Diacyl phospholipids

Since the fatty acid chains in each lipid were 18 carbons and 16 carbons, respectively, it is reasonable that they could form a mixed lipid phase. Furthermore the bis-dienoyl substitution of 15 favors the formation of crosslinked polymer networks. Ohno et al. showed that the dienoyl group associated with the sn-1 chain could be polymerized by lipid soluble initiators, e.g. AIBN, whereas the dienoyl in the sn-2 chain was unaffected by AIBN generated radicals. Conversely, radicals from a water-soluble initiator, e.g. azo-bis(2-amidinopropane) dihydrochloride (AAPD), caused the polymerization of the sn-2 chain dienoyl group, but not the sn-1 chain. These data provide clear evidence for the hypothesis of Lopez et al. that the same reactive group located in similar positions in the sn-1 and sn-2 chains of polymerizable 1,2-diacyl phospholipids are positionally inequivalent [23]. 

The dependence of the chain-melting or main phase transition temperature in a lipid bilayer, Tm, on the nature of the lipid head group as well as the length of the acyl chains and the degree and type (cis- or trans-) of unsaturation of the acyl chains has been studied exhaustively for some diacyl phospholipid classes (21). Bulkier head groups result in lower values of... 

The most detailed study has been carried out with a 50-fold purified enzyme preparation from Clostridium butylicium where the substrate is not the free olefinic fatty acid but a corresponding diacyl phospholipid (114). The enzyme displayed head group specificity for phosphatidylethanolamine and positional isomer preference of cyclopropanation for the acyl chain at C(l) of these glycerol-based phospholipids. For the monoolefinic C acyl chains, the A olefin (114) was a much better cyclopropanation substrate than the A olefins (even though the ratios of A /A hexadecenoates in these populations is 2/1) while for the Cig monoolefins, the A Lolefinic site was preferred kinetically 3 1 over the A -olefinic sites... 

Figure 5. Biosynthetic pathways for diacyl, plasmalogen and alkyl-ether molecular subclasses of phospholipids. Monoacyl dihydroxyacetone phosphate is the key branch-point intermediate whose utilization determines the phospholipid subclass distribution of newly synthesized phospholipids. Reduction of monoacyl dihydroxyacetone phosphate leads to the biosynthesis of diacyl phospholipids. Fatty alcohol exchange, catalyzed by alkyl dihydroxyacetone phosphate synthase, is the first committed step in the biosynthesis of alkyl-ether and plasmalogen subclasses of phospholipids.

Diacyl phospholipid molecules are extremely well fitted for their role as membrane constituents mainly because they exhibit... 

From studies on the incorporation of [3H]-leucine into proteins of the synaptic vesicles and nerve ending membranes, a half life of 20-22 days was calculated (Von Hungen et al., 1968). Lapetina et al. (1969) found a more rapid turnover for brain phospholipids, though different parts of the molecule seemed to turn over at different rates. Acylation of lysophospholipids to diacyl-phospholipids seems to be carried... 

Mahadevappa, V.G. and Holub, B.J. (1982). The molecular species composition of individual diacyl phospholipids in human platelets. Biochim. Biophys. Acta, 713, 73-79... 

Xia J, Hui YZ (1997) The Efficient Synthesis of Mixed Diacyl Phospholipids with Polyunsaturated Fatty Acid in sn-2 Position of Glycerol. Tetrahedron Asymm 8 3019... 

Plasmalogens are a group of phosphoglycerides which are phosphorylated derivatives of 1-alk-cw-l-enyl-2-acyl-5n-glyceroL The similarity in properties between plasmalogens and the corresponding 1,2-diacyl phospholipids has made isolation procedures difficult. Moreover, chemical synthesis is made... 

Early experiments on phospholipase A used an enzyme from snake venom which released one mole of fatty acid from a diacyl phospholipid. The original studies showed quite clearly that unsaturated fatty acids were released from egg or liver phosphatidylcholines and so it was quite natural to assume that the enzyme was specific for unsaturated acids. When fully saturated or fully unsaturated lipids were the substrates, however, one mole of fatty acid was still released from one mole of phospholipid and it was realized that the enzyme must be specific for one or other of the two positions. Early chemical analyses of the products of phospholipase A hydrolysis seemed to indicate that the enzyme released the fatty acid from the 1-position. When it was discovered that the enzyme also liberates fatty acid from plasmalogen (where it is known quite definitely to be located at the 2-position) the problem was reinvestigated more rigorously. 

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