Quantitative gas-liquid chromatographic analysis of ethanolamine, monomethyl ethanolamine, and dimethyl ethanolamine from lipids

Partial purification and characterization of phospholipid N-methyltransferases from murine thymocyte microsomes

Significant amounts of phospholipid N-methyltransferase activity in murine thymocytes were found to be distributed on the plasma membrane. The enzyme activity had an optimum pH of 9. The presence of divalent cations, Mg2+ (10 mM) or Ca2+ (1 mM), and EGTA separately in the assay had only a small effect on the enzyme activity. However, addition of both 10 mM Mg2+ and 1 mM Ca2+ increased the enzyme activity. The presence of two enzymes for each conversion of phosphatidylethanolamine (PE) to phosphatidylmonomethylethanolamine (PME) and PME to phosphatidylcholine (PC) was suggested by the result of the determination of the incorporated radioactivity into PME, phosphatidyldimethylethanolamine (PDE) and PC; the apparent Km values for S-adenosyl-L-methionine were 20 and 400-500 microM for the conversion of PE to PME and for the conversion of PME to PC they were 5 microM and 40 microM. S-Adenosyl-L-homocysteine (AdoHcy), a known inhibitor of enzymatic methylation, competitively inhibited [14C]methyl incorporation into total lipid. The apparent Ki value for AdoHcy was 44.7 microM. Two phospholipid N-methyltransferases were partially purified by extraction with sodium deoxycholate, gel filtration on Sephadex G-75, and affinity column chromatography on AdoHcy-Sepharose. One enzyme, mainly catalyzing the formation of PME, was purified approximately 1548-fold and the other catalyzing the formation of PDE and PC, was purified approximately 629- to 703-fold. However, the former still contained a little activity for PDE and PC formation and the latter contained a little activity for PME formation. In these partially purified phospholipid N-methyltransferase preparations, little contaminating protein O-carboxylmethyltransferase activity was observed; however, significant PC-phospholipase A2 activity was detected. This result may suggest that phospholipid N-methyltransferases associate with phospholipase A2 in the thymocyte plasma membrane.

Characterization of heart sarcolemmal phospholipid methylation

The transmethylation of phosphatidylethanolamine (PE) to phosphatidylcholine (PC) was studied in rat heart sarcolemmal membrane. Kinetically, three apparent Km values for S-adenosyl-L-methionine (AdoMet) were obtained when the total [3H]methyl groups incorporation into the phospholipids was examined in the presence of 0.01-250 microM AdoMet. A first methyltransferase active site having a very low Km (0.1 microM) for AdoMet showed a partial requirement for Mg2+ and an optimum pH of 8.0 with a major formation of phosphatidyl-N-monomethylethanolamine (PMME). Both Ca2+ and K+ were inhibitory to this site. A second active site with a Km of 3.6 microM showed an optimum pH of 7.0 with predominant formation of phosphatidyl-N,N-dimethylethanolamine (PDME) and no Mg2+ requirement; in addition, transmethylation activity was also observed over a broad alkaline pH range (9-11) with an optimum at pH 10.5. This site was insensitive to Ca2+ but was stimulated by Na+, while K+ had an inhibitory effect. A third active site with a Km of 119 microM showed an optimum pH of 10.5 with major formation of PC and no Mg2+ requirement. This site was also insensitive to Ca2+ but markedly inhibited by both K+ and Na+. Under optimal conditions, the activities of all three methyltransferase sites were linear for at least 30 min of incubation and the sensitivity to the inhibitory effect of S-adenosyl-L-homocysteine was different for each site. Addition of exogenous PMME and PDME as substrates enhanced the synthesis of the corresponding methylated products by 3-5-fold and 3-8-fold, respectively. In contrast, exogenous PE failed to increase methyltransferase activity. These results provide evidence for the existence of three distinct methyltransferase active sites in rat heart sarcolemma.

