Ethanolamine plasmalogen and cholesterol reduce the total membrane oxidizability measured by the oxygen uptake method

To investigate the effects of ethanolamine plasmalogen, phosphatidylethanolamine, cholesterol, and alpha-tocopherol on the oxidizability of membranes, various large unilamellar vesicles (LUVs) including these lipids and antioxidant were examined for their total membrane oxidizabilities, evaluated as R(p)/R(i)(1/2) value (where R(p) is rate of oxygen consumption and R(i)(1/2) is the square root of rate of chain initiation) by the oxygen uptake method with water-soluble radical initiator and inhibitor. Incorporation of bovine brain ethanolamine plasmalogen (BBEP) into vesicles as well as cholesterol led to lower the total membrane oxidizability dose-dependently. The effect of BBEP was more efficient in the presence of cholesterol in vesicles. On the other hand, diacyl counterpart, egg yolk phosphatidylethanolamine, and a typical radical scavenger, alpha-tocopherol, had no effect on the membrane oxidizability. Alpha-tocopherol only prolonged an induction period dose-dependently in the present oxidizing system, suggesting a novel antioxidant mechanism of ethanolamine plasmalogens besides the action of scavenging radicals.     

Ethanolamine plasmalogens protect cholesterol-rich liposomal membranes from oxidation caused by free radicals

The aim of the present study is to investigate the effect of ethanolamine plasmalogens on the oxidative stability of cholesterol-rich membranes by comparing it with that of diacyl glycerophosphoethanolamine, using bovine brain ethanolamine plasmalogen (BBEP) or egg yolk phosphatidylethanolamine (EYPE)-containing large unilamellar vesicles (LUVs) and the water-soluble radical initiator AAPH. Electron microscopic observation and particle size measurement visually demonstrated that ethanolamine plasmalogens protect cholesterol-rich phospholipid bilayers from oxidative collapse. Lipid analyses suggested that the effect of ethanolamine plasmalogens in stabilizing membranes against oxidation is partly due to the antioxidative action of plasmalogens involved in scavenging radicals at vinyl ether linkage.     

Structure and dynamics of plasmalogen model membranes containing cholesterol: a deuterium NMR study

Deuterium nuclear magnetic resonance (²H-NMR) was used to investigate the structure and dynamics of the sn-2 hydrocarbon chain of semi-synthetical choline and ethanolamine plasmalogens in bilayers containing 0, 30, and 50 mol% cholesterol. The deuterium NMR spectra of the choline plasmalogen yielded well-resolved quadrupolar splittings which could be assigned to the corresponding hydrocarbon chain deuterons. The sn-2 acyl chain was found to adopt a similar conformation as observed in the corresponding diacyl phospholipid, however, the flexibility at the level of the C-2 methylene segment of the plasmalogen was increased. Deuterium NMR spectra of bilayers composed of the ethanolamine plasmalogen yielded quadrupolar splittings of the C-2 segment much larger than those of the corresponding diacyl lipids, suggesting that the sn-2 chain is oriented perpendicular to the membrane surface at all segments. Cholesterol increased the ordering of the choline plasmalogen acyl chain to the same extent as in diacyl lipid bilayers. T₁ relaxation time measurements demonstrated only minor dynamical differences between choline plasmalogen and diacyl lipids in model membranes.     

Structure and dynamics of plasmalogen model membranes containing cholesterol: a deuterium NMR study

Deuterium nuclear magnetic resonance (2H-NMR) was used to investigate the structure and dynamics of the sn-2 hydrocarbon chain of semi-synthetical choline and ethanolamine plasmalogen in bilayers containing 0, 30, and 50 mol% cholesterol. The deuterium NMR spectra of the choline plasmalogen yielded well-resolved quadrupolar splittings which could be assigned to the corresponding hydrocarbon chain deuterons. The sn-2 acyl chain was found to adopt a similar conformation as observed in the corresponding diacyl phospholipid, however, the flexibility at the level of the C-2 methylene segment of the plasmalogen was increased. Deuterium NMR spectra of bilayers composed of the ethanolamine plasmalogen yielded quadrupolar splittings of the C-2 segment much larger than those of the corresponding diacyl lipids, suggesting that the sn-2 chain is oriented perpendicular to the membrane surface at all segments. Cholesterol increased the ordering of the choline plasmalogen acyl chain to the same extent as in diacyl lipid bilayers. T1 relaxation time measurements demonstrated only minor dynamical differences between choline plasmalogen and diacyl lipids in model membranes.     

