Phosphatidyl esters

This enzyme [EC 3.1.4.4] (also known as lipophosphodiesterase II, lecithinase D, and choline phosphatase) catalyzes the hydrolysis of a phosphatidylcholine to produce choline and a phosphatidate. The enzyme will also act on other phosphatidyl esters. See Micelle... 

Phosphatidyl esters are classed as phosphoacylglycerols. The natures of the fatty acids vary widely, as they do in triacylglycerols. As a result, the names of the types of lipids (such as triacylglycerols and phosphoacylglycerols) that contain fatty acids must be considered generic names. 

A phosphatidyl ester (phosphoacylglycerol). Glycerol is esterfied to two carboxylic acids, stearic acid and linoleic acid, as well as to phosphoric acid. Phosphoric acid, in turn, is esterified to a second alcohol, ROH. 

The classification of a phosphatidyl ester depends on the nature of the second alcohol esterified to the phosphoric acid. Some of the most important lipids in this class are phosphatidyl ethanolamine (cephalin), phosphatidyl serine, phosphatidyl choline (lecithin), phosphatidyl inositol, phosphatidyl glycerol, and diphosphatidyl glycerol (cardiolipin) (Figure 8.5). In each of these types of compounds, the nature of the fatty acids in the molecule can vary widely. All... 

The work of Nyc and associates on the lipids of Neurospora crassa is of particular interest in relation to the bios3mthesis of lecithin. The phospholipid of a choline-requiriug mutant contains the phosphatidyl esters of both mono- and di-methylaminoethanol (Hall and Nyc 1961). Nutritional studies with appropriate mutants of this organism have shown that lecithin may be formed (a) from choline by the cytosine nucleotide pathway (Reactions 11 and 12), (b) from phosphatidyl ethanolamine by the methylation pathway (Reactions 13,14 and 15) and (c) from mono- and di-methylaminoethanol by the incorporation of the base into lipid, presumably by way of CDP-intermediates, followed by methylation. The relative contributions of these pathways can be influenced by mutations and by nutritional supplementation of the culture medium (Crocken and Nyc 1964). 

The most common phospholipids are phosphatidyl esters in which phosphatidic acid is esterified with another hydroxy compound to form a diester of phosphoric acid. Incorporation of choline and ethanolamine as the hydroxy compound would result in the formation of phosphatidylcholine (lecithin) and phosphatidyl ethanolamine (cephaline), respectively. There are enzymes specifically hydrolyze one or more of the four ester bonds in phospholipids and/or galactolipid. Each of the enzymes, phospholipase Ai (PL-Ai), phospholipase A2 (PL-A2), phospholipase C (PL-C), and phospholipase D (PL-D), cleaves a phosphatidyl ester into two products as shown in Figure 1. 

Lecithin is a phosphoglyceride (phosphatide) natural product isolated commercially from soybean oil and, in much lower quantity, from egg yolks. The terminology is somewhat confused for historical reasons (7). While the term lecithin is sometimes used synonymously with a-phosphatidyl choline, it more correctly denotes a crude mixture also containing P-phosphatidyl choline and phosphatidyl esters of other compounds, chiefly ethanolamine, inositol, and serine. (While phosphatidyl choline and phosphatidyl ethano-lamine are amphoteric, phosphatidyl inositol and phosphatidyl serine are anionic.) For soybean lecithin, R and R represent C16 saturated fatty acid and Cjg saturated and unsaturated... 

Let us consider first lipid-lipid interaction. Urry et al, showed the existence of a positive CD band at 218 m/x and a negative CD band at about 192 m/z in phosphatidyl choline and phosphatidyl ethanolamine dissolved in trifluoroethanol (86). The 192-m/z band was not characterized in detail, but the 218-m/z band is of such position and shape that the addition of lipid and protein CD bands could produce a composite CD band, and hence an ORD Cotton effect, which is red shifted. As noted by Urry, the 218-m/z CD extremum of lecithin must arise from n — 7T transitions in the fatty acid ester groups. Although the optical activities of solutions of deproteinized membrane phospholipids determined at the same concentration as in the intact membrane are negligibly small, in membranes an ordered array of lipids could greatly enhance rotation. Such an effect could yield information on the nature of lipid-lipid association. This can be tested experimentally. Halobacterium cutirubrum offers a unique system since Kates has shown that the lipids in this extreme halophile contain ether bonds rather than ester bonds (43, 44), Hence, the n — tt transition essential to the CD band at 218 m/z in phospholipids does not exist. Nevertheless, we found that the ORD... 

