S-adenosyl-L-methionine synthetase and phospholipid methyltransferase are inhibited in human cirrhosis

We have measured the activity S-adenosyl-L-methionine synthetase in liver biopsies from a group of controls (n = 17) and in 26 cirrhotics (12 alcoholic and 14 posthepatic). The activity of this enzyme was markedly reduced in the group of cirrhotics (285 +/- 32 pmoles per min per mg protein) when compared with that observed in controls (505 +/- 37 pmoles per min per mg protein). No differences in S-adenosyl-L-methionine synthetase was observed between both groups of cirrhotics. Similarly, a marked reduction in the activity phospholipid methyltransferase was also observed in liver biopsies from the same group of cirrhotics (105 +/- 12 pmoles per min per mg protein) when compared with the control subjects (241 +/- 13 pmoles per min per mg protein). Again, no difference in the activity of this enzyme was observed between both groups of cirrhotics. These results indicated a marked deficiency in the metabolism of S-adenosyl-L-methionine in cirrhosis.

Differential effects of benzodiazepines on phospholipid methylation in hippocampus and cerebellum of rats

To elucidate the relationship between the occupancy of BDZ binding sites and phospholipid methylation in brain, we examined phosphatidylethanolamine-N-methyltransferase (PEMT) activity in synaptosomes of rat hippocampi and cerebella in the presence of BDZ ligands with different modes of action. We found that Ro 5-4864, a specific ligand for "peripheral type" receptors, increased PL methylation in hippocampal and cerebellar synaptosomes. This effect was directly related to receptor occupancy, since the specific antagonist PK 11195 inhibited the rise in PEMT activity induced by Ro 5-4864. Clonazepam, on the other hand, tended to reduce PL production in cerebellum and hippocampus except for hippocampal (3H)-phosphatidyl-N-monomethylethanolamine which was elevated by 40 to 70% at doses ranging from 10(-9) to 10(-6) M. When equimolar concentrations of the antagonist Ro 15-1788 were given in association the clonazepam-induced phosphatidyl-N-monomethylethanolamine increase was reduced by 70%. These data support the involvement of structural and functional membrane alterations in the action of BDZ.

Purification of phosphatidylethanolamine N-methyltransferase from rat liver

Phosphatidylethanolamine (PE) N-methyltransferase catalyzes the synthesis of phosphatidylcholine by the stepwise transfer of methyl groups from S-adenosylmethionine to the amino head group of PE. PE N-methyltransferase was solubilized from a microsomal membrane fraction of rat liver using the nonionic detergent Triton X-100 and purified to apparent homogeneity. Specific activities of PE N-methyltransferase with PE, phosphatidyl-N-monomethylethanolamine (PMME), and phosphatidyl-N,N-dimethylethanolamine (PDME) as substrates were 0.63, 8.59, and 3.75 mumol/min/mg protein, respectively. The purified enzyme was composed of a single subunit with a molecular mass of 18.3 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Methylation activities dependent on the presence of PE, PMME, and PDME and the 18.3-kDa protein co-eluted when purified PE N-methyltransferase was subjected to gel filtration on Sephacryl S-300 in the presence of 0.1% Triton X-100. All three methylation activities eluted with a Stokes radius 2.1 A greater than that determined for pure Triton micelles (molecular mass difference of 27.4 kDa). Two-dimensional analysis of PE N-methyltransferase employing nonequilibrium pH gradient gel electrophoresis and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicates that the enzyme is composed of a single isoform. Analysis of enzyme activity using PE, PMME, and PDME at various Triton X-100 concentrations indicated the enzyme follows the "surface dilution" model proposed for other enzymes that act at the surface of mixed micelle substrates. Initial velocity data for all three lipid substrates (at fixed concentrations of Triton X-100) were highly cooperative in nature. Hill numbers for PMME and PDME ranged from 3 at 0.5 mM Triton to 6 at 2.0 mM Triton. All three methylation activities had a pH optimum of 10. These results provide evidence that a single membrane-bound enzyme catalyzes all three methylation steps for the conversion of PE to phosphatidylcholine.

Yeast phosphatidylethanolamine methylation pathway. Cloning and characterization of two distinct methyltransferase genes

The structural genes (PEM1 and PEM2) encoding the enzymes involved in the yeast phosphatidylethanolamine (PE) methylation pathway were cloned by means of genetic complementation using yeast mutants. The cloned genes were expressed in a yeast mutant that was completely deficient in the PE methylation pathway. The membrane fraction obtained from the transformants carrying PEM1 only catalyzed the conversion of PE to phosphatidyl-N-monomethylethanolamine (PMME), the first step of the methylation pathway. Therefore, the enzyme encoded by PEM1 was designated as PE methyltransferase. In contrast, the membrane fraction from the transformants carrying PEM2 catalyzed the synthesis of phosphatidylcholine (PC) from PE, indicating that it contains all of the three methylation activities. PMME and phosphatidyl-N,N-dimethylethanolamine were found to be utilized more preferentially than PE. Because of its rather broad substrate specificity, the enzyme encoded by PEM2 is designated as phospholipid methyltransferase. The results of phospholipid composition analysis showed that the PEM1 transformant accumulated PMME whereas the PEM2 transformant contained a decreased amount of PC. Both genes were required for maintenance of the PC content of the yeast at a normal level. The results of nucleotide sequence analysis demonstrated that the coding frames of the PEM1 and PEM2 genes were capable of encoding 869- and 206-amino acid residues with calculated molecular weights of 101,202 and 23,150, respectively. The sizes of the PEM1 and PEM2 transcripts detected in the exponentially growing wild-type yeast were consistent with those of the deduced translation products. PE methyltransferase exhibits internal sequence homology as well as homology with phospholipid methyltransferase, suggesting that these two methyltransferase genes evolved through gene duplication. Furthermore, there was significant sequence homology between PE methyltransferase and bovine phenylethanolamine N-methyltransferase, and between phospholipid methyltransferase and Escherichia coli S-adenosylmethionine-6-N',N'-adenosyl (rRNA) dimethyltransferase.

A phospho-oligosaccharide mimics the effect of insulin to inhibit isoproterenol-dependent phosphorylation of phospholipid methyltransferase in isolated adipocytes

Addition of isoproterenol to isolated rat adipocytes prelabeled with [32P]phosphate caused an increase in the phosphorylation and activation of phospholipid methyltransferase. 32P-Labeled phospholipid methyltransferase was recovered by immunoprecipitation and gel electrophoresis. Analysis of 32P-labeled peptides revealed one site of phosphorylation regulated by isoproterenol, and analysis of phosphoamino acids demonstrated that the incorporation of [32P]phosphate was on phosphoserine. Incubation of adipocytes with isoproterenol in the presence of insulin or a phospho-oligosaccharide inhibited the phosphorylation and activation of this enzyme. The inhibitory effect of insulin on the phosphorylation of phospholipid methyltransferase was reversible, and it was mimicked by a phospho-oligosaccharide. The phospho-oligosaccharide was generated by hydrolysis of an isolated glycophospholipid with phosphatidylinositol-specific phospholipase C from Staphylococcus aureus. The insulin-like effect of this phospho-oligosaccharide on the phosphorylation of phospholipid methyltransferase was demonstrated in isolated adipocytes, and the effect was abolished by treatment of the phospho-oligosaccharide with 10% NH4OH, nitrous acid, or sodium periodate. These data suggest that in intact adipocytes the effect of insulin to inhibit the phosphorylation/activation of phospholipid methyltransferase is mediated by a phospho-oligosaccharide generated by a phosphatidylinositol-specific phospholipase C.