Regulation of phospholipid metabolism in differentiating cells from rat brain cerebral hemipheres in culture. II. Incorporation of [U-14C]ethanolamine into 1-alkenyl,2-acyl-and 1,2 diacyl-ethanolamine phosphoglycerides.

Cultured dissociated cells from rat embryo cerebral hemisphere incorporate [3H]-and [U-14C]ethanolamine into cellular lipids. Nearly all radioactivity in the lipid fractions is incorporated into 1,2-diacylethanolamine phosphoglycerides and 1-alkenyl,2-acylethanolamine phosphoglycerides (plasmalogen). Kinetic data suggest that the rate of labeling of both ethanolamine phospholipids from the phosphorylethanolamine is similar. A relative increase of the plasmalogen labeling is observed when free ethanolamine is continually present in the medium. The rate of incorporation of label from ethanolamine and phosphorylethanolamine into lipids was measured using a double label technique. Based upon these studies, an independent labeling pattern of the ethanolamine moiety of plasmalogens is suggested. A relative delay for the incorporation of label in plasmalogens could be explained by the presence of a variety of cell types which may differ in their capacity for phospholipid biosynthesis. The rate of incorporation of phosphorylethanolamine into the phosphatidylethanolamine was not affected by the presence of high concentrations of either choline or serine.     

Uptake of fluorescent plasmalogen analogs by cultured human skin fibroblasts deficient in plasmalogen

One of the consequences of hereditary peroxisomal dysfunction in the cerebro-hepato-renal (Zellweger) syndrome (CHRS) is a dramatic decrease in the biosynthesis and cellular content of ether lipids. In the present study effects of reduced cellular plasmalogen levels on membrane-membrane interactions were investigated. Cultured CHRS fibroblasts were incubated with unilamellar phospholipid vesicles consisting of 1-O-alkenyl-2-acyl- or 1,2-diacyl-sn-glycerophosphocholines and ethanolamines, carrying either the trans-parinaroyl or the 1,6-diphenyl-1,3,5-hexatriene propionyl group in position 2. Transfer of the fluorogenic phospholipids from vesicles to cells was followed by measuring the concomitant increase in fluorescence intensity. Transfer of phospholipids from cells to vesicles was monitored by incubating cells, prelabeled with [3H]oleic acid, in the presence of phospholipid vesicles. Fibroblasts from healthy donors or CHRS fibroblasts supplemented with the plasmalogen precursor 1-O-hexadecylglycerol served as controls. Plasmalogen-deficient cells exhibited a significantly increased tendency to take up exogenous choline or ethanolamine plasmalogens. Cellular plasmalogens were transferred from control cells to vesicles at a higher rate if the acceptor vesicles consisted of plasmalogens as compared to diacylglycerophosphocholine. Thus, it appears as if mechanisms existed which preserve cellular plasmalogen levels during interaction with exogenous phospholipid pools. Preliminary experimental evidence suggests that the observed exchange of phospholipids between cultured fibroblasts and vesicles occurs by a protein-catalyzed process.     

