Pitfalls and problems in studies on the methylation of phosphatidylethanolamine

Phospholipid methylation in mammals: from biochemistry to physiological function

Phosphatidylcholine is made in the liver via the CDP-choline pathway and via the conversion of phosphatidylethanolamine to phosphatidylcholine by 3 transmethylation reactions from AdoMet catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). PEMT is a 22.3kDa integral transmembrane protein of the endoplasmic reticulum and mitochondria-associated membranes. The only tissue with quantitatively significant PEMT activity is liver; however, low levels of PEMT in adipocytes have been implicated in lipid droplet formation. PEMT activity is regulated by the concentration of substrates (phosphatidylethanolamine and AdoMet) as well as the ratio of AdoMet to AdoHcy. Transcription of PEMT is enhanced by estrogen whereas the transcription factor Sp1 is a negative regulator of PEMT transcription. Studies with mice that lack PEMT have provided novel insights into the function of this enzyme. PEMT activity is required to maintain hepatic membrane integrity and for the formation of choline when dietary choline supply is limited. PEMT is required for normal secretion of very low-density lipoproteins. The lack of PEMT protects against diet-induced atherosclerosis in two mouse models. Most unexpectedly, mice that lack PEMT are protected from diet-induced obesity and insulin resistance. Moreover, mice lacking PEMT have increased susceptibility to diet-induced fatty liver and steatohepatitis. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Alteration of ethanolamine glycerophospholipid turnover in trembler dysmyelinating mutant: An analysis of the sciatic nerve by biochemistry and radioautography

The turnover of phospholipids was compared in peripheral nerves of Trembler dysmelinating mutant and control mice, after intraperitoneal and local injection of labeled ethanolamine. In the mutant sciatic nerve, neurochemical analysis showed that [(14)C]ethanolamine is incorporated into EGP (ethanolamine glycerophospholipids) of the sciatic nerve at a much higher rate in Trembler mutant than in control mice. Furthermore the decay rate of (14)C-labeled EGP is faster in Trembler than in normal animals. The accelerated turnover of EGP in Trembler sciatic nerve affects the diacyl-EGP while the renewal of the alkenylacyl-EGP (plasmalogens) is slower than in controls. Quantitative radioautographic study at the ultrastructural level corroborate that the initial increase of the label in Trembler nerve fibers was different in axons, Schwann cells and myelin sheaths. EM radioautographs reveal indeed that the high label content observed in Trembler axons takes place preferentially in the myelinated portions of axons and drops within 1 week. In both myelinated and unmyelinated segments of the axons, the majority of the radioactivity was contained in axolemma and smooth axoplasmic reticulum. The 10-fold increase of label found in the myelin sheath of Trembler nerve fibers at 1 day raises the question of the origin of the labeled EGP, either by a stimulated synthesis in Schwann cells or by transfer from axonally transported phospholipids. In contrast, the label of axons, Schwann cells and myelin sheaths of control nerve remains stable during the same period.

Physiological roles of phosphatidylethanolamine N-methyltransferase

Phosphatidylethanolamine N-methyltransferase (PEMT) catalyzes the methylation of phosphatidylethanolamine to phosphatidylcholine (PC). This 22.3 kDa protein is localized to the endoplasmic reticulum and mitochondria associated membranes of liver. The supply of the substrates AdoMet and phosphatidylethanolamine, and the product AdoHcy, can regulate the activity of PEMT. Estrogen has been identified as a positive activator, and Sp1 as a negative regulator, of transcription of the PEMT gene. Targeted inactivation of the PEMT gene produced mice that had a mild phenotype when fed a chow diet. However, when Pemt(-/-) mice were fed a choline-deficient diet steatohepatitis and liver failure developed after 3 days. The steatohepatitis was due to a decreased ratio of PC to phosphatidylethanolamine that caused leakage from the plasma membrane of hepatocytes. Pemt(-/-) mice exhibited attenuated secretion of very low-density lipoproteins and homocysteine. Pemt(-/-) mice bred with mice that lacked the low-density lipoprotein receptor, or apolipoprotein E were protected from high fat/high cholesterol-induced atherosclerosis. Surprisingly, Pemt(-/-) mice were protected from high fat diet-induced obesity and insulin resistance compared to wildtype mice. If the diet were supplemented with additional choline, the protection against obesity/insulin resistance in Pemt(-/-) mice was eliminated. Humans with a Val-to-Met substitution in PEMT at residue 175 may have increased susceptibility to nonalcoholic liver disease. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Phosphatidylethanolamine N-methyltransferase (PEMT) gene expression is induced by estrogen in human and mouse primary hepatocytes

Choline is an essential nutrient for humans, though some of the requirement can be met by endogenous synthesis catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). Premenopausal women are relatively resistant to choline deficiency compared with postmenopausal women and men. Studies in animals suggest that estrogen treatment can increase PEMT activity. In this study we investigated whether the PEMT gene is regulated by estrogen. PEMT transcription was increased in a dose-dependent manner when primary mouse and human hepatocytes were treated with 17-beta-estradiol for 24 h. This increased message was associated with an increase in protein expression and enzyme activity. In addition, we report a region that contains a perfect estrogen response element (ERE) approximately 7.5 kb from the transcription start site corresponding to transcript variants NM_007169 and NM-008819 of the human and murine PEMT genes, respectively, three imperfect EREs in evolutionarily conserved regions and multiple imperfect EREs in nonconserved regions in the putative promoter regions. We predict that both the mouse and human PEMT genes have three unique transcription start sites, which are indicative of either multiple promoters and/or alternative splicing. This study is the first to explore the underlying mechanism of why dietary requirements for choline vary with estrogen status in humans.

Phospholipid methylation in skeletal muscle membranes

Phospholipid methylation is thought to modulate such vital cellular processes as calcium transport, receptor function, and membrane microviscosity. As these processes are fundamental to the function of muscle cells and are thought to be altered in disease states, we have characterized several features of phospholipid methylation reactions in skeletal muscle and have defined appropriate assay conditions. In rat leg muscle, methyltransferase activity was assayed radiometrically by measuring the incorporation of methyl groups from S-adenosyl-L-[methyl-3H]methionine into membrane phospholipids, the methylated derivatives of which were separated by thin-layer chromatography. Contrary to previous investigations of whole muscle, phospholipid methyltransferase activity was clearly present in skeletal muscle membranes, being highly localized in sarcoplasmic reticulum and present to a lesser extent in sarcolemma. Both the reaction products and the reaction kinetics were consistent with sequential methylation of phospholipids by two methyltransferase enzymes. S-adenosylhomocysteine and its analogues were potent inhibitors of phospholipid methylation in sarcoplasmic reticulum. The predominant localization of phospholipid methyltransferase activity in sarcoplasmic reticulum suggests that its functional role in skeletal muscle may be in calcium transport.