Synthesis of acetylcholine from choline derived from phosphatidylcholine in a human neuronal cell line

Cholinergic neurons are unique among cells since they alone utilize choline not only as a component of major membrane phospholipids, such as phosphatidylcholine (Ptd-Cho), but also as a precursor of their neurotransmitter acetylcholine (AcCho). It has been hypothesized that choline-phospholipids might serve as a storage pool of choline for AcCho synthesis. The selective vulnerability of cholinergic neurons in certain neurodegenerative diseases (e.g., Alzheimer disease, motor neuron disorders) might result from the abnormally accelerated liberation of choline (to be used as precursor of AcCho) from membrane phospholipids, resulting in altered membrane composition and function and compromised neuronal viability. However, the proposed metabolic link between membrane turnover and AcCho synthesis has been difficult to demonstrate because of the heterogeneity of the preparations used. Here we used a population of purely cholinergic cells (human neuroblastoma, LA-N-2), incubated in the presence of [methyl-3H]methionine to selectively label PtdCho synthesized by methylation of phosphatidylethanolamine, the only pathway of de novo choline synthesis. PtdCho, purified by thin-layer chromatography, contained 90% of the label incorporated into lipids, demonstrating that LA-N-2 cells contained phosphatidylethanolamine N-methyltransferase. Three peaks of radioactive material that cochromatographed with authentic Ac-Cho, choline, and phosphocholine were observed when the water-soluble metabolites of the [3H]PtdCho were purified by high-performance liquid chromatography. Their identities were ascertained by subjecting them to enzymatic modifications with acetylcholinesterase, choline oxidase, and alkaline phosphatase, respectively. The results demonstrate that AcCho can be synthesized from choline derived from the degradation of endogenous PtdCho formed de novo by methylation of phosphatidylethanolamine.

Variations of phospholipid methyltransferase(s) activity in the rat pituitary: estrous cycle and sex differences

We previously demonstrated a specific stimulatory action of estrogens on phosphatidylethanolamine methylation in rat pituitary membranes. To investigate the physiological relevancy of this effect, the activity of methylating enzyme(s) was evaluated during the rat estrous cycle, a period in which both endogenous ovarian steroid levels and the sensitivity of pituitary membrane receptors fluctuate. Anterior pituitary membranes (P2) were prepared from adult female rats at different stages of the estrous cycle and assayed for phospholipid methylation in the presence of S-adenosyl-[methyl-3H]methionine as a donor of 3H-methyl groups. Methylated phospholipids were separated by TLC. Formation of phosphatidyl-mono- and dimethylethanolamine and that of phosphatidylcholine increased significantly in the morning, reaching maximal values on the afternoon of proestrus; they decreased thereafter during estrus, metestrus, and diestrus. Plasma estradiol concentrations increased in late diestrus and then varied similarly with the fluctuations of phospholipid methyltransferase activity throughout the cycle. In parallel, plasma levels of LH and PRL were significantly elevated during the afternoon of proestrus, but remained low throughout the rest of the cycle. Under the same experimental conditions, phospholipid methylation in membranes prepared from mediobasal-hypothalamic structures was not affected. These data demonstrate that under physiological conditions the increased pituitary methyltransferase activity is associated with the progressive increment of plasma estradiol levels occurring shortly before proestrus and precedes the release of LH and PRL. Ovariectomy significantly decreased methyltransferase activity; however, 17 beta-estradiol treatment of ovariectomized rats for 5 days restored the enzyme activity, which was further augmented after progesterone administration. Attempting to investigate variations of pituitary methyltransferase activity in male rats, we demonstrated that the intact males showed weaker activity than that of females; orchidectomy diminished the phospholipid methylation, but adrenalectomy had no effect.

The specificity of rat liver phospholipid methyltransferase for lyso derivatives and diacyl derivatives of phosphatidylethanolamine

When sonicated suspensions of 1-palmitoyl-2-lysophosphatidyl-N-monomethylethanolamine (lysoPME) or 1-palmitoyl-2-lysophosphatidyl-N,N-dimethylethanolamine (lysoPDE) were incubated with rat liver microsomes and [Me-3H]AdoMet or [Me-14C]AdoMet, one methyl group was added to these lipids. With dipalmitoylphosphatidyl-N-monomethylethanolamine (PME) or dipalmitoylphosphatidyl-N,N-dimethylethanolamine (PDE) as substrates, enzymic monomethylation was also observed. However, the methylation of the lyso compounds was biphasic with an optimum at 0.75 mM lysoPME and 0.5 mM lysoPDE. In contrast to PME or PDE, the lysophospholipids produced a decrease in PC synthesis by lysoPME was reversible and was accompanied by an up to 4-fold increase in PDE synthesis. Competition experiments between lysoPME or lysoPDE and PME or PDE, together with kinetic studies, indicate a connection with methylation of both lyso- and diacylphospholipids. The same active site or sites in close proximity may serve for the second and third methylations. Hence, the presence of two acyl groups on the phospholipid molecule is not a prerequisite for N-methylation of this class of compounds. On the contrary, suspensions of phosphatidylethanolamine, or 2-lysophosphatidylethanolamine (lysoPE) with acyl chains of different degrees of saturation or with one alkenyl at the C1 position of the glycerol were not substrates for PE-N-methyltransferase; the lysoPEs were inhibitory above 0.5 mM.

Hormonal regulation of phospholipid methyltransferase by 3',5'-cyclic adenosine monophosphate-dependent and independent mechanisms

Treatment of isolated rat adipocytes with epinephrine or isoproterenol caused a time- and concentration-dependent increase in phospholipid methyltransferase (PLMT) activity that was blocked by propranolol and unaffected by phentolamine. Forskolin mimicked the stimulatory effect on PLMT, and insulin inhibited this effect. In both the absence and presence of insulin, there was a linear relationship between PLMT activity and lipolysis. PLMT activity was also increased in response to oxytocin, which does not activate adenylate cyclase in adipocytes and does not stimulate lipolysis. The effects of oxytocin were inhibited by insulin and were additive with those of isoproterenol on PLMT. These data support the hypothesis that in adipocytes, PLMT is activated by a cAMP-dependent protein kinase and a cAMP-independent mechanism, both of which can be regulated independently, and both of which are sensitive to inhibition by insulin.

Enhancement by L-methionine of contractile responses to acetylcholine and high KCl in uterine segment

The contractile responses of isolated uterine segments from 17 beta-estradiol-3-benzoate-treated ovariectomized rats to acetylcholine (ACh) and high KCl in Ca-depleted modified Locke-Ringer solution on addition of CaCl2 were used as indicators of Ca2+ influxes through ACh receptor- and voltage-operated Ca2+ channels, respectively. L-Methionine (L-Met) significantly enhanced these responses. The enhancement depended on the time of treatment with L-Met and concentration of L-Met. 3-Deazaadenosine (3-DAA) plus homocysteine thiolactone (HCTL), which inhibit S-adenosylmethionine-dependent methylation, caused dose-dependent inhibition of these contractile responses to ACh and high KCl. These inhibitory effects of 3-DAA plus HCTL were significantly attenuated in the presence of L-Met. Protein carboxylmethyltransferase and phospholipid methyltransferase activities were detected in the isolated uterine segment under conditions similar to those in which the contractile responses were observed. 3-DAA plus HCTL inhibited these enzyme activities. These findings suggest that S-adenosylmethionine-dependent methylations of protein and/or phospholipid in isolated uterine segment are involved in the contractile responses to ACh and high KCl in Ca-depleted modified Locke-Ringer solution on addition of CaCl2.