Characterization of brain phosphatidylserine decarboxylase: localization in the mitochondrial inner membrane

A membranous fraction from calf brain, sedimenting at 10,000g, catalyzes the decarboxylation of exogenous phosphatidyl[14C]serine presented in an aqueous dispersion in detergent. The product formed by the enzymatic decarboxylation reaction is chemically and chromatographically identical to phosphatidyl[14C]ethanolamine. The calf brain decarboxylase activity: (1) did not require divalent cations; (2) was optimally active at neutral pH; (3) exhibited maximal activity in the presence of 0.1% Cutscum or sodium taurocholate; (4) was inhibited by hydroxylamine or p-hydroxymercuribenzoate; and (5) has an apparent Km = 2.4 mM for the phospholipid substrate. When this fraction was further separated by metrizamide density centrifugation, 90% of the phosphatidylserine decarboxylase activity was associated with the mitochondria. Resolution of the inner and outer membranes of the mitochondria revealed that greater than 99% of the decarboxylase activity was bound to the inner membrane. In contrast to this result, diacylglycerol ethanolaminephosphotransferase, another enzyme responsible for phosphatidylethanolamine biosynthesis in brain, was greatly enriched in the microsomal fraction. The highest level of phospholipid N-methyltransferase activity was also localized in the microsomal fraction. Thus, phosphatidylethanolamine formation via cytidine diphosphate ethanolamine in brain occurs at a membrane site where it should be available for the biosynthesis of phosphatidylcholine by stepwise methylation. In order for phosphatidylethanolamine formed by the decarboxylation reaction to be available for N-methylation, translocation from mitochondria to the microsomal site would be required.

High-performance liquid chromatography of methylated phospholipids

A rapid high-performance liquid chromatographic method for the separation of methylated phospholipids is described. The separation is accomplished on an amine column using acetonitrile--methanol--water as the eluting solvent and UV detection at 203 nm. The choice between gradient and isocratic elution for the separation depends upon the condition of column. The method is suitable for the isolation of phosphatidylcholine, sphingomyelin, lysophosphatidylcholine, phosphatidylethanolamine, phosphatidylmonomethylethanolamine, phosphatidyldimethylethanolamine and lysophosphatidylethanolamine from tissues. It is applicable to the study of reaction products in phosphatide methyltransferase assay mixtures. Choline and ethanolamine plasmalogens can be determined indirectly by converting them into lysophosphatidylcholine and lysophosphatidylethanolamine with exposure to hydrochloric acid fumes.

Properties of particulate and detergent-solubilized phospholipid N-methyltransferase activity from calf brain

Calf brain membranes catalyze the enzymatic transfer of [CH3-3H]methyl groups from S-adenosyl-L-[CH3-3H]methionine into endogenous phosphatidyl-N-methylethanolamine (PME), phosphatidyl-N,N-dimethylethanolamine (PDE), and phosphatidylcholine (PC). Phospholipid N-methylation can be stimulated by the addition of exogenous PME or PDE, added in aqueous dispersions with sodium taurocholate. When membranes are incubated in the presence of exogenous PME, [CH3-3H]PDE represents 86% of the labeled phospholipid products. When exogenous PME is replaced by PDE, 91% of the label is incorporated into PC. Thus, under these in vitro conditions it is possible to assay PME- and PDE-N-methyltransferase activity separately. The calf brain phospholipid N-methyltransferase activity has also been solubilized by treating the membranes ultrasonically in the presence of Triton X-100 and 10 mM monothioglycerol. When the detergent extracts are incubated in the presence of exogenous PME, [CH3-3H]PDE represents 86% of the enzymatically labeled products. In the presence of exogenous PDE, more than 97% of the label is incorporated into PC. Optimal conditions for the membrane-bound and detergent-solubilized PME- and PDE-N-methyltransferase activity have been established. These conditions have been used as a basis for testing the hypothesis that the conversion of PME to PC is catalyzed by a single enzyme in calf brain. In these studies, PME- and PDE-N-methyltransferase activities have been found to be similar, if not identical, with respect to: (1) extractability with Triton X-100; (2) pH optimum; (3) response to divalent cations; (4) apparent Km for S-adenosyl-L-methionine and K1 for S-adenosyl-L-homocysteine, (5) sensitivity to N-ethylmaleimide; and (6) thermal inactivation at 55 degrees. Overall, these results are consistent with the conclusion that in calf brain, PME and PDE are methylated by the same enzyme or by two phospholipid N-methyltransferases having very similar properties.