Dietary polyunsaturated fatty acids in gestation alter fetal cortical phospholipids, fatty acids and phosphatidylserine synthesis

Docosahexaenoic acid (DHA; 22:6 n-3) is high in brain phosphatidylserine (PS) and phosphatidylethanolamine (PE), but low in phosphatidylcholine (PC). PS is synthesized from PE or PC by exchange of ethanolamine or choline for serine. PS can be decarboxylated to PE, and PC is synthesized from PE by phosphatidylethanolamine N-methyltransferase (PEMT). We characterized the perinatal changes in rat brain cortex phospholipids and metabolism and determined if maternal dietary n-3 fatty acid intake alters newborn brain cortex phospholipids, serine base exchange, PS decarboxylase or PEMT activities. PE became increasingly predominant, with an increase in the cortex PC/PE ratio from 2:1 at gestation day 19 to 1:1 at postnatal day 20. DHA increased and n-6 docosapentaenoic acid (DPA; 22:5 n-6) decreased in all phospholipids during development. [3H]serine incorporation into PS was higher in the fetal than neonatal brain cortex. Newborn rats of dams fed an n-3 fatty acid-deficient diet with 0.02% energy alpha-linolenic acid (ALA; 18:3 n-3) had approximately 50% lower DHA and higher DPA in cortex PE, PS and PC than newborns of rats fed a control diet with 1.5% energy ALA. [3H]serine incorporation into PS was significantly lower in the brain cortex of n-3 fatty acid-deficient than control newborns. n-3 Fatty acid deficiency had no effect on newborn brain PEMT or serine decarboxylase activities. These studies show that maternal dietary n-3 fatty acid deprivation impairs fetal brain DHA accretion and PS metabolism; altered PS metabolism may change release of lipid mediators and neurotransmitter precursors important in brain function.

Hexadecylphosphocholine inhibits phosphatidylcholine synthesis via both the methylation of phosphatidylethanolamine and CDP-choline pathways in HepG2 cells

We reported in a recent publication that hexadecylphosphocholine (HePC), a lysophospholipid analogue, reduces cell proliferation in HepG2 cells and at the same time inhibits the biosynthesis of phosphatidylcholine (PC) via CDP-choline by acting upon CTP:phosphocholine cytidylyltransferase (CT). We describe here the results of our study into the influence of HePC on other biosynthetic pathways of glycerolipids. HePC clearly decreased the incorporation of the exogenous precursor [1,2,3-3H]glycerol into PC and phosphatidylserine (PS) whilst increasing that of the neutral lipids diacylglycerol (DAG) and triacylglycerol (TAG). Interestingly, the uptake of L-[3-3H]serine into PS and other phospholipids remained unchanged by HePC and neither was the activity of either PS synthase or PS decarboxylase altered, demonstrating that the biosynthesis of PS is unaffected by HePC. We also analyzed the water-soluble intermediates and final product of the CDP-ethanolamine pathway and found that HePC caused an increase in the incorporation of [1,2-14C]ethanolamine into CDP-ethanolamine and phosphatidylethanolamine (PE) and a decrease in ethanolamine phosphate, which might be interpreted in terms of a stimulation of CTP:phosphoethanolamine cytidylyltransferase activity. Since PE can be methylated to give PC, we studied this process further and observed that HePC decreased the synthesis of PC from PE by inhibiting the PE N-methyltransferase activity. These results constitute the first experimental evidence that the inhibition of the synthesis of PC via CDP-choline by HePC is not counterbalanced by any increase in its formation via methylation. On the contrary, in the presence of HePC both pathways seem to contribute jointly to a decrease in the overall synthesis of PC in HepG2 cells.

The inhibitory role of methylation on the binding characteristics of dopamine receptors and transporter

Excess methylation has been suggested to play a role in the pathogenesis of Parkinson's disease (PD), since the administration of S-adenosylmethionine (SAM), a biological methyl donor, induces PD-like changes in rodents. It was proposed that SAM-induced PD-like changes might be associated with its ability to react with the dopaminergic system. In the present study the effects of SAM on dopamine receptors and transporters were investigated using rats and cloned dopamine receptor proteins. Autoradiographic examination of SAM indicated its tendency to be localized and accumulated in rat striatal region after the intracerebroventricular injection into rat brain. Moreover, results showed that SAM significantly decreased dopamine D1 and D2 receptor binding activities by decreasing the Bmax and increasing the Kd values. At concentrations of 0.1, 0.25 and 0.5 mM, SAM was able to reduce the Bmax from the control value of 848.1 for dopamine D1-specific ligand [3H] SCH 23390 to 760.1, 702.6 and 443.0 fmol/mg protein, respectively. At the same concentrations, SAM was able to increase the Kd values from 0.91 for the control to 1.06, 3.84 and 7.01 nM of [3H] SCH 23390, respectively. The effects of SAM on dopamine D2 binding were similar to those of dopamine D1 binding. SAM also decreased dopamine transporter activity. The interaction of SAM with dopamine receptor proteins produced methanol from methyl-ester formation and hydrolysis. We propose that the SAM effect might be related to its ability to react with dopamine receptor proteins through methyl-ester formation and methanol production following the hydrolysis of the carboxyl-methylated receptor proteins.

S-Adenosyl-L-methionine and alcoholic liver disease in animal models: implications for early intervention in human beings