Phosphatidylethanolamine methyltransferase: evidence for influence of diet fat on selectivity of substrate for methylation in rat brain synaptic plasma membranes

Male weanling rats were fed diets containing 20% (w/w) fat differing in fatty acid composition for 24 days. Synaptic plasma membranes were isolated from the brain and the fatty acid composition of phosphatidylethanolamine and phosphatidylcholine was determined. In vitro assays of phosphatidylethanolamine methyl-transferase activity were performed on fresh membrane samples to assess effect of dietary fat on the rate of phosphatidylethanolamine methylation for phosphatidylcholine synthesis via the phosphatidylethanolamine methyltransferase pathway. Dietary level of n-6 and ratio of n-6 to n-3 fatty acids influenced membrane phospholipid fatty acid composition and activity of the lipid-dependent phosphatidylethanolamine methyltransferase pathway. Rats fed a diet rich in n-6 fatty acids produced a high ratio of n-6/n-3 fatty acids in synaptosomal membrane phosphatidylethanolamine, and elevated rates of methylation of phosphatidylethanolamine to phosphatidylcholine by phosphatidylethanolamine methyltransferases, suggesting that the pathway exhibits substrate selectivity for individual species of phosphatidylethanolamine containing long-chain homologues of dietary n-6 and n-3 fatty acids (20:4(n-6), 22:4(n-6), 22:5(n-6) and 22:6(n-3). It may be concluded that diet alters the membrane content of n-6, n-3 and monounsaturated fatty acids, and that change in phosphatidylethanolamine species available for methylation to phosphatidylcholine alters the rate of product synthesis in vivo by the phosphatidylethanolamine methyltransferase pathway.

Inhibition by insulin of glucagon-dependent phospholipid methyltransferase phosphorylation in rat hepatocytes

Addition of insulin to isolated rat hepatocytes prelabeled with [32P]phosphate inhibited glucagon-dependent phospholipid methyltransferase phosphorylation and activation. Insulin alone had no effect on either the phosphorylation of the enzyme or on its activity. The effect of insulin on glucagon-dependent phospholipid methyltransferase phosphorylation was dose-dependent and occurred at physiological doses of the hormone (10(-11)-10(-10) M). Analysis of 32P-labeled peptides after digestion with trypsin revealed only one site of phosphorylation regulated by glucagon (10(-8) M) in isolated rat hepatocytes. This site, as analyzed by HPLC and thin-layer chromatography, coincided with that phosphorylated by the cAMP-dependent protein kinase using purified rat liver phospholipid methyltransferase.

Biphasic changes in the sarcolemmal phosphatidylethanolamine N-methylation activity in catecholamine-induced cardiomyopathy

Phosphatidylethanolamine (PE) N-methylation activity was studied in rat heart sarcolemma at 1, 3, 9 and 24 h after an intraperitoneal injection of isoproterenol (40 mg/kg). Three reaction sites for PE N-methylation were examined by assaying the incorporation of radiolabeled methyl groups from S-adenosyl-L-methionine (AdoMet) into sarcolemmal PE molecules under optimal conditions. Total methylation activity at catalytic site I (studied by employing 0.055 microM AdoMet) was increased at 1 and 3 h after the isoproterenol injection and depressed at 24 h; 9 h samples showed no change. Similar biphasic alterations were seen for phosphatidyl-N-monomethylethanolamine, the major methylated product formed at site I. Alterations in the methylation activity at site I were associated with changes in Vmax values but the apparent affinity for AdoMet remained unaltered. No alterations were found in total methylation activities at sites II and III in isoproterenol treated preparations when studied by employing 10 and 150 microM AdoMet, respectively. An increase and a decrease in the PE N-methylation activity at site I were also observed in the sarcoplasmic reticular (microsomal) fraction from experimental hearts after 1 h and 24 h of the isoproterenol injection respectively, without changes at sites II and III. On the other hand, no changes were seen in the mitochondrial fraction. These results indicate biphasic alterations in the sarcolemmal and microsomal PE N-methylation activities during the development of catecholamine-induced cardiomyopathy.

Phosphatidylethanolamine methyltransferase and cAMP, cGMP phosphodiesterases in lymphocytes and monocytes in sarcoidosis

Among the various hypotheses proposed to explain immune cell defect in sarcoidosis, we examined thoroughly that of Faguet who described abnormalities of signal transmission at lymphocyte membrane level. Phosphatidylethanolamine methyltransferase and cAMP cGMP phosphodiesterases were studied in blood lymphocytes and monocytes from 8 subjects with sarcoidosis disease. Phosphatidylethanolamine methyltransferase (PMT1) plays an important regulatory role in membrane signal transmission. cAMP and cGMP phosphodiesterases (PDE) regulate cytoplasmic cyclic nucleotide levels and so participate in the modulation of the cell cycle. We observed a decreased PMT1 activity in lymphocytes and monocytes and a decreased cAMP and cGMP PDE activities in monocytes. It is not now possible to say if these abnormalities are primary or secondary. Whatever the origin of this dysfunctioning, these results evoke simultaneous disturbances of membrane signal transmission and cell cycle in monocytes and membrane abnormalities in lymphocytes. These abnormalities could explain some immune cell defects in sarcoidosis disease.

Stimulation of adipocyte phospholipid methyltransferase activity by phorbol 12-myristate 13-acetate. Differential regulation of phospholipid methyltransferase and lipolysis

The present studies demonstrate that treatment of rat adipocytes with the phorbol ester phorbol 12-myristate 13-acetate (PMA) causes a dose-dependent stimulation of phospholipid methyltransferase (PLMT) activity. The stimulatory effect of PMA was not additive with that of isoprenaline or forskolin. The sensitivity of stimulated PLMT activity to inhibition by insulin, however, was decreased in the presence of PMA. The inhibitory effect of a maximal concentration of insulin on PLMT was unchanged in the presence of PMA. In contrast with the effects on PLMT, the lipolytic response of adipocytes to isoprenaline and the anti-lipolytic response to insulin were unaffected by PMA. These data suggest that PLMT is, whereas hormone-sensitive lipase is not, an intracellular target for the action of PMA. The lack of effect of PMA on lipolysis suggests that PLMT and hormone-sensitive lipase can be regulated by separate mechanisms. Furthermore, phorbol esters do not interfere in the regulatory pathway whereby insulin inhibits PMLT or lipolysis.

