with or without methionine supplement. The effect of ethanolamine, an indirect methyl-group acceptor via phosphatidylethanolamine(PE) formation, was also compared with those of methyl-group acceptors. The methyl-group acceptors and ethanolamine decreased or tended to decrease plasma total cholesterol level when added to the 25% casein diet. These compounds also significantly depressed the methionine-induced enhancement of plasma cholesterol level. The activity of PE N-methyltransferase was decreased by the addition of glycine, guanidinoacetic acid, and nicotinamide, but not ethanolamine, to the reaction mixture when assayed using the postmitochondrial fraction of liver homogenate, suggesting that PE N-methyltransferase activity can be depressed by glycine N-methyltransferase, guanidinoacetic acid N-methyltransferase, and nicotinamide N-methyltransferase systems. The PE N-methyltransferase activity in liver microsomes, however, did not decrease in response to the dietary addition of methyl-group acceptors. The in vitro incorporation of [CH3-14C]methionine into PC of liver slices was also significantly inhibited by the addition of glycine and nicotinamide, but not guanidinoacetic acid and ethanolamine, to the incubation medium. It is suggested that methyl-group acceptors can decrease plasma cholesterol level at least in part through the depression of PC biosynthesis via PE N-methylation pathway, and that the mechanism for the plasma cholesterol-lowering effect of ethanolamine is different from that of methyl-group acceptors.

In vitro phosphorylation of phosphatidylethanolamine N-methyltransferase by cAMP-dependent protein kinase: lack of in vivo phosphorylation in response to N6-2'-O-dibutryladenosine 3',5'-cyclic monophosphate

Phosphorylation of rat liver phosphatidylethanolamine (PE) N-methyltransferase by cAMP-dependent protein kinase was investigated. The 18 kDa methyltransferase was found to be phosphorylated in vitro by cAMP-dependent protein kinase on a serine residue. The stoichiometry of phosphate incorporation reached a maximum of 0.25 mol phosphate/mol methyltransferase at 30 min. Resolution of the phosphorylated methyltransferase by two-dimensional gel electrophoresis showed that two isoproteins were substrates. Phosphorylation of the purified PE N-methyltransferase for up to 1 h had no effect on the methylation of PE, PMME or PDME. To test for in vivo phosphorylation, isolated rate hepatocytes were exposed to 0.5 mM N6-2'-O-dibutryladenosine 3':5'-cyclic monophosphate (DiB-cAMP) and the phosphorylation state of microsomal proteins evaluated by two-dimensional gel electrophoresis, nitrocellulose blotting and autoradiography. The same nitrocellulose blots were probed with a rabbit anti-PE N-methyltransferase antibody, immunochemically stained and aligned with the autoradiogram. No phosphorylated proteins co-migrated with the methyltransferase under non-phosphorylating conditions, or when hepatocytes were exposed to the cAMP analogue for up to 2 h. Oddly, DiB-cAMP increased both PE- and PMME-dependent activity in isolated microsomes, but decreased PE to PC conversion measured in intact hepatocytes. The results indicated that PE N-methyltransferase is poorly phosphorylated by cAMP-dependent protein kinase in vitro, and is not phosphorylated in intact hepatocytes treated with a cAMP analogue.

Mutations in the Saccharomyces cerevisiae opi3 gene: effects on phospholipid methylation, growth and cross-pathway regulation of inositol synthesis