In patients with severe alcoholic liver disease (i.e., cirrhosis), a deficiency of S-adenosylmethionine (SAMe) develops as a result of decreased SAMe synthetase activity. Whether a sizeable SAMe depletion occurs already at earlier stages of alcoholic liver disease has been the subject of debate. To address this issue, rats were fed alcohol (or isocaloric carbohydrate) in Lieber-DeCarli liquid diets containing adequate amounts of protein, vitamins, and lipotropic factors, including methionine. Alcohol feeding resulted in hepatic steatosis (without fibrosis) and unchanged SAMe synthetase activity, yet SAMe concentration was already greatly decreased. This most likely resulted from oxidative stress associated with the metabolism of alcohol and the induction of cytochrome P4502E1 (CYP2E1), which generates free radicals. Indeed, the decrease in hepatic SAMe correlated with parameters of oxidative stress, such as increased 4-hydroxynonenal (measured by gas chromatography-mass spectrometry) and diminished glutathione (GSH). Decreased GSH, occurring as a result of excessive GSH consumption caused by the oxidative stress, probably generated by enhanced utilization of SAMe, a precursor of GSH, thereby explaining the depletion of SAMe. In view of the known differences between rodents and primates in the metabolism of lipotropes, my colleagues and I have also studied the interaction between alcohol and SAMe in baboons and found again that, at early stages preceding the development of cirrhosis, there was already a significant lowering of hepatic SAMe concentration, associated with a striking oxidative stress documented by decreased levels and accelerated turnover of GSH. This was associated with increased lipid peroxidation and damage to cellular membranes, including those of the mitochondria, assessed by electron microscopy. Oral administration of SAMe resulted in its hepatic repletion with a corresponding attenuation of the ethanol-induced oxidative stress and liver injury, with significantly less GSH depletion, less increases in plasma aspartate aminotransferase (AST) levels, less leakage of mitochondrial glutamic dehydrogenase into the plasma, and fewer megamitochondria. In conclusion, (1) both in rodents and in non-human primates, significant SAMe depletion occurs already at early stages of alcoholic liver disease, despite the consumption of adequate diets; (2) the decreased hepatic SAMe concentration and the associated liver lesions, including mitochondrial injury, can be corrected with SAMe supplementation; and (3) accordingly, therapeutic administration of SAMe should be the subject of a comprehensive clinical trial to assess its capacity to attenuate early stages of alcoholic liver injury in human beings.

Alcoholic liver disease: new insights in pathogenesis lead to new treatments

Much progress has been made in the understanding of the pathogenesis of alcoholic liver disease, resulting in improvement of prevention and therapy, with promising prospects for even more effective treatments. The most successful approaches that one can expect to evolve are those that deal with the fundamental cellular disturbances resulting from excessive alcohol consumption. Two pathologic concepts are emerging as particularly useful therapeutically. Whereas it continues to be important to replenish nutritional deficiencies, when present, it is crucial to recognize that because of the alcohol-induced disease process, some of the nutritional requirements change. This is exemplified by methionine, which normally is one of the essential amino acids for humans, but needs to be activated to S-adenosylmethionine (SAMe), a process impaired by the disease. Thus, SAMe rather than methionine is the compound that must be supplemented in the presence of significant liver disease. Indeed, SAMe was found to attenuate mitochondrial lesions in baboons, replenish glutathione, and significantly reduce mortality in patients with Child A or B cirrhosis. Similarly, polyenylphosphatidylcholine (PPC) corrects the ethanol-induced hepatic phospholipid depletion as well as the decreased phosphatidylethanolamine methyltransferase activity and opposes oxidative stress. It also deactivates hepatic stellate cells, whereas its dilinoleoyl species (DLPC) increases collagenase activity, resulting in prevention of ethanol-induced septal fibrosis and cirrhosis in the baboon. Clinical trials with PPC are ongoing in patients with alcoholic liver disease. Furthermore, enzymes useful for detoxification, such as CYP2E1, when excessively induced, become harmful and should be downregulated. PPC is one of the substances with anti-CYP2E1 properties that is now emerging. Another important aspect is the association of alcoholic liver disease with hepatitis C: a quarter of all patients with alcoholic liver disease also have markers of HCV infection, with an even higher incidence in some urban areas but, at present, no specific therapy is available since interferon is contraindicated in that population. However, in addition to antiviral medications, agents that oppose oxidative stress and fibrosis should also be tested for hepatitis C treatment since these two processes contribute much to the pathology and mortality associated with the virus. In addition to antioxidants (such as PPC, silymarin, alpha-tocopherol and selenium), anti-inflammatory medications (corticosteroids, colchicine, anticytokines) are also being tested as antifibrotics. Transplantation is now accepted treatment in alcoholics who have brought their alcoholism under control and who benefit from adequate social support but organ availability is still the major limiting factor and should be expanded more aggressively. Finally, abstinence from excessive drinking is always indicated; it is difficult to achieve but agents that oppose alcohol craving are becoming available and they should be used more extensively.

Demonstration and partial characterisation of phospholipid methyltransferase activity in bile canalicular membrane from hamster liver

BACKGROUND/AIMS

Methylation of phosphatidylethanolamine to phosphatidylcholine predominantly takes place in mitochondrial-associated membrane and the endoplasmic reticulum of the liver. The transport of the phospholipids from endoplasmic reticulum to the bile canalicular membrane is via vesicular and protein transporters. In the bile canalicular membrane a flippase enzyme helps to transport phosphatidylcholine specifically to the biliary leaflet. The phosphatidylcholine then enters the bile where it accounts for about 95% of the phospholipids. We postulated that the increased proportion of phosphatidylcholine in the bile canalicular membrane and the bile compared to the transport vesicles may be due to a methyltransferase activity in the bile canalicular membrane which, using s-adenosyl methionine as the substrate, converts phosphatidylethanolamine on the cytoplasmic leaflet to phosphatidylcholine, which is transported to the biliary leaflet. The aim of our study was to demonstrate and partially characterise methyltransferase activity in the bile canalicular membrane.

METHODS

Organelles were obtained from hamster liver by homogenisation and separation by sucrose gradient ultracentrifugation. These, along with phosphatidylethanolamine, were incubated with radiolabelled s-adenosyl methionine. Phospholipids were separated by thin-layer chromatography and radioactivity was counted by scintigraphy.

RESULTS

We demonstrated methyltransferase activity (nmol of SAMe converted/mg of protein/h at 37 degrees C) in the bile canalicular membrane of 0.442 (SEM 0.077, n=8), which is more than twice that found in the microsomes at 0.195 (SEM 0.013, n=8). The Km and pH optimum for the methyltransferase in the bile canalicular membrane and the microsomes were similar (Km 25 and 28 microM, respectively, pH 9.9 for both). The Vmax was different at 0.358 and 0.168 nmol of SAMe converted/mg of protein/h for the bile canalicular membrane and the microsomes, respectively.

CONCLUSION

The presence of the methyltransferase activity in the bile canalicular membrane may be amenable to therapeutic manipulation.