Protein kinase C catalyses the phosphorylation and activation of rat liver phospholipid methyltransferase

When a partially purified rat liver phospholipid methyltransferase is incubated with [gamma-32P]ATP and rat brain protein kinase C, phospholipid methyltransferase (Mr 50,000, pI 4.75) becomes phosphorylated. Phosphorylation of the enzyme showed Ca2+/lipid-dependency. Protein kinase C-dependent phosphorylation of phospholipid methyltransferase was accompanied by an approx. 2-fold activation of the enzyme activity. Activity changes and enzyme phosphorylation showed the same time course. Activation of the enzyme also showed Ca2+/lipid-dependency. Protein kinase C mediates phosphorylation of predominantly serine residues of the methyltransferase. One major peak of phosphorylation was identified by analysis of tryptic phosphopeptides by isoelectrofocusing. This peak (pI 5.2) differs from that phosphorylated by the cyclic AMP-dependent protein kinase (pI 7.2), demonstrating the specificity of phosphorylation of protein kinase C. Tryptic-peptide mapping by h.p.l.c. of the methyltransferase phosphorylated by protein kinase C revealed one major peak of radioactivity, which could be resolved into two labelled phosphopeptides by t.l.c. The significance of protein kinase C-mediated phosphorylation of phospholipid methyltransferase is discussed.

Properties and topology of enzymes methylating phosphatidylethanolamine to phosphatidylcholine in sarcoplasmic reticulum

1. The synthesis of phosphatidylcholine (PC) by stepwise methylation of phosphatidylethanolamine (PE) is carried out by two enzymes in sarcoplasmic reticulum (SR) membrane of rabbit fast-twitch skeletal muscles. 2. Two methyltransferases (Met I and Met II) have a different pH optimum and affinity for methyl donor--S-adenosyl-L-methionine (SAM). 3. Met I is an integral SR membrane protein which active site faces the cytoplasmic surface of the membrane. 4. Met II is a peripheral, loosely bound protein, localized mainly on the extracytoplasmic (luminal) part of the SR membrane.

The polar head group of a novel insulin-sensitive glycophospholipid mimics insulin action on phospholipid methyltransferase

A phospholipid has been purified from rat liver membranes which copurified with an insulin-sensitive glycophospholipid isolated from H35 hepatoma cells. The polar head group of this phospholipid was generated by treatment with a phosphatidylinositol-specific phospholipase C from Staphylococcus aureus and purified through a C18 extraction column. Like insulin, the addition of this polar head group to isolated rat adipocytes inhibited the stimulatory effect of isoproterenol on phospholipid methyltransferase. The polar head group was also active on a subcellular fraction. The addition of the polar head group to microsomes isolated from isoproterenol-treated adipocytes produced a time-dependent inactivation of phospholipid methyltransferase, approaching basal activity. It is proposed that the effects of insulin on phospholipid methyltransferase may be mediated by this polar head group.

Estradiol activates methylating enzyme(s) involved in the conversion of phosphatidylethanolamine to phosphatidylcholine in rat pituitary membranes

17 beta-Estradiol (E2) affects the sensitivity of pituitary cells to several neurohormones as LHRH, TRH, or dopamine, presumably by modulating receptor coupling mechanisms. We attempted to pinpoint the membrane processes underlying this modulation and studied the effect of E2 on pituitary membrane phospholipid methylation. Anterior pituitary membranes prepared from ovariectomized (ovx) or ovx plus E2-treated rats were assayed for phospholipid methylation. Methylated phospholipids were separated by TLC. Incorporation of [3H]methyl groups into phospholipids increased with membrane concentration and incubation time with S-adenosyl-L-methyl [3H]methionine; it was not Mg2+ dependent and was inhibited in a dose-dependent manner by S-adenosyl-L-homocysteine, methyltransferase inhibitor. pH was found to be critical. Formation of phosphatidyl-monoethanolamine, phosphatidyl-dimethylethanolamine, and phosphatidylcholine was markedly stimulated by treatment with E2. The effect increased progressively when animals were killed 15 h to 5 days after E2 implantation. The response involved a shift in the maximum velocity (Vmax) although there was no change in the available substrate for the methylating enzyme. This change in Vmax probably reflects changes in the amount of the methylating enzyme itself. Administration of 17 alpha-estradiol, an inactive stereoisomer of E2 was ineffective, pointing to a stereospecific interaction. After differential centrifugation of pituitary membranes, the highest specific methyltransferase activity was found in light mitochondrial (L) and microsomal (P) fractions and the lowest in nuclei (N) and the heavy mitochondrial (M) fractions. After sucrose density gradient centrifugation, methylated phospholipids were preferentially recovered from fractions corresponding to the endoplasmic reticulum and/or secretory granules. E2 treatment for 5 days did not modify the subcellular distribution of methyltransferase activity but stimulated it in all fractions; in contrast, it did not modify the activity of the other enzymes measured as fraction markers. Under the same experimental conditions, phospholipid methylation in membranes prepared from cortex, and anterior and mediobasal hypothalamic structures was not affected by the steroid, with the exception of a slight increment of [3H]methyl incorporation into mediobasal hypothalamic membrane phospholipids after 5 days of E2 treatment. These results indicate that E2-induced changes in pituitary responsiveness might be concomitant with selective effects of the steroid on specific membrane enzymatic activities involved in coupling mechanisms.

Lymphocyte and granulocyte phosphatidylethanolamine N-methyltransferase: properties and activity in cystic fibrosis

Human lymphocyte and granulocyte membranes contain an enzyme, phosphatidylethanolamine N-methyltransferase (PEMT), which catalyzes the transfer of a methyl group from S-adenosylmethionine to the polar head group of phosphatidylethanolamine to form phosphatidylmonomethylethanolamine. This enzyme, in lymphocyte membranes, has Km for S-adenosylmethionine of 7.01 +/- 2.9 (SD) microM, and specific activity 0.57 +/- 0.31 pmol/mg protein/15 min, is inhibited by S-adenosylhomocysteine, displays optimal activity at pH 8.0-9.0, and is stimulated by isoproterenol in dose-dependent, propranolol-inhibitable fashion, to a lesser extent by epinephrine, but not by norepinephrine, prostaglandin E1, concanavalin A, or adenosine 3':5' cyclic monophosphate, with or without phosphodiesterase inhibitors. Granulocyte membrane PEMT has Km for S-adenosylmethionine of 4.4 microM and specific activity 0.54 +/- 0.51 pmol/mg protein/15 min, is inhibited by S-adenosylhomocysteine, displays optimal activity at pH 8.0-9.5, and is stimulated by isoproterenol greater than epinephrine greater than norepinephrine, but not by prostaglandin E1, serum-treated zymosan, formyl-methionyl-leucyl-phenylalanine, or adenosine 3':5' cyclic monophosphate. Because activation of PEMT reportedly contributes to several processes known to be abnormal in cystic fibrosis, including coupling of the beta-adrenergic receptor to adenylate cyclase, activity of PEMT was compared in lymphocyte and granulocyte membrane preparations from cystic fibrosis patients and healthy controls, in which abnormal coupling of beta-adrenergic receptor to adenylate cyclase had been demonstrated. For both cell types, the Km and specific activity of PEMT were comparable in normal and cystic fibrosis samples. Therefore, the hypothesis that reduced PEMT activity accounts for the impaired coupling of beta-adrenergic receptor to adenylate cyclase in lymphocytes and granulocytes in cystic fibrosis is rejected.