Effect of cholesterol on vesicle bilayer geometry of choline plasmalogen and comparison with dialkyl-, alkylacyl- and diacyl-glycerophosphocholines

Small unilamellar vesicles containing alkenylacyl-, alkylacyl-, dialkyl- or diacyl-glycerophosphocholine were prepared by sonication. Their size was determined from the average internal volume after chromatography on Sepharose 2B and from 31P-NMR linewidths. Alkenylacyl glycerophosphocholine (choline plasmalogen) was found to form the largest vesicles. By addition of 30 mol% cholesterol, the size of plasmalogen vesicles, but not of those containing the alkyl and acyl analogue lipids, was significantly increased. The presence of 50 mol% sterol led to highly increased vesicle sizes of alkylacyl, dialkyl and diacyl-glycerophosphocholine. Mixtures of plasmalogens with 50 mol% cholesterol did not form unilamellar vesicles upon sonication. Bilayer thickness and surface area per phospholipid molecule were determined by small angle X-ray scattering and measurement of partial specific volumes. There is little difference between alkenylacyl glycerophosphocholine and the corresponding diacyl-analog, whereas bilayers consisting of dioleoyl glycerophosphocholine are significantly thinner. Correspondingly their molecular surface area is by about 8% larger than that of the mixed-chain diradyl glycerophosphocholine, since the partial molar volumes are similar for all vesicles tested.     

On the plasmalogenation of myocardial choline glycerophospholipid during maturation of various vertebrates

1. The plasmalogen profiles of a series of hearts from fish to mammals were obtained by various TLC analyses. 2. All specimens (ventricular) contained ethanolamine plasmalogen and some choline plasmalogen, as well. 3. The distribution of these two plasmalogen species was relatable, in part, to (a) phylogeny and (b) ontogeny. 4. There were exceptions. 5. The appearance of choline plasmalogen was preceded by its alkylacyl precursor, suggesting plasmalogenation by a base-specific delta 1-alkyl desaturase. 6. From the data, we have raised some questions as to the metabolic role played by the plasmalogens and precursors as occupants of myocardial mitchondrial membranes.     

The utilization of ethanolamine and serine for ethanolamine phosphoglyceride synthesis by human Y79 retinoblastoma cells

Phospholipid synthesis was investigated in human Y79 retinoblastoma cells, a cultured cell line of retinal origin that retains many neural characteristics. Ethanolamine is taken up by Y79 cells through a high-affinity transport system and is utilized to synthesize ethanolamine and choline phosphoglycerides. High-affinity ethanolamine uptake has a K'm of 40.6 microM and a V'max of 1.06 nmol/min/mg protein, and the process is Na+ dependent. Choline is the only compound tested that reduced ethanolamine uptake, and very high choline concentrations were required to produce this effect. The cells incorporate ethanolamine into phosphatidylethanolamine and ethanolamine plasmalogen at equivalent rates, and the rates of catabolism of these phospholipids are similar. Only a small quantity of ethanolamine is incorporated into phosphatidylcholine, but the amount is not reduced by the addition of choline. Serine is incorporated into phosphatidylserine, which then is converted to phosphatidylethanolamine. Ethanolamine reduces but does not abolish this conversion. Unlike ethanolamine, only a small amount of serine is incorporated into ethanolamine plasmalogen. It is possible that the ethanolamine high-affinity uptake system is necessary to provide a neural cell with enough free ethanolamine for ethanolamine plasmalogen synthesis.     

High plasmalogen and arachidonic acid content of canine myocardial sarcolemma: a fast atom bombardment mass spectroscopic and gas chromatography-mass spectroscopic characterization

Canine myocardial sarcolemma was purified, and its phospholipid constituents were determined by gas chromatography-mass spectrometry, fast atom bombardment mass spectrometry, and conventional techniques. Canine myocardial sarcolemma contained 2.7 mumol of lipid Pi/mg of protein which was comprised predominantly of choline glycerophospholipids (47%), ethanolamine glycerophospholipids (28%), and sphingomyelin (11%). Sarcolemmal phospholipids contained 40% plasmalogen which was quantitatively accounted for by choline (57% of choline glycerophospholipid) and ethanolamine (64% of ethanolamine glycerophospholipid) plasmalogens. Choline plasmalogens contained predominantly the vinyl ether of palmitic aldehyde though ethanolamine plasmalogens were composed predominantly of the vinyl ethers of stearic and oleic aldehydes. The majority of sarcolemmal ethanolamine glycerophospholipids (75%) contained arachidonic acid esterified to the sn-2 carbon. Sphingomyelin was composed predominantly of long-chain saturated fatty acids (stearic and arachidic) as well as substantial amounts (8%) of odd chain length saturated fatty acids. The possible functional role of these unusual phospholipid constituents is discussed.     