Effects of tiadenol and di-(2-ethylhexyl)phthalate on the metabolism of phosphatidylcholine and phosphatidylethanolamine in the liver of rats: comparison with clofibric acid

Metabolic changes induced by 2,2'-(decamethylenedithio)diethanol (tiadenol) and di-(2-ethylhexyl)phthalate (DEHP) in the biosynthesis of phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) in rat liver were compared with changes induced by p-chlorophenoxyisobutyric acid (clofibric acid). Treatment of rats with either tiadenol or DEHP increased the hepatic contents of PtdCho and PtdEtn, as was observed with clofibric acid treatment. The administration of tiadenol, DEHP, or clofibric acid slightly, but significantly, increased, in common, the activity of CTP:phosphocholine cytidylyltransferase, a key enzyme for the synthesis de novo of PtdCho, and suppressed the activity of PtdEtn N-methyltransferase. With regard to the enzymes involved in the synthesis of PtdEtn, the three peroxisome proliferators enhanced the activity of phosphatidylserine (PtdSer) decarboxylase and markedly decreased the activity of CTP:phosphoethanolamine cytidylyltransferase. Treatment of rats with the three compounds markedly increased, in common, the content and the proportion of the molecular species of PtdCho containing oleic acid (18:1), but considerably decreased the proportion of the molecular species of PtdCho containing linoleic acid (18:2) in the liver, resulting in a striking decrease in the concentration of the molecular species of PtdCho containing 18:2 in the serum. The present study suggests that the administration of peroxisome proliferators to rats increases the contents of hepatic PtdCho and PtdEtn for hepatomegaly and proliferation of organelles by the same mechanism, irrespective of their chemical structures.

Alterations by perfluorooctanoic acid of glycerolipid metabolism in rat liver

The effects of perfluorooctanoic acid (PFOA) feeding on hepatic levels of glycerolipids and the underlying mechanism were investigated. Feeding of rats with 0.01% of PFOA in the diet for 1 week caused an increase in the contents of phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn), phosphatidylinositol (PtdIns), phosphatidylserine (PtdSer) and triglyceride (TG), which were 2.2, 2.4, 2.4, 1.6 and 5.2 times over control, respectively, on the basis of whole liver. The activities of glycerol-3-phosphate acyltransferase, diacylglycerol kinase and PtdSer decarboxylase were significantly increased upon PFOA feeding, whereas the activities of CTP:phosphoethanolamine cytidylyltransferase and PtdEtn N-methyltransferase were decreased. On the other hand, the activity of CTP:phosphocholine cytidylyltransferase was not increased by PFOA. Upon PFOA feeding, hepatic level of 16:0-18:1 PtdCho was markedly increased and, by contrast, the levels of molecular species of PtdCho which contain 18:2 were decreased, resulting in the reduced concentration of molecular species of serum PtdCho containing 18:2. The increase in the level of hepatic 16:0-18:1 PtdCho seemed to be due to 3-fold increase in the activities of both delta9 desaturase and 1-acylglycerophosphocholine (1-acyl-GPC) acyltransferase. The mechanism by which PFOA causes the accumulation of glycerolipids in liver was discussed.

Dietary methionine level affects linoleic acid metabolism through phosphatidylethanolamine N-methylation in rats

The effects of dietary methionine level on the profiles of fatty acids and phospholipids and on the plasma cholesterol concentration were investigated to confirm whether the methionine content of dietary proteins is one of the major factors that cause differential effects on lipid metabolism. The effect of dietary supplementation with eritadenine, which is shown to be a potent inhibitor of phosphatidylethanolamine (PE) N-methylation, was also investigated. Rats were fed six diets containing casein (100 g/kg) and amino acid mixture (86.4 g/kg) differing in methionine content (2.5, 4.5, and 7.5 g/kg) and without or with eritadenine supplementation (30 mg/kg) for 14 d. The ratio of arachidonic to linoleic acid of liver microsomal and plasma phosphatidylcholine (PC) was significantly increased as the methionine level of diet was elevated, indicating that dietary methionine stimulates the metabolism of linoleic acid. The PC/PE ratio of liver microsomes and the plasma cholesterol concentration were also increased by dietary methionine. These effects of methionine were completely abolished by eritadenine supplementation The S-adenosylmethionine concentration in the liver reflected the methionine level of diet. These results support the idea that the differential effects of dietary proteins on lipid metabolism might be ascribed, at least in part, to their different methionine contents, and that methionine might exert its effects through alteration of PE N-methylation.

Bezafibrate and clofibric acid are novel inhibitors of phosphatidylcholine synthesis via the methylation of phosphatidylethanolamine

The effects of bezafibrate and clofibric acid, fibrate hypolipidemic agents, on phosphatidylcholine (PC) synthesis via the phosphatidylethanolamine (PE) methylation pathway were studied. In cultured rat hepatocytes, bezafibrate and clofibric acid added to the medium rapidly and markedly reduced the conversion of ethanolamine-labeled PE to PC (IC50 30 and 150 microM, respectively). Furthermore, the methylation of PE derived from serine was also blocked by bezafibrate, as was the secretion of PC derived from either serine or ethanolamine. The microsomal activity of PE N-methyltransferase was inhibited by these agents. Perfluorooctanoic acid but not DCQVA, though both are potent peroxisome proliferators comparable to fibrates, produced this inhibition. The inhibitory effects produced by these agents were diminished by dithiothreitol (DTT) added to the assay or alkaline pH assay condition. Inhibition by oleic acid was also attenuated under these conditions, suggesting a common mechanism of inhibition. However, unlike fatty acids, fibrates did not have rapid stimulatory effects on the CDP-choline pathway in hepatocytes. These results suggest that fibrates may mimic fatty acids in regulating PC synthesis from the PE methylation pathway but not the CDP-choline pathway.

Phosphatidylethanolamine N-methyltransferase activity in isolated rod outer segments from bovine retina

Phosphatidylcholine (PC) can be synthesized in isolated rod outer segments from bovine retina by successive transfer of methyl groups from S-adenosyl-L-methionine (SAM) to phosphatidylethanolamine (PE), with the intermediate formation of phosphatidyl-N-monomethylethanolamine (PMME) and phosphatidyl-N,N-dimethylethanolamine (PDME). This reaction is time-protein-and SAM concentration-dependent. Phosphatidylethanolamine N-methyltransferase (PE N-MTase) has two pH optima, 8.5 and 10, at low (10 microM) and high (200 microM) SAM concentrations and requires magnesium ions for full activity. When ROS membranes were incubated at 5 to 200 microM SAM concentrations at pH 8.5 or pH 10, the major methylated product was PMME, followed by PC and PDME. The apparent Kms for SAM at pH 8.5 and at pH 10 were similar (37 and 38 microM, respectively). The Vmax was 13 pmol h-1 (mg protein)-1 at pH 8.5 and 12.50 pmol h-1 (mg protein)-1 at pH 10. Pulse-chase experiments demonstrated a precursor-product relationship with [3H]PC as the end product. The level of PE N-Mtase activity in the purified ROS preparation obtained from crude ROS fractions by discontinuous sucrose gradient centrifugation, was as high as 65% of the level found in the microsomal fraction obtained from the remainder of the retinas. The presence of microsomal and mitochondrial marker enzymes, however, was minimal in the ROS preparation. The radioactivity incorporated into ROS PC was measured in an upper and lower band of PC obtained by two-dimensional TLC. We found that the amount of [methyl-3H] groups incorporated into the upper PC band was 2.5-fold greater than that incorporated into the lower one. The fatty acid composition of the upper band was very different from that of the lower band, the former being enriched in very long-chain polyunsaturated fatty acids and the latter in saturated fatty acids. Phosphatidyl-ethanolamine N-methyltransferase activity increased in the presence of exogenous phospholipid substrates. PDME being augmented ten-fold and PC eight-fold when the incubations were carried out in the presence of PMME and PDME, respectively. At a 2 mM concentration, S-adenosyl-L-homocysteine (SAH) inhibited the methyl groups' incorporation into the endogenous phospholipids by 40%. When ROS membranes were selectively depleted of soluble or peripheral and soluble proteins, the PE N-MTase activity remained mainly associated to the membrane, suggesting that this enzyme (s) is an intrinsic membrane protein.(ABSTRACT TRUNCATED AT 400 WORDS)