Quantitation of phosphatidyl N-methyl-and N,N-dimethylaminoethanol in rat liver

The contents of phosphatidyl N-methyl- and N,N-dimethylaminoethanol were determined in the liver of rats injected with (Me-C14) methionine. Total phospholipids were extracted from aliquots of the liver and fractionated by two-dimensional thin layer chromatography after addition of carrier phosphatidyl-N-methyl- and N,N-dimethylaminoethanol. The radioactivity present in the two phosphatide spots was determined and used to calculate total disintegrations per min/100 g body wt. The remainder of the livers was pooled, and total phospholipids were isolated and subjected to acid hydrolysis. N-methyl- and N,N-dimethylaminoethanol were purified by thin layer chromatography, and their specific activity was determined after quantitation by gas liquid chromatography and radioactivity measurement. The liver contents of phosphatidyl N-methyl- and N,N-dimethylaminoethanol were determined by dividing disintegrations per min/100 g body wt by the specific activity of N-methyl- or N,N-dimethylaminoethanol.

Lipid composition of the electron transport membrane of Haemophilus parainfluenzae

The principal lipids associated with the electron transport membrane of Haemophilus parainfluenzae are phosphatidylethanolamine (78%), phosphatidylmonomethylethanolamine (0.4%), phosphatidylglycerol (18%), phosphatidylcholine (0.4%), phosphatidylserine (0.4%), phosphatidic acid (0.2%), and cardiolipin (3.0%). Phospholipids account for 98.4% of the extractible fatty acids. There are no glycolipids, plasmalogens, alkyl ethers, or lipo amino acid esters in the membrane lipids. Glycerol phosphate esters derived from the phospholipids by mild alkaline methanolysis were identified by their staining reactions, mobility on paper and ion-exchange column chromatography, and by the molar glycerol to phosphate ratios. Eleven diacyl phospholipids can be separated by two-dimensional thin-layer chromatography. Each lipid served as a substrate for phospholipase D, and had a fatty acid to phosphate ratio of 2:1. Each separated diacyl phospholipid was deacylated and the glycerol phosphate ester was identified by paper chromatography in four solvent systems. Of the 11 separated phospholipids, 3 were phosphatidylethanolamines, 2 were phosphatidylserines, and 2 were phosphatidylglycerols. Phosphatidylcholine, cardiolipin, and phosphatidic acid were found at a single location. Phosphatidylmonomethylethanolamine was found with the major phosphatidylethanolamine. Three distinct classes of phospholipids are separable according to their relative fatty acid compositions. (i) The trace lipids consist of two phosphatidylethanolamines, two phosphatidylserines, phosphatidylcholine, phosphatidic acid, and a phosphatidylglycerol. Each lipid represents less than 0.3% of the total lipid phosphate. These lipids are characterized by high proportions of the short (C(10) to C(14)) and long (C(19) to C(22)) fatty acids with practically no palmitoleic acid. (ii) The major phospholipids (93% of the lipid phosphate) are phosphatidylethanolamine, phosphatidylmonomethylethanolamine, and phosphatidylglycerol. These lipids contain a low proportion of the short (<C(14)) and long (>C(19)) fatty acids. Palmitic and palmitoleic acids represent over 80% of the total fatty acids. (iii) The fatty acid composition of the cardiolipin is intermediate between the other two classes. Both palmitoleic and the longer fatty acids represent a significant proportion of the total fatty acid.