Phospholipids lysophospholipids

This emphasises the importance of the cell containing sufficient free arachidonic acid for such an exchange to take place. The combination of these two reactions, if they occur simultaneously, is a cycle - the phospholipid/ lysophospholipid cycle (Figure 11.22). 

Although extraction of lipids from membranes can be induced in atomic force apparatus (Leckband et al., 1994) and biomembrane force probe (Evans et al., 1991) experiments, spontaneous dissociation of a lipid from a membrane occurs very rarely because it involves an energy barrier of about 20 kcal/mol (Cevc and Marsh, 1987). However, lipids are known to be extracted from membranes by various enzymes. One such enzyme is phospholipase A2 (PLA2), which complexes with membrane surfaces, destabilizes a phospholipid, extracts it from the membrane, and catalyzes the hydrolysis reaction of the srir2-acyl chain of the lipid, producing lysophospholipids and fatty acids (Slotboom et al., 1982 Dennis, 1983 Jain et al., 1995). SMD simulations were employed to investigate the extraction of a lipid molecule from a DLPE monolayer by human synovial PLA2 (see Eig. 6b), and to compare this process to the extraction of a lipid from a lipid monolayer into the aqueous phase (Stepaniants et al., 1997). 

Lipid phosphate phosphohydrolases (LPPs), formerly called type 2 phosphatidate phosphohydrolases (PAP-2), catalyse the dephosphorylation of bioactive phospholipids (phosphatidic acid, ceramide-1-phosphate) and lysophospholipids (lysophosphatidic acid, sphingosine-1-phosphate). The substrate selectivity of individual LPPs is broad in contrast to the related sphingosine-1-phosphate phosphatase. LPPs are characterized by a lack of requirement for Mg2+ and insensitivity to N-ethylmaleimide. Three subtypes (LPP-1, LPP-2, LPP-3) have been identified in mammals. These enzymes have six putative transmembrane domains and three highly conserved domains that are characteristic of a phosphatase superfamily. Whether LPPs cleave extracellular mediators or rather have an influence on intracellular lipid phosphate concentrations is still a matter of debate. 

Phosphatidylcholine, commonly known as lecithin, is the most commonly occurring in natnre and consists of two fatty add moieties in each molecule. Phosphati-dylethanolamine, also known as cephahn, consists of an amine gronp that can be methylated to form other compounds. This is also one of the abundant phospholipids of animal, plant, and microbial origin. Phosphatidylserine, which has weakly acidic properties and is found in the brain tissues of mammals, is found in small amounts in microorganisms. Recent health claims indicate that phosphatidylserine can be used as a brain food for early Alzheimer s disease patients and for patients with cognitive dysfunctions. Lysophospholipids consist of only one fatty acid moiety attached either to sn-1 or sn-2 position in each molecule, and some of them are quite soluble in water. Lysophosphatidylchohne, lysophosphatidylserine, and lysophos-phatidylethanolamine are found in animal tissues in trace amounts, and they are mainly hydrolytic products of phospholipids. 

Two possible pathways for the biosynthesis of 2-AG have been proposed (1) a phospholipase C (PLC) hydrolysis of membrane phospholipids followed by a second hydrolysis of the resulting 1,2-diacylglycerol by diacylglycerol lipase or (2) a phospholipase Ai (PLA,) activity that generates a lysophospholipid, which in turn is hydrolyzed to 2-AG by lysophospholipase C (Fig. 5) (Piomelli, 1998). Alternative pathways may also exist from either triacylglycerols by a neutral lipase activity or lysophosphatidic acid by a dephosphorylase. The fact that PLC and diacylglycerol lipase inhibitors inhibit 2-AG formation in cortical neurons supports the contention that 2-AG is, at least predominantly, biosynthesized by the PLC pathway (Stella, 1997). However, a mixed pathway may also be plausible. 

Mercury is known to exert an effect on the synthesis of membrane lipids. Mercuric chloride produces lipid alteration in pig kidney epithelial cells (LLC-PK, cells), with rapid accumulation of unesterified fatty acids (particularly arachidonic acid) and lysophospholipids and loss of cellular phospholipids... 

Phospholipids are also found which have similar structures (Table 12.6). These include lysophospholipids, which have only one of the two possible positions of glycerol esterified, almost invariably at carbon 1, and the plas-malogens, in which there is a long chain vinyl ether at carbon 1 instead of a fatty acid ester. These compounds also contain an amino alcohol, which may be either serine, ethanolamine or choline. Other rarer phospholipids are the monoacyl monoether, the diether and the phosphono forms. 

Phospholipids are digested and absorbed in a similar manner to that of triacylglycerol. Pancreatic lipase has some hydrolytic activity towards phospholipids and removes the fatty acid from the 1-position. The product is a lysophospholipid such as lysolecithin (Figure 4.12). It also acts as a detergent and contributes to the stability of the mixed micelles. 

Figure 4.12 Hydrolysis of a phospholipid (lecithin) in the lumen by a phospholipase. Lysolecithin is a lysophospholipid and is a detergent. At high concentrations it can damage membranes. It is also produced during repair of damaged phospholipids (Chapter 11)...

Fig. 8.9 The activity of phospholipase A2 can be determined by following the consumption of the triacylglycerol phospholipid and the formation of the lysophospholipid.