The mechanism for the increased supply of phosphatidylcholine for the proliferation of biological membranes by clofibric acid, a peroxisome proliferator

The metabolic changes induced by p-chlorophenoxyisobutyric acid (clofibric acid), a peroxisome proliferator, in hepatic glycerolipids for the supply of membrane phospholipids were studied. The administration of clofibric acid to rats caused hepatomegaly and an increase in hepatic contents of phosphatidylcholine (PtdCho) (1.13-fold on the basis of g liver and 1.50-fold on the basis of whole liver). The administration of the drug enhanced the formation in vivo of PtdCho from [3H]glycerol, which seemed to be due to the increase in activity of CTP:phosphocholine cytidylyltransferase. On the other hand, clofibric acid depressed the activity of phosphatidylethanolamine N-methyltransferase. The in vivo study using [3H]glycerol revealed that clofibric acid slightly reduced the secretion of PtdCho into circulation. On the other hand, the drug did not affect the turnover of PtdCho. These results may elucidate the metabolic alterations by which clofibric acid increases hepatic mass of PtdCho. The facilitated biosynthesis of PtdCho by the drug seemed to lead to the increased formation of phosphatidylserine and subsequently phosphatidylethanolamine. Physiological significance of the alterations in glycerolipid metabolism by clofibric acid was discussed in relation to biological action of the drug.

Phospholipid methylation in brain membrane preparations: kinetic mechanism

The methylation reactions which convert phosphatidylethanolamine (PE) to phosphatidylcholine (PC) have been studied kinetically using exogenously added intermediates and crude membrane preparations from brain. The addition of exogenous PE resulted in no change in the methylation rates compared to that of endogenous PE. The addition of the two intermediates, monomethylphosphatidylethanolamine (PMME) and dimethylphosphatidylethanolamine (PDME), resulted in significantly increased rates of methylation and allowed the kinetic analysis of these latter two methylation reactions. The mechanism for this enzyme appears to be similar to human RBC (Reitz et al. (1989) J. Biol. Chem. 264, 8097-8106) which was a rapid-equilibrium random Bi-Bi sequential mechanism. There were some slight differences between the brain enzyme and that from the RBC, but there is little reason to suggest a fundamentally different mechanism. It is more likely that the differences may relate to an additional dead-end complex for the enzyme from brain such that saturation with AdoMet cannot eliminate AdoHcy inhibition. The KM values for the two phospholipid substrates were 41-44 microM and 39 microM for the methylation of PMME and PDME, respectively. The KM for S-adenosylmethionine (AdoMet) was 7-9 microM with PMME and 4 microM with PDME as the other substrates. The Ki(lipid) varied from 54 microM with PMME to 225 microM with PDME, and the Ki(AdoMet) was 11 microM with PMME and 21 microM with PDME. The product from the use of AdoMet, S-adenosylhomocysteine (AdoHcy), was shown to be a noncompetitive inhibitor of both lipid substrates as well as AdoMet. The methylation of PMME was somewhat higher in cerebellum and brain stem compared to cortex and striatum, but the methylation of PDME was similar in cerebellum, brain stem and cortex.

Phospholipid N-methylation in diabetic erythrocytes: effects on membrane Na+, K+ ATPase activity

Phospholipid methylation was quantified in non-diabetic and streptozotocin diabetic rat erythrocytes. While the total mass of methylated lipids remained the same in both groups, the relative abundance of individual methylated lipid species differed significantly in diabetic erythrocytes. Moreover, incubation of erythrocytes membranes with S-adenosyl methionine, a substrate for methyl transferases, not only increased membrane lipid methylation but also decreased Na+, K+ ATPase activity significantly. These results suggest that phospholipid methylation may cause the observed depression of erythrocyte Na+, K+ ATPase activity in diabetes and could contribute to the altered rheology of erythrocytes in diabetes.

Characterization of membrane fraction lipid composition and function of cirrhotic rat liver. Role of S-adenosyl-L-methionine

The effect of S-adenosyl-L-methionine (SAM) administration on the lipid composition of the membrane fraction obtained from livers of cirrhotic rats was studied. Four groups of animals were used: group 1 received CCl4 for 8 weeks to induce cirrhosis. Animals in group 2 received 3 daily i.m. injections of SAM 20 mg/kg in addition to CCl4. Groups 3 and 4 were control groups of SAM and vehicles. Seventy-two h after the end of treatment all animals were killed and livers were studied to measure glycogen, cAMP contents and to isolate membrane fractions. The membrane activity of Na+,K(+)- and Ca(2+)-ATPases was measured and the lipid content was analyzed in extracts. Phospholipids were determined by thin-layer chromatography and fatty acids by gas chromatography. Chronic CCl4 treatment led to increases in cholesterol and in the cholesterol/phospholipid ratio. Analysis of phospholipids revealed an increase in phosphatidylserines. Saturated fatty acids increased, while unsaturated decreased significantly. The CCl4-treated group showed a decrease in glycogen and an increase in cAMP contents. Na+,K(+)- and Ca(2+)-ATPases activity were highly reduced in cirrhotic membranes. In the group receiving CCl4 + SAM the lipid composition and the function of liver membrane fraction showed no difference compared to normal controls, except for fatty acid composition which was similar to concentrations in the CCl4-treated group. Glycogen depletion was only partially prevented whereas cAMP levels were normalized in the CCl4 + SAM group. Our results showed that membrane lipid alterations were accompanied by changes in the activity of enzymes embedded in the membrane fraction derived from CCl4-cirrhotic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of phosphatidylethanolamine methyltransferase and phospholipid methyltransferase by phospholipid precursors in Saccharomyces cerevisiae

