Choline phosphatidylethanolamine

Several studies have evaluated the effects of oral di(2-ethylhexyl) adipate on various aspects of hepatic lipid metabolism. Feeding di(2-ethylhexyl) adipate (2% of diet) to male Wistar rats for seven days resulted in increased hepatic fatty acid-binding protein as well as in increased microsomal stearoyl-CoA desaturation activity (Kawashima et al., 1983a,b). Feeding the compound at this dose for 14 days resulted in increased levels of hepatic phospholipids and a decline in phosphatidyl-choline phosphatidylethanolamine ratio (Yanagita et al., 1987). Feeding di(2-ethyl-hexyl) adipate (2% of diet) to male NZB mice for five days resulted in induction of fatty acid translocase, fatty acid transporter protein and fatty acid binding protein in the liver (Motojima et al., 1998). 

The identity of the moiety (other than glycerol) esterified to the phosphoric group determines the specific phosphoHpid compound. The three most common phosphoHpids in commercial oils are phosphatidylcholine or lecithin [8002-45-5] (3a), phosphatidylethanolamine or cephalin [4537-76-2] (3b), and phosphatidjlinositol [28154-49-7] (3c). These materials are important constituents of plant and animal membranes. The phosphoHpid content of oils varies widely. Laurie oils, such as coconut and palm kernel, contain a few hundredths of a percent. Most oils contain 0.1 to 0.5%. Com and cottonseed oils contain almost 1% whereas soybean oil can vary from 1 to 3% phosphoHpid. Some phosphoHpids, such as dipaLmitoylphosphatidylcholine (R = R = palmitic R" = choline), form bilayer stmetures known as vesicles or Hposomes. The bdayer stmeture can microencapsulate solutes and transport them through systems where they would normally be degraded. This property allows their use in dmg deHvery systems (qv) (8). 

Fig. 1. Chemical stmcture of phosphatidylcholine (PC) (1) and other related phosphohpids. R C O represents fatty acid residues. The choline fragment may be replaced by other moieties such as ethanolamine (2) to give phosphatidylethanolamine (PE), inositol (3) to give phosphatidylinositol (PI), serine (4), or glycerol (5). IfH replaces choline, the compound is phosphatidic acid (6). The corresponding lUPAC-lUB names ate (1), l,2-diacyl-t -glyceto(3)phosphocholine (2), l,2-diacyl-t -glyceto(3)phosphoethanolamine (3), 1,2-diacyl-t -glyceto(3)phosphoinositol (4), 1,2-diacyl-t -glyceto(3)phospho-L-serine and (5), l,2-diacyl-t -glyceto(3)phospho(3)-t -glycetol.

Phosphatidylethanolamine synthesis begins with phosphorylation of ethanol-amine to form phosphoethanolamine (Figure 25.19). The next reaction involves transfer of a cytidylyl group from CTP to form CDP-ethanolamine and pyrophosphate. As always, PP, hydrolysis drives this reaction forward. A specific phosphoethanolamine transferase then links phosphoethanolamine to the diacylglycerol backbone. Biosynthesis of phosphatidylcholine is entirely analogous because animals synthesize it directly. All of the choline utilized in this pathway must be acquired from the diet. Yeast, certain bacteria, and animal livers, however, can convert phosphatidylethanolamine to phosphatidylcholine by methylation reactions involving S-adenosylmethionine (see Chapter 26). 

FIGURE 25.19 Diacylglycerol and CDP-diacylglycerol are the principal precursors of glycerolipids in eukaryotes. Phosphatidylethanolamine and phosphatidylcholine are formed by reaction of diacylglycerol with CDP-ethanolamine or CDP-choline, respectively. 

Phosphatidylethanolamine (cephalin) and ph os-phatidylserine (found in most tissues) differ from phosphatidylcholine only in that ethanolamine or serine, respectively, replaces choline (Figure 14-8). 

These compounds constimte as much as 10% of the phospholipids of brain and muscle. StmcmraUy, the plasmalogens resemble phosphatidylethanolamine but possess an ether link on the sn- carbon instead of the ester link found in acylglycerols. Typically, the alkyl radical is an unsamrated alcohol (Figure 14-10). In some instances, choline, serine, or inositol may be sub-stimted for ethanolamine. 

Figure 24-2. Biosynthesis of triaq/lglycerol and phospholipids. ( , Monoacylglycerol pathway (D, glycerol phosphate pathway.) Phosphatidylethanolamine may be formed from ethanolamine by a pathway similar to that shown for the formation of phosphatidylcholine from choline.

Ethanolamine Choline HOCH2CH2NHj HOCH2CH2N+(CH3)3 Phosphatidylethanolamine Phosphatidylcholine also called Lecithin PE PC... 