Alveolar macrophage membrane phospholipid methylation in sarcoidosis and hypersensitivity pneumonitis

Methylation of phospholipids seems to be an essential step in the recognition and transduction of regulatory signals by eukaryotic cells. Phosphatidylethanolamine methylation was compared in alveolar macrophage membrane from patients presenting with pulmonary sarcoidosis or hypersensitivity pneumonitis and control subjects. Phosphatidylethanolamine methyltransferase (PMT1) activity was determined by various measures of incorporation of tritiated methyl group from (3H) S-adenosyl-L-methionine in membrane phospholipids. Tritiated methyl group incorporation in macrophage membrane was higher in some patients presenting with sarcoidosis or hypersensitivity pneumonitis, than in controls. PMT1 activity was found to be higher in sarcoidosis patients with a positive gallium lung scan. As lipids play an important role during macrophage activation and cell interaction, although a wide heterogeneity was observed in PMT1 activity, increased membrane phospholipid methylation seems to be an important feature in pulmonary diseases where macrophages are involved.

Purification of phospholipid methyltransferase from rat liver microsomal fraction

Phospholipid methyltransferase, the enzyme that converts phosphatidylethanolamine into phosphatidylcholine with S-adenosyl-L-methionine as the methyl donor, was purified to apparent homogeneity from rat liver microsomal fraction. When analysed by SDS/polyacrylamide-gel electrophoresis only one protein, with molecular mass about 50 kDa, is detected. This protein could be phosphorylated at a single site by incubation with [alpha-32P]ATP and the catalytic subunit of cyclic AMP-dependent protein kinase. A less-purified preparation of the enzyme is mainly composed of two proteins, with molecular masses about 50 kDa and 25 kDa, the 50 kDa form being phosphorylated at the same site as the homogeneous enzyme. After purification of both proteins by electro-elution, the 25 kDa protein forms a dimer and migrates on SDS/polyacrylamide-gel electrophoresis with molecular mass about 50 kDa. Peptide maps of purified 25 kDa and 50 kDa proteins are identical, indicating that both proteins are formed by the same polypeptide chain(s). It is concluded that rat liver phospholipid methyltransferase can exist in two forms, as a monomer of 25 kDa and as a dimer of 50 kDa. The dimer can be phosphorylated by cyclic AMP-dependent protein kinase.

Inhibition of adenylate cyclase by transglutaminase-catalyzed reactions in pigeon erythrocyte ghosts

We report the occurrence in pigeon erythrocytes of a soluble Ca2+-dependent transglutaminase (TGase) activity. The effect of the erythrocyte ghost protein modifications, determined by TGase-catalyzed reactions, on adenylate cyclase, phospholipid methyltransferase I and II activities and on the lipidic matrix fluidity of the membrane was investigated by using a purified guinea pig liver TGase preparation. The results showed a significant inhibitory effect of such modifications both on the basal and on the variously stimulated (by NaF, Gpp(NH)p alone or in the presence of 1-isoproterenol) adenylate cyclase activity. By contrast, both the phospholipid methylation and the fluidity of the lipidic matrix of the membrane were unaffected by TGase-mediated reactions. These data suggest a new possible inhibitory mechanism of the cyclic AMP synthesis which might be triggered by the enhancement of the cytosolic Ca2+ concentration.

Regulation of rat liver phosphatidylethanolamine N-methyltransferase by cytosolic factors. Examination of a role for reversible protein phosphorylation

The nature of cytosolic factors which modulate the activity of rat liver phosphatidylethanolamine (PE) methyltransferase was investigated. The combined additions of cytosol, Mg X ATP, and NaF to incubations with rat liver microsomes produced a 1.6-fold activation of the methyltransferase at pH 9.2 and a 1.3-fold stimulation at pH 7.0. Nonhydrolyzable 5'-adenylylimidodiphosphate could not substitute for ATP, although GTP could. The activation was time dependent, stable to reisolation of the microsomes by ultracentrifugation, and partially preventable by other cytosolic components. Despite these indications that PE methyltransferase might be a substrate for cytosolic protein kinases, cAMP and Ca2+-calmodulin exerted little influence on the activation reaction. Furthermore, microsomal PE methyltransferase activity was unaffected by purified preparations of cAMP-dependent protein kinase, calmodulin-dependent protein kinase, and casein kinase II, nor was methyltransferase activity influenced by the purified catalytic subunits of protein phosphatases 1 and 2A. Cytosol also contained inhibitors of PE methyltransferase which could overcome the Mg X ATP X NaF-mediated activation of the enzyme, but were not affected by the thermostable phosphatase inhibitors 1 and 2. Part of this inhibitory activity (apparent molecular mass of 15 X 10(3) daltons) was insensitive to trypsin and chymotrypsin, stimulated by Mn2+, and partly inhibited by NaF. Therefore, regulation of methyltransferase by reversible phosphorylation, while still a tenable hypothesis, is apparently more complex than previously proposed.

Stimulation of phosphatidylcholine synthesis by insulin and ATP in isolated rat adipocyte plasma membranes

Added individually or together, insulin and/or ATP significantly and rapidly increased the concentration of phosphatidylcholine in an enriched plasma membrane preparation from rat adipocytes. The increase in phosphatidylcholine synthesis mediated by insulin or ATP was suppressed by the phospholipid methyltransferase inhibitor, S-adenosylhomocysteine. These results suggest that the activity of phospholipid methyltransferase from adipocyte plasma membranes may be increased by phosphorylation and that insulin may further increase the activity of the phosphorylated phospholipid methyltransferase by an alternative pathway.

S-adenosyl-L-methionine modulates phosphatidylethanolamine methyltransferase response to isoproterenol in brain

The effect of l-isoproterenol on S-adenosyl-L-methionine (Adomet)-mediated phospholipid methylation in rat brain cortex was examined. Under conditions which favor the activity of methyltransferase I (i.e. low Adomet), formation of phosphatidyl-N-methylethanolamine was inhibited by l-isoproterenol. On the other hand, methyltransferase II activity (i.e. high Adomet) was stimulated by l-isoproterenol. The results suggest that Adomet may play a regulatory role in the response of phospholipid methyltransferases to catecholamines in brain.

Phosphatidylcholine homeostasis in phosphatidylethanolamine-depleted Tetrahymena

The relative contributions of the two pathways of phosphatidylcholine biosynthesis, phosphatidylethanolamine N-methyltransferase (EC 2.1.1.17) and diacylglycerol: CDP-choline cholinephosphotransferase (EC 2.7.8.1), are altered in the ciliate protozoan Tetrahymena thermophila whose phospholipid composition has been modified by culturing the organism in the presence of one of several aminophosphonic acids, as determined by measuring the incorporation of [methyl-3H]choline and [methyl-14C]methionine into phosphatidylcholine in vivo. In control cells the phosphotransferase pathway provides about 40% of the phosphatidylcholine, while in cells grown with 2-aminoethylphosphonate (AEP), 3-aminopropylphosphonate (APP), and N,N,N-trimethylaminoethyl-phosphonate (TMAEP) the contribution of the phosphotransferase pathway to phosphatidylcholine formation is 75, 90, and 26%, respectively. In AEP- and APP-grown cells, in which 80% of the phosphatidylethanolamine has been replaced by the corresponding phosphonolipid, the methyltransferase is less active since the level of the substrate phosphatidylethanolamine is reduced and neither of the phosphonolipids is a substrate for the enzyme. In TMAEP-grown cells, TMAEP competes with and reduces the incorporation of phosphocholine by the phosphotransferase pathway, leading to a smaller contribution of the pathway to phosphatidylcholine biosynthesis. The relative amounts of the two different radioactive labels incorporated into diacylphosphatidylcholine vs alkylacylphosphatidylcholine are also altered in the phosphonate-grown cells. The exogenous AEP induces a change in the glyceryl ether content of the 2-aminoethylphosphonolipid--33% in the AEP-grown cells compared to 70% in the control cells--indicating that the exogenous AEP is entering the phospholipids by the ethanolamine-phosphotransferase pathway rather than by the route of the endogenous AEP.