Semisynthetic preparation of choline and ethanolamine plasmalogens

Choline and ethanolamine plasmalogens containing defined acyl chains are prepared by deacylation and reacylation of beef heart plasmalogens. During the reactions, the amino group of ethanolamine plasmalogens is protected by the trityl group. Deacylation is achieved by mild alkaline hydrolysis, and the lysoplasmalogens are reacylated with oleoylimidazolide in the presence of the methylsulfinylmethide anion. The protective group is removed from N-trityl ethanolamine plasmalogen by treatment with silicic acid in hexane. The choline and ethanolamine plasmalogens prepared by the procedures described are free of geometric, positional and steric isomers.     

Differential turnover of polyunsaturated fatty acids in plasmalogen and diacyl glycerophospholipids of isolated cardiac myocytes

To investigate the relative turnover of esterified polyunsaturated fatty acids in diacylglycerophospholipids and plasmalogens in isolated cardiac myocytes, we characterized the phospholipid composition and distribution of radiolabel in different phospholipid classes and in individual molecular species of diradyl choline (CGP) and ethanolamine (EGP) glycerophospholipids after incubation of isolated cardiac myocytes with [3H]arachidonate or [14C]linoleate. Plasmalogens in CGP (55%) and EGP (42%) quantitatively accounted for the total plasmalogen content (39%) of cardiac myocyte phospholipids. Plasmalogens comprised 86% and 51% of total arachidonylated CGP and EGP mass, respectively, and [3H]arachidonate was primarily incorporated into plasmalogens in both CGP (65%) and EGP (61%) classes. The specificity activity of [3H]arachidonylated diacyl-CGP was approximately 2- to 5-fold greater than that of [3H]arachidonylated choline plasmalogen, whereas comparable specific activities were found in the [3H]arachidonate-labeled ethanolamine plasmalogen and diacyl-EGP pools. Of the total linoleate-containing CGP and EGP mass, 54% and 57%, respectively, was esterified to plasmalogen molecular species. However, [14C]linoleate was almost exclusively incorporated into diacyl-CGP (96%) and diacyl-EGP (86%). The specific activities of [14C]linoleate-labeled diacyl-CGP and diacyl-EGP were 5- to 20-fold greater than that of the [14C]linoleate-labeled plasmalogen pools. The differential incorporation of polyunsaturated fatty acids in plasmalogens and diacylglycerophospholipids demonstrates that the metabolism of the sn-2 fatty acyl moiety in these phospholipid subclasses is differentially regulated, possibly fulfilling separate and distinct physiologic roles.     

Serine and ethanolamine incorporation into different plasmalogen pools: Subcellular analyses of phosphoglyceride synthesis in cultured glioma cells