The phosphoric acid esters of diacyl glycerides, phospholipids, are important constituents of cellular membranes. Lecithins (phosphatidyl cholines) from egg white or soybeans are often added to foods as emulsifying agents or to modify flow characteristics and viscosity. Phospholipids have very low vapor pressures and decompose at elevated temperatures. The strategy for analysis involves preliminary isolation of the class, for example by TLC, followed by enzymatic hydrolysis, derivatization of the hydrolysis products, and then GC of the volatile derivatives. A number of phospholipases are known which are highly specific for particular positions on phospholipids. Phospholipase A2, usually isolated from snake venom, selectively hydrolyzes the 2-acyl ester linkage. The positions of attack for phospholipases A, C, and D are summarized on Figure 9.7 (24). Appropriate use of phospholipases followed by GC can thus be used to determine the composition of phospholipids. 

In some cases separation of lecithins from phosphatidyl ethan-olamines and phosphatidyl serines is carried out prior to enzymatic hydrolysis. Composition of the phospholipids is virtually always determined finally by GLC of their fatty acid methyl esters. 

Fig. 2. General scheme for transduction of ACTH signal from plasma membrane to mitochondria in the adrenal cortex. CHX, cycloheximide TG. triglyceride CE, cholesterol ester Lts, leukotrienes PGs, prostaglandins SCP2, sterol carrier protein PE, PC, PI, phosphatidyl ethanolamine, choline and inositol (from Ref. 25, with permission).

Ajowan seed contains generally 8.9% moisture, 15.4% protein, 18.1% fat (ether extract), 11.9% crude fibre, 38.6% carbohydrates, 7.1% mineral matter, 1.42% calcium, 0.30% phosphorus and 14.6mg/100g iron, with a calorific value of 379.4 per 100g. The percentage of seed oil extracted with n-hex-ane is 31.80%, while that with ethanol is 28%. The neutral lipid component of the oil includes hydrocarbons, esters, sterol esters, triglycerides, free fatty acids, diglycerides, sterols and monoglycerides, whereas the polar lipid components are phosphatidyl ethanolamines and phosphatidyl cholines (Qasim and Khan, 2001). 

The term, phosphatidyl, is reserved for describing phosphoglycerides which bear two fatty acyl esters in a glycerophosphoric acid backbone. How... 

LPL exhibits no fatty acid specificity during hydrolysis of mixed triglycerides but does have strong positional specificity (Morley and Kuksis, 1977). It acts on primary ester bonds with some preference for the sn-1 over the sn-3 position of triglycerides (Somerharju et al., 1978) and can hydrolyze 2-monoglycerides only after their conversion to the sn-1 or sn-3 isomers (Nilsson-Ehle et al., 1973). It shows phospholipase Ai activity on phosphatidyl choline (i.e., it hydrolyzes the primary ester bond at the sn-1 position). This contrasts with most phospholipases A, which exhibit A2 activitiy. 

Benzyl 2,2,2-trichloroacetimidate (bp 106-114 °C/0.07 kPa) alkylates alcohols in the presence of trifluoromethanesulfomc acid.311 Esters, imides, isopropylidene and benzylidene acetals are unaffected. This method allows the formation of benzyl ethers in molecules that are base-sensitive. In the example [Scheme 4,167],312 benzylation of the P-hydroxy ester 167.1.1 under the usual basic conditions would lead to retro-aldol reactions and/or elimination. A synthesis of the cellular messenger L-a-phosphatidyl-D-myo-inositol 3,4-bisphosphate exemplifies the use of trityl cation-promoted benzylation of two adjacent hydroxyls [Scheme 4.168].313314... 

Although the title of this section is phospholipids , we in fact will, as a case study, concentrate on the most frequently studied class, viz. the diacylglycerophos-pholipid h The molecules of these compounds consist of two fatty acids and a phosphate-containing hydrophilic moiety, connected by ester bonds to glycerol. Some representative structures cire shown in Table 3.8. Of these the phosphatidyl-... 

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