Phosphatidylethanolamine methyltransferase (PEMT) and phospholipid methyltransferase (PLMT), which are encoded by the CHO2 and OPI3 genes, respectively, catalyze the three-step methylation of phosphatidylethanolamine to phosphatidylcholine in Saccharomyces cerevisiae. Regulation of PEMT and PLMT as well as CHO2 mRNA and OPI3 mRNA abundance was examined in S. cerevisiae cells supplemented with phospholipid precursors. The addition of choline to inositol-containing growth medium repressed the levels of CHO2 mRNA and OPI3 mRNA abundance in wild-type cells. The major effect on the levels of the CHO2 mRNA and OPI3 mRNA occurred in response to inositol. Regulation was also examined in cho2 and opi3 mutants, which are defective in PEMT and PLMT activities, respectively. These mutants can synthesize phosphatidylcholine when they are supplemented with choline by the CDP-choline-based pathway but they are not auxotrophic for choline. CHO2 mRNA and OPI3 mRNA were regulated by inositol plus choline in opi3 and cho2 mutants, respectively. However, there was no regulation in response to inositol when the mutants were not supplemented with choline. This analysis showed that the regulation of CHO2 mRNA and OPI3 mRNA abundance by inositol required phosphatidylcholine synthesis by the CDP-choline-based pathway. The regulation of CHO2 mRNA and OPI3 mRNA abundance generally correlated with the activities of PEMT and PLMT, respectively. CDP-diacylglycerol synthase and phosphatidylserine synthase, which are regulated by inositol in wild-type cells, were examined in the cho2 and opi3 mutants. Phosphatidylcholine synthesis was not required for the regulation of CDP-diacylglycerol synthase and phosphatidylserine synthase by inositol.

Effect of monomethylethanolamine, dimethylethanolamine, gangliosides, isoproterenol, and 2-hydroxyethylhydrazine on the conversion of ethanolamine to methylated products by cultured chick brain neurons

The sequential methylation of ethanolamine and its phosphorylated derivatives has been studied with chick neurons in culture in the presence of several pharmacological agents. Incubation with [3H]ethanolamine in the presence of monomethylethanolamine and dimethylethanolamine indicated that in these neurons the preferential conversion to choline-containing compounds is via the methylation of phosphorylethanolamine. The possibility that there are two separate enzymes, i.e., one responsible for the methylation of water-soluble ethanolamine-containing compounds and another for the ethanolamine phospholipids, was examined with agents believed to influence these conversions. Incubation of neurons in the presence of a mixture of exogenous gangliosides at 10(-8) M and 10(-5) M concentrations showed that these neuritogenic compounds stimulate the methylation of phosphatidylethanolamine and decrease that of phosphorylethanolamine. The inhibitor of phosphatidylethanolamine methyltransferase (EC 2.1.1.17), 2-hydroxyethylhydrazine, decreased the conversion of phosphatidylethanolamine to phosphatidylcholine and increased that of phosphorylethanolamine to phosphorylcholine. The possible effects of adrenergic stimulation were studied by the incubation of neurons with isoproterenol at 10(-6) M and 10(-5) M concentrations. There was a reduction of phosphorylethanolamine methylation and a stimulation of that of phosphatidylethanolamine, and these effects were counteracted by the presence of 5 x 10(-5) M propranolol.

Calmodulin- and Ca2(+)-insensitive fatty acid methyltransferase from RINm5F cells. Inhibition by trifluoperazine and W7

1. Methylation of endogenous lipids by homogenates of rat insulinoma cells was studied. 2. 3H-methyl groups (38 pmol/mg protein per 10 min) from [3H-methyl]S-adenosyl-L-methionine were incorporated into endogenous lipids, mainly (greater than 80%) into the neutral lipid fraction. 3. The reaction was sensitive to heat, was almost abolished by S-adenosyl-L-homocysteine, but insensitive to the addition of EGTA (5 mM), Ca2+ (5-100 microM) and/or calmodulin (15 microns). 4. At concentrations relevant for calmodulin antagonistic activity strong inhibition by W7 and trifluoperazine (25-100 microM each), but not by CGS 9343B (10 microM), was observed. 5. Calmodulin antagonists of phenothiazine- and sulfonamide-type appear to block the fatty acid methyltransferase in a way unrelated to calmodulin.

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.

S-adenosyl-L-methionine attenuates alcohol-induced liver injury in the baboon

Chronic ethanol consumption by baboons (50% of energy from a liquid diet) for 18 to 36 mo resulted in significant depletion of hepatic S-adenosyl-L-methionine concentration: 74.6 +/- 2.4 nmol/gm vs. 108.9 +/- 8.2 nmol/gm liver in controls (p less than 0.005). The depletion was corrected with S-adenosyl-L-methionine (0.4 mg/kcal) administration (102.1 +/- 15.4 nmol/gm after S-adenosyl-L-methionine-ethanol, with 121.4 +/- 11.9 nmol/gm in controls). Ethanol also induced a depletion of glutathione (2.63 +/- 0.13 mumol/gm after ethanol vs. 4.87 +/- 0.36 mumol/gm in controls) that was attenuated by S-adenosyl-L-methionine (3.89 +/- 0.51 mumol/gm in S-adenosyl-L-methionine-methanol vs. 5.22 +/- 0.53 mumol/gm in S-adenosyl-L-methionine controls). There was a significant correlation between hepatic S-adenosyl-L-methionine and glutathione level (r = 0.497; p less than 0.01). After the baboons received ethanol, we observed the expected increase in circulating levels of the mitochondrial enzyme glutamic dehydrogenase: 95.1 +/- 21.4 IU/L vs. 13.4 +/- 1.8 IU/L; p less than 0.001, whereas in a corresponding group of animals given S-adenosyl-L-methionine with ethanol, the values were only 30.3 +/- 7.1 IU/L (vs. 9.6 +/- 0.7 IU/L in the S-adenosyl-L-methionine controls). This attenuation by S-adenosyl-L-methionine of the ethanol-induced increase in plasma glutamic dehydrogenase (p less than 0.005) was associated with a decrease in the number of giant mitochondria (assessed in percutaneous liver biopsy specimens), with a corresponding change in the activity of succinate dehydrogenase, a mitochondrial marker enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the methyltransferases in the yeast phosphatidylethanolamine methylation pathway by selective gene disruption