In cultured glioma cells, plasma membrane (PM) is enriched in phosphatidylserine (PtdSer) and plasmalogens (1-O-alk-1-enyl-2-acyl-sn-glycero-3-phosphoethanolamine). Serine can be a precursor of headgroups of both ptdSer and ethanolamine phosphoglycerides (PE) including plasmalogens and non-plasmalogen PE (NP-PE). Synthesis of phospholipids was investigated at the subcellular level using established fractionation procedures and incorporation of [³H(G)]L-serine and [1,2-¹⁴C]ethanolamine. Specific radioactivity of PtdSer from [³H]serine was 2-fold greater in PM than in microsomes, reaching maximum by 2–4 h. Labeled plasmalogen from [³H]serine appeared in PM by 4 h and increased to 48 h, whereas almost no plasmalogen accumulated in microsomes within 12 h. In contrast, labeled plasmalogen from [1,2-¹⁴C]ethanolamine appeared in both PM and microsomes at early incubation times and became enriched in PM beyond 12 h. Thus, in glioma cells: (1) greater and faster accumulation of labeled PtdSer in PM may reflect direct synthesis from serine within PM; (2) PM is a major source of PtdSer for decarboxylation and PE synthesis; (3) NP-PE in both PM and microsome provides headgroup for synthesis of plasmalogen; and, (4) plasmalogen synthesis may involve different intracellular pools depending on headgroup origin.Abbreviations NP-PE nonplasmenylethanolamine phosphoglycerides including both diacyl and alkylacyl species - PE total ethanolamine phosphoglycerides: plasmalogen-plasmenylethanolamine or alkenylacyl ethanolamine phosphoglyceride (1-O-alk-1-enyl-2-acyl-sn-glycero-3-phosphoethanolamine) - PL phospholipid - PM plasma membrane - PtdCho phosphatidylcholine - PtdSer phosphatidylserine     

Kinetic properties of plasmalogenase from bovine brain.

Abstract— Plasmalogenase was assayed by measuring the disappearance of the plasmalogen by two-dimensional thin-layer chromatography. The enzyme was present in a glycerol-bicarbonate extract of an acetone-dried powder from bovine brain. With ethanolamine plasmalogens as the substrate, the K_m was 180 μM. Diacyl glycerophosphorylcholines, diacyl glycerophosphorylethanolamines and choline plasmalogens were competitive inhibitors. With choline plasmalogens as the substrate, the K_m was 208 μM and competitive inhibition was observed with diacyl glycerophosphorylcholines and ethanolamine plasmalogens. The same enzyme may be responsible for the hydrolysis of the alk-1-enyl moiety from both plasmalogens. Plasmalogenase activity was 5.1 μmol/h/g of dog brain, 3.9 μmol/h/g of rat brain and 3.4 μmol/h/g of gerbil brain. A lysophospholipase was also found in the glycerol-bicarbonate extract from the acetone-dried powder. The lysophospholipase was more active in hydrolysing acyl groups from 2-acyl-sn-glycero-3-phosphorylethanolamines than the plasmalogenase was active in hydrolyzing alk-1-enyl groups from 1-alk-1′-enyl-2-acyl-sn-glycero-3-phosphorylethanolamines.     

Enzymic synthesis of ether types of choline and ethanolamine phosphoglycerides by microsomal fractions from rat brain and liver.

The formation of product by ethanolamine phosphotransferases (EC 2.7.8.1) and cholinephosphotransferases (EC 2.7.8.2) in microsomal fractions from brains and livers of mature rats is increased several fold by 1,2-diacyl-sn-glycerols. With the addition of 1-alkyl-2-acyl-sn-glycerols, we have found an 11-fold increase with brain microsomes and a 20-fold increase with lvier microsomes in the synthesis of choline ether lipids (1-alkyl-2-acyl- and 1-alk-1'-enyl-2-acyl-sn-glycero-3-phosphorylcholines). For the synthesis of ethanolamine ether lipids (1-alkyl-2-acyl and 1-alk-1'-enyl-2-acyl-sn-glycero-3-phosphorylethanolamines), the stimulation of alkylacylglycerols was 7-fold for brain microsomes and 18-fold for liver microsomes. The alkylacyl glycerols (8 mM) also inhibited the synthesis of diacyl phosphoglycerides by 44 to 65%, indicating that the same ethanolaminephosphotransferases and cholinephosphotransferases are utilized for the synthesis of alkylacyl phosphoglycerides and diacyl phosphoglycerides. A desaturation of the alkyl groups may take place in the same reaction mixture. The rate of incorporation of phosphorylcholine into alkenylacyl glycerophosphorylcholines (choline plasmalogens) with alkylacylglycerols, cytidine diphosphate choline, and liver microsomes was 15 nmoles per mg protein per hour. The in vitro synthesis of choline plasmalogens with alkylacylglycerols had not been observed previously. The corresponding rate of incorporation of phosphorylethanolamine into ethanolamine plasmalogens was 10 nmoles per mg protein per hour, a value greater than any of the previously reported values for ethanolamine plasmalogen formation from alkylacyl glycerophosphorylethanolamines.     