We report the isolation of two new opi3 mutants by EMS mutagenesis, and construction of an insertion allele in vitro using the cloned gene. We have demonstrated that the opi3 mutations cause a deficiency in the two terminal phospholipid N-methyltransferase (PLMT) activities required for the de novo synthesis of PC (phosphatidylcholine). The opi3 mutants, under certain growth conditions, produce membrane virtually devoid of PC although, surprisingly, none of the mutants displays a strict auxotrophic requirement for choline. Although the opi3 mutants grow without supplements, we have shown that the atypical membrane affects the ability of the mutant strains to initiate log phase growth and to sustain viability at stationary phase. The commencement of log phase growth is enhanced by addition of choline or to a lesser extent DME (dimethylethanolamine), and retarded by addition of MME (monomethylethanolamine). The mutant cells lose viability at the stationary phase of the cell cycle in the absence of DME or choline, and are also temperature sensitive for growth at 37 degrees especially in media containing MME. These growth defects have been correlated to the presence of specific phospholipids in the membrane. The opi3 growth defects are suppressed by an unusual mutation in the phospholipid methylation pathway that perturbs the N-methyltransferase (PEMT) activity immediately preceding the reactions affected by the opi3 lesion. We believe this mutation, cho2-S, alters the substrate specificity of the PEMT. A secondary effect of opi3 mutations is disruption of the cross pathway regulation of the synthesis of the PI (phosphatidylinositol) precursor inositol. Synthesis of inositol is controlled through regulation of the INO1 gene which encodes inositol-1-phosphate synthase. This highly regulated gene is expressed constitutively in opi3 mutants. We have used the opi3 strains to demonstrate that synthesis of either PC or PD (phosphatidyldimethylethanolamine) will restore normal regulation of the INO1 gene.

Phosphatidylethanolamine N-methyltransferase in human red blood cell membrane preparations. Kinetic mechanism

The successive methylations of phosphatidylethanolamine to form phosphatidylcholine were measured using exogenously added intermediates and membrane preparations from human red blood cells. The addition of phosphatidylethanolamine resulted in no increase in methylation rate over that with endogenous substrate; however, the addition of monomethylphosphatidylethanolamine (PME) and dimethylphosphatidylethanolamine (PDE) markedly increased the reaction rate and allowed studies into the kinetic mechanism for the second and third methylation reactions. The data are consistent with catalysis of the last two methylations being by a single enzyme with a random Bi-Bi sequential mechanism. Analysis of PDE:phosphatidylcholine product ratios indicates that the enzyme can conduct multiple methylations of enzyme-bound phospholipid. The nature of the acyl chain (16:0 versus 18:1) of the phospholipid had only a small effect on the value of the kinetic constants. The maximal velocities obtained with the 18:1 substrate were less than 5% lower than those obtained with the 16:0 substrate. The Km values for the two phospholipids were 20-45 and 10-14 microM for the methylation of PME and PDE, respectively. The Km for S-adenosylmethionine (AdoMet) was 5-9 microM with PME and 4 microM with PDE as substrates. Depending on the acyl chain and the phospholipid, the Ki(AdoMet) varied from 8 to 19 microM, the Ki(PME) from 41 to 82 microM, and the Ki(PDE) from 35 to 61 microM. The Ki for S-adenosylhomocysteine (AdoHcy) was between 1.0 and 1.4 microM depending upon the variable substrate. The endogenous concentrations of PME and PDE in red blood cell membranes were estimated to be 0.49 and 0.24 mumol/liter packed cells, respectively. The product from the utilization of AdoMet, S-adenosylhomocysteine (AdoHcy), was shown to be a competitive inhibitor of its precursor, AdoMet, and a noncompetitive inhibitor of the two phospholipid substrates.

Co-ordinate control of CDP-choline and phosphatidylethanolamine methyltransferase pathways for phosphatidylcholine biosynthesis occurs in response to change in diet fat

Brain microsomal and synaptic plasma membrane phosphatidylcholine composition and biosynthetic activity were examined in relation to the composition of diet fat fed. Phosphocholinetransferase and methyltransferase activities are shown to be modulated by the diet, and by changes in the membrane phospholipid content of long-chain polyunsaturated fatty acids. This physiological modulation is co-ordinated such that the rate of phosphatidylcholine synthesis via one route is inversely regulated with activity of the alternate pathway.

