Phospholipids phosphatidylcholine

There is also inside-outside (transverse) asymmetry of the phospholipids. The choline-containing phospholipids (phosphatidylcholine and sphingomyelin) are located mainly in the outer molecular layer the aminophospholipids (phosphatidylserine and phos-phatidylethanolamine) are preferentially located in the inner leaflet. Obviously, if this asymmetry is to exist at all, there must be limited transverse mobility (flip-flop) of the membrane phospholipids. In fact, phospholipids in synthetic bilayers exhibit an extraordinarily slow rate of flip-flop the half-life of the asymmetry can be measured in several weeks. However, when certain membrane proteins such as the erythrocyte protein gly-cophorin are inserted artificially into synthetic bilayers, the frequency of phospholipid flip-flop may increase as much as 100-fold. 

The cells of all contemporary living organisms are surrounded by cell membranes, which normally consist of a phospholipid bilayer, consisting of two layers of lipid molecules, into which various amounts of proteins are incorporated. The basis for the formation of mono- or bilayers is the physicochemical character of the molecules involved these are amphipathic (bifunctional) molecules, i.e., molecules which have both a polar and also a non-polar group of atoms. Examples are the amino acid phenylalanine (a) or the phospholipid phosphatidylcholine (b), which is important in membrane formation. In each case, the polar group leads to hydrophilic, and the non-polar group to hydrophobic character. 

The rate of production of DAG in the cell does not occur linearly with time, but rather it is biphasic. The first peak is rapid and transient and coincides with the formation of IP3 and the release of Ca2+ this DAG is therefore derived from the PI-PLC catalyzed hydrolysis of phosphatidylinositols [1]. There is then an extended period of enhanced DAG production that is now known to be derived from the more abundant phospholipid phosphatidylcholine (PC), which has a different composition of fatty acid side chains [9]. Although DAG may be generated directly from PC through the action of PC-PLC, it can also be formed indirectly from PC. In this pathway, PC is first hydrolyzed by PLD to give choline and phosphatidic acid, which is then converted to DAG by the action of a phos-phatidic acid phosphatase [10,11 ]. 

Electroneutral phospholipids Phosphatidylcholine, phosphatidyl-ethanol, phosphatidylethanolamine... 

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. 

Mammalian ceU membranes consists of a variety of lipids. The phospholipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are quantitatively the most important lipids in cellular membranes. PC and PE are synthesized via different pathways in mammalian cells. 

Since phospholipids were not soluble in water, it was necessary to emulsify them or use organic solvents for the enzymatic phospholipid hydrolyzing. Consequently, the phospholipid (phosphatidylcholine) was hydrolyzed in a two phase system of diethylether and water by a Z-modified lipase. Table I shows the hydrolysis of phospholipid by a modified lipase. 

Figure 21-3 Major pathways of synthesis of fatty acids and glycerolipids in the green plant Arabidopsis. The major site of fatty acid synthesis is chloroplasts. Most is exported to the cytosol as oleic acid (18 1). After conversion to its coenzyme A derivative it is converted to phosphatidic acid (PA), diacylglycerol (DAG), and the phospholipids phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidylglycerol (PG), and phosphatidylethanolamine (PE). Desaturation also occurs, and some linoleic and linolenic acids are returned to the chloroplasts. See text also. From Sommerville and Browse.106 See also Figs. 21-4 and 21-5. Other abbreviations monogalactosyldiacylglycerol (MGD), digalactosyldiacylglycerol (DGD), sulfolipid (SL), glycerol 3-phosphate (G3P), lysophosphatidic acid (LPA), acyl carrier protein (ACP), cytidine diphosphate-DAG (CDP-DAG).

Figure 8.1. The structure of some common phospholipids ( ) phosphatidylcholine, (b) phosphatidylethanolamine and (c) phosphatidic acid.

Liposomes were first proposed for drug topical administration to the skin more than 25 years ago by Mezei and Gulusekharam [1,2]. The basic components of liposomes are phospholipids (phosphatidylcholine, phophatidylethanolamine, phophatidylserine, dipalmitoyl phosphatidylcholine, and others), cholesterol, and water. Liposomes may vary significantly in terms of size (from tens of nm to microns) and structure. In liposomes, one or more concentric bilayers surround an aqueous core generating small or large unilamellar vesicles (SUV, LUV) or multilamellar vesicles (MLV), respectively [3]. 

Answer Because silica gel is polar, the most hydrophobic lipids elute first, the most hydrophilic last. The neutral lipids elute first cholesteryl palmitate and triacylglycerol. Cholesterol and %-tetradecanol, neutral but somewhat more polar, elute next. The neutral phospholipids phosphatidylcholine and phosphatidylethanolamine follow. Sphingomyelin, neutral but slightly more polar, elutes after the neutral phospholipids. The negatively charged phosphatidylserine and palmitate elute last—phosphatidylserine first because it is larger and has a lower charge-to-mass ratio. 

The positional distribution of the fatty acids in two phospholipids, phosphatidylcholine and phosphatidylethanolamine, in bovine and human milk is described in Table 1.13 (Christie, 1995). The fatty acids in bovine milk phospholipids have a lower degree of unsaturation than those in human milk phospholipids. However, the distribution of fatty acids between the two positions is similar. Polyunsaturated fatty acids are attached preferentially at the sn-2 position, the monounsaturates are distributed similarly between the two positions, and 18 0 is esterified preferentially at the sn-1 position while the other saturated fatty acids (14 0, 16 0) are distributed more evenly (bovine phosphatidylethanolamine being an exception). The phospholipids in human milk show remarkable selectivity for 18 0 at the sn-1 and 18 2 at the sn-2 positions. 