Premalignant alterations in the lipid composition and fluidity of colonic brush border membranes of rats administered 1,2 dimethylhydrazine

Dimethylhydrazine (DMH) is a potent procarcinogen with selectivity for the colon. To determine whether alterations in the lipid composition and fluidity of rat colonic brush border membranes existed before the development of DMH-induced colon cancer, rats were injected s.c. with this agent (20 mg/kg body weight per wk) or diluent for 5, 10, and 15 wk. Animals were killed at these time periods and brush border membranes were prepared from proximal and distal colonocytes of each group. The "static" and "dynamic" components of fluidity of each membrane were then assessed, by steady-state fluorescence polarization techniques using limiting hindered fluorescence anisotropy and order parameter values of the fluorophore 1,6 diphenyl-1,3,5-hexatriene (DPH) and fluorescence anisotropy values of DL-2-(9-anthroyl) stearic acid and DL-12-(9-anthroyl) stearic acid, respectively. Membrane lipids were extracted and analyzed by thin-layer chromatography and gas-liquid chromatography. Phospholipid methylation activity in these membranes was also measured using S-adenosyl-L-methionine as the methyl donor. The results of these studies demonstrate that: the lipid composition and both components of fluidity of proximal DMH-treated and control membranes and their liposomes were similar at all time periods examined; at 5, 10, and 15 wk the "dynamic component of fluidity" of distal DMH-treated membranes and their liposomes was found to be higher, similar, and lower, respectively, than their control counterparts; the "static component of fluidity" of distal DMH-treated membranes and their liposomes, however, was similar to control preparations at all three time periods; and alterations in the lipid composition and phospholipid methylation activities appeared to be responsible for these differences in the "dynamic component of fluidity" at these various time periods.

Subcellular and submitochondrial localization of phospholipid-synthesizing enzymes in Saccharomyces cerevisiae

Using highly enriched membrane preparations from lactate-grown Saccharomyces cerevisiae cells, the subcellular and submitochondrial location of eight enzymes involved in the biosynthesis of phospholipids was determined. Phosphatidylserine decarboxylase and phosphatidylglycerolphosphate synthase were localized exclusively in the inner mitochondrial membrane, while phosphatidylethanolamine methyltransferase activity was confined to microsomal fractions. The other five enzymes tested in this study were common both to the outer mitochondrial membrane and to microsomes. The transmembrane orientation of the mitochondrial enzymes was investigated by protease digestion of intact mitochondria and of outside-out sealed vesicles of the outer mitochondrial membrane. Glycerolphosphate acyltransferase, phosphatidylinositol synthase, and phosphatidylserine synthase were exposed at the cytosolic surface of the outer mitochondrial membrane. Cholinephosphotransferase was apparently located at the inner aspect or within the outer mitochondrial membrane. Phosphatidate cytidylyltransferase was localized in the endoplasmic reticulum, on the cytoplasmic side of the outer mitochondrial membrane, and in the inner mitochondrial membrane. Inner membrane activity of this enzyme constituted 80% of total mitochondrial activity; inactivation by trypsin digestion was observed only after preincubation of membranes with detergent (0.1% Triton X-100). Total activity of those enzymes that are common to mitochondria and the endoplasmic reticulum was about equally distributed between the two organelles. Data concerning susceptibility to various inhibitors, heat sensitivity, and the pH optima indicate that there is a close similarity of the mitochondrial and microsomal enzymes that catalyze the same reaction.

Vasopressin-stimulated phosphorylation of rat liver phospholipid methyltransferase in isolated hepatocytes

Addition of vasopressin (1 microM) to isolated rat hepatocytes prelabeled with [32P]phosphate was accompanied by a 250% increase in the phosphorylation of phospholipid methyltransferase. Vasopressin-stimulated phospholipid methyltransferase phosphorylation was time- and dose-dependent. 32P-labeled phospholipid methyltransferase was recovered by immunoprecipitation and SDS-polyacrylamide gel electrophoresis. After electrophoresis, phospholipid methyltransferase was electroeluted from the polyacrylamide gel and subjected to tryptic digestion or HCl hydrolysis. Analysis of 32P-labeled peptides reveals only one site of phosphorylation and the analysis of [32P]phosphoamino acids indicates that phosphoserine is the only labeled amino acid.

Phospholipid methylation in rabbit aorta

The formation of phosphatidylcholine by successive methylations of phosphatidylethanolamine using S-adenosylmethionine as the methyl donor was studied in homogenates of rabbit aorta. Addition of phosphatidylmonomethylethanolamine and phosphatidyldimethylethanolamine, but not phosphatidylethanolamine, stimulated methyltransferase activity and this activity was further stimulated when the phospholipids were dispersed in taurocholate prior to addition to the assay system. No incorporation of radiolabel into sphingomyelin or lysolecithin was detected indicating minimal metabolism of newly formed phosphatidylcholine. The majority of methyltransferase activity was detected in the high-speed pellet of the aortic homogenate; however, since activity was also detected in the high-speed supernatant, the low-speed supernatant preparation was used as the source of enzyme. Methyltransferase activity was characterized in cultured arterial smooth muscle cells using methionine as the radiolabeled precursor. The major product formed was phosphatidylcholine. No difference in enzyme activity was seen as a function of the length of time that cells were in culture or anatomic location of the aortic explant used as a source of cells. Treatment of the cells with cycloheximide did not affect methyltransferase activity. The ability of catecholamine agonists and vasoactive peptides to influence methyltransferase activity was investigated both in the cell-free preparation and in cultured cells. These compounds did not appear to alter methyltransferase activity in rabbit aortic smooth muscle cells.

Phosphatidylethanolamine methyltransferase activity in chick liver microsomes

The synthesis of phosphatidylcholine from phosphatidylethanolamine is carried out by chick liver microsomes (Gallus domesticus). Different concentrations of PE, NPE and NNPE were used as exogenous substrates. Saturation of the S-adenosylmethionine has been found for the three different reactions with or without exogenous substrate. Kinetic parameters have been determined for this enzyme system in chick liver microsomes. The three methyl reactions had a similar pH profile with an optimum at pH = 8. Divalent ions such as Ca2+ or Mg2+ did not stimulate the enzyme activity. The results suggest that the synthesis of phosphatidylcholine from phosphatidylethanolamine by chick liver microsomes exhibits a kinetic pattern with different aspects than that described for other animal or human preparations.

The effect of copper deficiency on heart microsomal phosphatidylcholine biosynthesis and concentration

The effect of dietary copper deficiency on phosphatidylcholine biosynthetic enzymes, phosphatidylethanolamine methyltransferase, phosphatidyldimethyltransferase and choline phosphotransferase of heart microsomes was measured in rats. The data indicated that dietary copper deficiency can alter phosphatidylcholine biosynthesis and concentration in microsomal membranes of the heart. There was a significant decrease in the specific activity of choline phosphotransferase. There was a significant decrease in the concentration of total phospholipid-P, phosphatidylcholine-P, phosphatidylethanolamine-P, phosphatidylinositol-P, sphingomyelin-P and cardiolipin-P in the microsomes of the copper deficient animals. There was a significant decrease in the concentration of copper in microsomes of heart and liver in the copper deficient animals.