It can be seen from Figure 1 that the choline-containing phospholipids, phosphatidylcholine and sphingomyelin are localized predominantly in the outer monolayer of the plasma membrane. The aminophospholipids, conprising phosphatidylethanolamine and phosphatidylserine, by contrast, are enriched in the cytoplasmic leaflet of the membrane (Bretcher, 1972b Rothman and Lenard, 1977 Op den Kamp, 1979). The transmembrane distribution of the minor membrane lipid components has been determined by reaction with lipid-specific antibodies (Gascard et al, 1991) and lipid hydrolases (Biitikofer et al, 1990). Such studies have shown that phosphatidic acid, phosphatidylinositol and phosphatidylinositol-4,5-fc -phosphate all resemble phosphatidylethanolamine in that about 80% of the phospholipids are localized in the cytoplasmic leaflet of the membrane. 

Achve translocation of phospholipids aaoss the plasma membrane has been demonstrated both from the inner to the outer leaflet and from the outer to the inner leaflet. The translocation processes specifically transport phosphatidyserine and phosphatidylethanolamine from the cytoplasmic to the outer surface of the membrane while choline phosphatides are transported from the outer to the cytoplasmic surface. The rate of translocahon, in general, is greater for the amino phospholipids compared with the choline phospholipids. 

Figure 1. Pathways for the synthesis of phosphatidylcholine, phosphatidylethanolamine and sphingomyelin. Abbreviations CK, choline kinase CPT, cholinephosphotransferase CT, CTP phosphooholine cytidylyltransferase DAG, diacylglycerol PC, phosphatidylcholine PE, phosphatidylethanolamine PEMT, phosphatidylethanolamine-N-methyltransferase SM, sphingomyelin SMase, sphingomyelinase SMsyn, sphingomyelin synthase.

A minor pathway to synthesize PC, which is mainly active in liver cells, utilizes the enzyme phosphatidylethanolamine-A-methyltransferase (PEMT), which converts phosphatidylethanolamine (PE) to PC by the subsequent transfer of three methyl groups from S-adenosylmethionine (Vance et al, 1997). The PEMT pathway, which links PE synthesis to PC, was found to be critical for PC homeostasis in the Uver dining choline deficiency (Walkey et al, 1997). 

Figure 2. Effect of Ca-ceramide on phosphatidylcholine and phosphatidylethanolamine synthesis in rat-2 fibroblasts. Cells were treated for 2 h in the absence (open bars) or presence (hatched bars) of 25pM C6-ceramide and the following parameters were determined i) the incorporation of [ H]choline and [ H]ethanolamine into phosphatidylcholine (PC) and phosphatidylethanolamine (PE), respectively (panel A) and in CDP-choline and CDP-ethanolamine, respectively (panel B) and ii) the in vitro activity of choline- and ethanolaminephosphotransferase (CPT and EPT) (panel C).

Figure 11.21 Outline of synthesis of phosphatidylinositol, phosphatidylserine, phosphatidylethanolamine and phosphatidylcholine. Note in the synthesis of phosphatidylinositol, the free base, inositol, is used directly. Inositol is produced in the phosphatase reactions that hydrolyse and inactivate the messenger molecule, inositol trisphosphate (IP3). This pathway recycles inositol, so that it is unlikely to be limiting for the formation of phosphatidylinositol bisphosphate (PIP )- This is important since inhibition of recycling is used to treat bipolar disease (mania) (Chapter 12, Figure 12.9). Full details of the pathway are presented in Appendix 11.5. Inositol, along with choline, is classified as a possible vitamin (Table 15.3).

Phosphatidylcholine (lecithin) is the most abundant phospholipid in membranes. Phosphatidylethanolamine (cephalin) has an ethanolamine residue instead of choline, and phosphatidylserine has a serine residue. In phosphatidylinositol, phosphatidate is esterified with the sugarlike cyclic polyalcohol myo-inositol. A doubly phosphorylated derivative of this phospholipid, phosphatidylinositol 4,5-bisphosphate, is a special component of membranes, which, by enzymatic cleavage, can give rise to two second messengers, diacylglycerol (DAG) and inositol l,4,5trisphosphate (InsPsi see p.386). 

Transfer of a phosphocholine residue to the free OH group gives rise to phosphatidylcholine (lecithin enzyme l-alkyl-2-acetyl-glycerolcholine phosphotransferase 2.7.8.16). The phosphocholine residue is derived from the precursor CDP-choline (see p. 110). Phos-phatidylethanolamine is similarly formed from CDP-ethanolamine and DAG. By contrast, phosphatidylserine is derived from phosphatidylethanolamine by an exchange of the amino alcohol. Further reactions serve to interconvert the phospholipids—e.g., phosphatidylserine can be converted into phosphatidylethanolamine by decarboxylation, and the latter can then be converted into phosphatidylcholine by methylation with S-adenosyl methionine (not shown see also p. 409). The biosynthesis of phosphatidylino-sitol starts from phosphatidate rather than DAG. 