Inhibitory effects of GMCHA-OPhBut on phospholipid methylation and histamine release in mast cells activated by concanavalin A, anti-IgE, and antigen

[3H]Methyl group incorporation and histamine secretion in rat mast cells induced by anti-IgE and con A were strongly inhibited by trans-4-guanidinomethylcyclohexanecarboxylic acid 4-tert-butylphenyl ester (GMCHA-OPhBut), a strong and specific inhibitor for pH 7 tryptase (Muramatsu et al. (1988) Biol. Chem. Hoppe-Seyler 369, 617-625) which is present in rat mast cells. The IC50s for these events were of the order of 10(-6) M. Addition of GMCHA-OPhBut after the maximal increase in [3H]methyl group incorporation in rat mast cells activated by con A and anti-IgE induced rapid reduction of the methylated phospholipid, and the later histamine release was strongly suppressed. Mast cells were prepared with Mg2+-free Tyrode-HEPES solution, and challenged with anti-IgE with or without Mg2+. With Mg2+, [3H]methyl group incorporation was enhanced, and histamine was secreted time-dependently. Without Mg2+, [3H]methyl group incorporation fell to one-third, whereas histamine secretion was not affected. These results were incompatible with the above results. From these results it was strongly suggested that a trypsin-like protease, probably pH 7 tryptase, is involved not only in the early events, such as activation of phosphatidylethanolamine methyltransferase I and/or II, but also in the late events such as histamine release, and phospholipid methylation is not associated with histamine secretion.

Phosphatidylethanolamine levels and regulation of phosphatidylethanolamine N-methyltransferase

The activity of phosphatidylethanolamine (PE) N-methyltransferase in liver microsomes, measured using endogenous microsomal PE as a substrate, was elevated 2-fold in the choline-deficient state. However, methyltransferase activity assayed in the presence of a saturating concentration of phosphatidyl-N-mono-methylethanolamine or microsomal PE was unchanged by choline deficiency. Accompanying the increase in methyltransferase activity in liver homogenates and microsomes were increased PE concentrations and an increased PE to phosphatidylcholine ratio. The concentration of other phospholipids was unchanged. Immunoblot analysis of choline-deficient and choline-supplemented rat liver microsomes using a rabbit polyclonal anti-PE N-methyltransferase antibody revealed that the amount of enzyme protein was unaltered. The regulation of methyltransferase by PE levels was also investigated in cultured hepatocytes obtained from choline-deficient rat livers. Supplementation of deficient hepatocytes with 200 microM methionine resulted in a 50% reduction in cellular PE levels over a 12-h period. PE N-methyltransferase activity assayed with endogenous PE was also reduced by 50%, but phosphatidyl-N-monomethylethanolamine-dependent activity was unchanged. A 4-h supplementation with choline did not affect PE levels or methyltransferase activity. Either methionine or choline supplementation resulted in net synthesis of cellular phosphatidylcholine. Immunoblotting of membranes from methionine-supplemented hepatocytes revealed no change in enzyme protein, a further indication that enzyme mass was constitutive, and activity was regulated by the concentration of PE.

S-adenosylmethionine, S-adenosylhomocysteine and adenosine system. Age-dependent availability in rat brain

The changes in activity of N-methyltransferase 1 and [3H]-S-adenosylhomocysteine binding were determined in cortical membranes preparations from newborn and 1-, 2- and 10-month-old rats. These parameters were significantly higher: +130 and 200%, respectively, in 1-day compared to 2-month-old rats (adult). With maturation, S-adenosylmethionine levels decreased (-24%) whereas adenosine levels increased gradually from 0.17 to 6.10 nmol/mg protein in forebrain or whole brain between 1-day and 10-month-old rats. The ontogenesis of adenosine receptors was studied. The A2 receptors were not detectable at 1 day. For A1 receptors, Kd and Bmax increased with age while for A2 receptors Kd and Bmax were similar between 9 days and 2 months of age.

[The effect of ionizing radiation on the transmethylation of phospholipids in the high molecular complex acyl-tRNA-synthetase]

Transmethylation was found to contribute to the postirradiation redistribution of phospholipids which enter high molecular weight complexes of rat liver aminoacyl-tRNA-synthetases. The kinetic capacity of methyltransferases of codosome phospholipids was studied in normal conditions and at early times after irradiation of rats with a dose of 0.21 C/kg.