Modulation by the ratio S-adenosylmethionine/S-adenosylhomocysteine of cyclic AMP-dependent phosphorylation of the 50 kDa protein of rat liver phospholipid methyltransferase

The present results show that the catalytic subunit of cyclic AMP-dependent protein kinase phosphorylates the 50 kDa protein of rat liver phospholipid methyltransferase at one single site on a serine residue. Phosphorylation of this site is stimulated 2- to 3-fold by S-adenosylmethionine. S-adenosylmethionine-dependent protein phosphorylation is time- and dose-dependent and occurs at physiological concentrations. S-adenosylhomocysteine has no effect on protein phosphorylation but inhibits S-adenosylmethionine-dependent protein phosphorylation. S-Adenosylmethionine/S-adenosylhomocysteine ratios varying from 0 to 5 produce a dose-dependent stimulation of the phosphorylation of the 50 kDa protein. In conclusion, these results show, for the first time, that the ratio S-adenosylmethionine/S-adenosylhomocysteine can modulate phosphorylation of a specific protein.

Developmental changes in the activity of phosphatidylethanolamine N-methyltransferases in rat brain

The activity of phosphatidylethanolamine N-methyltransferase (PeMT), an enzymic system that catalyses the synthesis of phosphatidylcholine (PtdCho) via sequential methylation of phosphatidylethanolamine (PtdEtn) using S-adenosylmethionine (AdoMet) as a methyl donor, was examined in brain homogenates from rats of various ages. The data thus obtained were consistent with the existence of two distinct enzyme activities within this enzyme system, i.e. one catalysing the methylation of PtdEtn [to form phosphatidyl-N-monomethylethanolamine (PtdMeEtn)], and the other catalysing the methylations of PtdMeEtn and phosphatidyl-NN-dimethylethanolamine (PtdMe2Etn) (to form PtdMe2Etn and PtdCho, respectively). PeMT (PtdEtn-methylating) activity per g of brain was 4-fold higher in neonatal than in adult brains. The enzyme activity in adult brains exhibited Michaelis-Menten kinetics for AdoMet, and its affinity for AdoMet was high (apparent Km 1.6 microM). In neonatal brain the relationships between AdoMet concentrations and PtdMeEtn formation were more complex: a sigmoidal component (with a Hill coefficient of 2.7), requiring 90 microM-AdoMet for half-saturation predominated over the high-affinity component (similar to that of the adult brain). PeMT (PtdMe2Etn-methylating) activity per g of brain increased 2-fold between the 5th and the 20th postnatal days and remained constant thereafter; it was higher than that of PeMT (PtdEtn-methylating) activity at all ages studied, and its affinity for AdoMet was low (apparent Km 99 microM). No sexual dimorphism in brain PeMT activity was observed at any age. We conclude that PeMT (PtdEtn-methylating) catalyses the rate-limiting step in PtdCho synthesis in rat brain, and that PtdCho formation via this pathway may be greatest during the neonatal period.

Subcellular localization of phosphatidylethanolamine N-methylation activity in rat heart

Synthesis of phosphatidylcholine (PC) by S-adenosyl-L-methionine (AdoMet)-dependent methylation of phosphatidylethanolamine (PE) has been recently characterized in rat heart sarcolemma obtained by hypotonic shock-LiBr treatment method. The present study, employing different procedures for the isolation of purified cardiac sarcolemmal membranes in rat, confirms the existence of three catalytic sites which are specifically involved in the sequential methyl transfer reactions from PE to PC. Other subcellular organelles such as sarcoplasmic reticulum (microsomes) and mitochondria showed methyltransferase activity which was absent in myofibrils and in cytosolic fraction. Experiments with several concentrations of AdoMet revealed that the kinetic pattern of methyltransferase activity in both microsomes and mitochondria was comparable to that obtained in sarcolemma. In addition, the characteristics of three catalytic sites as identified by the synthesis of phosphatidyl-N-monomethylethanolamine, phosphatidyl-N,N-dimethylethanolamine and PC in these subcellular organelles were similar to those of sarcolemma. The results are consistent with the view that methyltransferase activity is localized in different membrane systems of the myocardium.

The effect of polar head group substitution on phospholipid methylation and the beta-adrenergic response in C6 glial cells

The membranes of intact C6 cells were enriched with phosphatidyldimethylethanolamine or phosphatidylmonomethylethanolamine. These cells showed enhanced rates of phospholipid methylation but this was not accompanied by an increased beta-adrenergic response. We conclude that phospholipid methylation is not coupled to the activation of adenylate cyclase in the beta-adrenergic response of C6 glial cells.

Phospholipid methyltransferase activity in pancreatic islets: activation by calcium

Pancreatic islet homogenates contain a Mg2+-requiring phospholipid methyltransferase activity, the activity of which was doubled by calcium (K0.5 less than 5 microM). Other divalent metal ions stimulated the activity from 11 to 35%, but zinc and strontium were inhibitory. Cyclic AMP had no effect on the enzyme activity and cyclic GMP inhibited it slightly. Calcium increased the Vmax of the enzyme without affecting its Km with respect to S-adenosylmethionine (6 microM). Chlorpromazine, trifluoperazine, and dibucaine inhibited the calcium-stimulatable activity without affecting the activity in the absence of calcium. Phosphatidylserine stimulated, and arachidonic acid and palmitic acid inhibited, the basal enzyme activity. The methylated products were found to be primarily mono- and dimethylphosphatidylethanolamine (30%) and phosphatidylcholine (43%) and an, as yet unidentified, nonpolar lipid fraction (27%), as judged by thin-layer chromatography. In the presence of calcium, incorporation of methyl groups into phosphatidylcholine, mono- and dimethylphosphatidylethanolamine, and nonpolar lipids was increased by 131, 60, and 46%, respectively. Based on the localization of the enzyme activity in the insulin secretory granule fraction, it is proposed that phospholipid methylation plays a role in coupling the stimulus to the initial events in insulin secretion, leading to the exocytosis of insulin.

A component of genetic variation among mice in activity of transmembrane methyltransferase I determined by the H-2 region

The effects of the mouse major histocompatibility complex, H-2, on phospholipid methyltransferase I and II activities were investigated on hepatocyte membranes from inbred, congenic and recombinant strains. Each methyltransferase was assayed individually by measuring the incorporation of radiolabel from S-adenosyl-L-[methyl-3H]methionine into endogenous phospholipids. Our results indicate that H-2 exerted a significant effect on methyltransferase I but not on methyltransferase II activity. Thus, as in lower eukaryotes, two distinct enzymes were involved in methylation of phosphatidylethanolamine (PE) to phosphatidylcholine (PC). In addition, this effect was localized to the K end of the major histocompatibility complex by the use of recombinant haplotypes.