Isolation of somatic cell mutants defective in the biosynthesis of phosphatidylethanolamine

An in situ autoradiographic assay for CDP-ethanolamine:1,2-sn-diacylglycerol ethanolamine phosphotransferase (EC 2.7.8.1) activity in Chinese hamster ovary cells was developed and used to screen approximately 10,000 individual mutagen-treated colonies attached to filter paper (Esko, J. D., and Raetz, C. R. H. (1978) Proc. Natl. Acad. Sci. U. S. A. 75, 1190-1193). A variant (strain 40.11) was isolated in which the ethanolamine phosphotransferase specific activity in vitro was 6-10-fold less than in the parent, but the level of CDP-choline:1,2-sn-diacylglycerol choline phosphotransferase (EC 2.7.8.2) activity was normal. In extracts, the mutant was also defective in the synthesis of ethanolamine plasmalogen. In vivo, the short term kinetics of labeling with [32P]phosphate or [14C]ethanolamine was correspondingly altered. However, the long tem growth rate and steady state phospholipid compositions of the mutant and parent were quite similar. These results show that the ethanolamine and choline phosphotransferases of Chinese hamster ovary cells are distinct as judged by genetic criteria, while the biosynthesis of phosphatidylethanolamine and its plasmalogen share common enzymatic component(s).     

Preferential synthesis of diacyl and alkenylacyl ethanolamine and choline glycerophospholipids in rabbit platelet membranes

In rabbit platelet membranes, the contents of alkenylacyl phospholipids (plasmalogen) were 56% of phosphatidylethanolamine and 3% of phosphatidylcholine. This uneven distribution of plasmalogens in each phospholipid class could be attributed to the different substrate specificity of ethanolaminephosphotransferase (EC 2.7.8.1) and cholinephosphotransferase (EC 2.7.8.2). The properties of the enzymes were studied, using endogenous diglycerides and CDP-[3H]ethanolamine or CDP-[14C]choline as substrates. The newly formed phospholipids were mainly diacyl and alkenylacyl and only rarely alkylacyl type. The ratios of the labeled alkenylacyl to diacyl type of phospholipids clearly varied with the concentrations of CDP-ethanolamine or CDP-choline. When 1, 10, and 30 microM CDP-[3H]ethanolamine were used, the labeled phospholipids contained 53, 37, and 27% of the alkenylacyl type, respectively. The apparent Km for CDP-ethanolamine to synthesize alkenylacyl and diacyl types were 2.2 and 8.1 microM. On the other hand, when 1, 10, and 30 microM CDP-[14C]choline were used, the labeled lipids contained 10, 17, and 24% alkenylacyl type, respectively. The apparent Km for CDP-choline to synthesize alkenylacyl and diacyl types were 24 and 4.3 microM. Further, the syntheses of diacyl type of phosphatidylethanolamine and the alkenylacyl type of phosphatidylcholine were markedly inhibited by unlabeled CDP-choline and CDP-ethanolamine, respectively. The two enzymes had opposite substrate specificities, and ethanolaminephosphotransferase showed a high preference to plasmalogen synthesis, especially in the presence of CDP-choline.     

