Phosphatidylcholine, positional Phosphatidylglycerol

Figure 4.9 Representative ESI-MS analysis of lipid classes resolved by intrasource separation. Lipid extracts from mouse liver samples were prepared by using a modified procedure of Bligh and Dyer [1]. MS analysis was performed with a TSQ Vantage triple-quadrupole mass spectrometer (Thermo Fisher Scientific, San Jose, CA) equipped with an automated nanospray apparatus (i.e., TriVersa, Advion Bioscience Ltd., Ithaca, NY) and Xcalibur system software. Mass spectra were acqnired directly from the diluted hpid extract in the negative-ion mode (a), after addition of 50 nmol LiOH/mg of protein in the diluted lipid extract and analyzed in the negative-ion mode (h), or the identical hpid solution to that in (b) in the positive-ion mode (c). IS denotes internal standard PC, PE, PG, PI, PS, TAG, NEFA, and CL stand for phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidyUnositoL phosphatidylserine, triacylglycerol, nonesterified fatty acid, and doubly charged cardioUpin, respectively.

Glycerophospholipids contain a glycerol skeleton to which two fatty acids are esterified saturated fatty acids occupy mostly sn-position 1, whereas unsaturated fatty acids are mainly present on sn-position 2. The third hydroxyl is linked to a phosphate group to which an organic base is mostly esterified (Fig. 1). The most important components of soybean lecithin are phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylinositol (PI). Phosphatidic acid (PA) may become important due to the presence of phospholipase D this enzyme slowly converts PC into PA in vegetable lecithins. Phosphatidylserine (PS), phosphatidylglycerol (PG), and lyso-phosphatidylcholine (LPC) are known as minor components lysophospholipids contain only one acyl group per molecule. Besides, ether phospholipids occur in which one or both fatty acyl... 

Phosphodiesterase (Hydrolysis) Activity. A rather broad substrate specificity is exhibited by the purified phospholipase D (phosphodiesterase activity). It can attack phosphatidylcholine, phosphatidylethanolamine, phospha-tidylserine, and phosphatidylglycerol. In most cases, Ca2+ was an activator, but variable results were obtained on the positive influence of diethyl ether on the catalytic activity of different sources of this enzyme. Usually the optimum pH was in the range from 5.0 to 7.0. Mammalian phospholipase D, containing both the phosphodiesterase and transphosphatidylase activities, exhibited a broad-range substrate specificity similar to that of the plant enzyme. However, the mammalian enzyme showed a dependency for the presence of oleic acid in the reaction system (Kobayashi and Kanfer, 1991). 

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]. 

Phospholipase A enzymes - particularly those from snake venoms or digestive secretions - have been widely studied. Phospholipase Ai is found in microsomal and liposomal fractions (cf. Newkirk and Waite, 1971 Gatt, 1968). It specifically deacylates phosphatidylcholine or phosphatidylethanolamine at the 1-position. Both these substrates are hydrolysed at the same rate by the adrenal medulla lysosomal enzyme, but that from brain prefers phosphatidylcholine as substrate. Detergents will increase phosphatidylethanolamine hydrolysis by the brain enzyme. A phospholipase Ai (which is relatively specific for phosphatidylglycerol) has been reported from the spores of some bacteria (Raybin et aL, 1972), but most bacterial enzymes are unspecific for either the 1- or the 2-positions. 

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