The other phospholipids can be derived from phosphatidates (residue = phosphatidyl). Their phosphate residues are esterified with the hydroxyl group of an amino alcohol choline, ethanolamine, or serine) or with the cyclohexane derivative myo-inositol. Phosphatidylcholine is shown here as an example of this type of compound. When two phosphatidyl residues are linked with one glycerol, the result is cardiolipin (not shown), a phospholipid that is characteristic of the inner mitochondrial membrane. Lysophospholipids arise from phospholipids by enzymatic cleavage of an acyl residue. The hemolytic effect of bee and snake venoms is due in part to this reaction. 

Regarding other pseudostationary phases for measurement of lipophilicity or lipophilicity-related properties (e.g., intestinal absorption, brain penetration), there are several reports on the use of vesicles such as phospholipid bilayer liposome (56-58), lysophospholipid micelle (59), DTAB/SDS vesicle (60), and double-chain synthetic surfactant vesicle (61), which are described in other chapters. 

Phospholipids are hydrolyzed by phospholipases, which remove a fatty acid from carbon 2, leaving a lysophospholipid, which may be further processed or absorbed. 

T6. The Action of Phospholipases The venom of the Eastern diamondback rattler and the Indian cobra contains phospholipase A2, which catalyzes the hydrolysis of fatty acids at the C-2 position of glycerophospholipids. The phospholipid breakdown product of this reaction is lysolecithin (lecithin is phosphatidylcholine). At high concentrations, this and other lysophospholipids act as detergents, dissolving the membranes of erythrocytes and lysing the cells. Extensive hemolysis may be life-threatening. 

Lysophospholipids have been found in butter serum by Cho et al. (1977). They characterized the sn-1 and -2 lysophosphatidylcholines and phosphatidylethanolamines. It is not known if these compounds are products of degradation or remnants of biosynthesis. Cho et al. (1977) searched for, but did not find, another possible product of enzymatic degradation of milk, phosphatidic acid. Phosphatidic acid can be formed by the action of phospholipase D on phosphatidylcholine, for example, but this enzymatic activity was not detected. The compound is also an important intermediate in the biosynthesis of lipids, but the concentration in tissue is always very low. The amount is also low in milk. Cho et al. (1977) found 1.2 and 0.9 (percent of total lipid P) of the lyso compounds above. The quantities of the other phospholipids were phosphatidylethanolamine, 27.3 -choline, 29.1 -serine, 13.4 -inositol, 2.5 and sphingomyelin, 25.6. 

There are several methods to selectively open up closed polymeric membrane compartments in order to release entrapped substances (Fig. 37). For uncorking a polymerized vesicle, its membrane has to contain destabilizable areas which could possibly be opened up by variation of pH 70), temperature increase71), photochemical destabilization 72), or enzymatic processes. Such an enzymatic process is the hydrolysis of a natural phospholipid by phospholipase A2 (Fig. 38). This enzyme cleaves the ester bond in position two of a natural phosphoglyceride producing a lysophospholipid and a fatty acid which are both water soluble. This leads to complete destruction of the membrane. 

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... 

However, Shaikh demonstrated that the aforementioned traditional methods are inappropriate to recover completely lysophospholipids as well as acidic phospholipids classical Folch gave 85-90% recovery of LPC and LPE, whereas Bligh and Dyer yielded only 75-80% recovery. Extraction with a mixture of chloroform and methanol, on the other hand, provided nearly complete recovery of acidic and lysophospholipids, but up to 15% losses were observed during subsequent washing, according to Folch. These losses could be circumvented by purification of the crude extract on Sephadex G-25, but this column chromatographic procedure is quite time-consuming. 

The progressive increase of lysophospholipids within the AZ due either to SPAN PLA2 mediated hydrolysis of phospholipids, or to partition of lysophospholipids present in the bathing medium (Rigoni et al. 2005), is suggested to cause a transition from inhibition (phase 1) to promotion of neuroexocytosis (phase 2) with the mechanisms discussed here below. 

Fig. 5 The snake PLA2 neurotoxin is depicted here as a snake, which binds to an active zone, i.e., a synaptic vesicle (SV) release site, and hydrolyses the phospholipids of the external layer of the presynaptic membrane (green) with formation of the inverted-cone shaped lysophospholipid (yellow) and the cone-shaped fatty acid (dark blue). Fatty acids rapidly equilibrate by trans-bilayer movement among the two layers of the presynaptic membrane. In such a way lysophospholipids, which induce a positive curvature of the membrane, are present in trans and fatty acid, which induce a negative curvature, are present also in cis, with respect to the fusion site. This membrane conformation facilitates the transition from a hemifusion intermediate to a pore. Thus, the action of the toxin promotes exocytosis of neurotransmitter (NT) (from the left to the right panel) and, for the same membrane topological reason, it inhibits the opposite process, i.e., the fission of the synaptic vesicle.

Rigoni M, Caccin P, Gschmeissner S, Koster G, Postle AD et al. (2005) Equivalent effects of snake pla2 neurotoxins and lysophospholipid-fatty acid mixtures. Science 310 1678-80 Rigoni M, Pizzo P, Schiavo G, Weston AE, Zatti G et al. (2007) Calcium influx and mitochondrial alterations at synapses exposed to snake neurotoxins or their phospholipid hydrolysis products. JBiolChem 282 11238-45... 

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