Continuous growth and phosphatidylcholine synthesis of rat hepatoma cells in choline-deprived chemically defined medium

A choline non-required cell line was established from a rat hepatoma cell line. The line designated R-Y121B X cho was able to grow in choline-deprived medium without serum and lipid. Choline is necessary for the biosynthesis of phosphatidylcholine, which is a main component of cell membranes. Phosphatidylcholine can be synthesized by the methylation of phosphatidylethanolamine in liver cells. Phospholipid composition and incorporation of radiolabeled serine into phosphatidylserine or phosphatidylethanolamine were quite similar in R-Y121B X cho and its parental cells. However R-Y121B X cho cells had higher phosphatidylcholine synthesis activity from radiolabeled methionine than parental cells. These results indicate that choline requirement of mammalian cells depends on the activity of phosphatidylethanolamine methyltransferase.

Rat brain phosphatidyl-N,N-dimethylethanolamine is rich in polyunsaturated fatty acids

Phosphatidyl-N,N-dimethylethanolamine (PDME), an intermediate in the formation of phosphatidylcholine (PC) by the sequential methylation of phosphatidylethanolamine (PE), was purified from rat brain and its fatty acid (FA) composition compared with those of brain PC and PE. The proportion of polyunsaturated fatty acids (PUFAs) in the PDME (29.8%) was similar to that of PE (27.7%) and much greater than in PC (2.8%). Like the PUFAs of PE, the major PUFAs found in PDME were arachidonic acid (20:4) and docosahexaenoic acid (22:6). An isotopic method was developed to quantify the PDME purified from brain; a tritiated methyl group from CH3I was transferred to the PDME in the presence of cyclohexylamine to form [3H]PC, and the radioactivity of the PC was then counted. The concentration of rat brain PDME obtained using this method (33.0 +/- 1.8 micrograms/g brain) was very similar to that obtained using quantitative GLC analysis of its FAs (36.9 +/- 1.8 micrograms/g). The FAs in the PE and PC of rat brain synaptosomes were also analyzed; too little PDME was present in synaptosomes to permit similar analysis. The percentage of unsaturated FAs insynaptosomal PE was even higher (43.4 vs. 27.7) than that in PE prepared from whole brain. Since synaptosomes have a very high activity of phosphatidyl-N-methyltransferase, the enzyme complex that methylates PE to form PC, this enzyme may serve, in nerve endings, to produce a particular pool of PC, rich in PUFAs, which may have a distinct physiological function.

Sterol control of the phosphatidylethanolamine-phosphatidylcholine conversion in the yeast mutant GL7

The relatively slow growth rate of the yeast mutant GL7, a sterol auxotroph, on medium containing cholesterol is markedly accelerated by supplementation with small amounts of ergosterol. Under these conditions (sterol synergism) cellular phospholipid synthesis is enhanced. We now find that one of the ergosterol-stimulated processes is the methylation of phosphatidylethanolamine to phosphatidylcholine. This is shown by comparing methyltransferase activities of membrane preparations derived from cells grown on either ergosterol, cholesterol, or the synergistic sterol pair. Incorporation of 32P from [gamma-32P]ATP into the yeast membranes is rapid and greater when ergosterol-grown cells rather than cholesterol-grown cells are the source of membranes.

Phospholipid methyltransferase phosphorylation by intact hepatocytes: effect of glucagon

We have obtained a rabbit antiserum that specifically immunoprecipitates the 50K and 25K proteins of rat liver phospholipid methyltransferase. Exposure of intact rat hepatocytes preincubated with [32P]phosphate to glucagon induces a time-dependent phosphorylation of the 50K protein of phospholipid methyltransferase. The incorporation of 32P into the 50K protein was only on phosphoserine. These data support the concept that the activation of rat liver phospholipid methyltransferase by glucagon is mediated by phosphorylation of the enzyme.

The phospholipid-N-methyltransferase of rat brain microsomes

The phospholipid-N-methyltransferase activity of rat brain microsomes had an optimum pH of 11.0 in the absence or presence of phosphatidylethanolamine (PE) but pH 10.0 in the presence of phosphatidylmonomethylethanolamine (PMME) or phosphatidyldimethylethanolamine (PDME). An apparent Km for S-adenosyl methonine from 0.10 to 0.12 mM was observed with exogenous methylated phospholipids PMME or PDME. Methylated neutral lipid was the major lipid produced in the absence of the exogenous acceptors. Two exogenous phospholipids, PMME and PDME, significantly stimulated microsomal phospholipid-N-methyltransferase activity and the predicted methylated phospholipids were the major products. PE additions did not cause any stimulation of methylated lipid formation. Preincubation of particles at temperatures from 40 to 100 degrees C resulted in a loss in the microsomal phospholipid-N-methyltransferase activity that was stimulated by PMME and PDME.

Phospholipid methylase activity, [3H]S-adenosyl-L-homocysteine binding, and S-adenosyl-L-methionine and S-adenosyl-L-homocysteine levels in rat brain during maturation

The changes in activity of phospholipid methyltransferase I and [3H]S-adenosyl-L-homocysteine ([3H]SAH) binding were determined in cortical membrane preparations from newborn rats and rats 1, 2, and 8 months old. The activity of phospholipid methyltransferase I and the [3H]SAH binding were significantly greater (respectively, +30 and +40%) in newborn rats than in 1-, 2-, and 8-month-old rats. The methylated products at days 1 and 30 were identical. These changes in methyltransferase activity may be correlated with variations in concentration of S-adenosyl-L-methionine (SAM) and SAH. The endogenous SAM level was higher and the SAH level was lower in newborn compared with adult rats. These data suggested that the processes of methylation were favored in newborn rats. The modifications observed after treatment with L-homocysteine reinforced this hypothesis.

Phosphatidylcholine produced in rat synaptosomes by N-methylation is enriched in polyunsaturated fatty acids

Rat brain synaptosomes contain an enzyme, phosphatidylethanolamine N-methyltransferase (EC 2.1.1.17), that catalyzes the methylation of phosphatidylethanolamine to form its mono-, di-, and trimethyl (phosphatidylcholine) derivatives. Synaptosomal phosphatidylethanolamine is much richer in polyunsaturated fatty acids (43.4%) than is synaptosomal phosphatidylcholine (4.6%). It thus seemed possible that the phosphatidylcholine derived via the N-methylation of phosphatidylethanolamine might also be especially enriched in polyunsaturated fatty acids. To test this hypothesis, we examined the incorporation of [3H]methyl groups into various molecular species of phosphatidylcholine, by incubating rat synaptosomes for 10, 30, or 90 min in a medium containing S-adenosyl[methyl-3H]methionine. Phosphatidylcholine was extracted and separated from other lipids by TLC, after which its molecular species were isolated by argentation TLC (which distinguishes among the phospholipid molecules by the degree of unsaturation of their fatty acid moieties.) We found that approximately 65% of the [3H]methyl incorporated into phosphatidylcholine during the incubation period was present in the fraction associated with pentaene or hexaene fatty acids; an additional 30% was present in the tetraene fraction, while the remaining phosphatidylcholine radioactivity was distributed between the dienes and monoenes. Similar distributions were observed among synaptosomes incubated for 10 or 30 min; however, after 90 min the phosphatidyl[3H]choline contained proportionately less of the tetraenes. These observations indicate that neuronal phosphatidylcholine molecules formed via N-methylation are especially richer in polyunsaturated fatty acids, and they raise the possibility that these molecules constitute a distinct pool with particular physiologic functions.