Rat and human mammary tissue can synthesize choline moiety via the methylation of phosphatidylethanolamine

The normal mammal requires large amounts of choline for maintenance and growth of tissue mass. Since milk, the only food for neonates, has many-fold higher free choline concentration than does maternal plasma, it is possible that mammary gland can synthesize choline molecules. The only known mammalian pathway for the synthesis de novo of choline molecules is catalysed by phosphatidylethanolamine N-methyltransferase (PeMT), which synthesizes phosphatidylcholine (PtdCho) via sequential methylation of phosphatidylethanolamine (PtdEtn) using S-adenosylmethionine (AdoMet) as a methyl donor. We identified PeMT activity in rat mammary tissue, and differences in affinities for substrate, as well as in activities as a function of pH, suggest that at least two distinct enzyme activities are involved [i.e. one catalysing the methylation of PtdEtn to form phosphatidyl-N-methylethanolamine (PtdMeEtn) and the other catalysing the methylation of PtdMeEtn and phosphatidyl-NN-dimethylethanolamine (PtdMe2Etn) to form PtdMe2Etn and PtdCho, respectively]. The relationships between AdoMet concentrations and PtdCho formation from endogenous PtdEtn in rat mammary homogenate were complex: a sigmoidal component (with a Hill coefficient of 2.2), requiring 55 microM-AdoMet for half saturation (Vmax. = 9 pmol/h per mg of protein), and a high affinity component (Kapparent = 8.7 microM and Vmax. = 3.8 pmol/h per mg of protein) were identified. When exogenous PtdMe2Etn was added as substrate, PtdCho formation exhibited Michaelis-Menten kinetics for AdoMet, and its affinity for AdoMet was high (Kapparent = 9 microM, Vmax. = 85 pmol/h per mg of protein). In the presence of endogenous substrates, the rates of PeMT-catalysed PtdCho formation within homogenates of rat mammary tissue were similar in tissue from lactating and non-lactating animals. When exogenous PtdMe2Etn was added to homogenates of rat mammary tissue, tissue from lactating rats made twice as much PtdCho as did tissue from non-lactating rats. Isolated mammary epithelial cells also exhibited PeMT activity; the rate of formation of PtdCho was much greater in intact versus broken cells. We also identified PeMT activity in homogenates of mammary tissue from non-lactating humans. The rate of PtdCho formation was of similar magnitude to that seen in rat tissue. This evidence supports the hypothesis that some of the choline found in milk could have been synthesized de novo in the mammary gland.

Specificity of rat hepatic phosphatidylethanolamine N-methyltransferase for molecular species of diacyl phosphatidylethanolamine

The specificity of phosphatidylethanolamine (PE) N-methyltransferase for molecular species of PE has been investigated. Phosphatidylcholine (PC), synthesized by incubation of [methyl-3H]S-adenosyl-L-methionine with microsomes or pure enzyme (Ridgway, N. D., and Vance, D. E. (1987) J. Biol. Chem. 262, 17231-17239) plus microsomal PE, had a distribution of methyl label in molecular species similar to the mole percent distribution of molecular species in the precursor PE. A similar lack of specificity was observed with PE that was synthesized from egg PC by transphosphatidylation with phospholipase D. Phosphatidyl-N-monomethylethanolamine (PMME) and phosphatidyl-N,N-dimethylethanolamine (PDME), both with the acyl composition of egg PC, were methylated by the pure enzyme and showed a distribution of labeled molecular species in PDME and PC, respectively, similar to the mole percent distribution of egg PC. Results with synthetic PEs and pure methyltransferase showed higher rates of methylation with more unsaturated species. Long chain saturated PEs (e.g. dipalmitoyl-PE) were not methylated by the enzyme. Maximal methylation rates were obtained with two or more double bonds in the substrate PE. Rates of methylation of the saturated and monoenoic PEs could be enhanced when 40 mol % polyunsaturated-rich microsomal PC was included in the mixed micelles. PC isolated from primary cultures of rat hepatocytes pulsed with [methyl-3H]methionine was analyzed by high performance liquid chromatography. Initially, the labeling pattern of PC molecular species varied slightly from that of total hepatocyte PE and hepatocyte microsomal PE. 1-Palmitoyl-2-docosahexaenoyl-PC had the highest specific activity at the end of the pulse and was preferentially labeled relative to the mole percent distribution of hepatocyte PE molecular species. During the 24-h chase period both the percent distribution of label and specific activity of this species of PC declined. In the same time period, there was a corresponding increase in specific activity and percent distribution of label in 1-palmitoyl and 1-stearoyl species with linoleate and arachidonate in the sn-2 position.