PEM1 and PEM2 are structural genes for the yeast phosphatidylethanolamine methylation pathway which mediates the three-step methylation of phosphatidylethanolamine to phosphatidylcholine. Selective disruption of each locus in the yeast genome was performed using the in-vitro-inactivated gene with insertion of yeast LEU2 or HIS3. Complementation test and spore analysis indicated that the disruptants were allelic with our previous mutants that were isolated by chemical mutagenesis and used for the cloning of PEM1 and PEM2. The methyltransferase activities of the disruptants were assayed using their membrane fractions. When the PEM1 locus was disrupted, the activity for the first methylation was greatly decreased but was still detectable, while the activities for the second and third methylations were well retained. The remaining three activities exhibited nearly identical pH optima and apparent Km values for S-adenosyl-L-methionine. The disruptant incorporated radioactivity from L-[methyl-14C]Met into phosphatidylcholine at a low but measurable rate and required choline for optimal growth. When choline was omitted from the culture medium, the phosphatidylcholine content of the cells significantly decreased, but was restored by the addition of N-monomethylethanolamine or choline. When the PEM2 locus was disrupted, the activities for the second and third methylations were totally lost, but that for the first methylation remained. This activity could be distinguished from those remaining in the pem1 disruptant by its different pH optimum and apparent Km for S-adenosyl-L-methionine. When incubated with [methyl-14C]Met, the pem2 disruptant accumulated the radioactivity in phosphatidylmonomethylethanolamine. This disruptant also required choline for optimal growth. In the absence of choline, it accumulated phosphatidylmonomethylethanolamine with a concomitant decrease in phosphatidylcholine and phosphatidylethanolamine. When both loci were disrupted, all phospholipid-methylating activities were lost and cells absolutely required choline for growth. The flux through the pathway became negligible. Thus, the PEM1-encoded methyltransferase was strictly specific to the first step while the PEM2-encoded methyltransferase exhibited a somewhat broader specificity with a preference for the second and third steps of the pathway. These two enzymes accounted for all the activities in the yeast phosphatidylethanolamine methylation pathway.

Characterization of the binding of S-adenosyl-L-methionine to plasma membranes of HL-60 promyelocytic leukemia cells

S-Adenosyl-L-methionine (AdoMet) has been found to bind specifically to the plasma membrane of promyelocytic leukemia cells, HL-60. The Kd for AdoMet is 4.2.10(-6) M and the Bmax is 4.0.10(-12) mol/10(7) HL-60 cells. The binding is not related to the adenosine receptor since neither adenosine, ADP, nor ATP affect the ligand-receptor reaction. When HL-60 cells were incubated with physiological concentrations of [methyl-3H]AdoMet (20 microM) at 36 degrees C, AdoMet did not equilibrate with the intracellular pool, nor were any [3H]methyl groups incorporated into nucleic acids or proteins. In contrast, significant amounts of [3H]methyl groups were incorporated into membrane phospholipids. When cells were incubated with 20 microM [methyl-3H]AdoMet, [3H]methyl groups were transferred to phosphatidylethanolamine, -monomethylethanolamine, and -dimethylethanolamine yielding phosphatidylcholine. However, the rate of methyl transfer with AdoMet was only 22% of that observed when cells were incubated with a comparable amount of [methyl-3H]methionine. Both the binding of AdoMet and the methylation of phospholipids were inhibited by exogenous S-adenosyl-L-homocysteine. Therefore, the binding may be linked to a phospholipid methyltransferase.

Differential methylation patterns in molecular species of phosphatidylethanolamine derivatives in rat liver membranes

The appearance of individual molecular species of phospholipids in the complete sequence of the transmethylation of phosphatidylethanolamine (PE) was examined in rat liver microsomes incubated with S-adenosyl-L-[methyl-14C]methionine. Reverse-phase HPLC analysis of phosphatidylcholine (PC), phosphatidyl-N,N-dimethylethanolamine (dimethyl-PE), or phosphatidyl-N-monomethylethanolamine (monomethyl-PE) showed that radioactivity was present in the same six principal molecules; a first group is constituted by 16:0/22:6, 16:0/20:4 and 16:0/18:2 and a second one by the homologous molecules with 18:0 instead of 16:0 at the sn-1 position of glycerol. In PC, 16:0/22:6 (23% of total radioactivity) was preponderant, and 18:0/20:4 was the lowest. The ratios cpm in PC/nmol in PE were in the order: 16:0/22:6 greater than 16:0/18:2 greater than 16:0/20:4 followed by the corresponding 18:0 molecules. On the other hand, in intermediate phospholipids, incorporation of methyl groups was most marked in 18:0/20:4 (24-27% of total). 16:0/22:6 and 16:0/18:2 were low in comparison to their relative values in PC. The ratio (18:0/20:4)/(16:0/22:6) was 4.5-5.6-times higher in monomethyl-PE and dimethyl-PE than in PC. These differences were found consistently, regardless of incubation time of microsomes (2.5-60 min) and of S-adenosyl-L-methionine (AdoMet) concentration (3 or 100 microM). In liver membranes, it would therefore seem that there is a different selectivity in methyl group transfer, depending upon whether the first two steps or the third step of the reaction are considered. Side reactions, such as deacylation/reacylation, are unlikely to account for this difference, which could rather be related to the enzyme itself.

Comparison of phospholipid N-methylation activity in rat submandibular salivary gland and liver

Successive phospholipid N-methylation from phosphatidylcholine to phosphatidylethanolamine in submandibular gland and liver microsomes proceeded without the addition of exogenous phospholipid substrate. Methylation activity in the submandibular microsomes showed different susceptibilities to various detergents than the liver enzyme and also partially required Mg2+. However, the three methylation steps could not be distinguished by their Mg2+ requirements. Ca2+ had no effect on the activity. The methylation activity in submandibular gland was much lower than in liver. Chronic administration of isoproterenol, which causes an increase of phosphatidylcholine in membrane phospholipids of salivary glands, decreased methylation activity in the submandibular gland. Thus the increase in phosphatidylcholine in isoproterenol-treated rat salivary glands may not be derived from the phospholipid methylation pathway, but may be due to stimulation of other routes of phosphatidylcholine metabolism.