Improvement of membrane separation technology has resulted in the isolation of MFGM-enriched material from commercially available products. A phospholipid-rich fraction can be extracted from whey (Boyd et al., 1999) and buttermilk (Sachedva and Buchheim, 1997) with a reported yield of 0.25 g of phospholipids/g of protein in buttermilk (Sachdeva and Buchheim, 1997). Microfiltration of whey derived from the Cheddar cheese process, using 0.2 pm ceramic filters results in a fraction containing two major phospholipids, phosphatidylcholine and phosphatidylethanolamine, and lesser amounts of phosphatidylinositol, phosphatidylserine, sphingomyelin and cerebrosides (Boyd et al., 1999). The phospholipid fraction separated from the total lipids contains a larger proportion of mono- and polyunsaturated fatty acids (mainly oleic, Cig i and linoleic, C ) compared to the total lipid and the neutral lipid fraction (Boyd et al., 1999). 

Figure 1.5 A phospholipid molecule. In a phospholipid molecule, one fatty acid is replaced with a phosphate group, to which is attached (X) a nitrogen-containing molecule, for example choline, ethanolamine, serine or inositol (giving the phospholipid phosphatidylcholine, phos-phatidylethanolamine, phosphatidylserine or phosphatidylinositol, respectively).

Figure 1.6 An amphipathic phospholipid (phosphatidylcholine) molecule. The amphipathic phospholipid molecule contains a polar head group and non-polar tail this is crucial to the ability of such molecules to self assemble in water to form lipid membranes.

Choline is an essential component of phospholipids - phosphatidylcholine (lecithin) is the major phospholipid in cell membranes and sphingomyelin is important in the nervous system. Acetylcholine is a transmitter in the central and parasympathetic nervous systems and at neuromuscular junctions, and has a role in the regulation of differentiation and development of the nervous system (Biagioni et al., 2000). Acetylcholine is also synthesized in mononuclear lymphocytes, where it has an autocrine or paracrine role in regulating immune function (Fujii and Kawashima, 2001). 

We found these conditions by introducing surface-active materials to a solution of a specially prepared catalytic complex containing molybdenum and stabilized by magnesium ions [22]. The structure of the complex which was isolated from the solution in its oxidized form is presented on Figure 5 [23]. When it is reduced to the Mo state the complex becomes an active catalyst of dinitrogen reduction to hydrazine and ammonia by sodium amalgam. Phospholipid (phosphatidylcholine)... 

Although not shown in the following simplified scheme, the phospatidic acid/ 1,2-diacylglycerol duo is also an intermediate in the pathway to the phospholipids (phosphatidylcholines, phosphatidylethanolamines, phosphatidylinositols, phospha-tidylinositols, and phosphatidylserines) and the mono- and digalactosyldiacylgly-cerols. 

The most common phospholipid, phosphatidylcholine, contains two molecules of fatty acid and one molecule of choline phosphate attached to a glycerol backbone (see Figure 1.11), Like all other phospholipids, it is amphipathic. The water-soluble end features a phosphate group, an amino group, and two keto groups. 

Bear bile contains bile acids, cholesterol, and phospholipids (phosphatidylcholine, phosphatidylethanolamine, and phos-phatidyhnositol). It has been used for centuries in traditional Chinese medicine to treat liver and eye complaints and convulsions, and, in combination tvith curcuma and capil-laris, gallstones and cholecystitis. More recently it has been touted as a treatment for stroke on the basis of animal experiments. It has few or no adverse effects, but by the same token probably has little or no efficacy, although it does contain ursodeoxycholic acid, which in purified form is effective in managing gallstones. 

The total amount of phospholipids (phosphatidylcholine, phosphatidylinositol and phosphatidylethanolamine). 

Those include C-, N- and 5-oxidations, N-, 0- and 5-deaIkylation, deaminations, and certain dehalogenations. Under anaerobic conditions, it can also catalyze reductive reactions. The CYP monooxygenase system is a multien-zymatic complex constituted by the CYP hemoprotein, the flavoprotein enzyme NADPH CYP reductase, and the unsatnrated phospholipid phosphatidylcholine. The isoforms involved in xenobiotic metabolism are membrane bonnd enzymes situated in the endoplasmic reticnlnm. After... 

Experimental work for the determination of phospholipids in olive oil is rather limited. Freshly produced virgin olive oil was found to contain 40-135 mg/kg (Tiscomia et al. 1982). Crude pomace oil has higher levels of phospholipids. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidic acid are the main phospholipids present in unprocessed oil. Phospholipids are partly bound to lipoproteins. 

Plants, however, can synthesize both the desaturases that introduce double bonds at the A and A positions are located in the ER and the chloroplast. The ER enzymes act not on free fatty acids but on a phospholipid, phosphatidylcholine, that contains at least one oleate linked to the glycerol (Fig. 21-14). Both plants and bacteria must synthesize polyunsaturated fatty acids to ensure membrane fluidity at reduced temperatures. 

---