This enzyme catalyzes the reaction of a phospholipid (for example, phosphatidylserine) with ethanolamine to produce phosphatidylethanolamine and the free base (/.c., the amine-containing metabolite serine), thereby preserving the phosphodiester linkage. Ethanolamine can be replaced with serine, choline, monomethyletha-nolamine, and dimethylethanolamine. 

FIGURE 21-28 Pathway for phosphatidylcholine synthesis from choline in mammals. The same strategy shown here (strategy 2 in Fig. 21-24) is also used for salvaging ethanolamine in phosphatidylethanolamine synthesis. 

Although the role of lipid composition in membrane function is not entirely understood, changes in composition can produce dramatic effects. Researchers have isolated fruit flies with mutations in the gene that encodes ethanolamine kinase (analogous to choline kinase Fig. 21-28). Lack of this enzyme eliminates one pathway for phosphatidylethanolamine synthesis, thereby reducing the amount of this lipid in cellular membranes. Flies with this mutation—those with the genotype easily shocked—exhibit transient paralysis following electrical stimulation or mechanical shock that would not affect wild-type flies. 

Mammalian cells have some pathways similar to those in bacteria, but somewhat different routes for synthesizing phosphatidylcholine and phosphatidylethanolamine. The head-group alcohol (choline or ethanolamine) is activated as the CDP derivative, then condensed with diacylglycerol. Phosphatidylserine is derived only from phosphatidylethanolamine. 

Source of choline and ethanolamine used for phospholipid synthesis Phosphatidylethanolamine (PE) and phosphatidylcholine (PC) are the most abundant phospholipids in most eukaryotic cells. The primary route of their synthesis uses choline and ethanolamine obtained either from the diet or from the turnover of the body s phospholipids. Because the amount of choline the body makes is insufficient for its need, choline is an essential dietary nutrient. 

Lysophospholipids have been found in butter serum by Cho et al. (1977). They characterized the sn-1 and -2 lysophosphatidylcholines and phosphatidylethanolamines. It is not known if these compounds are products of degradation or remnants of biosynthesis. Cho et al. (1977) searched for, but did not find, another possible product of enzymatic degradation of milk, phosphatidic acid. Phosphatidic acid can be formed by the action of phospholipase D on phosphatidylcholine, for example, but this enzymatic activity was not detected. The compound is also an important intermediate in the biosynthesis of lipids, but the concentration in tissue is always very low. The amount is also low in milk. Cho et al. (1977) found 1.2 and 0.9 (percent of total lipid P) of the lyso compounds above. The quantities of the other phospholipids were phosphatidylethanolamine, 27.3 -choline, 29.1 -serine, 13.4 -inositol, 2.5 and sphingomyelin, 25.6. 

Morrison et al (1965) reported the positional distribution of the fatty acids in phosphatidylethanolamine, -serine, and -choline. In contrast to the TGs, the phospholipids had no short chain acids and many more long chain unsaturates. There were more unsaturates in phosphatidylethanolamine than in -serine or -choline. The distribution of the acids between sn-1 and sn-2 is similar to that observed in other tissues, with... 

Hay and Morrison (1971) later presented additional data on the fatty acid composition and structure of milk phosphatidylethanolamine and -choline. Additionally, phytanic acid was found only in the 1-position of the two phospholipids. The steric hindrance presented by the four methyl branches apparently prevents acylation at the 2-position. The fairly even distribution of monoenoic acids between the two positions is altered when the trans isomers are considered, as a marked asymmetry appears with 18 1 between the 1- and 2-positions of phosphatidylethanolamine, but not of phosphatidylcholine. Biologically, the trans isomers are apparently handled the same as the equivalent saturates because the latter have almost the same distribution. There are no appreciable differences in distribution of cis or trans positional isomers between positions 1 and 2 in either phospholipid. Another structural asymmetry observed is where cis, cis nonconjugated 18 2s are located mostly in the 2-position in both phospholipids. It appears that one or more trans double bonds in the 18 2s hinders the acylation of these acids to the 2-position. 

Fig. 21-5, are also used for formation of both phosphatidylcholine and phosphatidylethanolamine. In both cases, the free base, choline, or ethanolamine180a b is phosphorylated with ATP. Choline phosphate formed in this manner is then converted by reaction with CTP to CDP-choline (Eq. 17-58).181 Phosphatidylcholine is formed from this intermediate1813/b while CDP-ethanolamine is used to form phosphatidylethanolamine (Fig. 21-5). These synthetic reactions occur within cell nuclei as well as on surfaces of cytoplasmic membranes.1810...