Changes in rat lung microsomal lipids after p-xylene: relationship to inhibition of benzo[a]pyrene metabolism

The relationship between p-xylene's effects on microsomal membranes, cytochrome P-450, and benzo[a]pyrene (BaP) metabolism was studied. p-Xylene (1 g/kg, ip, 1 h) inhibited 3-hydroxy BaP (3-OH) formation and decreased arylhydrocarbon hydroxylase (AHH) activity approximately 40% in rat lung microsomes. BaP dihydrodiol and quinone formation were unchanged by p-xylene administration. Cytochrome P-450 was below the limit of detection in lung microsomes from p-xylene-treated rats. Total phospholipid (PL) and phosphatidylcholine (PC) in microsomal membranes were decreased 28% and 17%, respectively. Cholesterol (CL), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), and sphingomyelin (SM) were unchanged. The net activity of enzymes involved in the synthesis of PC, phosphatidylethanolamine-N-methyltransferase I and II (PMT I and PMT II), was slightly elevated by p-xylene. PL/CL and PC/PE ratios, indicators of membrane fluidity, were decreased 34% and 13%, respectively, in microsomes from p-xylene-treated rats. Analysis of fluidity by fluorescence polarization showed that the actual fluidity of treated microsomes was slightly decreased (5%) as compared to controls. The decrease in P-450, PL, and PC is considered to contribute to the inhibition of BaP metabolism.

Specific loss of the high-molecular-weight form of S-adenosyl-L-methionine synthetase in human liver cirrhosis

We have measured the activity of S-adenosyl-L-methionine synthetase, the ratio between the high- and low-molecular-weight forms of this enzyme and the concentration of S-adenosyl-L-methionine in liver biopsies from a group of controls (n = 6) and in six cirrhotics (five posthepatitic and one alcoholic). The total activity of S-adenosyl-L-methionine synthetase was markedly reduced in cirrhosis (37.5% of that found in the control group). This was due to a specific reduction in the high-molecular-weight S-adenosyl-L-methionine synthetase in the group of cirrhotics (73.9 pmoles per min per mg protein) when compared with that observed in controls (460.3 pmoles per min per mg protein). Despite this reduction in the rate of synthesis of S-adenosyl-L-methionine (the high-molecular-weight form of the enzyme is 15 times more active than the low-molecular-weight form at physiological concentration of substrates), the concentration of this metabolite was the same in the control group (17.3 +/- 2.6 microM) and in the group of cirrhotics (17.8 +/- 3.1 microM). To explain these findings, it is postulated that in human liver, where the concentration of S-adenosyl-L-methionine is lower than the Km values of a variety of enzymes that use this metabolite (around 50 to 100 microM), a reduction in the synthesis of S-adenosyl-L-methionine is compensated by a reduction in the rate of utilization of this molecule without affecting the intrahepatic concentration of S-adenosyl-L-methionine.(ABSTRACT TRUNCATED AT 250 WORDS)

1,2-Dimethylhydrazine-induced premalignant alterations in the S-adenosylmethionine/S-adenosylhomocysteine ratio and membrane lipid lateral diffusion of the rat distal colon

Prior studies by our laboratory, utilizing the 1,2-dimethylhydrazine experimental model of colonic cancer, had shown that administration of this procarcinogen for 5 weeks was found to increase phospholipid methyltransferase activity and the fluidity of rat distal colonic brush-border membranes. The present studies were conducted to further explore these 'premalignant' colonic phenomena. Male albino rats of the Sherman strain were subcutaneously injected with dimethylhydrazine (20 mg/kg body weight per week) or diluent for 5 weeks. Animals from each group were killed, distal colonic tissue harvested and the levels of S-adenosylmethionine, S-adenosylhomocysteine and decarboxylated S-adenosylmethionine measured by high performance liquid chromatography. The activity of methionine adenosyltransferase was also examined in these tissues. Additionally, brush-border membranes were isolated from the distal colonocytes of control and treated-animals and examined and compared with respect to their phospholipid methylation activities as well as their lipid fluidity as assessed by the rotational mobilities of the probes 1,6-diphenyl-1,3,5-hexatriene and DL-12-(9-anthroyl)stearic acid and translational mobility of the fluorophore pyrenedecanoic acid. The results of these studies demonstrated: (1) phospholipid methyltransferase activity in rat colonic plasma membranes was increased concomitantly with increases in the cellular levels of S-adenosylmethionine and the S-adenosylmethionine/S-adenosylhomocysteine ratio in the distal colonic segment of treated-animals; and (2) the lateral diffusion of rat distal colonic brush-border membrane lipids, as assessed by the ratio of excimer/monomer fluorescence intensities of the fluorophore pyrenedecanoate, was also increased after dimethylhydrazine administration to these animals for 5 weeks.

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.

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.

Identification and partial characterization of phospholipid methylation in rat small-intestinal brush-border membranes

An earlier study (Biochim. Biophys. Acta 46 (1961) 205-216) failed to detect the enzymatic synthesis of phosphatidylcholine (PC) from phosphatidylethanolamine (PE) via a transmethylation pathway in rat small-intestinal microsomal membranes. This pathway was therefore assumed to be absent from this organ. Recently, however, in our laboratory it has been demonstrated that this pathway for the synthesis of phosphatidylcholine is present in rat colonic brush-border and basolateral membranes. It was therefore of interest to examine whether phospholipid methylation activity was present in rat small-intestinal brush-border membranes. The results of the present experiments demonstrate for the first time that this pathway for the synthesis of phosphatidylcholine exists in these plasma membranes. Evidence to support the enzymatic nature of this reaction include: loss of activity by heat denaturation and at 0 degree C, significant inhibition by S-adenosyl-L-homocysteine and saturation kinetics. The predominant product of this brush-border membrane phospholipid methyltransferase is phosphatidyl-N-monomethylethanolamine. This enzymatic activity has an apparent Km for S-adenosyl-L-methionine of 40 microM, a Vmax of 8.4 pmol/mg protein per 5 min, and a pH optimum of 8.0.

1,2-Dimethylhydrazine-induced alterations in colonic plasma membrane fluidity: restriction to the luminal region

Recently, work in this laboratory has shown that changes in the 'dynamic' component of fluidity, lipid composition and phospholipid methylation activity of distal colonic brush-border membranes could be detected after administration of 1,2-dimethylhydrazine to rats of the Sherman strain for 5-15 weeks, i.e., before the development of colon cancer. The present experiments were therefore conducted to: determine whether similar 'premalignant' biochemical changes could be detected in basolateral membranes of Sherman rats treated with this agent; and clarify the relationship of these membrane changes to the malignant transformation process by examining the effect of 1,2-dimethylhydrazine on these biochemical parameters in colonic antipodal plasma membranes of rats of the Lobund-Wistar strain. This particular strain of rats has previously been shown to be total resistant to the induction of tumors by 1,2-dimethylhydrazine. The results of the present experiments demonstrate that similar biochemical alterations could not be detected in the colonic plasma membranes prepared from either strain of rat treated with 1,2-dimethylhydrazine. These data support the contention that the prior biochemical membrane alterations noted in brush-border membranes of 1,2-dimethylhydrazine-treated animals are, in fact, related to the malignant transformation process and, furthermore, are confined to the luminal surface of distal colonic epithelial cells.

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.