Insulin inhibits changes in the phospholipid profiles in sciatic nerves from streptozocin-induced diabetic rats: A phosphorus-31 magnetic resonance study

Sciatic nerve phospholipids obtained from insulin-treated streptozocin-induced diabetic, non-treated streptozocin-induced diabetic, and healthy, control male Sprague-Dawley rats after eighteen weeks of diabetes were studied by ³¹P NMR spectrometry. Eleven phospholipids resonances were identified as follows: Phosphatidic acid (Chemical shift, 0.30 ppm), dihydrosphingomyelin (0.13 ppm), ethanolamine plasmalogen (0.07 ppm), phosphatidylethanolamine (0.03 ppm), phosphatidylserine (−0.05 ppm), sphingomyelin (−0.09 ppm), lysophosphatidylcholine (−0.28 ppm), phosphatidylinositol (−0.30 ppm), alkylacylglycerophosphorylcholine (−0.78 ppm), choline plasmalogen (−0.80 ppm), and phosphatidylcholine (−0.84 ppm). Diabetic rats showed that phosphatidylcholine was significantly elevated p > 0.05, and ethanolamine plasmalogen and choline plasmalogen were significantly lower when compared with both control and insulin treated rats. The choline ratio (choline-containing phospholipids over noncholine phospholipids) was significantly elevated in the diabetic group, when compared with both control and insulin-treated groups. The ethanolamine ratio (ethanolamine-containing phospholipids over nonethanolamine phospholipids) and the ratio of the ethanolamine ratio over the choline ratio, was significantly elevated in the control and the insulin-treated groups when compared with the diabetic group. The presence of phosphatidic acid and the significance in phosphatidylcholine and ethanolamine plasmalogen, suggested that insulin had a role in the phosphatidylcholine metabolism in the rat nerve.     

Identification of plasmalogen as the major phospholipid constituent of cardiac sarcoplasmic reticulum

The phospholipid molecular species of canine myocardial sarcoplasmic reticulum were identified by fast atom bombardment mass spectrometry, reverse-phase high-performance liquid chromatography, and other conventional techniques. Cardiac sarcoplasmic reticulum contains 1.4 mumol of lipid Pi/mg of protein which is comprised of 53% plasmalogen. Cardiac sarcoplasmic reticulum ethanolamine glycerophospholipid contains 73% plasmalogen that is predominantly comprised of moieties with 18-carbon vinyl ethers at the sn-1 position and arachidonic acid at the sn-2 position. In contrast, canine skeletal muscle sarcoplasmic reticulum contains only 19% plasmalogen that is predominantly comprised of ethanolamine plasmalogen (78% of skeletal muscle sarcoplasmic reticulum ethanolamine glycerophospholipid) with arachidonic and docosatetraenoic acids at the sn-2 position. The possibility that tetraenoic ethanolamine plasmalogens in both cardiac and skeletal muscle sarcoplasmic reticulum facilitate calcium translocation by their propensity for adopting a hexagonal II conformation at physiologic temperatures is discussed.     

A phospholipid serine base exchange enzyme

A membrane bound L-serine exchange enzyme which catalyzes the exchange reaction between L-serine and phospholipid-base was solubilized and separated from the ethanolamine-exchange enzyme by Sepharose 4B and DEAE-cellulose column chromatography. The separated fraction was purified approximately 37-fold with a yield of 2--5%. This fraction did not possess ethanolamine or choline exchange activity. The optimal pH was approx. 8.0, the incorporation rate of L-serine into phospholipid was linear up to 20 min incubation time and the activity was maximum at 10 mM CaCl2. The calculated Km value for L-serine was 0.4 mM. Ethanolamine phospholipid was the most effective acceptor for L-serine incorporation, particularly ethanolamine plasmalogen. The Km values obtained were: 0.25 mM for ethanolamine plasmalogen, 0.25mM for pig liver phosphatidylethanolamine and 0.66 mM for egg yolk phosphatidylethanolamine. These observations suggest that the hydrophobic moiety in ethanolamine phospholipid, as well as the base moiety, is important for the affinity of the L-serine exchange enzyme. Neither ethanolamine nor choline inhibited the L-serine exchange activity. There was no detectable conversion of phosphatidylcholine or phosphatidylethanolamine to phosphatidic acid by the partially purified enzyme.