The formation of phosphatidylserine and possibly other phospholipids in animal tissues may also be accomplished by exchange reactions (Eq. 21-10, step a). 82 83 At the same time, decarboxylation of phosphatidylserine back to phosphatidylethanolamine (Eq. 21-10, step b) also takes place, the net effect being a catalytic cycle for decarboxylation of serine to ethano-lamine. The latter can react with CTP to initiate synthesis of new phospholipid molecules or can be converted to phosphatidylcholine (step c). However, unless there is an excess of methionine and folate in the diet, choline is an essential human nutrient.184... 

Synthesis of most phospholipids starts from glycerol-3-phosphate, which is formed in one step from the central metabolic pathways, and acyl-CoA, which arises in one step from activation of a fatty acid. In two acylation steps the key compound phosphatidic acid is formed. This can be converted to many other lipid compounds as well as CDP-diacylglycerol, which is a key branchpoint intermediate that can be converted to other lipids. Distinct routes to phosphatidylethanolamine and phosphatidylcholine are found in prokaryotes and eukaryotes. The pathway found in eukaryotes starts with transport across the plasma membrane of ethanolamine and/or choline. The modified derivatives of these compounds are directly condensed with diacylglycerol to form the corresponding membrane lipids. Modification of the head-groups or tail-groups on preformed lipids is a common reaction. For example, the ethanolamine of the head-group in phosphatidylethanolamine can be replaced in one step by serine or modified in 3 steps to choline. 

Additional regulation of phosphatidylcholine and phosphatidylethanolamine biosynthesis occurs at the second step in the biosynthetic sequence (see fig. 19.4) where either CDP-choline or CDP-ethanolamine are made. For phosphatidylcholine biosynthesis, the activity of CTP phos-phocholine cytidylyltransferase (which makes CDP-choline) is governed by an unusual mechanism. The enzyme... 

The major (salvage) pathways for the formation of phosphatidylcholine and ethanolamine are illustrated in Figure 19.16. Free (dietary) choline and etha-nolamine are converted to their CDP derivatives, which then react with diacyl-glycerol to form phosphatidylcholine and ethanolamine. In the lungs, another pathway forms dipalmitoyl phosphatidylcholine, a powerful surfactant. Phos-phatidylethanolamine may be methylated by S-adenosylmethionine (SAM see Chapter 20) to yield phosphatidylcholine. The reaction is catalyzed by two enzymes the first methyl group is transferred via phosphatidylethanolamine N-methyltransferase I. The other two methyl groups are transferred by phosphatidylethanolamine N-methyltransferase II. Some authorities believe that the two enzymes are identical. It has also been proposed that methylation of phospha-... 

Methionine is intimately related to lipid metabolism in the liver. Methionine deficiency is one of the causes of the fatty liver syndrome. Lack of methionine prevents the methylation of phosphatidylethanolamine to phosphatidylcholine, resulting in an ability by the liver to build and export very low density lipoprotein. The syndrome can be treated by the administration of choline, and for this reason, choline has often been referred to as the lipotropic factor. 

Synthesis of phosphatidylcholine and phosphatidylethanolamine begins with activation of choline (or ethanolamine) with ATP via choline kinase to yield phosphocholine (phosphoethanolamine) + ADP the activated base is transferred via CTP and phosphocholine cytidyl transferase to form CDP-choline (CDP-ethanolamine) and PPi. The base is then transferred to the sn-3 of diacylglycerol via phosphocholine diacylglycerol transferase to yield phosphatidylcholine (phosphatidylethanolamine) + CMP. The cytidyl transferase is believed to be the rate-limiting or regulatory step in the pathway. Phospha-tidylserine is formed by a direct transfer and substitution of serine for ethanolamine in phosphatidylethanolamine. Phosphatidyl serine can be dec-arboxylated to form phosphatidylethanolamine. 

A typical biological membrane is a complex structure composed primarily of lipids and proteins. The major structural components of the bilayer are various lipids. In eukaryotes, the most common type of lipids are phosphatidylcholines, whereas in prokaryotes (such as Escherichia coli), the main lipids are typically phosphatidylethanolamines (1). One example of a typical eukaryotic neutral (zwitterionic) phospholipid is palmitoyl-oleoy 1-phosphatidylcholine (POPC). The molecular structure of POPC is compared to those of dimyristoylphosphatidyl-choline (DMPC) and the negatively charged dimyristoylphosphatidylglycerol (DMPG), commonly used in membrane mimetics, in Fig. 1. 

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