Phosphatidylcholines isomers

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. 

Hay and Morrison (1971) did not neglect the alkyl and alkenyl ethers in milk phospholipids, finding 4% of the latter in phosphatidylethanolamine and 1.3% in phosphatidylcholine. Trans isomers were not found. The authors postulated that the branched chain compounds in the alkenyl ethers were derived from rumen microbial lipids. 

Physical Methods. H.p.l.c. procedures for the separation and assay of ubiquinone and homologues278-280 and of menaquinone cis- and fra/ts-isomers, 2,3-epoxides, and chain-length homologues281 282 have been described. A XH n.m.r. study has been reported283 of the location and motion of ubiquinones in perdeuteriated phosphatidylcholine bilayers. Other aspects of the interaction of ubiquinone with phospholipid monolayers have been studied.284... 

Many monolayer studies have been made with phospholipids and sterols which have helped to clarify the details of the molecular properties of both types of molecule required to bring about the so-called condensation effect [21-23,27]. After various speculations and suggestions, it was shown that a cis double bond at the 9,10 position of the acyl chain was not necessary for condensation, and that trans isomers and even fully saturated phospholipid could bring about this condensation effect [24]. The condensation effects are not restricted to particular phospholipid classes but have been demonstrated to occur with phosphatidylcholines and phosphatidylethanolamines as well as with phosphatidic acid [25], sphingomyelin, phosphatidylglycerol and phospholipid derivatives [26]. 

Both sonicated and imsonicated dispersions of dipalmitoylglycerophosphocholine and cholesterol in water have been studied by proton nmr spectroscopy. The observed line widths provide a useful indication of the mobility of the system. Analysis of the relative intensities of the observed signals locates the steroid between the phospholipid hydrocarbon chains of the bilayer with the steroid hydroxyl group adjacent to the phosphate groups of the phosphatidylcholines [34]. The number of hydrocarbon chain gauche-trans isomers of the phosphohpid are reduced by the presence of the cholesterol molecules. 

Pisch s group [31], Phosphatidylinositol (PI) was synthesized from dioleoyl phosphatidylcholine (DOPC) using mutant PLD [32], although the yield was only 2.5% of PI isomers. While some PLD with plant origins were reported to catalyze PI synthesis, these enzymes were not easily available for industrial purposes [33]. The synthesis of cardiolipin (diphosphatidylglycerol) was also reported by Piazza et al. [34]. 

The polyhedral borane ion [n-B2oH,8] reacts with liquid ammonia in the presence of a suitable base to produce an apical-equatorial isomer of the [B2oH,7NH3] ion. This species undergoes acid-catalyzed rearrangement to an apical-apical isomer. The sodium salts of both isomers were encapsulated in unilamellar liposomes, composed of distearoyl phosphatidylcholine/cholesterol, and investigated as boron delivery agents for BNCT (Feakes et al. 1994 Shelly et al. 1992). 

Synthesis of triacylglycerols requires the enzyme diacylglycerol acyltransferase (EC 2.3.1.20). The enzyme is particulate and has been studied in many mammalian tissues (O Doherty, 1978). In some cases activity is stimulated by the additon of phosphatidylcholine (O Doherty et al, 1974) and there is also evidence that the Z protein may be involved in stimulating acyltransferase activity (O Doherty and Kuksis, 1975). 1,2-Isomers of diacylglycerol are about twice as effective as 2,3-isomers. The enzymes show little chain-length specificity, however (e.g. Hill etal, 1968). 

CLA and their metabolites have been shown to be incorporated mainly into neutral lipids from different tissues (Table 15.1). This shows that for acylation, CLA is behaving more like oleic acid than like linoleic acid. When fed in the same quantity, more 9c,llt-isomer than lOt, 12c-18 2 was found in the different tissues studied. Similarly, very few CLA metabolites were incorporated into the phospholipids (Table 15.2). On the contrary, using diets low in essential fatty acids (butter fat), Banni et al. (4) demonstrated that the conjugated arachidonic acid was enriched predominantly in phosphatidylserine and phosphatidylinositol. On the contrary, arachidonic acid was incorporated mainly into phosphatidylcholine and to a lesser extent into phosphatidylethanolamine. In mice, even using diets equilibrated in essential fatty acids, lOt, 12c-18 2 was shown to be converted into conjugated arachidonic acid (Table 15.3). This metabolite was incorporated preferentially into phospholipids, thus confirming the data of Banni et al (4). 

Nakanishi, H., lida, Y., Shimizu, T. and Taguchi, R. (2010) Separation and quantification of sn-1 and sn-2 fatty acid positional isomers in phosphatidylcholine by RPLC-ESIMS/MS. J. Biochem. 147, 245-256. 

The transfer of phosphatidylcholine from monolayer to vesicles catalyzed by phosphatidylcholine transfer protein shows remarlcable differences for the positional isomers (23). The PC transfer protein acts as a specific carrier of PC between membrane interfaces. This protein has a recognition site for the phosphorylcholine headgroup and binding sites for the sn-1 and sn-2 fatty acyl chain. Lysophosphatidylcholine is not transferred. It was found that the protein transferred C10 0/C18 l PC twice as fast as C18 l/C10 0 PC. Similar differences in rate were observed for C12 0/C18 l PC and C18 l/C12 0 PC but not for isomers carrying myristic acid (Table I). 

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