Kinetic mechanism of phosphatidylethanolamine N-methyltransferase

We have investigated the kinetic mechanism of phosphatidylethanolamine (PE) N-methyltransferase purified from rat liver using PE, phosphatidyl-N-monomethylethanolamine (PMME), and phosphatidyl-N,N-dimethylethanolamine (PDME) as substrates. We previously reported (Ridgway, N. D., and Vance, D. E. (1987) J. Biol. Chem. 262, 17231-17239) that initial velocity curves with PE, PMME, and PDME at a fixed concentration of Triton X-100 were sigmoidal, thus generating nonlinear inverse plots. Comparison with other integral membrane enzymes suggested this response resulted from the enzyme's requirement for a complete boundary layer of phospholipid. Hence, the effect of a nonsubstrate phospholipid on initial velocity patterns for PE, PMME, and PDME was examined. The sigmoidicity of initial velocity curves at constant Triton X-100 concentration and increasing PE, PMME, and PDME were converted to the more familiar hyperbolic response by the addition of egg phosphatidylcholine (PC). Hill coefficients for PE, PMME, and PDME at a fixed Triton concentration were 3.6, 2.5, and 4.7, respectively, but with the addition of 30 or 40 mol % of egg PC, coefficients were close to unity (0.9-1.2). The activation by egg PC of PE, PMME, and PDME methylation indicates that a secondary phospholipid binding site(s) plays a role in catalysis in mixed micelles. This site(s) may represent a transmembrane segment(s) in close association with a boundary layer of phospholipid. Kinetic analysis of initial velocity and product inhibition patterns for PMME and PDME methylation fit an ordered Bi Bi mechanism. Phospholipid substrates and products were the first to bind and the last to dissociate from the active site, respectively. As well, PE, PMME, and PDME compete for a single active site. The overall kinetic scheme for the methylation of PE to PC in mixed micelles involves the initial binding of PE, followed by successive steps where S-adenosyl-L-methionine is bound, the sulfonium methyl group is transferred, and S-adenosyl-L-homocysteine is released.

Dexamethasone-induced reduction of phospholipase D activity in the rat. Possible role of lipocortin

Subcutaneous injection of dexamethasone resulted in a reduction of demonstrable phospholipase D activity of rat brain and liver microsomes. Partially purified rat lung lipocortin inhibited the activity of both microsomal and partially purified rat brain phospholipase D. These results show that phospholipase D activity is suppressed by dexamethasone and one of the possible mechanisms of inhibition may be a phospholipase inhibitory protein, lipocortin.

Methyltransferase and phospholipase A2 activity in the cell membrane of neutrophils and lymphocytes from patients with Behçet's disease, systemic lupus erythematosus, and rheumatoid arthritis

Phospholipid methylation and phospholipase A2 activation in the membrane of neutrophils and lymphocytes, which participate in the induction of cell activation, were assessed in patients with Behçet's disease, systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). [3H-methyl] incorporation and phospholipase A2 activity of neutrophils from active cases of Behçet's disease and RA were significantly increased compared with normal controls. In lymphocytes from the patients with active Behçet's disease and RA, a significant increase in methyltransferase activity and a marked enhancement of phospholipase activity were found. A modest increase in these two membrane phospholipid enzyme activities was observed in lymphocytes of patients with active SLE. In addition, these enzyme activities were significantly enhanced in normal leukocytes preincubated with serum from patients with active SLE and malignant RA. The potentiated functions of neutrophils and lymphocyte abnormalities in the patients tested thus seem to be at least partly due to an increase in these enzymatic activities in the cell membrane.

Histamine increases phospholipid methylation and H2-receptor-adenylate cyclase coupling in rat brain

Histamine stimulated the enzymatic synthesis of phosphatidylcholine from phosphatidylethanolamine in crude synaptic membranes of rat brain containing the methyl donor S-adenosyl-L-methionine (SAM). In the presence of, but not in the absence of SAM, histamine increased cyclic AMP accumulation at the concentrations that stimulate phospholipid methylation. S-Adenosyl-L-homocysteine, an inhibitor of phospholipid methyltransferases, inhibited histamine-stimulated phospholipid methylation and histamine-induced cyclic AMP accumulation in the presence of SAM in a concentration-dependent manner. Histamine-induced [3H]methyl incorporation into phospholipids exhibited a marked regional heterogeneity in rat brain in the order of cortex greater than medulla oblongata greater than hippocampus greater than striatum greater than midbrain greater than hypothalamus. The regional distribution of histamine-induced cyclic AMP accumulation exactly paralleled histamine-stimulated [3H]methyl incorporation in rat brain. Histamine-induced cyclic AMP accumulation was inhibited by the addition of cimetidine or famotidine, but not by mepyramine or diphenhydramine. The accumulation of cyclic AMP in the presence of SAM was observed by the addition of impromidine or dimaprit, but not by 2-pyridylethylamine. These results indicate that phospholipid methylation is induced by histamine and may participate in H2-receptor-mediated stimulation of adenylate cyclase in rat brain.

Does rat liver Golgi have the capacity to synthesize phospholipids for lipoprotein secretion?

Phosphatidylcholine and phosphatidylethanolamine in lipoproteins secreted from cultured rat hepatocytes are derived from specific biosynthetic pools (Vance, J. E., and Vance, D. E. (1986) J. Biol. Chem. 261, 4486-4491). We have tested the hypothesis that some of the phospholipids destined for secretion with lipoproteins may be made in the Golgi. Golgi fractions were prepared by three different procedures. Although each procedure yielded membranes highly enriched in galactosyltransferase, the protein profiles on polyacrylamide gels were distinct for each preparation. Similarly, the presence of phospholipid synthetic enzyme activities differed among the preparations of Golgi. Two of the preparations were judged to be contaminated by no more than 15% with endoplasmic reticulum. Although an unequivocal conclusion that Golgi contains phospholipid biosynthetic enzymes is not possible, the available evidence is consistent with this hypothesis. Golgi prepared by one method (Croze, E. M., and Morré, D. J. (1984) J. Cell. Physiol. 119, 46-57) was studied in detail. This preparation contained activities for CTP:phosphocholine cytidylyltransferase, CDP-choline:1,2-diacylglycerol cholinephosphotransferase, CDP-ethanolamine:1,2-diacylglycerol ethanolamine-phosphotransferase, phosphatidylethanolamine-N-methyltransferase, and phosphatidylserine synthase. These enzyme activities in the Golgi displayed properties similar to the enzyme activities in endoplasmic reticulum with respect to Km values for substrates, pH optima, cofactor requirements, and inhibition by metabolites. Topology experiments suggested that these enzymes on endoplasmic reticulum and Golgi are all exposed to the cytosolic surface. Phosphatidylserine decarboxylase was not detected in the Golgi preparation. The results support the hypothesis that Golgi has the capacity to make certain phospholipids for lipoprotein secretion: phosphatidylcholine via the CDP-choline and methylation pathways, phosphatidylethanolamine by the CDP-ethanolamine pathway, and phosphatidylserine. Synthesis of phosphatidylethanolamine via decarboxylation of phosphatidylserine does not appear to occur in Golgi.